Pepro Tech
Originally published online as doi:10.1189/jlb.0208140 on July 18, 2008

Published online before print July 18, 2008
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow All Versions of this Article:
jlb.0208140v1
84/5/1238    most recent
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Bots, M.
Right arrow Articles by Medema, J. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Bots, M.
Right arrow Articles by Medema, J. P.
(Journal of Leukocyte Biology. 2008;84:1238-1247.)
© 2008 by Society for Leukocyte Biology

Serpins in T cell immunity

Michael Bots1 and Jan Paul Medema

Laboratory of Experimental Oncology and Radiobiology, Center for Experimental and Molecular Medicine, Academic Medical Center, Amsterdam, The Netherlands

1 Correspondence at current address: Gene Regulation Laboratory, Cancer Immunology Program, Peter MacCallum Cancer Centre, St Andrews Place, East Melbourne 3002, Victoria, Australia. E-mail: michael.bots{at}petermac.org


arrow
ABSTRACT
 
Serine protease inhibitors (serpins) are a family of proteins that are important in the regulation of several biological processes. This mainly involves the inhibition of serine proteases, although some serpins inhibit a different class of proteases or even function without inhibitory activity. In contrast to other protease inhibitor families, serpins inhibit their target proteases by a specific mechanism, which depends on a change in conformation. This review primarily focuses on one subgroup of serpins—ovalbumin (ov)-serpins. Different than most members of the family, this group of serpins lacks secretion signal sequences and therefore, mainly functions intracellularly. In addition to expression in most normal tissues, ov-serpins can be found in multiple different cells of the immune system. Interestingly, expression of ov-serpins in these cells is tightly regulated, indicating a role for these serpins in the regulation of immune responses. The role of serpins in the immune response will be the topic of this review.

Key Words: granzyme • apoptosis


arrow
INTRODUCTION
 
Multiple biological processes are regulated by enzymes with proteolytic activity, which are called proteases. Proteases have the capacity to cleave other proteins in a sequence-specific manner. Historically, proteases are subdivided according to the amino acid in the reactive site of the enzyme. Serine, cysteine, but also other amino acids such as threonine have been reported. The class of serine proteases is the largest class of proteases and consists of roughly 200 members in humans [1 ].

Proteases cleave a wide variety of target proteins, including hormones, structural proteins, signaling intermediates, and transcription factors. It is clear that such an activity requires tight regulation to prevent uncontrolled proteolysis. This regulation can be achieved in multiple ways. First of all, most members of the family are produced as inactive proforms or zymogens, which require proteolytic cleavage to become active [2 , 3 ]. This mechanism ascertains that proteases only become active upon cleavage by other proteases. Second, the expression of the protease or more often, the target is regulated in such a way that proteolysis only occurs when required. For example, the protease tissue plasminogen activator is expressed in response to factors related to the coagulation cascade, which ensures the local activation of plasmin [4 , 5 ]. Third, the substrate may be modified in a way that will target it for proteolytic degradation. For instance, degradation of I{kappa}B{alpha} by the proteasome only takes place after its phosphorylation and ubiquitination [6 ]. Alternatively, the localization of the protease may be tightly regulated. For instance, proteases may be secreted into the extracellular space to meet their substrate. Finally, protease activity may be under the regulation of endogenous protease inhibitors that prevent their activity. This latter regulatory mechanism is the basis of this review, which focuses on the family of serine protease inhibitors (serpins) and specifically on the role of ovalbumin serpins (ov-serpins) in immunity.

The serpin family of protease inhibitors is made up of a large group of homologous glycoproteins and was first described in the early 1980s [7 ]. In eukaryotes, this family consists of several hundred members [8 ]. However, expression of serpins is not only restricted to eukaryotes. Members of this family have been found in bacteria, archaea, and viruses [8 , 9 ]. Phylogenetic analysis of the protein sequences show that serpins can be classified into at least 16 groups, which are called "clades". In humans, serpins fall into nine different clades [8 , 10 ]. The majority of these human serpins plays a role in the extracellular processes. However, the so-called ov-clade consists of serpins that are primarily involved in intracellular processes [11 ].

Most serpins inhibit serine proteases, but some display so-called cross-class inhibition, as they target other classes of protease instead [12 ]. In addition, the serpin family includes several members that have no protease inhibitory activity. These non-inhibitory serpins function in diverse biological processes, such as hormone transport (thyroxine-binding globulin) [13 ], blood pressure regulation (angiotensinogen) [14 ], and angiogenesis (pigment epithelium-derived factor) [15 ]. However, the intriguing mechanism by which serpins inactivate serine proteases is the major reason this family has attracted enormous attention over the past decades.


arrow
STRUCTURE AND MECHANISM OF ACTION
 
In general, serpins have a core domain of 350–500 aa in size. Despite this relatively uniform core domain, the actual size of serpins is highly variable, mainly as a result of modifications such as glycosylation and N- and C-terminal extensions [16 ]. The eukaryotic family members have been characterized by at least 30% sequence homology with the prototypical serpin {alpha}1-antitrypsin and have an overall conserved tertiary structure. This conserved structure consists of nine {alpha}-helices, three β-sheets, and a flexible reactive center loop (RCL) and is essential for the function of serpins [17 18 19 ]. The RCL is ~20 aa in length and contains a sequence that resembles the natural substrate of the protease targeted by the serpin. The most important residue in the RCL is the so-called P1 residue, which primarily defines the inhibitory specificity of the serpin [20 , 21 ]. The surrounding residues (P4–P4') contribute to the recognition of the protease by enhancing the affinity of the interaction [22 ].

Crystal structures have revealed that in a native conformation, serpins fold in a metastable state [17 , 23 ]. This fold is crucial for their function. For instance, a well-known mutation (substitution of a glutamic acid to a lysine at residue P17) in {alpha}1-antitrypsin interferes with this folding and renders the serpin inactive [24 ]. In this metastable state, the RCL is placed above the body of the serpin and acts as "bait" for the target protease, which recognizes the RCL as a normal substrate, and the subsequent interaction results in cleavage of the serpin between the residues P1 and P1' [16 , 20 , 21 ]. As always in proteolysis, cleavage initially results in a covalent acyl ester intermediate between the active serine residue of the protease and the P1 residue of the substrate. In case of a natural substrate, the protease continues the process of proteolysis as a result of a water molecule in the active site that will hydrolyze the acyl ester bond to release the protease and the cleaved substrate [3 ]. However, upon initial cleavage of a serpin, this process is diverted. This is the result of the unique structure of serpins, which induces a conformational change, in which the cleaved RCL loop is inserted into the major β-sheet (Fig. 1 ). This conformational change is termed the "stressed to relaxed transition" and increases the stability of the serpin considerably. As the protease is still covalently bound to the RCL, the conformational change causes the translocation of the protease from top to bottom [20 , 21 , 25 ]. More importantly, the translocation distorts the active site of the protease in such a way that the hydrolytic water molecule can no longer be positioned to deacetylate the peptide bond [25 , 26 ]. In effect, the protease remains covalently bound to the serpin. This stable complex is then removed rapidly by binding to low-density lipoprotein receptor-related proteins [16 , 27 ]. This family of proteins hardly recognizes serpins or proteases on their own as compared with stable complexes. The exact domains on proteases and serpins that determine the binding of serpin–protease complexes to these receptors are not yet known. However, it has been suggested that these domains are only exposed after complex formation, explaining the preference of the receptors for binding serpin–protease complexes [20 ].


Figure 1
View larger version (14K):
[in this window]
[in a new window]

  		Figure 1. Schematic overview of the serpin mechanism of action. After serpin recognition, the protease cleaves the RCL between the P1 and P1' residues. Following this cleavage, the RCL is inserted into the body of the serpin. As the protease is still bound to the RCL, this conformational change causes the translocation of the protease and results in a stable complex without protease activity.


arrow
REGULATION OF SERPIN ACTIVITY
 
Serpins in an active (i.e., metastable) conformation thus rapidly and specifically inhibit the activity of serine proteases. This implies that serpin activity, like protease activity, needs to be controlled tightly. In human blood, for instance, where serpins are abundant, constitutive activity of serpins would prevent the activation of proteases and thereby could block the formation of blot clots with severe bleeding as an evident consequence. As is the case for proteases, multiple regulatory mechanisms are in place to control the activity of serpins. It is clear that regulated expression of serpins is an important mechanism to prevent unwanted protease inhibition. For instance, during inflammation, the expression of {alpha}1-antitrypsin in the circulation is induced threefold as a result of an increased production and primarily prevents unwanted damage in the respiratory tract [28 ]. Furthermore, localization is an important regulatory mechanism. Serpins involved in coagulation, such as antithrombin and heparin cofactor II, become active around sites of vascular injury. This appears to be regulated by the specific expression of cofactors such as heparin, which forms a third way to regulate the activity of serpins [29 ]. In their native form, antithrombin and heparin cofactor II are folded in a different conformation—their RCL is partially inserted into the top of the major β-sheet [20 , 30 , 31 ]. Because of this conformation, the target protease cannot interact with the important residues in the RCL, and in this native form, these serpins are therefore relatively poor inhibitors of their target proteases. Upon binding to a specific pentasaccharide sequence present in heparin molecules, the RCL is expelled and renders the RCL available for the protease interaction and inhibition [20 , 32 , 33 ]. Besides modifying the serpin structure, full-length heparin also serves as a platform binding to serpin and protease and thereby facilitates their interaction [34 , 35 ]. Next to the regulation of antithrombin and heparin cofactor II by heparin, activity of other serpins has been described to depend on the presence of cofactors. For instance, binding to vitronectin keeps plasminogen activator inhibitor-1 (PAI-1) to its active conformation [36 , 37 ].

Regulation of serpin activity is thus a complex and intriguing mechanism to control protease activity in diverse biological processes. As mentioned, most of these serpin–protease interactions occur extracellularly except for those mediated by the ov-subfamily. Below, we will review briefly what is known about the ov-subfamily and will then focus on the regulation of T cell immunity by ov-serpins.


arrow
OV-SERPINS
 
Roughly 30% of the identified serpins in humans belongs to the subfamily of ov-serpins [38 ], which were identified in the early 1990s on the basis of a high homology in amino acid sequence (>39%) with the prototypical ov-serpin [39 ]. The 13 human ov-serpins are encoded by genes that are located in two clusters on chromosomes 6 and 18, and all consist of seven or eight exons [40 ]. Other characteristics of ov-serpins are the absence of N- and C-terminal extensions and the lack of an N-terminal signal peptide [39 ]. As a consequence, ov-serpins are found intracellularly. Nevertheless, secretion of some ov-serpins has been reported. It remains to be determined though whether this serves a physiological function. Almost all human ov-serpins encode for true protease inhibitors (Table 1 ), although some exceptions exist. Maspin (serpinB5) and Epipin (serpinB11), for instance, encode for a noninhibitory serpin [41 , 42 ]. Maspin is suggested to suppress tumor growth and metastasis [85 , 86 ], but Epipin is currently without function. Similarly, Megsin (serpinB7) is suggested to encode for a noninhibitory serpin and appears to be involved in maturation of megakaryocytes [87 ].


View this table:
[in this window]
[in a new window]

  		Table 1. Overview of Human OV-Serpins

The murine ov-serpin family encompasses many more members as compared with the human [88 ]. Most of the additional members seem close homologs and derived from relatively recent gene duplications. For instance, PI-9 (serpinB9) contains seven close homologues in mice [88 , 89 ]. Of these, serine protease inhibitor-6 (SPI-6; serpinb9) is most likely the murine ortholog of PI-9 in that its expression profile and target protease (GrB) are similar [89 , 90 ]. Another member of this cluster is serine protease inhibitor involved in cytotoxicity inhibition (SPI-CI; serpinb9b) [89 ], which encodes for an inhibitor of GrM [91 ]. Although it is not completely clear why the mouse genome encodes many more ov-serpins, it seems reasonable to assume that this is a result of a multiplication of protease targets. In agreement, T cell activation in the mouse is accompanied by the expression of more granule proteases (granzymes) as compared with the human T cells, and it is well established that serpins play a role in the regulation of these proteases.


arrow
SERPIN INVOLVEMENT IN T CELL IMMUNITY
 
Immune responses regulated by T cells involve a plethora of proteolytic events, which are tightly regulated, especially by ov-serpin activity. As most of the available information about this regulation has been obtained from mouse studies, we will focus on the mouse T cell response and will provide links to the human system where possible.

T cells are critically involved in the cellular immune response against virally infected or malignant cells. Activation of T cells requires the involvement of DC [92 , 93 ]. DC take up antigen in the periphery and are stimulated by so-called danger signals upon which DC mature and migrate to lymphoid organs. According to the "licensing" model, in these organs, DC interact with naïve CD4+ Th cells [94 95 96 ]. Such Th cells recognize processed antigen presented on the DC in MHC class II and induce further maturation of DC, via, for instance, the CD40 ligand–CD40 interaction [95 , 96 ]. These fully matured DC are now capable of stimulating naïve CD8+ T cells via antigen presented in MHC class I molecules. Upon stimulation, CD8+ T cells are activated rapidly and become effector cells, which have the capacity to induce cell death. Effector CD8+ T cells, so-called CTL, divide rapidly and subsequently leave the lymphoid organs to find their target cells in the periphery. Recognition of targets by the CTL results in the induction of cell death by means of two distinct mechanisms [97 , 98 ]. The first involves oligomerization of death receptors, which include CD95 and TNFRI [99-101 ]. The second depends on the release of cytotoxic proteins from the granules of CTL [102 ]. Among these, cytotoxic proteins are the pore-forming protein perforin and several granzymes [103 , 104 ], including GrB [105 , 106 ]. Perforin is crucial for the entry of granzymes, and these subsequently induce death of the target cells [107 108 109 ].


arrow
OV-SERPINS IN DC–CTL INTERACTION
 
Although priming of CD8+ T cells is meant to result in eradication of unwanted cells in the periphery, DC are likely the first peptide-specific targets that CTL encounter. Interaction between CTL and DC then may result in elimination of DC and prevent the initiation of a proper CTL response. Several reports, however, have described the expression of the GrB inhibitor PI-9/SPI-6 in DC [67 68 69 ]. Expression of PI-9/SPI-6 correlated with the maturation status of DC; stimulation with LPS or anti-CD40 results in the maturation of DC and concomitant in the induction of PI-9/SPI-6 [68 ]. Importantly, in murine DC, it has been shown that the expression of SPI-6 renders DC resistant to CTL-induced apoptosis. In support of this observation, incubation with Th1 but not with Th2 cells has been found to induce the expression of PI-9/SPI-6 and protection of DC, although both CD4+ subsets induced maturation in DC [68 ]. Critical in the inhibition of PI-9/SPI-6 is the immunoregulatory cytokine IL-10, as coincubation with IL-10 and Th1 cells prevents the up-regulation of PI-9/SPI-6. Recent data suggest that expression of PI-9/SPI-6 is not only needed to protect DC against the killing machinery of CTL, as it appears a much more general feature of DC maturation (M. Bots, manuscript in preparation). For instance, stimulation of bone marrow-derived DC (BMDC) with polyinosinic:polycytidylic acid induced the expression of SPI-6 but not the release of the important Th1 cytokine IL-12p70. Moreover, LPS stimulation results in the induction of SPI-6 in MyD88-deficient BMDC, although peptide vaccination in the presence of LPS does not induce CD8+ T cell responses. These observations suggest that in DC, the expression of PI-9/SPI-6 is a feature of maturity in general and also necessary in other situations than during the priming of CD8+ T cells. Nevertheless, our studies and studies from others have shown that serpin activity in DC is closely associated with CTL priming.


arrow
OV-SERPINS IN CTL ACTIVATION
 
Besides the expression in DC, serpin expression is also required in the CTL itself. When CTL meet their targets, perforin and granzymes are efficiently directed toward these target cells via the formation of an immunological synapse. However, one can imagine that granzymes loop back on the CTL itself during this interaction. In addition, simple leakage of the cytotoxic granzymes from the granules during the lifespan of the CTL will also have severe consequences to CTL themselves. To prevent this from happening, CTL express factors that protect them from the action of granzymes, of which the serpin SPI-6 is the best-studied. As mentioned above, SPI-6 is expressed in activated CTL but is absent from naïve CD8+ T cells, a pattern that correlates with the expression of its target protease GrB [69, 90 ]. Similarly, SPI-CI, a serpin that inhibits GrM, has been found to be expressed upon activation in CD8+ T cells and other lymphoid killer cells such as NK cells and appears to be correlated with the levels of GrM [91 ]. Importantly, PI-9, the human ortholog of SPI-6, has been shown to localize around GrB-containing granules and was thus suggested to sequester GrB leaking from the granules [69 ]. In agreement, overexpression of PI-9 allowed for higher GrB expression within the CTL and thus increased cytolytic activity. For SPI-6, the role in protection of CTL from autolysis is even clearer. In vitro activation of CD8+ T cells deficient for SPI-6 resulted in increased levels of active GrB in the cytoplasm of these CTL as compared with wild-type CD8+ T cells [110 ]. Subsequently, this cytoplasmic GrB induced further granule breakdown and induced apoptosis in these SPI-6-deficient CTL. Expression of SPI-6 is also found to be crucial for the viability of CTL in vivo, as infection with lymphocytic choriomeningitis virus (LCMV) or Listeria monocytogenes resulted in lower numbers of specific CTL in SPI-6-deficient mice as compared with control mice [110 ]. As a consequence, SPI-6-deficient mice failed to clear LCMV. This indicates that SPI-6 is essential for the survival of GrB containing CTL and is thus crucial in the expansion phase of CTL (Fig. 2 ).


Figure 2
View larger version (10K):
[in this window]
[in a new window]

  		Figure 2. Expression kinetics of SPI-6 and SPI-2A in a CD8+ T cell response during expansion, contraction, and memory phases.


arrow
OV-SERPINS IN IMMUNE ESCAPE OF TUMORS
 
Activated CTL are crucial controllers of virus-infected and malignant cells. However, many reports have shown that such cells can escape CTL responses. Tumor immune escape can be a result of the presence of T regulatory cells (Tregs) and release of immunosuppressive cytokines, which affect the initiation of proper anti-tumor immune responses [111 , 112 ]. Moreover, down-regulation of MHC class I molecules or prevention of peptide processing on viral-infected cells or tumors impairs the recognition of target cells by CTL [113 , 114 ]. However, even when tumors or virally infected cells are recognized by activated CTL, they can still prevent their elimination via the expression of specific, antiapoptotic molecules [115 ]. As the perforin/granzyme pathway is essential in this elimination, it is not surprising that tumors have acquired a specific defense mechanism against this effector pathway. This appears largely as a result of the expression of serpins. Murine GrB inhibitor SPI-6 is expressed in several murine tumor cell lines derived from spontaneous tumors of different origins [116 ]. In vitro, SPI-6 (over)expression correlated with the resistance to apoptosis induced by CTL or recombinant GrB but not anti-CD95 [90 , 116 , 117 ]. More important, also, in vivo expression of SPI-6 protected tumor cells from CTL-induced killing [116 ]. Besides the expression of a GrB-inhibitory serpin, we found that murine colon carcinoma cell lines express another serpin, SPI-CI, which prevents death induced by GrM [91 ]. Down-regulation of SPI-6 and SPI-CI renders colon carcinoma cell line CMT93 sensitive to CTL-induced cytolysis. Importantly, overexpression of SPI-6 and SPI-CI did not prevent the induction of CTL-mediated cytolysis in lymphomas. It therefore appears likely that CMT93 contains even more inhibitory serpins.

Also, in human tumors, there is evidence that overexpression of PI-9 protects tumor cells against GrB-induced apoptosis. Endogenously, PI-9 was found to be expressed in tumors from different origins, such as colon carcinoma, melanoma, breast carcinoma, and lymphoma [116 ]. Expression in non-Hodgkin lymphomas was linked with high-grade malignancy, and in patients with anaplastic, large cell lymphoma, PI-9 expression was strongly related with a poor prognosis [118 , 119 ]. More convincing evidence that supports the hypothesis that PI-9 expression protects tumor cells from immune attack has recently been reported. In this study, PI-9 expression was shown to be an important determinant in disease-free survival time of melanoma patients following immunotherapy [120 ].

Intriguingly, it seems that some viruses have also managed to copy this system. Orthopoxviruses, for instance, express three proteins that belong to the serpin family: SPI-1, -2, and -3 [121 122 123 ]. Of these, SPI-3 has currently no known function, but SPI-1 and -2 clearly function to protect virus-infected cells from CTL-induced killing [124 ]. SPI-2 is suggested to do this via inhibition of GrB and caspases, although its affinity for GrB is relatively low [125 126 127 ]. SPI-1 is reported to inhibit chymotrypsin activity [128 ] and as such, may have similar target specificity as SPI-CI. Deletion of these serpins from the viral genome results in a much-less virulent strain, which supports the conclusion that orthopoxviruses use serpins to escape immune-mediated suppression. It thus seems reasonable to conclude that serpins are crucial in the regulation of killing by CTL on the side of the CTL and on the side of the target.


arrow
OV-SERPINS IN CONTRACTION/MEMORY PHASE
 
Fortunately, most CTL target interactions end in successful target elimination and subsequently, in a cessation of the immune response. An important process in this termination is the eradication of the activated CD8+ T cells. In this contraction phase, almost all activated CD8+ T cells are eliminated [129 ]. Several mechanisms have been described that may contribute to this elimination. For instance, studies performed with IFN-{gamma}-deficient mice showed a reduced contraction of specific CD8+ T cells after infection with LCMV or L. monocytogenes [130 , 131 ]. Additionally, the pore-forming protein perforin has been suggested to influence the elimination of CD8+ T cells during the contraction phase [132 133 134 ]. However, as perforin is essential for the induction of target cell death via the secretion of cytotoxic granules, this conclusion is potentially flawed. Failure to clear pathogens in perforin-deficient mice may result in a continuous presentation of antigen and thus, increased numbers of specific CD8+ T cells. If, however, perforin plays a role in T cell contraction, then it is likely to do so via its main function—delivery of granzymes into the cell. It is therefore of interest to determine whether mice deficient in one of these granzymes have a defect in the contraction phase. For GrB, this has been analyzed directly, but the role of GrB in contraction seems to be minimal, as the numbers of specific CD8+ T cells in GrB-deficient mice did not differ from those found in wild-type mice 15 days after LCMV infection [90 ]. In agreement with this observation, LCMV infection in SPI-6 transgenic mice, which display less GrB activity as a result of an increased inhibition, did not result in a reduced contraction of specific CD8+ T cells as compared with wild-type mice [90 ].

Recent evidence contradicts the role of granzymes or perforin in CTL contraction, as it was shown that release of lysosomal proteases in CD8+ T cells plays a prominent role in their own elimination. Upon infection with LCMV, the degree of contraction of specific CD8+ T cells was diminished in mice overexpressing the serpin SPI-2A, as compared with wild-type mice [135 ]. SPI-2A has been described to prevent the induction of cell death via the inhibition of the cysteine protease cathepsin B but is capable of inhibiting other cathepsins aswell [136 ]. This therefore suggests that release of cathepsin B from the lysosomes into the cytoplasm may be responsible for the contraction of CD8+ T cells. Importantly, in mice with a decreased expression of SPI-2A, the contraction was more severe as compared with wild-type mice [135 ]. These results indicate that expression of SPI-2A affects the level of contraction and thereby influences the formation of the memory CD8+ T cell pool (Fig. 2) . So far, a human homologue for SPI-2A was not described, and it is not yet clear whether human T cell downsizing is regulated in a similar manner.

Survival of memory CTL seems to be regulated differently by ov-serpins as compared with the contraction phase. In memory CD8+ T cells, the expression of SPI-6 is induced as compared with naïve CD8+ T cells, suggesting a role for SPI-6 in these memory CD8+ T cells [90 ]. A possible role for SPI-6 was proposed after SPI-6 transgenic mice and wild-type mice were infected with LCMV, and the CD8+ T cell response was followed. In SPI-6 transgenic mice, LCMV infection resulted in more memory CD8+ T cells as compared with wild-type mice [90 ]. However, in SPI-6 transgenic mice, the number of specific CTL in the effector or contraction phase is not different as compared with wild-type mice. Therefore, the expression of SPI-6 in CD8+ memory T cells is suggested to play an important role in their homeostasis and prevents cell death induced by leakage of GrB out of the granules. In support of this suggestion, it has been shown that memory CD8+ T cells still have 10 times as much GrB than naïve CD8+ T cells [90 ]. As SPI-6 inhibits GrB, one would suggest that similar experiments performed in GrB-deficient mice would result in increased numbers of memory CD8+ T cells as well. However, memory formation of CD8+ T cells in LCMV-infected, GrB-deficient mice has been shown to result in conflicting outcomes [90 , 137 ]. This indicates that further research is necessary to determine the role of SPI-6 and GrB in the homeostasis of memory CD8+ T cells. Nevertheless, from the above, it is clear that ov-serpins play a central role in the contraction and memory phase of CTL.


arrow
OV-SERPINS IN CD4+ CELLS
 
Although CD4+ T cells are generally thought to provide help in the induction of CTL or B cell responses, these cells have been shown to express granzymes as well. This expression suggests that CD4+ T cells may be functional in the direct killing of target cells. Indeed, CD4+ T cells have been reported to function as effector cells in response to viral infections [138 ]. In addition, the granzyme/perforin pathway has been found to be one of the mechanisms that CD4+ Tregs use to control immune responses [139 , 140 ]. Similar to what has been shown for CD8+ T cells, one would expect that these CD4+ T cells require protection from autolysis. Indeed, CD4+ T cells isolated from human peripheral blood are found to express PI-9 [69 ]. Next to expression of granzymes in CD4+ T cells with cytotoxic capacity, TCR triggering in Th cells induced the expression of GrB. Surprisingly, stimulation of Th2 but not of Th1 cells resulted in the induction of death [141 ]. This seems to be dependent on GrB, as Th2 cells could be rescued from death by addition of a GrB inhibitor or cathepsin C inhibitor, which prevents GrB processing and thereby activation. Apparently, Th1 cells were protected from GrB-induced death, which could be explained by an increase in SPI-6 protein in stimulated Th1 but not Th2 cells. Interestingly, stimulated Th2 cells expressed high levels of SPI-2A [141 ], which suggests that these cells are protected from cathepsin-induced death, although this has not been proven yet. Thus, even CD4+ T cells require ov-serpins to exert their function.

Taken together, this overview shows that ov-serpins are important regulators of T cell responses. These serpins not only prevent early death of DC but also protect activated CD4+ and CD8+ T cells from suicide. In addition, they serve a role in the contraction and memory phase (Table 2 ). Unfortunately, serpin expression can also render tumor and virally infected cells less susceptible to granzyme-mediated death. Silencing of serpins in tumor cells would provide CTL a potential therapeutic window to kill the target, but this will be counterbalanced by the negative impact on the CTL response itself. For this approach to work, we will need to design tumor-directed modifications of ov-serpin expression. This is clearly a challenging approach but worth the effort, as the potential benefit in tumor control would be substantial.


View this table:
[in this window]
[in a new window]

  		Table 2. Overview of the Modulatory Effects of Serpins during a CD8+ T Cell Response


arrow
ACKNOWLEDGEMENTS
 
The authors are supported by the Dutch Cancer Society and the Association for International Cancer Research.

Received February 27, 2008; revised May 22, 2008; accepted May 23, 2008.


arrow
REFERENCES
 
    1
  1. Puente, X. S., Sanchez, L. M., Overall, C. M., Lopez-Otin, C. (2003) Human and mouse proteases: a comparative genomic approach Nat. Rev. Genet. 4,544-558[CrossRef][Medline]
    2. 2
  2. Boatright, K. M., Salvesen, G. S. (2003) Mechanisms of caspase activation Curr. Opin. Cell Biol. 15,725-731[CrossRef][Medline]
    3. 3
  3. Hedstrom, L. (2002) Serine protease mechanism and specificity Chem. Rev. 102,4501-4524[CrossRef][Medline]
    4. 4
  4. Emeis, J. J. (1992) Regulation of the acute release of tissue-type plasminogen activator from the endothelium by coagulation activation products Ann. N. Y. Acad. Sci. 667,249-258[Medline]
    5. 5
  5. Oliver, J. J., Webb, D. J., Newby, D. E. (2005) Stimulated tissue plasminogen activator release as a marker of endothelial function in humans Arterioscler. Thromb. Vasc. Biol. 25,2470-2479[Abstract/Free Full Text]
    6. 6
  6. Ben Neriah, Y. (2002) Regulatory functions of ubiquitination in the immune system Nat. Immunol. 3,20-26[CrossRef][Medline]
    7. 7
  7. Hunt, L. T., Dayhoff, M. O. (1980) A surprising new protein superfamily containing ovalbumin, antithrombin-III, and {alpha} 1-proteinase inhibitor Biochem. Biophys. Res. Commun. 95,864-871[CrossRef][Medline]
    8. 8
  8. Irving, J. A., Pike, R. N., Lesk, A. M., Whisstock, J. C. (2000) Phylogeny of the serpin superfamily: implications of patterns of amino acid conservation for structure and function Genome Res. 10,1845-1864[Abstract/Free Full Text]
    9. 9
  9. Irving, J. A., Steenbakkers, P. J., Lesk, A. M., Op den Camp, H. J., Pike, R. N., Whisstock, J. C. (2002) Serpins in prokaryotes Mol. Biol. Evol. 19,1881-1890[Abstract/Free Full Text]
    10. 10
  10. Silverman, G. A., Bird, P. I., Carrell, R. W., Church, F. C., Coughlin, P. B., Gettins, P. G., Irving, J. A., Lomas, D. A., Luke, C. J., Moyer, R. W., Pemberton, P. A., Remold-O'Donnell, E., Salvesen, G. S., Travis, J., Whisstock, J. C. (2001) The serpins are an expanding superfamily of structurally similar but functionally diverse proteins. Evolution, mechanism of inhibition, novel functions, and a revised nomenclature J. Biol. Chem. 276,33293-33296[Free Full Text]
    11. 11
  11. Silverman, G. A., Whisstock, J. C., Askew, D. J., Pak, S. C., Luke, C. J., Cataltepe, S., Irving, J. A., Bird, P. I. (2004) Human clade B serpins (ov-serpins) belong to a cohort of evolutionarily dispersed intracellular proteinase inhibitor clades that protect cells from promiscuous proteolysis Cell. Mol. Life Sci. 61,301-325[CrossRef][Medline]
    12. 12
  12. Schick, C., Pemberton, P. A., Shi, G. P., Kamachi, Y., Cataltepe, S., Bartuski, A. J., Gornstein, E. R., Bromme, D., Chapman, H. A., Silverman, G. A. (1998) Cross-class inhibition of the cysteine proteinases cathepsins K, L, and S by the serpin squamous cell carcinoma antigen 1: a kinetic analysis Biochemistry 37,5258-5266[CrossRef][Medline]
    13. 13
  13. Pemberton, P. A., Stein, P. E., Pepys, M. B., Potter, J. M., Carrell, R. W. (1988) Hormone binding globulins undergo serpin conformational change in inflammation Nature 336,257-258[CrossRef][Medline]
    14. 14
  14. Kim, H. S., Krege, J. H., Kluckman, K. D., Hagaman, J. R., Hodgin, J. B., Best, C. F., Jennette, J. C., Coffman, T. M., Maeda, N., Smithies, O. (1995) Genetic control of blood pressure and the angiotensinogen locus Proc. Natl. Acad. Sci. USA 92,2735-2739[Abstract/Free Full Text]
    15. 15
  15. Dawson, D. W., Volpert, O. V., Gillis, P., Crawford, S. E., Xu, H., Benedict, W., Bouck, N. P. (1999) Pigment epithelium-derived factor: a potent inhibitor of angiogenesis Science 285,245-248[Abstract/Free Full Text]
    16. 16
  16. Gettins, P. G. (2002) Serpin structure, mechanism, and function Chem. Rev. 102,4751-4804[CrossRef][Medline]
    17. 17
  17. Elliott, P. R., Lomas, D. A., Carrell, R. W., Abrahams, J. P. (1996) Inhibitory conformation of the reactive loop of {alpha} 1-antitrypsin Nat. Struct. Biol. 3,676-681[CrossRef][Medline]
    18. 18
  18. Stein, P. E., Leslie, A. G., Finch, J. T., Turnell, W. G., McLaughlin, P. J., Carrell, R. W. (1990) Crystal structure of ovalbumin as a model for the reactive centre of serpins Nature 347,99-102[CrossRef][Medline]
    19. 19
  19. Wei, A., Rubin, H., Cooperman, B. S., Christianson, D. W. (1994) Crystal structure of an uncleaved serpin reveals the conformation of an inhibitory reactive loop Nat. Struct. Biol. 1,251-258[CrossRef][Medline]
    20. 20
  20. Huntington, J. A. (2006) Shape-shifting serpins—advantages of a mobile mechanism Trends Biochem. Sci. 31,427-435[CrossRef][Medline]
    21. 21
  21. Whisstock, J. C., Bottomley, S. P. (2006) Molecular gymnastics: serpin structure, folding and misfolding Curr. Opin. Struct. Biol. 16,761-768[CrossRef][Medline]
    22. 22
  22. Sun, J., Whisstock, J. C., Harriott, P., Walker, B., Novak, A., Thompson, P. E., Smith, A. I., Bird, P. I. (2001) Importance of the P4' residue in human granzyme B inhibitors and substrates revealed by scanning mutagenesis of the proteinase inhibitor 9 reactive center loop J. Biol. Chem. 276,15177-15184[Abstract/Free Full Text]
    23. 23
  23. Carrell, R. W., Owen, M. C. (1985) Plakalbumin, {alpha} 1-antitrypsin, antithrombin and the mechanism of inflammatory thrombosis Nature 317,730-732[CrossRef][Medline]
    24. 24
  24. Lomas, D. A., Evans, D. L., Finch, J. T., Carrell, R. W. (1992) The mechanism of Z {alpha} 1-antitrypsin accumulation in the liver Nature 357,605-607[CrossRef][Medline]
    25. 25
  25. Huntington, J. A., Read, R. J., Carrell, R. W. (2000) Structure of a serpin–protease complex shows inhibition by deformation Nature 407,923-926[CrossRef][Medline]
    26. 26
  26. Dementiev, A., Dobo, J., Gettins, P. G. (2006) Active site distortion is sufficient for proteinase inhibition by serpins: structure of the covalent complex of {alpha}1-proteinase inhibitor with porcine pancreatic elastase J. Biol. Chem. 281,3452-3457[Abstract/Free Full Text]
    27. 27
  27. Herz, J., Strickland, D. K. (2001) LRP: a multifunctional scavenger and signaling receptor J. Clin. Invest. 108,779-784[CrossRef][Medline]
    28. 28
  28. Kalsheker, N., Morley, S., Morgan, K. (2002) Gene regulation of the serine proteinase inhibitors {alpha}1-antitrypsin and {alpha}1-antichymotrypsin Biochem. Soc. Trans. 30,93-98[CrossRef][Medline]
    29. 29
  29. Pike, R. N., Buckle, A. M., le Bonniec, B. F., Church, F. C. (2005) Control of the coagulation system by serpins. Getting by with a little help from glycosaminoglycans FEBS J. 272,4842-4851[CrossRef][Medline]
    30. 30
  30. Skinner, R., Abrahams, J. P., Whisstock, J. C., Lesk, A. M., Carrell, R. W., Wardell, M. R. (1997) The 2.6 A structure of antithrombin indicates a conformational change at the heparin binding site J. Mol. Biol. 266,601-609[CrossRef][Medline]
    31. 31
  31. Schreuder, H. A., de Boer, B., Dijkema, R., Mulders, J., Theunissen, H. J., Grootenhuis, P. D., Hol, W. G. (1994) The intact and cleaved human antithrombin III complex as a model for serpin–proteinase interactions Nat. Struct. Biol. 1,48-54[CrossRef][Medline]
    32. 32
  32. Desai, U. R., Petitou, M., Bjork, I., Olson, S. T. (1998) Mechanism of heparin activation of antithrombin. Role of individual residues of the pentasaccharide activating sequence in the recognition of native and activated states of antithrombin J. Biol. Chem. 273,7478-7487[Abstract/Free Full Text]
    33. 33
  33. Olson, S. T., Bjork, I., Sheffer, R., Craig, P. A., Shore, J. D., Choay, J. (1992) Role of the antithrombin-binding pentasaccharide in heparin acceleration of antithrombin-proteinase reactions. Resolution of the antithrombin conformational change contribution to heparin rate enhancement J. Biol. Chem. 267,12528-12538[Abstract/Free Full Text]
    34. 34
  34. Li, W., Johnson, D. J., Esmon, C. T., Huntington, J. A. (2004) Structure of the antithrombin-thrombin-heparin ternary complex reveals the antithrombotic mechanism of heparin Nat. Struct. Mol. Biol. 11,857-862[CrossRef][Medline]
    35. 35
  35. Dementiev, A., Petitou, M., Herbert, J. M., Gettins, P. G. (2004) The ternary complex of antithrombin-anhydrothrombin-heparin reveals the basis of inhibitor specificity Nat. Struct. Mol. Biol. 11,863-867[CrossRef][Medline]
    36. 36
  36. Mottonen, J., Strand, A., Symersky, J., Sweet, R. M., Danley, D. E., Geoghegan, K. F., Gerard, R. D., Goldsmith, E. J. (1992) Structural basis of latency in plasminogen activator inhibitor-1 Nature 355,270-273[CrossRef][Medline]
    37. 37
  37. Zhou, A., Huntington, J. A., Pannu, N. S., Carrell, R. W., Read, R. J. (2003) How vitronectin binds PAI-1 to modulate fibrinolysis and cell migration Nat. Struct. Biol. 10,541-544[CrossRef][Medline]
    38. 38
  38. Law, R. H., Zhang, Q., McGowan, S., Buckle, A. M., Silverman, G. A., Wong, W., Rosado, C. J., Langendorf, C. G., Pike, R. N., Bird, P. I., Whisstock, J. C. (2006) An overview of the serpin superfamily Genome Biol. 7,216[CrossRef][Medline]
    39. 39
  39. Remold-O'Donnell, E. (1993) The ovalbumin family of serpin proteins FEBS Lett. 315,105-108[CrossRef][Medline]
    40. 40
  40. Scott, F. L., Eyre, H. J., Lioumi, M., Ragoussis, J., Irving, J. A., Sutherland, G. A., Bird, P. I. (1999) Human ovalbumin serpin evolution: phylogenic analysis, gene organization, and identification of new PI8-related genes suggest that two interchromosomal and several intrachromosomal duplications generated the gene clusters at 18q21–q23 and 6p25 Genomics 62,490-499[CrossRef][Medline]
    41. 41
  41. Askew, D. J., Cataltepe, S., Kumar, V., Edwards, C., Pace, S. M., Howarth, R. N., Pak, S. C., Askew, Y. S., Bromme, D., Luke, C. J., Whisstock, J. C., Silverman, G. A. (2007) SERPINB11 is a new noninhibitory intracellular serpin. Common single nucleotide polymorphisms in the scaffold impair conformational change J. Biol. Chem. 282,24948-24960[Abstract/Free Full Text]
    42. 42
  42. Pemberton, P. A., Wong, D. T., Gibson, H. L., Kiefer, M. C., Fitzpatrick, P. A., Sager, R., Barr, P. J. (1995) The tumor suppressor maspin does not undergo the stressed to relaxed transition or inhibit trypsin-like serine proteases. Evidence that maspin is not a protease inhibitory serpin J. Biol. Chem. 270,15832-15837[Abstract/Free Full Text]
    43. 43
  43. Benarafa, C., Cooley, J., Zeng, W., Bird, P. I., Remold-O'Donnell, E. (2002) Characterization of four murine homologs of the human ov-serpin monocyte neutrophil elastase inhibitor MNEI (SERPINB1) J. Biol. Chem. 277,42028-42033[Abstract/Free Full Text]
    44. 44
  44. Benarafa, C., Priebe, G. P., Remold-O'Donnell, E. (2007) The neutrophil serine protease inhibitor serpinb1 preserves lung defense functions in Pseudomonas aeruginosa infection J. Exp. Med. 204,1901-1909[Abstract/Free Full Text]
    45. 45
  45. Cooley, J., Takayama, T. K., Shapiro, S. D., Schechter, N. M., Remold-O'Donnell, E. (2001) The serpin MNEI inhibits elastase-like and chymotrypsin-like serine proteases through efficient reactions at two active sites Biochemistry 40,15762-15770[CrossRef][Medline]
    46. 46
  46. Remold-O'Donnell, E., Chin, J., Alberts, M. (1992) Sequence and molecular characterization of human monocyte/neutrophil elastase inhibitor Proc. Natl. Acad. Sci. USA 89,5635-5639[Abstract/Free Full Text]
    47. 47
  47. Dickinson, J. L., Bates, E. J., Ferrante, A., Antalis, T. M. (1995) Plasminogen activator inhibitor type 2 inhibits tumor necrosis factor {alpha}-induced apoptosis. Evidence for an alternate biological function J. Biol. Chem. 270,27894-27904[Abstract/Free Full Text]
    48. 48
  48. Hibino, T., Matsuda, Y., Takahashi, T., Goetinck, P. F. (1999) Suppression of keratinocyte proliferation by plasminogen activator inhibitor-2 J. Invest. Dermatol. 112,85-90[CrossRef][Medline]
    49. 49
  49. Kruithof, E. K., Baker, M. S., Bunn, C. L. (1995) Biological and clinical aspects of plasminogen activator inhibitor type 2 Blood 86,4007-4024[Free Full Text]
    50. 50
  50. Kumar, S., Baglioni, C. (1991) Protection from tumor necrosis factor-mediated cytolysis by overexpression of plasminogen activator inhibitor type-2 J. Biol. Chem. 266,20960-20964[Abstract/Free Full Text]
    51. 51
  51. Laug, W. E., Cao, X. R., Yu, Y. B., Shimada, H., Kruithof, E. K. (1993) Inhibition of invasion of HT1080 sarcoma cells expressing recombinant plasminogen activator inhibitor 2 Cancer Res. 53,6051-6057[Abstract/Free Full Text]
    52. 52
  52. Mueller, B. M., Yu, Y. B., Laug, W. E. (1995) Overexpression of plasminogen activator inhibitor 2 in human melanoma cells inhibits spontaneous metastasis in scid/scid mice Proc. Natl. Acad. Sci. USA 92,205-209[Abstract/Free Full Text]
    53. 53
  53. Yu, H., Maurer, F., Medcalf, R. L. (2002) Plasminogen activator inhibitor type 2: a regulator of monocyte proliferation and differentiation Blood 99,2810-2818[Abstract/Free Full Text]
    54. 54
  54. Cataltepe, S., Gornstein, E. R., Schick, C., Kamachi, Y., Chatson, K., Fries, J., Silverman, G. A., Upton, M. P. (2000) Co-expression of the squamous cell carcinoma antigens 1 and 2 in normal adult human tissues and squamous cell carcinomas J. Histochem. Cytochem. 48,113-122[Abstract/Free Full Text]
    55. 55
  55. Katagiri, C., Nakanishi, J., Kadoya, K., Hibino, T. (2006) Serpin squamous cell carcinoma antigen inhibits UV-induced apoptosis via suppression of c-JUN NH2-terminal kinase J. Cell Biol. 172,983-990[Abstract/Free Full Text]
    56. 56
  56. Suminami, Y., Nagashima, S., Vujanovic, N. L., Hirabayashi, K., Kato, H., Whiteside, T. L. (2000) Inhibition of apoptosis in human tumor cells by the tumor-associated serpin, SCC antigen-1 Br. J. Cancer 82,981-989[CrossRef][Medline]
    57. 57
  57. McGettrick, A. F., Barnes, R. C., Worrall, D. M. (2001) SCCA2 inhibits TNF-mediated apoptosis in transfected HeLa cells. The reactive center loop sequence is essential for this function and TNF-induced cathepsin G is a candidate target Eur. J. Biochem. 268,5868-5875[Medline]
    58. 58
  58. Schick, C., Kamachi, Y., Bartuski, A. J., Cataltepe, S., Schechter, N. M., Pemberton, P. A., Silverman, G. A. (1997) Squamous cell carcinoma antigen 2 is a novel serpin that inhibits the chymotrypsin-like proteinases cathepsin G and mast cell chymase J. Biol. Chem. 272,1849-1855[Abstract/Free Full Text]
    59. 59
  59. Coughlin, P., Sun, J., Cerruti, L., Salem, H. H., Bird, P. (1993) Cloning and molecular characterization of a human intracellular serine proteinase inhibitor Proc. Natl. Acad. Sci. USA 90,9417-9421[Abstract/Free Full Text]
    60. 60
  60. Scott, F. L., Coughlin, P. B., Bird, C., Cerruti, L., Hayman, J. A., Bird, P. (1996) Proteinase inhibitor 6 cannot be secreted, which suggests it is a new type of cellular serpin J. Biol. Chem. 271,1605-1612[Abstract/Free Full Text]
    61. 61
  61. Scott, F. L., Hirst, C. E., Sun, J., Bird, C. H., Bottomley, S. P., Bird, P. I. (1999) The intracellular serpin proteinase inhibitor 6 is expressed in monocytes and granulocytes and is a potent inhibitor of the azurophilic granule protease, cathepsin G Blood 93,2089-2097[Abstract/Free Full Text]
    62. 62
  62. Scott, F. L., Sun, J., Whisstock, J. C., Kato, K., Bird, P. I. (2007) SerpinB6 is an inhibitor of kallikrein-8 in keratinocytes J. Biochem. 142,435-442[Abstract/Free Full Text]
    63. 63
  63. Strik, M. C., Wolbink, A., Wouters, D., Bladergroen, B. A., Verlaan, A. R., van Houdt, I. S., Hijlkema, S., Hack, C. E., Kummer, J. A. (2004) Intracellular serpin SERPINB6 (PI6) is abundantly expressed by human mast cells and forms complexes with β-tryptase monomers Blood 103,2710-2717[Abstract/Free Full Text]
    64. 64
  64. Dahlen, J. R., Jean, F., Thomas, G., Foster, D. C., Kisiel, W. (1998) Inhibition of soluble recombinant furin by human proteinase inhibitor 8 J. Biol. Chem. 273,1851-1854[Abstract/Free Full Text]
    65. 65
  65. Leblond, J., Laprise, M. H., Gaudreau, S., Grondin, F., Kisiel, W., Dubois, C. M. (2006) The serpin proteinase inhibitor 8: an endogenous furin inhibitor released from human platelets Thromb. Haemost. 95,243-252[Medline]
    66. 66
  66. Strik, M. C., Bladergroen, B. A., Wouters, D., Kisiel, W., Hooijberg, J. H., Verlaan, A. R., Hordijk, P. L., Schneider, P., Hack, C. E., Kummer, J. A. (2002) Distribution of the human intracellular serpin protease inhibitor 8 in human tissues J. Histochem. Cytochem. 50,1443-1454[Abstract/Free Full Text]
    67. 67
  67. Bladergroen, B. A., Strik, M. C., Bovenschen, N., van Berkum, O., Scheffer, G. L., Meijer, C. J., Hack, C. E., Kummer, J. A. (2001) The granzyme B inhibitor, protease inhibitor 9, is mainly expressed by dendritic cells and at immune-privileged sites J. Immunol. 166,3218-3225[Abstract/Free Full Text]
    68. 68
  68. Medema, J. P., Schuurhuis, D. H., Rea, D., van Tongeren, J., de Jong, J., Bres, S. A., Laban, S., Toes, R. E., Toebes, M., Schumacher, T. N., Bladergroen, B. A., Ossendorp, F., Kummer, J. A., Melief, C. J., Offringa, R. (2001) Expression of the serpin serine protease inhibitor 6 protects dendritic cells from cytotoxic T lymphocyte-induced apoptosis: differential modulation by T helper type 1 and type 2 cells J. Exp. Med. 194,657-667[Abstract/Free Full Text]
    69. 69
  69. Hirst, C. E., Buzza, M. S., Bird, C. H., Warren, H. S., Cameron, P. U., Zhang, M., Ashton-Rickardt, P. G., Bird, P. I. (2003) The intracellular granzyme B inhibitor, proteinase inhibitor 9, is up-regulated during accessory cell maturation and effector cell degranulation, and its overexpression enhances CTL potency J. Immunol. 170,805-815[Abstract/Free Full Text]
    70. 70
  70. Annand, R. R., Dahlen, J. R., Sprecher, C. A., De Dreu, P., Foster, D. C., Mankovich, J. A., Talanian, R. V., Kisiel, W., Giegel, D. A. (1999) Caspase-1 (interleukin-1β-converting enzyme) is inhibited by the human serpin analogue proteinase inhibitor 9 Biochem. J. 342,655-665[CrossRef][Medline]
    71. 71
  71. Bladergroen, B. A., Strik, M. C., Wolbink, A. M., Wouters, D., Broekhuizen, R., Kummer, J. A., Hack, C. E. (2005) The granzyme B inhibitor proteinase inhibitor 9 (PI9) is expressed by human mast cells Eur. J. Immunol. 35,1175-1183[CrossRef][Medline]
    72. 72
  72. Buzza, M. S., Hirst, C. E., Bird, C. H., Hosking, P., McKendrick, J., Bird, P. I. (2001) The granzyme B inhibitor, PI-9, is present in endothelial and mesothelial cells, suggesting that it protects bystander cells during immune responses Cell. Immunol. 210,21-29[CrossRef][Medline]
    73. 73
  73. Hirst, C. E., Buzza, M. S., Sutton, V. R., Trapani, J. A., Loveland, K. L., Bird, P. I. (2001) Perforin-independent expression of granzyme B and proteinase inhibitor 9 in human testis and placenta suggests a role for granzyme B-mediated proteolysis in reproduction Mol. Hum. Reprod. 7,1133-1142[Abstract/Free Full Text]
    74. 74
  74. Sun, J., Bird, C. H., Sutton, V., McDonald, L., Coughlin, P. B., De Jong, T. A., Trapani, J. A., Bird, P. I. (1996) A cytosolic granzyme B inhibitor related to the viral apoptotic regulator cytokine response modifier A is present in cytotoxic lymphocytes J. Biol. Chem. 271,27802-27809[Abstract/Free Full Text]
    75. 75
  75. Young, J. L., Sukhova, G. K., Foster, D., Kisiel, W., Libby, P., Schonbeck, U. (2000) The serpin proteinase inhibitor 9 is an endogenous inhibitor of interleukin 1β-converting enzyme (caspase-1) activity in human vascular smooth muscle cells J. Exp. Med. 191,1535-1544[Abstract/Free Full Text]
    76. 76
  76. Riewald, M., Schleef, R. R. (1995) Molecular cloning of bomapin (protease inhibitor 10), a novel human serpin that is expressed specifically in the bone marrow J. Biol. Chem. 270,26754-26757[Abstract/Free Full Text]
    77. 77
  77. Riewald, M., Chuang, T., Neubauer, A., Riess, H., Schleef, R. R. (1998) Expression of bomapin, a novel human serpin, in normal/malignant hematopoiesis and in the monocytic cell lines THP-1 and AML-193 Blood 91,1256-1262[Abstract/Free Full Text]
    78. 78
  78. Schleef, R. R., Chuang, T. L. (2000) Protease inhibitor 10 inhibits tumor necrosis factor {alpha} -induced cell death. Evidence for the formation of intracellular high M(r) protease inhibitor 10-containing complexes J. Biol. Chem. 275,26385-26389[Abstract/Free Full Text]
    79. 79
  79. Askew, Y. S., Pak, S. C., Luke, C. J., Askew, D. J., Cataltepe, S., Mills, D. R., Kato, H., Lehoczky, J., Dewar, K., Birren, B., Silverman, G. A. (2001) SERPINB12 is a novel member of the human ov-serpin family that is widely expressed and inhibits trypsin-like serine proteinases J. Biol. Chem. 276,49320-49330[Abstract/Free Full Text]
    80. 80
  80. Abts, H. F., Welss, T., Mirmohammadsadegh, A., Kohrer, K., Michel, G., Ruzicka, T. (1999) Cloning and characterization of hurpin (protease inhibitor 13): a new skin-specific, UV-repressible serine proteinase inhibitor of the ovalbumin serpin family J. Mol. Biol. 293,29-39[CrossRef][Medline]
    81. 81
  81. Jayakumar, A., Kang, Y., Frederick, M. J., Pak, S. C., Henderson, Y., Holton, P. R., Mitsudo, K., Silverman, G. A., El Naggar, A. K., Brömme, D., Clayman, G. L. (2003) Inhibition of the cysteine proteinases cathepsins K and L by the serpin headpin (SERPINB13): a kinetic analysis Arch. Biochem. Biophys. 409,367-374[CrossRef][Medline]
    82. 82
  82. Shellenberger, T. D., Mazumdar, A., Henderson, Y., Briggs, K., Wang, M., Chattopadhyay, C., Jayakumar, A., Frederick, M., Clayman, G. L. (2005) Headpin: a serpin with endogenous and exogenous suppression of angiogenesis Cancer Res. 65,11501-11509[Abstract/Free Full Text]
    83. 83
  83. Spring, P., Nakashima, T., Frederick, M., Henderson, Y., Clayman, G. (1999) Identification and cDNA cloning of headpin, a novel differentially expressed serpin that maps to chromosome 18q Biochem. Biophys. Res. Commun. 264,299-304[CrossRef][Medline]
    84. 84
  84. Welss, T., Sun, J., Irving, J. A., Blum, R., Smith, A. I., Whisstock, J. C., Pike, R. N., von Mikecz, A., Ruzicka, T., Bird, P. I., Abts, H. F. (2003) Hurpin is a selective inhibitor of lysosomal cathepsin L and protects keratinocytes from ultraviolet-induced apoptosis Biochemistry 42,7381-7389[CrossRef][Medline]
    85. 85
  85. Zou, Z., Anisowicz, A., Hendrix, M. J., Thor, A., Neveu, M., Sheng, S., Rafidi, K., Seftor, E., Sager, R. (1994) Maspin, a serpin with tumor-suppressing activity in human mammary epithelial cells Science 263,526-529[Abstract/Free Full Text]
    86. 86
  86. Sheng, S., Carey, J., Seftor, E. A., Dias, L., Hendrix, M. J., Sager, R. (1996) Maspin acts at the cell membrane to inhibit invasion and motility of mammary and prostatic cancer cells Proc. Natl. Acad. Sci. USA 93,11669-11674[Abstract/Free Full Text]
    87. 87
  87. Tsujimoto, M., Tsuruoka, N., Ishida, N., Kurihara, T., Iwasa, F., Yamashiro, K., Rogi, T., Kodama, S., Katsuragi, N., Adachi, M., Katayama, T., Nakao, M., Yamaichi, K., Hashino, J., Haruyama, M., Miura, K., Nakanishi, T., Nakazato, H., Teramura, M., Mizoguchi, H., Yamaguchi, N. (1997) Purification, cDNA cloning, and characterization of a new serpin with megakaryocyte maturation activity J. Biol. Chem. 272,15373-15380[Abstract/Free Full Text]
    88. 88
  88. Kaiserman, D., Knaggs, S., Scarff, K. L., Gillard, A., Mirza, G., Cadman, M., McKeone, R., Denny, P., Cooley, J., Benarafa, C., Remold-O'Donnell, E., Ragoussis, J., Bird, P. I. (2002) Comparison of human chromosome 6p25 with mouse chromosome 13 reveals a greatly expanded ov-serpin gene repertoire in the mouse Genomics 79,349-362[CrossRef][Medline]
    89. 89
  89. Sun, J., Ooms, L., Bird, C. H., Sutton, V. R., Trapani, J. A., Bird, P. I. (1997) A new family of 10 murine ovalbumin serpins includes two homologs of proteinase inhibitor 8 and two homologs of the granzyme B inhibitor (proteinase inhibitor 9) J. Biol. Chem. 272,15434-15441[Abstract/Free Full Text]
    90. 90
  90. Phillips, T., Opferman, J. T., Shah, R., Liu, N., Froelich, C. J., Ashton-Rickardt, P. G. (2004) A role for the granzyme B inhibitor serine protease inhibitor 6 in CD8+ memory cell homeostasis J. Immunol. 173,3801-3809[Abstract/Free Full Text]
    91. 91
  91. Bots, M., Kolfschoten, I. G., Bres, S. A., Rademaker, M. T., de Roo, G. M., Kruse, M., Franken, K. L., Hahne, M., Froelich, C. J., Melief, C. J., Offringa, R., Medema, J. P. (2005) SPI-CI and SPI-6 cooperate in the protection from effector cell-mediated cytotoxicity Blood 105,1153-1161
    92. 92
  92. Banchereau, J., Steinman, R. M. (1998) Dendritic cells and the control of immunity Nature 392,245-252[CrossRef][Medline]
    93. 93
  93. Guermonprez, P., Valladeau, J., Zitvogel, L., Thery, C., Amigorena, S. (2002) Antigen presentation and T cell stimulation by dendritic cells Annu. Rev. Immunol. 20,621-667[CrossRef][Medline]
    94. 94
  94. Ridge, J. P., Di Rosa, F., Matzinger, P. (1998) A conditioned dendritic cell can be a temporal bridge between a CD4+ T-helper and a T-killer cell Nature 393,474-478[CrossRef][Medline]
    95. 95
  95. Bennett, S. R., Carbone, F. R., Karamalis, F., Flavell, R. A., Miller, J. F., Heath, W. R. (1998) Help for cytotoxic-T-cell responses is mediated by CD40 signaling Nature 393,478-480[CrossRef][Medline]
    96. 96
  96. Schoenberger, S. P., Toes, R. E., van der Voort, E. I., Offringa, R., Melief, C. J. (1998) T-cell help for cytotoxic T lymphocytes is mediated by CD40–CD40L interactions Nature 393,480-483[CrossRef][Medline]
    97. 97
  97. Barry, M., Bleackley, R. C. (2002) Cytotoxic T lymphocytes: all roads lead to death Nat. Rev. Immunol. 2,401-409[Medline]
    98. 98
  98. Russell, J. H., Ley, T. J. (2002) Lymphocyte-mediated cytotoxicity Annu. Rev. Immunol. 20,323-370[CrossRef][Medline]
    99. 99
  99. Kagi, D., Vignaux, F., Ledermann, B., Burki, K., Depraetere, V., Nagata, S., Hengartner, H., Golstein, P. (1994) Fas and perforin pathways as major mechanisms of T cell-mediated cytotoxicity Science 265,528-530[Abstract/Free Full Text]
    100. 100
  100. Lowin, B., Hahne, M., Mattmann, C., Tschopp, J. (1994) Cytolytic T-cell cytotoxicity is mediated through perforin and Fas lytic pathways Nature 370,650-652[CrossRef][Medline]
    101. 101
  101. Braun, M. Y., Lowin, B., French, L., Acha-Orbea, H., Tschopp, J. (1996) Cytotoxic T cells deficient in both functional fas ligand and perforin show residual cytolytic activity yet lose their capacity to induce lethal acute graft-versus-host disease J. Exp. Med. 183,657-661[Abstract/Free Full Text]
    102. 102
  102. Trapani, J. A., Smyth, M. J. (2002) Functional significance of the perforin/granzyme cell death pathway Nat. Rev. Immunol. 2,735-747[CrossRef][Medline]
    103. 103
  103. Bots, M., Medema, J. P. (2006) Granzymes at a glance J. Cell Sci. 119,5011-5014[Free Full Text]
    104. 104
  104. Lieberman, J. (2003) The ABCs of granule-mediated cytotoxicity: new weapons in the arsenal Nat. Rev. Immunol. 3,361-370[CrossRef][Medline]
    105. 105
  105. Heusel, J. W., Wesselschmidt, R. L., Shresta, S., Russell, J. H., Ley, T. J. (1994) Cytotoxic lymphocytes require granzyme B for the rapid induction of DNA fragmentation and apoptosis in allogeneic target cells Cell 76,977-987[CrossRef][Medline]
    106. 106
  106. Shresta, S., MacIvor, D. M., Heusel, J. W., Russell, J. H., Ley, T. J. (1995) Natural killer and lymphokine-activated killer cells require granzyme B for the rapid induction of apoptosis in susceptible target cells Proc. Natl. Acad. Sci. USA 92,5679-5683[Abstract/Free Full Text]
    107. 107
  107. Kagi, D., Ledermann, B., Burki, K., Seiler, P., Odermatt, B., Olsen, K. J., Podack, E. R., Zinkernagel, R. M., Hengartner, H. (1994) Cytotoxicity mediated by T cells and natural killer cells is greatly impaired in perforin-deficient mice Nature 369,31-37[CrossRef][Medline]
    108. 108
  108. van den Broek, M. E., Kagi, D., Ossendorp, F., Toes, R., Vamvakas, S., Lutz, W. K., Melief, C. J., Zinkernagel, R. M., Hengartner, H. (1996) Decreased tumor surveillance in perforin-deficient mice J. Exp. Med. 184,1781-1790[Abstract/Free Full Text]
    109. 109
  109. Voskoboinik, I., Smyth, M. J., Trapani, J. A. (2006) Perforin-mediated target-cell death and immune homeostasis Nat. Rev. Immunol. 6,940-952[CrossRef][Medline]
    110. 110
  110. Zhang, M., Park, S. M., Wang, Y., Shah, R., Liu, N., Murmann, A. E., Wang, C. R., Peter, M. E., Ashton-Rickardt, P. G. (2006) Serine protease inhibitor 6 protects cytotoxic T cells from self-inflicted injury by ensuring the integrity of cytotoxic granules Immunity 24,451-461[CrossRef][Medline]
    111. 111
  111. Khong, H. T., Restifo, N. P. (2002) Natural selection of tumor variants in the generation of "tumor escape" phenotypes Nat. Immunol. 3,999-1005[CrossRef][Medline]
    112. 112
  112. Terabe, M., Berzofsky, J. A. (2004) Immunoregulatory T cells in tumor immunity Curr. Opin. Immunol. 16,157-162[CrossRef][Medline]
    113. 113
  113. Marincola, F. M., Jaffee, E. M., Hicklin, D. J., Ferrone, S. (2000) Escape of human solid tumors from T-cell recognition: molecular mechanisms and functional significance Adv. Immunol. 74,181-273[Medline]
    114. 114
  114. Seliger, B., Maeurer, M. J., Ferrone, S. (2000) Antigen-processing machinery breakdown and tumor growth Immunol. Today 21,455-464[CrossRef][Medline]
    115. 115
  115. Igney, F. H., Krammer, P. H. (2002) Death and anti-death: tumor resistance to apoptosis Nat. Rev. Cancer 2,277-288[CrossRef][Medline]
    116. 116
  116. Medema, J. P., de Jong, J., Peltenburg, L. T., Verdegaal, E. M., Gorter, A., Bres, S. A., Franken, K. L., Hahne, M., Albar, J. P., Melief, C. J., Offringa, R. (2001) Blockade of the granzyme B/perforin pathway through overexpression of the serine protease inhibitor PI-9/SPI-6 constitutes a mechanism for immune escape by tumors Proc. Natl. Acad. Sci. USA 98,11515-11520[Abstract/Free Full Text]
    117. 117
  117. Bots, M., Van Bostelen, L., Rademaker, M. T., Offringa, R., Medema, J. P. (2006) Serpins prevent granzyme-induced death in a species-specific manner Immunol. Cell Biol. 84,79-86[CrossRef][Medline]
    118. 118
  118. Bladergroen, B. A., Meijer, C. J., ten Berge, R. L., Hack, C. E., Muris, J. J., Dukers, D. F., Chott, A., Kazama, Y., Oudejans, J. J., van Berkum, O., Kummer, J. A. (2002) Expression of the granzyme B inhibitor, protease inhibitor 9, by tumor cells in patients with non-Hodgkin and Hodgkin lymphoma: a novel protective mechanism for tumor cells to circumvent the immune system? Blood 99,232-237[Abstract/Free Full Text]
    119. 119
  119. ten Berge, R. L., Meijer, C. J., Dukers, D. F., Kummer, J. A., Bladergroen, B. A., Vos, W., Hack, C. E., Ossenkoppele, G. J., Oudejans, J. J. (2002) Expression levels of apoptosis-related proteins predict clinical outcome in anaplastic large cell lymphoma Blood 99,4540-4546[Abstract/Free Full Text]
    120. 120
  120. van Houdt, I. S., Oudejans, J. J., van den Eertwegh, A. J., Baars, A., Vos, W., Bladergroen, B. A., Rimoldi, D., Muris, J. J., Hooijberg, E., Gundy, C. M., Meijer, C. J., Kummer, J. A. (2005) Expression of the apoptosis inhibitor protease inhibitor 9 predicts clinical outcome in vaccinated patients with stage III and IV melanoma Clin. Cancer Res. 11,6400-6407[Abstract/Free Full Text]
    121. 121
  121. Kotwal, G. J., Moss, B. (1989) Vaccinia virus encodes two proteins that are structurally related to members of the plasma serine protease inhibitor superfamily J. Virol. 63,600-606[Abstract/Free Full Text]
    122. 122
  122. Smith, G. L., Howard, S. T., Chan, Y. S. (1989) Vaccinia virus encodes a family of genes with homology to serine proteinase inhibitors J. Gen. Virol. 70,2333-2343[Abstract/Free Full Text]
    123. 123
  123. Turner, P. C., Moyer, R. W. (1992) An orthopoxvirus serpinlike gene controls the ability of infected cells to fuse J. Virol. 66,2076-2085[Abstract/Free Full Text]
    124. 124
  124. Macen, J. L., Garner, R. S., Musy, P. Y., Brooks, M. A., Turner, P. C., Moyer, R. W., McFadden, G., Bleackley, R. C. (1996) Differential inhibition of the Fas- and granule-mediated cytolysis pathways by the orthopoxvirus cytokine response modifier A/SPI-2 and SPI-1 protein Proc. Natl. Acad. Sci. USA 93,9108-9113[Abstract/Free Full Text]
    125. 125
  125. Ray, C. A., Black, R. A., Kronheim, S. R., Greenstreet, T. A., Sleath, P. R., Salvesen, G. S., Pickup, D. J. (1992) Viral inhibition of inflammation: cowpox virus encodes an inhibitor of the interleukin-1 β converting enzyme Cell 69,597-604[CrossRef][Medline]
    126. 126
  126. Quan, L. T., Caputo, A., Bleackley, R. C., Pickup, D. J., Salvesen, G. S. (1995) Granzyme B is inhibited by the cowpox virus serpin cytokine response modifier A J. Biol. Chem. 270,10377-10379[Abstract/Free Full Text]
    127. 127
  127. Turner, S. J., Silke, J., Kenshole, B., Ruby, J. (2000) Characterization of the ectromelia virus serpin, SPI-2 J. Gen. Virol. 81,2425-2430[Abstract/Free Full Text]
    128. 128
  128. Moon, K. B., Turner, P. C., Moyer, R. W. (1999) SPI-1-dependent host range of rabbitpox virus and complex formation with cathepsin G is associated with serpin motifs J. Virol. 73,8999-9010[Abstract/Free Full Text]
    129. 129
  129. Sprent, J., Tough, D. F. (2001) T cell death and memory Science 293,245-248[Abstract/Free Full Text]
    130. 130
  130. Badovinac, V. P., Tvinnereim, A. R., Harty, J. T. (2000) Regulation of antigen-specific CD8+ T cell homeostasis by perforin and interferon-{gamma} Science 290,1354-1358[Abstract/Free Full Text]
    131. 131
  131. Haring, J. S., Badovinac, V. P., Harty, J. T. (2006) Inflaming the CD8+ T cell response Immunity 25,19-29[CrossRef][Medline]
    132. 132
  132. Kagi, D., Odermatt, B., Mak, T. W. (1999) Homeostatic regulation of CD8+ T cells by perforin Eur. J. Immunol. 29,3262-3272[CrossRef][Medline]
    133. 133
  133. Matloubian, M., Suresh, M., Glass, A., Galvan, M., Chow, K., Whitmire, J. K., Walsh, C. M., Clark, W. R., Ahmed, R. (1999) A role for perforin in downregulating T-cell responses during chronic viral infection J. Virol. 73,2527-2536[Abstract/Free Full Text]
    134. 134
  134. Catalfamo, M., Henkart, P. A. (2003) Perforin and the granule exocytosis cytotoxicity pathway Curr. Opin. Immunol. 15,522-527[CrossRef][Medline]
    135. 135
  135. Liu, N., Phillips, T., Zhang, M., Wang, Y., Opferman, J. T., Shah, R., Ashton-Rickardt, P. G. (2004) Serine protease inhibitor 2A is a protective factor for memory T cell development Nat. Immunol. 5,919-926[CrossRef][Medline]
    136. 136
  136. Liu, N., Raja, S. M., Zazzeroni, F., Metkar, S. S., Shah, R., Zhang, M., Wang, Y., Brömme, D., Russin, W. A., Lee, J. C., Peter, M. E., Froelich, C. J., Franzoso, G., Ashton-Rickardt, P. G. (2003) NF-{kappa}B protects from the lysosomal pathway of cell death EMBO J. 22,5313-5322[CrossRef][Medline]
    137. 137
  137. Zajac, A. J., Dye, J. M., Quinn, D. G. (2003) Control of lymphocytic choriomeningitis virus infection in granzyme B deficient mice Virology 305,1-9[CrossRef][Medline]
    138. 138
  138. Casazza, J. P., Betts, M. R., Price, D. A., Precopio, M. L., Ruff, L. E., Brenchley, J. M., Hill, B. J., Roederer, M., Douek, D. C., Koup, R. A. (2006) Acquisition of direct antiviral effector functions by CMV-specific CD4+ T lymphocytes with cellular maturation J. Exp. Med. 203,2865-2877[Abstract/Free Full Text]
    139. 139
  139. Grossman, W. J., Verbsky, J. W., Barchet, W., Colonna, M., Atkinson, J. P., Ley, T. J. (2004) Human T regulatory cells can use the perforin pathway to cause autologous target cell death Immunity 21,589-601[CrossRef][Medline]
    140. 140
  140. Grossman, W. J., Verbsky, J. W., Tollefsen, B. L., Kemper, C., Atkinson, J. P., Ley, T. J. (2004) Differential expression of granzymes A and B in human cytotoxic lymphocyte subsets and T regulatory cells Blood 104,2840-2848[Abstract/Free Full Text]
    141. 141
  141. Devadas, S., Das, J., Liu, C., Zhang, L., Roberts, A. I., Pan, Z., Moore, P. A., Das, G., Shi, Y. (2006) Granzyme B is critical for T cell receptor-induced cell death of type 2 helper T cells Immunity 25,237-247[CrossRef][Medline]




This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow All Versions of this Article:
jlb.0208140v1
84/5/1238    most recent
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Bots, M.
Right arrow Articles by Medema, J. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Bots, M.
Right arrow Articles by Medema, J. P.