EXPRESSION OF CD21 ANTIGEN ON MYELOMA CELLS AND ITS INVOLVEMENT IN THEIR ADHESION TO BONE-MARROW STROMAL CELLS

The mature myeloma cells express very late antigen 5 (VLA-5) and MPC-1 antigens on their surface and adhere to bone marrow (BM) stromal cell s more tightly than the VLA-5(-)MPC-1(-) immature myeloma cells in vit ro. The VLA-5 and MPC-1 antigens possibly function as two of the molec ules responsible for interaction of mature myeloma cells with BM strom al cells. However, the immature myeloma cells do interact with BM stro mal cells, and it is unclear which adhesion molecules mediate their in teraction. In this study, we found that both immature and mature myelo ma cells expressed CD21, an adhesion molecule known to bind to CD23. C D21 was also detected on normal plasma cells. To evaluate the role of CD21 expression on myeloma cells, two myeloma cell lines, NOP-2 (VLA-5 (-)MPC-1(-)) and KMS-5 (VLA-5(+)MPC-1(+)), were used as representative s of immature and mature myeloma cell types, respectively, and an adhe sion assay was performed between the myeloma cell lines and BM stromal cells. Antibody-blocking results showed that adhesion of the mature t ype KMS-5 to KM102, a human BM-derived stromal cell line, or to short- term cultured BM primary stromal cells was inhibited by monoclonal ant ibodies (MoAbs) against CD21, VLA-5, and MPC-1, and inhibition of adhe sion of the immature type NOP-2 to KM102 by the anti-CD21 MoAb was obs erved as well. Furthermore, CD23 was detected on KM102. Treatment of K M102 with an anti-CD23 MoAb also inhibited adhesion of either KMS-5 or NOP-2 to KM102. Therefore, we propose that CD21 expressed on myeloma cells likely functions as a molecule responsible for the interaction o f immature myeloma cells as well as mature myeloma cells with BM strom al cells, and CD23 may be the ligand on the stromal cells for the CD21 -mediated adhesion. (C) 1995 by The American Society of Hematology.