The CD28-related molecule ICOS is required for effective T cell-dependent immune responses

While CD28 is critical for expansion of naive T cells, recent evidence sugg ests that the activation of effector T cells is largely independent of CD28 /B7. We suggest that ICOS, the third member of the CD28/CTLA-4 family, play s an important role in production of IL-2, IL-4, IL-5, and lFN gamma from r ecently activated T cells and contributes to T cell-dependent B help in viv o. Inhibition of ICOS attenuates lung mucosal inflammation induced by Th2 b ut not Th1 effector populations. Our data indicate a critical function for the third member of the CD28 family in T cell-dependent immune responses.