Mouse mammary tumour virus-related antigens in core-like density fractions from large samples of women's milk

Specimens of human milk (500 ml) were processed applying methods useful for isolating oncornavirus cores from mouse milk. Treatment with Triton X 100 and diethyl ether as well as banding in sucrose density gradients were included in the preparation procedure. Out of 28 women investigated, five exhibited mouse mammary tumour virus (MuMTV)-related antigens in fractions of density 1.26–1.28 g/ml. By means of a sensitive radioimmunodiffusion (RID) test, these antigens were shown to be identical to at least two main antigenic constituents of MuMTV-B particles and intracytoplasmic A particles (iAp) of mouse mammary tumours. They were precipitated by rabbit antisera against MuMTV-B particles, iAp, and isolated iAp polypeptides (Ap 37, Ap 14). The antigens were reproducibly detected in 22 out of 23 core-like density fractions prepared from the milk of the five positive women between the 5th and 28th week after delivery. By indirect immunofluorescence technique, a rabbit hyperimmune serum against antigen-positive human milk fractions was seen to react with clusters of iAp within mouse mammary tumour cells. There were no immunological cross-reactions with Mason-Pfizer monkey virus main core antigen p 27 and Mason-Pfizer-like viruses replicating in a cell line of human origin (HEp- 2). All women found positive for MuMTV-antigens in their milk, possessed the previously described serum antibody which reacts with murine iAp and inner constituents of B particles. Antigenpositive human milk fractions absorbed the antibody from autologous and homologous sera. Thus, in summary, the presence is indicated of an antigen-antibody system in man that is closely related to MuMTV.