CXCR4 acts as a costimulator during thymic β-selection

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Abstract
Pre-TCR signaling is essential for the passage of thymocytes through the β-selection checkpoint. Ravichandran and co-workers find that CXCR4 acts as a costimulator for the pre-TCR and is needed for optimal progression through β-selection. Passage through the β-selection developmental checkpoint requires productive rearrangement of segments of the T cell antigen receptor-β gene (Tcrb) and formation of a pre-TCR on the surface of CD4−CD8− thymocytes. How other receptors influence ββ-selection is less well understood. Here we define a new role for the chemokine receptor CXCR4 during T cell development. CXCR4 functionally associated with the pre-TCR and influenced β-selection by regulating the steady-state localization of immature thymocytes in thymic subregions, by facilitating optimal pre-TCR-induced survival signals, and by promoting thymocyte proliferation. We also characterize functionally relevant signaling molecules downstream of CXCR4 and the pre-TCR in thymocytes. Our data designate CXCR4 as a costimulator of the pre-TCR during β-selection.