Mechanisms and consequences of gut commensal translocation in chronic diseases

Abstract

Humans and other mammalian hosts have evolved mechanisms to control the bacteria colonizing their mucosal barriers to prevent invasion. While the breach of barriers by bacteria typically leads to overt infection, increasing evidence supports a role for translocation of commensal bacteria across an impaired gut barrier to extraintestinal sites in the pathogenesis of autoimmune and other chronic, non-infectious diseases. Whether gut commensal translocation is a cause or consequence of the disease is incompletely defined. Here we discuss factors that lead to translocation of live bacteria across the gut barrier. We expand upon our recently published demonstration that translocation of the gut pathobiont Enterococcus gallinarum can induce autoimmunity in susceptible hosts and postulate on the role of Enterococcus species as instigators of chronic, non-infectious diseases.

Keywords: Gut vascular barrier; TLR7; autoimmune liver disease; autoimmunity; bacterial translocation; enterococcus; gut lymphatic barrier; intestinal permeability; lupus; microbiota; tight junctions; vancomycin resistance.

Figure 1.

Gut barrier breach by the gut pathobiont E. gallinarum in autoimmune-prone hosts. Beyond the intestinal epithelial barrier (IEB), the gut vascular (GVB) and gut lymphatic barriers (GLB) shield the internal organs from colonization by commensals breaching the inner mucus layer and epithelial lining. (a) In (NZW x BXSB)F₁ mice, a leaky gut barrier allows E. gallinarum to translocate beyond the intestinal epithelium. In monocolonized mice, E. gallinarum downregulates barrier molecules related to both the GVB and GLB,⁵ suggesting that it can breach all barrier components in the gut. Alternatively, it could be carried by host cells into host tissues. Once past the IEB and inside the lamina propria, E. gallinarum travels via the mesenteric veins (blue) and lymphatics (green) to the so-called “firewalls”, the liver and mesenteric lymph nodes, respectively. Within these organs, E. gallinarum interacts with host immune and epithelial cells to promote autoimmunity.⁵ (b) In a non-autoimmune-prone host, the intestinal epithelia, GVB, and GLB are intact. At steady state, antigen-presenting cells (orange) sample luminal bacteria to induce homeostatic IgA responses. T- and B-lymphocytes (blue) participate in this process leading to T cell-dependent and -independent IgA. The GVB is supported by enteric glial cells (yellow) and pericytes (dark green) surrounding a fenestrated endothelium sealed by tight junctions.^8.

Figure 2.

Factors influencing translocation of bacteria and extraintestinal autoimmunity. Autoimmunity arises in genetically susceptible hosts that are exposed to environmental stimuli that accelerate loss of self-tolerance. Barrier leakiness can either be intrinsic to the host or induced by external factors such as diet or medications. Bacteria that are normally contained in the gut lumen can gain access to extraintestinal tissues in response to enhanced intestinal permeability and via intrinsic bacterial mechanisms. The capability of bacteria to translocate depends on virulence factors, their abundance and proximity to the epithelial barrier, and their ability to compete within the gastrointestinal niche and evade immune defenses. Once translocated, bacteria such as E. gallinarum colonizing internal organs can directly induce autoimmunity by interacting with host tissues or indirectly via metabolites and their influence on the innate and adaptive immune system. The examples listed for each factor (host genetics, gut bacteria, modulators/accelerators) can impact translocation, autoimmunity or both (for instance, diet can influence barrier function as well as autoimmune responses). MHC, major histocompatibility complex; NSAIDs, non-steroidal anti-inflammatory drugs; PPIs, proton pump inhibitors.

Figure 3.

Small intestinal organoids from autoimmune-prone mice downregulate gut barrier molecules upon exposure to E. gallinarum. Ileum tissue was dissected from 12-week-old (NZW x BXSB)F₁ mice. Crypts were isolated and cultured for 7 days in IntestiCult Organoid Growth Medium (STEMCELL Technologies) and Matrigel Matrix (Corning). On day 7, heat-killed E. gallinarum (EG) or EG RNA, as prepared in ref. 5, or medium was added to the organoid cultures (n = 3) for 1.5, 3, 6, and 12 h. RNA was extracted and RT-qPCR performed as described in ref. 5. RNA expression of the barrier molecules claudin-3 and claudin-5, as well as the mucus protein mucin-2, were quantified in relation to actin. Blue lines, media alone; red lines, EG lysate; green lines, EG RNA. Data are presented as mean ± SD in (a) to (c); *P < 0.05, **P < 0.01, ***P < 0.001; ANOVA followed by Bonferroni multiple-comparisons test.

Figure 4.

Vancomycin resistance gene restriction fragment length patterns for E. gallinarum and E. faecalis. E. gallinarum was grown in increasing concentrations from 1 μg/ml to 8 μg/ml of vancomycin. Restriction digest with MspI and multiplex PCR was performed as described in ref. 56 to determine vancomycin resistance genes. E. gallinarum normally carries the low-level vancomycin resistance vanC-1 operon (fragment sizes at 230/237 bp). E. faecalis carries the high-level vancomycin resistance vanB operon (fragment sizes at 136, 160, 188/189 bp). Horizontal transfer of this operon to E. gallinarum can occur within gut microbiomes (not shown). Depicted below the gel is the in vitro growth of E. gallinarum without the vanB operon. Normal growth at concentrations between 1 and 4 μg/ml is noted with diminishing growth starting at 8 μg/ml and little to no growth at 20 μg/ml and beyond (not shown).

Figure 5.

Network of predicted biosynthetic gene clusters (BGCs) from E. faecium, E. faecalis, E. gallinarum, and E. casseliflavus genomes according to Rashidi et al. Figure reproduced with permission from ref. 60. Unique strains are represented by colored nodes. Connecting lines indicate at least 75% similarity of BGCs between two strains. The weight of each line is proportional to the number and strength of BGC connections between two strains. Thicker lines represent more frequent higher-similarity pathways. Secondary metabolite profile predictions revealed distinct clusters. Intrinsically vancomycin-resistant E. gallinarum (orange) and E. casseliflavus (red) cluster together whereas E. faecalis (green) and E. faecium (blue) cluster independently from each other.