Immunophenotypic Characterization and Purification of Neoplastic Cells from Lymph Nodes Involved by T‐Cell/Histiocyte‐rich Large B‐cell Lymphoma by Flow Cytometry and Flow Cytometric Cell Sorting

Background T‐cell/histiocyte rich large B‐cell lymphoma (THRLBCL) is B‐cell lymphoma in which rare neoplastic cells are embedded in a reactive infiltrate. We describe the first characterization of the neoplastic cells by flow cytometry (FC). Methods Using FC, we immunophenotyped the neoplastic cells of 11 cases of THRLBCL and 11 cases of DLBCL, NOS (controls). Neoplastic THRLBCL cells were also purified by flow cytometric cell sorting (FCCS). Results A neoplastic THRLBCL population was detected by FC in 9 of 11 cases (82%). Neoplastic THRLBCL cells demonstrated an aberrant germinal center B‐cell immunophenotype (bright CD20, bright CD40; positive for Bcl‐6 and CD75; weakly positive for CD32; negative for IgH). With regard to adhesion molecules, CD54 was overexpressed, CD58 expression varied between cases, and CD50 expression was intermediate. Evaluation of immunomodulatory receptors demonstrated that PD‐L2 was weakly expressed and PD‐L1 was variably expressed. Finally, FCCS of two cases showed large multi‐lobated cells with morphology consistent with neoplastic cells of THRLBCL. Conclusions The immunophenotype identified and the morphology of the FCCS purified cells confirms the FC defined populations are neoplastic cells from THRLBCL. While the cohort is small, neoplastic THRLBCL cells lack surface immunoglobulins. CD40, CD50, and CD54 were overexpressed in THRLBCL relative to DLBCL, NOS, perhaps contributing to the predominance of T cells in THRLBCL. Expression of CD32, PD‐L1, and PD‐L2 may be useful in distinguishing THRLBCL and NLPHL. Finally, the FC assays will be useful for purifying neoplastic cells of THRLBCL and for diagnostic immunophenotyping of THRLBCL. © 2019 International Clinical Cytometry Society