Immunosuppression Induced In Vitro by Cell-Free Extracts of Friend Leukemia Virus-Infected Splenocytes 2

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Abstract
The immunologic responsiveness by normal BALB/c spleen lymphocytes immunized in vitro with sheep erythrocytes was markedly suppressed by clarified cell-free homogenates of Friend leukemia virus (FLV)-infected mouse spleens. Suppression was achieved with a 3,000Xg supernatant of FLV-containing homogenates freed of cellular debris but not with crude unclarified homogenates or pelleted material after centrifugation. The immunosuppressive effects of the virus were dose dependent whether the virus was added directly to the target spleen cells or separated from them by a 0.4-μ Nuclepore filter. Suppression was prevented by heating of the virus at 100° C for 10 minutes or by neutralization with antiserum to FLV. Addition of the virus as late as 48 hours after in vitro immunization of splenocytes affected the immune response. However, suppression was maximum when the clarified virus was added to the splenocytes 1 or 2 days before immunization. The agent in the FLV-infected spleen homogenates responsible for immunosuppression appeared to be the virus per se; however, virus-associated soluble factor(s) might have been involved.

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