Download IMMUNOLOGY ADVANCED

IMMUNOLOGY
ADVANCED:
THE GENETIC
PATHOPHYSIOLOGY OF
PRIMARY
IMMUNODEFICIENCY
DISORDERS (PIDs)
Dr. Peter Vickers
23/05/2017
1
CONTENTS
Introduction
 Genetic classification of PIDs
 Antibody deficiencies: 1. XLA
 Antibody deficiencies: 2.CVID
 Antibody deficiencies: 3. SIgA
 T-cell/combined deficiencies – ADA SCID
 Summary
 References & Further reading

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INTRODUCTION



PADs can be classified as being caused by:
1. a single gene mutation (e.g. XLA)
2. a multi-genetic susceptibility (e.g. CVID)
The 3 main modes of genetic inheritance are:
 Autosomal recessive
 Autosomal dominant
 X-linked recessive

 although there are other ways of obtaining
genetic defects, e.g. spontaneous mutation .
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
Within these 3 modes of inheritance, genetic
problems can be further subdivided by
mutations within genes responsible for certain
tasks, for example:

Some PIDs are caused by mutations within
the genes that are necessary for the
development of a functionally mature cell line
– for example Btk which is necessary for the
development of B-cell lymphocytes
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
Other mutations are caused by mutations in
genes that are responsible and necessary for
the development and functioning of a single
precursor

A third grouping of genetic mutations occurs
when a gene that is expressed in multiple
types of tissues/cells becomes mutated.
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Goals of Common
Disease Genetics
Phenotype
Genotype
• Work out the distribution of diseases or traits
within families and across populations.
• Identify the genes that predispose to a
disease or phenotype
• Determine if the particular diseases or
intermediate phenotypes have a genetic
component.
• Understand how those genes work towards
predisposition (e.g. interaction).
• Describe the pre-clinical natural history
• Identify the interactions between the gene
and the environment
WHY?
We can better understand
the disease pathogenesis
We can identify the
high risk populations
This allows us to:
•Develop specific preventive and therapeutic measures
•Apply individualized medicine
23/05/2017
Adapted
from
Rotter
2004
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GENETIC CLASSIFICATION OF
PIDs

Things are complicated by the fact that a specific
clinical phenotype may occur as a result of mutations
within multiple genes, which may sometimes occur
with different modes of inheritance.

An example of this is severe combined
immunodeficiency (SCID):
◦ X-linked SCID is caused by mutations of the common
gamma chain (γ-chain)
◦ Recessive SCID is caused by mutations of the JAK3 gene.
However, both types clinically present as T- B+
NK- SCID.
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Another consideration is that mutations of the same
gene can cause strikingly different clinical phenotypes.
 This is dependent upon the type of mutation and its
position within the gene.
 Example:
RAG-1/RAG-2 mutations usually result in T- B- NK+
SCID, but a different type of mutation within the gene
– for example a missense mutation, which still allows
the expression of a mutated gene (as opposed to a
nonsense mutation which stops the production and
expression of a gene) may result in Omenn Syndrome
(T+ B- NK+), with the added clinical symptoms of
erythematous rash, raised serum IgE, and eosinophilia.

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The fact that there are such a large number of genes
that, if mutated, are linked to PIDs just serve to
demonstrate how complex the immune system is.
 For example, genes involved in the maturation of
lymphocyte subsets, such as Btk, RAG-1 and RAG-2 are
responsible for immune cytopaenias and resulting
immune defects, whilst mutations of genes involved in
CD40 cell signalling cause problems with the signals that
are sent out by various cells - and so on.
 In terms of the most common PIDs and genetic
mutations, a large proportion of them have an X-linked
inheritance, many have an autosomal recessive mode of
inheritance, whilst a few are inherited via the autosomal
dominant route.

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


So it can be seen that genetically, the immune system
is very prone to many and varied mutated genes, just
because so many genes are involved in our immune
system, which can lead to the many and variable
presentations of PIDs.
This session will look at just 4 of the many different
PIDs so far identified in order to show how a PID can
develop.
This is important because it is only by an
understanding of the genetics of PIDs and, in
particular, the actual mutations that occur within the
genes responsible for the immune system, that we can
truly be able to treat/cure /care for children and
adults with these disorders.
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X-LINKED
AGAMMAGLOBULINAEMI
A
(Bruton’s Disorder)
B-CELL DEFICIENCY
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


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The gene responsible for XLA is called
Bruton's tyrosine kinase (Btk)
This is a member of a family called Tec
kinases, and Btk is a protein tyrosine kinase
This protein is expressed in both B cells and
neutrophils, but in XLA only the B cells are
defective
It is also expressed in erythroid precursors,
myeloid cells, mast cells, monocytes,
megakaryocites and platelets – but it is NOT
expressed in T-cells and NK-cells.
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




It is required for both B-cell survival and activation.
This gene provides instructions for making the BTK
protein, which is important for the development of B
cells and normal functioning of the immune system.
Mutations in the gene for Btk prevent the B cells from
maturing because Btk is a crucial signalling molecule
which regulates the development of B cells into plasma
cells
Only cells in which the normal allele of Btk is active can
develop into mature B cells
Thus, those people with defective Btk have impaired B
cell development and poor or absent antibody
production
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



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Most mutations in the Btk gene prevent the production of
any BTK protein. The absence of functional BTK protein
blocks B cell development and leads to a lack of
antibodies.
Signalling through the pre-B-cell receptors and B-cell
antigen receptors controls these developmental stages
and transitions.
Consequently, in Btk-deficient cells, these processes do
not happen and so the affected cells fail to proliferate, and
are destroyed through apoptosis (cell suicide).
Thus, in XLA patients, mature B-cells generally make up
only about 0.1% of circulating lymphocytes.
Typically about 1/3 of X-linked PIDs are new, sporadic
mutations
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Genetics of X-linked
Agammaglobulinaemia
There are more than 300 known mutations
and most of them cause a truncation of the
protein.
 These mutations are either nonsense
mutations or frameshift mutations (caused by
insertions, deletions or splice site defects)

 (Vihinen M 2003)
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Clinical Features & Pathophysiology:
Immunoglobulin G (IgG) is actively transported
across the placenta, so XLA patients will generally
have normal levels of IgG at birth and few, if any
symptoms.
 Soon, though, the maternal IgG becomes
catabolised, leading to hypogammaglobulinaemia
and an increased susceptibility to infections.
 Bruton’s disorder – boys with Bruton’s usually
present with recurrent pyogenic infections (fevers)
between the ages of 4 months and 2 years.

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
Over 90% of these infections will be in the
respiratory tract, with 20% of them affecting
the gastro-intestinal tract, 13% the central
nervous system, 5% the genito-urinary tract, 5%
the bones, and 9% affecting the skin and other
organs.

A medical history will usually show that the boy
has had mainly recurrent infections of the
upper and lower respiratory tract, whilst other
usual presenting sites may well include:
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Skin sepsis – boils, abscesses and cellulitis
 Urinary tract infections
 Episodes of diarrhoea and vomiting
 Arthritis
 Meningitis

◦ with the commonest infecting agents being
pyogenic bacteria (e.g. streptococci, Haemophilus
influenza and Streptococcus pneumonia, although
they may also have presented with less common
infectious organisms.
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
However, because cell-mediated immunity (Tcells) is still present in these boys, then they
tend not to have problems with viral or fungal
infections – but be aware that continual
bacterial infections may eventually leave the
body open to secondary viral and/or fungal
infections.

Laboratory analysis will tend to demonstrate
absent or very few circulating mature B-cells,
with normal or increased T-cells.
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
Thus, the diagnosis of an antibody deficiency (not
just XLA) is made from:





Medical history
Signs & Symptoms
Low serum levels of all immunoglobulin classes
Absence of mature circulating B-cells
Family history of affected male relatives (
particularly when diagnosing Bruton’s XLA)

A warning: in very exceptional circumstances and very
rarely, female siblings of affected male children could
also be antibody deficient.
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COMMON VARIABLE
IMMUNODEFICIENCY (CVID)
“CVID is currently viewed as a heterogeneous group of
disorders with an intrinsic B-cell defect or a B-cell
dysfunction related to abnormal T-cell-B cell interaction. In
the majority of cases, there is no family history of a related
or similar defect. However, in 10% to 20% of families,
another member may have selective IgA deficiency, IgG
deficiency, or much more rarely, CVID”
 (Ochs et al. 2004 page 375)

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

CVID is often a ‘catch-all’ category for a
group of immune deficiency disorders
presenting in either childhood or adulthood –
but the majority of people with this condition
are usually not diagnosed until they reach
adult status.
So, CVID is an immune deficiency disorder
that can present at any age, and is
characterised by:
◦ Recurrent bacterial infections
◦ Hypogammaglobulinaemia
◦ Impaired antibody responses (even though B-cells
are usually present)
◦ Normal/near normal T-cell immunity.
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
Because the genetic elements of this disease
are unpredictable and the fact that there is
so much variability in age of onset, as well as
the clinical and laboratory anomalies, it is
believed that CVID is not caused by a single
genetic defect.
To date, no precise molecular defect has
been identified for most cases of CVID
 (Ochs et al, 2004)

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


CVID is thought of as a heterogonous group of
disorders with an intrinsic B-cell defect or a Bcell dysfunction which are related to abnormal
T-cell - B-cell interactions.
In the majority of cases, there is no family
history of a related/similar defect, although
another family member may have selective IgA
deficiency, IgG deficiency, or – rarely – CVID.
Although diagnosed in 1953, the fundamental
cause(s) of CVID remains unknown.
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Due to the unclear genetic nature of CVID, a
clear pattern of inheritance has not been defined.
 In some instances, more than one family member
is found to be deficient in one or more types of
immunoglobulins. - for example, it is not too
unusual for one family member to have CVID
whilst another may have selective IgA deficiency.
 Because of the variability in age at onset and also
within the clinical and laboratory findings, as well
as the evidence of an unpredictable genetic
component, it is thought that CVID is not caused
by a single gene defect.

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In the past few years, mutations in several different
genes have been found to be associated with CVID.
 These include inducible co-stimulatory (ICOS) in
one family and a protein on B-cells (CD19) in
several families as causes of autosomal recessive
CVID
 Mutations in a cell receptor (TACI) for two factors
(BAFF or APRIL) needed for normal growth and
regulation of B-cells have also been found in about
10% of patients with CVID.


A causative role of these mutations in the immune
defect is not yet clear since some of these mutations
can be found in people with normal immunoglobulins.
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The main phenotype of CVID is a lack of immune
globulin, and hence of antibodies, which occur as a
result of a variable block in B-cell differentiation.
 Most patients with CVID have normal numbers of
B-cells (although a few do have low B-cell
numbers).
 In addition, a few males diagnosed with CVID may
actually have an atypical XLA, and should be
checked for Btk gene mutations.
 Unlike the mature B-memory cells found in nonantibody deficient people, B-cells found in people
with CVID whilst possessing the characteristics of
immature B-cell lymphocytes, have deficiencies in
actual numbers and the activation of memory Bcells.

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


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Some patients with CVID also have associated
T-cell defects. In these patients, although T-cell
subsets are generally normal, many of them
have lymphopaenia.
Then there is a subgroup of CVID patients (2530%) who have increased numbers of CD8+
lymphocytes with normal or decreased
numbers of CD4+ lymphocytes, and a reduced
CD4/CD8 ratio.
Finally, many of these patients have a possible
persistence of naϊve T-cells.
These findings may be as a result of the T-cells
continually being required for chronically
infected tissues.
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◦ Other potential problems for patients with CVID
include:
◦ Reduced expression of cell surface molecules
involved in adhesion (adhesion-switching defects).
◦ Despite the main characteristic of CVID being
profound antibody defects, several types of immune
over-activation could also contribute to some of the
clinical pathology, and hence the signs and symptoms
of CVID.
◦ Monocyte/macrophage defects which may cause the
immune system to be altered away from antibody
production to monocyte production.
◦ This monocyte activation may also be involved in the
pathogenesis of the chronic inflammatory and
granulomatous complications experienced by some
patients with CVID
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Some CV ID patients have inflammatory bowel
disease (either Crohn’s disease or ulcerative
cholitis) presenting with atypical inflammatory
disease resulting in diarrhoea, malabsorption &
weight loss.
 Some may also have chronic malabsorption
syndrome .
 Patients are also at risk of GI infections, which
may be caused by:

◦
◦
◦
◦
◦
Giardia lamblia
Salmonella
Shigella
Campylobacter
Other odd rarer enteropathogenic organisms
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Autoimmune disorders are more frequent in CVID
patients (and their relatives) than in the general
population.
 20-25% of patients with CVID develop one or more
autoimmune conditions, such as:

◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
Rheumatoid arthritis
Dermatomyositis
Scleroderma
SLE
Autoimmune haemolytic anaemia
Immune thrombocytopaenic purpura
Autoimmune neutropaenia
Chronic active hepatitis
Parotitis
Alopaecia
Primary billiary cirrhosis
Guillon-Barré syndrome
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Both adult and paediatric CVID patients may
develop noncaseating granulomas of the lung,
spleen, liver, skin and other tissues – these
resemble sarcoidosis
 There is also an unusually high incidence of
lympho-reticular and gastrointestinal
malignancies in older patients with CVID
 However, as with other congenital
immunodeficiencies, the lymphomas in CVID
tend to be extranodal in origin and are of Bcell lineage.

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Selective Immunoglobulin A
Deficiency (SIgA)
Selective IgA deficiency is a primary immunodeficiency
disease and is the most common of the primary
antibody deficiencies.
 Total immunoglobulin A deficiency (IgA) is defined as an
undetectable serum immunoglobulin A (IgA) level at a
value < 5 mg/dL (0.05 g/L), whilst partial IgA refers to
detectable but decreased IgA levels that are more than
2 standard deviations below normal age-adjusted
means.
 IgA is commonly associated with normal B lymphocytes
in peripheral blood, normal CD4+ and CD8+ T cells, and,
usually, normal neutrophil and lymphocyte count

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Anti-IgA autoantibodies of the IgG and/or IgE
isotype may be present.
 Peripheral blood may also be affected by
autoimmune cytopaenias, e.g., autoimmune
thrombocytopaenia, and patients may have other
autoimmune phenomena.
 Thus IgA deficiency is a heterogeneous disorder,
and the results of intensive study are beginning to
identify genetic loci and molecular pathogeneses
that contribute to various subtypes of this
disorder.
 Several lines of evidence suggest that, in many
cases, IgA and common variable
immunodeficiency (CVID) have a common
pathogenesis, whilst other data indicate different
genetic risk factors.

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Family studies show variable inheritance
patterns.
 Familial inheritance of IgA deficiency occurs in
approximately 20% of cases, whilst, within a
number of families, IgA and CVID are
associated.
 Many IgA deficient patients are asymptomatic
and are identified by finding a laboratory
abnormality, without any apparent associated
clinical disease.
 Some patients with IgA deficiency may have
the following associated conditions:

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Deficits in one or more immunoglobulin G (IgG)
subclasses (this accounts for 20-30% of IgA-deficient
patients, many of whom may have total IgG levels
within the normal range) or
 A deficient antibody response to pneumococcal
immunisation (specific polysaccharide antibody
deficiency).
 Some patients with IgA later develop CVID, although
family members of patients with CVID may have only
selective IgA deficiency.
 Primary IgA deficiency is permanent, and belownormal levels have been found to remain static and
to persist for more than 20 years, although,
interestingly, there is one report documents a rare
case of reversion (Desar et al, 2007).

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SEVERE COMBINBED
IMMUNE DEFICIENCY (SCID)

T-CELL/COMBINED IMMUNE
DEFICIENCY
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ADA SCID


Just a (very) brief look at another type of PID
ADA SCID is an autosomal recessive disorder
ADA = adenosine deaminase (an enzyme of
purine metabolism)
 PNP = purine nucleoside phosphorylase
 ADA (and PNP) acts sequentially on purine
nucleosides - namely adenine and guanine.

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
ADA is expressed in all cells, but it serves an
extremely essential protective role in
lymphocytes.
Lymphocytes are exceptionally sensitive to
metabolites from the breakdown of nuclear
acids if ADA is absent.
This results particularly in the accumulation of
two metabolites - namely :
dATP (deoxyadenosine triphosphate) and
dGTP (deoxyguanosine triphosphate)

These metabolites are toxic to lymphocytes.




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OTHER MUTATED
GENES CAUSING SCID
JAK3 = 19p13.1 (Janus Kinase 3)
 RAG1 = 11p13 (Recombinase-Activating Gene 1)
 RAG2 = 11p13 (Recombinase-Activating Gene 2)
 CD45 = 1q31-q32
 X-linked SCID = Xq13.1

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ADA SCID MUTATIONS
To date, there have been 54 unique
mutations identified for this gene, as
opposed to only 3 for the CD45 gene and
as many as 169 for the IL2RG gene
 Of these 54 unique mutations, 37 are
missense mutations, 9 are splice-site
mutations, 4 are frameshift deletions, 3 are
nonsense mutations, and 1 is an inframe
and gross deletion.

 (Kalman et al 2004)
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SUMMARY



These are just a very few of the many, many
different types of primary immunodeficiency
disorders, and mainly we have looked at just 3
42antibody deficiencies.
Antibody deficiencies make up most of the PIDs
that nurses will see and deal with, which is why this
session concentrated on them.
Other PID groups – combined/severe combined
immune deficiencies, phagocyte deficiencies and
complement deficiencies are also very interesting
but time would only allow for SCID to be very
briefly discussed.
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REFERENCES

Desar IM, Weemaes CM, van Deuren M, van der Meer JW.
Reversible hypogammaglobulinaemia. Neth J Med. Nov
2007;65(10):381-5.

Kalman L, Lindagren ML, Kobrynski L et al. (2004)
Mutations in genes for T-cell development: IL7R, CD45,
L2RG, JAK3, RAG1, RAG2, AETEMIS and ADA, and severe
combined immunodeficiency: HuGE review Genetics in
Medicine 6:1 16-26

Ochs HD, Stiehm, ER, Winkelstein JA (2004) Antibody
Deficiencies in Stiehm ER, Ochs HD, Winkelstein JA (eds)
Immunologic Disorders in Infants and Children (5th. Ed)
Philadelphia: Elsevier Saunders
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
Rotter JI (2004) Approaching the Genetics of
Common Diseases PowerPoint Slides; Director
of Research and Co-Director, Medical Genetics
Institute, Cedars-Sinai, Board of Governors’
Chair in Medical Genetics, Cedars-Sinai,
Director, Division of Medical Genetics, CedarsSinai, Professor of Medicine, Pediatrics & Human
Genetics, UCLA
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FURTHER READING

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Chapel, H., Haeney, M., Misbah, S., Snowden, N. (2014)
Essentials of Clinical Immunology (6th. Ed.) WileyBlackwell, Oxford, UK
Male, D., Brostoff, J., Roth, D.B. Roitt, I. (2012)
Immunology (8th. Ed.) Elsevier, Saunders, Philadelphia,
USA
Murphy, K. (2011) Janeway's Immunobiology (8th. Ed.)
Garland Science, New York, USA
Nairn, R., Helbert, M. (2007) Immunology for Medical
Students (2nd. Ed.) Elsevier Mosby, Edinburgh, UK
Stiehm, E.R., Ochs, H.D., Winkelstein, J.A. (2004)
Immunologic Disorders in Infants and Children (5th. Ed.)
Elsevier Saunders, Philadelphia, USA
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