Abstract 3215: Cystatin F changes in melanoma patients during therapy with immune checkpoint inhibitors

Abstract Although immune checkpoint inhibitors (ICI) have achieved undeniable success in the treatment of metastatic melanoma, primary and secondary resistance to ICI therapy continues to pose a significant challenge for melanoma patients. Around 40-60% of patients respond to immunotherapy, achieving long-term benefits, but over two-thirds experience immune-related adverse events (irAEs) with ICIs. About 10% show pseudoprogression, marked by transient inflammation, while up to 30% experience hyperprogression with rapid cancer advancement. Despite irAEs, treatment interruption often leads to better outcomes and prolonged survival. The lack of clinically validated predictive biomarkers remains a major factor contributing to the unpredictable effects of immunotherapy. Implementing biological markers in clinical practice is essential to enhance personalized treatment and predict responses. We have identified a protease inhibitor cystatin F as a potent down regulator of cytotoxic efficacy of cytotoxic T lymphocytes (CTLs). Its expression was shown to be increased in hypo responsive and exhausted CTLs but also in the cancer tissue in several types of cancer. In order to elucidate its role in CTLs of patients with melanoma and correlation its expression with response to treatment we have examined cystatin F expression, along with the CTL PD-1 expression and IFNγ in peripheral blood CTLs form patients with advanced melanoma (TNM classification, stage IIID and IV) receiving first-line therapy with ICIs. Study-specific data were collected at three different time points: up to 4 weeks before the start of treatment, at the 12th (+/−2 weeks) and 28th week (+/−2 weeks) and the expression of markers on whole blood preparations or isolated CTLs was analysed using flow cytometry. We characterized the cellular sources of cystatin F in tumor tissue using multicolor immunofluorescence staining and confocal analysis on formalin-fixed melanoma tissue samples. Our findings reveal cystatin F expression in predominantly in non-lymphoid cells within the tumor microenvironment. Analysis of cystatin F in peripheral blood CTLs shows higher expression in non-responders and patients with irAEs, suggesting its potential as a marker for enhancing personalized treatment and predicting treatment responses. Citation Format: Milica M. Perisic Nanut, Emanuela Senjor, Tanja Mesti, Simona Miceska, Janja Ocvirk, Janko Kos. Cystatin F changes in melanoma patients during therapy with immune checkpoint inhibitors [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 3215.