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ICOS costimulation regulates the development of effector memory CD4 T cells

Inducible costimulator (ICOS) is a member of the CD28/B7 family of costimulatory molecules. ICOS costimulation has been shown to enhance T‐helper responses in vivo, particularly Th2 responses. ICOS is upregulated upon T cell activation and is highly expressed on memory phenotype CD4 T cells, but the role of ICOS costimulation on the development and survival of memory CD4 T cells is unclear. To address the influence of ICOS costimulation on the development of memory cells, we utilized an adoptive transfer model to track the development of memory cells from naïve ICOS−/− and wild‐type precursors. The ICOS−/− T cells had an initial defect in proliferation and differentiation. Interestingly, the initial contraction of the ICOS−/− population was slower than that of the wild‐type population, but was more prolonged so that fewer memory cells developed from ICOS−/− precursors. Thus, the ICOS−/− T cells produced significantly less memory CD4 T cells, specifically effector memory (EM) cells. In support of these findings, mice lacking ICOS costimulation accumulated polyclonal endogenous EM CD4 T cells at a significantly slower rate than wild‐type mice. These data suggest that ICOS costimulation selectively enhances the development of the EM cells in vivo, probably by enhancing proliferation and proliferation‐associated differentiation.