Evaluation of primary mediastinal large B cell lymphoma by flow cytometry

Primary mediastinal large B cell lymphoma (PMLBCL) is a B cell non-Hodgkin lymphoma (B-NHL) that shows morphologic, immunophenotypic, and genetic similarities to classical Hodgkin lymphoma (CHL). We evaluated the neoplastic and reactive infiltrate of PMLBCL by flow cytometry (FC).

A total of 24 cases of PMLBCL were retrospectively characterized using FC combinations for B-NHL, T-NHL, and CHL.

The CHL assay identified the neoplastic population (NP) in all cases, while the B-NHL assay identified the NP in 18 of 24 cases. In four cases, the neoplastic population was retrospectively identified in the B-NHL tube, given the CHL tube-derived immunophenotype. Neoplastic cells of PMLBCL displayed a B cell immunophenotype (CD19/CD20⁺ ), with CD40 consistently and brightly expressed. Most cases lacked immunoglobulin light chains, CD10, and CD15; CD30 was frequently expressed. All cases showed expression of CD71 and CD95. The reactive infiltrate in PMLBCL showed increases in T cells relative to reactive tissues. Lower CD4/CD8 ratios in PMLBCL relative to CHL and reactive tissues were also observed. Although not seen as commonly as with CHL, 41% of cases showed a reactive CD3/CD4/CD7^bright /CD45^bright+ T cell population. Reactive populations seen in PMLBCL were similar to that seen in diffuse large B cell lymphoma, not otherwise specified.

FC can detect the NP of PMLBCL and a FC assay for CHL performed better than a B-NHL assay in this regard. The neoplastic cells showed a B cell immunophenotype with over-expression of CD40. The reactive infiltrate in PMLBCL shows unique features including a prominent CD8⁺ T cell infiltrate. © 2017 International Clinical Cytometry Society.