Understanding the role of TMEM33 in modulating host STING and the tumour immune microenvironment

The Endoplasmic Reticulum (ER) is a central immunoregulatory hub which orchestrates multiple processes underpinning tumour immunosurveillance, including antigen presentation, calcium flux, and innate immune signalling. These functions are fine-tuned by numerous membrane-embedded E3 ubiquitin ligases and their cofactors. STING (STimulator of INterferon Genes) is an ER-localised immune adaptor protein which in host antigen presenting cells (APCs) drives type I interferon-dependent anti-tumour immunity. Understanding its endogenous regulation is paramount for enhancing the clinical efficacy of STING agonists. This project examined the roles of ER-resident cofactor TMEM33, which associates with RING-domain E3s RNF26 and RNF5, in modulating STING in APCs, and its broader impact on the cancer immunity cycle in the host. Functional and transcriptomic analyses revealed nuanced TMEM33 activity in STING modulation, with varied effects on STING responses depending on cell models, methods of TMEM33 perturbation, and agonist treatment. A potential role for TMEM33 in governing mitochondrial homeostasis was also unveiled in macrophages. Strikingly, Tmem33-/- mice showed significantly improved protection to B16F10-OVA tumour growth. This correlated with increased CD8+ T cell infiltration, and reduced M2-like tumour­-associated macrophage polarisation, consistent with attenuated IL-4-induced M2-­like skewing observed in Tmem33-/- bone marrow-derived macrophages. Importantly, restricted tumour growth persisted in Tmem33-/-Sting1-/- mice, suggesting that the protective effect is STING-independent. CD8+ T cell enrichment also occurred in MC38 tumour bearing Tmem33-/- hosts, but did not correlate with improved tumour control. CD8+ T cell-focused spectral flow cytometry in B16F10-OVA tumours and draining lymph nodes (DLNs) indicated increased tumour enrichment of antigen-specific progenitor exhausted TCF-1+PD-1+ populations in Tmem33-/- hosts, accompanied by augmented cytotoxic and effector profiles, as well as reduced inhibitory receptor expression. In DLNs, OVA-targeting cells exhibited enhanced effector memory phenotypes and elevated CXCR3, suggesting improved tumour-migratory potential. Conventional dendritic cell (cDC1) abundance remained largely unchanged, however. Together, this work has identified novel, putative roles for TMEM33 in shaping tumour-immune immune dynamics and T cell fitness beyond its initial link to STING modulation.