Use of PD-1, CD1a, and S-100 in Differentiating Pseudolymphomatous Folliculitis and Indolent Primary Cutaneous B-Cell Lymphomas

Background: Pseudolymphomatous folliculitis (PLF) is a non-neoplastic lymphoid proliferation that clinically and histopathologically mimics primary cutaneous extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma). In this study we assessed the diagnostic value of three immunohistochemical markers, PD-1, CD1a, and S100. Methods: We evaluated 25 cases of cutaneous lymphoid proliferations with established diagnoses, including 9 patients with PLF, 11 with MALT lymphoma, and 5 with cutaneous lymphoid hyperplasia. The clinical, histopathologic, and immunohistochemical characteristics were reviewed and three major characteristics assessed: 1) proportion of T cells expressing PD-1, 2) pattern of expression of CD1a by dendritic cells, and 3) pattern of expression of S100 by dendritic cells. Results: We found PLF to have a significant increase in PD-1+ T cells compared to MALT lymphoma (p<0.0001). The pattern of CD1a staining is also informative: MALT lymphoma is significantly more likely to demonstrate a peripheral concentration of CD1a+ dendritic cells around lymphoid nodules than PLF (p<0.0003) or CLH (p<0.05). PLF demonstrates an interstitial distribution of CD1a+ cells more often than MALT lymphoma (p<0.04). S100 staining was not a helpful discriminator. Conclusions: Histopathological factors including PD-1 and CD1a staining patterns may allow for more certainty in distinguishing PLF from MALT lymphoma.