Absence of 4-1BB increases cell influx into the peritoneal cavity in response to LPS stimulation by decreasing macrophage IL-10 levels

CD137 expressed on neutrophils plays dual roles in antibacterial responses against Gram-positive and Gram-negative bacterial infections

Severe sepsis and septic shock caused mainly by bacterial infections are life-threatening conditions that urge the development of novel therapies. However, host responses to and pathophysiology of sepsis have not been clearly understood, which remains a major obstacle for the development of effective therapeutics. Recently, we have shown that stimulation of a costimulatory molecule, CD137, enhanced survival of mice infected with the Gram-positive (G(+)) intracellular bacterium Listeria monocytogenes but decreased survival in a polymicrobial sepsis model. Herein, we report that CD137 deficiency or blocking of CD137 signaling decreased antibacterial responses of mice infected with G(+) bacteria (Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis) but increased these responses in mice infected with Gram-negative (G(-)) bacteria (Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica serovar Typhimurium). Consistent with these findings, stimulation of CD137 by administration of agonistic antibody enhanced responses against G(+) bacteria, whereas it decreased these responses against G(-) bacteria. Neutrophils were responsible for CD137-mediated opposite roles in control of G(+) and G(-) bacterial infections. Stimulation of CD137 enhanced activities of neutrophils against S. aureus but decreased these activities against E. coli, while CD137 blocking produced opposite results with the stimulation of CD137 in vivo and in vitro. Furthermore, we found that combined signaling of CD137 and Toll-like receptor 2 (TLR2) induced synergistic production of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) by neutrophils, but combined signaling of CD137 and TLR4 did not. Our data strongly suggest that CD137 may play a dual role in sepsis in association with TLRs.

Lipoxin a4 increases survival by decreasing systemic inflammation and bacterial load in sepsis

Sepsis is characterized by systemic inflammation with release of a large amount of inflammatory mediators. If sustained, this inflammatory response can lead to multiple organ failure and/or immunoparalysis. In the latter condition, the host may be susceptible to opportunistic infections or be unable to clear existing infections. Therefore, it is potentially beneficial to resolve inflammation by reducing inflammation without compromising host defense. We examined the effect of lipoxin A4 (LXA4), a compound with inflammatory resolution properties, in the cecal ligation and puncture (CLP) model of sepsis. Cecal ligation and puncture rats were given either saline or LXA4 (40 μg/kg, i.p.) 5 h after surgery. Lipoxin A4 administration increased 8-day survival of CLP rats, which lived longer than 48 h, and attenuated tissue injury after 8 days. Therefore, we investigated the effects of LXA4 on systemic inflammation and bacterial load 48 h after CLP sepsis. Plasma IL-6, monocyte chemotactic protein 1, and IL-10 levels were reduced in LXA4-treated rats compared with CLP rats given saline vehicle. Lipoxin A4 reduced phosphorylation of the p65 subunit of nuclear factor κB (NF-κB) at serines 536 and 468 in peritoneal macrophages, suggesting that LXA4 reduced production of proinflammatory mediators through an NF-κB-mediated mechanism. Lipoxin A4 reduced blood bacterial load and increased peritoneal macrophage number without affecting phagocytic ability, suggesting that LXA4 reduced blood bacterial load by enhancing macrophage recruitment. It also suggests that LXA4 reduced systemic inflammation and NF-κB activation without compromising host defense. Increased macrophage recruitment was in part due to a direct effect of LXA4 as LXA4 increased peritoneal macrophage recruitment in sham controls and partly due to reduced production of IL-10 as LXA4 decreased macrophage IL-10 release (a known inhibitor of macrophage migration) after CLP. The results suggest that LXA4 increased survival in sepsis by simultaneously reducing systemic inflammation as well as bacterial spread.

Blockade of CD137 signaling counteracts polymicrobial sepsis induced by cecal ligation and puncture

Sepsis, a leading cause of death worldwide, involves proinflammatory responses and inefficient bacterial clearance. Previously, we have shown that CD137 (4-1BB), a member of the tumor necrosis factor receptor superfamily, plays critical roles in eradicating infective Listeria monocytogenes, a gram-positive bacterium, and that stimulation of CD137 protects mice from sepsis-induced death. In this study, we unexpectedly found that CD137 activation aggravated polymicrobial sepsis due to mixed gram-positive and gram-negative bacterial infection induced by cecal ligation and puncture (CLP). CD137-deficient (CD137(-/-)) mice showed significantly lower mortality than CD137-sufficient (CD137(+/+)) mice in the CLP model. Administration of an agonistic anti-CD137 monoclonal antibody (MAb) to CD137(+/+) mice decreased their survival in this infection model, while administration of a blocking anti-CD137 ligand MAb (TKS-1) to such mice increased their survival. CD137(-/-) mice and TKS-1-treated CD137(+/+) mice had lower levels of chemokines/proinflammatory cytokines (monocyte chemoattractant protein 1, interleukin-6 [IL-6], tumor necrosis factor alpha, IL-12) and an anti-inflammatory cytokine (IL-10), exhibited improved bacterial clearance in the peritoneum, liver, and blood, and had greater numbers of infiltrated peritoneal neutrophils and macrophages in the CLP model than control mice. Our data suggest that CD137 activation aggravates polymicrobial sepsis induced by CLP.

Identification of regulatory functions for 4-1BB and 4-1BBL in myelopoiesis and the development of dendritic cells

The costimulatory molecule 4-1BB and its ligand 4-1BBL can control adaptive immunity, but here we show that their interaction also suppressed myelopoiesis. We found that 4-1BBL was expressed on hematopoietic stem cells, differentiating common myeloid progenitors and granulocyte-macrophage progenitors, and 4-1BB was inducible on activated myeloid progenitors. Steady-state numbers of granulocyte-macrophage progenitors, myeloid-lineage cells and mature dendritic cells were higher in 4-1BB- and 4-1BBL-deficient mice, indicative of a negative function, and we confirmed that result with bone marrow chimeras and in vitro, where the absence of interactions between 4-1BB and 4-1BBL led to enhanced differentiation into dendritic cell lineages. The regulatory activity was mediated by 4-1BBL, with binding by 4-1BB inhibiting differentiation of myeloid progenitors. Thus, 4-1BB and 4-1BBL have a previously unknown function in limiting myelopoiesis and the development of dendritic cells.