The inducible costimulatory molecule (ICOS) is expressed on activated T cells and participates in a variety of important immunoregulatory functions. After the induction of experimental allergic encephalomyelitis in SJL mice with proteolipid protein (PLP), brain ICOS mRNA and protein were up-regulated on infiltrating CD3+ T cells before disease onset. ICOS blockade during the efferent immune response (9-20 days after immunization) abrogated disease, but blockade during antigen priming (1-10 days after immunization) exacerbated disease. Upon culture with PLP and compared with immunized controls, splenocytes produced either decreased interferon-gamma (IFN-gamma, in efferent blockade) or excessive IFN-gamma (in priming blockade). PLP-specific immunoglobulin G1 was decreased in animals treated with anti-ICOS during antigen priming, but not in other groups.