Discovery and characterization of the mechanisms of a vaccinia viral Bcl-2 homolog, F1L, on Inhibition of caspase-9 and NLRP1

The innate immune system detects invading pathogens and provides immediate response to eliminate pathogens and infected cells, and to trigger secondary immune response to inhibit infection. Programmed cell death (Apoptosis) and cytokine secretion, which are mediated by activation of different caspase-family protease cascades, are examples of the numerous mechanisms of our innate immunity against viral infections. Nonetheless, the virus has evolved a variety of strategies to cope with the anti- viral responses of the host. In this dissertation, I discovered that the vaccinia virus (VACV)-encoded Bcl-2 homologue, F1L, directly inhibits caspase-9, the apical caspase in the mitochondrial cell death pathway, and NLRP1 (also known as NALP1), one of the inflammasome proteins that facilitates caspase-1 activation and cytokine secretion. We provide multiple lines of evidence that F1L directly and selectively binds and suppresses caspase-9 and NLRP1. In cells, F1L specifically represses caspase-9- dependent apoptosis and NLRP1-dependent IL-1[Beta] secretion. We identified two conserved motifs at the N- terminus of F1L preceding the Bcl-2-like domain by mutagenesis studies that are responsible for interaction and inhibition of caspase-9 and NLRP1, respectively. I further show that two short peptides derived from these motifs are sufficient to inhibit caspase-9 and NLRP1, respectively. Moreover, F1L is critical for suppression of caspase-1 activation and IL-1[Beta] secretion during VACV infection in macrophage cultures. In mice, I show that F1L is a prominent virulence factor that inhibits cytokine secretion upon VACV infection. In summary, we revealed two novel functions of F1L that are important for the virulence of VACV. F1L is the first caspase-9 inhibitor that is a Bcl-2-like protein, and the first example of viral protein inhibitor of the NLRP family protei