Immunological aspects of 22q11.2 deletion syndrome

Abstract

Chromosome 22q11 deletion is the most common chromosomal deletion syndrome and is found in the majority of patients with DiGeorge syndrome and velo-cardio-facial syndrome. Patients with CHARGE syndrome may share similar features. Cardiac malformations, speech delay, and immunodeficiency are the most common manifestations. The immunological phenotype may vary widely between patients. Severe T lymphocyte immunodeficiency is rare—thymic transplantation offers a new approach to treatment, as well as insights into thymic physiology and central tolerance. Combined partial immunodeficiency is more common, leading to recurrent sinopulmonary infection in early childhood. Autoimmunity is an increasingly recognized complication. New insights into pathophysiology are reviewed.

Introduction

Deletion of the q11.2 region of chromosome 22 is found in a heterogeneous group of disorders sharing a common genetic basis. It is the most common chromosomal deletion syndrome with an estimated incidence of 1:4,000 live births [1]. The majority of patients with DiGeorge syndrome and velo-cardio-facial syndrome have monosomic deletions of chromosome 22q11.2 [1]. Other syndromes have occasionally been associated with this deletion including conotruncal anomaly face syndrome, Opitz/GBB, and CHARGE (coloboma, heart disease, choanal atresia, retarded growth and central nervous system development, genital hypoplasia and ear abnormalities and/or deafness) syndrome [2]. Cardiac malformations, speech delay, and immunodeficiency are the most common manifestations [2] but there is a wide phenotypic spectrum including neuropsychiatric disorders and otolaryngological disorders. No clinical features are diagnostic and size of the deletion does not predict disease severity [3]. The clinical phenotype may vary widely between patients, including monozygotic twins with identical mutations, suggesting non-genetic factors contribute to disease phenotype [4].

The pharyngeal arches and pouches are a common embryonic precursor for the thymus, parathyroid, and conotruncal regions of the heart. Defects in these organs in 22q11.2 deletion syndrome are believed to be caused by impaired migration of neural crest cells into pouch ectoderm [5]. Disruption in the pathways of neural crest cell development in mice results in malformations similar to the 22q11.2 deletion phenotype [4]. This review will focus on the immunological features associated with 22q11.2 deletion.

Genetics of 22q11.2 deletion syndrome

The clinical association of thymic aplasia and hypoparathyroidism was first recognized as a syndrome when Dr. Angelo DiGeorge described the postmortem findings of a group of infants with a congenital absence of the thymus and parathyroid glands [6]. Subsequently, conotruncal cardiac abnormalities and facial dysmorphism were added as characteristic features, but it was not until the early 1980s that deletions within chromosome 22 were identified as the cause of DiGeorge syndrome [7, 8]. Subsequently, the same deletion has been discovered in other disorders including velo-cardio-facial syndrome [9], conotruncal anomaly face syndrome [10], and sporadic cases of Opitz/GBB syndrome [11] and CHARGE syndrome [12]. The classic features of DiGeorge syndrome have been described in patients with point mutations in the gene TBX1, found in the 22q11.2 region [13], and with other chromosomal re-arrangements including chromosome 10p deletion [14–16]. The nomenclature is somewhat confusing and evolving. Patients without the classic 22q11 deletion, but phenotypic features of DiGeorge syndrome are often described as having 22q11.2 deletion syndrome, and the immunological phenotype bears no relationship to the size of the deletion, or its absence [17]. Many patients with CHARGE syndrome have a deletion in the gene CHD7 [18], and some have a similar immunological phenotype [19]. This review will focus on immunological defects found in patients with genetic or clinical features found above, using the term 22q11.2 deletion syndrome to encompass all of the phenotypes and diagnoses, particularly for DiGeorge and CHARGE syndrome.

Approximately 90% of individuals with the classical 22q11 deletion have a heterozygous deletion of identical three million base pairs (3 Mb) of 22q11.2, the ‘common deletion’ [20]. This region is prone to rearrangements, likely due to homologous recombination events occurring within two complex highly homologous low-copy repeat areas that are 3 Mb apart [21]. The 3 Mb sequence of genomic DNA contains approximately 30–50 genes [22]. A further 8% of patients have smaller deletions of 1.5 Mb encompassing 24 genes [21]. However, there are no consistent phenotype differences associated with smaller deletions [23]. Numerous candidate genes have been proposed based on their location within the region or expression pattern consistent with a DiGeorge syndrome phenotype. So far it has not been possible to identify which, if any, features of the phenotype can be specifically linked to one gene or combinations of genes affected by this deletion [24].

Murine models identified Tbx1 as a candidate gene for the 22q11.2 deletion syndrome. TBX1 belongs to a family of transcription factors that contain a DNA-binding domain called “T-box”. A homozygous deletion of the murine Tbx1 gene is embryonic lethal. However, phenotypic features of 22q11.2 deletion were detectable in the embryos including abnormal facial features, thymic and parathyroid hypoplasia, and cardiac abnormalities. Heterozygous mutants exhibited a less penetrant phenotype, with varying degrees of absence or reduction in fourth pharyngeal arches [25]. Other implicated genes include the Crkl gene, which encodes an adaptor protein implicated in growth factor and adhesion molecule signaling, which is highly expressed in neural crest derived tissue during development. Homozygous Crkl gene deletion results in gestational death in mice with multiple defects in neural crest derivatives including aortic arch arteries, thymus, and craniofacial structures [26], but heterozygous mutants do not display this phenotype. Compound heterozygosity of murine homologs of CRKL and TBX1, results in a more penetrant phenotype compared to heterozygosity at either locus, suggesting that dose-sensitive interaction of these genes is important to establish the phenotype [27]. It is clear that at least some of the genes in the deleted region are critical for development of pharyngeal ectoderm, endoderm, and mesenchyme, and disruption of these genes leads to disruption of structures of the pharyngeal arch, including the cardiac outflow tract, parathyroid glands, and thymus, as well as craniofacial morphology. Some of these effects may be mediated through downstream signaling [28], and it is not yet clear why there is such a variable morphological and clinical phenotype.

CHD7, mutated in CHARGE syndrome, is a member of the chromodomain helicase DNA-binding domain family of adenosine-5′-triphosphate-dependent chromatin remodeling enzymes. The related CHD1, CHD3, and CHD4 proteins contribute to nucleosome remodeling and histone deacetylation, which regulates dynamic changes in chromatin structure during transcription, recombination, repair, and replication [29, 30], and is important in regulating early embryonic development and cell-cycle control. These proteins lead to transcriptional activation or repression of a gene or region. It is likely that CHD7 has a similar function [31]. CHD7 is expressed throughout the neural crest containing mesenchyme of the pharyngeal arches [31, 32]. Seven of ten CHARGE fetuses were found to have thymic hypoplasia or agenesis [32]. The clinical overlap of CHARGE and 22q11.2 deletion syndrome reported previously [33, 34] suggests a common neural crest defect. In 22q11.2 deletion syndrome, defects in thymus, parathyroid and conotruncal regions of the heart are caused by impaired migration of neural crest cells into pouch exoderm. TBX1 contains a DNA-binding domain, and may be a functional target for CHD7, perhaps explaining some of the overlapping features in CHARGE syndrome and 22q11.2 deletion syndrome.

Thymic development

Segmentation of the posterior pharynx initiates thymic development. There follows a series of incompletely understood developmental events culminating in endodermally derived epithelial cells of the third pharyngeal pouch forming the thymus anlage [35]. Neural crest-derived mesenchymal cells of the pharyngeal arches give rise to the thymic connective tissue as well as smooth muscles of the great cephalic arteries and aorto-pulmonary septum, and the parathyroid gland connective tissue [36]. Thymic mesenchyme promotes thymic epithelium development and signaling between the two cell types controls initial thymic morphogenesis and mesenchymal cells regulate proliferation and differentiation of immature thymic epithelial cells. Once thymic development is able to support immature thymocytes, further thymic epithelial differentiation is largely independent of mesenchymal cells.

Lymphoid stem cells begin to populate the thymus by the eighth week of embryogenesis [37]. T lymphocyte progenitors colonize the thymus before it is vascularized, transmigrating through the mesenchyme to the epithelial cells. Molecular interaction between the developing lymphoid and the epithelial cell is critical for further thymic development [38]. Defects in genes that promote T lymphocyte development lead to defective thymic development. Complete deficiency of thymocytes (e.g., null mutations in AK2, IL2RG, ADA) results in a severely atrophic thymus with lack of corticomedullary demarcation. Thymocyte development arrested at a later developmental stage (e.g., null mutations in RAG) leads to normal cortical development but only rudimentary medullary regions [39].

T lymphocyte development

The thymus plays a crucial role in T lymphocyte development. Immature T lymphocytes develop from the common lymphoid progenitor in the bone marrow and traffic to the thymus. Developing CD4–CD8- (double negative) T lymphocytes (thymocytes) enter the thymus at the cortico-medullary junction. During cortical migration, double-positive thymocytes express both CD4 and CD8. Following successful T lymphocyte receptor gene rearrangement, positive selection occurs in the thymic cortex upon successful engagement of the T lymphocyte receptor in the context of MHC class I or class II molecules to differentiate into single positive CD8+ or CD4+ T lymphocytes, respectively. Only cells that recognize MHC expressed by thymic cortical epithelial cells are selected. Single positive cells are predominantly found in the medulla where they closely interact with thymic epithelial cells. Bi-directional cross-talk between thymocytes positively selected in the cortex is essential to preserve thymic architecture, and hence function. Positively selected thymocytes promote maturation of thymic epithelial cells and formation of thymic medullary regions through molecular signaling [40]. Some mature medullary thymic epithelial cells and thymic dendritic cells, directed by the transcriptional regulator, AIRE, express organ-specific self-antigen. Autoreactive thymocytes that recognize these self-antigens are deleted (negative selection) [41], although some auto-reactive thymocytes may be converted into FOXP3+ regulatory T lymphocytes through interaction with thymic dendritic cells [42]. Surviving thymocytes leave the thymic medulla and enter the peripheral circulation.

The importance of all of these elements of immune development is emphasized by examining human disease severely affecting thymic and T lymphocyte development. Null mutations leading to absence of T lymphocyte development lead to thymic atrophy, reversed when developing thymocytes re-populate the thymus, which then develops normally [43]. Hypomorphic mutations, which lead to restricted T lymphocyte development, give rise to abnormal thymic development, resulting in partial immuno-deficiency in which T lymphocyte and B lymphocyte deficiency are manifest, as well as autoimmune features [38]. Defects in AIRE expression give rise to normal T and B cell immunity, but a predisposition to T lymphocyte-mediated organ-specific autoimmune polyendocrinopathy [44], caused by the escape of normally deleted auto-reactivce thymocytes into the peripheral circulation. Lack of FOXP3+ regulatory T lymphocytes, as seen in IPEX syndrome, gives rise to auto-immunity manifest particularly by cytopenias, severe autoimmune enteritis, and insulin-dependent diabetes mellitus caused by pancreatic islet cell destruction by auto-reactive T lymphocytes [45].

Given the critical nature of molecular interaction between developing lymphoid and thymic epithelial cells for thymic development, and the disordered immune function that can arise from primary partially permissive T lymphocyte development defects leading to secondary disordered thymic architecture [46], it can be conjectured that primary disordered thymic architecture may give rise to secondary disordered immunity. It is likely that the following descriptions of immunodeficiency found in patients with 22q11.2 deletion syndrome and CHARGE syndrome relate to this.

Immunological features of 22q11.2 deletion syndrome

Immunodeficiency is one of the key features of 22q11.2 deletion syndrome, and was one of the initial triad described by Angelo DiGeorge [6]. Defective immunity is also increasingly recognized in CHARGE syndrome [19, 47]. The observed immunodeficiency is secondary to thymic aplasia or hypoplasia with subsequent impaired thymocyte development. The spectrum of severity of the immune deficit varies widely and the frequency and severity of the immunodeficiency does not appear to be related to other phenotypic features [17]. Patients with complete thymic aplasia present with severe or complete T lymphopenia, manifesting with a severe combined immunodeficiency phenotype. However, the degree of immunodeficiency seen in patients with 22q11.2 deletion is highly variable and may include defects of T lymphocyte number and function as well as humoral defects (Table 1). Careful evaluation of patients is necessary to ensure prompt and appropriate treatment (Table 2).

Table 1.

Initial immunological investigations

Full blood count and differential leukocyte count

Immunoglobulins (IgM, IgA, IgG,)

Lymphocyte phenotyping (CD3, CD4, CD8, CD19, or CD20, CD16/CD56)

Lymphocyte proliferations to phytohemagglutinin if T lymphocyte counts low

Post-vaccination antibody responses to tetanus, and polysaccharide-protein conjugated Haemophilus influenzae type B capsular (Hib) and/or pneumococcal antigens

Beyond 2 years of age—assessment of polysaccharide antigen response

Assessment of autoantibodies, if clinically indicated, including direct antiglobulin test and thyroid antibodies

Table 2.

Key immunological management decisions

Irradiated, CMV negative blood products if immune status severely affected or unknown

Urgent referral to specialist center for further treatment if absent or very low T lymphocytes

Assess vaccination status—live viral vaccines not contra-indicated unless severe immunocompromisation is present. (If tetanus or Hib, responses are normal and CD4 > 400, should receive measles mumps, and rubella vaccine - MMR)

If recurrent respiratory infection—refer to an immunologist to exclude underlying immunodeficiency

Consider antibiotic prophylaxis if recurrent respiratory infection or evidence of poor specific antibody response to vaccine antigens

Patients with recurrent or severe respiratory symptoms should be assessed by a respiratory pediatrician or physician.

Regular monitoring for autoimmunity, particularly autoimmune cytopenias, and thyroid disease.

Severe T lymphocyte defects

Presenting features

Complete thymic aplasia with severe or complete T lymphopenia, presenting with a severe combined immunodeficiency phenotype (T-B + NK+) is the most widely appreciated immunodeficiency associated with 22q11.2 deletion. It is however, extremely rare, with less than 1.5% of patients having this phenotype [48]. Patients present within the first few months of life with similar clinical features to infants with other forms of severe combined immunodeficiency, namely, failure to clear infection and presentation with persistent viral respiratory tract or gut infection, and failure to thrive. Occasionally, infants present with disseminated BCG. Other features of 22q11.2 deletion syndrome may be apparent, including facial dysmorphism, hypocalcemia due to hypoparathyroidism, occasionally presenting with seizures or neonatal tetany, and congenital cardiac defects. The diagnosis may be delayed, as normal cytogenetic analysis may fail [49], due to the fact that leucocyte cultures for chromosome analysis rely on phytohemagglutinin to stimulate T lymphocytes into a transient blastic transformation so that metaphase chromosomes can be obtained after 3 days in culture. Similarly, cytogenetic analysis specifying fluorescent in situ hybridization for the 22q11.2 region will fail to give a result. Investigation usually shows severe lymphopenia from birth, demonstrated on a complete blood count and differential leucocyte count. Lymphocyte phenotyping shows severely depleted T-lymphocyte numbers, typically completely absent or present in extremely low numbers (<50 CD3+ cells/mm³); B lymphocytes and NK cells are normal. There is an absent or extremely reduced proliferative response to mitogens such as phytohemagglutinin. Mitogen responses are usually absent. Immunoglobulin estimations show low levels of IgG, IgA, and IgM although residual trans-placental maternal IgG in the first few weeks of life may give a falsely reassuring result. Chest radiographs show an absent thymus and, if infection is present, hyperinflation and/or interstitial pneumonia (Table 1).

Occasional patients show unusual patterns of mature T lymphocyte markers; in such cases, maternal engraftment should be excluded [50]. Congenital GvHD occurs because infants with severe combined immunodeficiency are unable to reject foreign lymphocytes acquired either from the mother in utero [51], or from a non-irradiated blood transfusion, particularly likely if severe congenital heart disease, requiring urgent open heart surgery, is present. When clinically apparent, there is typically a mild reticular skin rash, sometimes with abnormal liver function tests. GvHD following transfusion with non-irradiated blood or white cell or platelet concentrates is generally more severe and can be fatal. In these cases, the skin rash is severe and lymphadenopathy and hepatosplenomegaly may be present.

In atypical forms, characterized by oligoclonal peripheral T lymphocyte expansion, often associated with erythroderma and lymphadenopathy giving an Omenn syndrome-like picture, the diagnosis is based on a lack of naive T lymphocytes (<50 CD3+ CD45RA+ CD62L+ cells/mm³), indicating absence of recent thymic emigrants [52].

Investigation and treatment

General

Children with absent T lymphocytes should be referred urgently to a designated specialist immunology center for further evaluation and treatment. If erythrocyte transfusions (for cardiac surgery) are required before the immunological results are available, they should be from cytomegalovirus seronegative donors, and irradiated to prevent potential transfusion-associated graft versus host disease. Anti-pneumocystis, antiviral, and anti-fungal prophylaxis, and immunoglobulin replacement therapy should be commenced.

Adoptive transfer of mature T lymphocytes

Athymic patients with completely absent T lymphocytes, or with an atypical oligoclonal T lymphocyte expansion, present a therapeutic challenge. Unlike patients with other forms of severe combined immunodeficiency, the absence of T lymphocytes is due to an absence of thymic environment rather than an intrinsic hematopoietic defect. Patients receiving hematopoietic stem cells can achieve peripheral engraftment of post-thymic donor T lymphocytes, but do not demonstrate ongoing T lymphopoiesis [53]. Indeed, T lymphocyte-depleted grafts unsurprisingly fail to demonstrate donor T lymphopoiesis [19]. Adoptive transfer of cells of hematopoietic origin, achieving long-term survival of the patient, has been published in a few cases only [53, 54]. Overall survival is poor (41–48%) as compared to other forms of severe combined immunodeficiency (>80%) (Table 3) [55]. Severe pre-existing medico-surgical conditions are a major cause of death, as is graft versus host disease (GvHD), which often seems more severe than expected given that most patients receive cells from well-HLA-matched donors, and not all receive chemotherapy conditioning [53]. Indeed, it could be argued that there is no rational for giving chemotherapy prior to transplantation, except perhaps in patients with oligoclonal T lymphocytes and an Omenn-like presentation, as there is no requirement for donor myeloid cells or B lymphocytes, and lymphocyte progenitors will not lead to T lymphopoiesis given the lack of thymic machinery.

Table 3.

Survival after bone marrow or peripheral T lymphocyte transplant in DiGeorge syndrome

Donor	Number transplanted	Survival (%)
Matched family	13	8 (62)
Marrow	5	4
Peripheral blood lymphocytes	8	4
Matched unrelated	9	3 (33)
Marrow	6	2
Peripheral blood lymphocytes	3	1
Mismatched related	1	0 (0)
Unrelated cord	4	2 (50)
Total	27	13 (48)

Compiled from Janda et al. [53]

Most patients who survive long-term remain well without frequent infections. Humoral immunity in survivors shows good antibody responses to protein or conjugate vaccines when tested. Few survivors require ongoing immunoglobulin replacement. Information on T lymphocyte-mediated immunity is incomplete, but where tested, the majority had CD3+ and CD4+ lymphocytopenia and an absence of naive T lymphocytes with a skewed T lymphocyte receptor repertoire. Data from other studies indicates that these patients may remain at risk of late complications, particularly if lymphocyte numbers decline with time [56].

There are a number of possible explanations for the severity of GvHD. Firstly, the underlying condition may predispose to chronically inflamed tissue acting as a substrate for GvHD, for instance lung inflammation secondary to gastro-esophageal reflux and pulmonary aspiration. Secondly, an absence of thymus-derived FOXP3 + regulatory T lymphocytes may predispose to prolonged and severe GvHD. Thirdly, GvHD could be related to homeostatic expansion of donor T lymphocytes as neo-T lymphopoiesis does not take place in completely athymic patients.

The failure of neo-lymphopoiesis may also explain the poor outcome of patients presenting with pre-existing viral infection, with only 2/6 surviving [53], compared with an overall survival of 59% for patients with other forms of severe combined immunodeficiency, even after haploidentical transplantation, in the European series [55].

Thymic transplantation

In athymic individuals with a typical or atypical presentation, an alternative treatment to hematopoietic stem cell transplantation is allogeneic thymic transplantation. Initially, fetal thymic tissue was used [57, 58]. While some success was achieved, with evidence of T lymphocyte reconstitution, hematopoietic stem cell transplantation superseded as the treatment of choice. More recently, trials of postnatal allogeneic thymus transplantation have been pioneered at Duke University Medical Center, North Carolina [59], and Great Ormand Street Hospital, London, UK (G Davies, pers. comm.). Thymic tissue, taken from infants less than 9 months old undergoing open cardiac surgery, is cultured for 12–21 days to ensure removal of donor thymocytes, which might otherwise cause GvHD, before being transplanted into the quadriceps muscle of the recipient [60]. Immunosuppression is not required, unless the recipient has atypical, Omenn-like features, in which case the patient is pre-treated with anti-thymocyte globulin and receives a calcineurin inhibitor until there is evidence of thymopoiesis.

While direct comparison with the results following hematopoietic stem cell transplantation is not possible, as some patients are excluded from the thymic transplantation on the basis of pre-existing medical complications, the procedure seems successful. The results from 60 patients have been reported [34, 60]. Forty-three of 60 subjects are alive (72%). The main cause of death was infection, with four others dying from causes unrelated to the transplant. Immune reconstitution was very good, with evidence of thymopoiesis in the donor thymus (Fig. 1). T lymphocyte numbers are lower than normal for age, but naive T lymphocytes are present, indicating thymopoiesis, the cells proliferate when stimulated with PHA and there is a diverse T lymphocyte repertoire, which persists [34, 60–62]. Tolerance of recipient T lymphocytes to donor thymic tissue, has been demonstrated [63]. Subsequent to transplantation, tolerance to donor lymphocytes is also seen.

Fig. 1.

Biopsy from the thigh of a patient with thymic aplasia, following thymic transplantation. Skeletal muscle is shown with thymic tissue at the top right. Cytokeratin (CK) staining identifies thymus tissue (inset a), CD1a and Ki-67 identify cortical thymocytes (inset b, c), and CD3 identifies T lymphocytes (inset d). (Courtesy of Dr. Graham Davies, Great Ormand Street Hospital, London—copyright retained)

The major long-term adverse events in this patient group relate to autoimmunity. Thirteen patients had autoimmune thyroid disease, and nine patients experienced 1-, 2-, or 3-lineage hematological cytopenia. Other events included nephritic syndrome and autoimmune enteritis.

The most interesting concept surrounds the issue of HLA matching. Thymus transplantation is performed without matching the HLA antigens of the thymus donor to the recipient, yet outcome is not dependent on the degree of HLA matching between the donor thymus, and the recipient, and, perhaps surprisingly, there seem to be no adverse events associated with transplanting across HLA barriers [64]. Despite the thymus being the site at which developing T lymphocytes undergo both negative and positive selection, a completely HLA-mismatched allogeneic thymus permits development of recipient T lymphocytes that protect the recipient from infection. The process by which this occurs is incompletely understood. Negative selection is effected by CD83+ dendritic cells, derived from hematopoietic stem cells and which migrate from the bone marrow to the thymus, and are located throughout the medulla and at the corticomedullary junction [65]. Recipient dendritic cells are likely to populate a transplanted thymus, and so mediate negative selection, allowing deletion of T lymphocytes with receptors demonstrating high affinity to self MHC. As these dendritic cells also present organ-specific self-antigens regulated by AIRE, auto-reactive T lymphocytes will be deleted [66]. The process of positive selection presents more of a dilemma in the HLA-mismatched thymic transplant. It has been believed that only T lymphocytes that recognize MHC expressed by thymic cortical epithelial cells are positively selected. Thus, when the donor thymus is HLA-mismatched with respect to the recipient, it can be predicted that the positively selected T lymphocytes will be restricted to thymus donor MHC. Subsequently, T lymphocytes emigrating from the thymus should not be able to interact with antigen-presenting cells carrying recipient MHC in the periphery and these T lymphocytes, because of MHC mismatching, will not be functional. The successful reconstitution of recipient T lymphocyte function in the thymus recipients suggests that alternative cells may provide positive selection in the allogeneic donor thymus. Recipient cells may contribute to positive selection in the transplanted thymus. Studies in mice have demonstrated thymocytes that present self MHC to each other [67, 68], which enables positive selection to recipient MHC. At present, however, the mechanisms of positive selection in allogeneic thymus transplantation are not defined and further investigation is required. The opportunity to further unravel the mechanism of central tolerance is anticipated.

Partial combined immunodeficiency

Presenting and immunological features

While the majority of pediatricians associate DiGeorge syndrome with severe immunodeficiency, it is clear that the severe phenotype as described above is rare. Much more common is partial combined immunodeficiency, manifest predominantly as recurrent upper respiratory tract infection (URTI) and more rarely, lower respiratory tract infection. This presents after the first 6 months of life with frequent coughs, colds, and sino-pulmonary infections, often due to encapsulated bacteria. Concomitant velo-pharyngeal dysfunction contributes to the increased frequency of URTI, which are common in these patients even in the absence of immunological abnormalities. There is often mild to moderate antibody impairment often with a T lymphocytopenia. This is most marked in infancy [69, 70] and there is a normal age-related decline in T lymphocyte counts, although blunted in patients [71]. There is a reduction in naive cells exiting the thymus [72–74] and in older patients, an accelerated post-thymic conversion of naive to memory T lymphocytes due to homeostatic proliferation [75].

Consequently, abnormalities in the TCRVB repertoire have been described including expanded or contracted TCRVB families in CD4+ and CD8+ sub-populations, and oligoclonal TCRVB populations demonstrated on flow cytometry and by VDJ region sequencing. Restricted TCR diversity has been demonstrated by spectratyping, with altered CDR3 profiles in most TCRVB families investigated, with oligoclonality demonstrated by VDJ region sequencing [75, 76, McClean-Tooke, pers. comm.]. There are conflicting accounts regarding the gdTCR population with reductions and expansions reported [71, 77]. A few patients with T lymphocytopenia appear to be at greater risk of significant viral or candidal infection or early infectious death, particularly those with a combined CD4 and CD8 lymphocytopenia and diminished thymic output, or associated hypoparathyroidism [78, 79].

A wide spectrum of antibody deficiencies has been described. Isolated low IgM may be associated with recurrent infection [80–83], and there may be poor specific IgM responses, with low isohemagglutinins. Low IgG with low IgG subclasses are also described [69, 83]. Poor or absent specific antibody responses to polysaccharide antigen, particularly pneumococcal antigen, are relatively common [82–84], which may be associated with otherwise normal IgG levels. Absent, low, or elevated IgA levels are described [69, 71, 81, 83, 85, 86] and may be associated with recurrent infection or autoimmunity. In many patients, initial low immunoglobulin levels normalize with increasing age [71]. In some patients, B lymphocytopenia is a feature, particularly in infancy, but with numbers normalizing over time [69]. Others have described normal B lymphocyte numbers in patients but low CD27+ IgM+ IgD+ (memory) and CD27+ IgM− IgD− (class-switched memory) B lymphocyte numbers. It is not clear if this is a direct effect of 22q11.2 deletion on B lymphocytes or impaired T lymphocyte/B lymphocyte interaction [73, 77, 81]. There is also evidence that in patients with humoral deficiency, there is a normal IgV_H repertoire, but somatic hypermutation is reduced [87]. Interestingly, in a patient with severe T lymphocyte immunodeficiency, there was also a normal IgV_H repertoire, but no somatic hypermutation.

Investigation and treatment

In pre-school children, lymphocyte phenotype should be evaluated, immunoglobulin levels measured, and the IgG antibody response to tetanus, Hib, and pneumococcal antigens evaluated. Inadequate responses should be re-evaluated after further vaccination. Patients with recurrent or persistent lower respiratory tract infection should be considered for thoracic high-resolution computerized tomographic imaging of the chest and lung function assessment.

Treatment for this partial form of 22q11 deletion is largely symptomatic. Patients generally improve as they become older. Bacterial sinopulmonary infection should be treated promptly. Prophylactic antibiotics may be given, particularly over the winter months, but may be required throughout the year in occasional patients. For a few with symptomatic hypogammaglobulinemia, or with recurrent bacterial infection despite adequate antibiotic prophylaxis, immunoglobulin replacement may be required [83]. Live vaccines are generally safe to give, even in those with CD4 counts <600cells/μl and are effective at preventing disease [88–90]. However, vaccine-related infection has occasionally been described in patients with low T lymphocyte counts [91], and poor T lymphocyte responses to specific antigens can be found in patients with T lymphocyte counts <10th centile of normal values [92]. It seems prudent, therefore, that live vaccines are withheld in patients with total CD4 counts less than 400 cells/μl [93].

Autoimmunity

Presenting and immunological features

As more patients are identified with 22q11 deletion, it is becoming clear that autoimmunity is found more commonly than in the normal population. Currently, there are no biomarkers to identify or predict which patients will develop autoimmune features [71]. A range of autoimmune features have been described including cytopenias, systemic autoimmunity, particularly rheumatoid arthritis, and organ-specific autoimmunity, most notably autoimmune thyroid disease [71, 83, 84, 86, 94–97]. Other immunological abnormalities, including IgA deficiency may be associated [85].

The mechanism by which autoimmunity occurs is unclear, and is likely to be multifactorial. Infections may lead to autoimmune disease through a variety of mechanisms including the release of sequestered antigens through tissue damage; ‘bystander’ activation of autoreactive T lymphocytes by inflammatory cytokines and microbial products; and ‘molecular mimicry’ due to structural similarity between microbial and endogenous peptides [98]. Patients with immunodeficiency are less able to clear infectious agents. This results in chronic immune responses and tissue damage, a situation which favors the breaking of peripheral tolerance [99]. However, the development of autoimmunity does not appear to be related to the number of infections in 22q11-deleted patients. There is some evidence to implicate skewing of T lymphocyte subsets to a T_H2 phenotype through homeostatic peripheral expansion, leading to a dysregulated B lymphocyte compartment, which may contribute to autoimmunity [100].

As previously noted, abnormal thymic development, resulting in partial immuno-deficiency, can lead to autoimmune features [38]. Detailed histological examination of thymii from patients with 22q11 deletion and partial T and B lymphocyte deficiency has not been reported, but it is possible that abnormal expression of AIRE may contribute to some autoimmune features, at least in some patients. There is little work detailing FOXP3 + regulatory T lymphocytes in these patients, but in one study, patients had proportionately fewer cells than normal controls [73], although there were no differences in patients with or without autoimmune features. Interestingly, comparison of TCRVB repertoire abnormalities between the CD4+ and CD4+ CD25^Bright population showed no significant differences for individual patients, but significant differences were seen between the CD8+ and CD4+ CD25^Bright T lymphocytes and expansions/contractions were seen (McLean-Tooke submitted).

Investigation and treatment

Beyond early childhood, it is a good practice to review patients annually for evidence of autoimmune disease. History and examination should be directed towards symptoms of autoimmunity. Investigations, directed by the clinical picture, should include appropriate auto-antibodies, thyroid function, full blood count, and film and direct anti-globulin test. Treatment should be directed toward specific symptoms.

Conclusions

Immunological disorders are common in patients with 22q11 deletion, and are not restricted to severe or recurrent infection. However, most studies are cross-sectional, and there is still little data on the longitudinal immunological history. While it is clear that autoimmunity is more common in these patients, it is not possible to predict who is at risk of developing disease. Further work on defining mechanisms of autoimmunity is needed. An awareness of these problems by physicians caring for children and for adults is important, so that patients are managed appropriately. For patients with a severe phenotype, thymic transplantation offers an exciting alternative to hematopoietic stem cell transplantation, although it may be that the procedure transforms a patient with complete T lymphocyte deficiency into one with partial combined immunodeficiency, with the associated problems of autoimmunity and immune dysregulation.