Type 2 Cystatins and Their Roles in the Regulation of Human Immune Response and Cancer Progression

SIMPLE SUMMARY: Type 2 cystatins are a group of small secreted protease inhibitors that regulate cysteine protease cathepsins and legumain. These enzymes regulate important cellular processes that are linked to the immune response and tumor progression, playing important roles in both autoimmune diseases and various types of cancers. This review aims to explore the roles of type 2 cystatins in immune regulation and cancer development, shedding light on their significance in maintaining health. ABSTRACT: Cystatins are a family of intracellular and extracellular protease inhibitors that inhibit cysteine cathepsins—a group of lysosomal cysteine proteases that participate in multiple biological processes, including protein degradation and post-translational cleavage. Cysteine cathepsins are associated with the development of autoimmune diseases, tumor progression, and metastasis. Cystatins are categorized into three subfamilies: type 1, type 2, and type 3. The type 2 cystatin subfamily is the largest, containing 10 members, and consists entirely of small secreted proteins. Although type 2 cystatins have many shared biological roles, each member differs in structure, post-translational modifications (e.g., glycosylation), and expression in different cell types. These distinctions allow the type 2 cystatins to have unique biological functions and properties. This review provides an overview of type 2 cystatins, including their biological similarities and differences, their regulatory effect on human immune responses, and their roles in tumor progression, immune evasion, and metastasis.