The role of complement in monoclonal antibody-mediated protection against virulent Semliki Forest virus.

Monoclonal antibodies (MAs), specific for either the E1 or E2 glycoproteins of Semliki Forest virus (SFV), and belonging to various immunoglobulin subclasses (IgM, IgG2a, IgG2b and IgG3), effected lysis of SFV-infected L cells in co-operation with guinea-pig complement. In this antibody-dependent complement-mediated cytolysis (ADCMC) test, IgG1 MAs were not effective although these antibodies recognize the viral antigens on the surface of SFV-infected L cells. The latter was shown with horseradish peroxidase (HRPO)-labelled MAs in a direct enzyme immunoassay. The binding reactivities of HRPO-labelled MAs to infected L cells at selected time-intervals after infection correlated well with the amount of cytolysis in a parallel ADCMC test. Cytolysis was dependent on the duration of incubation with antibodies: more cytolysis was measured after a 4-hr incubation period with MA, starting at 4 hr after infection, compared to a 1-hr incubation period starting after 7 hr of infection. However, in the latter case (1-hr period) the amount of cytolysis measured correlated better to neutralization and/or protection by MAs than after the extended period (4 hr) of incubation. Complement (C3) depletion by cobra venom factor treatment led to a higher mortality and viraemia of mice prophylactically injected with critically protective doses of either the neutralizing MA UM 8.4 (IgM) or the non-neutralizing MA UM 4.2 (IgG2a). The results suggest a co-operative role of MA with complement in mediating protection against SFV. Passive immunization by administration of low amounts (0.1 micrograms/mouse) of neutralizing MA UM 5.1 resulted in protection of normal mice against a lethal infection with SFV. Mice immunosuppressed by cyclophosphamide were not protected by these doses. If the doses were increased however, these mice were protected both prophylactically and therapeutically. These results indicate that, using critical doses of MAs, an intact immune system ensures survival in normal mice after infection with virulent SFV.