Immuno 1 - The immune response to infection/Primary immune deficiencies Part 1

Question 1

Clinical features suggestive of immunodeficiency

Answer 1

•	Infection
o	Two major or one major and recurrent minor infections in one year
o	Atypical organisms 
o	Unusual site
o	Poor response to treatment

• Features to suggest primary immune deficiency
o FH
o Young age at presentation
o Failure to thrive

Question 2

3 receptors that are expressed by cells of the innate immunity

Answer 2

• Express receptors for cytokines/chemokines  to detect inflammation

• Express pattern recognition receptors  to detect pathogens
o Pattern recognition receptors (Toll-like receptors or mannose receptors) – recognise generic motifs known as pathogen-associated molecular patterns (PAMPs) such as bacterial sugars, DNA, RNA

• Express Fc receptors for Ig  to detect immune complexes

Question 3

Which are the phagocytes

Answer 3

monocytes, macrophages, neutrophils, tissue dendritic cells, and mast cells

Question 4

What do natural killer cells recognse

Answer 4

natural cytotoxicity receptors that recognise heparan sulfate proteoglycans

kill altered self as in malignant or virus infected cells - these cells downregulate their self-HLA molecules so that NK cells can be activated

Question 5

Describe the functions of

Neutrophils
Macrophages
Natural killer cells
Dendritic cells

Answer 5

Neutrophils – polymorphonuclear cells capable of phagocytosing pathogens and killing by oxidative and non-oxidative mechanisms

Macrophages – derived from monocytes and resident in peripheral tissues

Natural killer cells – lymphocytes that express inhibitory receptors capable of recognizing HLA class I molecules and have cytotoxic capacity

Dendritic cells – immature cells are adapted for pathogen recognition and uptake whilst mature cells are adapted for antigen presentation to prime T cells

Question 6

Microbial killing mechanisms – oxidative killing

Answer 6

• NADPH oxidase complex converts – Oxygen  reactive oxygen species (superoxide, hydrogen peroxide)
• Myeloperoxidase catalyses – Hydrogen peroxide + Chloride  hydrochlorous acid

Question 7

Microbial killing mechanisms – non-oxidative killing

Answer 7

• Release of bactericidal enzymes e.g. lysozyme, lactoferrin into the phagolysosome

 Lysozyme and antimicrobial peptides directly kill invading pathogens
 Lactoferrin acts to starve invading bacteria of iron

Question 8

Fates of neutrophils and macrophages after phagocytosis

Answer 8

• Process of phagocytosis depletes neutrophil glycogen reserves  neutrophil cell death
• Macrophages survive and go on to communicate with T-cells

Question 9

Describe

• Oxidative killing
• Non-oxidative killing
• Pathogen recognition
• Opsonisation

Answer 9

• Oxidative killing – describes killing mediated by reactive oxygen species generated by the action of the NADPH oxidase complex
• Non-oxidative killing – may be mediated by bactericidal enzymes such as lysozyme
• Pathogen recognition – is mediated by Toll like receptors which recognise pathogen associated molecular patterns
• Opsonisation – may be mediated by antibodies, complement components or acute phase proteins and facilitates phagocytosis

Question 10

Reticular dysgenesis

Answer 10

Most severe form
AR SCID
• Failure of stem cells to differentiate along myeloid or lymphoid lineage

 Mutation in mitochondrial energy metabolism enzyme adenylate kinase 2 (AK2)
 Failure of production of – neutrophils, lymphocytes, monocytes/macrophages, platelets

Question 11

Kostmann syndrome

Answer 11

• Specific failure of neutrophil maturation
mutation in HCLS1-associated protein X-1 (HAX1)
Congenital neutropenia
AR

Question 12

Cyclic neutropenia

Answer 12

• Specific failure of neutrophil maturation
 AD episodic neutropenia every 4-6 weeks
 Mutation in neutrophil elastase (ELA-2)

Question 13

Leukocyte adhesion deficiency

Answer 13

defect of phagocyte migration

o Deficiency of CD18
o CD11a/CD18 and CD11b/CD18 are usually expressed on neutrophils – bind to ligand ICAM-1 on endothelial cells – therefore regulate neutrophil adhesion/transmigration

o In leukocyte adhesion deficiency – neutrophils lack these adhesion molecules – neutrophils fail to exit the bloodstream

o	Characteristics
	Very high neutrophil counts in blood 
	Immunodeficiency – can’t get the neutrophils into the site of infection (neutrophils can get into the blood but can’t get out)
	Absence of pus/abscess formation 
	Delayed umbilical cord separation

Question 14

Chronic granulomatous disease

characteristics
ix
mx

Answer 14

• Chronic granulomatous disease
o Absent respiratory burst
 Deficiency of one of the components of NADPH oxidase
 Inability to generate oxygen free radials

o Excessive inflammation
 Persistent neutrophil/macrophage accumulation
 Failure to degrade antigens
o Granuloma formation
o Lymphadenopathy and hepatosplenomegaly

o Susceptibility to bacteria esp. catalase positive bacteria (PLACESS – Pseudomonas, Listeria, Aspergillus, Candida, E.coli, Staph Aureus, Serratia)

• Investigation of chronic granulomatous disease –
Normally activated neutrophils stimulate respiratory burst + produce H2O2
o Nitroblue tetrazolium (NBT) test
 NBT is a dye that changes colour from yellow to blue following interaction with hydrogen peroxide (free radical)
o Dihydrorhodamine (DHR) flow cytometry test
 DHR is oxidised to rhodamine which is strongly fluorescent following interaction with hydrogen peroxide
o In people with chronic granlomatus disease – no change in colours

• Treatment of chronic granulomatous disease
o Interferon gamma

Question 15

Cytokine deficiency

Answer 15

o IL12, IL12R, IFNγ or IFNγR deficiency
o IL12-IFNγ network important in control of mycobacteria infection
o Susceptibility to infection with mycobacteria (TB and atypical), BCG, Salmonella
o Inability to form granulomas

o Infection with mycobacteria activates IL-12-IFNγ network
 Infected macrophages stimulated to produce IL12  IL12 induces T cells to secrete IFNγ  IFNγ feeds back to macrophages + neutrophils  stimulates production of TNF  activates NADPH oxidase  stimulates oxidative pathways

Question 16

Pus or no pus?

Kostmann syndrome
Leukocyte adhesion deficiency
Chronic granulomatous disease

Answer 16

Kostmann syndrome + Leukocyte adhesion deficiency  No pus

Chronic granulomattus disease  Pus

Question 17

NK cell deficiencies

when to suspect

abnormalities in

classical NK deficiency
Functional NK deficiency

treatment

Answer 17

• NK cell deficiencies
o Recurrent episodes of infection
o Virus infection – Herpes virus (HIS + HSII), VZV, EBC, CMV, papillomavirus)
o Malignancy – papillomavirus associated cancers

classical NK deficiency
o Abnormalities described in GATA2 or MCM4 genes in subtypes 1 and 2

Functional NK deficiency
o Abnormality described in FCGR3A gene in subtype 1

• Treatment
o Prophylactic antiviral drugs e.g. acyclovir, ganciclovir
o Cytokines to stimulate NK cytotoxic function e.g. IFNα
o HSCT in severe phenotypes

Question 18

Complement definition

Answer 18

• >20 tightly regulated linked proteins
o Produced by the liver
o Present in circulation as inactive molecules
• When triggered enzymatically, activate other proteins in a biological cascade  results in rapid, highly amplified response

Question 19

How does the complement get activated?

classical pathway

Answer 19

B cell has developed into an IgG producing plasma cell after the germinal centre reaction  produces IgG which binds to the extracellular pathogen  immune complex activates complement

• Activated by antibody-antigen immune complexes
o Results in change in antibody shape – exposes binding site for C1
o Binding of C1 to the binding site on antibody  activation of the cascade
• Depends on activation of the adaptive immune system/ acquired immune response (antibody)
o This requires you to already have activated the adaptive immune response

Question 20

How does the complement get activated?

mannose binding lectin pathway

Answer 20

• Activated by the direct binding of MBL to microbial cell surface carbohydrates
• Directly stimulates the classical pathway, involving C2 and C4 (not C1)
• Not dependent on adaptive immune system/acquired immune response

Question 21

How does the complement get activated?

alternative pathway

Answer 21

• Directly triggered by binding of C3 to bacterial cell wall components
o E.g. lipopolysaccharide of gram -ve bacteria
o Teichoic acid of gram +ve bacteria
• Not dependent on adaptive immune system/acquired immune response

Question 22

Deficiency of complement/alternative pathway/C3/terminal common pathway consequences

Answer 22

increased susceptibility to encapsulated bacterial infections
(remember using acronym NHS)
 Neisseria meningitides – esp. properidin and C5-C9 deficiency
 Haemophilus influenzae
 Streptococcus pneumoniae

Question 23

Which factors does the alternative pathway invovle

Answer 23

• Involves factors B, I, Properidin
• Factor H – control protein
o Normally properdin stabilizes C3 convertase  triggers MAC complex

Question 24

Deficiency of classical pathway consequences

Answer 24

• Classical pathway deficiency (C1q, C2)

Classical pathway necessary against infection and phagocyte mediated clearance of apoptotic cells and immune complexes

Susceptibility to SLE

C2 deficiency is the most common in SLE

o Does not affect c3 or c4 – these will be normal
o However, active SLE leads to consumption of C3 +C4  secondary deficiency
 Active lupus causes persistent production of immune complexes + consequent consumption of complement  functional complement deficiency  low C3 + low C4

o Clinical phenotype
 Almost all patients with C2 deficiency have SLE
 Severe skin disease
 Increased number of infections

(((( no phagocytosis of dead cells  increased nuclear debris that isn’t being cleared up  increased load of self-antigens (particularly nuclear components)  danger of stimulating antibodies against nuclear components  development of SLE and immune complexes
 Failure to clear immune complexes  immune complex disposition in blood vessels  inflammation in skin, joints and kidneys  vasculitis  SLE )))

Question 25

MBL deficiency

Answer 25

o MBL2 mutations are common but not usually associated with immunodeficiency
o Tends to be relatively benign
o Associated with increased infection in patients who have another cause of impairment
 Premature infants
 Chemotherapy
 HIV infection
 Antibody deficiency

Question 26

What is secondary C3 deficiency

Answer 26

o C3 nephritic factors lead to consumption of C3
o Nephritic factors are auto-antibodies directed against components of the complement pathway
o Nephritic factors stabilise C3 convertases  this results in C3 activation + consumption
o Often associated with glomerulonephritis – classically membranoproliferative
o May be associated with partial lipodystrophy (abnormal fat distribution)

Question 27

How can autoimmune disease lead to complement deficiency

Answer 27

o SLE in someone with normal complement system will lead to CONSUMPTION of the complement  low C3 + low C4
o AUTOANTIBODIES directed against components of the complement pathway may lead to consumption of the complement (usually C3)

Question 28

Importance of

C3
C1
C9
MBL

Answer 28

C3 – cleavage of this protein may be triggered via the classical, MBL or alternative pathways
C1 – binding of immune complexes to this protein triggers the classical pathway of complement activation
C9 – part of the final common pathway resulting in the generation of the membrane attack complex
MBL – binds to microbial surface carbohydrates to activate the complement cascade in an immune complex independent manner