CD69 acts downstream of interferon-α/β to inhibit S1P1 and lymphocyte egress from lymphoid organs

Abstract

Naive lymphocytes continually enter and exit lymphoid organs in a recirculation process that is essential for immune surveillance. During immune responses, the egress process can be shut down transiently¹. When this occurs locally it increases lymphocyte numbers in the responding lymphoid organ; when it occurs systemically it can lead to immunosuppression as a result of the depletion of recirculating lymphocytes. Several mediators of the innate immune system are known to cause shutdown, including interferon α/β (IFN-α/β) and tumour necrosis factor^2,3,4,5, but the mechanism has been unclear. Here we show that treatment with the IFN-α/β inducer polyinosine polycytidylic acid (hereafter ‘poly(I:C)’) inhibited egress by a mechanism that was partly lymphocyte-intrinsic. The transmembrane C-type lectin CD69 was rapidly induced and CD69^-/- cells were poorly retained in lymphoid tissues after treatment with poly(I:C) or infection with lymphocytic choriomeningitis virus. Lymphocyte egress requires sphingosine 1-phosphate receptor-1 (S1P₁), and IFN-α/β was found to inhibit lymphocyte responsiveness to S1P. By contrast, CD69^-/- cells retained S1P₁ function after exposure to IFN-α/β. In coexpression experiments, CD69 inhibited S1P₁ chemotactic function and led to downmodulation of S1P₁. In a reporter assay, S1P₁ crosslinking led to co-crosslinking and activation of a CD69–CD3ζ chimaera. CD69 co-immunoprecipitated with S1P₁ but not the related receptor, S1P₃. These observations indicate that CD69 forms a complex with and negatively regulates S1P₁ and that it functions downstream of IFN-α/β, and possibly other activating stimuli, to promote lymphocyte retention in lymphoid organs.