Cutaneous B-cell lymphoma with loss of CD20 immunoreactivity after rituximab therapy

Background: Antibodies to the B-cell-specific antigen CD20 are widely used for immunohistochemical identification of B-cell lymphomas, approximately 95% of which are strongly CD20-positive. Methods: We report a 51-year-old male with a CD20-positive systemic B-cell lymphoma who developed a CD20-negative relapse with secondary cutaneous involvement after therapy with the anti-CD20 monoclonal antibody rituximab (Rituxan). Results: Biopsy of a skin nodule demonstrated an atypical lymphocytic infiltrate that was negative for CD20, CD3, lysozyme, and myeloperoxidase, but strongly positive for CD45rb (LCA) and the B-cell marker CD79a. Conclusions: We conclude that loss of CD20 expression in cutaneous B-cell lymphoma (both primary and secondary) is important to recognize, that immunohistochemistry for CD79a, another widely expressed B-cell marker, is useful in the identification of CD20-negative B cells in such cases, and that loss of CD20 expression may become more common, as use of rituximab is expected to increase.