The innate immune system is responsible for the immediate response to pathogens and unlike adaptive immunity it is non-specific and has no lasting memory. It is generally involved in bacterial killing, antigen
presentation, and initiating the further release of pro-inflammatory cytokines. In the gut of IBD patients, the innate immune response is triggered when microbes infiltrate the disrupted epithelial cell barrier in genetically susceptible hosts. There are several important cell types within the innate immune system that work synergistically to provide a rapid response to invading pathogens. Several features of the structure and physiology of the gut provide protection and have a valuable role in the innate immune system and are discussed below.
Epithelial cells provide a physical barrier with tight junctions, adheren junctions, and apical junction complexes between cells, to protect against the luminal contents, also relying on an inner and outer mucous barrier for protection. Within the epithelial barrier goblet cells release gel-forming mucins to form a protective barrier, where the inner layer is sterile due to the release of anti-microbial peptides from paneth cells, specialised to the base of the crypts of the small intestine. The epithelial barrier in UC patients if often reported to be deficient in goblet cell production of mucins which expose the host epithelial barrier to microbes. CD patients have a deficiency in anti-microbial peptides released from the epithelial wall, in particular the β-defensins HBD2, HBD3, and HBD4 (Wehkamp, 2003).
The toll-like receptors and NOD-like receptors are vital first responders which recognise the structural motifs of micro-organisms termed pattern associated molecular patterns (PAMPS), resulting in downstream cytokine release and T-cell activation which can bridge into the adaptive immune response. In the healthy gut, TLR activation results in a tolerance to pathogens and a down-regulation of pattern recognition receptors in order to promote mucosal tissue healing and repair, whereas in IBD the continued immune response results in an increased epithelial barrier permeability and an impaired mucosal healing process triggering IL-8 release from epithelial cells and attracts neutrophils and resident macrophages.
Nucleotide-binding oligomerisation domain-containing protein 1 and 2 (NOD1 and NOD2) are cytosolic proteins within epithelial cells and paneth cells which respond to bacteria infiltration activating NF-κB and MAPK gene transcription for downstream innate and adaptive immune cells and mediators (Barnich, 2005). The NOD proteins are important in maintaining the integrity of the epithelial barrier by contributing to tissue homeostasis thought to be through tonic stimulation by gut microbiota which results in the release of defensins, although the precise mechanisms are not yet fully understood (Rubino, 2012).
In 2001, NOD2 became the first gene to be associated with CD and remains a strong risk factor to developing the disease (Hugot, 2001). In the absence of NOD2, mouse models show significant epithelial layer damage likely as a result of defects in the recovery of mucosal tissue damage (Kobayashi, 2005). Although important in the development of disease, NOD2 signalling results in downstream release of several pro-inflammatory cytokines and chemokines such as TNFα, IL-6, IL-8, CCL2, CXCL2, which attract neutrophils and monocytes and play a role in driving Th2 immunity (Rubino, 2012). In general, TLR’s are understood to stimulate Th1 immunity and NOD signalling drives Th2 immunity through activating dendritic cells, regardless of the fact that Th2 immunity is classically derived from extracellular pathogens. In addition, a NOD2 variant has also been reported to inhibit the expression of the anti-inflammatory cytokine IL-10 in human monocytes suggesting that NOD2 mutations may also result in compromised immune regulation (Noguchi, 2009). Dendritic cells reside along the epithelial barrier and reach into the luminal contents to sample the local environment. Once activated they signal through IL-23 to naive CD4+ T-cells which proliferate into Th or T- reg cells to induce the adaptive immune response.
Macrophages are leukocytes responsible for the detection, phagocytosis, destruction of bacteria and dead cells, and the releases of cytokines and proteases which further promote inflammatory cell involvement. Traditionally, they are categorised as either M1 (classical) or M2 (non-classical), where M1 macrophages proliferation from CD14+ monocytes is induced by IFN-ɣ and microbial products resulting in phagocytosis of microbes and the release of pro-inflammatory cytokines such as IL-1β, TNFα, IL-6, IL-8, IL-23, and further INF- ɣ release. The M2 macrophages typically proliferate from CD14+/CD16+ monocytes induced by IL-4 and IL-13 cytokines and promote anti-inflammatory signalling (Busch-Dienstfertig, 2012). Approximately 90% of macrophages observed in IBD are M1, and active CD also results in reduced M2 activity (Hunter, 2010). Therapies targeting IBD may also indirectly effect the ratio of macrophage subtypes with anti-TNFα therapies reportedly increasing the level of M2 macrophages within the mucosa which may contribute to the treatment effect (Vos, 2011).
Innate lymphoid cells (ILC’s) have been studied for a long time, distinctly NK cells, but emerging research focuses on a relatively recent series of 3 subsets (Bull, 1977). Together, ILC’s respond rapidly to cytokine and microbial signals to release cytokines which help modulate the adaptive immune response. ILC’s are
particularly enriched in tissues such as the skin, lung, and intestinal mucosa and exhibit both anti- and pro- inflammatory characteristics.
Type 1 ILC’s express t-bet in response to signals from IL-12. They produce and release TNFα and INF-ɣ in response to intracellular pathogens (Fuchs, 2013). The more widely known example of type 1 ILC’s include NK cells, but there are subsets within type 1 ILC’s themselves.
Type 2 ILC’s respond to signals from IL-25 and IL-33 and elicit an immune response to extracellular parasites. They express the GATA3 transcription factor and when activated produce and release several cytokines such as IL-4, IL-5, IL-9, and IL-13, to encourage an inflammatory response (Moro, 2010; Neill, 2010). Due to the bacteria infiltration associated with IBD from epithelial barrier dysfunction, the type 2 ILC’s have been the focus of intense IBD research.
The type 3 ILC’s act in a similar manner to Th17 cells and respond to cytokine signalling from IL-1β, IL-23 and IL-6, where they produce and release IL-17 and IL-22, particularly in response to extracellular bacteria and fungi (Buonocore, 2010; Cupedo, 2009). Several studies have indicated that IBD patients suffer from a reduced expression of type 3 ILC’s and research is currently focused on restoring the potential benefits of these ILC’s (Buonocore, 2010; Geremia, 2011).
1.16.2. Adaptive immune system
The innate immune response is a prerequisite for excessive activation of the adaptive response which is the main driver for inflammatory damage in IBD. Unlike the innate response however, adaptive immunity is highly specific and offers long term immunity through B-cell activation. The specificity of the adaptive response comes from the proliferation of Th0 cells which differentiate based on specific signalling mechanisms (see figure 20 & 21). Th1 cells were first recognised for their ability to target intracellular pathogens and have been strongly associated with CD where they are induced by elevated mucosal levels of IL-12 and IL-18, and macrophages in CD have been shown to release high concentrations of IL-12 (Monteleone, 1997). The Th1 cells release various cytokines such as TNFα, IL-2, IL-10, signalling through STAT1 and STAT4 pathways. In addition to releasing TNFα themselves, Th1 cells also induce the release of TNFα by activating mucosal macrophages, which causes the differentiation of stromal cells into
myofibroblasts, thereby releasing MMP’s. The Th2 cells focus on targeting parasitic invasions, and are crucial in allergic reactions. Several Th2 cytokines such as IL-5, IL-1β, IL-6, are elevated in UC compared with CD leading researchers to suggest that UC is a Th2-dominant disease, which has been shown to signal through STAT6 and GATA3 (Rosen, 2011). Although CD and UC present with Th1- or Th2-dominant pathologies, the Th17 cell type shares common involvement with both diseases. The Th17 cells target extracellular pathogens and fungi, and in IBD release cytokines such as IL-17, IL-21, and IL-23, significantly contributing to the pathogenesis of IBD. They differentiate as a result of signals from cytokines such as IL-6, IFN-ɣ, and TGF-β, and the cytokines IL-21 and IL-23 also promote Th17 cell proliferation which in turn release more IL-23 in a positive feedback loop. Furthermore, IL-23R polymorphisms have been associated with IBD in genomic studies suggesting that this specific cytokine pathway may be pivotal in IBD (Newman, 2009). IL-21 can stimulate myofibroblasts to produce and release MMP’s leading to local tissue destruction, and IL-21 also leads to the attraction of further T-cells at inflammatory sites. The IL-17 released from Th17 cells has a significant role in IBD. IL-17 can attract neutrophils to local sites of inflammation which in turn releases cytokines such as IL-8. IL-17 is also central in the up-regulation of nitric oxide and IL-1β, both of which are pro-inflammatory mediators, and stimulates macrophage release of further pro-inflammatory cytokines (Awane, 1999).
T-regulatory cells (Treg) are a subset of CD4+ T-cells that suppress Th
0 proliferation thereby suppressing the
immune response (Schevach, 2009). IBD models have established that dysfunction of Treg cells can lead to increases in transcription factors t-bet, STAT1, and NF-kB, potentiating T-cell activation and the adaptive immune response (Ishimaru, 2008).
There are several subsets of Treg cells within multiple locations in the body each with differing characteristics. Natural Treg (nTreg) cells are found within the thymus and secrete IL-10 and TGFβ to have a broad range of suppressive actions on T-cell proliferation, dendritic cell and T-helper cell activity. They become activated predominantly through the IL-2 signalling pathway and mice with IL-2 signalling deficiencies develop spontaneous IBD-like pathology (Setoguchi, 2005). Peripheral Treg (iTreg) cells have similar actions to nTreg cells but express CD4 and FOXP3 markers with an absence of CD25. They develop from CD4+ T-cells during the immune response through TFGβ signalling (Chen, 2003). The tissue-resident
FOXP3+/CD25+ Treg cells that are a main interest in IBD mucosal tissue are generated from nTreg cells and are highly expressed within GI tissue. The Treg cells negatively regulate T-cell function by signalling through released IL-10, IFN-ɣ, and TFGβ, and have been shown to specifically down-regulate the Th17 response in mice via TGFβ (Schevach, 2009; Read, 2006; Harrison, 2015; Nava, 2010). Transfer of Treg cells into IBD mouse models has also demonstrated this inflammatory control whereby IBD-like lesions and mucosal inflammation were reduced, suggesting that Treg cell dysfunction plays a major role not only in the pathogenesis of IBD but also during active disease (Martin, 2004).
Treg cells have been shown to be reduced in IBD patients although some authors reports that the suppressive properties remain active (Maul, 2015). An endogenous inhibitory molecule for Treg function has been identified as a therapeutic target with promising results in animal models (Monteleone, 2001; Fahlén, 2005). Anti-TNFα treatment in IBD patients has been shown to positively effect FOXP3+/CD25+ Treg frequency and potentiate the suppressive properties which coincide with the inflammatory control through reduced TNFα activity (Boschetti, 2011). Therefore current treatments may enhance Treg actions and contribute to inflammatory control.
Figure 5. The pathogenesis of Crohn’s disease IL-8 CCL20
IL-6
IL-12
TNFα
T-bet RORɣT IFN-ɣ IL-6 TNFα IL-17A IL-17F IL-21 IL-22CD
(Th17)
(Th1)
IL-23Th17
Treg
IL-23
IL-21 IL-23Figure 6. The pathogenesis of ulcerative colitis
Dendritic cell: NIH Public domain T-cell/ neutrophil: Blausen.com Staff (2014)
B cell: https://www.biooncology.com/pathways/cancer-tumor-targets/b-cell.html NK cell: biolegend.com NK cell Antibodies e.g.pANCA