Chapter 10- Preventing Infection at Mucosal Surfaces

Question 1

Mucosa

Answer 1

Epithelial tissue that lines the respiratory, intestinal, and urogenital tracts and secretes mucus. Also includes the conjunctiva, lacrimal glands, salivary glands, pancreas, and lactating breasts. Mucosal epithelium communicates with the external environment and is the route of entry for most pathogens

Question 2

Mucus

Answer 2

The protective secretion produced by the mucosa. It is made of large glycoproteins, proteoglycans, peptides, and enzymes, which are produced by goblet cells in many internal epithelia. The glycopeptides are composed of polypeptide chains that are 10,000 amino acids in length.

Question 3

Mucosa physiological functions

Answer 3

Gas exchange, food absorption, sensory activity (eyes, nose, mouth, and throat), and reproduction (uterus, vagina, breast). These functions require mucosal surfaces to be dynamic, thin, and permeable barriers to the interior of the body. These properties make the mucosa vulnerable to infection, but they have defense mechanisms

Question 4

Secretory IgA

Answer 4

The dimeric form of IgA, which is present at mucosal surfaces. Produced by the plasma cells in mucosal tissues

Question 5

Specialized defense mechanisms of mucosal surfaces (4)

Answer 5

1. Mucus, enzymes, and antimicrobial peptides
2. Normal microbiota
3. Mucosal-associated lymphoid tissue (MALT)
4. Immune response- T and B cell responses

Question 6

Characteristics of mucus

Answer 6

Mucus is viscous, which impedes microbes from accessing the internal environment. Glycosylation retains water to prevent dehydration. It also has a polyanionic (negatively charged) surface. It binds defensins, antimicrobial peptides, and IgA

Question 7

Defensins

Answer 7

Proteins that are 35-40 amino acids in length and are secreted by Paneth cells in the gut. They are produced constitutively and are effective against enteric bacteria and viruses. Defensins are considered amphipathic molecules that disrupt microbial membranes. Examples include the alpha defensins HD5 and HD6

Question 8

GI tract

Answer 8

The GI tract is 30 feet long and is composed of the small and large intestine. The small intestine is responsible for nutrient absorption, while the large intestine is responsible for water absorption and waste transport. Bacteria (called commensal or normal flora) colonize the entire GI tract- there are greater than 750 species. They play key roles in digestion and mucosal immunity, and are required for normal immune system development

Question 9

Microorganism-mediated protection in the GI tract

Answer 9

There are 3 parts, all with different bacteria- upper small intestine, distal ileum, and large bowel. The upper small intestine contains 1000 organisms per mL, and the distal ileum contains 10^5 to 10^8 organisms per mL. The large bowel contains mostly anaerobic bacteria, 10^10 to 10^12 organisms per mL. This region undergoes dynamic population growth and maintenance

Question 10

Roles of commensal gut microbes (5)

Answer 10

1. Synthesize metabolites
2. Breakdown of food products
3. Detoxification
4. Block attachment and colonization of pathogenic microorganisms
5. Required for functional development of secondary lymphoid tissues

Question 11

Which metabolites are synthesized by gut microbes?

Answer 11

Gut microbes provide metabolic building blocks that can’t be made by human cells. One example is the menaquinone precursors used to make vitamin K (essential for blood clotting)

Question 12

How do microbes assist with digestion?

Answer 12

Bacteria enhance the efficiency with which humans digest plant-based foods by providing enzymes that covert plant fibers, which are indigestible by human enzymes, into energy-rich metabolites. They create CAZymes- carbohydrate-active enzymes like dextran, plant degrading enzymes like cellulose, and play a role in the fermentation of polysaccharides

Question 13

How do microbes assist in detoxification?

Answer 13

They convert toxic substances in food or secreted by pathogens into safe derivatives. This includes environmental pollutants

Question 14

Development of the gut microbiota

Answer 14

Gut microbes are required for the functional development of secondary lymphoid tissues. They are first acquired at birth, traveling through the birth canal and consumed via breast feeding. Additionally, they are introduced with the introduction of solid foods into the diet

Question 15

Germ-free mice

Answer 15

Mice with reduced microbiota. This leads to defects in intestinal lymphoid tissue and poor immune functions. This demonstrates how important gut flora is for immune system development

Question 16

Mucosa-associated lymphoid tissues (MALT)

Answer 16

Includes NALT (nasal), BALT (bronchus), and GALT (gut)

Question 17

Waldeyer’s ring

Answer 17

A ring of lymphoid tissues around the entrance to the gut and airways, formed by the tonsils and adenoids. The ring is necessary because the mouth is a major entry point for microbes. The tonsils and adenoids can therefore become painful and swollen during infections. These organs are required for appropriate sIgA responses to vaccination- removal leads to poorer sIgA responses

Question 18

Gut-associated lymphoid tissue

Answer 18

Consists of 2 compartments- inductive and effector. The inductive compartment is directly beneath the epithelium and is responsible for development of the adaptive immune response. Interactions between antigens, dendritic cells, and lymphocytes occur here. In contrast, the effector compartment is a residence for effector T cells, plasma cells, macrophages, and mast cells. Made up of the lamina propria

Question 19

Lamina propria

Answer 19

The connective tissue underlying the epithelium and the lymphoid tissues in the gut

Question 20

Villi

Answer 20

The small intestine acts as the major site of nutrient absorption because its surface is deeply folded into finger-like projections called villi. Villi have a large surface area for absorption. It is the part of the gut most heavily invested with lymphoid tissue

Question 21

Peyer’s patches

Answer 21

Characteristic secondary lymphoid organs of the small intestines- they are organized GALT. The patches are integrated into the intestinal wall and are dome-like aggregates of lymphocytes which cause the wall to bulge out into the intestinal lumen. The patches vary in size and contain between 5-200 B cell follicles with germinal centers, interspersed with areas of T cells and dendritic cells

Question 22

Why is the systemic immune response considered reactive? (4)

Answer 22

1. Activation of tissue resident macrophages (PRRs and PAMPs)
2. Released cytokines causes inflammation and recruitment of neutrophils, NK cells, and effector T cells
3. Pathogen killed, but tissues are often damaged and disrupted
4. Inflammation and immunity are suppressed to allow repair of damaged tissues

Question 23

Why are mucosal immune responses considered proactive?

Answer 23

Mucosal tissues anticipate potential infections. They make continual adaptive responses to gut microbiota. They also have sIgA in the lumen and effector and memory T and B cells in the lamina propria to prevent infection. Inflammation is tightly regulated here, preventing tissue damage

Question 24

How is inflammation regulated in the mucosa?

Answer 24

The macrophages in the intestine are not inflammatory, and Treg cells secrete IL-10 to suppress inflammation. Gut inflammation typically exacerbates infection and can cause chronic diseases, like Crohn’s.

Question 25

Intestinal epithelial cells

Answer 25

These cells are very active in the uptake of nutrients and other materials from the gut lumen. They also have TLRs on their apical and basolateral surfaces, and NOD receptors in the cytoplasm. TLR and NOD receptor signals lead to activation of the transcription factor NFκB and formation of the inflammasome by NOD-like receptor P3

Question 26

TLRs on intestinal epithelial cells

Answer 26

TLRs are located on both the apical and basolateral surfaces. On the apical surface, they allow intestinal epithelial cells to sense bacteria that overcome the defenses of the mucus and reach the epithelium. On the basolateral surfaces, the cells sense the invading bacteria that have penetrated the epithelium

Question 27

NOD1 and NOD2 receptors

Answer 27

Located in the cytoplasm of intestinal epithelial cells. They detect components of bacterial cell walls

Question 28

NFκB secretion and inflammasome formation lead to

Answer 28

The production and secretion of antimicrobial peptides (like defensins), chemokines, and cytokines like IL-1 and IL-6 by the epithelial cells. The defensins kill the bacteria, while chemokines attract neutrophils and other cells from the blood. The inflammatory response from these cells is quick and localized, and is usually sufficient to kill the pathogen

Question 29

Chemokines in the intestine

Answer 29

During infection, chemokines attract neutrophils (via CXCL8), monocytes (via CCL3), eosinophils (via CCL4), T cells (via CCL5), and immature dendritic cells (via CCL20) from the blood

Question 30

Intestinal macrophages

Answer 30

Intestinal macrophages populate the lamina propria and are the first line of defense against infection. These macrophages have a life span of only a few months and are replenished by monocytes arriving from the bloodstream. Intestinal macrophages maintain gut homeostasis- limit any potential tissue damage and leaking of microbes from the gut

Question 31

How do intestinal macrophages develop?

Answer 31

They arrive in the gut as a functioning non-mucosal macrophage, but their functions are altered by TGF-β secreted by intestinal epithelium, stromal cells, and mast cells

Question 32

Functions of intestinal macrophages (2)

Answer 32

1. Perform phagocytosis 2. Kill pathogens

Question 33

What functions do intestinal macrophages not have? (6)

Answer 33

1. No respiratory burst 2. Activate naïve T cell (lack B7) 3. Produce cytokines 4. Perform functions of a professional APC 5. Express FcRs for IgA and IgG 6. Send signals to activate NFkB

Question 34

M (microfold) cells

Answer 34

A specialized cell type in the intestinal epithelium, through which antigens and pathogens enter GALT (Peyer's patches and lymphoid tissue) from the intestines. M cells do not have villi, their name comes from microfolds/ruffles on their surface

Question 35

Role of M cells

Answer 35

M cells express receptors and adhesion molecules for microbial antigens. They have use endocytosis, phagocytosis, or transcytosis to deliver antigens to APCs such as dendritic cells and B lymphocytes. They also generate adaptive immune responses in the gut- effector T cells, and B cells differentiate into antibody secreting plasma cells

Question 36

How do M cells carry out endocytosis? (3)

Answer 36

1. Endocytic vesicle transports through cell 2. Fuses with basolateral epithelial membrane 3. Intraepithelial pocket in lymphoid tissue allows microbes and antigens to contact Dendritic cells, T cells, and B cells

Question 37

Oral tolerance

Answer 37

The characteristic tolerance of the immune system for "foreign" antigens like food, when it is ingested into the GI tract

Question 38

Which cytokine receptor do dendritic cells express?

Answer 38

Dendritic cells in the GI tract express CCR6, which is the receptor for the chemokine CCL20, produced by the follicle-associated epithelial cells. When processing antigens, dendritic cells secrete the immunosuppressive cytokine IL-10, which prevents any T cells that are activated by the dendritic cells from making inflammatory cytokines

Question 39

How does oral tolerance occur?

Answer 39

In a healthy gut, antigens from food are transported through M cells and taken up by a subset of CD103-expressing dendritic cells in the lamina propria, which travel to the mesenteric lymph nodes. Then, the dendritic cells present the antigens to T cells, driving their differentiation to Treg cells that express the transcription factor FoxP3. These cells actively suppress the immune response to food antigens.

Question 40

How do dendritic cells deal with microbes?

Answer 40

A commensal microorganism is treated as a pathogen if it breaches the epithelial barrier. Specific IgA antibodies are made against commensal species and secreted into the gut lumen to prevent infections by commensal microbes. In a healthy gut, small numbers of commensal species enter the GALT. Dendritic cells take up the microbes and present their antigens to MHC class 2 molecules

Question 41

What is the purpose of dendritic cells presenting antigens to MHC molecules?

Answer 41

On activation and differentiation into CD4 TFH cells, helper T cells form cognate pairs with antigen-specific B cells that have also taken up microbes and are presenting the antigens to MHC class 2. This union drives the B cells to differentiate into plasma cells, which secrete pentameric IgM and then switch to secreting dimeric IgA. Therefore, IgA is made against gut microbiota species

Question 42

Advantages and disadvantages of the M cell delivery system

Answer 42

Advantage- allows monitoring of the gut microbiota. Disadvantage- offers pathogens easy access to the tissues underlying the gut epithelium, when IgA has not been made against these pathogens

Question 43

In the presence of infection, what is the function of dendritic cells?

Answer 43

Dendritic cells in the lamina propria and outside the organized lymphoid tissues become more mobile and capture pathogens independently of M cells. The dendritic cells move into the epithelium, or send processes through it that capture microbes and antigens without disturbing the integrity of the epithelium. Once they have obtained their antigen cargo, dendritic cells move into the T cell area of the GALT. Or, they can travel in the draining lymph to the T cell area of a mesenteric lymph node to stimulate antigen-specific T cells

Question 44

How do the GALT attract lymphocytes?

Answer 44

Peyer's patches and mesenteric lymph nodes release chemokines CCL21 and CCL19, which bind to chemokine receptor CCR7, expressed by naive B cells and T cells. The naive lymphocytes are induced to leave at blood at the high endothelial venules and enter the secondary lymphoid tissue

Question 45

What happens if a specific antigen is not encountered in GALT?

Answer 45

The naive cells leave the tissues in the efferent lymph and recirculate. If lymphocytes find their specific antigen, they are retained in the lymphoid tissue. Dendritic cells present specific antigen to activate naive T cells, causing them to proliferate and differentiate into effector T cells- including TFH cells

Question 46

What happens to effector T and B cells that are activated in Peyer's patch?

Answer 46

They pass through the mesenteric lymph node to the thoracic duct, and then to the blood

Question 47

Effector cells and the lamina propria

Answer 47

Effector cells gain access to the lamina propria via the blood. Here, their functions include CD8 killing, CD4 cytokine secretion, and IgA secreting plasma cells

Question 48

Effector T cell homing

Answer 48

When antigen-activated T cells in mucosal lymphoid tissue become effector cells, which leave in the lymph and then populate mucosal tissue from the blood. Homing is mediated by integrin α4β7 on the effector T cell, which binds to MAdCAM-1 on the blood vessel. Naive lymphocytes that are activated in the GALT can then enter and function in other mucosal tissues, helping to defend all mucosal sites

Question 49

How are effector T cells directed to home to the lamina propria?

Answer 49

Cells in the lamina propria increase their expression of CCR9, which binds to CCL25 secreted by the lamina propria cells. Once in the lamina propria, T cells express αE:β7, which binds to E-cadherin. This enhances the cells' interaction with the gut epithelium.

Question 50

Components of a healthy lamina propria (5)

Answer 50

1. CD4 T cells 2, Plasma cells 3. Dendritic cells 4. CD8 T cells (intraepithelial lymphocyte; IEL) 5. Mast cells or eosinophils (rare)

Question 51

When are neutrophils present in the lamina propria?

Answer 51

Neutrophils are present only during inflammation, infection, or other disease

Question 52

Effector cells of the gut mucosa

Answer 52

The majority of the effector cells are T cells (both γδ T cells and αβ T cells). CD8 cells predominate in the epithelium and CD4 cells predominate in the lamina propria. The effector B cells are almost all plasma cells, which secrete either pentameric IgM or dimeric IgA

Question 53

Intraepithelial lymphocyte

Answer 53

Distinctive types of CD8 T cell and γδ cell that are integrated into the epithelial layer of the small intestine. Intraepithelial lymphocytes are the products of antigen activation. They contain intracellular granules like those of CD8 T cells. They also express TCRs with a narrow range of antigen specificities, and have a distinctive combination of chemokine receptors and adhesion molecules

Question 54

Fates of effector B cells

Answer 54

Effector B cells can either travel to the lamina propria and differentiate to plasma cells secreting IgM, or remain in the B-cell area of the GALT

Question 55

What happens to B cells that remain in the B cell area of the GALT?

Answer 55

They undergo affinity maturation and isotype switch to IgA (requires TGF-β). After switching, the cells recirculate and travel to the lamina propria. High affinity, dimeric sIgA, which is dominant in mucosal tissue, has been produced, and delivered to the lumen surface by transcytosis. sIgA becomes attached to mucus, ready to trap bacteria

Question 56

Where does dimeric IgA predominate?

Answer 56

It is the dominant immunoglobulin in tears, saliva, milk, and intestinal fluid/

Question 57

Where does IgG predominate?

Answer 57

It predominates in secretions of the nose, lower respiratory tract, and male and female urogenital tracts. Monomeric IgG is transported via FcRn receptors. IgG selectively opsonizes virulent bacteria, and confers protection against systemic infection by GI pathogens (E.coli, Salmonella typhi)

Question 58

Secreted IgA function

Answer 58

sIgA has little capacity to activate complement or act as an opsonin. It limits the access of pathogens and commensal bacteria to the mucosal surface to avoid damage to tissues. Also removes pathogens and toxins from the lamina propria

Question 59

IgA deficiency

Answer 59

Selective IgA deficiency is due to defects in isotype switching from IgM to IgA. However, people with this deficiency are generally healthy. Deficient infants receive secretory IgA from breast milk. With aging, other isotypes compensate for the loss of IgA- there is increased secretion of pentameric IgM and increased FcRn transport of IgG. Patients are susceptible to some bacterial lung infections and parasitic gut infections

Question 60

Distinctive anatomical features of the mucosal immune system (3)

Answer 60

1. Intimate interactions between the mucosal epithelia and lymphoid tissues 2. Discrete compartments of diffuse lymphoid tissue and more organized structures such as Peyer's parches, isolated lymphoid follicles, and tonsils 3. Specialized antigen-uptake mechanisms provided by M cells in Peyer's patches, adenoids, and tonsils

Question 61

Mucosal immune system effector mechanisms (2)

Answer 61

1. Activated effector T cells predominate even in the absence of infection 2. Plasma cells are in the tissues when antibodies are needed

Question 62

Mucosal immunoregulatory environment mechanisms (2)

Answer 62

1. Dominant and active downregulation of inflammatory immune responses to food and other innocuous environmental antigens 2. Inflammation-anergic macrophages and tolerance-inducing CD103+ dendritic cells

Question 63

Strategies for evading host immune defenses (4)

Answer 63

1. Suppression 2. Hiding 3. Molecular mimicry 4. Parasites

Question 64

How does suppression evade host immune defenses?

Answer 64

Suppression alters CD4 helper T cell responses and reduce antigen processing and presentation

Question 65

How do pathogens "hide" from the immune system?

Answer 65

Viral latency is one example. Additionally, the polysaccharide capsule found in S. pneumoniae limits complement binding. Antigenic variation is another example- pathogens alter their surface antigens to avoid the adaptive immune response

Question 66

Molecular mimicry

Answer 66

The sharing of antigenic determinants between the pathogen and the host. Induces anergy or autoimmunity. Pathogens can also use decoy cytokines and cytokine receptors

Question 67

How do parasites evade the immune system?

Answer 67

Parasites like Trypanosomes utilize antigenic variation

Question 68

Complement system

Answer 68

A cascade of 30+ proteins, which initiate innate and adaptive immunity. Extracellular pathogens are attacked by the deposition of complement protein C3b on its surface. This results in pathogen destruction through the formation of MAC. It also facilitates the process of phagocytosis, leading to microorganism degradation. Patients who are deficient for C3 are susceptible to encapsulated bacteria

Question 69

Capsule

Answer 69

A polymer structure of variable material, which varies by the bacterial species. For example, the capsule is a polysaccharide in S. pneumoniae and a sialic acid in N. meningitidis. The capsule aids to bacterial adhesion to surfaces

Question 70

Capsule production inhibits

Answer 70

The deposition of C3b on the pathogen surface, by preventing the formation of C3 convertase. This decreases opsonization and MAC formation

Question 71

Immunity against S. pneumoniae

Answer 71

S. pneumoniae strains differ in the structure of their capsular polysaccharides- there are 90 different serotypes. Antibodies that are made against one serotype do not protect against infection with another serotype. The pneumococcal vaccine contains purified polysaccharides from 23 serotypes

Question 72

Antigenic drift

Answer 72

Point mutations in the HA and NA proteins (outer membrane proteins) in influenza virus. These mutations eliminate recognition by pre-existing antibodies and are the cause of seasonal flu epidemics

Question 73

Antigenic shift

Answer 73

The reassortment of viral RNA segments between influenza viruses infecting the same cell, which creates a completely new virus. Antigenic shift arises when human and bird viruses infect pigs. Causes of flu pandemics

Question 74

Trypanosomes

Answer 74

Parasites with part of their life cycle in mammals and part in insects. It is the cause of African sleeping sickness. Symptoms include relapsing fever, lymphadenopathy, and the sleeping sickness stage. This stage occurs when the parasite invades the CNS- causes behavior changes, apathy, fatigue, confusion, meningocephalopathy, and death

Question 75

Trypanosomes antigenic variation

Answer 75

Trypanosomes have over a thousand genes that encode variable surface glycoproteins (VSGs). Only one VSG is expressed at a time, and gene conversion causes replacement of active VSG with a new gene. Infecting trypanosomes are a heterogeneous population for VSG. If the infection stimulates antibody production to dominant VSG, the infection is cleared. The selective pressure results in a minority expressing new VSG to become dominant, stimulating new antibody production

Question 76

Bacterial superantigens

Answer 76

Stimulates a massive, but ineffective CD4 T cell response, involving 2-20% of the body's CD4 T cells. Also causes excessive production of IL-2, IFN-γ, and TNF-α. Examples include S. aureus enterotoxin B and S. aureus toxic shock syndrome toxin

Question 77

Superantigen interactions (3)

Answer 77

1. The superantigen first forms a stable interaction with MHC class 2 2. Binds to the beta chain of the TCR 3. Binds to CD28 on the T cell to stimulate activation

Question 78

Mechanisms of viral evasion of the host immune response (6)

Answer 78

1. Virus sequestration – infect specific host cells; establish latency 2. Blocking APC function – MHC I directed to cytosol for destruction. 3. Cytokine evasion (fake receptors) 4. Inhibition of apoptosis – viral upregulation or block degradation of Bcl (cell protein inhibits apoptosis) 5. Evade NK cell mediated killing – disruption of MHC I by decoy molecules 6. Avoid complement-antibody elimination – variation/hyper-variability antigenic determinants

Question 79

Herpesviruses

Answer 79

Viruses with large DNA genomes. They establish latency in host cells (often nerve cells). Includes HSV1, HSV2, varicella zoster virus, CMV, and EBV

Question 80

Viral latency

Answer 80

Few, if any, viral proteins are expressed during latency, so there are no viral peptides to be loaded onto MHC class 1 to stimulate CD8 T cells

Question 81

How are latent viruses re-activated?

Answer 81

Reactivation may be caused by stress or a change in environmental conditions. It results in replication and production of peptides, as well as restimulation of viral-specific CD8 T cells. The cycle can be repeated many times throughout life, as with shingles and cold sores

Question 82

CMV suppression of antigen presentation

Answer 82

CMV expresses 10 proteins that are known to interfere with MHC class 1 molecule's ability to stimulate NK cells and CD8 T cells. The proteins stimulate degradation of MHC class 1 molecule, disrupt proteasome function, interfere with peptide transport by TAP, and retain MHC class 1 in the ER. They also have additional mechanisms to suppress NK cell activation