Abstract LB-074: Antigen-cross presentation promotes development of terminally differentiated CD8 T cells in young individuals
Authors: Moustaki Ardiana , Crawford Jeremy Chase , Alli Shanta , Zamora Anthony , Fan Yiping , Boi Shannon , McDonald Natalie M. , Thomas Paul G. , Pappo Alberto S. , Dyer Michael A. , Stewart Elizabeth , Federico Sara , Youngblood Ben 
Published: September 2020

Abstract
Recent advances in T cell-based immunotherapies have revolutionized treatment strategies for several types of cancers; however, approaches that rely on endogenous T cell responses have experienced limited success in pediatric populations, a pattern primarily attributed to the relatively low mutational burden characteristic of pediatric tumors. Here, we report our analysis of CD8 T cells isolated from a diverse set of pediatric solid tumors, documenting an enrichment of CD8 T cells with an antigen-experienced phenotype (i.e., high PD1 expression). The limited ability of immune checkpoint blockade therapy (ICBT) to trigger anti-tumor responses in pediatric populations, despite the presence of activated CD8 T cells in these tumors prompted us to explore alternative mechanisms restricting the endogenous T cell response. Using a novel, transplantable mouse tumor model that expresses a well-characterized epitope coupled to a mCherry marker, we identified antigen cross-presentation by tumor-infiltrating myeloid cells as a key mediator of CD8 T cell effector function in tumors. Specifically, we show that tumor microenvironment (TME)-mediated suppression of CD8 T cell polyfunctionality occurs even in the absence of direct antigen-presentation by tumor cells, suggesting that antigen cross-presentation is sufficient to enforce this dysfunctional state. Strikingly, age-related changes in the TME had a significant impact on the differentiation of tumor-specific CD8 T cells. Cross-presentation of tumor antigens in the young animals skewed the effector differentiation of tumor-specific CD8 T cells toward a TCF7lowGzmBhi terminally differentiated state. Profiling of tumor-infiltrating antigen-presenting cells by scRNAseq revealed the differential polarization of the myeloid compartment towards M1 and M2 phenotypes between the young and adult tumors, respectively. Consistent with our mouse findings, analysis of immune infiltrates from human pediatric solid tumors revealed a strong correlation between the expression of PDL1 on myeloid cells and enrichment of tumor-associated CD8 T cells with an exhaustion phenotype (PD1hiTim3+). Collectively, these data indicate that the “young” microenvironment of an actively developing tissue/individual contributes to the generation of an immune response skewed towards a terminally differentiated state with limited plasticity, thus narrowing the window for immunotherapeutic interventions.
Citation Format: Ardiana Moustaki, Jeremy Chase Crawford, Shanta Alli, Anthony Zamora, Yiping Fan, Shannon Boi, Natalie M. McDonald, Paul G. Thomas, Alberto S. Pappo, Michael A. Dyer, Elizabeth Stewart, Sara Federico, Ben Youngblood. Antigen-cross presentation promotes development of terminally differentiated CD8 T cells in young individuals [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-074.