Oligoclonal Peripheral T-Cell Lymphocytosis as a Result of Aberrant T-Cell Development in a Cortical Thymoma
A 42 year old man presented with a locally invasive cortical thymoma. Before chemotherapy was commenced 36 months after presentation, a peripheral lymphocytosis of 19×109/1 had slowly developed over time. After the first course of chemotherapy, the lymphocytosis showed a sharp decline to normal absolute cell numbers and subsequently remained at normal levels. Currently, the patient is in stable partial remission and doing well.
Immunophenotypic analysis showed 95% CD3+ve cells; 36% CD4+ve and 56% CD8+ve cells with a mature T-cell phenotype. 78% of the T-cells expressed TcR-aß and 16% TcR-γδ as assessed by immunocytochemistry. No CD4/CD8 double positive population was detected. 4% CD16/CD56+ve NK-cells could be demonstrated. Southern blot analysis of peripheral blood mononuclear cells before chemotherapy showed a striking oligoclonal pattern with 13–20 rearranged fragments of different intensity for theTcRß-gene. TcRγ also showed a pattern compatible with a oligoclonal proliferation with 6 of the 8 theoretically possible rearranged fragments in EcoRI-digested DNA. After treatment, when absolute blood counts had returned to normal, the distribution of subsets still showed a slightly aberrant pattern with a CD4/CD8 ratio of 3.25 and 80%TcR-aß and 16% TcR-γδ expressing cells. Immunophenotypic analysis of a blood sample taken 6 months later, also at normal absolute cell counts, showed an aberrant phenotype of late thymocytes (CD34−, TdT−, CD1+, CD4/CD8 single positive) and of mature T-cells (CD1−). TcRß- and TcRγ-gene rearrangment of the same sample showed a polyclonal pattern.
Thymomas may rarely be associated with peripheral lymphocytosis. Immunophenotypic analysis showed a T-cell phenotype in all reported cases. We describe a patient with a locally invasive cortical thymoma, who presented with an oligoclonal peripheral lymphocytosis of mature T-cell phenotype and subsequent polyclonal emergence of late thymocytes. These findings may be interpreted as the result of aberrant positive and negative selection and development of thymocytes in the microenvironment of neoplastic thymic epithelial cells and clonal selection through continuous peripheral stimulation.
KeywordsThymic Carcinoma Immunophenotypic Analysis Thymic Epithelial Tumor Thymic Epithelium Invasive Thymoma
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