Macrophage accumulation in mice is inhibited by low molecular weight products from murine leukemia viruses.

Low m.w. extracts from three known oncogenic viruses, Friend, Moloney, and Rauscher, inhibited the accumulation of macrophages at sites of delayed inflammatory reactions in mice. The potential biologic significance of these proteins is suggested by their potency: as little as 1.2 ng of viral protein inhibited (p less than 0.02) macrophage accumulation when injected at a site distant to the inflammatory reaction. A virus envelope protein fraction of 15,000 daltons (p15E) was likewise found to inhibit macrophage accumulation and may in part represent the active factor of the virus extracts. Certain oncogenic viruses may thus exert their immunosuppressive activity by release of potent inhibitors of systemic macrophage function.