Lack of β₂- Microglobulin on the surface of canine transmissible venereal tumor cells2,3

β₂-Microglobulin (β₂m) expression on the cell surface of the naturally occurring, allotransplantable canine transmissible venereal tumor (TVT) was investigated by use of indirect membrane immunofluorescence and radioimmunoassay. Two cell populations were identified in animal-derived, collagenase-disag-gregated TVT cell suspensions. About 80% of the cells lacked surface β₂m expression, whereas about 20% of the cells strongly reacted with anti-dog β₂m serum. With the use of a cell separation technique, β₂m-negative cells were demonstrated to carry TVT markers on their surface, whereas the β₂m-positive cells did not express the tumor markers. The β₂m-positive cells seemed, therefore, to be tumor-infiltrating host cells. These findings were supported by fluorescence staining studies of frozen sections of the TVT. The lack of β₂m expression on the surface of TVT cells might explain the allotransplantability of this neoplasm, since β₂m expression on the cell surface appeared to be obligatory for the expression of class I major histocompatibility complex antigens.