Progressive peripheral CD8+ T lymphocytosis complicated by pure red cell aplasia following immunosuppressive therapy for thymoma-associated myasthenia gravis.

We herein report a unique case of type B2 thymoma-associated myasthenia gravis which was ameliorated by immunosuppressive therapy in combination with chemotherapy. However, the patient subsequently developed pure red cell aplasia and marked lymphocytosis after additional chemotherapy aimed at improvement of thymoma. While a separate immunosuppressive regimen was effective for anemia, lymphocytosis was exacerbated. The biopsied thymoma specimen contained CD4+, CD8+, and CD4+/CD8+ T cells, some of which were CD3-, suggesting immature thymocytes. In contrast, majority of the peripheral lymphocytes were polyclonal CD3+/CD8+/T cell receptor (TCR)αβ+ T cells. The CD4/CD8 ratio in the present patient might be affected by immunosuppressive agents, resulting in CD8+ T cell expansion associated with pure red cell aplasia. Although several cases of thymoma accompanied by peripheral T cell lymphocytosis were reported, marked CD8+ T cell proliferation is extremely rare.