The surface expression of a tumor-associated antigen on human kappa myeloma cells

The monoclonal antibody K-1-21 defines an antigen, KMA (kappa myeloma antigen) on the surface of human kappa myeloma cells. K-1-21 also recognizes human kappa light chains in free form but not when covalently bonded to heavy chains. To examine the relationship between KMA and this determinant on free kappa chains, the surface expression of KMA was examined on the IgG, kappa myeloma line LICR LON/HMy2 (HMy2). No patching or capping was observed in the presence of K-1-21 alone, but KMA could be capped if the cells were incubated with K-1-21 followed by fluorescein isothiocyanate-conjugated sheep F(ab')2 anti-mouse immunoglobulin. Capping was not affected by the inhibitors calcium ionophore or dibucaine. When IgG molecules were removed from the cell surface by capping with anti-IgG antiserum both KMA and free kappa light chains could still be detected with K-1-21 and a polyvalent anti-kappa antiserum, respectively. By contrast, after removal of all surface kappa chains with the polyvalent anti-kappa serum, no staining was observed with K-1-21 indicating that KMA may be an epitope on free kappa chains inserted in the membrane of kappa myeloma cells but absent from normal cells. KMA cell surface expression varied with the stage of the cell cycle. Flow cytometric analysis of K-1-21-stained HMy2 cells from either continuous cultures or from elutriated fractions enriched for various cell cycle phases showed that, within the cycling population, cells in G2 + M expressed KMA at a higher frequency and density than did cells in G1.