Type D Primate Retroviruses: A Review1

The prototype virus of the type D retroviruses is the Mason-Pfizer monkey virus (MPMV). MPMV was originally isolated from a breast carcinoma of a female rhesus monkey (an Old World monkey). MPMV is of obvious importance in that it is the only retrovirus thus far isolated from a mammary tumor of a primate and has been shown to have transforming potential for primate cells in vitro. Subsequent to the isolation of MPMV, viruses morpholog ically and ¡mmunologicallyindistinguishable from MPMV have been isolated from normal placenta and lactating mammary glands of other rhesus monkeys in captivity. Recently, viruses morphologically resembling MPMV have been isolated from a langur monkey (another Old World monkey) and from squirrel monkeys (a New World mon key). Based on nucleic acid hybridization studies, the latter 2 viruses represent endogenous viruses in their species of origin, whereas MPMV appears to be a horizon tally transmitted virus containing gene sequences par tially related to the langur monkey isolate. Studies on the immunological relatedness of the type D retroviruses have demonstrated interspecies cross-reactivities be tween the major internal and external proteins of the viruses. Furthermore, these viruses also share crossreactivity of their major external glycoproteins with those of the type C baboon endogenous virus. These interspe cies reactivities can also be demonstrated in natural sera from both imported and laboratory-bred monkeys. The demonstration of these interspecies cross-reactivities shared by distantly related primate retroviruses provides a means for detecting determinants that are representa tive of all primate retroviruses presently known and yet to be isolated and may provide new assays for detection of a human retrovirus. techniques. The retrovirus associated with the induction of mammary cancer in mice, MMTV,2 is morphologically dis tinct from the type C viruses and is the prototype of the type B virus group (30). A retrovirus group of primate origin, which has properties similar to the type C and type B viruses but is morphologically distinct from them, has been designated genus Oncornavirus D (30). The prototype virus of this genus is the MPMV. MPMV was originally isolated from a mammary carcinoma of a female rhesus monkey (an Old World monkey) (19, 60). Because mammary carcinomas represent the most prevalent form of neoplasia in humans and, furthermore, because the incidence of mammary tu mors in subhuman primates is rare (61, 62, 70, 78), MPMV is of obvious potential importance because it is the only retrovirus thus far isolated from a primate mammary tumor. Recently, there have been reports that viruses morpho logically and immunologically similar to MPMV have been isolated from certain rhesus monkey and human tissues and cell lines (6, 8, 10, 44, 49, 55, 57-59, 72, 81, 107-109). Virus morphologically resembling MPMV have also been isolated from normal tissues of a langur monkey (99) (an other Old World monkey) and from squirrel monkeys (53, 99) (a New World monkey). Only the langur isolate shares significant immunological cross-reactivity and nucleic acid sequence homology with MPMV. The discovery of these 2 viruses makes it apparent that MPMV is a member of a larger group of viruses of both Old and New World pri mates. Furthermore, the large accumulation of information on MPMV and the new type D primate isolates warrants a comprehensive review of these viruses. This review will consider the unique characteristics of type D retroviruses as well as their distribution in the primate population and their relevance to human cancer.