Immunosuppression in mice after inoculation with 334C, a murine lymphatic leukemia-inducing virus

334C murine leukemia virus, which induces a high incidence of lymphatic leukemias (80-90%) in susceptible mice following a long latency period, was found to cause a severe in vivo suppression of direct plaque-forming cells from the spleen, following antigenic stimulation with sheep red blood cells. Neonatally infected inbred BALB/c and outbred Ha/ICR Swiss mice, which develop a sustained viremia, were highly susceptible to the immunosuppressive effect of this virus as early as 1 week after virus infection, long before any detectable histologic evidence of leukemia development. Ha/ICR Swiss mice, which are highly resistant to the leukemogenic potential of this virus following infection in adult life, were highly resistant to its immunosuppressive action; only a moderate and transient suppression, without viremia, occurred 2 weeks after virus infection. In marked contrast, BALB/c mice were highly susceptible to the immunosuppressive action of 334C murine leukemia virus following infection in adult life; a severe and sustained suppression was observed as early as 1 week after virus infection and was followed by a sustained viremia, beginning at 2 weeks, with a 55-60% incidence of leukemia observed over a period of 1 year. Infectious virus was essential to produce theimmunosuppressive effect; heat-inactivated (56 degrees C/30 min) and attenuated (4 degrees C/4 1/2 mo) virus preparations were ineffective. The plaque-forming response of spleen cells from lethally irradiated syngeneic adult BALB/c mice was markedly suppressed following reconstitution with thymus-dependent (T) or thymus-independent (B) cells from the thymus and bone marrow, respectively, of virus-infected mice, in combination with each other, or with the appropriate cell populations from normal mice.