Background Autoreactive T cells play a central role for the pathogenesis of autoimmune diseases. They recruit proinflammatory cells of the innate immune system to sites of inflammation and cause tissue destruction through the secretion of cytokines. Even more important, the special subpopulation of T follicular helper cells (TFH) provides help for B cells which is a prerequisite for the production of autoantibodies. The inducible T cell costimulator ICOS is an important regulator of T cell responses. We previously showed that T cells in SLE patients express enhanced levels of ICOS and interact directly with B cells in kidney infiltrates.
Objectives To analyze the role of ICOS in the cooperation of T and B cells in detail we moved to an in vivo mouse model which would allow to track antigen-specific T and B cells in all phases of the immune response.
Methods Our in vivo T/B cooperation system is based on the adoptive transfer of ovalbumin- (OVA-) specific T cells and nitrophenol- (NP-) specific B cells into syngeneic immunocompetent B6 mice which are immunized subcutaneously with NP coupled to OVA. Antigen-specific T and B cells can be tracked and characterized by flow cytometry and immunohistology.
Results Using this system and ICOS-deficient mice we could define two important functions of ICOS. First, the early expansion of antigen-specific T and B cells was significantly reduced in the absence of ICOS costimulation. Second, the differentiation of antigen-specific T cells towards a TFH phenotype was strongly impaired. As a direct consequence the development of germinal center B cells was also diminished.
Interestingly, at early time points (day 1-4) T cell activation and TFH differentiation was not influenced by the absence of ICOS signaling. In particular, ICOS-deficient T cells upregulated the master transcription factor Bcl-6 to a normal extent. In addition, expression of other TFH signature molecules like production of IL-21 and downregulation of CCR7 was comparable between wild-type and ICOS knock-out T cells. Antigen-specific ICOS-deficient T cells also migrated normally into B cell follicles.
Only at later time points (day 7-10) we observed a dramatic reduction in the number of TFH cells. For this reason, we analyzed the effects of missing ICOS costimulation specifically at late phases of the immune response by applying an anti-ICOS-L monoclonal antibody. Late ICOS-L blockade led to an almost complete disappearance of already differentiated TFH cells within 48 h. However, the absolute number of antigen specific T cells remained constant, indicating that they reversed to a non-TFH effector phenotype.
Conclusions Our results clearly show that ICOS is not important for the generation of TFH cells. Instead, continuous ICOS costimulation is required to maintain the TFH phenotype at later phases. In addition, on a more general basis, our data show that the TFH phenotype is not a stable endpoint of T cell differentiation.
Disclosure of Interest None Declared
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