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The major 35,000-molecular-weight internal antigen (p35) of the squirrel monkey retrovirus (SMRV) was isolated and partially characterized. Immunological analysis of SMRV p35 led to the demonstration of antigenic determinants common to SMRV and the Mason-Pfizer monkey virus (MPMV). A broadly reactive competition immunoassay was developed utilizing antiserum to MPMV to precipitate 125I-labeled SMRV p35. Although the major structural proteins of MPMV and SMRV competed with equal efficiency in this assay, type B and type C oncornavirus proteins lacked detectable reactivity. Antibodies reactive with the major structural proteins of both MPMV and SMRV were observed in sera of several normal rhesus monkeys with known prior exposure to MPMV-infected animals. These findings demonstrate the ability of sera from naturally immunized primates to recognize broadly reactive interspecies antigenic determinants shared by the major structural proteins of type D oncornaviruses, and they suggest possible horizontal transmission of MPMV among rhesus monkeys. Although sera from a number of squirrel monkeys contained antibody to SMRV p35, the possibility that this latter reactivity was due to endogenous virus activation rather than horizontal transmission cannot be ruled out.

Approximately 20% of normal male and female feral mice (Mus musculus) from areas with populations having either high [Lake Casitas (LC) and La Puente] or low (Bouquet Canyon) spontaneous lymphoma incidence expressed murine mammary tumor virus (MuMTV) gp52 in specific tissues. Sera from a low percentage (6%) of mice from the same trapping areas contained precipitating antibody specific for MuMTV. Although moderate to high levels of MuMTV gp52 were expressed in mammary tumor tissues of 3 of 7 LC mice and 3 of 3 (C57BL/10ScSn X LC)F1 mice, the same animals showed no detectable MuMTV-precipitating antibody. Neither MuMTV antibody nor tumor-associated MuMTV gp52 was defected in 10 LC mice bearing lymphomas or in 5 LC mice bearing hepatomas. Low levels of MuMTV gp52 expression and MuMTV antibody were also detected in subspecies of M. musculus and in the more distantly related species M. cervicolar. Compared with normal and tumor-bearing inbred mice of high (C3H/HeN) and low (C3H/HeN foster-nursed on NIH Swiss) mammary tumor strains, normal and tumor-bearing feral mice express MuMTV gp52 and MuMTV-precipitating antibodies at low frequency.