TLR2-mediated neutrophil depletion exacerbates bacterial sepsis.

T he host antimicrobial defense response is initiated by the innate immune system, the most ancient of the 2 limbs of the immune system. The innate immune system is activated as soon as a pathogen crosses the host external defense barriers, lasts for a few hours, and is aimed at the elimination of the invading microorganism. Detection of microbial pathogens is first carried out by sentinel cells [macrophages and dendritic cells (DCs)] located in tissues that are in direct contact with the host’s natural environment or are rapidly recruited to the site of infection (neutrophils). This process involves coordinated actions of soluble and cellular molecules comprising components of the complement system, acute phase proteins such as the lipopolysaccharide-binding protein, extracellular or intracellular patternrecognition types of molecules including the Toll-like receptors (TLR), the nucleotide-binding oligomerization domain-like receptors, RIG-I-like helicases, C-type lectin, and scavenger receptors. Recognition of invasive pathogens by immune cells relies on their capacity to detect microbial molecular motifs (for example, endotoxin, peptidoglycan subcomponents, lipopeptides, glucans, mannans, flagellin, and nucleic acids) via pattern-recognition receptors or molecules (1, 2). Granulocytes–neutrophils, also known as polymorphonuclear cells, are phagocytic cells that play a crucial role in the host defense mechanisms against bacterial infections, as illustrated by the increased susceptibility to infection and sepsis of individuals with congenital or acquired neutropenia or defects of neutrophil functions (3, 4). Neutrophils are produced and terminally differentiate in the bone marrow by a highly-regulated process (granulopoiesis) before being released into the blood stream where they account for 60% of all circulating leukocytes. The majority of mature neutrophils resides in the bone marrow and circulating neutrophils represent only 1–2% of the total neutrophil pool (lifespan of 6–10 h). The bone marrow provides a huge reserve of neutrophils that are rapidly mobilized during infection. The main hematopoietic cytokine regulating granulopoiesis is granulocyte colonystimulating factor (G-CSF). Blood concentrations of G-CSF are very low in healthy individuals and increase markedly during infection to increase neutrophil number and enhance the host antimicrobial defense response. Recombinant G-CSF is used to prevent or reverse neutropenia in various clinical settings and as experimental adjunctive therapy for sepsis that yielded disappointing results (5, 6). In this issue of PNAS, Navarini et al. (7) report the results of studies of the role of innate immune-induced neutrophil depletion in an experimental model of infection caused by Listeria monocytogenes, an opportunistic Gram-positive bacillus and food-borne pathogen causing serious infections in pregnant women and immunocompromised hosts associated with preterm deliveries, neonatal sepsis, and meningo-encephalitis (8). Whereas mice survived infection caused by a low L. monocytogenes inoculum, they rapidly succumbed from systemic infection when challenged with a high inoculum (Fig. 1). Analyses of cellular responses during the early phase of L. monocytogenes infection revealed striking differences in the kinetics of neutrophil counts that were associated with the outcome of infection. The natural response to sublethal listeriosis was characterized by a robust and transient infiltration of liver and spleen by neutrophils associated with bacterial clearance. Bone marrow neutrophil counts were reduced during the first 3 days of a sublethal infection because of cell egress into blood and migration to infected organs. This was followed by a rapid return to subnormal counts caused by stimulation of granulopoiesis by increased G-CSF concentrations. In contrast, challenge with a high bacterial inoculum led to a massive depletion of bone marrow neutrophils within 3 days after the onset of infection and a rapid disappearance of neutrophil liver and spleen infiltrates despite the presence of large numbers of bacteria and death (Fig. 1). Immuno-depletion of granulocytes with antigranulocyte receptor-1 (Gr-1) antibodies was also shown to enhance the susceptibility to listeriosis, confirming the critical role played by neutrophils in the outcome of infection. What was the cause of neutrophil depletion in the bone marrow during the course of L. monocytogenes infection? Navarini et al. (7) convincingly showed that neutrophil apoptosis was massively augmented between days 1 and 3 in mice challenged with a high, but not a low, bacterial inoculum. Interestingly, mortality was increased by the injection of heat-killed L. monocytogenes during the course of an otherwise nonlethal infection. This led Navarini et al. to explore the role of bacterial cell wall components in this process. Pam2Cys, a synthetic lipopeptide analogous to the