Intestinal macrophages and response to microbial encroachment

Abstract

Macrophages in the gastrointestinal mucosa represent the largest pool of tissue macrophages in the body. In order to maintain mucosal homeostasis, resident intestinal macrophages uniquely do not express the lipopolysaccharide (LPS) co-receptor CD14 or the IgA (CD89) and IgG (CD16, 32, and 64) receptors, yet prominently display Toll-like receptors (TLRs) 3-9. Remarkably, intestinal macrophages also do not produce proinflammatory cytokines in response to TLR ligands, likely because of extracellular matrix (stromal) transforming growth factor-β (TGF-β) dysregulation of nuclear factor (NF)-κB signal proteins and, via Smad signaling, expression of IκBα, thereby inhibiting NF-κB-mediated activities. Thus, in noninflamed mucosa, resident macrophages are inflammation anergic but retain avid scavenger and host defense function, an ideal profile for macrophages in close proximity to gut microbiota. In the event of impaired epithelial integrity during intestinal infection or inflammation, however, blood monocytes also accumulate in the lamina propria and actively pursue invading microorganisms through uptake and degradation of the organism and release of inflammatory mediators. Consequently, resident intestinal macrophages are inflammation adverse, but when the need arises, they receive assistance from newly recruited circulating monocytes.

Figure 1. Mucosal exposure to pathogens and host response

The mucosal epithelium presents a formidable structural and chemical barrier to ill-intentioned microbes, which gain entry into the lamina propria (1) by dendritic cells, (2) through disrupted or injured epithelium, (3) between or through intact epithelial cells, and (4) through M cells. In noninflamed lamina propria, macrophages phagocytose and kill microbes without an inflammatory response. In the present of inflammation and dysregulated mucosal homeostasis, newly recruited monocyte-derived macrophages (and other innate host defense cells) generate transforming growth factor-β (TGF-β) and chemokines to recruit monocytes and other leukocyte populations to contain the infection. Microbes and/or their products are transported by dendritic cells in the lymphatics to draining lymph nodes where they induce an adaptive immune response. Failed clearance of the microbe and/or an inability to restore homeostatic regulation because of genetic and environmental factors can perpetuate inflammation locally and/or systemically (illustration by Lydia Kibiuk, NIH).

Figure 2. Myeloid cell recruitment, activation, and regulation

Pluripotent stem cells differentiate into myeloid progenitors in the bone marrow where, under the influence of growth and colony stimulating factors, the cells differentiate into monoblasts. Monoblasts in turn develop into monocytes and migrate from the bone marrow into the blood where they circulate for several days and then are distributed to the tissues either as resident macrophages or as recruited macrophages at sites of infection and inflammation. Resident macrophages are phenotypically and functionally influenced by their environment, which varies with the anatomical location. During an inflammatory reaction, circulating monocytes adhere and migrate across the endothelium in response to chemoattractant gradients generated by stromal and inflammatory cells. Depending on the extracellular milieu, the cells may become polarized, exhibiting characteristics of M1 and/or M2 macrophages.

Figure 3. Influx of macrophages into the lamina propria in response to infection or inflammation

(a) Normal intestinal tissue (ileum) in which CD68⁺ macrophages are distributed throughout the lamina propria. (b) Infection of an immunocompromised individual with M. avium (identified by red acid fast stain) results in striking increases in macrophage accumulation and phagocytosis of the bacteria in the lamina propria. (c,d) Conditioned medium obtained from normal/noninflamed intestinal tissues and from inflamed (inflammatory bowel disease (IBD)) tissues (ileum) reveals a significant elevation of monocyte-targeted chemokines including MCP-1/CCL2 (monocyte chemotactic protein-1/chemokine (C-C motif) ligand 2) and MIP-1β/CCL4 (macrophage inflammatory protein-1/chemokine (C-C motif) ligand 4), which contribute to the recruitment of monocytes into inflamed or infected intestinal sites. Chemokine levels were determined using a bead-based multiplex cytokine assay (Biosource International, Camarillo, CA).

Figure 4. Constitutive and inducible expression of transforming growth factor-β (TGF-β)

(a) Section of intestinal mucosa shows staining in epithelial cells (arrows) and cells morphologically consistent with lymphocytes, and mast cells (arrows), and the lamina propria matrix. (b) TGF-β is secreted as a large latent complex that contains extracellular matrix (ECM) binding domains and is retained in a latent state while attached to ECM. Once TGF-β is activated and released from the ECM, it can interact with TGF-β receptors on target cells to initiate Smad-dependent and -independent signaling (adapted from ref. 133). (c e) Lymphocytes and macrophages were isolated from noninflamed intestinal mucosa and RNA prepared for reverse transcriptase PCR (RT-PCR) for (c) TGF-β, (d) TGF-βR1 and TGF-βR2, and (e) forkhead box p3 (Foxp3). Results were obtained using the 2^−ΔΔCt method and expressed as fold difference relative to isolated peripheral blood monocyte populations.