Published online before print April 23, 2008
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Laboratory of Allergic Diseases, National Institute of Allergy and Infectious Disease, National Institutes of Health, Bethesda, Maryland, USA
1Correspondence: E-mail: hrosenberg{at}niaid.nih.gov
The manuscript "Th-1 cytokines inhibit and Th-2 cytokines promote fibrocyte differentiation" was selected as a Pivotal Advance because it is the first publication to address the roles of T cell derived cytokines in promoting differentiation of the hematopoietic-derived components of fibrotic lesions known as fibrocytes. Specifically, Dr. Darrell Pilling and his colleagues examined the interplay of the Th-2 cytokines, IL-4 and IL-13, and Th-1 cytokines, IL-12 and IFN-
, as they promote or inhibit, respectively, the differentiation of CD14+ monocytes into this newly characterized cell lineage.
Dr. Pilling, to begin, for the readers of JLB who may not be familiar with this field, can you explain exactly what a fibrocyte is? What is the difference between a fibrocyte and a fibroblast?
DP: A fibrocyte is a monocyte-derived, spindle-shaped, fibroblast-like cell. Upon differentiation from monocytes, fibrocytes lose markers characteristic of the monocyte/macrophage lineage, such as CD14 and CD16, and begin to express markers more typically associated with fibroblasts, such as collagen and 
-smooth muscle actin. However, fibrocytes do retain markers that indicate their hematopoietic origin, including CD43 and CD45. Finally, fibrocytes differentially express markers such as CD34 as they mature. We and others are still trying to define fibrocytes with a more consistent scheme.
Among other properties of these cells, fibrocyte precursors are a subset of monocytes that can become fibrocytes, these can be found both in the circulation as well as in tissue. Mature fibrocytes are elongated fibroblast-like cells, and have been found in healing wounds as well as in fibrotic lesions. Markers for these cells include CD34, CD45, and collagen I. Myofibrocytes are derived from mature fibrocytes and express -smooth muscle actin in response to stimulation with TGF-β, and are similar to myofibroblasts.
In many respects, the main difference between fibrocytes and fibroblasts is their origin, as fibrocytes differentiate from hematopoietic stem cells, whereas fibroblasts are mesenchymal stem cell/stromal in origin. To some extent, it appears that these cell types display a sort of convergent evolution, where both cell types display elongated cell shape and secrete similar molecules in order to interact effectively with their stromal environments.
How did you and your laboratory group first become interested specifically in fibrocyte biology?
DP: Interestingly, these studies came about due to a chance meeting between me and Richard Gomer at a cell biology meeting in the UK in 2000. I was hoping for a quiet lunch and Richard was looking for someone to talk to! We soon realized that although we worked on different model systems, Richard on Dictyostelium discoideum and I on human leukocytes, we had similar interests in identifying and characterizing soluble survival factors. Soon thereafter, I was awarded a traveling fellowship from my funding agency, the Arthritis Research Council in the UK, to work with Richard on a project directed at defining the role of soluble survival factors in preventing T cell apoptosis. Although that specific project never really worked out, together we made the somewhat unrelated observation related to fibrocytes from peripheral blood, and found specifically that fibrocyte differentiation was inhibited by serum. I decided to stay on at Rice University, where I subsequently became a faculty fellow. As we pursued this line of work, we were able to identify a specific serum factor that inhibited fibrocyte differentiation as serum amyloid P (SAP), and we have since shown that SAP is capable of inhibiting fibrocyte differentiation both in vitro and in vivo [1 ]. We are currently investigating the mechanism by which SAP and cytokines regulate fibrocyte differentiation, and are trying to understand the basic biology of monocyte to fibrocyte differentiation.
In the Introduction to your manuscript, you note that fibrosis may result from the "classical" pathway, meaning, interstitial fibroblasts migrating into inflammatory areas, or via the "alternative" hypothesis involving circulating fibrocytes. Are these pathways mutually exclusive? Are there certain fibrotic states which result predominantly from one source or the other?
DP: To answer the first part of the question, as to whether these pathways are mutually exclusive, the simple answer is no. Many publications including those from the groups of Bucala, Tredget, Strieter, and Mattoli and as well as those from our group have shown the presence of fibrocytes and fibroblasts in the same areas, whether it be in a fibrotic lesion or during wound healing. As for which cell type predominates, that may well depend on the type of lesion. For example, small, sterile lesions seem to heal primarily through proliferation of stromal fibroblasts, with little or no input from blood-derived fibrocytes. However, in a large lesion, where a significant amount of tissue has been lost and there is a clear need to "fill in the gaps" rather rapidly, we then see many fibrocytes, and they may well be the major cell type in this sort of lesion. Regarding the second part of the question, one of our aims is to identify markers that can accurately distinguish fibrocytes from fibroblasts in a pathological biopsy. This may then allow clinicians to determine whether the appropriate course of therapeutic action is to reduce inflammation (i.e., if fibrocytes are the dominant cell population) or to regulate stromal/mesenchymal activation (i.e., if fibroblasts are the main pathological cell type).
One of your more intriguing findings was not reported directly in the manuscript, but mentioned in passing—specifically, the variability in the number of differentiating fibrocytes isolated from different individuals. What degree of variability do you typically observe? Is there any gender or age bias? Once you observe differentiation (in response to IL-4 and IL-13) is there likewise variability in susceptibility to inhibition (by IFN-)?
DP: There is clearly variability (up to 
10-fold) in the number of fibrocytes that we can isolate and culture in vitro from a given individual. The variation between individuals does not appear to be age or gender related, but does appear to be a stable phenotype over time. Variability also does not correlate directly with the number of circulating monocytes, and is maintained when we add cytokines, suggesting that the differential response is based on differences in genetic makeup or intrinsic regulation, as opposed to the in vivo milieu. My colleagues Richard Gomer and Varsha Vakil are currently trying to determine what specific factors regulate this variable response.
In your final figure, you note that serum amyloid protein (SAP) inhibits IL-4/IL-13-induced fibrocyte differentiation. Can you comment at all on the mechanism by which this might occur?
DP: The final figure shows that SAP appears to be dominant over IL-4 and IL-13. These data suggest that SAP, when activating downstream signaling cascade following Fc receptor activation, especially those involving syk- and src-related tyrosine kinases, provides a powerful inhibitory signal, and that the STAT-6 pathway initiated by IL-4 and IL-13 cannot overcome this effect. How these pathways interact is an ongoing area of research.
Is there anything else that you would like to add to your earlier comments?
DP: I would like to credit Diane Shao, the first author on this Pivotal Advance manuscript. She performed the majority of this work as an undergraduate student here at Rice University, and is now in the MD/PhD program at Harvard University. I would also like to acknowledge the efforts of Rahul Suresh, who is also an undergraduate student, who is soon to begin his future studies at University of Michigan. I also thank all the members of the research group here at Rice for their contributions to this work.
 View larger version (171K): [in a new window] |
Figure 1. Corresponding author, Darrell Pilling.
|
Related Article
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
