The Mitochondrial Phosphatase PGAM5 Is Dispensable for Necroptosis but Promotes Inflammasome Activation in Macrophages.

The cytokine IL-1ß is intimately linked to many pathological inflammatory conditions. Mature IL-1ß secretion requires cleavage by the inflammasome. Recent evidence indicates that many cell death signal adaptors have regulatory roles in inflammasome activity. These include the apoptosis inducers FADD and caspase 8, and the necroptosis kinases receptor interacting protein kinase 1 (RIPK1) and RIPK3. PGAM5 is a mitochondrial phosphatase that has been reported to function downstream of RIPK3 to promote necroptosis and IL-1ß secretion. To interrogate the biological function of PGAM5, we generated Pgam5(-/-) mice. We found that Pgam5(-/-) mice were smaller compared with wild type littermates, and male Pgam5(-/-) mice were born at sub-Mendelian ratio. Despite these growth and survival defects, Pgam5(-/-) cells responded normally to multiple inducers of apoptosis and necroptosis. Rather, we found that PGAM5 is critical for IL-1ß secretion in response to NLRP3 and AIM2 inflammasome agonists. Moreover, vesicular stomatosis virus-induced IL-1ß secretion was impaired in Pgam5(-/-) bone marrow-derived macrophages, but not in Ripk3(-/-) bone marrow-derived dendritic cells, indicating that PGAM5 functions independent of RIPK3 to promote inflammasome activation. Mechanistically, PGAM5 promotes ASC polymerization, maintenance of mitochondrial integrity, and optimal reactive oxygen species production in response to inflammasome signals. Hence PGAM5 is a novel regulator of inflammasome and caspase 1 activity that functions independently of RIPK3.

Virus-induced natural killer cell lysis of T cell subsets.

In addition to direct anti-viral activity, NK cells regulate viral pathogenesis by virtue of their cytolytic attack on activated CD4 and CD8 T cells. To gain insight into which differentiated T cell subsets are preferred NK targets, transgenic T cells were differentiated in vitro into Th0, Th1, Th2, Th17, Treg, Tc1, and Tc2 effector cells and then tested for lysis by enriched populations of lymphocytic choriomeningitis virus (LCMV)-induced activated NK cells. There was a distinct hierarchy of cytotoxicity in vitro and in vivo, with Treg, Th17, and Th2 cells being more sensitive and Th0 and Th1 cells more resistant. Some distinctions between in vitro vs in vivo generated T cells were explainable by type 1 interferon induction of class 1 histocompatibility antigens on the effector T cell subsets. NK receptor (NKR)-deficient mice and anti-NKR antibody studies identified no one essential NKR for killing, though there could be redundancies.

Weak vaccinia virus-induced NK cell regulation of CD4 T cells is associated with reduced NK cell differentiation and cytolytic activity.

Natural killer (NK) cells control antiviral adaptive immune responses in mice during some virus infections, but the universality of this phenomenon remains unknown. Lymphocytic choriomeningitis virus (LCMV) infection of mice triggered potent cytotoxic activity of NK cells (NKLCMV) against activated CD4 T cells, tumor cells, and allogeneic lymphocytes. In contrast, NK cells activated by vaccinia virus (VACV) infection (NKVACV) exhibited weaker cytolytic activity against each of these target cells. Relative to NKLCMV cells, NKVACV cells exhibited a more immature (CD11b-CD27+) phenotype, and lower expression levels of the activation marker CD69, cytotoxic effector molecules (perforin, granzyme B), and the transcription factor IRF4. NKVACV cells expressed higher levels of the inhibitory molecule NKG2A than NKLCMV cells. Consistent with this apparent lethargy, NKVACV cells only weakly constrained VACV-specific CD4 T-cell responses. This suggests that NK cell regulation of adaptive immunity, while universal, may be limited with viruses that poorly activate NK cells.