CD23 expression in mediastinal large B-cell lymphomas

Aims: Mediastinal large B-cell lymphoma (MLBCL) is a subtype of diffuse large B-cell lymphoma (DLBCL) in the WHO classification with peculiar features, such as female prevalence, young patient age and bulky presentation. It shows a B-cell phenotype with variable expression of surface immunoglobulin, negative CD21 and CD10 and positive CD30 in a large number of cases. An origin from activated thymic B cells has been suggested in several studies. A subpopulation of large, dendritic cells (asteroid cells) strongly expressing CD23 has been identified amongst thymic B cells and these could represent the normal cellular counterpart for this type of primary mediastinal large cell lymphoma. Methods and results: To explore this possibility, we immunostained 24 cases of primary mediastinal lymphomas and 100 cases of non-mediastinal, nodal and extranodal, DLBCLs for CD23 in routinely processed paraffin-embedded tissues. Conclusions: Our results show that a vast majority (70%) of mediastinal lymphomas strongly express CD23 whilst the same antigen is expressed in only 15% of non-mediastinal nodal DLBCLs and 9% of non-mediastinal extranodal DLBCLs. These results support the hypothesis that most cases of MLBCL arise from activated dendritic thymic B cells. We also suggest that CD23 should be included in the panel of antibodies currently used to characterize this subtype of DLBCL.