Monocyte Recruitment, Activation, and Function in the Gut-Associated Lymphoid Tissue during Oral Salmonella Infection

Abstract

Neutrophils, monocytes, and dendritic cells (DC) are phenotypically and functionally related phagocytes whose presence in infected tissues is critical to host survival. Their overlapping expression pattern of surface molecules, the differentiation capacity of monocytes, and the presence of monocyte subsets underscores the complexity of understanding the role of these cells during infection. In this study we use five- to seven-color flow cytometry to assess the phenotype and function of monocytes recruited to Peyer’s patches (PP) and mesenteric lymph nodes (MLN) after oral Salmonella infection of mice. The data show that CD68highGr-1int (intermediate) monocytes, along with CD68intGr-1high neutrophils, rapidly accumulate in PP and MLN. The monocytes have increased MHC-II and costimulatory molecule expression and, in contrast to neutrophils and DC, produce inducible NO synthase. Although neutrophils and monocytes from infected mice produce TNF-α and IL-1β upon ex vivo culture, DC do not. In addition, although recruited monocytes internalize Salmonella in vitro and in vivo they did not induce the proliferation of OT-II CD4+ T cells after coincubation with Salmonella expressing OVA despite their ability to activate OT-II cells when pulsed with the OVA323–339 peptide. We also show that recruited monocytes enter the PP of infected mice independently of the mucosal addressin cell adhesion molecule-1 (MAdCAM-1). Finally, recruited but not resident monocytes increase in the blood of orally infected mice, and MHC-II up-regulation, but not TNF-α or iNOS production, occur already in the blood. These studies are the first to describe the accumulation and function of monocyte subsets in the blood and GALT during oral Salmonella infection.

Monocyte and neutrophil recruitment during oral Salmonella infection is driven by MyD88‐derived chemokines

“…First, the arrival of the first myeloid cells to PP and MLN ([3] and data not shown) coincides with increased expression of CCL2, CXCL9 and CXCL2 at day 2 post infection. Second, monocytes and neutrophils in the blood of Salmonella-infected mice express CCR2 and CXCR2, which are the receptors for CCL2 and CXCL2, respectively [3], and a small subset of monocytes express CXCR3, the receptor for CXCL9 [24]. Consistent with the results at the mRNA level ( Fig.…”

“…Murine monocytes in the blood are a heterogeneous population and have been divided into two main subsets based on whether they are present in steady state or recruited under inflammatory conditions [1]. Inflammatory Gr-1 hi CCR2 hi CX 3 CR1 low monocytes, which have also been called TipDC upon entering a tissue, are a source of molecules important in controlling bacterial infection, particularly iNOS and TNF-a [2][3][4].Salmonella enterica serovar Typhimurium (S. typhimurium) is a Gram-negative facultative intracellular bacterium that infects via the oral route. Orally acquired Salmonella exits the gut lumen primarily by crossing the follicle-associated epithelium overlying Peyer's patches (PP) via M cells (reviewed in [5]).…”

“…The bacteria are then further spread to the MLN, most likely by DC, and to the spleen and liver via the blood [5,6]. After oral infection with S. typhimurium, neutrophils and Gr-1 hi CCR2 hi inflammatory monocytes accumulate in PP and MLN, the first organs encountering the bacteria, beginning 2-3 days after infection [3].Chemokines are the main mediators involved in the recruitment and migration of leukocytes to and within tissues. An array of chemokines is induced by inflammation and recruits monocytes, neutrophils and other cells to sites of infection.…”

“…To gain insight into the mechanism of this early recruitment, we assessed which chemokines are induced in PP and MLN at two time points. The first is day 2, which coincides with the arrival of the first phagocytes, and the second is day 4 when cells are rapidly accumulating [3]. Thus, the expression of nine chemokines reported to be involved in recruiting myeloid lineage cells was assessed in PP and MLN ( Fig.…”

“…We thus used an anti-iNOS antibody to identify monocytes in situ. Ly6G, which is not expressed by inflammatory monocytes [3,28], was used to identify neutrophils.As predicted, no iNOS-positive monocytes or Ly6G-positive neutrophils, were detected in PP from naïve mice, and the tissue www.eji-journal.eu structure was smooth with a well-defined FAE (Fig. 3A).…”