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

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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.