CD55 expression is induced during T cell maturation and protects recent thymic emigrants from complement-mediated killing

Abstract Newly generated T cells must undergo phenotypic and functional maturation. Previously, we showed that the absence of NKAP severely curtailed T cell maturation, and NKAP-deficient recent thymic emigrants (RTEs) fail to enter the long-lived naïve peripheral T cell pool. The paucity of peripheral T cells in CD4-cre NKAP conditional knockout (cKO) mouse is not due to apoptosis as neither loss of Bax nor Bcl-2 or Bcl-xL transgenes rescued the maturation defect. Here, we show that CD55, a complement regulatory protein functioning to disassemble C3 convertases, plays a role in T cell maturation. The expression of CD55, but not Crry or CD59, increases concurrently with T cell maturation by two orders of magnitude. NKAP-deficient peripheral T cells display a diminished expression of CD55 and an increase in complement C3 deposition, indicating CD55 is required for peripheral T cell fitness of RTEs. Reconstitution of CD4-cre NKAP cKO bone marrow into C3-deficient host partially rescues the maturation defect as measured by upregulation of Qa2 and CD55, and an increased proportion of mature naïve T cells. Thus, successful T cell maturation results in increased expression of CD55 that protects RTEs from complement-mediated killing.