Rabbit hyperimmune antisera against Sindbis (SIN) or Semliki Forest (SF) virus were absorbed with purified SIN virus or SIN virus-infected cells, or with SF virus or SF virus-infected cells. Residual antibody titers were determined by hemagglutination inhibition (HAI) and antibody-dependent, complement-mediated cytolysis (ADCMC) assays. It appeared that absorption with virus-infected cells removed ADCMC-detectable cross-reactive antibody much more efficiently than did absorption with either virus. HAI assays with the same absorbed antisera indicated that both virus and virus-infected cells removed HAI-detectable crossreactive antibody. On the basis of these and other data, there appeared to be a cross-reactive antigen present on virus-infected cells which was detectable by ADCMC and was distinct from the cross-reactive antigen assayed by HAI.

SUMMARYAfter immunization with Sindbis virus, T-cell deficient nude mice, compared to normal littermates, were equally protected against challenge with Sindbis virus. However, the nude mice showed about one-tenth the protection observed with normal littermates after challenge with Semliki Forest virus at a dose of too LDs0. This is consistent with our previous interpretation that sensitized T-cell populations play a major role in cross protection between the two togaviruses. The remaining low level of specific cross protection in nude mice (detectable only at a challenge dose of IO LDs0 ) could not be attributed to an anamnestic response of neutralizing antibody to the challenge virus or to an effective antibody-dependent, complement-mediated cytolysis of infected cells in vivo. Other possible compensatory mechanisms to explain the low level of specific cross protection in nude mice are discussed.

Temperature-sensitive (ts) mutants of Sindbis virus (SIN) were used to aid in the identification of alphavirus cross-reactive proteins on the surface of infected cells by antibody-dependent, complement-mediated cytolysis. Antisera prepared in rabbits against purified SIN or Semliki Forest viruses were highly cytotoxic for cells infected with wild-type SIN and for cells infected at the permissive temperature with maturation-defective, ts mutants of SIN belonging to several distinct complementation groups. When these SIN mutants were analyzed by antibody-dependent, complement-mediated cytolysis at the restrictive temperature only cells infected with the SIN mutant of complementation group E, ts2O, participated in both homologous (with anti-SIN serum) and heterologous (with anti-Semliki Forest virus serum) antibody-dependent, complement-mediated cytolysis reactions. These data and the known defect of ts2O suggested that the cellassociated viral El glycoprotein was a functional target antigen for homologous and cross-immunoreactivity in alphavirus-infected cells. At the restrictive temperature there were quantitative differences in antibody-dependent, complementmediated cytolysis reactivity of ts2Oversus wild type-infected cells consistent with the suggestion that ts2O-infected cells do not fully express all of the homologous or the cross-reactive antigenic determinants found in wild-type infection. Additional potential sites for antigenic determinants involved in alphavirus-immune cross-reactivity are discussed in relation to events in virus maturation.

SUMMARYMice immunized with Sindbis virus, strains HR (heat-resistant) or AR339, by a dual injection given intracerebrally (i.c.) and intraperitoneally (i.p.) at day zero, were cross-protected from challenge at day IO with Semliki Forest virus (SFV). Neutralizing and haemagglutination-inhibition antibodies to Sindbis are detected in high titre by day 6 after immunization but no cross-reacting antibodies to SFV were found up to day 42. Immunized mice that were challenged with SFV showed an 8 to IO day delay in the appearance of antibody to SFV compared to mice that were sham-inoculated. Thus, cross-protection in Sindbis-immunized animals was correlated with a temporary suppression of antibody formation to the challenge virus (SFV), while the appearance of high titre antibody to SFV in shamimmunized mice after challenge with SFV did not protect.

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