However, minor, albeit significant, changes were observed in the percentage of pre-marginal zone, marginal zone, T2 and B1 B cells. Although the

meaning of this observation is presently unclear, this finding suggests that Treg cells may also contribute to maintaining overall homeostasis of splenic B-cell populations. In addition to disrupting Treg-cell activity Trichostatin A order with administration of anti-GITR mAb, a large number of studies have examined the role of Treg cells in immune responses using a depleting anti-CD25 mAb.51–55 High-dose anti-CD25 treatment deletes most but not all Treg cells, because a minority of Foxp3+ T cells in secondary lymphoid tissues are CD25.1–47,52 BALB/c mice were injected with 250 μg of either anti-CD25 mAb (PC61) or control rIgG on days −2, +1, +5 with injections continued twice weekly until the mice were killed. Mice were immunized with SRBC on day 0 and splenic GCs were examined on days 8–24. As opposed to continuous anti-GITR mAb treatment, extended anti-CD25 mAb treatment did not lead to mortality, probably because of the protective activity of residual CD25− Treg cells. Similar to mice treated with anti-GITR mAb, however, injection of anti-CD25 mAb resulted in a larger total GC response and a progressive imbalance

of switched to IgM+ GC B cells (see Supplementary material, Fig. S2). Regardless of the means by which Treg-cell activity was inactivated, therefore, GC responses were markedly dysregulated. Although both anti-GITR mAb and anti-CD25 mAb treatments are well Vincristine datasheet accepted methods for inactivating Treg cells in vivo, it is possible that the mAbs may have direct effects on GC B cells. To rule out this possibility, GC B cells were tested at days 8, 12 and 18 post-immunization for expression of GITR and CD25. As shown in Supplementary material, Fig. S3, GC B cells were negative for these molecules at all time-points tested.

To ensure that Treg-cell control of GC responses was strain independent, C57BL/6 mice were similarly challenged with SRBC and treated with either anti-GITR mAb or control Thalidomide rIgG (Fig. 2). Even though control-treated C57BL/6 mice generated a smaller splenic GC reaction after SRBC immunization compared with BALB/c mice (Fig. 2a,b), the response was again characterized by a steady ratio of IgM+ to switched B cells at all time-points (Fig. 2c). Importantly, anti-GITR mAb administration resulted in a larger proportion and total number of GC B cells (Fig. 2b), especially at the early time-points, and a disproportionate percentage and number of switched GC B cells throughout the response (Fig. 2c). Similar to findings in BALB/c mice, there was also a significant increase in the percentage of IgG1+ GC B cells at day 8 in anti-GITR mAb compared with rIgG-treated mice (data not shown).