Neonatal low-density granulocytes internalize and kill bacteria but suppress monocyte function using extracellular DNA

Abstract

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.

The Enigma of Low-Density Granulocytes in Humans: Complexities in the Characterization and Function of LDGs during Disease

“…In a recent report, adult LDNs were shown to phagocytose opsonized latex beads at a significantly higher rate than NDNs [49]. We also demonstrated similar findings in that the frequency of neonatal LDGs capable of phagocytosing large numbers of pathogenic E. coli O1:K1:H7 is greater than neonatal NDNs [3]. However, in another study focusing on the effects of LDNs in Mtb patients, La Manna et al found that these cells did not phagocytose Mtb compared to NDNs [2].…”

“…In our most recent publication, we describe our neonatal LDGs (from cord blood mononuclear cells, or CBMCs) as CD66 hi , CD33 + , CD14 lo , and HLA-DR − , a phenotype established in a 2013 report on neonatal gMDSCs by Rieber et al [ 3 , 7 ]. Other studies have used similar cell surface markers to characterize LDGs, gMDSCs, and LDNs ( Table 1 ).…”

“…Suppression of T cell proliferation has indeed become a functional hallmark for MDSC assignment, but there are lapses in demonstration of this characteristic that are found throughout the literature. In addition, recent work in our lab has shown that LDGs inhibit innate immunity, specifically some aspects of monocyte killing of bacteria during co-culture [ 3 ]. This finding is consistent with the release of extracellular DNA from LDGs, and the inclusion of DNAse during co-culture increases bacterial viability [ 3 ].…”

“…In addition, recent work in our lab has shown that LDGs inhibit innate immunity, specifically some aspects of monocyte killing of bacteria during co-culture [ 3 ]. This finding is consistent with the release of extracellular DNA from LDGs, and the inclusion of DNAse during co-culture increases bacterial viability [ 3 ]. Cell-free conditioned media from LDG cultures further impaired monocyte phagocytosis of bacteria.…”

“…Low-density granulocytes (LDGs) are characterized as important immune cells during healthy and disease states in humans, with elevated numbers occurring in numerous microbial infections, cancers, and autoimmune disorders [ 1 , 2 , 3 , 4 ]. Many studies working with LDGs within these disease states classify LDGs as granulocytic myeloid-derived suppressor cells (gMDSCs) and low-density neutrophils (LDNs), with similar cell surface marker expression and functional characterizations utilized for each cell type.…”