Targeting nicotinamide N-methyltransferase-mediated immunosuppression to enhance immune checkpoint blockade in lung cancer: role of myeloid-derived suppressor cells

Immunotherapy has revolutionized the treatment of lung cancer. However, the clinical efficacy of immune checkpoint inhibitors (ICIs) in non-small cell lung cancer (NSCLC) remains limited (< 25%). The lack of response to ICIs has been partially attributed to the immunosuppressive nature of the tumor microenvironment (TME). The TME comprises a heterogeneous population of cells, including myeloid-derived suppressor cells (MDSCs), which play a central role in mediating intratumoral immunosuppression and promoting cancer progression through different mechanisms. Recent studies have shown that MDSCs-derived metabolites, such as itaconate and methylglyoxal, act as potent immunoregulators of T cell activity. This thesis aimed to investigate whether additional MDSCs-dependent metabolic events contribute to immunosuppression and to identify non-invasive biomarkers associated with ICIs response in NSCLC. To achieve this, an optimized workflow combining RosetteSep pre-enrichment, density gradient centrifugation and magnetic cell separation (Rosette-MACS) was developed to enable the robust and functionally reliable isolation of human MDSCs. Transcriptomic profiling by RNA sequencing of paired circulating and tumor-infiltrating MDSCs (tMDSCs) revealed nicotinamide N-methyltransferase (NNMT) as the most significantly upregulated enzyme-encoding gene in tMDSCs. NNMT catalyzes the methylation of nicotinamide (NAM) using S-adenosylmethionine (SAM) as a methyl donor to generate 1-methylnicotinamide (1-MNA), thereby linking NAM metabolism to methionine cycling, polyamine biosynthesis and tryptophan catabolism. NNMT overexpression has been linked to more aggressive tumor behaviour, such as increased metastasis and poor prognosis while 1-MNA has been described as an immunoregulatory metabolite which promotes T cell dysfunction in ovarian cancer. Notably, MDSCs were found to secrete extracellular 1-MNA at levels comparable to those observed in cancer cells and cancer-associated fibroblasts (CAFs), suggesting a previously unrecognized immunoregulatory role for NNMT in MDSCs-mediated suppression. Pharmacological inhibition of NNMT in tMDSCs and CD8⁺ T cell co-cultures partially rescued T cell function. The functional relevance of NNMT in tMDSCs was further evaluated using a NSCLC mouse model, in which NNMT inhibition reduced tumor growth, decreased MDSCs infiltration and enhanced anti-PD-1 efficacy. Finally, reduced circulating MDSCs levels and a lower serum 1-MNA/NAM ratio correlated with improved prognosis and response in NSCLC patients treated with chemotherapy combined with ICIs, supporting their potential as non-invasive predictive biomarkers. Overall, these findings demonstrate that NNMT overexpression in tMDSCs contributes to the intratumoral pool of 1-MNA, establishing a novel immunoregulatory mechanism mediated by MDSCs and highlighting NNMT as a potential metabolic target to alleviate TME immunosuppression and enhance the therapeutic efficacy of ICIs in NSCLC.