Functional Analysis of Granzyme M and Its Role in Immunity to Infection

Abstract

Cytotoxic lymphocytes express a large family of granule serine proteases, including one member, granzyme (Grz)M, with a unique protease activity, restricted expression, and distinct gene locus. Although a number of Grzs, including GrzM, have been shown to mediate target cell apoptosis in the presence of perforin, the biological activity of Grz has been restricted to control of a number of viral pathogens, including two natural mouse pathogens, ectromelia, and murine CMV (MCMV). In this article, we describe the first reported gene targeting of GrzM in mice. GrzM-deficient mice display normal NK cell/T cell development and homeostasis and intact NK cell-mediated cytotoxicity of tumor targets as measured by membrane damage and DNA fragmentation. GrzM-deficient mice demonstrated increased susceptibility to MCMV infection typified by the presence of more viral inclusions and transiently higher viral burden in the visceral organs of GrzM-deficient mice compared with wild-type (WT) mice. The cytotoxicity of NK cells from MCMV-infected GrzM-deficient mice remained unchanged and, like WT control mice, GrzM-deficient mice eventually effectively cleared MCMV infection from the visceral organs. In contrast, GrzM-deficient mice were as resistant as WT control mice to mouse pox ectromelia infection, as well as challenge with a number of NK cell-sensitive tumors. These data confirm a role for GrzM in the host response to MCMV infection, but suggest that GrzM is not critical for NK cell-mediated cytotoxicity.

Granzyme M targets host cell hnRNP K that is essential for human cytomegalovirus replication

“…GrM knockout mice are more susceptible to MCMV infections, indicating a role of GrM in CMV clearance at least in mice. 18 If GrM has a major role in controlling HCMV infections in humans, one would expect that GrM is present in HCMVspecific cytotoxic lymphocytes. Therefore, we analyzed GrM expression in CD8 þ T cells of healthy human HCMV-seropositive individuals as well as in seronegative renal transplant recipients receiving a kidney from an HCMV-seropositive donor, followed by primary HCMV infections.…”

“…The physiological importance of GrM as an antiviral mediator has been demonstrated in GrM-deficient mice, which are more susceptible to MCMV infections. 18 However, both murine GrM and MCMV have been largely diverged from their human counterparts, 1,21 emphasizing that caution is essential when extrapolating data from mice to humans. In this study, we demonstrated for the first time that human GrM is expressed in HCMV-specific CD8 þ T cells after primary infection and during latency (Figure 1).…”

“…[12][13][14][15][16][17] In mice, GrM has been shown to be important for murine cytomegalovirus (MCMV) clearance. 18 We have recently discovered that human GrM can efficiently inhibit HCMV replication in vitro in the absence of host cell death. 17 GrM efficiently cleaves HCMV phosphoprotein 71 (pp71) and completely abolishes its function to transactivate the MIEP, 17 which is indispensable for effective HCMV replication.…”

Mechanisms of granule-dependent killing

“…Indeed, granzyme M-deficient mice have recently been generated, and although NK cells from these animals showed no cytotoxic defect, a delay in the clearance of MCMV suggests that granzyme M may play a role in the eradication of some viral infections. 154 With regard to the tumorigenic function of CTL/NK cells, granzyme deficiency has produced conflicting results and, to date, an increased susceptibility of granzyme B-deficient mice to lymphoma has not been reported. In addition, granzyme A/ B-deficient animals were shown to retain potent in vivo antitumor activity, suggesting that these granzymes are not required for tumor clearance.…”

A quarter century of granzymes

“…The only alteration in cell-mediated immunity was a partially impaired response to MCMV. 85 GrM functions outside of direct killing have also been proposed, as GrM proteolytically inactives a GrB endogenous inhibitor, PI9. 86 The investigators proposed a model wherein GrM could assist GrB-induced killing by relieving PI9 inhibition of GrB.…”