The study of pathogenic mechanisms of disease can be greatly facilitated by studying genetic differences in susceptibility to infection. In the present study, we compared the severity of pneumococcal infection in C57BL/6 (B6) and 129Sv mice. The results showed that 129Sv mice were remarkably more susceptible to pneumococcal infection than B6 mice. Bacterial clearance, proinflammatory mediators, leukocyte recruitment, and phagocyte activities were measured to examine potential immune factors associated with differences in susceptibility to pneumococcal infection. The greater susceptibility of 129Sv mice was associated only with inadequate alveolar macrophage bacterial killing, as indicated by significantly decreased initial bacterial clearance from the respiratory tract. Effective pneumococcal clearance was not dependent upon Toll-like receptor 2 (TLR2) expression, oxidative stress, or matrix metallopeptidase 12 (MMP-12) expression. Furthermore, phagocytosis analysis suggested that the deficiency found in 129Sv alveolar macrophages was not due to a lack of bacterial recognition but, rather, to reduced bacterial uptake. In conclusion, our findings indicate a crucial role of alveolar macrophage phagocytosis during innate defense against pneumococcal infection, which may explain the association of host genetic risk factors with predisposition to pneumococcal infection.There are over 90 Streptococcus pneumoniae serotypes, based on their capsular polysaccharide, and different serotypes vary in the ability to cause disease on the basis of the virulent factors expressed (19,20). On the other hand, host genetic components are also involved in the outcome of pneumococcal infection (2, 4, 22), and host immune status associated with age, respiratory viral infection, chronic diseases, and immunosuppression are all significant risk factors for the development of severe pneumococcal infection (13,16).Although pneumococcus is a major human pathogen, the majority of information about pathogenesis and disease has resulted from mouse studies. It is recognized that there are significant variations in susceptibility to pneumococcal infection among inbred mouse strains (9, 12), and understanding the underlying immune protective mechanisms may explain the association of genetic risk factors with predisposition of the human host to pneumococcal infection.Earlier studies using immunocompetent strains have indicated that mice exhibit a spectrum of resistance to bacterial infection linked to neutrophil recruitment (7, 9, 12). We hypothesized that genetic variation in the capacity of alveolar macrophage-mediated bacterial killing may also exist. Our previous studies have demonstrated a pivotal role of alveolar macrophages in pneumococcal clearance, especially after a sublethal bacterial inoculation (24). However, the overall importance of alveolar macrophages in maintaining the sterility of the lower respiratory tract remains to be firmly established, and the impact of genetic variation on alveolar macrophage function against extracellular...