Immunosuppression by Moloney leukemia virus: lack of correlation between virus replication and the immunosuppressive effect.

Young adult mice were infected with 10(4) plaque-forming units (PFU) or Moloney murine leukemia virus M-MuLV. Two different virus preparations were used: a) M-MuLV obtained from serial passage in mice [animal passage (AP)] and b) tissue culture (TC)-grown virus harvested after three in vitro passages of the AP M-MuLV in fibroblasts. Replication of TC and AP M-MuLV in spleen cells was determined by an infectious center (IC) assay at 1 and 2 weeks after the infection. Immune responsiveness of spleen cells was evaluated in challenge with sheep red blood cells (SRBC) and subsequent enumeration of antibody plaque-forming cells (PFC). TC M-MuLV replicated faster in the spleen than did AP M-MuLV and reached about 10- to 100-fold higher titers. However, the response of anti-SRBC PFC, suppressed to the same degree in the spleens of mice infected with TC or AP virus, was from 10 to 50% of the control response. A comparison of virus replication with the anti-SRBC response in aliquots from the same spleens showed no correlation between virus IC and antibody PFC. Both TC and AP M-MuLV induced the expression of virus-specific, cell membrane antigen on spleen cells. These findings indicated a divergence between virus replication on the one hand and the immunosuppressive effect and the cell membrane alteration on the other.