Follicular helper T (TFH) cells participate in humoral responses providing selection signals to germinal center B cells. Recently, expression of CXCR5, PD-1, and the transcription factor Bcl-6 has allowed the identification of TFH cells. We found that a proportion of follicular T cells, with phenotypic characteristics of TFH cells and expressing Foxp3, are recruited during the course of a germinal center (GC) reaction. These Foxp3+ cells derive from natural regulatory T cells. To establish the in vivo physiologic importance of Foxp3+ follicular T cells, we used CXCR5-deficient Foxp3+ cells, which do not have access to the follicular region. Adoptive cell transfers of CXCR5-deficient Foxp3+ cells have shown that Foxp3+ follicular T cells are important regulators of the GC reaction following immunization with a thymus-dependent Ag. Our in vivo data show that Foxp3+ follicular T cells can limit the magnitude of the GC reaction and also the amount of secreted Ag-specific IgM, IgG1, IgG2b, and IgA. Therefore, Foxp3+ follicular regulatory T cells appear to combine characteristics of TFH and regulatory T cells for the control of humoral immune responses.

Induction of transplantation tolerance with certain therapeutic nondepleting monoclonal antibodies can lead to a robust state of peripheral “dominant” tolerance. Regulatory CD4+ T cells, which mediate this form of “dominant” tolerance, can be isolated from spleens of tolerant animals. To determine whether there were any extra-lymphoid sites that might harbor regulatory T cells we sought their presence in tolerated skin allografts and in normal skin. When tolerated skin grafts are retransplanted onto T cell–depleted hosts, graft-infiltrating T cells exit the graft and recolonize the new host. These colonizing T cells can be shown to contain members with regulatory function, as they can prevent nontolerant lymphocytes from rejecting fresh skin allografts, without hindrance of rejection of third party skin. Our results suggest that T cell suppression of graft rejection is an active process that operates beyond secondary lymphoid tissue, and involves the persistent presence of regulatory T cells at the site of the tolerated transplant.

Germinal center (GC) responses are controlled by T follicular helper (T) and T follicular regulatory (T) cells and are crucial for the generation of high-affinity antibodies. Although the biology of human circulating and tissue T cells has been established, the relationship between blood and tissue T cells defined as CXCR5Foxp3 T cells remains elusive. We found that blood T cells are increased in Sjögren syndrome, an autoimmune disease with ongoing GC reactions, especially in patients with high autoantibody titers, as well as in healthy individuals upon influenza vaccination. Although blood T cells correlated with humoral responses, they lack full B cell-suppressive capacity, despite being able to suppress T cell proliferation. Blood T cells have a naïve-like phenotype, although they are absent from human thymus or cord blood. We found that these cells were generated in peripheral lymphoid tissues before T-B interaction, as they are maintained in B cell-deficient patients. Therefore, blood CXCR5Foxp3 T cells in human pathology indicate ongoing humoral activity but are not fully competent circulating T cells.