T-cell lymphocytosis associated with invasive thymomas

Area of Science:

• Oncology
• Immunology
• Hematology

Background:

• Invasive thymomas rarely present with peripheral T-cell lymphocytosis and bone marrow infiltration.
• The origin of circulating lymphocytes in such cases is debated, with hypotheses including tumor spillover, neoplastic proliferation, or reactive T-cell proliferation due to thymoma-induced immune dysregulation.

Observation:

• Two cases of invasive thymoma with lymphocyte-rich, cortical histology exhibited peripheral T-cell lymphocytosis.
• Flow cytometry revealed T cells constituted up to 99% of circulating lymphocytes, expressing a mature phenotype (CD2+, CD3+, CD4+/CD8+, CD5+, CD7+) with alpha/beta T-cell receptor.
• Bone marrow biopsies showed infiltrates of small T cells, with no T-cell receptor beta gene monoclonal rearrangement and normal female karyotypes.

Findings:

• The observed T-cell population exhibited a mature phenotype consistent with medullary thymic or peripheral T cells.
• Absence of T-cell receptor gene rearrangement and normal karyotypes support a reactive, rather than neoplastic, origin for the peripheral lymphocytosis.
• A subset of peripheral T cells with low-intensity CD5 expression suggests a thymic origin.

Implications:

• Findings support a reactive origin for peripheral T-cell lymphocytosis in invasive thymoma.
• Tumor invasiveness may augment the release of mature thymoma-associated T cells into circulation, contributing to lymphocytosis.
• Further research into thymoma-induced immunoregulatory disorders is warranted to understand lymphocyte proliferation in situ.