Abstract 580: APP as a novel immune checkpoint: Expression in T cells and its clinical implications for immuno-oncology drug refractory
Authors: Cha Mi Young , Jeon Bu-Nam , Kim Gihyeon , Jeong Areum , Park Sang-Moo , Park Kyung Mi , Park Hansoo 
Published: March 2024

Abstract
Amyloid precursor protein (APP) is a type I transmembrane protein expressed in various cell types and tissues. We previously confirmed that APP expressed in T cells is a binding partner protein of a novel immune checkpoint contactin 4 (CNTN4), and APP is a central molecule for conveying the immunosuppressive signal of CNTN4 to T cells. We further profiled an expression of APP in tumor-infiltrating T cells from tumors of NSCLC patients who received anti-PD-1 as a neoadjuvant treatment combined with chemotherapy using single-cell RNA analysis. As a result, the proportion of APP-expressing CD4+ Teff or CD8+ Teff cells was higher in stable disease (SD) patients than in partial response (PR) patients. In contrast, we observed higher PDCD1-expressing CD4+ Tnaïve and CD4+ Teff cell proportions in PR patients than in SD patients. When we performed a Differential Expression Gene analysis between the PR and SD patients within Teff cells, we observed that the PR patients’ CD8+ Teff cells manifested a higher IFNG and PDCD1, while the SD patients’ CD8+ Teff cells predominantly exhibited elevated APP levels. Subsequent pathway analysis also suggested a negative relationship between immune-related pathways and APP expression in CD8+ Teff cells. Such findings imply that the predominant presence of APP in T cells is associated with I/O refractory. From a biological view, we demonstrated increased APP expression in T cells by stimulation with an anti-CD3 antibody or antigen-presenting cell. We also identified that upon stimulation with an anti-CD3 antibody, transcription factors for T-cell activation bind to the proximal promoter of APP and increase its expression. Furthermore, it was confirmed that APP in T cells not only increases its expression upon anti-CD3 antibody stimulation but also translocates from the cytoplasm to the membrane through the Golgi apparatus. When anti-CD3 antibody stimulation of T cells persists, the membrane expression of APP increases for 14 to 24 h, then gradually decreases when the stimulation disappears and becomes the same as before stimulation at 48 h. T cells differentiate into various subsets and play an essential role in adaptive immunological responses to foreign substances. In particular, CD4+ (helper) T cells differentiate into different T-cell subsets, such as Th1, Th2, Th17, Th19, Th22, Tfh, and Treg, to govern the overall immune response by producing cytokines that can recruit other cells of the immune system. So, we investigated whether APP is expressed on T cell subsets including naïve, effector, memory, and exhausted T-cells. We identified that APP is expressed in various CD4+ and CD8+ T cell subsets upon stimulation with anti-CD3 antibody. Taken together, our results suggest that APP is expressed on multiple functional T cells and that increased expression of APP on T cells in TIL may increase resistance to immunotherapy.
Citation Format: Mi Young Cha, Bu-Nam Jeon, Gihyeon Kim, Areum Jeong, Sang-Moo Park, Kyung Mi Park, Hansoo Park. APP as a novel immune checkpoint: Expression in T cells and its clinical implications for immuno-oncology drug refractory [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 580.