X-Linked Immunodeficiency

Etiology

Chronic granulomatous disease (CGD) is a genetically heterogeneous disease progressing with frequent and recurrent life-threatening bacterial and fungal infections and granuloma formation. 

Hyper immunoglobulin M syndrome is a heterogeneous syndrome that resulted from a defect in class switch recombination (CSR) and characterized by normal or high IgM levels, low IgG, and IgA levels. The most prevalent form of the disease is the X-linked form resulting in mutation in the CD40LG gene coding the CD40 ligand. 

X-linked lymphoproliferative syndrome typically indicates an irregular response of the immune system to Epstein Barr virus infection. Dysgammaglobulinemia, lymphoma, and fulminant mononucleosis are the 3 cardinal symptoms. Also, lymphocytic vasculitis, aplastic anemia, and lymphoid granulomatosis can be seen.

Wiskott-Aldrich syndrome is an X-linked disorder occurring as a result of a mutation in the gene coding Wiskott-Aldrich syndrome protein (WASp). The disease is characterized by susceptibility to infections (due to both adaptive and innate immune disorders), microthrombocytopenia, and eczema. The course of the disease can both be severe depending on the mutations in the WAS gene (classical form) and relatively be in mild forms such as X-linked thrombocytopenia and X-linked neutropenia. 

X-linked severe combined immunodeficiency (X-SCID) is the most prevalent and severe immunodeficiency occurring as a result of a defect in the common gamma chain IL-2 receptor gamma and causing mortality within the first 2 years if not treated.[3]

X-linked agammaglobulinemia (XLA) is a primary immunodeficiency characterized by profound hypogammaglobulinemia, antibody deficiency, and susceptibility to infections.[4] Clinical symptoms usually occur in male children when they are 3-18 months old.[5] XLA is associated with a disorder/absence of the signal transduction molecule named Bruton tyrosine kinase (BTK). 

IPEX is an abbreviation established by using the initials of immune deficiency, polyendocrinopathy, enteropathy, and X-linked expressions. IPEX is a rare but fatal X-linked disorder.

Epidemiology

The prevalence of X-linked CGD is 1 in about 200,000 live births, and since more than half of the mutations are X-linked, the disease is more common in men. X-linked CGD may occur at any age from infancy to late adolescence, however, cases are diagnosed usually below 5 years of age.

Males affected by hyper IgM syndrome start getting symptomatic by one year of age, and the majority develop symptoms by four years.

It has been estimated that XLP affects 1-3 patients in the birth of 1 million males. However, since many patients with fatal fulminant infectious mononucleosis cannot be considered XLP, the actual number of cases is considered to be higher than this.[6] Clinical symptoms occur at an average age of 2.5.

WAS is a rare disease and the estimated prevalence is one in 100,000 live birth.[7] The disease is prevalent in males due to being X-linked.

X-SCID is the most common SCID form.

It is highly difficult to predict the definitive incidence and prevalence of XLA disease with the absence of population screening, but it is presumed to be 1 in every birth of 190000 males based on the data acquired from the United States registry system.[8]

Prevalence and incidence of IPEX thereof remain clearly unknown. However, retrospective analyses suggest that IPEX disease has been diagnosed or reported less than expected.

Pathophysiology

Phagocytes, for the purpose of forming reactive oxygen radicals, use nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system. A deficiency in the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex causes a functional defect leading to insufficiency in phagocytes killing pathogenic microorganisms. This system is comprised of 6 different proteins, however, a functional loss or inactivation in any of these proteins may cause the disease. Gp91phox gene mutation is observed in 65% to 70% of the cases and this gene defect is associated with the X-linked CGD.

The main problem in X-linked hyper IgM syndrome is in CSR (class-switch recombination). CSR is the transformation of the immunoglobulin mu chain into other immunoglobulin chain isotypes with different properties. Costimulatory signals on the T-cell surface serve an important function in CSR. Activated CD4 (+) follicular helper T cells express CD40 ligand and secrete various cytokines to recognize the antigen. Cytokines secreted by activated CD4+ T cells bind on the cognate receptors expressed on B-cells and direct the CSR to specific immunoglobulin isotypes. CD40L binds on CD40 expressed by B-cells.[9]

Sixty percent of the XLP cases occur as a result of a mutation in the gene coding the SLAM-related (signaling lymphocyte activation molecule) protein.[10] XLP is an X-linked disorder and de novo mutation is quite rare. EBV infections are the predominant infection in patients with XLP due to the possible B-cell tropism of EBV. Although clinical symptoms are generally triggered by EBV infections, many immunological disorders occur prior to EBV exposure. 

WASp is synthesized in the cytoplasm of hematopoietic cells and critical in the actin remodeling of the cytoskeleton. Immunological synapses cannot occur between T-cells and antigen-presenting cells such as dendritic cells.[11] Thus, insufficient migration and adhesion as well as an insufficient synapse between other cells occur accordingly. Disorder in T cells also disrupts the functions of B-cells.

The defect in cytokine receptor subunit gamma-c gene on the X-chromosome causes the X-SCID. This receptor subunit is common in IL-2, 4,7, 9, 15, and 21.[12] The mutation in this gene leads to serious issues in many cytokine pathways within the immune system and disrupting the development and functions of lymphocyte. This condition results in the absence of T and NK-cell and non-functional B-cells.[13]

XLA disease caused by a mutation in the BTK gene has been named the Bruton gene to honor the clinician who diagnosed the first case. Approximately half of the patients are born in families previously diagnosed with XLA and this condition refers to the fact that the other half of the patients are contracted with the disease as a result of spontaneous mutations. B-cell development defect occurring in the patients as a result of BTK mutation results in decreased B-cell count in tissue and blood and reduced differentiation in plasma cell and ultimately in decreased levels of immunoglobulin along with the decreased specific antibody responses.[4] The absence of immunoglobulin involved in opsonization and neutralization of the encapsulated bacterias leads to susceptibility for infections caused by the encapsulated bacterias in particular.[14]

IPEX occurs as a result of a mutation in the FOXP3 transcription factor. FOXP3 is crucial for the function of Treg cells also known as regulatory T lymphocyte. Treg cells have a strong immune suppressive effect and serve a function in establishing immune balance. Moreover, they are involved in transplantation tolerance as well as in the regulation of autoimmune and allergic events.

History and Physical

Infections in X-linked CGD have often been caused by catalase-positive microorganisms. Infections most prevalently occur in the lungs, skin, lymph nodes, and liver. Granuloma formation, which is a complication of inflammation, often causes problems for the lungs, gastrointestinal tract, and urinary system. CGD associated colitis have been observed in 30% to 40% of the patients (6). Hepatomegaly, splenomegaly, and lymphadenopathy can be observed in the patients depending on granulomas.[15]

The characteristics of X-linked Hyper IgM syndrome vary depending on the nature of the genetic defect. CD40 ligand and CD40 deficiency are within the combined immunodeficiency class. CD40 ligand or CD40 deficiency is X-linked and is the most prevalent form of the disease. CD40L or CD40 deficiency reveal similar clinical characteristics because in both cases, the interaction between the CD4 (+) T cells and antigen-presenting cells are disrupted. Patients are presented with frequent and recurrent upper respiratory tract infections with encapsulated microorganisms. Furthermore, opportunistic infections such as pneumocystis, cryptosporidium, and Histoplasma can be observed. Forty percent of the patients have been reported Pneumocystis jirovecii pneumonia.[16] X-linked Hyper IgM further includes increased risk of hepatocarcinoma, cholangiocarcinoma, lymphoma, and peripheral neuroectodermal tumor. 

The most prevalent phenotypes of XLP are fulminant infectious mononucleosis, dysgammaglobulinemia, and B-cell lymphoproliferative disease. Patients develop an apparent cytotoxic cellular response during EBV infection. B and T-cell counts are normal or increased, but functions thereof are abnormal. IgG levels are low and lgM levels are often increased. NK cell activity is reduced.

Mutations in the WAS gene result in various clinical phenotypes. The clinical phenotype may vary over time, and the clinical profile may not be established fully under 2 years of age.  In the classic WAS, male children typically suffer from bleeding disorder due to thrombocytopenia, recurrent bacterial, viral, and fungal infections, and common eczema. Lymphadenopathy and hepatosplenomegaly can also be detected. However, adenoid tissue is not prevalently encountered. Patients with WAS are prone to autoimmune complications and lymphoma or other malignancies, and this causes premature mortality.[17]

Patients with X-SCID who have not been diagnosed with the neonatal screening program are suffered from recurrent opportunistic infections, chronic diarrhea, oral candidiasis, otitis media, and growth retardation as of the first year of life. X-SCID patients are diagnosed by the scantiness or absence of the T-cell receptor excision circles in the neonatal screening program (Trecs). T-B+NK- phenotype is available in the majority of the patients. T-cells are either absent or in few numbers (less than 300 cells/microL). The naive CD4+ T-cell rate is less than 20%. B-cells are mostly normal but non-functional. NK cells are either less in number or absent.[18]

The only characteristic physical examination finding of XLA is the absence of secondary lymphoid tissues rich in B cells, such as tonsils and adenoids. Also, palpable lymphoid tissue is not available in certain patients. However, peripheral nodes can be observed depending on the hypertrophy of the T-cell areas. Although it is diagnosed at an early stage due to the existence of a previously affected individual in the family, many patients are presented with recurrent infections. Diagnosis is established at the age of 2.6 in male children with family history, it is the age of 5.4 years in male children with no family history. Term neonates are under a passive control due to immunoglobulin transferred from mother in the last trimester and thus, the clinical symptoms in the patients may not be observed in up to 3 months.[19]

The clinical symptoms of the IPEX in a male patient are autoimmune-induced life-threatening chronic diarrhea, autoimmune endocrinopathy (neonatal type 1 diabetes mellitus or thyroiditis), and often eczematous dermatitis, respectively. Growth retardation is quite prevalent in children. Besides, autoimmune-induced cytopenias and food allergies can be encountered accordingly.

Evaluation

In X-linked CGD, increased sedimentation rate, C-reagent protein, and hypergammaglobulinemia are expected due to chronic inflammation in the laboratory investigation. Furthermore, chronic disease anemia and hypoalbuminemia, especially in patients with gastrointestinal involvement, can be experienced. Reduced memory B-cells and CD4 (+) T cell lymphopenia are frequently observed in the flow cytometry analysis. Male patients with a history of frequent and recurrent infections, especially in cases of abscesses and infections with CGD-related pathogens, X-linked CGD should be suspected and patients should be tested for neutrophil function.[20] In case a disorder is identified in the neutrophil function test, a rapid functional and genetic screening by genotyping is required accordingly.[21] In families with a history of CGD, it is extremely important to identify potential patients with prenatal or neonatal screening programs.

B-cell defect is experienced in the entire X-linked Hyper IgM syndromes. CD40 ligand deficiency is also coexistent with T-cell defect and dendritic cell/monocyte defect. Levels of IgG, IgM, and IgE are distinctively lower. IgM levels are normal or high. Antibody response to protein and polysaccharide antigens is insufficient. B-cell count is normal, however, memory B-cell count is low and significantly lower for switched memory B-cell count.[22] Hyper IgM syndrome is required to be suspected in the patients with appropriate clinical complaints and with low levels of lgG and lgA or increased levels of IgM. A definitive diagnosis requires mutation analysis.

T-cells with reduced count and function, insufficient antibody response to vaccines, abnormal immunoglobulin isotypes, normal or increased NK cell count but decreased NK cell function have been observed in the laboratory examination. Treg functions can also be reduced accordingly.[23] WAS is required to be suspected in male children with petechiae, ecchymosis, and congenital or early-onset thrombocytopenia. Deletion mutation is required to be demonstrated in the WAS gene for the diagnosis to be confirmed.[17] The existence or absence of WAS protein is able to be demonstrated on lymphocytes using anti-WASp antibodies in flow cytometry.[24]

X-SCID should be suspected in all male patients with the T-B + NK-SCID phenotype. The diagnosis is confirmed by the demonstration of a heterozygous mutation in the IL2RG gene by means of next-generation sequencing. X-SCID can also be diagnosed by demonstration of the absence of a common gamma chain (CD132) on the lymphocytes by flow cytometry. This method can yield faster results based on genetic analysis.

In XLA, hypogammaglobulinemia/agammaglobulinemia, insufficiency of antibody response to vaccines, lower or absence of B-cell count in peripheral blood is available in the laboratory examination. The entire antibodies are reduced in XLA and the mature B-cell rate is below 2%.[25] Likewise, no plasma cell is observed in lymphoid tissues, bone marrow, and lamina propria of rectal mucosa.[25]

IPEX is required to be regarded in the entire male patients with chronic persistent diarrhea, growth retardation, and/or infantile-onset type 1 diabetes mellitus. The existence of dermatitis, autoimmune cytopenia, or thyroiditis support the diagnosis, however, are not a requirement for the diagnosis. The definitive diagnosis in patients suspected to be with IPEX is ensured by the mutation analysis of the FOXP3 gene.

Treatment / Management

Antimicrobial and immunomodulatory treatments, early diagnosis of infections, and aggressive treatment of complications of infections establish the basis of the treatment in X-linked CGD.[26] These patients have been recommended with life-time antibacterial and antifungal prophylaxis. Immunomodulatory therapy can be added based on the clinical profile of the patients. Trimethoprim-sulfamethoxazole can be preferred as antibacterial prophylaxis and itraconazole can be preferred as an anti-fungal treatment. Interferon-gamma treatment can serve as an add-on for this treatment.[27] Oral glucocorticoids are commonly used to control inflammatory complications of the disease. Anti-inflammatory agents such as azathioprine and sulfasalazine can be required to avoid long-term adverse effects of the steroid. The definitive treatment of X-linked CGD is HCT.[20]

The entire patients with X-linked Hyper IgM syndrome are required to be treated with immunoglobulin replacement. Moreover, these patients are required to be assessed in terms of liver function at a frequency of 6 months and the existence of cryptosporidium and microsporidium is required to be investigated accordingly. The patients with infection identified with these organisms are required to be treated with azithromycin or nitazoxanide.[28] The sole curative treatment of the disease is HCT.(B2)

Treatment of XLP is trilateral as to be the treatment of acute disease symptoms, prevention from possible sequelae, and curative treatment. Anti-viral agents and immunoglobulin therapy are beneficial in the acute treatment of the disease. Another alternative treatment is ablative B-cell treatment with rituximab.[29] Immunoglobulin treatment is also able to be used to prevent primary infections and EBV reactivation, however, the efficacy of IGRT in this regard remains unclear.[30] Although gene therapy is promising, the sole curative treatment of XLP is HCT. (B2)

General precautions and protective approaches, intravenous immunoglobulin therapy, low dose IL-2 treatment are involved for the purpose of protection against infections in the treatment of WAS.[31] Moreover, splenectomy along with stem cell transplantation with immunosuppressive therapy can be required for the management of autoimmune complications.[32] Gene therapy is a promising treatment approach in this regard.[17]

The definitive treatment of X-SCID is allogeneic hematopoietic cell transplantation (HCT). Gene therapy is currently at the stage of clinical research. The formation of T, B, and NK engraftment is required to ensure complete immune functions with HCT. In the case of engraftment of only T-cell, the patients are required to be administered with IGRT due to abnormal B-cell function.[3](B2)

The basis of the treatment is comprised of immunoglobulin replacement therapy. Moreover, general protective precautions such as avoidance from infections and immunization by inactive vaccines are recommended accordingly. Immunoglobulin treatment reduces the incidence of infection and hospitalization rates. Immunoglobulin treatment further prevents the development of long-term pulmonary insufficiency and reduces the risk of a systemic enteroviral infection.

IPEX treatment or management is subdivided into acute and long-term treatment. The patients may present with serious dehydration or metabolic disorders due to infant-onset diabetes mellitus or diarrhea associated with enteropathy prior to or after the diagnosis. Also, the disease may flare up due to infections, vaccination, dietary changes, or other unknown triggers. Management of acute complications requires a multi-disciplinary endeavor. Long-term disease management includes dietary modifications and nutritional optimization for the purpose of avoiding intense immunosuppression and food allergens. HCT is the sole curative treatment of the disease. Although the disorder is recovered after the transplantation, endocrinopathies are persistent due to end-organ damage.

Differential Diagnosis

In X-linked CGD; Cystic fibrosis, hyper IgE syndrome, glucose-6-phosphate dehydrogenase (G6PD) deficiency, glutathione synthetase deficiency, and Crohn disease are required to come to mind in the differential diagnosis.

In X-linked hyper IgM; PI3K- delta syndrome, ataxia telangiectasis, Nijmegen breakage syndrome, and CVID (common variable immune deficiency) are required to be considered in the differential diagnosis. Moreover, congenital rubella, use of phenytoin, T-cell leukemia, lymphoma, and nephrotic syndrome can also mimic X-linked hyper IgM syndrome.

In XLP; Hemophagocytic lymphohistiocytosis, CVID, Chediak-Higashi syndrome, Griscelli syndrome ve X-linked inhibitor of apoptosis (XIAP) deficiency are required to be considered in the differential diagnosis. Moreover, the entire male patients with severe infectious mononucleosis or lymphoma are required to be tested in terms of XLP.

In WAS; Eczema such as Omenn syndrome, IPEX, Netherton syndrome, hyper IgE syndrome, DOCK8 deficiency, and atopic dermatitis are required to be considered in the differential diagnosis. 

In X-SCID, Janus kinase 3 (JAK3) deficiency with autosomal recessive transition causing other T-B + NK- SCID phenotype is required to be considered in the differential diagnosis. The differentiation of both diseases can be performed only by genetic tests.

In XLA; Transient hypogammaglobulinemia of infants, CVID, autosomal recessive agammaglobulinemia, and combined agammaglobulinemia with T and B-cell failure should be considered in the differential diagnosis.

In IPEX; IPEX-like syndromes, neonatal enteropathy, eosinophilic enteropathies, severe combined immune deficiencies are required to be considered in the differential diagnosis.

Prognosis

Survival rates in patients of a young age are much better in X-linked CGD. Although the disease is named fatal granulomatous of childhood when it was first described, the disease-related mortality and morbidity rates have been increased apparently with the use of prophylactic antimicrobial and immunomodulatory treatments.[33] Nowadays, patients' life expectancy has exceeded 40 years of age.

Severe and opportunistic infections, liver/biliary tract diseases, and malignancies are considered to be the most prevalent cause of mortality in patients with X-linked hyper IgM syndrome. Survival rates have been increased with HCT and the life expectancy of 20 years has reached 90%.[34] Early transplantation also prevents development of the liver failure.[34][35]

In XLP, the mortality has been reported as 70% for 10 years of age prior to HCT while as 100% for 40 years of age. Rituximab and intravenous immunoglobulin administration and HCT reversed the mortality rates in the patients with EBV naive. Long-term mortality rates have been reduced below 30%.[36]

In WAS, life expectancy in the patients not received treatments of hematopoietic cell transplantation or gene therapy has decreased due to infections, bleeding, autoimmune diseases, and malignancies. Bleeding is the principal cause of mortality.[37] 

The average diagnosis age of XLA has been significantly reduced within the last 50 years.[8] In addition to this, there have been developments in the immunoglobulin treatment. Therefore, in the last few decades, patients with XLA achieving optimal treatment have no difficulties to reach to adult ages.

In case the patients with IPEX are not treated, they generally experience death in the infancy period.[38]  Regardless of the fact that the disease is endeavored to be controlled by HCT and immunosuppressive treatments and even though the improvements in prognosis have been reported in the studies released over the years, the prognosis of the disease is still not good enough.

Complications

In chronic granulomatous disease, infections affect the lung, skin, lymph node, and liver. If it is not recognized or treated on time, it can be complicated by abscesses (skin, lung, organ abscesses), suppurative adenitis, osteomyelitis, pneumonia, and superficial skin infections.

X-linked hyper IgM syndrome may be complicated by bronchiectasis due to recurrent lung infections, biliary tract diseases, and CMV-related cirrhosis, and cholangiocarcinoma, if not diagnosed early and treated appropriately.

XLP patients can be complicated by fulminant infectious mononucleosis and lymphoma, as well as lymphocytic vasculitis, aplastic anemia.

The classic WAS, may be complicated by EBV-associated lymphoma. Autoimmune events have also been reported in about half of the cases.