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* Division of Vaccine Research, Institute of Human Virology, University of Maryland Biotechnology Institute, and Departments of
Microbiology and Immunology and
Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore
Correspondence: George K. Lewis, Ph.D., Division of Vaccine Research, Institute of Human Virology, University of Maryland at Baltimore, 725 W. Lombard Street, Baltimore, MD 21201. E-mail: lewisg{at}umbi.umd.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: adjuvant • cholera toxin • enterotoxin
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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subunits of G proteins (Gi). ADP-ribosylation of Gi prevents the inhibition of adenylate cyclase and leads to a sustained increase in intracellular cyclic adenosine monophosphate (cAMP) concentration [6
7
8
9
].
Like PT, the B pentamers of CT and LT target them to the membranes of cells [10
, 11
], and these toxins enter the cytoplasm by exploiting the ERAD [5
]. In the cytosol, the A1 subunits of CT and LT catalyze the transfer of an ADP-ribose from NAD to stimulatory -subunits of G proteins (Gs). After ADP-ribosylation, Gs binds to adenylate cyclase and constitutively activates it, leading to a sustained increase in intracellular cAMP concentration [12
].
AT is also an AB toxin, but unlike the AB5 enterotoxins PT, CT, and LT, AT is composed of a single polypeptide comprised of 1706 amino acids. The NH2-terminal 400 amino acids contain the catalytic domain [13 ], and the remaining 1306 amino acids are involved in binding the toxin to target cells and entry of the catalytic domain into the cytosol [14 ]. Like PT, CT, and LT, the toxicity associated with AT is the result of elevated intracellular cAMP levels in target cells. However, the mechanism through which AT elevates intracellular cAMP levels is different from that of the AB5 toxins described above. AT does not alter the activity of the host adenylate cyclase but is itself a functional adenylate cyclase enzyme [15 , 16 ]. AT also enters target cells through a different mechanism than PT, CT, and LT. Although AT can enter a wide variety of cells in a receptor-independent manner, the toxin binds with high affinity only to the CD11b/CD18 complex found on leukocytes such as macrophages and dendritic cells (DC) [17 ]. A two-step model was proposed for the receptor-independent entry of AT into target cells [18 ]. First, the carboxy-terminal domain of the toxin facilitates receptor-independent binding of the toxin-to-plasma membranes. Second, a conformational change allows the catalytic domain of the toxin to penetrate into the cytosol. It was further proposed that the binding of AT to the CD11b/CD18 complex facilitates the interaction of its carboxy-terminal domain with the plasma membrane [17 ].
PT, like CT and LT [19
], is a powerful mucosal immunogen and adjuvant [20
, 21
]. By contrast, no such adjuvant effects have been reported for AT. When delivered mucosally, PT, CT, and LT induce strong primary and secondary antibody responses and long-lasting immunologic memory to themselves and to coadministered antigens [20
, 21
]. Although PT, CT, and LT are extensively used as adjuvants in animal models, their toxicity makes them unsuitable for human use. For this reason, a number of investigators have attempted to identify nontoxic derivatives of these toxins that retain adjuvanticity by removing the A domain [22
] or by rendering it enzymatically inactive by site-directed mutagenesis [20
, 21
, 23
24
25
]. Studies addressing the adjuvant effects of recombinant B subunits of CT and LT have generated conflicting results. Several studies reported an adjuvant effect for the recombinant B (ganglioside-binding) chain of CT (rCTB) or chain of LT (rLTB) [26
27
28
], and others have reported no adjuvant effect [29
30
31
]. A number of mutants of these toxins have been generated that have abrogated or reduced enzymatic activity. For example, enzymatically inactive mutants of PT, CT, and LT [enzymatically inactive mutant of PT (PT9K/129G), of CT (CTK63), and of LT (LTK63)] have been studied [20
, 21
, 23
24
25
]. Several studies reported an adjuvant effect for these mutants [20
, 21
, 23
24
25
]; however, these responses were generally weaker than those induced by the wild-type toxins. In addition, PT9K/129G was genetically modified to deliver cytolytic T lymphocyte (CTL) epitopes, and this modified construct markedly enhanced CTL responses [32
, 33
]. Of particular interest, one study compared the adjuvanticity of LTK63, which lacks enzymatic activity, with that of LTR72, which has reduced enzymatic activity. The rank order of adjuvanticity was LT 
LTR72 > LTK63 >> LTB, showing that LT derives its adjuvanticity from enzymatic activity and the intrinsic structure of the AB complex itself [34
]. Although these in vivo studies are informative, the cellular loci at which these molecules exert their action were not addressed. There is emerging data suggesting that DC are one of the major cellular targets for these AB5 toxins in vivo [35
, 36
]. In this regard, it was reported recently that CT activates monocyte-derived DC (MDDC) in vitro, as defined by increased surface-marker expression and enhanced alloantigen presentation in the mixed lymphocyte reaction (MLR; ref. [37
] and in press). Large numbers of DC can be generated by culturing monocytes with granulocyte macrophage-colony stimulating factor (GM-CSF) and interleukin (IL)-4 [38
]. These MDDC are phenotypically equivalent to immature DC [38
]. MDDC incubated with lipopolysaccharides (LPS) or CT undergo maturation and display phenotypic characteristics of the mature DC [37
, 39
]. CT not only activates MDDC but also inhibits IL-12 production by MDDC stimulated with the strong IL-12-inducing stimuli LPS (refs. [37
, 40
] and in press).
It appears that the enzymatic activity and the intrinsic structure of the AB complexes of PT, CT, and LT contribute to their adjuvanticity of in vivo [34 ]; however, we found that only enzymatic activity is involved in the activation of MDDC by CT and LT (in press). In this regard, the enzymatically inactive derivatives of CT and LT (CTK63, LTK63, rCTB, and rLTB) failed to activate MDDC, and the pharmacological elevators of intracellular cAMP levels [dibutyryl-cAMP (d-cAMP) and Forskolin] mimic the activation of MDDC induced by the wild-type toxins (in press). In addition, the cAMP-dependent protein kinase A (PKA) inhibitor Rp-8-Br-cAMPs inhibited the ability of CT, LT, d-cAMP, and Forskolin to activate MDDC (in press). In the studies described below, we assayed PT and AT for the ability to activate MDDC and determined the mechanism through which they achieve this activation. We found that like CT and LT, PT and AT are potent activators of MDDC in vitro and that this activation is strictly dependent on the ability of the toxins to elevate intracellular cAMP. Together with the results of our previous study (in press), these results strongly support the hypothesis that AB enterotoxins that exert their toxicity through the elevation of intracellular cAMP in target cells also activate MDDC predominantly, if not solely, by the elevation of intracellular cAMP. In summary, this study shows for the first time that PT and AT activate human MDDC to mature, that the activation is cAMP-dependent, and that the intrinsic AB structure of PT does not contribute to this activation aside from its enzymatic activity.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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T cell proliferation medium
T cell proliferation medium consisted of -minimal essential medium without ribonucleosides and deoxyribonucleosides (Life Technologies) supplemented with 10% human AB serum (Sigma Chemical Co.), 1% sodium pyruvate, 4 mM L-glutamine, 20 mM Hepes buffer (Life Technologies), 100 µg/ml streptomycin (Life Technologies), 100 U/ml penicillin G (Life Technologies), and 50 µM 2-ME.
DC preparations
All human specimens were obtained under informed consent as approved by the University of Maryland at Baltimore Institutional Review Board. MDDC were generated as described previously with minor modifications [38
]. Briefly, human peripheral blood mononuclear cells (PBMC) were isolated by Ficoll-Hypaque density gradient centrifugation and were enriched for CD14+ monocytes by negative selection using a cocktail of monoclonal antibodies (mAb) from StemCell Technologies (Vancouver, Canada) according to the manufacturer’s instructions. The isolated monocytes were adhered to plastic by plating at 1 x 106 cells/ml in RPMI for 2 h. Adherent monocytes were washed with RPMI medium and then cultured at 1 x 106 cells/ml in DC culture medium supplemented with 50 ng/ml rGM-CSF and 1000 units/ml rIL-4 (R&D Systems, Minneapolis, MN). The day the cultures were initiated is designated as day zero.
Cell treatments
CT, PT, AT, B (ganglioside-binding) domain of PT (rPTB; List Biological Laboratories, Campbell, CA), LPS, LT, d-cAMP, Forskolin, n-butyric acid, H-89 (Sigma Chemical Co.), Rp-8-Br-cAMPs (Biolog Life Science Institute, Breman, Germany), and PT9K/129G (generously provided by Nicholas Carbonetti, University of Maryland at Baltimore) were added directly to MDDC cultures in individual wells at the concentrations indicated below. At the times indicated below, the cells were harvested, washed, and stained for phenotypic analysis by flow cytometry.
Flow cytometry
Cells were incubated for 30 min at 4°C with the following murine mAb specific for CD80, CD83, CD86, and human leukocyte antigen (HLA)-DR (BD Pharmingen, San Diego, CA), washed, and then fixed with 2% paraformaldehyde for analysis using a FACScalibur flow cytometer (Becton Dickinson, San Jose, CA). Data analysis was carried out using FlowJo software (Tree Star Inc., San Carlos, CA). The fraction of MDDC that responds by up-regulation of activation markers on the cell surface was calculated with Flowjo software by overlaying the histograms of treated and untreated MDDC and Overton subtraction of the curves.
Calculation of % inhibition induced by Rp-8-Br-cAMPs or H-89
Percent inhibition = ((X-Y)/X) x 100, where X = the fraction of cells that up-regulated a marker in the absence of the inhibitor, and Y = the fraction of cells that up-regulated a marker in the presence of the inhibitor.
Endotoxin quantification
LPS concentrations were monitored in the toxin preparations using the Limulus assay (BioWhittaker, Walkersville, MD). LPS concentrations were maximally 40 pg/ml in the final dilutions of toxins, and their derivatives were used in the studies. In addition, the possibility of LPS contamination contributing to the observed activation of MDDC was ruled out by boiling preparations of agonists for 10 min before addition to MDDC. Boiling PT or AT completely abolishes their ability to activate MDDC (data not shown).
MLR
MDDC for the MLR were prepared as described above and washed three times with T cell proliferation medium before addition to naïve, allogeneic CD4+ T cells. Naïve CD4+ T cells were enriched from PBMC by negative selection using a cocktail of mAb (StemCell Technologies) according to the manufacturer’s instructions. Naïve CD4+ T cells (
93% pure, as determined by the expression of CD45R0 and CD62L using flow cytometry) were cultured in replicates of five at 1 x 105 cells/well with 1000 allogeneic MDDC in 96-well U-bottom plates. The cells were pulsed with 1 µCi/well 3H-thymidine (Perkin Elmer Life Sciences, Boston, MA) for the last 18 h of culture before measuring thymidine incorporation using a Wallac 1450 Microbetta Trilux liquid scintilation spectrometer (Wallac, Turku, Finland) on the fifth day. The SD of replicate wells in the same experiment was typically <20%.
Cytokine enzyme-linked immunosorbent assay (ELISA)
Concentrations of tumor necrosis factor (TNF-) and IL-12 in supernatants were determined by ELISA (R&D Systems) according to the manufacturer’s instructions.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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An enzymatically active A domain is necessary for PT to activate MDDC
Enzymatically inactive derivatives of PT as well as those of CT and LT have been reported to retain adjuvant activity [20
, 21
, 23
24
25
]. In the case of CT and LT, there is evidence that a portion of this activity is a result of the intrinsic AB structure of the toxins, independent of enzymatic activity [20
, 21
, 23
24
25
, 34
]. For this reason, we determined whether these derivatives share the ability of the wild-type toxins to activate MDDC. Previously, we found that the activation of MDDC by CT and LT is strictly dependent on enzymatic activity, as the enzymatically inactive derivatives of CT and LT (CTK63, LTK63, rCTB, and rLTB) failed to activate MDDC (in press), showing that at least for MDDC activation, the intrinsic AB structure in the absence of enzymatic activity is not sufficient to elicit a response. Against this backdrop, we determined if the ability of PT to activate MDDC is also strictly dependent on enzymatic activity. A comparable AT derivative was not available. We investigated the ability of the enzymatically inactive mutant of PT, PT9K/129G, and rPTB to activate MDDC. Day 4 MDDC were incubated with PT, PT9K/129G, or rPTB at a final concentration of 1 µg/ml for a total of 20 h and were then harvested; activation was assayed by up-regulation of surface-activation markers. As shown in Figure 3
, PT9K/129G and rPTB failed to activate MDDC. In this regard, the expression of CD80, CD83, CD86, and HLA-DR on MDDC treated with PT9K/129G or rPTB was similar to untreated MDDC. The failure of these molecules to activate MDDC is unlikely to be a result of insufficient concentration of the molecules or incubation time, as 1 µg/ml PT is approximately 100-fold above the minimal concentration necessary to induce the maturation of these cells (see Fig. 2
), and the level of expression of the activation markers did not increase at later time points (48 and 72 h; data not shown). Taken together, these results show that an enzymatically active A domain is required for PT to activate MDDC as judged by the up-regulation of activation markers.
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production by LPS-activated MDDC in the supernatants were determined by ELISA. Figure 6B shows that MDDC activated with LPS alone produce high levels of IL-12, approximately 115-fold over those produced by untreated MDDC. Likewise, Figure 6C
shows that MDDC activated with LPS alone produce high levels of TNF-, approximately 580-fold over those produced by untreated MDDC. By contrast, as shown in Figure 6B and 6C
, MDDC activated by CT, PT, AT, or Forskolin in the presence or absence of LPS produce no more than fourfold the IL-12 or 60-fold the TNF- of untreated MDDC. This corresponds to 28-fold and tenfold reductions in the levels of IL-12 and TNF-, respectively, as compared with MDDC activated by LPS alone. Collectively, these results strongly suggest that PT and AT, like CT and LT, dominantly inhibit cytokine production by MDDC via a cAMP-dependent mechanism. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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production by LPS-activated MDDC, and d-cAMP or Forskolin mimics this effect. Several of these findings warrant additional comment. Although PT9K/129G, CTK63, and LTK63 have been shown to retain adjuvanticity [20 , 21 , 23 24 25 ], we found that these mutants do not share the ability of the wild-type toxins to activate MDDC. It has been suggested that the AB5 structure of CT and LT displays adjuvanticity separate from the adjuvanticity associated with their enzymatic activity [37 ]. This is plausible in that these toxins appear to traffic retrograde from the cell surface to the cytoplasm via the ERAD protein quality-control pathway [5 ]. This could potentially generate signals that activate MDDC. At the level of the DC, however, we found that the enzymatic activity of these toxins is essential for activation. This was true for PT in the studies above. Unfortunately, an enzymatically inactive AT derivative was not available for our studies, so we have not been able to make a direct test of whether the AB structure of this toxin possesses activity apart from its enzymatic activity. Be that as it may, the ability of d-cAMP and Forskolin to mimic the effects of CT, LT, PT, and AT on MDDC and the ability of PKA inhibitors to inhibit these effects, including that of AT, are consistent with the elevation of intracellular cAMP being the mechanism through which these toxins activate DC. Together, these results strongly support the central role for elevated cAMP levels in the activation of MDDC by toxins whose principal mechanism of toxicity is the elevation of cAMP levels in target cells. For these reasons, we speculate that any adjuvant effects as a result of the nontoxic AB complexes of CT, LT, and PT are mediated at sights other than DC.
Previously, we and others have shown that d-cAMP and Forskolin share the ability of CT to dominantly inhibit inflammatory cytokine production by LPS-stimulated DC (refs. [37
, 40
] and in press). In this study, we have extended those findings by showing that PT and AT also dominantly inhibit IL-12 and TNF- production. This lends further credence to the hypothesis that this effect is also cAMP-dependent. As AT shares the ability of CT, LT, and PT to activate MDDC to mature, and the maturation of DC is necessary for the initiation of primary immune responses, it is reasonable to speculate that AT would share the adjuvanticity of these AB5 toxins. In this regard, AT specifically targets the CD11b/CD18 complex, which is expressed on DC. To our knowledge, no such adjuvanticity has been described for AT. To this end, we are testing AT for mucosal and/or systemic adjuvant activity. In summary, the results of this study, coupled with the results of our previous study (in press), strongly support the requirement for an enzymatically active A domain in the ability of cAMP-elevating toxins to activate MDDC and suggest that there is little or no contribution of the intrinsic structure of the AB complex to this step in the immune response. It should be emphasized that our studies do not rule out such an effect for other cell types or for in vivo responses. Nevertheless, at the level of MDDC, our results show that this activation is highly likely to be a direct result of elevated levels of cAMP and that cAMP-independent pathways are not involved in this phenomenon.
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ACKNOWLEDGEMENTS |
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Received May 22, 2002; revised May 22, 2002; accepted July 15, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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