CD46, a membrane cofactor expressed on all nucleated human cells, plays an essential role in suppressing autoimmune reactions and protecting host cells from complement-mediated attack. Human transgenic CD46 homozygousmice (CD46(+/+)) are prone to lethal sepsis upon infection with Neisseria meningitidis (N. meningitidis). However, the underlying mechanisms are poorly understood. Here, we determined thatCD46(+/+) mice produce large numbers of M1 type macrophages with enhanced surface expression of MHC II and production of pro-inflammatory mediators such as IL-6, TNF, IL-12, and IL-1 beta In the presence of M-CSF or GM-CSF, CD46 signaling enhances monocyte-macrophage differentiation. Additionally, CD46(+/+) macrophages rapidly undergo apoptosis upon LPS challenge or meningococcal infection, which could contribute to uncontrolled bacterial dissemination in vivo. Adoptive transfer of CD46(+/+) peritoneal macrophages aggravated septic responses in wild-type mice, but the depletion of macrophages partially alleviated septic reactions in CD46(+/+) mice after N. meningitidis infection. Our findings reveal a novel role of CD46 in accelerating inflammatory responses upon meningococcal infection or LPS stimulation by regulating the functional polarization and survival of macrophages.