Critical Role for SLAM/SAP Signaling in the Thymic Developmental Programming of IL-17- and IFN-γ-Producing γδ T Cells

During thymic development, mouse γδ T cells commit to either an IFN-γ- or an IL-17-producing phenotype through mechanisms that remain unclear. In this study, we investigated the extent to which the SLAM/SAP signaling pathway regulates the functional programming of γδ T cells. Characterization of SLAM family receptor expression revealed that thymic γδ T cell subsets were each marked by distinct coexpression profiles of SLAMF1, SLAMF4, and SLAMF6. In the thymus, Vγ1 and Vγ4 T cells that exhibited an SLAMF1⁺SLAMF6⁺ double positive phenotype were largely contained within immature CD24⁺CD73^- and CD24⁺CD73⁺ subsets, whereas SLAMF1 single positive, SLAMF6 single positive, or SLAMF1SLAMF6 double negative cells were found within mature CD24^-CD73⁺ and CD24^-CD73^- subsets. In the periphery, SLAMF1 and SLAMF6 expression distinguished IL-17- and IFN-γ-producing γδ T cells, respectively. Disruption of SLAM family receptor signaling through deletion of SAP resulted in impaired thymic Vγ1 and Vγ4 T cell maturation at the CD24⁺CD73^-SLAMF1⁺SLAMF6⁺ double positive stage that was associated with a decreased frequency of CD44⁺RORγt⁺ γδ T cells. Impaired development was in turn associated with decreased γδ T cell IL-17 and IFN-γ production in the thymus as well as in peripheral tissues. The role for SAP was subset-specific, as Vγ1Vδ6.3, Vγ4, Vγ5, but not Vγ6 subsets were SAP-dependent. Together, these data suggest that the SLAM/SAP signaling pathway plays a larger role in γδ T cell development than previously appreciated and represents a critical checkpoint in the functional programming of both IL-17- and IFN-γ-producing γδ T cell subsets.