CD19-negative non-IgM type lymphoplasmacytic lymphoma: a case report and literature review

Lymphoplasmacytic lymphoma (LPL) that does not fulfill the diagnostic criteria for Waldenström's macroglobulinemia (WM), termed non-IgM type LPL, represents a rare entity that poses significant diagnostic challenges. Here, we report a rare case of CD19-negative non-IgM type LPL. A 69-year-old man was incidentally found to have elevated M-protein and globulin levels during a routine health examination. Subsequent evaluation revealed elevated serum IgG, an IgG-λ monoclonal protein, and markedly elevated urinary λ free light chains. Bone marrow biopsy demonstrated normocellular marrow with approximately 40% cellularity. Immunohistochemistry revealed that CD20⁺ B lymphocytes predominated over CD3⁺ T lymphocytes, exhibiting an interstitial infiltration pattern. PAX5 staining highlighted B lymphocytes, comprising approximately 30% of total nucleated cells. CD138 staining revealed plasma cells in a scattered and clustered distribution, accounting for 3-5% of nucleated cells. Flow cytometry identified a monoclonal B lymphocyte population that was CD19-negative and CD20-positive, with restricted λ light chain expression, coexisting with a monoclonal plasma cell population showing restricted cytoplasmic λ light chain expression. Although the bone marrow biopsy lacked characteristic lymphoid aggregates, molecular testing confirmed the presence of the MYD88 L265P mutation and the absence of the CXCR4 mutation. The integration of morphological, immunophenotypic, and molecular findings with clinical features established the diagnosis of CD19-negative non-IgM type LPL. This case underscores the critical role of a comprehensive, multi-platform diagnostic approach and effective clinician-laboratory communication in accurately diagnosing hematologic malignancies with atypical immunophenotypes.