T Cell Lymphoblastic Lymphoma Resulting From Expression of Self-Reactive TCRs During Early Thymopoiesis

Abstract 2057

Survivin has been considered a potential tumor antigen due to high expression in most cancers and limited expression in normal tissues. To explore the potential for survivin reactive TCRs to mediate antitumor effects in mice, we generated several founders of TCR transgenic (Tg) mice with specificity for the H-2b restricted immunodominant epitope of survivin. In survivin TCR Tg mice, survivin reactive T cells were predominantly CD8⁺ and mediated specific immune reactivity toward survivin peptide pulsed targets. Some antitumor reactivity was observed, but it was not potent, and the survivin reactive transgenic T cells were unable to mediate objective tumor regression of survivin bearing tumors in vivo. Surprisingly, spontaneous T cell acute lymphoblastic leukemia (T-ALL) was observed beginning at 4–6 months of age in both survivin TCR+Rag^+/− and survivin TCR+Rag^−/− mice. By one year of age, all mice had succumbed to T-cell ALL. The leukemic cells were CD3⁺, survivin TCR+, and CD8⁺ or CD4⁻/CD8⁻. Analysis of alpha gene rearrangements in tumor tissues revealed oligoclonality but the cells were malignant since they grew continuously in vitro without growth factors and induced tumors in C57BL/6 immunocompetent recipients. The occurrence of T-ALL in 3 founders suggests that the transgene itself, rather than insertional mutagenesis, is causative. We postulate that the survivin reactive TCR serves as an oncogene via recognition of survivin peptides within the thymus, leading to expansion of early thymic progenitors. In support of this, survivin itself is expressed in thymic tissue and premalignant survivin TCR Tg+ thymi show expanded frequencies and absolute numbers of CD4⁻CD8⁻CD44⁻CD25⁻ thymocytes and increased BrdU incorporation within this subset compared to controls. Subsequent to the premalignant phase characterized by expansion of early thymic progenitors, surviving TCR Tg+ cells acquired NOTCH mutations and upregulated CD25, consistent with NOTCH signaling as a 2^nd hit in this oncogenic process. At least one NOTCH1 mutation was found in all leukemias, with mutations in the PEST domain being most common (8/8), but 5' deletions (19/25) and mutations in the heterodimerization domain were also observed. Interestingly, T cell acute lymphoblastic leukemia with NOTCH mutations were also observed, albeit at reduced frequencies, in TCR Tg mice with specificity to WT1 and gp100. We propose a 2-hit model of oncogenesis for self-reactive TCR expression in the thymus. Early thymic expression of TCRs recognizing self antigens expressed in the thymus induces proliferation of early thymocytes, followed by acquisition of NOTCH mutations and ultimately lymphoblastic leukemia. We conclude that genetic engineering aimed at endowing hematopoietic or T lymphoid progenitors with the capacity to recognize tumor antigens expressed in the thymus could pose a risk for neoplasia, independent of insertional mutagenesis.