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, an unusual peripheral lymphocytosis of 19 x 10(9)/l 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 a mature T-cell phenotype with 78% TcR-a beta and 16% TcR-gamma delta in the absence of an immature component. Pretreatment Southern blot analysis of peripheral blood mononuclear cells showed an oligoclonal pattern with 13-20 rearranged fragments of different intensity for the TcR beta-gene. TcR gamma also showed a pattern compatible with an oligoclonal proliferation. After treatment, after normalization of absolute blood counts, the distribution of T-cell subsets still showed a slightly aberrant pattern. Immunophenotypic analysis of a blood sample taken 6 months later, also at normal absolute cell counts, showed an increase of thymocytes as well as of mature T cells with a polyclonal pattern on Southern blot analysis. 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. Moreover, this case stresses the importance of integrated interpretation of clinical, morphological, immunophenotypical, and molecular data to gain insight in unusual clinical problems.