Frequent downregulation or loss of CD79a expression in plasma cell myelomas: potential clue for diagnosis.

Plasma cell myeloma is a frequent hematogeneous disorder that occurs mainly in older people. Not only bone marrow smears but also clots and/or biopsied specimens are often taken for confirmation of pathological diagnosis. Some specimens show sheet-like plasma cell proliferation associated with immunoglobulin monotype on immunohistology, which readily leads to diagnosis, but many samples do not clearly show light-chain restriction. The aim of the present study was therefore to examine CD79a expression because some samples had reduced expression or none at all. The immunoreactivity of CD79a was categorized into three groups: positive, weakly positive and negative, compared with scattering non-neoplastic plasma cells in the same specimen. Out of 100 specimens of plasma cell myeloma, 48% were positive for CD79a, 15% were weakly positive, and 37% were negative. In contrast, overexpression of cyclinD1 was detected in 26% of examined samples. CD79a-negative cases had a significantly lower percentage of positive staining for cyclinD1 than CD79a-positive or weakly positive cases. Clinicopathological data showed that CD79a-negative expression was associated with decreased platelet numbers in patients. The present study indicates that downregulation or loss of CD79a and/or overexpression of cyclin D1, observed in 59% of neoplastic plasma cell samples, could provide a strong diagnostic clue without regard to the results of immunoglobulin light-chain restriction.