Flow Cytometric Characteristics of Extrathymic Thymocytes in Adenoid Tissue: A Case Report and Comparison to Normal Thymus and Thymoma

Background: Normal thymocyte precursors in secondary lymphoid organs have previously been described. It is important to recognize normal thymocyte precursors by flow cytometry to differentiate them from T-cell lymphoblastic leukemia. Methods: A 3-year-old boy status 2 years postallogenic cardiac transplant underwent adenoidectomy to exclude post-transplant lymphoproliferative disorder. Microscopic, immunohistochemical, and flow cytometry analyses of the adenoid were performed. Results: By flow cytometry, a population of CD45+(dim)/CD7+(bright)/CD3- cells were observed at 1.0% of lymphocytes. These cells expressed CD10, partial CD34 and exhibited acquisition of CD4 followed by CD8. Within the brighter CD45+ lymphocytes, a population of CD3-/CD4+/CD8+ thymocytes and a similarly sized population of CD4+/CD8+ cells exhibiting acquisition of low-density CD3 were identified. By immunostaining, clusters of TdT+/CD1a+/CD4+/CD8+ T-cells were identified in the interfollicular areas. Compared to normal thymus, thymocytes in the adenoid tissue lacked the classic CD4xCD8 winged differentiation profile but showed a normal early precursor pattern. Conclusions: Thymocytes in adenoid show a similar differentiation pattern to thymus and thymoma. However, the classic winged pattern of common thymocyte differentiation may not be readily apparent in thymocytes differentiating outside of the thymus. Recognition of the early thymocyte precursor antigen acquisition profile can be crucial to correct interpretation.