Differential contribution of bacterial N-formyl-methionyl-leucyl-phenylalanine and host-derived CXC chemokines to neutrophil infiltration into pulmonary alveoli during murine pneumococcal pneumonia

Despite the development of new potent antibiotics, Streptococcus pneumoniae remains the leading cause of death from bacterial pneumonia. Polymorphonuclear neutrophil (PMN) recruitment into the lungs is a primordial step towards host survival. Bacterium-derived N-formyl peptides (N-formyl-methionyl-leucyl-phenylalanine [fMLP]) and host-derived chemokines (KC and macrophage inflammatory protein 2 [MIP-2]) are likely candidates among chemoattractants to coordinate PMN infiltration into alveolar spaces. To investigate the contribution of each in the context of pneurnococcal pneumonia, CD1, BAILB/c, CBA/ca, C57BL/6, and formyl peptide receptor (FPR)-knockout C57BL/6 mice were infected with 10(6) or 10(7) CFU of penicillin/erythromycin-susceptible or -resistant serotype 3 or 14 S. pneumoniae strains. Antagonists to the FPR, such as cyclosporine H (CsH) and chenodeoxycholic acid, or neutralizing antibodies to KC and MIP-2 were injected either 1 h before or 30 min after infection, and then bronchoalveolar lavage fluids were obtained for quantification of bacteria, leukocytes, and chemokines. CsH was effective over a short period after infection with a high inoculum, while anti-CXC chemokine antibodies were effective after challenge with a low inocullum. CsH prevented PMN infiltration in CD1 mice infected with either serotype 3 or 14, whereas antichemokine antibodies showed better efficacy against the serotype 3 strain. When different mouse strains were challenged with serotype 3 bacteria, CsH prevented PMN migration in the CD1 mice only, whereas the antibodies were effective against CD1 and C57BL/6 mice. Our results suggest that fMLP and chemokines play important roles in pneumococcal pneumonia and that these roles vary according to bacterial and host genetic backgrounds, implying redundancy among chemoattractant molecules.