What are the two main functions of GALT? How does GALT differ from the systemic immune system?
Generally, how is an immune response initiated in GALT?
The GALT has two main goals: the clearance of pathogens and maintenance of homeostasis. In contrast to the systemic immune system whose focus is to act quickly in seconds of encountering foreign antigen, the GALT is poised to respond, but is often tolerant, rejecting foreign antigen but allowing beneficial ones to persist without evoking an immune response. In general, this is in part because to elicit a mucosal immune response, antigens must be transported across the epithelium before they can be processed and presented to cells of immune system. Despite developing multiple effective defense mechanisms the mucosal immune system is still vulnerable to infection due to this unique environment.
T or F: Intestinal epithelial cells (IECs) are active immune cells.
What stimulates IEC activation? What receptors do these molecules act on?
What is secreted from activated IECs?
What are Paneth cells? Where are they located? What is their function?
Innate mechanisms of the mucosal immune system include the mucosal barrier, PAMPs, antimicrobial peptides, immune cells, and the intestinal flora as compared to adaptive immune cells such as T cells and IgA (review). Innate immune system is the first line of defense from a stimulus, has multiple cell types, an absence of specificity, and lack memory. Intestinal Epithelial Cells (IECs) form the physical barrier in GALT as described above, but IECs are also active immune cells that constitutively participate in antigen trafficking and recognize and react to components of bacteria or viruses (pathogen associated molecular patterns, PAMPs) via pattern recognition receptors (PRRs). PRRs include either Toll-Like Receptors (TLRs) on both the apical and basal surface of the cell or NOD1/2 which are intracellular. TLRs recognize various microorganism components of bacteria such as lipopolysaccaride (LPS) which is recognized by TLR-4. In addition, specialized IECs, Paneth cells of the small intestine can secrete antimicrobial peptides which can rupture the bacterial cell and limit bacterial translocation. Reacting to pathogens epithelial cells can produce pro-inflammatory cytokines (IL-1, TNF-D) or anti-inflammatory cytokines such as (IL-10, PPAR-J) depending on the stimulus. Commensal bacteria are essential to interact with IEC via TLR and maintain normal small bowel and colonic health.
Where is the lamina propria (LP) in relation to the epithelium of the gut? What is contained within the LP?
The Lamina Propria (LP) located just beneath the epithelium contains billions of activated plasma cells, IgA, as well as CD4/CD8 T cells, macrophages, NK cells, dendtritic cells, eosinophils, and mast cells.
What type of T-cell (CD4 or CD8) is typically found in the epithelium? What kind of integrin do these T-cells express? What does it bind to? What is the function of these cells?
What kind of integrin do T-cells of the LP express?
What molecule do T-cells of both the LP and epithelium express that recruits them from the blood stream?
What frequently occurs to T and B cells of the LP to limit the inflammatory response?
1. Intraepithleial Lymphocytes (IELs) are specialized lypmphoctyes present in the epithelium and not the lamina propria. They usually number 10-15 lymphocytes/100 epithelial cells, and are predominately CD8+ T-cells. They have an activated appearance and contain perforin and granzyme in intracellular granules. IELs will kill an infected, stressed, or damaged IECs. CD8 T-cells in the epithelium express αEβ7 integrin which binds to E cadherin on epithelial cells.
2. T-cells of the LP (mostly CD4) express α4β7 integrin.
3. T-cells of both the LP and epithelium have the CCR9 receptor which recruits them from the circulation.
4. T and B cells in LP frequently undergo apoptosis limiting the normal inflammatory effects.
What is oral tolerance? What is oral tolerance dependent upon?
What immune cell regulates this process? What cytokines does it secrete?
Of the macromolecules, which is the most immunogenic?
Despite the gut being the largest lymphoid organ of the body, active inflammation is not normally present. The lymphoctyes of the lamina propria have a massive influx and activation known to be caused by colonization of bacteria, but do not have normal expansion or inflammation. This well documented phenomena is related to several key concepts such as oral tolerance and bystander suppression. Oral tolerance can be defined as the active, antigen specific non-response to antigens administered orally. An example of oral tolerance is the development a decreased systemic response to ovalbumin in mice following oral ingestion. (Maureen G et al., J of Immunology, 1986). Oral tolerance is dependent on the dose of exposure and form of exposure. Proteins are the most immunogenic with carbohydrates and fat less so. Large antigen doses induce more tolerance than small. This process appears to be mediated through expansion of T regulatory cells (CD4+, CD25+, FoxP3) that promote the production of transforming factor-β (TGF-β). In addition, co-administration of an antigen after oral tolerance causes bystander suppression, showing the effector arm of oral tolerance can be antigen non-specific.
What is the first layer of the intestinal barrier? What is the function of this barrier? What is the physical component of the intestinal barrier?
Where in the GI tract is there the most commensal bacteria? What are the functions of commensal bacteria? What can loss of commensal microbiota lead to? Why does this occur?
The barrier of epithelial cells is a biologically active structure that interacts with the environment. The first layer of the intestinal barrier is a mixture of glycoproteins (mucins) that decrease the ability of microorganisms to penetrate the intestine and trap antigens. Beneath the mucins are the epithelial cells joined by tight junction proteins complexes which is the physical component of the barrier. Substances can pass through the actual epithelial cell (transcellular) or through the intercellular spaces (paracelluar). The commensal microbiota are also an important component of the normal intestinal barrier and loss of intestional bacteria can lead to infection (C.Diff). The colon has the largest amount of commensal bacteria.
- May be refered to as normal flora, commensals, or microbiota
- but there is no "normal"!
- but microbiome is different in healthy and diseased individuals
- >400 species of commensals
- Compete with pathogenic bacteria
- Assist with enzymatic breakdown of food
What are defensins? What is their function? What cells secrete them?
- short peptides
- disrupt the cell membrane of bacteria and fungi
- alpha defensins
- produced by neutrophils, some macrophages and Paneth cells of the SI
- beta defensins
- produced by macrophages, granulocytes, NK cells and some epithelial cells
Where are M cells located? What is their function? Describe their characteristics.
What is contained within Peyer's patches?
Adaptive (acquired) Immune response include functions of antigen uptake and antigen presentation. The GI immune system has develop several unique mechanisms to achieve this. Peyer’s Patches (containing B-cells, T-cells, and APCs) are follicle associated epithelium which contain microfold (M cells). These structures allow special uptake and sampling of antigen. M cells contain no microvilli; a limited mucin layer, broader microfolds, and do not secrete lysozymes so are constantly exposed to antigen. M cells transport antigen from gut lumen to immune cells across epithelial barrier by endocytosis and deliver it APCs essentially bypassing normal IECs transport.
What are the professional APCs? What are the non-professional APCs?
How can DCs acquire antigens independent of M-cells and transport across the epithelium?
Where in the gut are DCs most abundant? Where can they go once exposed to antigen? What is their most frequent response to antigen?
What do DCs frequently interact with in the gut to maintain the controlled/physiologic inflammation characteristic of the mucosal immune system?
Dendritic Cells are APCs that can acquire antigens across an intact epithelial barrier without the need for M cells or transport of antigen across the epithelial barrier. They are abundant mainly in the lamina propria, and may travel to mesenteric lymph nodes once exposed to antigen to interact with naïve T cells, but generally produce an anti-inflammatory response such as IL-10. DCs frequently interact with commensal bacteria and both are essential to maintain the normal in the controlled/physiologic inflammation that is characteristic of the mucosal immune system.
What cytokines do TH17 cells secrete? What is their functin?
TH17 cells secrete IL-17, IL-21, IL-22, and IL-26 to enhance the neutrophil response to fungal and extracellular bacterial infections.
What is the major site of synthesis of IgA? What is the effect of IgA?
IgA is the hallmark of GALT immune responses, the major effector of the adaptive humoral immune response in the GI tract, and due to the large surface area of the GI tract is the most abundant antibody in the body. The lamina propria is the major site of sIgA synthesis, and is secreted in a soluble form (SIgA). Secretory IgA (SIgA) is a dimeric form of two IgA monomers bound together by a J chain. Subsequently, the dimer binds to a secretory component known as polymetric Ig receptor, a highly specialized 55-kd glycoprotein produced by IECs. SIgA is then transported into the intestinal lumen by a within a vesicle to the apical surface. SIgA has a mainly an anti-inflammatory effect and does not activate classic complement components. SIgA will bind to luminal antigens and promotes agglutination and removal of the antigen in the mucous layer overlying the epithelium.
How common is IgA deficiency? What are the sx of this disease? What is the reason for the presentation of individuals with IgA deficiency?
• Common 1:500 to 1:700 of Caucasians
• Most individuals have no symptoms
• IgM may replace IgA in secretions
• IgM is also J-chain linked and binds poly-Ig receptor
What is Celiac disease?
What molecule is present in patients with Celiac disease? What is the function of this molecule in the disease? What are the downstream effects of this molecule that lead to the pathlology of Celiac disease? State the cells and molecules involved.
What is the earliest pathologic abnormality of Celiac disease? What other pathologic abnormalities occur?
Celiac Disease is a specific autoimmune of the small bowel that is cause a reaction to a protein, gluten, that is found in wheat.
Celiac pts test positive for tissue transglutaminase (TTG) which deamidates a gluten protein (α-gliadin) which convers glutamine to glutamic acid.
This negatively charged reside binds strongly to MHC II HLA-DQ2 on APCs. APCS present to CD4+ T-cells and activate them. Activated CD4+ T-cessl induce production of IFN-γ which activates the epithelial cell and other immune cells. Also, CD4+ cells can kill mucosal epithelial cells through Fas ligand.
Epithelial cells undergo stress-infection, altered cell growth, and are exposed to toxi peptide (such as gluten protein). These stressed epithelial cells upregulate non-classical MHC molecules such as MIC which bind to NKG2D on IELs triggering apoptosis of the epithelial cell. Also, epithelial cells secret IL-15 which activated IELs.
Diagnostically, small bowel biopsies are graded by the “Marsch Classification” which includes the earliest pathologic abnormality seen in celiac disease which is an increase in the number of IEL on small bowel biopsy. Further exposure to the antigen can lead to villous blunting, malabsorption, and weight loss. Small bowel biopsies may be normal if the patient is on a gluten free diet, so gluten should be reintroduced into the diet prior to testing. Patients with celiac disease are also known to have increased intestinal permeability.
What are the two types of IBD? Where do they typically present? (what part of the GI tract)
IBD is a chronic autoimmune condition of unclear etiology. It has two phenotypes: Crohn’s Disease (CD) & Ulcerative Colitis (UC). CD has transmural involvement, commonly affect the distal small bowel (ileocolic area), but can involve any part of the GI tract. UC affects only the colon, is a more superficial disease, and always involves the rectum.
What is the general response to antigens in IBD? What do patients often report prior to onset of IBD?
IBD is known to have an altered mucosal immune response to antigens, and many patients report a GI infection that preceded development of the disease.
How is the intestinal permeability and translocation of luminal proinflammatory bacteria altered in IBD?
How are cytokine levels/balance affected in IBD?
Polymorphisms of what proteins increase risk of developing IBD?
Patients with IBD are known to have increased intestinal permeability, and translocation of luminal proinflammatory bacteria and endotoxins. Also polymorphisms in the NOD2 gene, which are the intracellular component of PAMPs, increase the risk of developing IBD 20-40x. Immunologically patients with IBD are known to have a loss of balance between inflammatory (IL-1, IL-6, TNF-α) versus tolerance (IL-10, TGF-β) provoking cytokines. This balance in IBD is also seen in T cell regulation between inflammatory Th-17 cells and T regs. Th-17 cells, which produce IL-23, are important in autoimmune inflammation and polymorphisms of the IL-23 receptor also increase the risk of developing IBD.