Adenosine A2a receptor (A2aR) signaling acts as a barrier to autoimmunity by promoting anergy, inducing regulatory T cells (Tregs), and inhibiting effector T cells. However, in vivo effects of A2aR signaling on polyclonal CD4 T cells during a primary response to foreign antigen has yet to be determined. To address this problem, we immunized mice with peptide antigen 2W1S coupled to PE in CFA and treated with the selective A2aR agonist CGS-21680 (CGS). 2W1S:I-Ab-specific tetramer-binding CD4 T cells did not become anergic or differentiate into Foxp3+ Tregs. Additionally, CGS treatment did not inhibit Th1 or Th17 differentiation. However, CGS did abrogate germinal center (GC) T follicular helper (Tfh) cells, and blunted PE-specific GC B cell responses. The use of A2aR-deficientCD4 T cells established that this CGS effect was T cell-intrinsic. Therefore, this study has identified a unique role for A2aRs in regulating CD4 T cell differentiation during vaccination.