More recently, Rosen and colleagues demonstrated that the majority of autoantigens targeted in human systemic autoimmune diseases are efficiently cleaved by granzyme B in vitro and during CTL-induced cell death, generating unique fragments not observed during apoptosis [145, 146]. Interestingly, GrB cleaved several autoantigens previously reported as not susceptible to cleavage during cell death, such as Ku-70, Jo-1, CENP-B, and PM-Scl, but failed to cleave other protease-resistant autoantigens, including SSA/Ro, Ku-80, ribosomal P proteins, histones, and the Sm proteins [145, 146]. In vivo killing of target cells by CTLs generated low amounts of the unique autoantigen fragments produced by GrB in vitro but favored the production of fragments corresponding to those generated by caspases during apoptosis, indicating that caspase-mediated proteolysis is the predominant pathway used during GrB-mediated apoptosis. However, the production of GrB-specific fragments was enhanced in the presence of the cas-pase-specific inhibitor AC-DEVD-CHO, suggesting that GrB may facilitate cell death independent of caspase-activation by directly cleaving intracellular substrates. This implies that under conditions where caspase activation is blocked by either viral proteins or endogenous inhibitors, GrB may generate modified forms of autoantigens that might be immunostimulatory in a proinflammatory context.
In a recent study, Rosen's group demonstrated that the nucleolar autoantigen B23 was efficiently cleaved by GrB in vitro but was highly resistant to cleavage by GrB during CTL-induced cell death of many different types, with the exception of differentiated vascular smooth muscle cells, suggesting that the cleavage of this autoantigen is dependent upon cell type . Given that B23 is associated with pulmonary vascular phenotype in scleroderma, it was concluded that GrB-mediated proteolytic modification of autoantigens may occur selectively in the target tissue and may play a role in shaping the phenotype-specific autoimmune response. According to this hypothesis, the immunizing microenvironment might play a central role in determining autoantibody responses in human systemic autoimmunity. To further test this hypothesis, it would be important to determine whether other autoantigens targeted in vitro by GrB (e.g., topo I, fibrillarin, CENP-B, PARP, and NuMA) are susceptible to GrB-mediated cleavage specifically in the tissues that are most affected in the autoimmune disease associated with these autoantigens. More importantly, it would be necessary to use mouse models deficient in GrB in order to determine whether the presence ofGrB is required for the generation of autoantibodies to intracellular autoantigens.
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