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[.]
1224 S E C T I O N X I 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 standard for making a diagnosis of IPEX, although sequencing needs to encompass noncoding areas of the gene, including 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 adjunct to gene sequencing, although only ,25% of patients have mutations that are predicted to completely abrogate FOXP3 protein 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 supportive 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 currently the only curative therapy for IPEX Early HSCT using a nonmyeloablative conditioning regimen before the onset of autoimmune-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 necessary 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 affected patients have both an infectious susceptibility and a bleeding disorder The bleeding problems are caused by the platelets being small (mean platelet volume ,5 fL), dysfunctional, and decreased in number (usually platelet counts ,70,000/µL) Patients 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 Treatment of WAS initially involves supportive care, including treatment of any acute infections and management of any bleeding 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 complications 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 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 leukemias 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 characterized by skeletal dysplasia with short-limb dwarfism (metaphyseal chondrodysplasia), sparse hypoplastic hair, gastrointestinal problems (Hirschsprung disease), skin hypopigmentation, and a variable 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 immunodeficiency that can range from a mild humoral defect with decreased vaccine responses to a SCID-like phenotype associated with progressive 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 malignancy 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 doublestranded DNA breaks in the cell ATM phosphorylates key proteins involved in activation of the DNA damage repair checkpoint, 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 rearrangement 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 and years of age) with cerebellar ataxia that progresses to unsteady gait and, over time, choreoathetosis Telangiectasias (small tufts of dilated blood vessels under the surface 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 swallowing; thus, it is hard to determine whether respiratory infections 1224.e1 Genotype/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, eczema, 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 dysfunction persists but the immunodeficiency is mild Point mutations 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 CHAPTER 103 Congenital Immunodeficiency occur more as a result of the immunodeficiency or the motor defects Most affected individuals have elevated serum a-fetoprotein levels, which can be useful diagnostically after 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 chromosomal 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 syndrome (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 retardation, and a strong propensity to develop lymphomas but 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 chromatid 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 immunodeficiency.88 Mammalian susceptibility to mycobacterial disease is discussed on ExpertConsult.com 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 phenotypes 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 neurodegeneration 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 syndrome (XLP) share susceptibility for HLH—an accelerated inflammatory 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 hemophagocytosis 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 lymphohistiocytosis.92 1225 • Fig 103.3 Peripheral smear showing mature and band neutrophils (top) with basophilic cytoplasmic inclusion consistent with Chédiak-Higashi syndrome 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 receptors (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 associated 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 Interestingly, patients with IRAK4 or MyD88 deficiency tend to have problems with invasive pyogenic bacterial infections in childhood, but these subside over time, presumably as a result of the adaptive immune response covering for this innate defect Mutations 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 immunodeficiency 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 occur in isolation or as part of a broader clinical syndrome such as in 1225.e1 Mammalian Susceptibility to Mycobacterial Disease Under normal circumstances, intracellular pathogens such as mycobacteria induce production of IL-12 and IL-23, which play a role in driving maturation of naïve 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 IFNinducible genes Multiple molecules in this signaling pathway have been found to be defective in patients with intact cellular immunity 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 tyrosine 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 infections (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 mycobacterial 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 1226 S E C T I O N X I Pediatric Critical Care: Immunity and Infection APECED (autoimmune polyendocrinopathy, candidiasis, ectodermal dystrophy) Recent discoveries have determined that, in almost all cases, CMC is associated with abnormalities in development of Th17 effector T cells or with the production of, or responses to, the cytokine IL-17 These include autosomal recessive mutations in IL-17F or the IL-17 receptor a-subunit, dominant loss-of-function mutations in STAT3, dominant gain-offunction 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 syndrome (ALPS) is characterized by massive lymphadenopathy and hepatosplenomegaly in most cases In addition, patients have problems with recurrent episodes of autoimmune hemolytic anemia 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 percentage 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 domain of FAS In addition to elevated double-negative T cells, patients with ALPS frequently have elevated levels of IL-10 and vitamin 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 (CASP8) were previously classified as ALPS but are now considered to be a unique syndrome that may have lymphadenopathy and splenomegaly, but recurrent respiratory tract infections and mucocutaneous herpesvirus infections are prominent, both unusual 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 numbers and function for each of the four immune system compartments A recommended workup using this approach is outlined in eTable 103.3 Diagnostic Testing: Complement Screening for complement deficiency should be performed in patients 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 imperative that prompt testing of the complement system be conducted for these individuals The screening test of choice for complement deficiencies measures functional classical complement 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 specimen was handled incorrectly or in patients with autoimmune disease, such as SLE or mixed connective tissue disease Once an abnormal 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, neutrophil counts may need to be evaluated three times weekly for to 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 oxidative 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 antibody response can be done by measuring specific antibody 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 weeks after vaccination Patients who respond appropriately to protein antigens but not respond to carbohydrate antigens may have specific antibody deficiency and may require additional workup Recent guidelines regarding the interpretation and use of diagnostic vaccination have been published.104 ... 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... evaluation In simple terms, this workup should include evaluation of numbers and function for each of the four immune system compartments A recommended workup using this approach is outlined in... 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