Immuno 2 - The immune response to infection/Primary immune deficiencies Part 2

Question 1

B cells emerge from the bone marrow as Ig_?__ expressing B cells

Answer 1

IgM

Question 2

Reticular dysgenesis mutation

Answer 2

mitochondrial energy metabolism enzyme adenylate kinase 2 (AK2)

Question 3

Which features would make you suspicious of SCID

Answer 3

Unwell by 3 months of age*
Infections of all types
Failure to thrive
Persistent diarrhoea
o	Unusual skin disease
	Colonisation of infant’s empty BM by maternal lymphocytes  graft vs host disease

FHx of early infant death

*(before – protected by IgG from mother across placenta and then colostrum)
• Normal baby will start to produce their own IgG over the course of 6 months

Question 4

X-linked SCID mutation

Answer 4

Mutation of common γ chain of IL2 on Chr Xq13.1

shared by cytokine receptors IL-
2
7
9
15
21

Question 5

X-linked SCID

T cells
NK cells
B cells
IgG

Answer 5

o Cells mature but are unable to respond to cytokines

early arrest of T cell + NK cell development
production of immature B cells

• Very low/absent T cells
• Very low/absent NK cells
• Normal or increased B cells (but don’t function well)
• Low IgG

Question 6

ADA deficiency

T cells
NK cells
B cells

Answer 6

Adenosine Deaminase deficiency

inability to respond to cytokines

• Very low/absent T cells
• Very low/absent NK cells
• Very low/absent B cells

Question 7

What do CD8 cells vs CD4 cells recognise

Answer 7

CD8
o Peptides derived from intracellular proteins in association with HLA class I (HLA-A, HLA-B, HLA-C)
o Malignant or virus infected cells

CD4
o	Peptides derived from extracellular proteins presented on HLA class II molecules (HLA-DR, HLA-DP, HLA-DQ)

Question 8

Function of
• Treg
• T follicular helper cells (Tfh)

cells

Answer 8

• Treg – important in negatively regulating immune responses, keeping them under control
• T follicular helper cells (Tfh) – important in the development of B cell responses– play an important role in promoting germinal centre reactions and differentiation of B cells into IgG and IgA secreting plasma cells

Question 9

DiGeorge syndrome features

characteristics
T cells
B cells
Immune function

Answer 9

Defects in T cell maturation/selection in thymus

CATCH22
o Complex congenital heart disease (esp. tetralogy of fallot)
o Abnormal facies (high forehead, low set ears, abnormally folded ear, cleft palate, small mouth, small jaw, oesophageal atresia)
o Thymic aplasia (T cell lymphopenia)
o Cleft palate
o Hypocalcaemia, Hypoparathyroidism
o 22 – chromosome - deletion at 22q11.2

o Reduced T cells
 Over time T cells proliferate to fill up the compartment and so their immune function tends to improve with age

o Normal B cells
 Low IgA, Low IgG (no T follicular helper cells to help differentiation to IgG or IgA producing B cells in germinal centre reaction)
o Homeostatic proliferation with age

 Immune function usually only mildly impaired + improves with age

Question 10

Bare lymphocyte syndrome

where is the problem
T cells
B cells
IgG
IgA

Answer 10

Defects in T cell maturation/selection in thymus

• Defect in one of the regulatory proteins involved in Class II gene expression
o Regulatory factor X
o Class II transactivator

• Absent expression of MHC Class II molecules  Profound deficiency of CD4+ cells

o Normal numbers of CD8+
o Low numbers of CD4+ (profound deficiency of CD4+ cells)
o Normal numbers of B cells
o Low IgG or IgA antibodies (lack of CD4+ T cell help, no class switching)

Question 11

Syndromes due to Defects in T cell maturation/selection in thymus

Answer 11

DiGeorge syndrome - 22q11.2 deletion

Bare lymphocyte syndrome type 2

Question 12

Causes of disordered T activation/ cell effector function

Answer 12

Failure of
• Cytokine production – IFN
• Cytokine receptors – IL12 receptor
• T-B cell communication

• Cytotoxicity

Question 13

Clinical features of T cell deficiency

Answer 13

• T cell deficiency clinical features
o Viral infections – CMV
o Fungal infection – pneumocystis, cryptosporidium
o Some bacterial infections, esp. intracellular organisms – TB, Salmonella
o Early malignancy

Question 14

T cell deficiencies ix

Answer 14

• Total WCC + differential
o Remember that lymphocyte counts are normally much higher in children than in adults

• Lymphocyte subsets
o Quantify CD8 T cells, CD4 T cells, B cells, NK cells

• Immunoglobulins
o If CD4 T cell deficient (because if there is CD4 T cell deficiency, you wont be able to generate your immunoglobulins in the germinal centre reaction)

• Functional tests of T cell activation + proliferation
o Useful if signalling or activation defects are suspected

• HIV test

Question 15

Management of immunodeficiency involving T cells

Answer 15

• Aggressive prophylaxis/ treatment of infection
• Ig replacement

• Enzyme replacement therapy
o PEG-ADA for ADA SCID

• HSCT
o To replace abnormal populations in SCID
o To replace abnormal cells – class II deficient APCs in BLS

• Thymic transplantation
o To promote T cell differentiation in Di George syndrome
o Cultured donor thymic tissue transplanted in quadriceps muscle

• Gene therapy
o Stem cells treated ex-vivo with viral vectors containing missing components
o Transduced cells have survival advantage in vivo

Question 16

What are Th1 cells?

Answer 16

Subset of lymphocytes
express CD4
secrete IL-2, IFNγ
Help CD8 cells + macrophages

Question 17

Describe what happens before and during the germinal centre reaction

Answer 17

o Dendritic cells prime the CD4+ T cells in the lymph nodes

Take up antigens in peripheral tissues, process them into proteolytic peptides, and load these peptides onto major histocompatibility complex (MHC) class I and II molecules.

o CD4+ T cells help B cell differentiation – requires CD40L:CD40 interaction

• CD4+ cells interact w B cells germinal centres (found in secondary lymphoid organs)

in the germinal centre reaction, the B cells undergo
o Somatic hypermutation  they edit their receptor on successive ranks of antigen engagement until it becomes very high affinity = stronger and more specific response to the antigen = affinity maturation
o Isotype switching from IgM to IgG, IgA or IgE
o B cell proliferation

Question 18

Immunoglobulins

What determines the antibody class
Antigen is recognised by...
Effector function is determined by...
IgA is a 
IgM is a 
IgE, IgD, IGg are

Answer 18

What determines the antibody class - Heavy chain

Antigen is recognised by antigen binding regions (Fab) on both heavy + light chains
• Antibodies identify pathogens and toxins (Fab mediated)

Effector function is determined by constant region on the heavy chain (Fc)
• Interact with other components of immune response to remove pathogens (Fc mediated)
o Fc binds to Complement, Phagocytes, NK cells to help remove antigens and toxins

IgA is a dimer
IgM is a pentamer
IgE, IgD, IgG are monomers

Question 19

Response to successive exposures to antigen is dominated by

Answer 19

IgG

Question 20

Bruton’s x-linked hypogammaglobulinemia/ agammaglobulinemia

What is wrong
When does it present
How does it present

Answer 20

• Abnormal BTK (B cell tyrosine kinase) gene – mutation in B cell tyrosine kinase
• Pre-B cells cannot develop into mature B cell  Absence of mature B cells  no antibody production

• No circulating IgG 3 months after birth (agammaglobulinemia - not even IgM)
o Boys (X-linked!) present in first few years of life
o Recurrent bacterial infections – otitis media, sinusitis, pneumonia, osteomyelitis, septic arthritis, gastroenteritis
o Viral, fungal, parasitic infections – enterovirus, pneumocystis
o Absent/scanty lymph nodes and tonsils (1o follicles and germinal centres absent)
o Failure to thrive

Question 21

X-linked hyper IgM syndrome

Where is the problem
Mode of inheritance
What does this problem cause

Clinical phenotype

Features
o	Number of circulating B cells
o	Number of T cells 
o	serum IgM
o	serum IgA, IgE, IgG

Answer 21

o Mutation in CD40 ligand gene (CD40L, CD154) (encoded in Xq26)
• X-linked recessive

• Inability of B cells to class switch causing production of only IgM
• Lack of isotype switch due to a failure of T cell co-stimulation

• B cell maturation defect
o IgM B cells cannot develop into IgM memory and plasma cells
o IgM B cells can’t undergo a germinal centre reaction

• Clinical phenotype
o Boys (X linked!) present in first few years of life
o Recurrent infections – bacterial
o Subtle abnormality in T cell function – predisposition to pneumocystis jiroveci infection, autoimmune disease and malignancy
o Failure to thrive

• Features
o Normal number of circulating B cells
o Normal number of T cells but activated cells do not express CD40L
o No germinal centre development within lymph nodes + spleen
 Failure of isotype switching
• Elevated serum IgM
• Undetectable IgA, IgE, IgG

Question 22

Common variable immune deficiency

Features
o Number of B cells
o serum IgM
o serum IgA, IgE, IgG

Where is the problem

Definition

Answer 22

o Number of B cells - normal
o serum IgM - normal
o serum IgA, IgE, IgG - low

o Failure of normal B cell maturation –> Reduction in immunoglobulins
o Mutation in MHC III

Defined by
o Marked reduction in IgG, with low IgA or IgM
o Poor/absent response to immunisation/vaccination
o Frequent infections
o Absence of other defined immunodeficiency

Associated with NHL, autoimmune diseases

Question 23

Common variable immune deficiency

Clinical features

Answer 23

• Clinical features – adults and children

o Recurrent bacterial infections
 Pneumonia, persistent sinusitis, gastroenteritis
 Often with severe end-organ damage

o	Pulmonary disease
	Interstitial lung disease
	Granulomatous interstitial lung disease (also LN, spleen)
	Obstructive airways disease
	Bronchiectasis 

o GI disease
 Inflammatory bowel like disease
 Sprue like illness
 Bacterial overgrowth

o	Autoimmune disease 
	Autoimmune haemolytic anaemia or thrombocytopenia
	Rheumatoid arthritis
	Pernicious anaemia
	Thyroiditis
	Vitiligo 

o Malignancy
 NHL

Question 24

Selective IgA deficiency

What percentage of people are symptomatic

what kind of infections do they suffer from

Answer 24

• 2/3 individuals asymptomatic
• 1/3 have recurrent respiratory tract infections
• Also GI infections

Question 25

Failure of B cell maturation Failure of T cell costimulation Failure of IgA production Failure of production of IgG antibodies

Answer 25

Failure of B cell maturation Bruton’s X-linked agammaglobulinaemia Failure of T cell costimulation X-linked hyper IgM syndrome Failure of IgA production Selective IgA deficiency Failure of production of IgG antibodies Common variable immune deficiency Selective antibody deficiencY

Question 26

Investigation of B cell deficiencies

Answer 26

• Total WCC + differentials – do they have lymphocytes? • Lymphocyte subsets o B cells, CD4, CD8 T cells, NK cells • Serum immunoglobulins and protein electrophoresis o IgG production is dependent on both the B cells and the CD4 T cells  Production of IgG is surrogate maker of CD4 T cell helper function o Therefore serum IgG if CD4 deficient • Functional tests of B cell function o Specific antibody responses to known pathogens/immunisations – measure IgG antibodies against tetanus, Haemophilus influenzae B, Strep. Pneumoniae o If specific antibody levels are low, immunise with appropriate killed vaccine and repeat antibody measurement 6-8 weeks later to see if they have responded • HIV

Question 27

Protein electrophoresis for investigation of B cell deficiencies which band gives you the antibodies? findings in Bruton's X-linked hypogammaglobulinaemia Monoclonal gammopathy (abnormal production of a single clone of plasma cells/antibodies)

Answer 27

On protein electrophoresis, it’s the γ band that gives you the antibodies – it’s all the antibodies, not just IgG e.g. Brutons  absent γ band monoclonal gammopathy (abnormal production of a single clone of plasma cells/antibodies)  high narrow peak in the γ band

Question 28

``` CD4 CD8 Bcell IgM IgG IgA in ``` ``` SCID Bruton's X-linked HyperIgM X-linked Selective IgA CIVD ```

Answer 28

Look at table - ix of B cell deficiencies

Question 29

Management of immunodeficiency involving B cells

Answer 29

• Aggressive prophylaxis/treatment of infection • Immunoglobulin replacement if required o Derived from pooled plasma from thousands of donors o Contains IgG antibodies to a wide variety of common organisms o Aim of maintaining trough IgG levels within normal range o Treatment is lifelong, every 3 weeks • Immunisation o For selective IgA deficiency (because it will boost the other responses) o Not otherwise effective because of defect in IgG antibody production • (for B cell immunodeficiencies you don’t usually do transplantations because what you are missing is the immunoglobulins and you can replace these)

Question 30

Primary lymphoid organs Secondary lymphoid organs

Answer 30

Primary lymphoid organs • Sites of B and T cell development • BM - B + T cell production, B cell maturation • Thymus - T cell maturation ``` Secondary lymphoid organs • Anatomical sites of interaction between naïve lymphocytes and microorganisms • Spleen • Lymph nodes • Mucosal associated lymphoid tissue ```

Question 31

Summary ``` Pre-B cells IgA IgG secreting plasma cells IgM secreting plasma cells Primary lymphoid organs Thoracic duct Thymus Germinal centre ```

Answer 31

Pre-B cells – exist within the bone marrow and develop from haematopoietic stem cells IgA – divalent antibody present within mucous which helps provide a constitutive barrier to infection IgG secreting plasma cells – cell dependent on the presence of CD4 T cell help for generation IgM secreting plasma cells – are generated rapidly following antigen recognition and are not dependent on CD4 T cell help Primary lymphoid organs – include both the bone marrow and the thymus; sites of B and T cell development Thoracic duct – carries lymphocytes from lymph nodes back to the blood circulation Thymus – site of deletion of T cells with inappropriately high or low affinity for HLA molecules and of maturation of T cells into CD4+ or CD8+ cells Germinal centre – area within secondary lymphoid tissue where B cells proliferate and undergo affinity maturation and isotype switching

Question 32

Types of primary immunodeficiency affecting Phagocytes Natural Killer cells Complement

Answer 32

Phagocytes Kostmann syndrome Leukocyte adhesion deficiency Chronic granulomatous disease Natural Killer cells Classical NK deficiency Functional NK deficiency Complement Classical pathway deficiencies Alternative pathway deficiencies Terminal pathway deficiencies

Question 33

Types of primary immunodeficiency affecting Haematopoietic stem cells Cytokines Lymphoid precursors

Answer 33

Haematopoietic stem cells Reticular dysgenesis Cytokines IL12 and IL12 receptor deficiency IFNg and IFNg receptor deficiency Lymphoid precursors Severe combined immunodeficiency

Question 34

Types of primary immunodeficiency affecting T cells B cells

Answer 34

T cells 22q11.2 deletion syndromes Bare lymphocyte syndrome ``` B cells X-linked agammaglobulinaemia X-linked hyperIgM syndrome Common variable immunodeficiency IgA deficiency ```

Question 35

Laboratory Tests for Primary Immunodeficiency White cells Immunoglobulins Complement

Answer 35

``` White cells Full blood count Lymphocyte subsets Special tests for white cell migration/function - Adhesion molecules – eg CD18 - Test for oxidative killing – DHR test ``` Immunoglobulins IgM, IgG, IgA Specific Igs and response to vaccination Complement Complement function - CH50 and AP50 Individual complement component

Question 36

What kind of conditions are seen in T cell deficiency

Answer 36

Intracellular + maligancy Viral infections (CMV) Fungal infection (Pneumocystis, Cryptosporidium) Some bacterial infections (esp. intracellular TB, Salmonella) Early malignancy

Question 37

What kind of conditions are seen in B cell deficiency

Answer 37

Bacterial infections (Staph, Strep) Toxins (Tetanus, Diptheria) Some viral infections (enterovirus)