CST7 (cystatin F) is a cysteine protease inhibitor with emerging roles in immune regulation and disease pathology. As a peptidase inhibitor, CST7 exhibits lower affinity for papain and cathepsin L compared to other cystatins [UniProt annotation]. Primary functions include regulating antigen processing and presentation, with subcellular localization to endolysosomal compartments including lysosomes, endosomes, and the Golgi apparatus. CST7 expression marks distinct immune cell populations with functional significance. It identifies exTreg cells—inflammatory, cytotoxic CD4+ T cells arising from regulatory T cells in atherosclerosis 1. In neutrophils, CST7 expression is regulated by transcription factor SPI1 and marks subpopulations critical for neutrophil extracellular trap formation during myocardial ischemia/reperfusion injury 2. CST7+ neutrophils coordinate with macrophages during SARS-CoV-2 infection for viral clearance 3. CST7 also marks activated CD8+ MAIT cells with enhanced cytotoxicity in NSCLC patients responding to anti-PD-1 immunotherapy 4. Clinically, elevated CST7 expression is associated with improved outcomes in cervical cancer and Alzheimer's disease contexts. In cervical cancer, CST7 upregulation reduces tumor cell proliferation and invasion 5. In Alzheimer's disease, higher CST7 levels in disease-associated microglia correlate with slower tau accumulation and cognitive decline 6. However, CST7 plays sexually dimorphic roles in microglial function, with complex interactions between endolysosomal and inflammatory pathways 7. In chr20 hepatitis B, CST7 upregulation drives Th2 cell differentiation, promoting disease chr20 8.