The indirect immunofluorescence technique was used to investigate the expression of virus antigens in a Swiss albino mouse strain infected by mouse mammary tumour virus (MMTV). The antisera used were monospecific for: the glycopolypeptides gp160 and gp47, and the internal polypeptides p28 and p8.

In mice infected with milk-borne MMTV, all four antigens were detected in the mammary gland during the first pregnancy and then throughout life, and in mammary tumours. Antigen gp160, located at the secretory border of all glandular cells, is shared by the plasma membrane of the mammary cell, whether virus-producing or not, and by the virus envelope, as shown by the use of absorbed antisera. The other three antigens are virus-specific. Anti-p47 serum stained the secretory border of cells whereas the fluorescence related to p28 and p8 consisted of a few large intracytoplasmic spots. The specific antibodies for p28 and p8 were only absorbed by disrupted virions, confirming that these polypeptides are internal antigens.

With serial sections of glands from pregnant and old female mice, the alveoli stained with anti-p28 or anti-p8 serum were found to react also with anti-gp47 serum. However, during lactation, the presence of p28 and p8 could not be detected in the alveoli stained with anti-gp47 serum. Thus, infected alveoli express virus antigens differently. In the mammary glands of all mice studied, some alveoli did not display any staining when the latter three antisera were used. The specific antigenic pattern, maintained in differentiated mammary tumours, became a diffuse and unlocalized staining after transformation into the undifferentiated state.

With the exception of the kidney, where gp47 was found in the glomeruli, all other organs of the mouse did not stain with any of the antisera. The presence of gp47 in the glomeruli, probably related to immune-complex deposits, increases with the age of the mouse and is most noticeable in tumour-bearing females

In mice free of milk-borne MMTV, examined during the first week of lactation, antigen gp47 could not be detected. Internal antigens p28 and p8 were detected in nearly all mammary cells as numerous small cytoplasmic dots. This suggests a limited expression of an endogenous virus genome; this expression would be modified when milk-borne virus is produced.


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