Diminished expression of CD19 in B-cell lymphomas.
CD19 is expressed on most B-cell lymphomas; however, the frequency and types of B-cell lymphomas with low-level expression of CD19 are not well characterized. We reviewed flow cytometric histograms specifically for decreased CD19 expression on 349 cases analyzed by the Flow Cytometry Laboratory at University Hospitals of Cleveland (Cleveland, Ohio). Results of flow cytometry were correlated with the morphologic diagnosis. Of the cases reviewed, 125 (36%) showed a visible decrease in CD19 expression compared with normal B lymphocytes. Decreased CD19 expression was noted in 79% of follicular lymphomas (27 of 34), 36% of small lymphocytic lymphomas/chronic lymphocytic leukemias (82 of 228), 31% of mantle cell lymphomas (4 of 13), 24% of diffuse large B-cell lymphomas (8 of 33), and 13% of marginal zone B-cell lymphomas/lymphoplasmacytoid lymphomas (4 of 30) and was not observed in any Burkitt lymphoma (0 of 5) or hairy cell leukemia (0 of 6). Decreased CD19 expression was significantly more frequent in follicular lymphomas than in other lymphoma subtypes (P < 0.001). No significant difference was observed in the frequency of decreased CD19 expression based on histologic grade of follicular lymphoma. Diminished expression of CD19 expression occurs frequently in B-cell lymphomas, in particular follicular lymphoma, and may be helpful in identifying B-cell lymphoma cells in complex cell mixtures such as bone marrow specimens.