Species-specific differences in regulation of macrophage inflammation by the C3a-C3a receptor axis.

Complement is an important arm of the innate immune system. Recent studies have shown that products of complement pathway activation can interact directly with other innate immune signaling molecules, including TLRs and inflammasome family members, during some infectious and chronic inflammatory disorders. Activation of the complement system generates anaphylatoxins, such as C3a and C5a, which modulate inflammation. However, the biological effects of interactions between the anaphylatoxins with their receptors may vary across species. In this study, we demonstrate that human complement and rat complement differ in the way they modulate the inflammatory response to the human pathogen, Neisseria gonorrhoeae, as well as purified pathogen-associated ligands, such as LPS. While rat serum down-regulates MyD88-dependent pro-inflammatory cytokine responses in macrophages, human serum has no effect, or in some cases an enhancing effect. Further, the inhibitory effect of rat serum on otherwise pro-inflammatory stimuli is mediated by complement, specifically C3a-C3a receptor interactions, via an undefined signaling mechanism that down-regulates the transcription factor, NF-κB and NLRP3 inflammasome-mediated caspase-1 activation. This study highlights important functional differences between rodent and human complement that could explain some of the differences in immune responses between these two species. Understanding the crosstalk between complement and other arms of the innate immune system will facilitate the development of better anti-inflammatory therapeutics.

Mycobacterium tuberculosis induces the miR-33 locus to reprogram autophagy and host lipid metabolism.

Mycobacterium tuberculosis (Mtb) survives in macrophages by evading delivery to the lysosome and promoting the accumulation of lipid bodies, which serve as a bacterial source of nutrients. We found that by inducing the microRNA (miRNA) miR-33 and its passenger strand miR-33*, Mtb inhibited integrated pathways involved in autophagy, lysosomal function and fatty acid oxidation to support bacterial replication. Silencing of miR-33 and miR-33* by genetic or pharmacological means promoted autophagy flux through derepression of key autophagy effectors (such as ATG5, ATG12, LC3B and LAMP1) and AMPK-dependent activation of the transcription factors FOXO3 and TFEB, which enhanced lipid catabolism and Mtb xenophagy. These data define a mammalian miRNA circuit used by Mtb to coordinately inhibit autophagy and reprogram host lipid metabolism to enable intracellular survival and persistence in the host.

Netrin-1 promotes adipose tissue macrophage retention and insulin resistance in obesity.

During obesity, macrophage accumulation in adipose tissue propagates the chronic inflammation and insulin resistance associated with type 2 diabetes. The factors, however, that regulate the accrual of macrophages in adipose tissue are not well understood. Here we show that the neuroimmune guidance cue netrin-1 is highly expressed in obese but not lean adipose tissue of humans and mice, where it directs the retention of macrophages. Netrin-1, whose expression is induced in macrophages by the saturated fatty acid palmitate, acts via its receptor Unc5b to block their migration. In a mouse model of diet-induced obesity, we show that adipose tissue macrophages exhibit reduced migratory capacity, which can be restored by blocking netrin-1. Furthermore, hematopoietic deletion of Ntn1 facilitates adipose tissue macrophage emigration, reduces inflammation and improves insulin sensitivity. Collectively, these findings identify netrin-1 as a macrophage retention signal in adipose tissue during obesity that promotes chronic inflammation and insulin resistance.

Novel blocking human IgG directed against the pentapeptide repeat motifs of Neisseria meningitidis Lip/H.8 and Laz lipoproteins.

Ab-initiated, complement-dependent killing contributes to host defenses against invasive meningococcal disease. Sera from nonimmunized individuals vary widely in their bactericidal activity against group B meningococci. We show that IgG isolated from select individuals can block killing of group B meningococci by human sera that are otherwise bactericidal. This IgG also reduced the bactericidal efficacy of Abs directed against the group B meningococcal protein vaccine candidates factor H-binding protein currently undergoing clinical trials and Neisserial surface protein A. Immunoblots revealed that the blocking IgG was directed against a meningococcal Ag called H.8. Killing of meningococci in reactions containing bactericidal mAbs and human blocking Abs was restored when binding of blocking Ab to meningococci was inhibited using either synthetic peptides corresponding to H.8 or a nonblocking mAb against H.8. Furthermore, genetic deletion of H.8 from target organisms abrogated blocking. The Fc region of the blocking IgG was required for blocking because F(ab')(2) fragments were ineffective. Blocking required IgG glycosylation because deglycosylation with peptide:N-glycanase eliminated blocking. C4b deposition mediated by an anti-factor H-binding protein mAb was reduced by intact blocking IgG, but not by peptide:N-glycanase-treated blocking IgG, suggesting that blocking resulted from inhibition of classical pathway of complement. In conclusion, we have identified H.8 as a meningococcal target for novel blocking Abs in human serum. Such blocking Abs may reduce the efficacy of select antigroup B meningococcal protein vaccines. We also propose that outer membrane vesicle-containing meningococcal vaccines may be more efficacious if purged of subversive immunogens such as H.8.

Phosphorylation-driven assembly of the RIP1-RIP3 complex regulates programmed necrosis and virus-induced inflammation.

Programmed necrosis is a form of caspase-independent cell death whose molecular regulation is poorly understood. The kinase RIP1 is crucial for programmed necrosis, but also mediates activation of the prosurvival transcription factor NF-kappaB. We postulated that additional molecules are required to specifically activate programmed necrosis. Using a RNA interference screen, we identified the kinase RIP3 as a crucial activator for programmed necrosis induced by TNF and during virus infection. RIP3 regulates necrosis-specific RIP1 phosphorylation. The phosphorylation of RIP1 and RIP3 stabilizes their association within the pronecrotic complex, activates the pronecrotic kinase activity, and triggers downstream reactive oxygen species production. The pronecrotic RIP1-RIP3 complex is induced during vaccinia virus infection. Consequently, RIP3(-/-) mice exhibited severely impaired virus-induced tissue necrosis, inflammation, and control of viral replication. Our findings suggest that RIP3 controls programmed necrosis by initiating the pronecrotic kinase cascade, and that this is necessary for the inflammatory response against virus infections.