Macrophages dictate both initiation and resolution of inflammation. During acute inflammation classically activated macrophages (M1) predominate, and during the resolution phase alternative macrophages (M2) are dominant. The molecular mechanisms involved in macrophage polarization are understudied. MicroRNAs are differentially expressed in M1 and M2 macrophages that influence macrophage polarization. We identified a role of miR-21 in macrophage polarization, and found that cross-talk between miR-21 and the lipid mediator prostaglandin E2 (PGE2) is a determining factor in macrophage polarization. miR-21 inhibition impairs expression of M2 signature genes but not M1 genes. PGE2 and its downstream effectors PKA and Epac inhibit miR-21 expression and enhance expression of M2 genes, and this effect is more pronounced in miR-21-/- cells. Among potential targets involved in macrophage polarization, we found that STAT3 and SOCS1 were enhanced in miR-21-/- cells and further enhanced by PGE2. We found that STAT3 was a direct target of miR-21 in macrophages. Silencing the STAT3 gene abolished PGE2-mediated expression of M2 genes in miR-21-/- macrophages. These data shed light on the molecular brakes involved in homeostatic macrophage polarization and suggest new therapeutic strategies to prevent inflammatory responses.

causes a wide range of diseases that together embody a significant public health burden. Aided by metabolic flexibility and a large virulence repertoire, has the remarkable ability to hematogenously disseminate and infect various tissues, including skin, lung, heart, and bone, among others. The hallmark lesions of invasive staphylococcal infections, abscesses, simultaneously denote the powerful innate immune responses to tissue invasion as well as the ability of staphylococci to persist within these lesions. In this article, we review the innate immune responses to during infection of skin and bone, which serve as paradigms for soft tissue and bone disease, respectively.

The chronic systemic inflammation in type I diabetes mellitus (T1DM), which is driven by signaling through the interleukin-1 (IL-1) 1 receptor (IL1R) and the adaptor protein myeloid differentiation factor 88 (MyD88), may be associated with the enhanced susceptibility of diabetics to systemic bacterial infection (sepsis). We hypothesized that low insulin concentrations trigger the enzyme 5-lipoxygenase (5-LO) to produce the lipid mediator leukotriene B4 (LTB4), serving as a trigger of systemic inflammation and increased susceptibility to polymicrobial sepsis in T1DM. In support of this hypothesis, we found that the abundance of MyD88 and its direct transcriptional regulator, STAT-1 were higher in peritoneal macrophages from two mouse models of T1DM compared to nondiabetic mice. Expression of Alox5, synthesis of LTB4, and concentrations of the proinflammatory cytokine IL-1β were also increased in peritoneal macrophages and serum from T1DM mice. Insulin treatment restored LTB4 concentrations and Myd88 and Stat1 expression in T1DM mice. T1DM mice lacking Alox5 or treated with a 5-LO inhibitor showed reduced Myd88 and Il1b mRNA expression and increased IL-1 receptor antagonist concentration. The transcription factor cJun drove LTB4-dependent transcription of Stat1 in macrophages from T1DM mice. Compared to wild-type or untreated diabetic mice, T1DM mice lacking 5-LO or treated with a 5-LO inhibitor survived polymicrobial sepsis and showed reduced production of proinflammatory cytokines and decreased bacterial counts, suggesting that high LTB4 concentrations contribute to enhanced susceptibility to sepsis in T1DM. These results uncover a role for LTB4 in promoting sterile inflammation in diabetes and enhanced susceptibility to sepsis in T1DM.

cOuter surface protein C (OspC) is one of the major lipoproteins expressed on the surface of Borrelia burgdorferi during tick feeding and the early phase of mammalian infection. OspC is required for B. burgdorferi to establish infection in both immunocompetent and SCID mice and has been proposed to facilitate evasion of innate immune defenses. However, the exact biological function of OspC remains elusive. In this study, we showed that the ospC-deficient spirochete could not establish infection in NOD-scid IL2r␥ null mice that lack B cells, T cells, NK cells, and lytic complement. The ospC mutant also could not establish infection in anti-Ly6G-treated SCID and C3H/HeN mice (depletion of neutrophils). However, depletion of mononuclear phagocytes at the skin site of inoculation in SCID and C3H/HeN mice allowed the ospC mutant to establish infection in vivo. In phagocyte-depleted mice, the ospC mutant was able to colonize the joints and triggered neutrophilia during dissemination. Furthermore, we found that phagocytosis of green fluorescent protein (GFP)-expressing ospC mutant spirochetes by murine peritoneal macrophages and human THP-1 macrophage-like cells, but not in PMN-HL60, was significantly higher than parental wild-type B. burgdorferi strains, suggesting that OspC has an antiphagocytic property. In addition, overproduction of OspC in spirochetes also decreased the uptake of spirochetes by murine peritoneal macrophages. Together, our findings provide evidence that mononuclear phagocytes play a key role in clearance of the ospC mutant and that OspC promotes spirochetes' evasion of macrophages during early Lyme borreliosis. L yme disease, the most prevalent vector-borne illness in the United States (1), is a multisystem inflammatory disorder caused by infection with the spirochete Borrelia burgdorferi (2, 3). This spirochete is maintained in nature through a complex enzootic cycle involving Ixodes ticks and various small-mammal hosts. Humans, as accidental hosts, become infected after B. burgdorferiinfected ticks feed on them (4). Spirochetes replicate in the skin, spread locally, and induce an inflammatory response with a symptom known as erythema migrans, observed in most patients (2, 4). During disseminated infection, B. burgdorferi colonizes multiple tissues, leading to different clinical manifestations, including arthritis, myocarditis, and neurological and/or cutaneous abnormalities (2, 4). This acute, disseminated stage of human Lyme disease is largely recapitulated using inbred mouse strains which are susceptible to B. burgdorferi infection and develop carditis and subacute arthritis (5). Thus, the murine model provides a powerful tool to elucidate the role of spirochete virulence factors and host immunological responses during Lyme disease pathogenesis (4).The B. burgdorferi genome encodes a large number of surface lipoproteins, many of which are expressed during mammalian infection (4, 6, 7). One of these lipoproteins is the major outer surface protein C (OspC), whose production is induced within inf...