De Novo Unclassifiable CD20-Negative Diffuse Large B-Cell Lymphoma: A Diagnostic and Therapeutic Challenge.

CD20-negative diffuse large B-cell lymphomas (DLBCLs) constitute a rare and heterogeneous group of aggressive lymphomas. Known well-documented variants include plasmablastic lymphomas, primary effusion lymphomas, anaplastic kinase-positive large B-cell lymphomas, and large B-cell lymphomas arising in human herpesvirus 8 (HHV8)-associated multicentric Castleman disease. They impose diagnostic challenges for pathologists and therapeutic confrontations for clinicians. CD20 loss in B-cell lymphomas is a well-known phenomenon after rituximab therapy. De novo loss of CD20 has been reported in human immunodeficiency virus (HIV)-positive patients. Rare cases of primary CD20-negative DLBCLs that did not meet the criteria of the well-established subtypes of CD20-negative DLBCLs have been reported. This might expand the spectrum of unclassifiable CD20-negative DLBCLs with aberrant genetic and immunophenotypes. This imposes further diagnostic and therapeutic challenges. We report a case of a primary CD20-negative DLBCL in an HIV-infected female patient with an Epstein Barr virus (EBV) coinfection, who presented with generalized lymphadenopathy and fever. The nodal neoplastic immunoblasts were positive for LCA, PAX5, CD30, OCT2, BOB1, MUM1, CD79a, and CD19. Ki67 proliferation index was 100%. They were negative for CD20, CD3, ALK, EMA, CD138, CD38, EBV, and HHV8. Our case did not meet the criteria of the known variants of CD20-negative DLBCLs. The aim of this study is to highlight the diagnostic challenges associated with CD20-negative DLBCLs. De novo unclassifiable CD20-negative DLBCLs might raise an insight into the complex genetic mechanisms of CD20 concealment with variable immunoprofiles and resistance to conventional chemotherapies.