THE CELL‐MEDIATED IMMUNE RESPONSE TO ECTROMELIA VIRUS INFECTION

Summary An in vitro culture method was used to study secondary cellmediated responses to ectromelia virus infection in mice. Infected, syngeneic spleen cells or peritoneal cells were efficient “stimulator” cells when cultured with “responder” cells obtained from mice infected with ectromelia 4–6 weeks previously. The kinetics of generation of cytotoxic cells in cultures were determined; a peak occurred on days 4–5. A separation procedure performed on the cytotoxic cells showed that activity was associated mainly with the Ig‐negative subpopulation (T cell‐rich) and that H‐2 compatibility between cytotoxic cells and target cells was required. The secondary response was virus‐specific, at the level of both induction and target cell lysis, at least so far as ectromelia and lymphocytic choriomeningitis (LCM) viruses are concerned. Separation of responder cells prior to culture showed that a potent secondary response was generated with the Ig‐negative (T cell‐rich) subpopulation and only a weak response was observed when the responder cells were Ig‐positive (rich in B cells). Infected stimulator cells did not appear to secrete significant amounts of soluble antigen into the medium over 4 days of culture. Thus, antigenic patterns effective in memory T cell stimulation may be largely associated with the surfaces of infected cells. Pretreatment of ectromelia virus with UV‐ or γ‐irradiation did not impair its ability to induce antigenic changes in stimulator cells. Stimulator cells treated with UV‐ or γ‐irradiated virus for 1 h and then immediately treated with pactamycin to inhibit further viral protein synthesis and replication were efficient stimulators, thus indicating that antigenic changes are induced very rapidly on the surface of stimulator cells after uptake of virus. These treatments are being used to further characterize the cellular requirements in the stimulator population.