Abstract 4859: TMEM33 loss improves CD8+ T cell fitness and tumor control

Abstract The Endoplasmic Reticulum (ER) is a critical immunomodulatory hub governing multiple processes relating to anti-tumor immunity. TMEM33 is an ER-resident membrane protein implicated in various ER-associated functions including cholesterol metabolism, calcium oscillations, and proteostasis. We previously reported TMEM33 as a negative modulator of the innate immune signaling protein STING (STimulator of INterferon Genes). Here we identify a central role of TMEM33 in tumor immunology where its deficiency profoundly impacts infiltrating immune populations and T cell phenotype. STING-mediated ISG (interferon-stimulated gene) expression was augmented in Tmem33-/- bone marrow-derived macrophages and TMEM33-depleted Flp-In T-REx 293 cells. Strikingly, Tmem33-/- mice exhibited significantly attenuated B16F10-OVA tumor growth and enhanced CD8+ T cell tumor infiltration compared to wild-type (WT) controls, while immunosuppressive immune populations including Treg cells, monocytic myeloid-derived suppressor cells and M2-like macrophages were diminished. CD8+ T cell enrichment was also observed in MC38 tumor-bearing Tmem33-/- hosts. Unchallenged mice displayed no overt phenotypic or immunological differences compared to WT, however. High dimensional spectral flow cytometry was leveraged to further examine effects of Tmem33-/- on T cell compartments in B16F10-OVA tumors and draining lymph nodes (DLNs). Progenitor exhausted (Tpex) TCF-1+PD-1+ tumor infiltrating lymphocytes (TILs) were significantly enriched in Tmem33-/- hosts, notable given that Tpex are responsive to checkpoint blockade therapy and associate with improved patient prognosis. Compared to WT counterparts, OVA-specific Tmem33-/- TILs exhibited increased cytotoxicity and effector function, expressing elevated granzyme-B and TNF-α. Moreover, EOMESloT-bethi TILs were more frequent among TCF-1+PD-1+ and TCF-1+ populations in Tmem33-/- hosts further suggestive of a more persistent CD8+ precursor pool, while inhibitory receptor (LAG-3, TIGIT) abundance was blunted, indicating reduced exhaustion. In DLNs of Tmem33-/- mice, antigen-specific CD8+ populations demonstrated enhanced effector memory (CD44+CD62L-) phenotypes alongside elevated CXCR3 expression compared to WT, suggesting increased tumor-migratory potential. In summary, here we identify TMEM33 as a novel immunoregulatory factor, the loss of which in hosts majorly reshapes the tumor-immune landscape and ameliorates multiple aspects of CD8+ T cell fitness including stemness, exhaustion, memory, cytotoxicity and migratory capacity. Further analysis of T cell populations from tumor-bearing mice and ex vivo analysis will enable mechanistic refinement of the role of TMEM33. Moreover, our study supports the importance of ER-resident proteins in fine-tuning immune cell function, underlining their potential as promising targets for therapeutic intervention. Citation Format: Matthew T. Jackson, Tianming Zhao, Isabela Pedroza-Pacheco, Amit Grover, Dmitry Gabrilovich, David Withers, Jan Rehwinkel, Eric Honore, John C. Christianson, Eileen E. Parkes. TMEM33 loss improves CD8+ T cell fitness and tumor control [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4859.