Background: Cancer development is in part a failure of the immune system. Advancements in our understanding of the immune system and different cellular processes responsible for modulating anti-tumor response has led to development of immune-based therapies, such as immune checkpoint inhibitors, which have revolutionized treatment of some previously lethal malignancies. Understanding regulatory mechanisms involved in T-cell activation has become even more critical with the development of chimeric antigen receptor T-cells (CAR T-cells), which have emerged as a very promising cancer treatment, succeeding where other available treatments had failed, however its most prevalent setback is T-cell exhaustion. Abelson interactor-1 (ABI1) is an adaptor protein known to regulate actin polymerization, adhesion molecules and tyrosine kinases. T-cell activation requires proper intercellular interactions mediated by cytoskeletal rearrangements and adhesion molecules, along with intracellular signal transduction via phosphorylation by tyrosine kinases (Lck and Fyn). Previously, ABI1 has been shown to localize to the immunological synapse in an antigen-dependent manner and its involved in the regulation of IL-2 secretion. Additionally, ABI1 has been linked to hyperactivation of SFKs/STAT3/NF-kB signaling. Published data from our lab shows that ABI1 has high affinity binding to SH2 domains of Src family kinases, Fyn and Lck, both of which are essential in T-cell activation. SFKs play a critical role in the signal propagation through the T-cell receptor, thus in this study we sought to delineate the role of ABI1 in T-cell activation and signaling. Methods: We have generated a conditional knock-out mouse model (ABI1(fl/fl)CD4Cre) to evaluate the role of ABI1 in CD4 and CD8 T-cells in-vivo. In addition, we generated an ABI1 KO cell line model in Jurkat T cells using CRISPR-Cas9. The efficiency of both the Cre-Lox system and CRISPR-Cas9 were evaluated by Western Blotting and genomic alterations confirmed by DNA sequencing. To identify ABI1-dependent transcriptional targets and signaling pathways, we performed RNAseq analysis of primary murine T-cells (ABI1 KO vs. WT controls). Results: We have generated the first T-cell specific ABI1 KO mouse model. In vitro studies of primary murine CD3+ T-cells showed increased levels of NF-kB and STAT3. Additionally, we demonstrate the effect of ABI1 loss on the expression of SOS1 and ESPS8L1 in T-cells. Conclusion: Our preliminary studies show that ABI1 is a potential modulator of T-cell activation through regulation of Lck/Fyn. Together, our data suggests ABI1 may play as a molecular switch negatively regulating T-cell activation by connecting phosphorylation-dependent signal transduction and actin cytoskeletal dynamics. Suggesting ABI1 as a new therapeutic to improve efficacy and longevity of T cell-based cancer immunotherapies. Citation Format: Maria Altagracia Ortiz, Marcin Kortylewski, Andras Perl, Leszek Kotula. Defining ABI1 role in T-Cell signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1761.

Background: The immune system plays a critical part in cancer development and progression. The increase in our understanding of mechanisms regulating immune-mediated cancer eradication has led to the development of immune-based therapies which have revolutionized treatment of some previously lethal malignancies. T-cell activation requires proper intercellular interactions mediated by cytoskeletal rearrangements and adhesion molecules, along with intracellular signal transduction via phosphorylation by tyrosine kinases (Lck and Fyn). Abelson interactor-1 (ABI1) is an adaptor protein known to regulate actin polymerization, adhesion molecules and tyrosine kinases. Previously, ABI1 has been shown to localize to the immunological synapse in an antigen-dependent manner and has been linked to regulation of IL-2 secretion. Published data from our lab shows that ABI1 has high affinity binding to SH2 domains of Src family kinases, Fyn and Lck, both of which are essential in T-cell activation. Additionally, ABI1 has been linked to hyperactivation of SFKs/STAT3/NF-kB signaling. In this study, we aim to delineate the role of ABI1 in T-cell activation, differentiation and signaling. Methods: We have generated a novel conditional knock-out mouse model to evaluate the effect of a T-cell specific ABI1 KO at the organismal level (ABI1(fl/fl)CD4Cre (ACD4). T-cells isolated from control (Abi1(fl/fl)CD4Cre(-)) and ABI1 KO mice (Abi1(fl/fl)CD4Cre(+)) were used for ex-vivo analysis and further characterization of ABI1 KO cellular phenotype with: Flow Cytometry (CD3+ subpopulations in immune organs), Immunofluorescence (actin polymerization), Western blotting (TCR-mediated T-cell activation) and RNAseq (transcriptional reprogramming). Results: In vitro studies of ABI1 KO primary murine CD3+ T-cells showed increased levels of NF-kB and STAT3. Flow cytometry analysis of CD3+ T cells subpopulations showed a difference in the percentage of CD4+ and CD8+ T cells; percentage of CD8+ T cells was increased, while percentage of CD4+ T cells was decreased in ABI1 KO vs. WT control mice. Furthermore, RNA seq analysis of CD8+ ABI1 KO T cells showed a significant decrease in the mRNA expression of Thpok (Zbtb7b) compared to WT. Conclusion: Our preliminary studies show that ABI1 is a potential modulator of T-cell activation through regulation of Lck/Fyn. This suggests ABI1 may play as a molecular switch negatively regulating T-cell activation by connecting phosphorylation-dependent signal transduction and actin cytoskeletal dynamics. Suggesting ABI1 as a new therapeutic to improve efficacy and longevity of T cell-based cancer immunotherapies. In addition, ABI1 appears to be implicated in the regulation of the percentage of CD4 and CD8 T-cells, a novel discovery which warrants further investigation. Citation Format: Maria Altagracia Ortiz, Andras Perl, Marcin Kortylewski, Leszek Kotula. Characterization of mouse and cellular phenotype of ABI1 KO T-cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2097.