Eur. J. Immunol. 2017. 47: 119-130 Macrophages play an essential role in controlling bacterial infection, as they efficiently eliminate pathogens, provide a significant source of cytokines, and represent and link between innate and adaptive immunity [6]. Both the environmental milieu and cellular mitochondrial metabolism [7] can alter the functional polarization of macrophages, which can be broadly classified as classically activated (M1) and alternatively activated (M2) [8]. M1 macrophages triggered by IFN-γ alone or in combination with bacterial LPS are characterized by the production of type 1 inflammatory cytokines and bactericidal mediators such as nitric oxide and reactive oxygen intermediates. Thus, M1 macrophages play a dominant role in controlling acute infections, but also have the potential to induce a cytokine storm that is associated with tissue injury. In contrast, M2 macrophages triggered by IL-4/IL-13, glucocorticoids, IL-10, or immune complexes produce elevated amounts of anti-inflammatory cytokines and Th2-recruiting chemokines. M2 macrophages exhibit efficient phagocytic activity as well as increased expression of arginase 1, and therefore contribute to tissue healing and resolution of harmful inflammation. However, prolonged M2 polarization can generate a survival niche that leads to chronic microbial infection [9]. Initial overwhelming inflammatory responses, followed by macrophage dysfunction during the later stages of infection, have been widely observed in patients with severe sepsis. This probably occurs due to a switch in the macrophage phenotype from M1 to M2, implying a close association between macrophage phenotype and the progression of sepsis [10].The transmembrane protein CD46 was initially identified as a co-factor for complement factor I, which is involved in protecting host cells from autologous complement-mediated attack [11]. CD46 also serves as a receptor for several pathogens including measles virus [12] (Fig. 1A). Systemic meningococcal dissemination was well controlled in the WT mice, but not in the CD46 +/+ mice (Fig. 1B).To investigate whether this phenomenon was a general response to gram-negative bacterial infection, we repeated the experiments, challenging the mice with E. coli LPS. A non-lethal dose (20 mg/kg) for WT mice led to 67% mortality in CD46 +/+ mice within 4 days (Fig. 1C). A reduction in body weight, a common indicator of the severity of sepsis, was observed in the CD46 +/+ mice. Body weight was maintained in WT mice even though an initial decline was observed (Fig. 1D). The levels of circulatory cytokines IL-10, IL-6, and TNF, which are closely associated with sepsis severity and a fatal outcome [27], were significantly higher in the CD46 +/+ mice than in the WT mice upon meningococcal infection (Fig. 1E). Stronger cytokine responses were also observed in CD46 +/+ mice, compared to that in WT mice, following LPS challenge even though the effect was transient and lost after 24 h for IL-10 and TNF ( hand, LPS degradation in vivo might also contribute t...