L10-12: Microbiome and Immunopathologies

Question 1

Define: Microbiome

Answer 1

Collection of all genomes of microbes in an ecosystem

Question 2

Define: Microbiota

Answer 2

Microbes that collectively inhabit a given ecosystem (symbiotic - commensals, NOT parasites)

Question 3

Define: Pathobionts

Answer 3

Typically benign endogenous microbes with the capacity, under altered conditions to elicit pathogenesis

Question 4

Prebiotics

Answer 4

Nutritional substrates that promote the growth of microbes that confer health benefits in the host

Question 5

Probiotics

Answer 5

Live microbes that confer health benefits when administered in adequate amounts in the host

Question 6

Synbiotics

Answer 6

Formulations consisting of a combination of probiotics and prebiotics

Question 7

T or F:

In utero environment is sterile in healthy conditions.

Answer 7

False, DNA-based microbiota studies have detected bacterial species in the placentas of healthy mothers, in amniotic fluid of preterm infants and in meconium.

Question 8

Key cells involved in Innate immunity

Answer 8

Monocytes, macrophages, NK cells, dendritic cells,

Question 9

Role of dendritic cells in innate immunity

Answer 9

Recognition of microbial patterns (PRRs/TLRs)

Co-stimulation for T lymphocytes - instructing naive T cell to differentiate into different subsets

Question 10

Cells/molecules involved in adaptive immunity

Answer 10

T lymphocytes - secretion of cytokines/chemokines

B lymphocytes - secretion of antibodies

Question 11

T lymphocytes

Answer 11

Helper T (Th)
Cytotoxic T (Tc)
Regulatory T cells (Treg)

Question 12

B lymphocytes

Answer 12

Produce Ab
B1/B2
Marginal zone B cells (MZB)
Follicular B cells

Question 13

Immunoglobulin class types: primary vs. secondary response

Answer 13

Primary response predominantly IgM

Secondary response predominantly IgG, IgA

Question 14

Most abundant Ig class type

Answer 14

IgG

Question 15

GC/Follicular pathway

Answer 15

Typically associated with protein antigens
FDC presents Ag to B cell via the BCR which receives further help from a helper T cell via MHCII/TCR and CD40/CD40L interactions. This causes B cell to differentiate into plasma cells which produce Ab and memory B cells

Question 16

Non-GC or extrafollicular pathway

Answer 16

T-independent antigens include large polysaccharides (pneumococcal PS) that can cross-link BCRs
No or few somatic hypermutations
Short-living plasma cells
No memory B cells

Question 17

Regulatory T cells

Answer 17

Control inflammation, allergy and autoimmunity

Question 18

Two types of Treg cells

Answer 18

1. Naturally-occurring: CD4+CD25+FoxP3+ emerge from the thymus during development
2. Inducible (can be induced in the periphery): CD4+CD25-FoxP3- (Tr1; IL-10) and CD4+CD25+/-
   FoxP3+ (Th3; TGF-β)

Question 19

Primary immunodeficiency

Answer 19

Genetic (inherited/present) at birth, approximately 150 different conditions, most rare
Symptoms may not appear until adulthood

Question 20

Secondary immunodeficiency

Answer 20

Due to factors other than genetics e.g. HIV

Question 21

Majority of primary immunodeficiencies are ___ related

Answer 21

antibody

Question 22

Types of PID

Answer 22

• Antibody deficiencies (B-cell related):
– Common Variable Immune Deficiency (CVID)
– Specific (selective) antibody deficiency (SAD)
• T-lymphocyte disorders:
– Severe Combined Immune Deficiency (SCID)
– Immunodysregulation polyendocrinopathy enteropathy X-linked syndrome (IPEX) (Treg)
• Innate deficiencies:
– IRAK-4 deficiency, CGD

Question 23

Common Variable Immunodeficiency (CVID)

Answer 23

One of the most common PIDs - 1:50,000 affected. Characterized by low levels of serum Igs (hypogammaglobulinemia). Increased susceptibility to infections, predominantly lung. Enlarged spleen and lymph nodes, polyarthritis. Genetic causes unknown (10-25% inherited) - TACI (Transmembrane activator and calcium modulator and cyclophilin ligand interactor).
B cell function, not numbers, are affected. B cells fail to differentiate into plasma cells, and therefore lack of Ab production. Deficient memory B cells, and poor response to vaccines.

Question 24

Treatment of CVID

Answer 24

Effective treatment allows patients to lead a normal life

• immunoglobulin replacement therapy (IVIg)
• antibiotics for chronic infections

Question 25

Specific Antibody Deficiency (SAD)

Answer 25

Patients have normal Ig levels and normal Ig subclasses, however deficient in IgG2 important in responses to encapsulated bacteria (fails to produce protective Ab levels in response to polysaccharide antigens e.g. Streptococcus pneumoniae). Responses to protein Ag/vaccines normal.

Question 26

Pneumococcal disease and SAD

Answer 26

Encapsulated bacteria – the capsule is a T-independent antigen (no T cell help). Significant pathogen causing pneumonia, otitis media and sinusitis. Also more serious invasive diseases such as meningitis and sepsis.

Question 27

SAD – Laboratory Diagnosis

Answer 27

* Evaluate response to pneumococcal polysaccharide vaccine – 23 serotypes, response to at least half of serotypes (4-fold rise in IgG or >1.3μg/mL) * Memory B cell numbers indicative of SAD * Treated with IVIg and/or antibiotics

Question 28

Severe Combined Immunodeficiency (SCID)

Answer 28

Rare, potentially fatal (‘bubble boy’), affects 1 in 58,000 (US data) Lack of T-lymphocyte differentiation – Lack of various T cell subsets – Also B cells (through lack of T cell help) Clinical presentation <6 months - diarrhoea, failure to thrive, pneumonia, persistent infections

Question 29

Genetic defects identified associated with SCID

Answer 29

Most common is X-linked (45%) - common IL-2 receptor gamma chain mutation. Also IL-7a, Jak3, CD3, CD45

Question 30

Typical vs. Leaky SCID

Answer 30

Typical SCID - <300 autologous T cells/uL, lymphocyte function <10% of lower limit of normal range, deleterious SCID gene mutations Leaky SCID - 300-1,499 autologous T cells, lymphocyte function 10-25%, hypomorphic SCID gene mutation

Question 31

IPEX syndrome

Answer 31

Very rare condition caused by mutation in Foxp3 Lack of functional Treg cells Multiple autoimmune disorders - diabetes, thyroiditis, haemolytic anaemia

Question 32

IPEX pathology

Answer 32

* Absence of small bowel mucosa * Inflammatory infiltrate in many organs * Liver: fatty change * Kidney: nephritis * Skin: eczematous * Duodenal villous atrophy * No goblet cells

Question 33

Chronic Granulomatous Disease

Answer 33

X-linked disorder, incidence 1:500,000 • Defect in intracellular bacterial killing by neutrophils and monocytes due to mutations in NADPH oxidase (> 400 known) • Increased susceptibility to infections by ‘catalase’ organisms

Question 34

IRAK-4 deficiency

Answer 34

Interleukin-1 receptor-associated kinase-4 • Extremely rare condition (48 worldwide as at 2010) • Essential role in TLR and IL-1 receptor signalling – Innate immune receptors for pathogen binding – Affects NFκB signalling – Inability to activate T cells • Susceptible to pyogenic bacteria, not viruses, fungi – predominately S. pneumoniae • Vaccination can be beneficial

Question 35

T cell tolerance mechanisms

Answer 35

Central tolerance occurs as T cell matures: - Occurs in the thymus - Positive selection - T cell has to recognise Ag in context of MHC - Negative selection - affinity too high ``` Peripheral tolerance: – Occurs in peripheral tissues – ‘Regulatory’ responses involving mature T cells – Clonal anergy (lack of costimulation) – Ignorance (do not encounter Ag) – Suppression by cytokines (e.g. TGF-β) – Specific regulation (Treg induction) – Negative regulation (engagement of CTLA4) ```

Question 36

B cell tolerance

Answer 36

``` Occurs in the bone marrow • Needs T cell tolerance to be intact • Self-reactive B cells can be: – Deleted when high affinity for antigen – Made anergic when antigen is soluble and at high concentration – Ignorance when lack of T cell help or low antigen concentration ```

Question 37

Molecular mimicry

Answer 37

* Similar/identical epitopes between microbe and host | * Important when host Ag has important biological function eg. Streptococcal protein and bacterial endocarditis

Question 38

Treg function and mechanism

Answer 38

Downregulation of receptors on APC or T cell directly in cell-cell contact inhibition Secretion of immunosuppressive cytokines (IL-4, IL-10, TGF-β)

Question 39

Milgrom and Witebsky’s criteria for | autoimmune diseases

Answer 39

* Lymphocytic infiltration of the target organ * Presence of circulating autoantibodies and/or cellular immunity against the target organ * Identification of the specific antigen(s) * Production of humoral and/or cellular autoimmune response in animals sensitized by autologous antigen * Close association with other autoimmune disorders

Question 40

Diabetes

Answer 40

Group of diseases affecting insulin production and/or function • Organ-specific autoimmune disease (Type 1) • Targets the islets of Langerhans cells of the pancreas – important for insulin secretion • Secondary damage to kidneys, eyes, nerves, blood vessels • Onset from first year of life to older adulthood

Question 41

Type 1 diabetes

Answer 41

5-10% of cases • pancreatic β cell destruction • Insulin deficiency

Question 42

Type 2 diabetes

Answer 42

90-95% of cases • ‘relative insulin deficiency’ • Inadequate production or peripheral resistance to insulin action • Defect in signaling to Glut-4 from insulin receptors

Question 43

Latent Autoimmune Diabetes in Adults (LADA)

Answer 43

A form of autoimmune diabetes which is diagnosed in individuals who are older than the usual age of onset of type 1 diabetes. • Also known as “Late-onset Autoimmune Diabetes of Adulthood”, "Slow Onset Type 1" diabetes, and sometimes also "Type 1.5” • LADA patients often thought to have Type 2 diabetes due to their age at diagnosis

Question 44

Immunological events in diabetes

Answer 44

Failure of T cell tolerance: – Defective clonal deletion (central) – Regulatory response altered (peripheral) Prior to clinical symptoms: – Autoreactive T cells become activated – Autoantibodies are produced – Destroy islets of Langerhans in pancreas – Occur over many years until nearly all beta cells affected

Question 45

Autoantibodies in diabetes

Answer 45

Major autoantibodies are reactive to 4 islet autoantigens (termed islet cell autoantibodies, ICA): – Insulinoma-associated antigen-2 (ICA512) – Insulin (micro-insulin autoantibodies, IAA) – Glutamic acid decarboxylase 65 (GAD65) – Zinc transporter 8 (ZnT8)

Question 46

Genetics of type I diabetes

Answer 46

Major linkage to MHC class I and II genes (HLA), also linked to polymorphisms in CTLA4

Question 47

Pathogenesis of type Idiabetes

Answer 47

Dysfunction in Tregs leading to breakdown in self-tolerance to islet-autoantigens

Question 48

Pathology of type I diabetes

Answer 48

Insulitis (inflammatory infiltrate of T cells and macrophages) β-cell depletion and islet atrophy

Question 49

Systemic Lupus Erythematosus

Answer 49

* Multi-system chronic autoimmune disease - remitting and relapsing * Type III hypersensitivity (immune complex) - anti-DNA complexes * Acute or insidious in onset * Many organs affected but main affected sites are kidneys and small blood vessels - Glomerulonephritis and skin involvement

Question 50

Diagnostic feature of SLE

Answer 50

Antinuclear antibodies to Sm antigen and dsDNA Malar rush Photosensitivity

Question 51

Pathogenesis of SLE

Answer 51

Genetics: HLA-DQ, complement components

Question 52

Type III hypersensitivity reactions

Answer 52

* Immune complexes formed mainly in organs where blood is filtered e.g. kidneys, joints * acute inflammatory response * IgG and IgM (Complement fixing) * phagocytosis

Question 53

Treatment for SLE

Answer 53

Corticosteroids, immunosuppressants

Question 54

Most common cause of death in SLE

Answer 54

renal failure