A Natural Genetic Variant of Granzyme B Confers Lethality to a Common Viral Infection

Many immune response genes are highly polymorphic, consistent with the selective pressure imposed by pathogens over evolutionary time, and the need to balance infection control with the risk of auto-immunity. Epidemiological and genomic studies have identified many genetic variants that confer susceptibility or resistance to pathogenic micro-organisms. While extensive polymorphism has been reported for the granzyme B (GzmB) gene, its relevance to pathogen immunity is unexplored. Here, we describe the biochemical and cytotoxic functions of a common allele of GzmB (GzmB(W)) common in wild mouse. While retaining ‘Asp-ase’ activity, GzmB(W) has substrate preferences that differ considerably from GzmB(P), which is common to all inbred strains. In vitro, GzmB(W) preferentially cleaves recombinant Bid, whereas GzmB(P) activates pro-caspases directly. Recombinant GzmB(W) and GzmB(P) induced equivalent apoptosis of uninfected targets cells when delivered with perforin in vitro. Nonetheless, mice homozygous for GzmB(W) were unable to control murine cytomegalovirus (MCMV) infection, and succumbed as a result of excessive liver damage. Although similar numbers of anti-viral CD8 T cells were generated in both mouse strains, GzmB(W)-expressing CD8 T cells isolated from infected mice were unable to kill MCMV-infected targets in vitro. Our results suggest that known virally-encoded inhibitors of the intrinsic (mitochondrial) apoptotic pathway account for the increased susceptibility of GzmB(W) mice to MCMV. We conclude that different natural variants of GzmB have a profound impact on the immune response to a common and authentic viral pathogen.