In multiple myeloma, only a single stage of neoplastic plasma cell differentiation can be identified by VLA-5 and CD45 expression.

The nature of the proliferating fraction in myeloma is still not known and understanding the characteristics of this fraction is central to the development of effective novel therapies. However, myeloma plasma cells typically show a very low rate of proliferation and this complicates accurate analysis. Although the level of CD45 and/or VLA-5 has been reported to identify proliferating 'precursor' plasma cells, there are discrepancies between these studies. We have therefore used a rigorous sequential gating strategy to simultaneously analyse cycle status and immunophenotype with respect to CD45, VLA-5 and a range of other integrin molecules. In 11 presentation myeloma patients, the proliferative fraction was distributed evenly between CD45+ and CD45- cells, however, cycling plasma cells were consistently VLA-5-. There was close correlation between the expression of VLA-5 and a range of other integrin molecules (CD11a, CD11c, CD103), as well as the immunoglobulin-associated molecules CD79a/b (Spearman, n = 10, P < 0.0001). In short-term culture, cells that were initially VLA-5-showed increasing VLA-5 expression with time. However, simultaneous analysis of the DNA-binding dye 7-amino-actinomycin D demonstrated that this was not as a result of differentiation, as VLA-5+ plasma cells were all non-viable. This was confirmed in freshly explanted plasma cells from nine patients. Discrete stages of plasma cell differentiation could not be distinguished by the level of CD45 or VLA-5 expression. The results indicate that there is a single stage of plasma cell differentiation, with the phenotype CD38+CD138+VLA-5-. These findings support the hypothesis that neoplastic bone marrow plasma cells represent an independent, self-replenishing population.