Spleen cells and serum from mice immunized with ectromelia virus suppressed the immune response to infectious virus when transferred intravenously into recipient mice given an immunizing virus dose. The suppression was reflected in decreased cytotoxic T-cell activity directed against H-2 compatible virus-infected target cells in the spleens of recipients. Suppression was observed when immune cells or serum were transferred 1-2 h or 1 day after immunization of recipients, but not 2 days after, and was maximal when 6-day immune spleen cells were used as suppressor cells. H-2 compatibility between donor and recipient mice was necessary for suppression to be expressed. Use of recombinant mice showed that I-region compatibility was neither sufficient nor necessary, and that D-region compatibility was sufficient. Specificity of suppression was suggested by the finding that cells and serum from mice immunized with Listeria monocytogenes, a bacterium, had no suppressive activity on the antiviral response. Anti-theta treatment eliminated the ability of immune cells to suppress, and the suppressive effect was not markedly dose-dependent with respect to both cell dose and virus dose under the conditions employed. Virus levels in the spleens of recipients were significantly reduced after injection of immune cells. Adult thymectomy had no effect on the primary immune response to ectromelia virus infection, thus indicating no role for T1 cells in the suppressive mechanism. The results obtained therefore suggested that suppression in this system was due to effector T cells which triggered clearance of virus (and thus, of virus-induced antigens) necessary for the induction of precursors of effector T cells, and that this simple feed-back mechanism normally plays an important role in the regulation of the primary immune response to ectromelia infection at the level of precursor induction. The existence of other postinduction regulatory mechanisms, however, is unknown and under investigation.

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