How I Diagnose Primary Mediastinal (Thymic) Large B-Cell Lymphoma

Sarah L. Ondrejka, DO; German Ott, MD


Am J Clin Pathol. 2021;156(4):497-512. 

In This Article

Immunohistochemistry/Flow Cytometry

PMBL strongly expresses B-lineage antigens CD19, CD20, CD22, PAX5, and CD79a,[31–33] and characteristically lacks surface and cytoplasmic immunoglobulin expression despite consistent and uniform expression of B-cell transcription factors such as OCT2, BOB1, and PU.1. Unique to PMBL, there is discordance between mb-1/CD79a expression and immunoglobulin expression, with the majority of cases retaining the CD79a molecule despite loss of surface immunoglbulin.[30] CD30 is expressed by most cases, with a weak intensity and heterogenous distribution,[34] although it has not shown utility as a predictive marker to chemoimmunotherapy with brentuximab vedotin in PMBL.[35] CD23 is positive in 70% of cases.[36] Tumor cells are generally positive for BCL2, variably positive for BCL6, and positive for MUM1, suggesting that they are derived from cells that have passed through the germinal center reaction.[16] A minor subset of cases, however, ranging between 20% and 30% express the CD10 protein,[30,37] and some studies suggest a germinal center cell derivation of this lymphoma based on a high frequency of somatically mutated IgVH genes and BCL6 alterations.[37,38] EBV is uncommon and does not entirely exclude the diagnosis, but it is not typical.[15,39] Flow cytometry can be helpful in identifying a B-cell population with abnormal forward scatter characteristics and undetectable surface κ and λ light chains. In some circumstances, there is a suboptimal yield of malignant B cells by flow cytometry because of either impaired viability or fibrosis that affects recovery.

The molecular signature of PMBL includes activation of the NFĸB pathway,[10,11,26] whereby nuclear translocation of c-Rel–containing transcriptional complexes causes upregulation of the NFĸB target TRAF1 protein. IHC analysis for cytoplasmic TRAF and nuclear c-REL protein expression revealed that the combination of these 2 markers showed high specificity for reliably distinguishing PMBLs from other DLBCLs, although sensitivity was not optimal and would require correlation with other clinical and immunophenotypic features. Because TRAF1 is expressed by CHL, it is not as helpful in that distinction.[40]

Expression of MAL protein, an integral membrane protein found in lipid rafts, supports derivation of PMBL from thymic medullary B cells and has been demonstrated as a distinct molecular and immunophenotypic marker of primary mediastinal lymphoma compared with nonmediastinal DLBCLs. MAL is expressed by approximately 70% of PMBLs Figure 6. In normal tissues, MAL protein expression is confined to a minor subpopulation of thymic medullary B cells and to some mature plasma cells in tonsil and lymph nodes but is more widely expressed in T cells, including thymocytes and a large proportion of peripheral CD4-positive T cells.[41] MAL protein is not helpful in distinguishing PMBL from MGZL because it is expressed in 70% of these lymphoma types. In contrast, CHL expresses MAL protein in 10% of cases, and less than 3% of DLBCL NOS cases are positive for the MAL protein.[40–43]

Figure 6.

Example of primary mediastinal )thymic) large B-cell lymphoma with typical histology (A, ×500), positive for MAL protein (B, ×500).

Finally, CD200 expression in PMBL is highly consistent. Although expressed by many B-cell neoplasms, CD200 is a practical and useful marker for the diagnosis of PMBL. It is available by IHC or flow cytometry. In an analysis comparing a series of 35 cases of PMBL with 30 cases of DLBCL, CD200 outperformed CD23, MAL, TRAF, and c-REL in terms of staining sensitivity and exhibited a high specificity of 93%.[44] In another study of CD200 and a commercially available MAL antibody, there was no substantial advantage to combining MAL with CD200 for differentiating PMBL from DLBCL NOS. In this analysis, the MAL antibody was favored as a diagnostic marker alone because of its higher specificity, despite a lower sensitivity than CD200.[45]