Intracytoplasmic type A particles from mammary tumours and leukaemias of strain ICRC mice

Integration of type B retroviral DNA in virus-induced primary murine thymic lymphomas

In previous studies we described the isolation and characterization of a highly leukemogenic virus, DMBA-LV, isolated from a transplanted, chemical carcinogen-induced thymic lymphoma. The virus is composed of a mixture of two unrelated retroviral genomes, one highly related to type B milk-borne mouse mammary tumor virus isolates and the other partially related to type C viral genomes. In the present study, primary thymic lymphomas induced by DMBA-LV in CFW/D, NIH Swiss, C3H/Bi/Ka, and C57BL/Ka mice were assessed for the presence of newly integrated type B retroviral DNA. All 46 primary thymic lymphomas examined contained one to four newly acquired murine mammary tumor virus proviruses. Based on the sizes of provirus-cell DNA junction fragments, the integration of newly acquired murine mammary tumor virus proviruses did not appear to be random.

Tissue and organ distribution of mammary tumor virus antigens in low and high mammary cancer strain mice

Expression of mammary tumor virus (MTV) antigen was measured in a wide variety of organs and tissues of a series of high (GR, SHN, SHNf, DD, SLN, SLNf) and low (DDf, DDD, DDDf, KF, KFf, ddY, C57BL, BALB/c) mammary cancer strain mice. Tests were carried out by microimmunodiffusion (micro-ID) and immunoperoxidase tests on formalin-fixed tissues and radioimmunoassays in extracts for 2 viral proteins, MTVp27 from the viral core and MTVgp52 from the viral envelope. Organs with exocrine function, i.e. the mammary gland, salivary gland, coagulating gland and prostate, were mostly positive. The secretory epithelial cells of these organs showed viral antigen expression. Less positivity was encountered in brain, pancreas, stomach, urinary bladder, epididymis, uterus, thymus, spleen, lymph nodes and kidney, plasma and blood cell pools. Unexpectedly the uterus extracts showed MTV antigen expression, occasionally even by immunodiffusion, especially in mice of various DD stocks, but also in the GR strain. Another striking observation was the detection of MTV antigen expression in salivary glands in C57BL strain mice; most other organs of this strain (including the mammary glands) were negative. The implications of results of this extensive survey for MTV antigen expression are discussed for tumorigenesis by MTV of mammary gland and perhaps other tissues in the mouse.

ML antigen of DBA/2 mouse leukemias: expression of an endogenous murine mammary tumor virus

Spontaneous, transplantable leukemias of DBA/2 mice express an antigen (ML) which cross-reacts with antigens of murine mammary tumor virus (MuMTV). The MuMTV cross-reactive antigen of the DBA/2 leukemias (ML cells) was found to be a glycoprotein of 78,000 molecular weight containing antigenic determinants of the major MuMTV glycoprotein gp52. No MuMTV particles were produced by the ML cells, although they did contain type A particles--the pronucleocapsids of MuMTV. The ML antigen appeared to be an aberrant form of the intracellular MuMTV env precursor molecular prgp70, which was not processed properly but instead acquired extra carbohydrate groups and was expressed in uncleaved form on the cell surface. Isolation of MuMTV core protein p28 from the leukemic cells and subsequent tryptic peptide mapping analysis showed that the p28 from leukemia cells differed from the p28 of MuMTV isolated from DBA/2 mouse milk. These observations indicate that the MuMTV expressed in DBA/2 leukemic spleen cells is of a different strain than the virus secreted in lactating mammary glands of DBA/2 mice and probably represents the expression of an endogenous DBA/2 provirus.