Whereas granulopoiesis during
Gram-negative bacterial infection is accelerated through activation of
toll-like receptor 4 (TLR4), it has not been elucidated whether
Gram-positive bacterial infection can stimulate granulopoiesis. Using the well-known TLR2 agonist
peptidoglycan (PGN), it was shown that neutrophils in bone marrow and spleen and plasma
granulocyte colony-stimulating factor were increased in mice that had received intraperitoneal administration of PGN. Incorporation of
bromodeoxyuridine into bone marrow neutrophils increased in mice administered PGN, demonstrating that PGN promotes granulopoiesis. These results illustrate that bacterial recognition by TLR2 facilitates granulopoiesis during
Gram-positive bacterial infection. Thus, granulopoiesis is accelerated to suppress
bacterial infection, but some bacteria can still cause severe
infections. Clostridium perfringens is a Gram-positive, anaerobic pathogenic bacterium and causes life-threatening
gas gangrene in humans. Of the many toxins produced by C. perfringens, α-toxin is known to be a major
virulence factor during
infection. Recently, it has been revealed that C. perfringens α-toxin impairs the innate immune system by inhibiting neutrophil differentiation, which is crucial for the pathogenesis of C. perfringens. Moreover, the toxin also attenuates erythropoiesis, which would cause severe
anemia in clinical settings. The findings provide new insight to understand how hosts strengthen innate immunity to fight pathogenic bacteria and how they evade the hosts' immune systems.