Immunomodification and the expression of virulence in mice by defined strains of Semliki Forest virus: the effects of cyclophosphamide.

Mice were infected i.c. or i.p. by an avirulent clone of Semliki Forest virus at various times before or after the i.p. administration of a single dose of cyclophosphamide. Consequent changes in the patterns of viraemia and antibody production are interpreted in terms of the potentiation of the initial infection or the decreased resistance to subsequent lethal challenge. These change demonstrate two distinct modes of immunomodification determined by the time of cyclophosphamide administration in relation to virus infection, but not correlated with the death or protection of individual mice. These results are discussed in relation to the early regulatory functions of T and B cells which may influence the expression of virulence but escape conventional assay during the critical first 3 days following infection.