Forum Abstract

Carbon monoxide (CO), once perceived to be a detrimental metabolite of heme degradation is now considered by many to be a key modulator of inflammation and immunity. In recent years, CO has been ascribed an additional novel role as a ‘bactericidal agent’ by harnessing and modulating the function of the macrophage. We previously demonstrated that exogenous administration of CO at 250ppm, accelerated the clearance of bacteria by cultured peritoneal macrophages that involved TLR-4 expression (Cell Mol Biol 51:433-40).  
                                                              
We have now expanded on these observations further to determine the role of CO in macrophage-mediated bacterial killing. In vivo, mice injected intra-peritoneally with purified Enterococci faecalis (EF) and administered exogenous CO 1h post-infection, all survived long up to 72 hr while  air-treated controls died by 48 hr. Similarly in vitro, administration of exogenous CO for 4h to ­post-infected cultures of peritoneal macrophages augmented the macrophage killing machinery when compared  to air in an agar plate assay; 8.3 x10 CFU/ml compared to 5.83 x 103 CFU/ml respectively. Part of the protective mechanism involves inducible nitric oxide (NOS2) as CO, when administrated post-infection, did not improve survival of nos2-/- mice vs wild type mice. Furthermore, in vitro cultures of peritoneal macrophages isolated from nos2-/- mice and infected with EF were not able to clear bacteria in the absence or presence of CO suggesting that macrophage-derived nitric oxide contributes significantly to the bacterial defense mechanism of macrophages which is heightened with exposure to CO.  The viability of EF was not affected by CO gas [100ppm-10,000ppm] unlike that observed with a CO releasing molecule, which supports the need for the macrophages to enhance killing. The heightened macrophage killing effects in response to CO are in part, prefaced by EF binding to macrophage receptor with collagenous structure (MARCO). Bone marrow derived macrophages isolated from marco-/- mice when exposed to CO were unable to kill EF as their wt counterparts suggesting that CO, in part, augments binding of the bacteria at the cell surface. In summary, low non-toxic levels of CO when administered post-infection in vivo and in vitro, functions as a therapeutic by modulating MARCO and NOS2 activity in macrophages. This suggests that endogenously generated CO by heme oxygenase-1 may play an important role in innate immunity and may serve as a novel therapeutic defense mechanism against bacterial infections.

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