T-cell lymphocytosis associated with invasive thymomas.

Peripheral T-cell lymphocytosis and bone marrow infiltration is a rarely associated feature of invasive thymomas. Hypotheses for the origin of the circulating lymphocytes include a spillover of tumor lymphocytes into the circulation, a neoplastic lymphoid proliferation, and a reactive proliferation of peripheral lymphocytes resulting from a thymoma-induced immunoregulatory disorder. The authors describe two cases of peripheral T-cell lymphocytosis associated with invasive thymomas with a predominantly lymphocyte-rich, cortical type histologic appearance. In both cases T cells constituted as much as 99% of the circulating lymphoid cell population, according to flow-cytometric analysis before and after treatment, and expressed a mature phenotype consistent with medullary thymic or peripheral T cells (CD2+, CD3+, CD4+ or CD8+, CD5+, CD7+) with predominant expression of the alpha/beta T-cell receptor heterodimer. Bone marrow biopsy specimens showed nodular and interstitial infiltrates of small T cells. No monoclonal rearrangement of the T-cell receptor beta gene was observed. Both cases had a normal female karyotype. Our findings confirm those of previous studies supporting a reactive origin for the peripheral lymphocytosis. Although an immunoregulatory disorder is probably involved in lymphocyte proliferation in situ, we postulate that peripheral lymphocytosis results from augmented release of the more mature thymoma-associated lymphocytes into the circulation through tumor invasiveness. A thymic origin is supported by the presence of a subset of peripheral T cells with low-intensity CD5 expression.