The microbiota regulates neutrophil homeostasis and host resistance to Escherichia coli K1 sepsis in neonatal mice

Abstract

Acquisition of microbes by the neonate, which begins immediately during birth, is influenced by gestational age and mother’s microbiota and modified by exposure to antibiotics1. In neonates, prolonged duration of antibiotic therapy is associated with increased risk of sepsis after 4 days of life, known as late-onset sepsis (LOS)2, a disorder critically controlled by neutrophils3, but a role for the microbiota in regulating neutrophil behavior in the neonate has not been described. We exposed pregnant mouse dams to antibiotics in drinking water to limit transfer of maternal microbes to the neonates. Antibiotic exposure of dams decreased the total number of microbes in the intestine, altered the structure of intestinal microbiota and changed the pattern of microbial colonization. These changes were associated with decreased numbers of circulating and bone marrow neutrophils and granulocyte/macrophage restricted progenitor cells in the bone marrow. Antibiotic-exposure of dams attenuated the postnatal granulocytosis by reducing the number of interleukin (IL) 17-producing cells in intestine and consequent production of granulocyte colony stimulating factor (G-CSF). Relative granulocytopenia contributed to increased susceptibility of antibiotic-exposed neonatal mice to Escherichia coli K1 and Klebsiella pneumoniae sepsis, which could be partially reversed by administration of G-CSF. Restoration of normal microbiota, through TLR4- and MYD88-dependent mechanism, induced accumulation of IL17-producing type 3 innate lymphoid cells (ILC) in the intestine, promoted granulocytosis, and restored the IL17-dependent resistance to sepsis. Specific depletion of ILCs prevented the IL17- and G-CSF-dependent granulocytosis and resistance to sepsis. These data support a role for the intestinal microbiota in regulation of granulocytosis and host resistance to sepsis in the neonates.

Neutrophils in cancer: neutral no more

“…Because IL-17 is upstream of G-CSF 52,53 , lower levels of IL-17 equate to reduced expression of G-CSF and steady-state release of neutrophils from the bone marrow 46 . Commensal bacteria and enterocyte-derived CXCL5 in the gut also play a role in neutrophil homeostasis by increasing or inhibiting IL-17 production, respectively 54,55 . IL-1β that is released from dying cells or upregulated in response to inflammatory stimuli is another potent inducer of the IL-17-G-CSF axis 56,57 .…”

The regulation of host defences to infection by the microbiota

“…Because of the lack of direct contact with live microbes, it has long been presumed that the microbiota does not influence these systemic defences. We now know that this assumption is incorrect and that the production and function of cells that constitute systemic defences is greatly influenced by the microbiota 29, 30, 39, 66, 67, 68…”

“…Reduced neutrophil production renders these animals more susceptible to systemic infection by numerous bacteria including E. coli and Listeria monocytogenes 39, 69. Studies have shown that recognition of bacteria and/or bacterial products from the microbiota by PRRs is the first step in driving this microbiota‐dependent increase in neutrophil production 67, 69. This PRR activation is not restricted to a single site, as PRR ligands in the circulation and at the mucosa are able to drive increased neutrophil production 67, 69.…”

“…Studies have shown that recognition of bacteria and/or bacterial products from the microbiota by PRRs is the first step in driving this microbiota‐dependent increase in neutrophil production 67, 69. This PRR activation is not restricted to a single site, as PRR ligands in the circulation and at the mucosa are able to drive increased neutrophil production 67, 69. The signals required downstream of PRRs, in contrast, are less well defined.…”

“…The signals required downstream of PRRs, in contrast, are less well defined. A number of studies have shown that IL‐17 is important, as are members of the colony‐stimulating factor family 67. In addition to neutrophils, the production of macrophages and monocytes in the spleen is also promoted by the microbiota through currently poorly understood mechanisms 39.…”