The GS Protein-coupled A2a Adenosine Receptor Controls T Cell Help in the Germinal Center

T follicular helper (T_FH) cells have been shown to be critically required for the germinal center (GC) reaction where B cells undergo class switch recombination and clonal selection to generate high affinity neutralizing antibodies. However, detailed knowledge of the physiological cues within the GC microenvironment that regulate T cell help is limited. The cAMP-elevating, G_s protein-coupled A2a adenosine receptor (A2aR) is an evolutionarily conserved receptor that limits and redirects cellular immunity. However, the role of A2aR in humoral immunity and B cell differentiation is unknown. We hypothesized that the hypoxic microenvironment within the GC facilitates an extracellular adenosine-rich milieu, which serves to limit T_FH frequency and function, and also promotes immunosuppressive T follicular regulatory cells (T_FR). In support of this hypothesis, we found that following immunization, mice lacking A2aR (A2aRKO) exhibited a significant expansion of T follicular cells, as well as increases in T_FH to T_FR ratio, GC T cell frequency, GC B cell frequency, and class switching of GC B cells to IgG1. Transfer of CD4 T cells from A2aRKO or wild type donors into T cell-deficient hosts revealed that these increases were largely T cell-intrinsic. Finally, injection of A2aR agonist, CGS21680, following immunization suppressed T follicular differentiation, GC B cell frequency, and class switching of GC B cells to IgG1. Taken together, these observations point to a previously unappreciated role of G_S protein-coupled A2aR in regulating humoral immunity, which may be pharmacologically targeted during vaccination or pathological states in which GC-derived autoantibodies contribute to the pathology.