Monoclonal plasma cells with normal phenotype can lead to diagnosis of small B‐cell lymphoma

Abstract

A 66-year-old man was referred for non-regenerative anaemia (Hb 114 g/L) and monoclonal IgG Kappa gammopathy (29.9 g/L). Biochemical test results showed normal serum creatinine and calcium and slight increase beta2-microglobulin (3.21 mg/L). Bone marrow examination revealed 3% plasma cells (Figure 1A May-Grünwald-Giemsa, ×100 objective, blue arrows) and 9% lymphocytes (Figure 1A red arrows) without morphological changes related to malignancy. Flow cytometric immunophenotyping showed CD38-positive, CD138positive plasma cells with Kappa monotypic expression and without aberrant expression of CD45, CD19, CD117, CD56, CD81 and CD27 (Figure 1B). Next generation sequencing of sorted plasma cells revealed MYD88 L265P mutation (VAF 0.22). These results were not in favour of monoclonal gammopathy of undetermined significance (MGUS) and led us to look for B-cell lymphomas. Flow cytometric immunophenotyping of lymphocytes, showing Kappa monotypic Bcells population (Figure 1B) with coexpression of CD19, CD20, CD79a and without CD5, CD23, CD10, was finally indicative of lymphoplasmacytic lymphoma. Plasmacytic differentiation in B-cells lymphomas is a well-known phenomenon [1, 2] that can be challenging for differential diagnosis, especially with MGUS and multiple myeloma (MM). In MGUS and MM, plasma cells typically exhibit aberrant phenotype with loss of CD19 and CD45, expression of CD56 and/or CD117 and/or lack of CD81 and/or CD27 [3, 4]. Thus, the presence of a monoclonal population of plasma cells with a normal phenotype should prompt us to look for B-cells lymphomas [5].