PD-1 expression in T-cell lymphomas and reactive lymphoid entities: potential overlap in staining patterns between lymphoma and viral lymphadenitis.

Peripheral T-cell lymphomas are a heterogeneous group that often requires the use of ancillary testing for accurate diagnosis. This is particularly applicable to the diagnosis of angiommunoblastic T-cell lymphoma (AITL) and peripheral T-cell lymphoma, unclassified (PTCLU), because of their histologic and immunophenotypic overlap with reactive lymphoid proliferations. Recently, immunohistochemistry for programmed death-1 (PD-1), a marker of follicular helper T cells, was shown to be sensitive in the detection of AITL and PTCLU. The sensitivity of this marker in reactive entities, however, has not been adequately evaluated. We confirm that PD-1 staining is a highly sensitive marker in the diagnosis of peripheral T-cell lymphomas: increased extrafollicular PD-1-positive cells were seen in 93% (76/82) of AITL, 62% (16/26) of PTCLU, and 11% (2/18) of anaplastic-lymphoma-kinase (ALK)-negative anaplastic large-cell lymphomas. The majority of reactive lymphadenopathies including Cat-scratch disease, Kikuchi lymphadenitis, Castleman disease, and reactive follicular hyperplasia showed no PD-1 staining outside follicles. Some reactive lymph nodes, showed increased extrafollicular PD-1-positive cells in a pattern similar to AITL and PTCLU, and include progressive transformation of germinal centers, viral lymphadenitis (Epstein-Barr virusand human immunodeficiency virus) and Rosai-Dorfman disease. This study shows that PD-1-positive cells may be increased in a number of settings other than T-cell lymphomas. We conclude that staining for PD-1 in reactive and atypical lymphadenopathies should be interpreted with caution and in the context of other ancillary immunophenotypic and molecular studies before a diagnosis of AITL or PTCLU is entertained.