In none of the groups reported here were we able to score arthritis above the baseline, suggesting that peripheral tolerance Selleck CB-839 is intact in all groups (data not shown). This further emphasizes the conclusion that clonal deletion is not a critical contributor to the development of such tolerance in the case of chronic peripheral self-antigen stimulation. The absence of clonal deletion in the lower frequency group, prompted us to examine if the other major mechanisms of peripheral tolerance are intact in the model — namely anergy and conversion to a Treg-cell fate. We examined the latter by staining for the canonical marker Foxp3 and did not find significant conversion in the chronic hosts (Fig. 3A, closed bars in 3B) with

only a minimal conversion in the acute hosts (Fig. 3A, open bars in 3B). While this argues against skewing of the autoreactive T cell itself, it does not, of course, rule out the possibility that endogenous Treg cells

participate in the peripheral tolerance process. Finally, we tested if the T cells that persist for such extended periods in the presence of chronic antigen, are in fact anergic. The in vivo parallel of anergy, known as adaptive tolerance, is typically marked by a severe blunting of the signaling cascades downstream of the TCR leading to a reduction in the ability of the T cell to secrete cytokines such as IL-2 [18]. Consistent with this, 5C.C7 T cells recovered 13 days later from 103 injected PCC-transgenic mice failed to make IL-2 (detected by capture assay as shown in Figure 3C). This contrasted with the robust IL-2 detected in similar CP-690550 chemical structure cells that were acutely immunized with PCC in antigen deficient mice (open bar Methane monooxygenase in Figure 3D). Therefore, in this model, at near physiological precursor frequencies, the induction of anergy seems to operate but without

the accompaniment of clonal deletion or the conversion to a regulatory Foxp3 lineage. These results are strikingly similar to the fate of the T cells in a lymphopenic model where we observe anergy but no deletion or suppression [19]. In this context, however, it must be emphasized that the choice of a nondeletional tolerance mechanism is not simply restricted to anergy. In fact, in similar models, under lymphopenic conditions, T cells have been shown to develop anergy in concert with a suppressive phenotype [7]. The variables that allow this phenotype to develop in specific models may relate to TCR affinity, antigen presentation, etc., but are not well understood. The mechanisms controlling T-cell numbers in vivo remains an enduring mystery. Recent work suggests that clonal competition regulates the pool of memory T cells generated after acute immunization. We suggest that it seems to be less of a factor in the case T cells responding to chronic, self-antigens. Interestingly, these T cells can also persist in vivo for extended periods with no evidence of clonal deletion or conversion to Treg cells.