T-lymphoblastic lymphoma is an aggressive neoplasm requiring prompt clinical treatment. Conversely, indolent T-lymphoblastic proliferation mimics T-lymphoblastic lymphoma but consists of a proliferation of non-neoplastic TdT+ T cells, requiring no treatment. Recently, we identified several cases of indolent T-lymphoblastic proliferations in extrathymic lymphoid tissues: 1 in a patient suffering from Castleman disease (CD) associated with a follicular dendritic cell sarcoma/tumor, 1 in a patient with a history of angioimmunoblastic T-cell lymphoma (AITL), and 1 in association with acinic cell carcinoma. Interestingly, in the case of the patient with a history of AITL, these TdT+ T cells were seen in multiple anatomic sites over the span of 5 years. Here we review these 3 cases and extend our findings by demonstrating that TdT+ T-lymphoblastic populations are increased in lymph nodes of patients with CD (P=0.011), CD in association with follicular dendritic cell tumors, and AITL (P<0.01) compared with other T-cell or B-cell lymphomas or reactive lymph nodes. Finally, analysis of 352 nonhematolymphoid tumors including carcinomas, melanomas, and sarcomas demonstrates that TdT+ T cells are not increased in these tumors. Our studies not only present several detailed cases of indolent T-lymphoblastic proliferations, but also correlate these populations with specific hematologic diseases.

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3238 A central dogma of human immunology is that proliferation of immature T-cells, and their development after release from the bone marrow, occurs in the central thymus. Recently, we identified several patients with aberrant polyclonal immature TdT+ precursor T-cell populations in extra-thymic lymphoid tissues. Here we demonstrate that immature precursor T-cell populations, with a cortical thymocyte phenotype, in fact, are expanded in extra-thymic lymphoid tissues of patients with Castleman disease (P < 0.001; n = 29), and angioimmunoblastic T-cell lymphoma (P = < 0.001; n =31) and increased in cases of Castleman disease in association with follicular dendritic cell sarcoma (Figures 1 and 2). Analysis of the proliferation marker, MiB-1, and the morphologic presence of mitoses reveal that these populations are undergoing extra-thymic proliferation and expansion, arguing against simple release from the central thymus and sequestration in these extra-thymic organs. Finally, these populations of immature T-cells are not associated with a particular anatomic site (i.e. neck or mediastinum). These findings challenge the dogma that proliferation of immature human T-cell populations occurs nearly exclusively in the central thymus and demonstrates that stimulation and significant proliferation of extra-thymic immature T-cells does, and can occur in a subset of patients. Disclosures: No relevant conflicts of interest to declare.

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