Putative myeloma precursor cells expressing 2,6 sialic acid-modified antigens actually belong to the erythroid lineage

The Golgi enzyme α2,6-sialyltransferase modifies glycoconjugates by adding sialic acid. In lymphocytes, different epitopes that result from this modification have been identified by the B cell-related CDw75, CDw76, HB4 or HB6 Ab. We previously described positive staining with these Ab of a highly transferrin receptor-positive (CD71) cell type in the bone marrow of multiple myeloma patients. These cells were distinct from plasma cells, but did contain Ig of the same isotype and idiotype as seen in the plasma cells. We postulated a precursor role for this cell type in myeloma. Here, we report that this CD71⁺(HB4/HB6/CDw75/CDw76)⁺ cell is an erythroid precursor cell instead. RT-PCR did not detect Ig mRNA, and from immune electron microscopy Ig appeared to be endocytosed rather than synthesized by these cells. At their cell surface the erythroid/megakaryocytic markers CD36 and CD41, and the erythroid-specific glycophorin A can be detected, while haemoglobin can be detected antigenically in the cytoplasm. Finally, purified cells proliferate in vitro upon addition of erythropoietin. Uptake of Ig could be explained by the presence of FcγRIII(CD16), which has also been found on other haematopoietic precursor cells.