SURFACE EXPRESSION OF IMMUNOGLOBULIN ISOTYPES ON PRIMARY HUMAN B-CELLS - NO EVIDENCE FOR GLYCOSYL-PHOSPHATIDYLINOSITOL LINKAGE

Surface expression of membrane (m) IgM molecules requires the associat ion of two disulphide-linked transmembrane (TM) glycoproteins, encoded by the B-cell-specific genes mb-1 and B29. We have shown that mIgM, m IgD and mIgG are associated with structurally related heterodimers on primary human B cells. Transfection studies in murine plasmacytoma cel ls, however, have demonstrated mb-1-independent expression of both mIg D and mIgG. The recent finding that mIgD is expressed on these cells t hrough glycosyl-phosphatidylinositol (GPI) linkage may be interesting in view of the function of mIgD on primary B cells. We therefore inves tigated whether GPI linkage serves as an additional mechanism for expr ession of mIgD and the other mIg isotypes on primary human B cells. Ho wever, we were unable to demonstrate the release of mIg molecules upon treatment with phosphatidylinositol-specific phospholipase C (PI-PLC) in either immunofluorescence analysis or Ig heavy (H) chain-specific enzyme-linked immunosorbent assay (ELISA). We conclude that primary hu man B cells, which constitutively express the mb-1 and B29 genes, do n ot express the mIg isotypes in a GPI-linked manner.