Costimulation through the inducible costimulator ligand is essential for both T helper and B cell functions in T cell–dependent B cell responses

Costimulation through the inducible costimulator (ICOS) and its ligand (ICOSL) is essential for T cell–dependent B cell responses, but the cellular and temporal dynamics underlying its in vivo effects are poorly defined. Here we have shown that Icosl−/− and Icos−/− mice had similar phenotypes and that ICOS-ICOSL costimulation modulated the early but not late phases of IgG1 affinity maturation. Exploiting the adoptive transfer of T or B cells from primed Icosl−/− mice, we provided genetic evidence that costimulation through ICOSL was essential for primary but not secondary helper T cell responses and for the control of both T and B cell activities, resulting in T cell–dependent IgG1 production.