Interleukin-6 gene expression is preferentially restricted in VLA-5^-MPC-1^- immature but not in VLA-5⁺MPC-1⁺ mature myeloma cells

Interleukin-6 (IL-6) is essential for the proliferation of myeloma cells, and IL-6 is considered to be produced from not only myeloma cells themselves but also microenvironments including stromal cells. To clarify which subpopulation of myeloma cells can produce IL-6, we examined IL-6 mRNA expression in immature and mature myeloma cells and normal plasma cells by RT-PCR. IL-6 mRNA expression was found in all (10/10) specimens of sorted VLA-5^--MPC-1^- immature myeloma cells and 27% (3/11) of VLA-5^-MPC-1⁺ myeloma cells. On the contrary, no IL-6 mRNA was expressed in VLA-5⁺MPC-1⁺ mature myeloma cells (0/4) and CD19⁺CD56^- normal plasma cells (0/5). IL-6R gene expression was detected in all normal and malignant plasma cells without exception. Therefore, these findings suggest that IL-6 production is preferentially restricted in immature not mature myeloma cells, and this may explain why immature myeloma cells show greater proliferative activity.