1222 SECTION XI Pediatric Critical Care Immunity and Infection deficiency alone are virtually unheard of In the patients who do have symptoms, they are typically more suggestive of immune dysregulatio[.]
1222 S E C T I O N X I Pediatric Critical Care: Immunity and Infection deficiency alone are virtually unheard of In the patients who have symptoms, they are typically more suggestive of immune dysregulation and autoimmunity (e.g., allergy, arthritis, diarrhea, celiac disease) than immunodeficiency (sinusitis, otitis media, bronchitis, and pneumonia) In patients who have no peripheral IgA at all, sensitization to IgA itself can be problematic, leading to anaphylactic reactions during infusions of blood products that contain immunoglobulin, red blood cells, and platelets However, this is quite uncommon and may be managed by pretreatment with antihistamine, acetaminophen, and corticosteroids.57 Specific Disorders: T Cells 22q11 Deletion Syndrome (DiGeorge Syndrome) 22q11.2 deletion syndrome, or DiGeorge syndrome (DGS), is among the few disorders with isolated T-cell deficiency Hemizygous deletion within the 22q11.2 region of the long arm of chromosome 22 has been associated with a spectra of clinical syndromes, including DGS, velocardiofacial syndrome (VCFS), conotruncal anomaly face syndrome (CTFS), CATCH22 syndrome, and others DGS is the name most commonly associated with immunodeficiency The most common disease-causing deletion includes a region containing the TBX1 gene, a transcription factor involved in the development of branchial arch structures Impaired embryogenesis of the third and fourth pharyngeal arches causes abnormal development of the thymus and parathyroid While DGS is a complex syndrome that has been associated with a wide array of symptoms, the diagnostic criteria proposed by the European Society of Immune Deficiency (ESID) are relatively straightforward These propose that a diagnosis of DGS should be strongly considered in patients who have less than 500 CD31 T cells/mL and any two of the following three characteristics: (1) conotruncal cardiac defect (truncus arteriosus, tetralogy of Fallot, interrupted aortic arch, or aberrant right subclavian); (2) hypocalcemia requiring therapy for more than weeks; and (3) deletion of chromosome 22q11.2 This chromosomal deletion syndrome occurs in approximately in 4000 to 5000 births and causes haploinsufficiency of the genes encompassed in the deletion that can extend to include as much as Mb of the chromosome.58,59 The characteristic T-cell lymphopenia of DGS is thought to arise primarily from the hypoplastic thymic tissue; at times, the diagnosis is suspected when the cardiac surgeon correcting a congenital heart defect finds little or no thymic tissue in the mediastinum Most affected infants have low but not absent T cells, and absolute counts tend to improve over the first year of life Both CD41 and CD81 T cells are low, yet CD81 T-cell numbers tend to be more affected in most patients Despite low T-cell counts, most patients not have significant problems with recurrent or severe viral or fungal infections Some patients may have recurrent candidiasis Bacterial infections of the upper respiratory tract, including otitis media and sinusitis, occur but may be related more to anatomic issues associated with the facial anomalies than to the immunodeficiency per se Rare patients (1%) have severe T-cell lymphopenia with essentially no T cells and are termed complete DGS as opposed to partial DGS These patients may have a clinical phenotype similar to SCID and require hematopoietic stem cell transplantation.58,59 Other prominent clinical features include hypocalcemia, which can be severe and persistent due to parathyroid hypoplasia; dysmorphic facial features that include small, low-set ears, hypertelorism, and micrognathia; renal anomalies, including horseshoe kidney and a duplicated collecting system; and developmental delay, including problems with speech acquisition, learning disabilities, and behavioral problems Patients with DGS have also been found to have an increased incidence of autoimmunity, including cytopenias (particularly affecting red cells and platelets), juvenile idiopathic arthritis, and thyroiditis In symptomatic patients, the diagnosis of DGS is typically made by confirming a deletion within the 22q11.2 region by fluorescence in situ hybridization or quantitative polymerase chain reaction for deletion of the TBX1 gene that lies within the deletion.60 In approximately 10% of patients, deletion of this region cannot be detected despite presence of the classic clinical features Treatment of DGS initially involves supportive care that may include cardiac support and calcium supplementation In patients with severe T-cell lymphopenia or evidence of decreased T-cell function, prophylaxis against PJ pneumonia and IgG supplementation may be used Blood transfusion for these patients should be irradiated to prevent the risk of graft-versus-host disease (GVHD) For those patients with the severe complete form of the syndrome, grafting of allogeneic thymus slices into the thigh muscle has proven to be successful in recovering the T-cell lymphopenia, improving T-cell responses, and correcting the infectious susceptibility.61,62 Hematopoietic stem cell transplantation (HSCT) has been used in a handful of patients with mixed results.63,64 In general, HSCT restores the T-cell counts and protects patients against further infection However, in the absence of a thymus, the T-cell graft is thought to consist primarily of long-lived, committed lymphoid progenitor T cells and not of cells derived from donor bone marrow stem cells As a result, there is concern that the T-cell grafts may senesce over time, once again leaving the patient lymphopenic and susceptible to infection There are currently no long-term therapies that can successfully correct the parathyroid defect Severe Combined Immunodeficiency SCID is among the most severe immunodeficiencies It is now known that this category of diseases is made up of a variety of related disorders caused by mutations in more than 20 different genes The one thing that is common to all forms of SCID is deficiency of one or more subsets of T cells In many cases, there are no circulating T cells Depending on the genetic defect, patients may also lack B cells and/or NK cells This has led to the useful convention of defining cases of SCID by their cellular phenotype (i.e., T2B1NK1, T2B1NK2, T2B2NK1, T2B2NK2) The cellular phenotype suggests what the underlying genetic defect may be (eTable 103.3).2 Because of the absence of functional T cells, patients with SCID typically come to medical attention because of severe or chronic viral infections, fungal infections, or autoimmunity Physical examination findings in SCID that may provide some clues to the diagnosis include a paucity of palpable lymph nodes and absence of a thymic shadow on chest radiograph In the overwhelming majority of SCID cases, this leads to death in infancy or early childhood if patients not undergo curative treatment, such as HSCT or gene therapy Unfortunately, the presence of infections makes HSCT or gene therapy much more complicated and substantially decreases the chances of survival Because of this, efforts have been underway in the United States and other countries to perform screening of all newborns using dried blood spot cards obtained at birth to identify those who may have SCID These efforts have led to the recognition that the incidence of SCID is approximately in 40,000 live births in the United States.65,66 1222.e1 eTABLE 103.3 Suggested Laboratory Tests for Basic Evaluation of Each Immune Compartment INNATE ADAPTIVE Complement Phagocytes Numbers: Plasma levels of specific complement components (e.g., C1 esterase inhibitor, C2, C3, C4) Function: CH50—functional test for classical pathway AH50—functional test for alternative pathway Numbers: CBC with differential: Are neutrophil counts normal? Function: Expression of the CD11b/CD18 integrin on leukocytes Neutrophil oxidative burst test: dihydrorhodamine 123 B Cells/Antibodies T Cells Numbers: CBC with differential: Are lymphocyte counts normal? Lymphocyte subsets: Are T, B, and NK cell counts normal? Function: Vaccine titers: protein antigens (e.g., tetanus or diphtheria titers) Vaccine titers: carbohydrate antigens (pneumococcal titers) Numbers: CBC with differential: Are lymphocyte counts normal? Lymphocyte subsets: Are T, B, and NK cell counts normal? Function: T-cell proliferation to mitogens/antigens Vaccine titers: protein antigens (e.g., tetanus or diphtheria titers) The laboratory evaluation for immunodeficiency should be individualized to each patient’s clinical presentation CBC, Complete blood cell count; NK, natural killer CHAPTER 103 Congenital Immunodeficiency X-Linked Severe Combined Immunodeficiency The X-linked form of SCID (X-SCID) is the most commonly diagnosed type of SCID, accounting for approximately 40% of all SCID cases It occurs as a result of mutations in the IL2RG gene, encoding the g-receptor chain (gc) used by a number of cytokine receptors including those for interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21 In the absence of a functional gc chain, cells are unable to respond to these cytokines Because IL-2 and IL-7 are key growth factors for T cells and IL-15 is the key growth factor for NK cells, patients with X-SCID lack T and NK cells but usually have normal numbers of B cells in their blood The ability to make antibodies is, nevertheless, severely impaired due to the lack of T-cell help If not identified by newborn screening, infants with X-SCID typically present with severe infections PJ pneumonia is common and often accompanied by severe viral infections with CMV, EBV, respiratory syncytial virus, rotavirus, human metapneumovirus, and others Adenovirus is particularly lethal in SCID, causing not only pneumonitis but also a severe, fulminant hepatitis In countries where BCG is still used for vaccination, X-SCID patients may develop severe systemic BCG-osis Other symptoms that are frequently associated with X-SCID include diarrhea, failure to thrive, and candidiasis Flow cytometry testing to evaluate tyrosine phosphorylation of the STAT3 transcription factor in response to IL-21 stimulation can provide a rapid assessment of whether a patient may have an IL2RG or JAK3 defect (see later discussion) Under normal circumstances, IL-21 stimulation of B cells causes rapid tyrosine phosphorylation in B cells that is absent in patients with IL2RG or JAK3 deficiency The diagnosis is ultimately confirmed, however, by sequencing of the IL2RG gene HSCT has been the mainstay of therapy for X-SCID using a variety of pretransplant conditioning regimens Some have strongly advocated the use of T-cell–depleted grafts from one of the haploidentical parents because they are readily available, but many patients transplanted using that approach have failed to obtain significant donor chimerism in the B-cell compartment; thus, they have remained dependent on IgG supplementation posttransplant X-SCID was also the first disorder successfully treated by gene therapy Unfortunately, the retroviral gene therapy vector used to deliver the normal copy of the IL2RG gene had an unknown propensity to integrate within the T-cell oncogene LMO2, causing T-cell leukemias in a subset of treated patients.67–69 Recent studies using alternative viral vectors have shown a better safety profile and successful resolution of the immunodeficiency.69a Detailed discussion of the pathogenesis of JAK3-deficient SCID, interleukin-7 receptor, CD3 components, and CD45deficient SCID, DNA recombination and repair defects in SCID, Zap-70–Deficient SCID, and defects in antigen presentation by MHCI or MHCII causing SCID are provided at ExpertConsult.com Adenosine Deaminase– and Purine Nucleotide Phosphorylase–Deficient Severe Combined Immunodeficiency Adenosine deaminase (ADA) deficiency was the first molecularly defined immunodeficiency with discovery of patients with SCID who lacked the enzymatic activity of ADA in the peripheral blood in the early 1970s ADA catalyzes the conversion of adenosine triphosphate (ATP), guanosine triphosphate (GTP), and their deoxy counterparts to adenosine diphosphate (ADP) and guanosine 1223 diphosphate (GDP) When enzyme activity is impaired or absent, intracellular levels of deoxy-ATP (dATP) rise to interfere with ribonucleotide reductase, such that DNA synthesis and repair are slowed, and lymphocyte apoptosis is increased Purine nucleotide phosphorylase (PNP) works further down this same metabolic pathway to convert deoxyinosine to hypoxanthine Mutations interfering with this function cause deoxy-GTP accumulation and the inhibition of ribonucleotide reductase and DNA synthesis In both cases, affected infants typically have a T2B2NK2/low cellular phenotype due to the toxic nature of the accumulated metabolites on lymphocyte development Patients typically present with failure to thrive, chronic diarrhea, and severe or recurrent viral or sinopulmonary infections in the first year of life However, some patients can present with late-onset disease due to the presence of residual activity In addition, one-third of patients with PNP deficiency will have autoimmune manifestations, such as cytopenias, thyroiditis, vasculitis, or arthritis Both syndromes are associated with other nonimmune symptoms, including skeletal abnormalities and osteochondrosis dysplasia in ADA deficiency and neurologic deficits, developmental delay, and spasticity in PNP deficiency In both cases, the diagnosis can be made by detecting high levels of the toxic metabolites that build up in the peripheral blood in the absence of enzymatic activity in the erythrocytes Sequencing of the ADA1 or PNP genes can be performed to confirm the diagnosis ADA deficiency is the second most common cause of SCID and accounts for 20% or more of autosomal-recessive cases Aside from bone marrow transplantation, ADA-deficient infants can be treated with polyethylene glycol–conjugated ADA, which is injected intramuscularly twice weekly Treatment efficacy is gauged by measuring plasma ADA activity and red cell dATP levels Enzyme treatment has the advantage of speed of response coupled with safety.73,74 The treatment is expensive, and many treated patients develop antibodies to the bovine protein, which may neutralize the effect of the enzyme HSCT is effective in ADA deficiency but less so in PNP deficiency, as it does not correct the neurologic deficits Gene therapy for ADA deficiency has been quite successful, and unlike X-linked SCID and Wiskott-Aldrich Syndrome (WAS), there have been no reported cases of leukemia despite the retroviral gene therapy vector backbone being similar.75 Immune dysregulation, polyendocrinopathy, enteropathy, Xlinked (IPEX) is a rare syndrome of regulatory T-cell deficiency Natural regulatory T cells comprise a subset of CD41 T cells that regulate immune responses and play a critical role in peripheral immune tolerance IPEX is caused by mutations in the FOXP3 gene located on the X chromosome, which encodes a key transcription factor that is required for the generation of functional regulatory T cells (Treg) In the absence of FOXP3, patients present with severe, early-onset, systemic autoimmunity that is the result of having decreased Treg cell function Almost all patients with IPEX present with enteropathy within the first months of life The enteropathy is typically characterized by profuse, watery diarrhea (often nonbloody) and villus atrophy The majority of patients have an eczematous dermatitis that typically begins in the first months of life More than half (60%–70%) of patients also develop an early-onset endocrinopathy that is almost exclusively either thyroiditis or type I diabetes The most consistent laboratory abnormality among patients is a significantly elevated serum IgE level and eosinophilia In addition to these characteristic clinical and laboratory features, patients also have a high incidence of other severe autoimmune disorders, including hemolytic anemia, thrombocytopenia, neutropenia, hepatitis, and renal disease 1223.e1 Pathogenesis of Selected Genotype/Phenotypes of SCID JAK3-Deficient Severe Combined Immunodeficiency JAK3 is the tyrosine kinase immediately downstream of the common gamma chain (gc) It transduces signals from cytokine receptors to the STAT5 transcription factor and to other intracellular signaling molecules Approximately 30 different mutations have been identified that impair JAK3 function to varying degrees Those that are severe have a T2B1NK2 cellular phenotype similar to mutations in IL2RG Approximately one-third of patients have milder mutations that allow some JAK3 function.70,71 These patients can develop some T cells, although the T cells that develop are functionally abnormal The clinical presentation is similar to that observed in gc-deficiency Sequencing of the JAK3 gene is required to confirm the diagnosis Interleukin-7 Receptor, CD3 Components, and CD45-Deficient Severe Combined Immunodeficiency Defects in the a-chain of the IL-7 receptor (IL7RA), CD3 subunits (g, d, e), or CD45 can cause SCID with a T2B1NK1 phenotype As noted earlier, IL-7 is one of the two key growth and differentiation factors for T cells As a result, mutations in the IL-7 receptor a-chain (CD127) lead to selective T-cell deficiency but allow normal development of B and NK cells Similarly, absence of any of the CD3 subunits specifically affects the ability of T cells to develop because CD3 makes up an essential part of the T-cell receptor complex The T-cell receptor, including CD3 subunits, assembles in the endoplasmic reticulum and Golgi complex before trafficking to the cell surface If any of the receptor subunits are absent (including any of the CD3 chains), the receptor is recycled and never makes it to the outer cell membrane As a result, cells are unable to receive the TCR stimulation needed as they traffic through the thymus and they die by “neglect.” Because the number of T cells entering the thymus is normal, the CD3 defects are unusual among SCID disorders in that the size of the thymus is normal Of the three known CD3 defects (g, d, and e), CD3g deficiency may have a somewhat milder clinical phenotype that can present with moderate T-cell lymphopenia and recurrent pneumonia and otitis CD45 is a transmembrane tyrosine phosphatase present on most white blood cells It regulates intracellular Src-family tyrosine kinases that are important for T-cell receptor function and activation Autosomal recessive mutations of CD45 lead to T-cell deficiency and a SCID phenotype in affected infants DNA Recombination and Repair Defects in Severe Combined Immunodeficiency Rearrangement of the V, D, J, and constant regions of the T-cell receptor gene and B-cell immunoglobulin gene requires that double-stranded breaks be made in the DNA and that these breaks then be repaired after removal of intervening chromosomal fragments The recombinase activating genes RAG1 and RAG2 are expressed in T and B cells and play an essential role in creating the necessary double-stranded DNA breaks in the TCR and immunoglobulin loci A large complex of proteins—including artemis (DCLRE1C), DNA-PKcs, cernunnos/XLF (XRCC4-like factor), and DNA ligase IV (LIG4)—then play a role in repair of the DNA double-stranded breaks needed to create the final, recombined TCR or immunoglobulin locus Because productive rearrangement of the TCR or immunoglobulin locus is a necessary developmental step during T- and B-cell maturation, complete failure in one of these processes results in lymphopenia, with few if any T or B cells in the blood, while generation of NK cells is generally unaffected Because radiation induces double-strand breaks in DNA, cells from patients with mutations in any of the proteins involved in DNA repair demonstrate decreased survival in vitro after radiation exposure (radiation sensitivity) This can be a valuable diagnostic clue in making a diagnosis Patients who have null mutations of any of these proteins have a clear T2B2NK1 cell phenotype However, there are many mutations, particularly missense mutations or small in-frame deletions, that allow expression of a partially functional protein that can occasionally productively recombine a TCR or immunoglobulin gene locus The cell in which this occurs is then able to pass the developmental block and proliferate in an attempt to fill the lymphopenic void that is otherwise present in SCID These forms of SCID are termed “leaky” because they allow a small number of T (and occasionally B) cells to “leak” past the developmental block Omenn syndrome, originally described in leaky RAG1 or RAG2 mutants, is a clinical phenotype that results from this leaky form of SCID Affected infants typically develop a desquamative erythroderma after birth that is associated with the presence of oligoclonal, activated T cells in the skin, hepatosplenomegaly, lymphadenopathy, eosinophilia, and elevated IgE Some develop autoimmune enteropathy and cytopenias Affected patients are typically treated with T-cell–directed immunosuppression such as cyclosporine or FK506, followed by bone marrow transplantation Leaky forms of SCID have now been described in a number of SCID-associated molecular defects, including RAG1, RAG2, artemis, ligase IV, IL7RA, IL2RG, RMRP, and others.72 Zap-70–Deficient Severe Combined Immunodeficiency Zap-70 is a tyrosine kinase that is downstream of CD3 in the T-cell receptor signaling pathway Absence of functional enzyme results in a characteristic SCID phenotype with decreased CD41 T cells but virtually absent CD81 T cells Immunoglobulin levels are often decreased, and antibody responses to vaccination are typically absent because of the lack of effective T-cell help The CD41 T cells that are present not respond to antigen or mitogen stimulation, which uses the T-cell receptor, but they proliferate and make cytokines in response to phorbol and ionomycin stimulation, which mimics TCR stimulation by activating kinases and calcium flux through downstream mechanisms Bone marrow transplantation is effective in this disorder This mutation is mainly found in families with consanguinity Defects in Antigen Presentation by Major Histocompatibility Complex Class I or II The class II MHC generally presents peptide antigen derived from ingested material (either by phagocytosis or via the B-cell antigen receptor) Defects that interfere with the assembly or cell-surface expression of the class II MHC–peptide complex prevent effective antigen recognition by CD41 T cells, resulting in a rare SCID syndrome that presents in infancy with chronic diarrhea, failure to thrive, and, occasionally, autoimmune cytopenias MHC class II deficiency, also known as bare lymphocyte syndrome type II, can result from defects in one of four DNA-binding proteins that regulate transcription of the MHC class II gene (CIITA, RFXANK, RFX5, and RFXAP) Founder mutations (particularly in RFXANK) 1223.e2 that are common in the Mediterranean region have led to most affected patients having some familial ties to this geographic area Because MHC class II presentation of antigen is abnormal, CD41 T cell counts are often low while CD8 counts are normal A rapid preliminary diagnosis can be made by evaluating peripheral blood cells for expression of the class II complex by flow cytometry, which will detect reduced expression The detection of gene mutations can confirm the diagnosis Some patients can have a mild disease course and live well into adulthood with only supportive care, while others with the same mutation (even in the same family) may have a more severe phenotype Bone marrow transplantation for MHC class II deficiency has been challenging, with survival rates typically approaching approximately 50% In addition, there is a high rate of severe GVHD in these patients, possibly because thymic epithelial cells that play a role in positive and negative T-cell selection are not replaced by transplant and still not express MHC class II.76 The class I MHC presents peptide antigens derived from intracellular proteins where proteolytically derived peptides are bound to the MHC class I as it is generated and trafficked through the endoplasmic reticulum Peptides that are generated in the cytosol are transported into the inner lumen of the endoplasmic reticulum by the peptide transporters TAP1 and TAP2 Mutations in either of these, or of tapasin to which they bind, have been identified TAP mutations are typically associated with recurrent bacterial sinopulmonary infections but not with viral infections, a milder phenotype than the MHC class II The diagnosis is usually suspected when serologic typing for human leukocyte antigen (HLA) class I antigens fails to give interpretable results, but molecular typing indicates intact MHC DNA CD81 T-cell and NK-cell counts are frequently (but not uniformly) decreased in these patients Other cell subsets are typically normal Heterogeneity in the phenotype may reflect differences in the underlying mutation, in the gene affected, and in the residual normal MHC class I expression Sequencing of the TAP1, TAP2, and TAPBP genes can confirm the diagnosis Some patients may survive into the third decade The role of HSCT in MHC class I deficiency is unclear given MHC class I expression is not restricted to hematopoietic cells ... nucleotide phosphorylase (PNP) works further down this same metabolic pathway to convert deoxyinosine to hypoxanthine Mutations interfering with this function cause deoxy-GTP accumulation and the... and calcium flux through downstream mechanisms Bone marrow transplantation is effective in this disorder This mutation is mainly found in families with consanguinity Defects in Antigen Presentation... patients can present with late-onset disease due to the presence of residual activity In addition, one-third of patients with PNP deficiency will have autoimmune manifestations, such as cytopenias, thyroiditis,