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Cystatin C (CyC) is a secreted cysteine protease inhibitor and its biological functions remain insufficiently characterized. Plasma CyC is elevated in many patients, especially when receiving glucocorticoid (GC) treatment. Endogenous GCs are essential for life and are appropriately upregulated in response to systemic stress. Here we empirically connect GCs with systemic regulation of CyC. We used genome-wide association and structural equation modeling to determine the genetics of the latent trait CyC production in UK Biobank. CyC production and a polygenic score (PGS) capturing germline predisposition to CyC production predicted elevated all-cause and cancer-specific mortality. We then demonstrated that CyC is a direct target of GC receptor, with GC-responsive CyC secretion exhibited by macrophages and cancer cells. Using isogenic CyC-knockout tumors, we discovered a markedly attenuated tumor growth in vivo and found abrogated recruitment of Trem2+ macrophages, which have been previously linked to failure of cancer immunotherapy. Finally, we showed that the CyC-production PGS predicted checkpoint immunotherapy failure in a combined clinical trial cohort of 685 metastatic cancer patients. Taken together, our results demonstrate that CyC may be a direct effector of GC-induced immunosuppression, acting through recruitment of Trem2+ macrophages, and therefore could be a target for combination cancer immunotherapy.