lecture 11

Question 1

What is the primary purpose of the immune system?

Answer 1

• to protect the host from disease

Question 2

By what mechanisms does our immune system protect us?

Answer 2

Non-specific mechanisms

• physical/chemical barriers
• innate immunity

Specific mechanisms
- adpative immunity

Question 3

What are the key physical and chemical barriers?

Answer 3

• skin
• pH
• antimicrobials
• commensals (beneficial bacteria)

Question 4

What are the three ‘lines of defense’?

Answer 4

Innate (nonspecific immunity) 
1st line of defence 
- intact skin
- mucous membranes and their secretions 
- normal microbiota 
2nd line 
- natural killer cells and phagocytic white blood cells 
- inflammation 
- fever 
- antimicrobial substances 

Adaptive (acquired) immunity
3rd line
- specialised lymphocytes: T cells and B cells
- antibodies

Question 5

What is innate immunity?

Answer 5

• first line of defence
• non-specific but rapid
• does not have capacity to ‘remember’
• key cells involved:
• • monocytes/macrophages (APCs)
• • NK cells
• • Dendritic cells
• involve cytokines, complement, antimicrobials

Question 6

What are dendritic cells?

Answer 6

• central role in generating immune responses
• act as sentinels
• interface between innate and adaptive immunity
• functions include:
• • recognition of microbial patterns (PRRs/TLRs)
• • costimulation for T lymphocytes
• • response determined by environment (e.g. cytokines)

‘master manipulators’

Question 7

What is the adaptive immune system?

Answer 7

• highly specific but slower
• able to eliminate foreign antigens
• has a critical memory function
• cells/molecules involved:
• • T lymphocytes: secrete cytokines/chemokines
• • B lymphocytes: secrete antibodies
• involved in self/non-self discrimination

Question 8

What are some of the main lymphocytes?

Answer 8

T lymphocyte: produce cytokines

• helper T lymphocytes (Th)
• cytotoxic T lymphocytes (Tc)
• regulatory T lymphocytes (Treg)

B lymphocytes - produce antibodies

• B-1/B-2
• marginal zone B cells (MZB) (CD27 marker)
• follicular B cells

Question 9

What are the kinetics of immune protection?

Answer 9

• primary response: IgM, low (e.g. vaccination)

- exposure to infection, massive response (greater magnitude), class switched IgG, IgA

Question 10

What are the sizes of the different antibodies?

Answer 10

IgM - secreted as pentamer - 950kD

• IgD - 175
• IgG - 150
• IgA - 160 - 400
• IgE - 190

Question 11

What are the functions of the different Igs?

Answer 11

IgM: first response, pentamer, low affinity, high avidity
IgD: less known, potentially involved in development, function not well described
IgG: complement activating, placental transport
IgA: mucosal
IgE: mast sensitising

Question 12

How might an immune response be carried out?

Answer 12

• antigen presented to an immune cell (APC)
• maybe cytotoxic pathway (T cells)
• can directly interact with B cell
• memory cells and plasma cells that secrete antibodies
• after T cell help B cells undergo class switching and affinity maturation

Question 13

What are T-independent antigens?

Answer 13

• include polysaccharides (pneumoccocal PS)

- B cells produce short lived plasma cells without the help of T cells (can’t recognise PS)

Question 14

What are T-dependent antigens?

Answer 14

• typically proteins

- B cells produce long lived plasma cells and memory cells with the help of T cells

Question 15

How are Regulatory T cells important in the control of immunity?

Answer 15

• involved in controlling a number of the inflammatory responses
• e.g. suppression of effector Th17 cells
• lots of suppression: inhibition of allergic type cells
• IL10 - unwanted or unwarranted allergy

Question 16

What are regulatory T cells (Treg)?

Answer 16

• Identification of CD25 by Shimon Sakaguchi in 1995 led to advances in Treg biology
• Also referred to as ‘suppressor’ T cells:
• Two types:
• • naturally-occurring: CD4+, CD25+, FoxP3+
• • inducible: CD4+CD25-FoxP3- (Tr1;IL-10) and CD4+CD25+/-FoxP3+ (Th3; TGF-beta)
• controls inflammation, allergy and autoimmunity
• Foxp3 transcription factor is key marker

Question 17

How is the immune system imperfect?

Answer 17

• defects in the immune system can happen
• can be genetically pre-determined or a consequence of aberrant regulation
• outcomes include:
• • immunodeficiency
• • autoimmunity
• • allergy

Question 18

What is immunodeficiency?

Answer 18

• a group of disorders where part of the immune system is missing or defective
• two classifications:
• • primary immunodeficiency (genetic)
• • secondary immunodeficiency (HIV)

Question 19

What are Primary Immunodeficiencies?

Answer 19

• approx 150 different conditions; most rare
• inherited (present at birth)
• defect in one or more components of the immune system (innate and adaptive)
• symptoms may not appear until adulthood

Question 20

What are the types of primary immunodeficiency?

Answer 20

• antibody deficiencies (majority)
• complement deficiency
• phagocytic
• cellular deficiency
• combined immunodeficiency

Question 21

How do we classify PIDs?

Answer 21

• classified in terms of the response and the cells involved
  (8 classes as defined by IUIS)

Question 22

What are examples of PIDs?

Answer 22

• combined B and T lymphocyte immunodeficiencies
• MHC Class I/II deficiency
• Hyper IgM syndrome (CD40-CD40L deficiency)
• Agammaglobunemias (Ab deficiency)
• Common Variable Immunodeficiency (CVID)
• Selective IgG, IgM or IgA (and subclass) deficiencies
• Wiskott-Aldrich disease
• DiGeorge Syndrome
• Chronic granulomatous deficiency (phagocytes)
• NK deficiencies, Complement deficiencies

Question 23

What are clinical signs of PIDs?

Answer 23

• > 10 episodes acute otitis media per year (infants and children)
• > 2 episodes consolidated pneumonia per ear
• > 2 life-threatening infections per lifetime
• two or more serious sinus infections within 1 year
• abnormal response to microbes

Diagnosis is a combination of clinical history and laboratory evaluation of immune system

Question 24

What are pathological features of PIDs?

Answer 24

• recurrent deep skin or organ abscesses
• two or more deep-seated infections such as meningitis, osteomyelitis, cellulitis or sepsis
• persistent oral thrush or candida infection elsewhere on the skin (>1yr age)
• recurrent autoimmune phenomena
• dysmorphic features associated with recurrent infection
• infections that worsen chronic disorders (e.g. asthma)
• infections with pathogen despite vaccine (e.g. pneumococcal)

Question 25

What is CVID?

Answer 25

- common variable immune deficiency - one of the most common PIDs: 1:50,000 affected - characterised by low levels of serum Igs - hypogammaglobulinaemia - increased susceptibility to infections (mainly bacterial) - genetic causes unknown (10-25% inherited e.g. TACI (transmembrane activator and calcium modulator and cyclophilin ligand interactor) - nature and type of deficiency varies with patient - affects multiple organs: Lungs, GIT, spleen, blood, ears (middle, predominantly)

Question 26

To what is CVID normally due?

Answer 26

- normal B cell numbers - B cells fail to differentiate into plasma cells - Lack of Ab production (IgG mainly, also IgA, IgM) - deficient memory B cells - poor response to vaccines - also lack of T cell help

Question 27

What are the pathological outcomes of CVID?

Answer 27

- no physical abnormality - recurrent and severe lung infections: bacteria such as pneumococcus, H. influenzae type b - enlarged lymph node, spleen - polyarthritis

Question 28

What are infections associated with CVID?

Answer 28

- sinusitis - pneumonia (biggest killer of children under 5) - bronchitis - otitis - abscess - rec herpes zoster - sepsis - meningitis - cellulitis - osteomyelitis

Question 29

How is CVID treated?

Answer 29

- effective treatment allows patients to lead a normal life - - immunoglobulin replacement therapy (IVIg) - - antibiotics for chronic infections

Question 30

What is specific antibody deficiency (SAD)?

Answer 30

- Patients have normal Ig levels and normal Ig subclasses: lacking IgG2 - responses to protein antigens/vaccines normal - failure to produce protective antibody levels in response to polysaccharide antigens - - streptococcus pneumoniae (pneumococcus) - IgG2: detects polysaccharide antigen - susceptible to infections with encapsulated bacteria - may also have IgG4 and IgA deficiency

Question 31

What is a major complication of SAD?

Answer 31

- pneumococcal disease - S. pneumoniae (pneumococcus): important pathogen globally (1-2 million children die each year) - affects young children, older adults and immunocompromised - often described as a commensal organism (carrier = asymptomatic) - can cause bacterial meningitis and acute otitis media (middle ear infection)

Question 32

What is pneumococcal disease?

Answer 32

- gram + diplococci - encapsulated bacteria - the capsule is a T-independent antigen (no T cell help) - many types: more than 90 different serotypes, different PS - significant pathogen causing pneumonia, otitis media (can lead to long term hearing impairment) and sinusitis - also more serious invasive diseases such as meningitis and sepsis - therefore diagnosis and management of SAD is critical

Question 33

How do you diagnose SAD in the lab?

Answer 33

- evaluate response to pneumococcal polysaccharide vaccine - 23 serotypes - respond to at least half the serotypes - 4-fold rise in IgG or >1/3µg/mL - memory B cell numbers indicative of SAD - treated with IVIg and/or antibiotics

Question 34

What is severe combined immunodeficiency (SCID)?

Answer 34

- rare, potentially fatal (bubble boy) - lack of T and B cell function (also NK) - several genetic defects identified (12 so far) - - most common is X-linked (45%) - c-gamma mutation - IL7-alpha, Jak3, CD3, CD45 - extreme susceptibility to infections - - cytomegalovirus (CMV), HSC, EBV - - implications for live virus vaccines

Question 35

What is IPEX syndrome?

Answer 35

- immunodysregulation polyendocrinopathy enteropathy X-linked syndrome - very rare condition caused by mutation in FoxP3 - lack of functional Treg cells - multiple autoimmune disorders - - diabetes, thyroiditis, haemolytic anaemia - allergic phenotypes also occur - mainly resulting in unwanted infection, malnutrition etc

Question 36

What is the pathology of IPEX?

Answer 36

- absence of small bowel mucosa - inflammatory infiltrate in many organs - Liver: fatty change - Kidney: nephritis - Skin: eczematous - duodenal villous atrophy - no goblet cells - inflammatory cell infiltrate

Question 37

What is chronic granulomatous disease?

Answer 37

- 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 38

What is IRAK-4 deficiency?

Answer 38

- 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 NFkB signalling - - inability to activate T cells - susceptible to pyogenic bacteria, not viruses, fungi - - predominantly S. pneumoniae - vaccination can be beneficial - much fewer numbers of CD4 and CD8

Question 39

How do we diagnose PIDs in the lab?

Answer 39

Antibody deficiency - IgG, IgM, IgA (and subclasses) - total and specific antibody (post vaccine) - B cell analysis (total, memory) Cellular deficiency - blood counts (total white blood cells) - lymphocyte proliferation assays (antigen stimulation) - oxidative burst (neutrophil function) Can only be done with reference to appropriate controls