PROJECT SUMMARY Treatment options for triple-negative breast cancer (TNBC) have been very limited. However, specific molecular features such as high PD-L1 expression, elevated tumor mutational burden, and increased tumor infiltrating lymphocytes in the tumor immune microenvironment (TIME) make TNBC an ideal candidate for immune checkpoint blockade (ICB)-based therapy. Poor clinical response of TNBC to the ICB monotherapy necessitates the identification of novel targets capable of enhancing the anti-tumor effects of immunotherapy. Effective cytotoxic T cell (CTL)-mediated immune responses against cancer cells rely on adequate tumor antigen presentation (AP). Insufficient tumor AP compromises the efficacy of ICB therapies, presenting a significant obstacle in solid tumor immunotherapy. Enhancing surface MHC-I expression emerges as a plausible strategy to potentiate ICB-based immunotherapy. Genetic alterations in the antigen processing and presentation (APP) machinery or dysregulations in the intrinsic endolysosomal network are the main causes of diminished tumor AP. While genetic alterations in the APP machinery have been extensively studied, factors regulating the endolysosomal network in tumor AP remain underexplored. Our laboratory utilized the “Inference of Cell Types and Deconvolution” algorithm and identified PIK3C2A as a promising druggable target in TNBC cells that regulates phosphoinositide metabolism. Depletion of PIK3C2A in TNBC cells demonstrated significant upregulation of tumor AP and enhanced cytotoxicity of specific CD8+ T cells. We hypothesize that inhibition of PIK3C2A alters PI(3,4)P2 metabolism, resulting in dysfunctional endocytic vesicle formation, preventing the internalization of the antigen:MHC-I complex, and thereby increasing the tumor AP. PIK3C2A is a dual-function protein with both kinase and non-kinase activities. Its kinase activity regulates the endocytosis pathway through PI(3,4)P2 generation around the neck of endocytic vesicles. Conversely, the non-kinase function of PIK3C2A prevents chromosomal instability. Due to this dual functionality, the exact molecular mechanism contributing to enhanced tumor AP remains unclear. The overall goal of this project is to characterize the function of PIK3C2A in TNBC cells and its impact on the TNBC TIME. Aim 1 will elucidate the mechanism underpinning enhanced tumor AP upon PIK3C2A depletion in TNBC cells through transcriptomic profiling, PI(3,4)P2 quantification, and examining the Ova/MHC-I turnover rate in vitro. In Aim 2, we will use a PIK3C2A isoform-selective inhibitor (PITCOIN1) and anti-PD-1 combination to treat orthotopic TNBC tumors in syngeneic mouse model as a proof- of-concept study to determine whether this combination therapy inflames the TIME and promotes potent tumor regression in vivo. This project aligns with my training as a future physician-scientist, whose research goal is to focus on the discovery and development of solid tumor immunotherapy.