Abstract 4142: CD4 T cell-driven response to immunotherapy against mouse melanoma tumors
Authors: Erbe Amy K. , Feils Arika S. , Hampton Alina , Spiegelman Dan , Tsarovsky Noah , Hoefges Anna , Carlson Peter M. , Pieper Alex , Haertle Callie , Heck Mackenzie , VandenHeuvel Sabrina , Frankel Lizzie , Zebertavage Lauren , Heaton Alexa , Morris Zachary S. , Patel Ravi , Rakhmilevich Alexander , Sondel Paul M. 
Published: April 2023

Abstract
Using an in situ vaccine (ISV) regimen that includes a combination therapy of radiation (given at D0) with hu14.18-IL2 immunocytokine [anti-GD2 linked to IL2, (Anyxis Immuno-Oncology GmbH (Austria)); given at D5-D9], we can cure mice of large B78 melanoma tumors (B78s). GD2 is expressed on many solid tumors, including most melanomas, and is not often expressed on immune cells. Mice cured with ISV of B78 tumors demonstrate long-term immune memory, with mice rejecting tumor rechallenge &gt;180 days post initial cure of tumor. Traditionally, immune effector memory is thought to be mediated via CD8 T cells, which require antigen presentation via MHC Class I (MHCI). However, B78s express little to no MHCI, but do express MHCII when stimulated with IFNγ. Expressed on 50-70% of melanomas in humans, the role of MHCII on response is unclear. We explored implications of MHCII and MHCI expression on response (initial response to ISV and memory response).
Mice bearing B78s depleted of NK cells or CD8 T cells during ISV respond to therapy, but mice depleted of CD4 T cells fail to respond to ISV. Likewise, B78-cured mice depleted of NK and CD8 T cells during rechallenge are able to reject the B78 tumor rechallenge, whereas those depleted of CD4 T cells failed to reject their B78 rechallenge. Together these data suggest that CD4 T cells are required for the initial antitumor response to ISV as well as for immune memory formation and function.
Tumors and tumor draining lymph nodes were harvested during ISV treatment in B78-bearing mice at D8 and assessed by scRNAseq, flow cytometry, IncuCyte and ImageStream (live-cell imaging assays), and IsoPlexis (cytokine profile). Though not required for initial or memory response, both CD8 and NK cells are activated and exhibit cytotoxic phenotypes in the tumor microenvironment (TME). Furthermore, a population of cells co-expressing both GD2 and CD45 was observed in the TME following ISV that was not observed in untreated mice. These GD2+/CD45+ cells were predominantly CD4+ T cells incorporate patchy GD2 expression on their surface via trogocytosis. We hypothesize that the interaction between MHCII on tumor cells and CD4 T cell receptors mediates the trogocytosis, activating CD4 T cells to function as helper T cells, initiating a cascade of antitumor immunity, as well as causing direct CD4 T cell-mediated cytotoxicity.
MHCII is expressed on some melanoma tumors, and its expression has been correlated with a positive response to immunotherapies. MHCII expression on tumors can directly engage CD4 cytotoxic T cells, suggesting an important role in the response to immunotherapy for CD4 T cells in melanoma tumors that express MHCII. Understanding the cellular and molecular mechanisms involved in the ISV-induced immune recognition and destruction of B78 may guide future improvements of this clinically-relevant immunotherapy regimen.
Citation Format: Amy K. Erbe, Arika S. Feils, Alina Hampton, Dan Spiegelman, Noah Tsarovsky, Anna Hoefges, Peter M. Carlson, Alex Pieper, Callie Haertle, Mackenzie Heck, Sabrina VandenHeuvel, Lizzie Frankel, Lauren Zebertavage, Alexa Heaton, Zachary S. Morris, Ravi Patel, Alexander Rakhmilevich, Paul M. Sondel. CD4 T cell-driven response to immunotherapy against mouse melanoma tumors. [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 4142.