Abstract 3566: Targeting CD99 in T-cell neoplasms

Abstract Introduction: CD99 is a 32 kDa transmembrane protein that regulates T cell maturation and the transendothelial migration of leukocytes. CD99 is also routinely used as a biomarker to aid in the diagnosis of T-cell acute lymphoblastic lymphoma/leukemia (T-ALL), Ewing sarcoma, and neuroendocrine tumors. We have previously shown that monoclonal antibodies (mAbs) directed against CD99 effectively induce cytotoxicity in acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), however, it remains unclear whether CD99 is a viable therapeutic target in T-cell neoplasms. Design: CD99 protein expression was assessed on 115 lymphoma patient samples by immunohistochemistry (IHC) using tissue microarrays. CD99 expression was also measured by flow cytometry on mature T-cell lymphoma cell lines, T-ALL cell lines, and peripheral blood (PB) T-cells. Cell lines exhibiting the highest levels of CD99 expression were used for in vitro cytotoxicity assays and were incubated with 40 μg/ml of anti-CD99 mAbs (DN16, O13, or 12E7) in the presence of increasing concentrations of an anti-IgG cross-linking antibody (Ab) (50-100 μg/ml). Cell survival and morphology were assessed by flow cytometry and light microscopy 72 hours following incubation with anti-CD99 mAbs. Results: Various T-cell neoplasms showed CD99 expression by IHC based on a 5% cut-off: 11/20 (55%) T lymphoblastic lymphomas, 7/16 (44%) angioimmunoblastic T-cell lymphomas, 4/13 (31%) anaplastic large cell lymphomas (ALCLs), 10/63 (16%) peripheral T-cell lymphomas, and 0/3 (0%) NK/T cell lymphomas. Flow cytometric analysis revealed that CD99 is up-regulated on Karpas-299 (ALCL) and Kopt-K1 (T-ALL) cell lines relative to PB T-cells (normalized MFI’s: 56.1, 39.5, 20.5), while low levels of CD99 were expressed on Mac2A (ALCL), Hut78 (sezary syndrome), and Hut102 (mycosis fungoides) cell lines (normalized MFI’s: 0.19, 10.9, 0.5, respectively). Incubations of Kopt-K1 cells with DN16 or 12E7 induced significant cell death in the presence of increasing concentrations of anti-IgG Ab relative to IgG isotype control (DN16: 70-81% cell death, p≤.0002; 12E7: 42-78% cell death, p≤.0003). Karpas-299 incubations with DN16 or O13 also led to significant reductions in cell numbers in the presence of anti-IgG Ab (O13: 25% cell death with 75 μg/ml anti-IgG Ab, p<.0001; DN16: 38-43% cell death with 50-100μg/ml anti-IgG Ab, p≤.0021). Consistent with previous findings in B-ALL, cell death was associated with marked cell aggregation. Conclusions: Our studies show that CD99 is expressed on T-ALL blasts and T-cell lymphomas, particularly in ALCL. Anti-CD99 mAbs are cytotoxic to T-ALL and ALCL cell lines in the presence of anti-IgG Ab, suggesting that cross-linking or multimerization of CD99 is required to induce cell death. Collectively, these studies indicate that targeting CD99 with therapeutic mAbs may represent a novel strategy in the treatment of multiple T-cell neoplasms. Citation Format: Montreh Tavakkoli*, Dong H. Lee, Stephen S. Chung, Christopher Y. Park. Targeting CD99 in T-cell neoplasms. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3566. doi:10.1158/1538-7445.AM2015-3566