Peripheral blood B cell labeling indices are a measure of disease activity in patients with monoclonal gammopathies

Labeling indices (LI) provide a rapid measure of the bone marrow (BM) plasma cell proliferation rate and are useful in the diagnosis and prognosis of monoclonal gammopathies. Because circulating B cells may be a part of the neoplastic clone, we examined peripheral blood B cells that were producing the same cytoplasmic light chain isotype as the patient's monoclonal protein (M-protein) and determined the peripheral blood LI (PBLI) by a two-color immunofluorescence bromodeoxyuridine method. The 105 patients studied were divided into three disease activity groups by standard clinical criteria. Median PBLI was 0.2% for the 29 patients with inactive monoclonal gammopathies (monoclonal gammopathy of undetermined significance [MGUS] and smoldering multiple myeloma [SMM]), 0.8% for the 35 patients with new, untreated multiple myeloma (MM), and 1.7% for the 41 patients with relapsed MM. These differences between groups were statistically significant (P < .001, Wilcoxon). Four patients had high PBLI but clinically inactive gammopathy at the time of study, and all developed active MM within 6 months that required treatment. In 92 patients a BMLI was performed simultaneously with the PBLI (rank correlation coefficient, 0.69). In patients with new, untreated MM, use of both tests identified 72% of patients (23 of 32) with high LI, rather than 56% (18 of 32) by BMLI alone or 63% (20 of 32) by PBLI alone. These results suggest that PB B cells bearing the same cytoplasmic light chain isotype as the monoclonal protein are part of the malignant clone and can be kinetically active. The LI of these cells can provide a measure of disease activity and may help to differentiate active from inactive disease.