Optimal germinal center responses require a multistage T cell:B cell adhesion process involving integrins, SLAM-associated protein, and CD84.

CD4(+) T cells deficient in signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) exhibit a selective impairment in adhesion to antigen-presenting B cells but not dendritic cells (DCs), resulting in defective germinal center formation. However, the nature of this selective adhesion defect remained unclear. We found that whereas T cell:DC interactions were primarily integrin dependent, T cell:B cell interactions had both an early integrin-dependent phase and a sustained phase that also required SAP. We further found that the SLAM family member CD84 was required for prolonged T cell:B cell contact, optimal T follicular helper function, and germinal center formation in vivo. Moreover, both CD84 and another SLAM member, Ly108, mediated T cell adhesion and participated in stable T cell:B cell interactions in vitro. Our results reveal insight into the dynamic regulation of T cell:B cell interactions and identify SLAM family members as critical components of sustained T cell:B cell adhesion required for productive humoral immunity.