Immunometabolic network interactions of the kynurenine pathway in cutaneous malignant melanoma

Abstract

Cancer is the second leading cause of mortality worldwide, and melanoma represents the most aggressive and deadliest form of skin cancer. Despite the various therapeutic approaches, metastatic melanoma is a disease with a poor prognosis. Recently, the evolution of immune checkpoint inhibitors (ICIs) led to a substantial improvement of the overall survival of patients. However, the long-term effectiveness of such treatments is restricted by the sometimes rapidly emerging resistance to treatment. Several molecular mechanisms underlying this resistant phenotype have begun to be elucidated. The Kynurenine pathway activity via indoleamine 2, 3-dioxygenase 1 (IDO1), is one such mediator of immunosuppression and resistance to ICIs.

Studies included in this thesis, therefore, aim to clarify the role of the kynurenine pathway (KP) in metastatic cutaneous melanoma. To this end, we established an in vitro co-culture model consisting of CD4 +T cells in culture with different melanoma cell lines (MCLs) to investigate the implication of KP modifications on CD4 + T-cell function. We found that in addition to IDO1, other KP enzymatic activities such as KMO may regulate CD4 + T-cell immunity (Study I). Following this finding, we evaluated the immune-metabolic network interactions of KP in CMM patients to explore the link between KP metabolites (KPMs) and regulation of the anti-tumour immune response. Our data showed a significant association between MAPKIs treatments and alterations of 3-HK and 3HAA concentrations. These results suggest that KP is clinically relevant in CMM patients (Study II). We further aimed to identify possible KP-related predictive biomarkers of response to ICIs treatment (Study III, IV). Our findings demonstrate the elevated S100A9+ monocytes among PBMCs of the CMM patients who are not responding to the PD-1 inhibition (Study III). Subsequently, by using the PBMCs and plasma of CMM patients on ICI therapy, we observed that kynurinase (KYNU) and LGALS3 (Galectin-3) expression in protein and RNA levels are negatively linked to clinical outcomes. Moreover, we found that the KYNU-LGALS3 network in monocytes is connected to the CD74-MYC network in CD4+ T-cells. These results suggest that LGALS3, MYC, CD74, and KYNU are biologically connected, and perturbing their interaction will possibly modulate ICI efficacy in CMM patients (Study IV).

In summary, this thesis provides insights into the induction of n immune-suppressive phenotype by KP activation in CD4+ T-cells and demonstrates the therapeutic potential of targeting KP in the treatment of malignant melanoma.