| Abstract: | The tumor necrosis factor receptor family molecule 4-1BB (CD137) has diverse roles in adaptive and innate immune responses. However, little is known of its role in bacterial infections. Previously, we showed that 4-1BB-deficient mice have enhanced susceptibility to Listeria monocytogenes infection, and mice pretreated with agonistic anti-4-1BB antibody (3E1) were much more resistant to L. monocytogenes infection than mice treated with control antibody. In this study, we report that stimulating 4-1BB by administering 3E1 in the early phase of L. monocytogenes infection is critical for promoting the survival of mice by inducing rapid infiltration of neutrophils and monocytes into L. monocytogenes-infected livers. The levels of tumor necrosis factor alpha, interleukin 6, and monocyte chemoattractant protein 1 in the livers of 3E1-treated mice increased as early as 30 min postinfection and peaked by 1 to 2 h, while those in mice treated with control antibody started to increase only at 16 h postinfection. Monocytes and neutrophils from the 3E1-treated mice had higher levels of activation markers, phagocytic activity, and reactive oxygen species than those from control mice. In vitro stimulation of 4-1BB induced the production of the inflammatory cytokines/chemokines of neutrophils, but not those of monocytes. These results suggest that 4-1BB stimulation of neutrophils in the early phase of L. monocytogenes infection causes rapid production of inflammatory cytokines/chemokines and that the subsequent infiltration of neutrophils and monocytes is crucial for eliminating the infecting L. monocytogenes.Listeria monocytogenes is a gram-positive intracellular pathogen responsible for listeriosis, a life-threatening infection in immunocompromised patients, newborns, or elderly people. The murine model of listeriosis has been used to investigate immune responses to bacterial infection (11). L. monocytogenes infects both phagocytic and nonphagocytic cells, escapes from intracellular vacuoles into the cytosol by secreting listeriolysin, replicates, and spreads to neighboring cells by actin-based motility. Intravenous (i.v.) bacteria are rapidly cleared from the bloodstream; most of them are taken up by the liver and spleen within 10 min of infection. Although T-cell-dependent adaptive immune responses are required to clear L. monocytogenes infection, they take several days to develop. Therefore, early control of the infection is critical for the survival of mice and primarily depends on innate immunity (5); indeed, it has even been shown that lymphocytes are detrimental during the early innate immune response to L. monocytogenes (4). Neutrophils and monocytes/macrophages are thought to be the main cells responsible for killing L. monocytogenes during the innate immune response. Thus, depletion of neutrophils from mice by using anti-Gr-1 antibodies greatly enhances their susceptibility to infection with L. monocytogenes (35), and the increased number of neutrophils resulting from deficiency in LFA-1 in mice confers resistance to listeriosis (30). Recruitment of monocytes is also essential to eradicate L. monocytogenes from infected mice, as indicated by reports that blocking complement receptor 3 of monocytes exacerbates listeriosis (38), and CC chemokine receptor 2-deficient mice have defects in the emigration of monocytes from the bone marrow and are highly susceptible to L. monocytogenes infection (22, 40). Although recruitment of neutrophils/monocytes is critical for eradication of L. monocytogenes during the early phase of infection, the molecular mechanisms of bacterial killing and the receptor molecules responsible for activation of neutrophils/monocytes against the bacteria are not clearly defined.The 4-1BB (CD137) receptor, a member of the tumor necrosis factor receptor superfamily (TNFRSF 9), is expressed on activated T cells (43), and the in vivo effects of 4-1BB activation on T-cell-dependent immune responses, such as eradication of established tumors (29), antiviral responses (1), and enhancement of the memory pool of antigen-specific CD8+ T cells (34), have been well defined. However, recent findings indicate that 4-1BB activation also plays an important role in other immune cells. 4-1BB is constitutively expressed on innate immune cells, including neutrophils (24), dendritic cells (10), natural killer (NK) cells (28), mast cells (31), and eosinophils (9). Its activation results in proliferation, gamma interferon secretion, and tumor rejection by NK cells (28); the production of cytokines by dendritic cells and the expression of costimulatory molecules on these cells (10); proliferation, survival, and cytokine production in human monocytes (21); and abrogation of the granulocyte-macrophage colony-stimulating factor-mediated antiapoptotic functions of human neutrophils (17). Previously, we reported that 4-1BB-deficient (4-1BB−/−) mice are very susceptible to L. monocytogenes infection because the antibacterial activity of their neutrophils is defective (24). Furthermore, pretreatment of agonistic anti-4-1BB monoclonal antibody (MAb) markedly increased the survival of L. monocytogenes-infected 4-1BB+/+ mice. In this study, we further characterized the mechanism of 4-1BB-mediated protection of L. monocytogenes-infected mice. We found that activation of 4-1BB in the early phase of L. monocytogenes infection rapidly stimulated the induction of proinflammatory cytokines/chemokines and the subsequent recruitment and activation of neutrophils and monocytes into the bacterium-infected livers. |
