Microbial infections early in life remain a major cause of infant mortality worldwide. This is consistent with immune deficiencies in this population. Interleukin (IL)‐27 is suppressive toward a variety of immune cell types, and we have shown that the production of IL‐27 is elevated in humans and mice early in life. We hypothesize that elevated levels of IL‐27 oppose protective responses to infection during the neonatal period. In this study, we extended previous findings in neonatal mice to identify a population of IL‐27 producers that express Gr‐1 and were further identified as myeloid‐derived suppressor cells (MDSCs) based on the expression of surface markers and functional studies. In neonates, MDSCs are more abundant and contribute to the elevated pool of IL‐27 in this population. Although the ability of MDSCs to regulate T lymphocyte activation has been well‐studied, sparingly few studies have investigated the influence of MDSCs on innate immune function during bacterial infection. We demonstrate that macrophages are impaired in their ability to control growth of Escherichia coli when cocultured with MDSCs. This bacterium is a significant concern for neonates as a common cause of bacterial sepsis and meningitis. The suppressive effect of MDSCs on macrophage function is mediated by IL‐27; inclusion of a reagent to neutralize IL‐27 promotes improved control of bacterial growth. Taken together, these results suggest that the increased abundance of MDSCs may contribute to early life susceptibility to infection and further highlight production of IL‐27 as a novel MDSC mechanism to suppress immunity.

Neonates are at increased risk for bacterial sepsis. We established that the immune-suppressive cytokine interleukin-27 (IL-27) is elevated in neonatal mice. Similarly, human cord blood-derived macrophages express IL-27 genes and secrete more cytokine than macrophages from adults. In the present work, we hypothesized that increased levels of IL-27 predispose neonatal mice to more severe infection during Gram-negative sepsis. Serum IL-27 levels continued to rise during infection. Peripheral tissue analysis revealed systemic IL-27 expression, while myeloid cell profiling identified Gr-1-and F4/80-expressing cells as the most abundant producers of IL-27 during infection. Increased IL-27 levels were consistent with increased mortality that was improved in IL-27 receptor ␣ (IL-27R␣) Ϫ/Ϫ mice that lack a functional IL-27 receptor. Infected IL-27R␣ Ϫ/Ϫ pups also exhibited improved weight gain and reduced morbidity. This was consistent with reduced bacterial burdens and more efficient bacterial killing by Ly6B.2 ϩ myeloid cells and macrophages compared to WT neonates. Live animal imaging further supported a more severe and disseminated infection in WT neonates. This is the first report to describe the impact of elevated early-life IL-27 on the host response in a neonatal infection model while also defining the cell and tissue sources of cytokine. IL-27 is frequently associated with suppressed inflammation. In contrast, our findings demonstrate that IL-27 indirectly promotes an inflammatory cytokine response during neonatal sepsis by directly compromising control of bacteria that drive the inflammatory response. Collectively, our results suggest that IL-27 represents a therapeutic target to limit susceptibility and improve infectious outcomes in neonatal sepsis.

Low-density granulocytes (LDGs) are found abundantly in neonatal blood. However, there is limited mechanistic understanding of LDG interactions with bacteria and innate immune cells during acute infection. We aimed to determine how human neonatal LDGs may influence control of the bacterial burden at sites of infection, both individually and in the presence of mononuclear phagocytes. LDGs from human umbilical cord blood do phagocytose E. coli O1:K1:H7 and traffic bacteria into acidic compartments. However, LDGs were significantly less efficient at bacterial uptake and killing compared to monocytes, and this activity was associated with a reduced inflammatory cytokine response. The presence of bacteria triggered the release of DNA (eDNA) from LDGs into the extracellular space that resembled neutrophil extracellular traps, but had limited anti-bacterial activity. Instead, eDNA significantly impaired monocyte control of bacteria during co-culture. These results suggest that LDG recruitment to sites of bacterial infection may compromise host protection in the neonate. Furthermore, our findings reveal novel insights into LDG activity during infection, clarify inflammatory contributions relative to monocytes, and identify a novel LDG mechanism of immunosuppression.

Neonates are at an increased risk of an infectious disease. This is consistent with an increased abundance of myeloid-derived suppressor cells (MDSCs) compared with older children and adults. Using a murine model of neonatal bacterial sepsis, we demonstrate that MDSCs modulate their activity during an infection to enhance immune suppressive functions. A gene expression analysis shows that MDSCs increased NOS2, Arg-1 and IL-27p28 expression in vitro and in vivo in response to Escherichia coli O1:K1:H7 and this is regulated at the level of the gene expression. Changes in the effector gene expression are consistent with increased enzymatic activity and cytokine secretion. The neonatal MDSCs express toll-like receptor (TLR) 2, 4 and 5 capable of recognizing pathogen-associated molecular patterns (PAMPS) on E. coli. However, a variable level of effector expression was achieved in response to LPS, peptidoglycan or flagellin. Individual bacterial PAMPs did not stimulate the expression of Arg-l and IL-27p28 equivalently to E. coli. However, the upregulation of NOS2 was achieved in response to LPS, peptidoglycan and flagella. The increased immune suppressive profile translated to an enhanced suppression of CD4+ T cell proliferation. Collectively, these findings increase our understanding of the dynamic nature of MDSC activity and suggest that these cells abundant in early life can acquire activity during an infection that suppresses protective immunity.

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