Specificity of human antibodies to intracytoplasmic type-A particles of the murine mammary tumor virus.

Large-scale studies showed that antibodies previously detectable in women with proliferating mastopathy or breast cancer were directed to intracytoplasmic type-A particles (iAp) of mouse mammary tumor virus. Immunofluorescence revealed the human antibodies to be bound only by those tumors producing a certain amount of iAp clusters visible by light microscopy. The intensity of the reaction corresponded to the iAp content of every tumor tested as revealed by electron microscopy and rabbit antisera to iAp. The fluorescence patterns obtained with positive human sera were similar to those obtained with rabbit antisera specific for iAp and resembled the tissue distribution patterns of iAp inclusions stained by acid fuchsin. The reaction with human sera was entirely blocked by rabbit antisera to iAp and, less so, by rabbit or mouse antisera to B particles. The human antibody activity was exhaustively absorbed by purified iAp or purified and disrupted B particles, which indicated that the human antibodies were directed to antigenic components shared by iAp and B particles. Preliminary immunoperoxidase studies supported the assumption that the human antibodies were bound to the iAp membrane; technical details might have accounted for the finding that the human antibodies reacted with the iAp but not with B particles in situ.