Abstract 4429: ‘Free’ ISG15 acts as an immunoadjuvant to enhance CD8+ T cell-mediated anti-tumor immunity
Authors: Oladejo Mariam , Wood Laurence 
Published: June 2023

Abstract
Interferon-stimulated gene 15 (ISG15) is a ubiquitin-like molecule that is primarily involved in antiviral responses. ISG15 can either exist in the conjugated form, where it is covalently bound to proteins in a process termed ISGylation or can be released extracellularly in the cytokine-like “free” form. Although conjugated ISG15 often indicates poor prognosis in cancers, ‘free’ ISG15 is suggested to have immunostimulatory properties, owing to its ability to stimulate the production of IFN-gamma from lymphocytes and natural killer (NK) cells. While the immunomodulatory properties of free ISG15 are known, its ability to improve the antigen-specific T-cell response mediated by tumor immunotherapies has not been fully elucidated. The need for CD8+ T cell-based immunoadjuvant in cancer immunotherapy is underscored by the current failure in the generation of optimum CD8+ T cell response by the multitude of cancer vaccines in clinical trials. In this study, we demonstrated that ISG15 is vital for the generation of antigen-specific adaptive immunity; we showed that ISG15 deficient (ISG15-/-) mice do not develop effective systemic CD8+ T cell response after ovalbumin protein vaccination in non-tumor bearing mice. Upon tumor challenge of ISG15-/- mice and wild type C57/BL6 mice with B16-OVA melanoma tumors, effective OVA-specific CD8+ T cell-mediated anti-tumor immunity was not elicited, and the tumors progressed at a faster rate in the ISG15-/- mice. We then utilized a clinically relevant model to evaluate the therapeutic benefits of ‘free’ ISG15. We used a synergistic therapeutic approach that involved the concomitant administration of a DNA-construct encoding ‘free’ ISG15(mutISG15) and a Listeria-monocytogenes(Lm)-based cancer vaccine directed against the tumor vasculature (Lm-LLO-CD105A) to intervene in murine models of renal cell carcinoma (Renca) and breast cancer (4T1-Luc). The combination of Lm-LLO-CD105A and mutISG15 led to a significant reduction in tumor burden in both syngeneic models. This tumor control was mediated by the expansion of a population of highly polyfunctional CD8+ T cells in the combinatorial therapy group. These CD8+ T cells were characterized by the robust production of multiple cytokines, including IFN-gamma, IL-2, and TNF-alpha.Moreover, we confirmed that mutISG15 significantly increased the antigen (CD105)-specific anti-tumor response generated by Lm-LLO-CD105A. Not surprisingly, in-vitro studies demonstrated that mutISG15 probably potentiates the induction of maturation, activation, and antigen-presentation of dendritic cells to facilitate the activation and effective functionality of tumor-specific T cells. Conclusively, our results positions ‘free’ ISG15 as a powerful T-cell stimulant that may provide a suitable therapeutic option for improving the effectiveness of therapeutic cancer vaccines.
Citation Format: Mariam Oladejo, Laurence Wood. ‘Free’ ISG15 acts as an immunoadjuvant to enhance CD8+ T cell-mediated anti-tumor immunity. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4429.