Several studies have suggested that elevated iNOS expression may contribute to pulmonary hemorrhage in leptospirosis.49, 53, 54 iNOS is an important component of innate immunity, acting as a protective barrier against pathogens, mediating bacterial clearance and controlling infection,55, 56 while simultaneously contributing to cell and tissue injury.14 In this study, the defective expression of proinflammatory mediators by TLR4−/− mice is probably responsible, to some extent, for the ineffective control of bacterial replication. This evidence concerns the gene TLR4 and infection.