Gastrointestinal immunology

At body surfaces, mammalians share their life with a complex commensalic and symbiotic microflora (SUCHODOLSKI 2011). Therefore, it is important to adapt to this microbial load to protect the specific ecosystem while ensuring body integrity.

Alongside the gastrointestinal border, which holds the largest microbial population, many unspecific and innate defence mechanisms are established to prevent the penetration of the host (TIZARD 2012a). For instance, the intestinal mucosa is covered by a continuous epithelial layer closely connected by tight junctions. This cellular border establishes the anatomical separation of luminal organisms from the mucosa, expresses the pathogen recognition receptors, such as toll-like receptors (TLRs), and secretes several antimicrobial peptides (LOTZ et al. 2007). Additionally, specialised secreting goblet cells produce large amounts of mucin that form a mucous layer covering the epithelial surface and thus prevent microbial epithelial penetration (CORFIELD et al. 2000). Moreover, commensal bacteria themselves can block the proliferation of pathogens by influencing the local environment, such as pH and oxygen concentration (FUKUDA et al. 2011).

In addition to unspecific defence mechanisms, the gut contains a complex adaptive immunological network (Figure 1) that allows antigen specific interventions and ensures ‘toleration of the friend but elimination of the foe’ (WORBS et al. 2006;

RAMIRO-PUIG et al. 2008). This network includes mesenteric lymph nodes (MLNs) and the so-called gut-associated lymphoid tissue (GALT) comprising organised lymphoid structures – Peyer’s patches (PPs) and isolated lymphoid follicles (ILFs) – as well as diffuse parts including the lamina propria (LP) and surface epithelium (BRANDTZAEG et al. 2008).

M cell

Figure 1. Organisation of the adaptive intestinal immune system.

The mucosal immune system comprises structured parts, such as PPs and MLNs, and diffuse elements including the LP and surface epithelium. Antigen uptake, processing and presentation to naïve T and B cells take place within PPs and MLN resulting in the induction of adaptive immune responses (inductive sites).

Subsequently, generated effector cells, including plasma cells and T lymphocytes, migrate to the intestinal mucosa, where they are randomly scattered throughout either the LP or the epithelium. Therefore, diffuse compartments host many effector functions, such as plasma cells contributing to the formation of S-IgA (effector sites).

However, antigen uptake may also occur within the LP.

DC = dendritic cell; F = follicle; GC = germinal centre; IFR = interfollicular region; LP

= lamina propria; M cell = microfold cell; MLN = mesenteric lymph node; PP = Peyer’s patch; SED = subepithelial dome; S-IgA = secretory immunoglobulin A.

PPs – firstly described in 1677 by the Swiss anatomist and physician Joseph Hans Conrad Peyer (MAKALA et al. 2002) – are prominent lymphoid structures of the small intestine that are believed to be the main source of immunoglobulin (Ig) A-producing plasma cells (FAGARASAN and HONJO 2003). Anatomically, PPs are covered by a

follicle-associated epithelium (FAE) containing distinct cells with a microfold apical cellular membrane (M cells) that are specialised in uptake of luminal antigens (VON ROSEN et al. 1981). Basally, M cells transfer the antigens to dendritic cells (DCs), which are located in subepithelial domes (SEDs). Subsequently, DCs present those antigens to naïve lymphocytes located in underlying follicles, which is followed by the formation of germinal centres and eventually results in the generation of specific effector lymphocytes (MAKALA et al. 2002). In addition to antigen presentation and generation of effector cells, which classically takes place in secondary lymphoid organs, the ileal PPs of sheep, cattle and pigs are important for primary B cell development and therefore act as primary lymphoid tissue (BARMAN et al. 1997;

YASUDA et al. 2006). In dogs, the small intestine comprises a total of 26-29 PPs that differ between anatomical parts (HOGENESCH et al. 1987). The jejunum and upper ileum contain small discrete PPs, while the PP that totally encircles the distal ileum measures 26-30 cm in length (HOGENESCH and FELSBURG 1992). Additionally, canine duodenal PPs normally show intrafollicular invaginations of the SED (HOGENESCH and FELSBURG 1990).

Although PPs were initially described in the small intestine, similar structures exist in the large bowel of several species, for instance as so-called lymphoglandular complexes in dogs (ATKINS and SCHOFIELD 1972). In addition to PPs and their large intestinal counterparts, the canine stomach contains many ILFs that are randomly scattered throughout the gastric mucosa without association to a FAE (KOLBJORNSEN et al. 1994).

Once they are generated in the so-called inductive sites, mature effector cells migrate into diffuse GALT parts where they exert their specific effector functions (BRANDTZAEG and JOHANSEN 2005; AGACE 2008). Therefore, the LP is a main reservoir of mature B cells, plasma cells and cluster of differentiation (CD) 4 positive T cells – making it difficult to decide whether the mucosa is inflamed or not by means of histopathological examination (DAY et al. 2008). LP plasma cells are proficient in the production of specialised mucosal dimeric immunoglobulins known as IgA (WOOF and KERR 2006). Once they are produced, IgA dimers bind to the basal surface of enterocytes followed by their transepithelial transport and secretion

(secretory IgA, S-IgA) into the intestinal lumen (JOHANSEN and KAETZEL 2011).

Luminal S-IgA is capable to agglutinate antigenic particles, to neutralise viruses and to prevent the adherence of invading microbes to the intestinal surface (WOOF and MESTECKY 2005). Similar to M cells of PPs, the intestinal LP also contains cells responsible for the detection and uptake of antigens followed by their migration to MLNs (BIMCZOK et al. 2005; SCHULZ et al. 2009).

In addition to the LP, the intestinal epithelium contains many intraepithelial lymphocytes (IELs) of which the majority are T cells predominantly expressing CD8 (HAYDAY et al. 2001). Functionally, IELs are suggested to exhibit both cytotoxic and regulatory properties (CORAZZA et al. 2000; LUCKSCHANDER et al. 2009).

Although they are not covered by the definition of organised GALT, the draining MLNs are important parts of the intestinal immune system as they perform antigen processing and generation of related effector cells similar to PPs (BRANDTZAEG et al. 2008). Furthermore, their role in intestinal homoeostasis seems to be independent of the presence of PPs, as mice lacking PPs but possessing MLNs can indeed develop oral tolerance (SPAHN et al. 2002). However, it remains unknown whether PPs and MLNs exhibit different functions or if they just comprise a double layer of defence.