The Role of Granzyme B in Tumor Migration and Metastasis

Abstract

The granule exocytosis pathway has classically been described as our most potent response to pathogens, cancers, as well as autoreactive disorders. This is executed by the action of perforin and a family of granule-associated serine proteases known as granzymes (Gzm). Many studies have shown that these proteases exert their effects by killing affected cells, creating an obvious correlation with cytotoxic T cells (CTLs) and natural killer cells (NK) greatly expressing GzmB. Studies have found that the tumor cells ability to create a suppressive environment is dependent on Gzm expression by regulatory T lymphocytes (Treg). Research has shown GzmB can target and cleave extracellular molecules such as fibronectin, vitronectin and laminin. Bird et al published data showing that GzmB aids in the transmigration of CTLs, however the ability of GzmB to aid in the migration of non-T cells has not been investigated. We hypothesize that GzmB secretion may cause extracellular matrix (ECM) remodeling in the tumor microenvironment, aiding in the outgrowth of the tumor cells via promoting higher rates of invasion and/or metastasis.

First, we designed experiments to test in-vivo and in-vitro conditions that generate GzmB production in multiple cell types. We were able to induce GzmB production by Tregs both in-vitro and in-vivo. We also identified multiple mechanisms that allow for CD8 T cells to produce GzmB. We next investigated mechanisms that would allow for GzmB to be found in the extracellular environment. We found that CD8 T cells have the ability to secrete GzmB without the need of an immunological synapse. In fact, GzmB secretion is not dependent on exogenous IL-2.

GzmB improves the migration and invasive potential of tumor cells. In Treg-specific GzmBko (Foxp3creGzmBfl/fl) mice, GzmB+ Tregs increase the rate of tumor metastasis in the lungs of the mice. In addition, GzmB+ CD8 T cells can potentiate tumor metastasis, in contrast to GzmB-deficient CD8 T cells. These findings shed light on the protumorigenic potential of secretory GzmB, which could serve as a unique biomarker for predicting metastasis.