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 develo pment of effective novel therapies. However, myeloma plasma cells typically show a very low rate of proliferation and this complicates accurate analys is. Although the level of CD45 and/or VLA-5 has been reported to identify p roliferating 'precursor' plasma cells, there are discrepancies between thes e studies. We have therefore used a rigorous sequential gating strategy to simultaneously analyse cycle status and immunophenotype with respect to CD4 5, VLA-5 and a range of other integrin molecules. In 11 presentation myelom a 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 immunoglo bulin-associated molecules CD79a/b (Spearman, n = 10, P < 0.0001) In short- term culture, cells that were initially VLA-5-showed increasing VLA-5 expre ssion with time. However, simultaneous analysis of the DNA-binding dye 7-am ino-actinomycin D demonstrated that this was not as result of differentiati on, as VLA-5(+) plasma cells were all non-viable. This was confirmed in fre shly explanted plasma cells from nine patients. Discrete stages of plasma c ell 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 c ell differentiation, with the phenotype CD38(+)CD138(+)VLA-5. These finding s support the hypothesis that neoplastic bone marrow plasma cells represent an independent, self-replenishing population.