1224 SECTION XI Pediatric Critical Care Immunity and Infection Recognition of the clinical features of IPEX is the first step in diagnosing this disorder Sequencing of the FOXP3 gene remains the gold[.]
Trang 1Recognition of the clinical features of IPEX is the first step in
diagnosing this disorder Sequencing of the FOXP3 gene remains
the gold standard for making a diagnosis of IPEX, although
se-quencing needs to encompass noncoding areas of the gene,
in-cluding the upstream noncoding exon and the polyadenylation
signal sequence in order to cover all regions in which pathogenic
mutations have been identified.77 , 78 Flow cytometry to measure
FOXP3 protein expression and FOXP31 Tregs is a helpful
ad-junct to gene sequencing, although only ,25% of patients have
mutations that are predicted to completely abrogate FOXP3
pro-tein expression The remainder of patients have varying degrees of
FOXP31 Treg deficiency due to the fact that mutant FOXP3 may
not support normal Treg development As a result, flow cytometry
by itself is not considered to be a sufficiently reliable screening test
for IPEX
Initial therapy for IPEX typically consists of aggressive
sup-portive care (e.g., parenteral nutrition, insulin, thyroid hormone)
combined with T-cell–directed immune regulator using agents
such as tacrolimus, cyclosporine, or rapamycin HSCT is
cur-rently the only curative therapy for IPEX Early HSCT using a
nonmyeloablative conditioning regimen before the onset of
au-toimmune-mediated organ damage usually leads to the best
outcome and limits the adverse effects of therapy.79 , 80 Because
Tregs constitutively express the high-affinity IL-2 receptor, they
have a selective growth advantage in vivo As a result, complete
donor engraftment in all hematopoietic lineages may not be
nec-essary because preferential engraftment of donor Tregs can be
sufficient to control the disease.81
Specific Disorders: Other Complex or Combined
Immunodeficiencies
Wiskott-Aldrich Syndrome
WAS is unique among immunodeficiency disorders because
af-fected patients have both an infectious susceptibility and a
bleed-ing disorder The bleedbleed-ing problems are caused by the platelets
being small (mean platelet volume ,5 fL), dysfunctional, and
decreased in number (usually platelet counts ,70,000/µL)
Pa-tients with WAS typically present in infancy with bloody diarrhea
and/or bruising, recurrent upper respiratory tract infections, and
eczema The incidence of hematopoietic malignancies is high IgE
levels are often elevated Serum IgG levels and T-cell counts are
often normal in infancy but may decrease over time Responses to
vaccination, particularly with carbohydrate antigens such as
Pneumovax, are often abnormal
WAS, with X-linked recessive inheritance, is caused by mutations
in the WAS gene, located on the short arm of the X chromosome.
Genotype/phenotype correlations of mutations in WAS are
discussed on ExpertConsult.com
remains controversial.85 Some have advocated that the benefits for curing the bleeding problems are worth the risk if a matched sibling donor is available Gene therapy has been successful in a small number of patients with WAS However, like the X-SCID gene therapy trials, some treated patients developed T-cell leuke-mias as a result of integration of the viral gene therapy vector near
an oncogene.86
Cartilage-Hair Hypoplasia
Cartilage-hair hypoplasia (CHH) is a complex disorder character-ized by skeletal dysplasia with short-limb dwarfism (metaphyseal chondrodysplasia), sparse hypoplastic hair, gastrointestinal prob-lems (Hirschsprung disease), skin hypopigmentation, and a vari-able immunodeficiency CHH is caused by mutations in the
RMRP gene, which encodes the 267 base-pair RNA components
of the RNase MRP complex, which plays a role in processing of precursor ribosomal RNA The mechanism by which autosomal
recessive mutations in RMRP cause the clinical features of CHH
is unknown The largest populations of CHH patients have been identified among the Old Order Amish and Finnish populations Virtually all patients with CHH have some degree of immunode-ficiency that can range from a mild humoral defect with decreased vaccine responses to a SCID-like phenotype associated with pro-gressive lymphopenia and severe infections with bacteria, viruses, and fungi A leaky-SCID phenotype has also been described in RMRP deficiency The diagnosis of CHH can be made on the basis of the clinical phenotype and confirmed by sequencing of
the RMRP gene For those with a major cellular immune defect
related to CHH, HSCT could be a cure immunologically but not for the other features that persist.87 Supportive therapies such as immunoglobulin replacement therapy or prophylactic antibiotics should be implemented for milder phenotype
Radiation-Sensitive Disorders: Ataxia Telangiectasia and Nijmegen Breakage Syndrome
Ataxia telangiectasia (AT) is a disorder associated with progressive neurologic decline, immunodeficiency, and propensity to
malig-nancy It is caused by autosomal-recessive mutations in the ATM
gene, which encodes a serine/threonine kinase that acts together with the NBS1 protein as one of the major sensors of double-stranded DNA breaks in the cell ATM phosphorylates key pro-teins involved in activation of the DNA damage repair check-point, leads to cell-cycle arrest, and then leads to double-stranded DNA break repair In the absence of functional ATM or NBS1, cells have a marked sensitivity to ionizing radiation Because
rear-rangement of the TCR gene and the immunoglobulin gene loci
also require double-stranded DNA break repair, these processes may be affected as well
Patients with AT usually present in early childhood (most commonly between 2 and 5 years of age) with cerebellar ataxia that progresses to unsteady gait and, over time, choreoathetosis Telangiectasias (small tufts of dilated blood vessels under the sur-face of the skin or mucous membranes) typically develop, first on the conjunctivae and later on the nose, ears, and shoulders They can be an important diagnostic clue in a child with progressive ataxia The majority of patients have immunoglobulin deficiency
of varying degrees and can develop sinopulmonary symptoms and sepsis Progressive neurodegeneration can compromise swallow-ing; thus, it is hard to determine whether respiratory infections
Treatment of WAS initially involves supportive care, including
treatment of any acute infections and management of any
bleed-ing episodes In general, repeated platelet transfusions are avoided
because of the concern that patients will become sensitized to a
wide array of HLA types, which may increase the risk of
compli-cations during subsequent HSCT Splenectomy can increase
platelet counts, but it also increases the risk that patients may die
of sepsis with encapsulated organisms Thus, there continues to be
significant controversy surrounding the role of splenectomy in
WAS Patients with the full WAS phenotype should be evaluated
for HSCT with matched related or unrelated donor bone marrow
or cord blood.84 Haploidentical transplants have proven risky in
this disease and are generally avoided The role of HSCT in XLT
Trang 2Genotype/Phenotype Correlations of Mutations
in Wiscott Aldrich Syndrome
There is a distinct genotype/phenotype correlation of mutations
in WAS: mutations that destroy WAS protein (WASp) expression
lead to the full syndrome of combined immunodeficiency,
ec-zema, and platelet dysfunction Mutations that allow expression
of a mutant WASp are typically associated with a milder X-linked
thrombocytopenia (XLT) phenotype in which the platelet
dys-function persists but the immunodeficiency is mild Point
muta-tions in the CDC42 binding domain of the WASp lead to a third
phenotype of X-linked neutropenia (XLN), which is generally not
accompanied by bleeding abnormalities.82 , 83
The WASp is expressed primarily in lymphoid and myeloid cells, where it functions to nucleate actin polymerization in the cell Patients with WAS therefore have problems with directed migration of neutrophils and with clustering and signaling through T-cell and B-cell antigen receptors (this causes decreased signaling into the cell and abnormal proliferative responses) A diagnosis of WAS can be confirmed by demonstrating the absence
of WASp in cells by flow cytometry or Western blotting or by
identifying a mutation in the WAS gene.
Trang 3occur more as a result of the immunodeficiency or the motor
defects Most affected individuals have elevated serum
a-fetopro-tein levels, which can be useful diagnostically after 6 months of
age Malignancies are an important complication as a result of the
DNA-repair defect Acute T-cell leukemias are common and often
demonstrate chromosomal translocations that affect the
chromo-somal regions involved in T-cell receptor gene rearrangements
B-cell lymphomas also occur and are usually associated with
11q22-23 chromosomal deletions There is also an increase in the
frequency of epithelial tumors in both homozygotes and ATM
mutation carriers Affected patients usually die in the second or
third decade of life from pulmonary complications or cancer
Management of immunodeficiency includes immunoglobulin
replacement and prophylactic antimicrobials
Other DNA repair defects that cause varying degrees of ataxia
and/or mild mental retardation include an ataxia-like syndrome
caused by mutations in MRE11 and the Nijmegen breakage
syn-drome (NBS), caused by mutations in NBS1 In addition to an
immunodeficiency like that observed in AT, patients with NBS
have marked microcephaly, mild developmental delay/mental
re-tardation, and a strong propensity to develop lymphomas but do
not have telangiectasia or elevated a-fetoprotein levels Another
DNA repair defect, Bloom syndrome, caused by mutations in a
DNA helicase (RecQ proteinlike-3), results in excess sister
chro-matid exchanges; it is associated principally with lymphomas and
cancer Multiple primary tumors occurring at an early age are
common Reduced growth in childhood results in a proportional
dwarfism that, with cutaneous telangiectasia, is a useful physical
sign Inheritance is autosomal recessive, and the disease occurs
with increased frequency in Ashkenazi Jewish populations
Chronic lung disease occurs and may be related to humoral
im-munodeficiency.88
Mammalian susceptibility to mycobacterial disease is discussed
on ExpertConsult.com
Toll-Like Receptors and Innate Signaling Pathway Defects
Toll-like receptors are a family of at least 10 pattern recognition receptors that are expressed in varying combinations on a broad array of immune and nonimmune cells (see Chapter 100) They recognize particular types (patterns) of molecules derived from pathogens (e.g., bacterial lipopolysaccharide, flagellin, mannan, CpG dinucleotides, viral dsDNA) Triggering of toll-like recep-tors (TLRs) activates intracellular signaling pathways, many of which converge on a common pathway using the IRAK proteins and MyD88, which, in turn, activate the IkB kinase complex (NEMO/IkKa/IkKb), ultimately leading to phosphorylation and degradation of IkBa and activation of the nuclear factor-kB (NF-kB) transcription factor complex
Mutations in TLR signaling pathways have now been
associ-ated with major infectious susceptibilities: mutations in IRAK4 and MyD88 have been identified in patients with susceptibility to
invasive pyogenic bacterial infections, including particularly
Streptococcus pneumoniae, Staphylococcus aureus, and Pseudomonas aeruginosa, characteristically presenting with a lack of fever and
signs of inflammation (ESR/CRP) despite active infection Inter-estingly, patients with IRAK4 or MyD88 deficiency tend to have problems with invasive pyogenic bacterial infections in child-hood, but these subside over time, presumably as a result of the adaptive immune response covering for this innate defect
Muta-tions in TLR3, TRIF, TRAF3, and UNC-93B have been identified
in patients with susceptibility to herpesvirus encephalitis Defects
in NEMO and IkBa also cause susceptibility to infections with pyogenic bacteria and mycobacteria and are associated with an anhidrotic ectodermal dysplasia phenotype The severity of im-munodeficiency and ectodermal dysplasia is quite variable and depends on the specific mutation that is present.93
Chronic Mucocutaneous Candidiasis Syndromes
Chronic mucocutaneous candidiasis (CMC) is a clinical syndrome
associated with chronic and recurrent Candida albicans infections
of mucosae, particularly oral and genital, together with nail-bed infections but generally without systemic infection CMC can oc-cur in isolation or as part of a broader clinical syndrome such as in
Susceptibility to Hemophagocytic
Lymphohistiocytosis and Severe Epstein-Barr
Virus Infection
Several defects affecting intracellular trafficking of vesicles and
granules are associated with susceptibility to hemophagocytic
lymphohistiocytosis (HLH) This group includes some
pheno-types that are quite distinctive, including Chédiak-Higashi
syndrome (CHS) and Griscelli syndrome (GS), which are both
associated with a partial oculocutaneous albinism Patients with
CHS have pyogenic lung and skin infections and periodontitis
Their neutrophils have reduced chemotaxis and contain giant
lysosomes (Fig 103.3) Patients with GS also have neurologic
defects ranging from developmental delay to fatal
neurodegen-eration Their neutrophils are dysfunctional, but they lack the
giant granules of CHS In vitro tests show NK cell and cytotoxic
cell function CHS, GS, and X-linked lymphoproliferative
syn-drome (XLP) share susceptibility for HLH—an accelerated
in-flammatory process that, in the case of XLP, may be triggered by
EBV This is characterized by hyperproliferation of activated
lymphocytes that infiltrate tissues and cause end-organ damage
with fever, hepatosplenomegaly, pancytopenia, and
hemophago-cytosis The genes responsible for these syndromes are LYST,
RAB27A, SH2D1A, and XIAP/BIRC4, respectively, whereas
genes for a further group of proteins (Perforin, Munc 13-D, and
Syntaxin 11) are associated with familial hemophagocytic
lym-phohistiocytosis.92
with basophilic cytoplasmic inclusion consistent with Chédiak-Higashi syndrome.
Trang 4Mammalian Susceptibility to Mycobacterial
Disease
Under normal circumstances, intracellular pathogens such as
my-cobacteria induce production of IL-12 and IL-23, which play a
role in driving maturation of nạve T cells into activated T helper
type I (Th1) cells that produce interferon-g (IFN-g) IFN-g then
acts on a variety of cells, including phagocytes (neutrophils,
monocytes, and dendritic cells) to induce a number of IFN-
inducible genes Multiple molecules in this signaling pathway have
been found to be defective in patients with intact cellular
immu-nity who have invasive, nontuberculous mycobacterial infections
There are mutations in the genes encoding the IL12 p40 subunit
(IL12B), the IL-12 receptor b1 chain (IL12RB1), the TYK2
tyro-sine kinase that is associated with the IL-12 receptor and required
for phosphorylation of the STAT4 transcription factor (TYK2),
the IFN-g receptor subunits (IFNGR1 and IFNGR2), and the
STAT1 transcription factor that is activated in response to IFN-g
(STAT1) Mutations in this pathway are also associated with other
infections, including invasive salmonellosis and severe viral
infec-tions (STAT1) In addition to these genetic defects, patients with
invasive mycobacterial disease have now been identified with neutralizing autoantibodies to IFN-g or IL-12 Together, these discoveries demonstrate a major role for the IL-12/IL-23/IFN-g axis in normal human immunity.89 For patients with the milder, autosomal-dominant IFN-g receptor defects, IL-12 p40 defects, IL-12 receptor defects, and TYK2 defects, antimicrobial therapy and IFN-g treatment are often sufficient to treat invasive myco-bacterial infections and prevent future infections For patients with the more severe, autosomal-recessive IFN-g receptor defects, IFN-g supplementation provides no benefit and HSCT is warranted.90 , 91
Trang 5APECED (autoimmune polyendocrinopathy, candidiasis,
ectoder-mal dystrophy) Recent discoveries have determined that, in
al-most all cases, CMC is associated with abnormalities in
develop-ment of Th17 effector T cells or with the production of, or
responses to, the cytokine IL-17 These include autosomal
reces-sive mutations in IL-17F or the IL-17 receptor a-subunit,
domi-nant loss-of-function mutations in STAT3, domidomi-nant
gain-of-function mutations in STAT1, recessive deficiency of DOCK8
(hyper-IgE syndrome), and recessive deficiency of CARD9.94
Recently, the CMC associated with APECED was found to be
the result of autoantibodies to IL-17 and IL-22.95 , 96 APECED is
caused by mutations in AIRE1, a nuclear protein that regulates
the expression of self-antigens by thymic medullary epithelial
cells, where it plays a role in T-cell selection and maturation
Other features of this disorder include polyendocrinopathies
(hypoparathyroidism, adrenal insufficiency, gonadal failure, and
insulin-dependent diabetes), alopecia, nail dystrophy, and vitiligo
Autoimmune Lymphoproliferative Syndrome
The clinical phenotype of autoimmune lymphoproliferative
syn-drome (ALPS) is characterized by massive lymphadenopathy and
hepatosplenomegaly in most cases In addition, patients have
problems with recurrent episodes of autoimmune hemolytic
ane-mia and thrombocytopenia The defects that have been identified
in patients with this group of disorders are all associated with
abnormalities in lymphocyte apoptosis These include FAS
(CD95), Fas ligand (FASL), FADD, caspase 10 (CASP10), NRAS,
and KRAS Peripheral blood T- and B-cell counts are generally
normal However, in almost all cases, there is an increased
per-centage of ab-TCR1 double-negative T cells that lack expression
of both CD4 and CD8 A predisposition to developing lymphoid
malignancies has been described in ALPS but is thought to be
primarily in those patients who have mutations in the death
do-main of FAS In addition to elevated double-negative T cells,
pa-tients with ALPS frequently have elevated levels of IL-10 and
vi-tamin B12 in the blood, which can be valuable diagnostically in
patients with suspected ALPS Defective in vitro Fas-mediated
apoptosis and mutation analysis of known ALPS gene can provide
a definitive diagnosis Patients with mutations in caspase 8
(CASP8) were previously classified as ALPS but are now
consid-ered to be a unique syndrome that may have lymphadenopathy
and splenomegaly, but recurrent respiratory tract infections and
mucocutaneous herpesvirus infections are prominent, both
un-usual features in ALPS.97 , 98
A variety of immunosuppressive therapies have been tried in
ALPS to control the recurrent autoimmune cytopenias and severe
hepatosplenomegaly Most of these were only modestly successful,
as was splenectomy However, recent studies have demonstrated a
dramatic response to rapamycin therapy in many patients with
ALPS, often causing shrinkage of the lymph nodes and spleen
back to their normal size.99 , 100
Laboratory Evaluation of the Immune System
A basic laboratory workup to screen for significant defects in
each of the four major compartments of the immune system
can be done by most practitioners before making a referral to
a clinical immunologist for further detailed evaluation In
simple terms, this workup should include evaluation of
num-bers and function for each of the four immune system
com-partments A recommended workup using this approach is
outlined in eTable 103.3
Diagnostic Testing: Complement
Screening for complement deficiency should be performed in pa-tients with recurrent or severe episodes of bacteremia, meningitis,
or disseminated gonorrhea Since more than 30% of patients with
recurrent Neisseria infections have complement deficiency, it is
im-perative that prompt testing of the complement system be con-ducted for these individuals The screening test of choice for com-plement deficiencies measures functional classical comcom-plement activity in the plasma If an alternative pathway defect is suspected, however, an analogous test alternative pathway evaluation can be performed For the complement assays to give accurate results, the blood specimen needs to be handled carefully because complement
is very heat labile In general, it is recommended that any abnormal classical pathway test should be repeated to confirm a complement deficiency The classical pathway is typically very low or absent in patients with a complement component deficiency A result that is only moderately low is often seen in situations in which the speci-men was handled incorrectly or in patients with autoimmune dis-ease, such as SLE or mixed connective tissue disease Once an ab-normal functional complement test is confirmed, specialized testing
to identify the specific complement component that is absent can
be performed, including C3 and C4 serum level
Diagnostic Testing: Phagocytes
Assessment of patients for a possible phagocytic disorder requires that both the number and the function of phagocytes to be evaluated Numbers are easily evaluated using a complete blood count with differential If there is a concern for cyclic neutropenia, neutro-phil counts may need to be evaluated three times weekly for 3 to
4 weeks to identify the nadir.101 Functional testing includes evaluation of CD11/CD18 integrin expression on myeloid cells
by flow cytometry if the patient has symptoms suggestive of LAD In cases of suspected CGD, evaluation of neutrophil oxi-dative burst function is essential Traditionally, this was done using nitroblue tetrazolium (NBT) but is now performed using dihydrorhodamine (DHR), a reagent that permeates neutrophils and fluoresces when reduced by a normal neutrophil oxidative burst Fluorescence is measured by flow cytometry The DHR test is sensitive enough to differentiate most cases of X-linked CGD from autosomal recessive CGD, making it a particularly useful clinical assay.102 , 103
Diagnostic Testing: B Cells and Antibodies
The diagnosis of antibody deficiency needs to evaluate both the quantity and quality of the antibody response Quantity is easily evaluated by measuring quantitative immunoglobulin levels (IgG, IgM, IgA, and IgE) in the blood and comparing these with the age-appropriate normal ranges Evaluating the quality of the anti-body response can be done by measuring specific antianti-body titers
to vaccines that the patient has received Generally, responses to both protein antigens (e.g., tetanus, diphtheria, hepatitis B) and carbohydrate antigens (23-valent unconjugated Pneumovax) need
to be assessed to confirm normal antibody responses Antibody titers will be optimal if measured 4 weeks after vaccination Pa-tients who respond appropriately to protein antigens but do not
respond to carbohydrate antigens may have specific antibody defi-ciency and may require additional workup Recent guidelines
re-garding the interpretation and use of diagnostic vaccination have been published.104