Primary immunodeficiency

Primary immunodeficiencies" refers to a heterogeneous group of rare, congenital diseases of the immune system. In the IUIS classification of 2015, this family of diseases isdivided into 9 groups with nearly 300 different, mostly molecularly defined immunodeficiencies (Picard C et al. 2015; see Appendix, International Union of Immunological Societies classification 2015) - see below Immunodeficiencies primary (classification).

The overarching collective terms SCID, C ID, CVID, HIGM, HIES, VEO-IBD, XLP, etc. are clinically useful to narrow down an immunodeficiency. However, the terminologies should be complemented by subtle molecular diagnostics. Only this allows a better prognosis estimation, the important genetic counseling. Furthermore, molecular diagnostics also allows the application of adequate and specific therapeutic concepts.

Classification into disease groups (IUIS classification, cf. Primary immunodeficiencies , classification -Picard C et al. 2015)

1. Primary immunodeficiencies/combined T/B/(NK)-cell immunodeficiencies incl. "SCID".
2. Primary immunodeficiencies/T and B cell immunodeficiencies with syndromal features.
3. Primary immunodeficiencies/antibody deficiency as leading symptom
4. Primary immunodeficiencies/immune dysregulation
5. Primary immunodeficiencies/defects of phagocyte number and/or function
6. Primary immunodeficiencies/defects of intrinsic and natural immunity
7. Primary immunodeficiencies/autoinflammatory diseases
8. Primary immunodeficiencies/complementary defects
9. Primary immunodeficiencies/bone marrow failure
10. Primary immunodeficiencies/phenocopies (DD of primary immunodeficiencies)

The following warning signs are considered worthy of attention for primary immunodeficiencies in children and adults (Picard C et al. 2015):

• Pathological susceptibility to infection "ELVIS"(pathogen, localization, course, intensity,total).
• Immundysregulation "GARFIELD"(granulomas, autoimmunity, recurrent fevers, unusual eczema, lymphoproliferation, chronic intestinal inflammation).
• Failure to thrive Weight loss, usually with diarrhea
• Conspicuous family history (consanguinity, immunodeficiency, pathological susceptibility to infections, immune dysregulation, lymphoma)
• Hypogammaglobulinemia, persistent or recurrent lymphopenia, neutropenia, thrombocytopenia
• A genetic indication of a primary immunodeficiency or a positive newborn screening for primary immunodeficiencies.

There are no data on the prevalence of the various immunodeficiencies, most of which are defined by molecular genetics (Picard C et al. 2015). From data from the USA, the prevalence for a clinically relevant immunodeficiency is estimated between 1:1200 and 1:2000 (Boyle JM et al. 2007).

Pathological susceptibility to infection as a leading symptom of primary immunodeficiencies: Pathological susceptibility to infection is usually the leading symptom of a primary immunodeficiency. Differentiation from physiological susceptibility to infection is difficult because no current epidemiological data are available on what number, type, and course of infectious diseases are considered normal at what age (Monto AS et al. 1993). The influence of factors such as social structures, family size or attendance at a day-care centre on the frequency of infection makes it even more difficult to specify an upper limit for the physiological frequency of infection. Signs of pathological susceptibility to infection may include infections caused by unusual pathogens that rarely lead to severe disease in immunocompetent individuals, such as pneumonia caused by Pneumocystis jirovecii or CMV, Candida sepsis, intestinal and/or biliary tract infection caused by cryptosporidia or microsporidia, or disseminated infection caused by nontuberculous mycobacteria (NTM) (Bustamante J et al. 2008). Furthermore, recurrent severe infections with "common" pathogens such as pneumococci or herpes simplex viruses may indicate a primary immunodeficiency (Bustamante J et al. 2008). The isolated pathogens may already provide a first indication of the underlying immunodeficiency.

Localization of infection: The localization of infection can also be an indication of pathological susceptibility to infection. Monotopic infections are more likely to indicate local anatomical causes, whereas polytopic infections are more likely to indicate a systemic immune deficiency. Pathological susceptibility to infection may also be characterized by atypical localizations of infections, e.g., a brain abscess caused by Aspergillus spp. or a liver abscess caused by S. aureus (Patiroglu T et al. 2010).

Atypical course of infections: The protracted course of infections or inadequate response to antibiotic therapy are also common indications of pathological susceptibility to infection (Cunningham-Rundles C et al. 1999). For example, a systematic review of the literature on persistent chronic rhinosinusitis reported that up to 50% of patients who did not respond to adequate therapy ultimately had a primary immunodeficiency. Unusual courses after pathogen exposure include infectious complications due to attenuated pathogens that may occur after live vaccinations, such as BCG vaccination, MMR, varicella, or rotavirus vaccination (Cunningham-Rundles C et al. 1999; Marciano BE et al. 2014).

Major/minor infections: Finally, the severity (intensity) of infectious diseases can be an expression of pathological susceptibility to infection. Here, the term "major infections" is used to distinguish pneumonia, meningitis, sepsis, osteomyelitis and invasive abscesses from so-called "minor infections", such as otitis media, sinusitis, bronchitis and superficial skin abscesses. Although the occurrence of major infections predominates in primary immunodeficiencies, persistent or recurrent minor infections above the level can also be an expression of a primary immunodeficiency (Aghamohammadi A et al. 2008; Owayed A et al. 2016). The number of infections (the sum) is often perceived as a leading symptom, especially by the affected person or patient's relatives. Infections should be distinguished here from episodes of fever without focus or infection-like symptoms (e.g. obstructive bronchitis).

Disturbance of immune regulation as a leading symptom of primary immunodeficiencies: The immune system is subject to complex regulation. Thus, genetic disorders of defense processes can often also affect the homeostasis of the immune system, including immunological tolerance. Disturbances of immune regulation may be manifested, for example, by fever, autoimmunity, lymphoproliferation, chronic "eczematous" skin manifestations, chronic intestinal inflammation, or granuloma formation. These clinical signs of impaired immune regulation can be significant, sometimes sole, symptoms of a primary immunodeficiency (Arason GJ et al. 2010). In some primary immunodeficiencies, atopic diseases or allergies are found clustered (e.g., in DOCK8 deficiency, WAS, IPEX syndrome, DiGeorge syndrome, antibody deficiency diseases, CVID) (Blazina S et al. 2016; Engelhardt KR et al. 2015; Lugo Reyes SO et al. 2016; MacGinnitie A et al. 2011; Chan SK et al. 2015; Mohammadinejad P et al. 2015; Harp J et al. 2015). However, due to the prevalence of allergies in the general population, this symptomatology is usually only suggestive in the context of other clinical manifestations.

GARFIELD -symptoms indicative of a disorder of immunoregulation: The following manifestations are considered more specific (Farmand S et al. 2017): they can be classified under the acronym "GARFIELD" as an acronym for a disorder of immunoregulation. Thus, non-necrotizing, small-heart, epithelioid cell granulomas ("sarcoid-like lesions") can be the first manifestation of an immunodeficiency. They occur mainly in the lung, lymphoid tissues, intestine but also in the skin (Harp J et al. 2015; Park MA et al. 2008; Jesenak M et al. 2014). Association with pathogens (e.g., nontuberculous mycobacteria) is possible, but usually no causative germs can be identified. Granulomatous inflammation is typical for chronic granulomatosis (mainly intestine, urogenital tract, liver), but it is also detected in about 8% to 20% of CVID patients, in CID and SCID variants (mainly RAG defect) or radiosensitive immunodeficiencies (e.g. AT, NBS).

Autoimmunity and primary immunodeficiencies: Autoimmunity is a common manifestation in primary immunodeficiencies (Carneiro-Sampaio M et al. 2015). The most common are autoimmune cytopenias (mainly in humoral immunodeficiencies, CID, ALPS and other lymphoproliferative immunodeficiencies, WAS, IPEX) and autoimmune thyroiditis. Approximately 12% to 20% of patients with CVID develop autoimmune cytopenia, which often also precedes presentation with diagnosis-related susceptibility to infection. Other autoimmune manifestations in immunodeficiencies may be mediated by autoantibodies but also by T cells and include rheumatoid arthritis, JIA, vasculitis, glomerulonephritis, hepatitis, celiac disease/autoimmune enteropathy, alopecia, vitiligo, type 1 diabetes mellitus, Addison's disease, or hypoparathyroidism. Complement deficiencies, partial/selective antibody deficiency disorders, CVID, and CGD (especially carriers) may be associated with SLE (Dimitriades VR et al. 2016).

Unexplained recurrent fever: Recurrent fever without an infectious focus is the main manifestation in periodic fever syndromes where the additional presence of skin lesions, cervical lymphadenopathy, aphthae, arthritides, myalgias or abdominal complaints helps in the differential diagnostic classification. Recurrent unexplained fever may also be the first manifestation of cyclic neutropenia. Chronic dermatitic skin diseases are often a sign of primary immunodeficiencies. These include, in particular, early-onset dermatitides that are often difficult to treat, up to erythroderma with/without alopecia (e.g., in OS, IPEX syndrome, WAS, HIES, SCID) (Sillevis Smitt JH et al. 2013; Berron-Ruiz A et al. 2000). Lymphoproliferation describes the pathological enlargement of spleen, liver and lymph nodes or the establishment of tertiary lymphoid tissue especially in the lung and gastrointestinal tract. Lymphoproliferation is the main leading symptom in ALPS, XLP, APDS, in activating STAT3 mutations or CTLA4 deficiency (Kuehn HS et al. (2014). In FHL (Familial and acquired hemophagocytic lymphohistiocytosis), hepatosplenomegaly is a diagnostic criterion (Janka GE 2007). In antibody deficiency diseases (e.g. CVID, some HIGM), splenomegaly is particularly common. Furthermore, nodular lymphoid hyperplasia is often found in the intestine. Lymphoproliferation can also be a major manifestation in CID (Blazina S et al 2016).

Inflammatory bowel disease: Inflammatory bowel disease is increasingly recognized as a primary manifestation of congenital immunodeficiencies. In particular, early onset and/or refractory course of chronic diarrhea may be important indicators of immunodeficiency. Early-onset colitis is typical for IPEX syndrome, but may also be the key leading symptom in CGD, NEMO defect, XIAP deficiency, defects in the IL-10/IL-10 pathway or CID, among others. Chronic intestinal inflammation is also common in antibody deficiency syndromes (Cunningham-Rundles C et al. 1999). The acronym "GARFIELD" (granulomas, autoimmunity, recurrent fever, unusual eczema, lymphoproliferation, chronic intestinal inflammation) was introduced in the initial version of the guideline for the typical manifestations of impaired immune regulation in primary immunodeficiency. The choice of these criteria is supported by epidemiological studies and case reports from recent years.

Other leading symptoms for primary immunodeficiencies: Malignancies, especially lymphomas, may also be the first clinical manifestation of immunodeficiencies. Lymphomas in immunodeficiencies are often B-cell lymphomas, frequently occur in younger children, more often show extranodal involvement, often respond less well to therapy, and more often show recurrences.

Malignant diseases are often virus-associated (EBV, HPV). Malignancies are particularly common in primary immunodeficiencies with chromosomal breakage (e.g., AT, NBS, Bloom syndrome, ligase IV deficiency, Artemis deficiency), XLP-1/SAP deficiency, ALPS, and cartilage-hair hypoplasia (Salavoura K et al. 2008; Shapiro RS 2011). 30-50% of patients with GATA2 deficiency develop myelodysplastic syndrome (MLS) or AML (van der Werff Ten Bosch J et al 2016). Lymphoma is also common in CID. Furthermore, ectopic EBV-associated soft tissue tumors or HHV-8-associated Kaposi's sarcoma may be expressions of a primary immunodeficiency (Schober T et al. 2017; Byun M et al. 2013). Knowledge of an underlying immunodeficiency can be decisive for therapy (e.g. chemotherapy with reduced toxicity in the case of chromosomal fragility, early decision for allogeneic stem cell transplantation if necessary).

Syndromal phenomena such as:

• Dysmorphia (e.g. DiGeorge syndrome, STAT3-deficient HIES),
• albinism (e.g. Chediak-Higashi syndrome, Griscelli syndrome, Hermansky-Pudlak syndrome
• Type 2),
• microcephaly (e.g., NBS, Cernunnos/XLF deficiency, DNA ligase IV deficiency),
• short stature (e.g. cartilage-hair hypoplasia, Schimke syndrome)
• ectodermal dysplasia (e.g. anhidrotic ectodermal dysplasia with immunodeficiency)

may indicate an immunodeficiency (International Union of Immunological Societies Expert Committee on Primary I et al. 2009).

Neurological abnormalities: These include progressive neurodegeneration of the cerebellum in AT and the presence of chronic aseptic meningitis in NOMID. Developmental abnormalities up to mental retardation are found e.g. in NBS, Cernunnos/XLF deficiency, DANN ligase IV deficiency, LAD II, ß-actin deficiency, ADA-SCID, PNP-SCID. Inflammatory neurological manifestations are frequently found in Chediak-Higashi syndrome and FHL. These may also occur, for example, in ADA2, DOCK8, or CTLA4 deficiency and in activating STAT3 mutations. Teleangiectasia of the conjunctiva and ocular motility disorders, for example, are typical of AT and usually appear after the age of 3 years.

Delayedcord drop: Delayed cord drop (>21 days after birth) may be indicative of granulocyte dysfunction (e.g., LAD, RAC2 deficiency) or a defect in Toll-like receptor-mediated signal transduction (e.g., MyD88/IRAK-4 deficiency). However, delayed cord drop can usually be explained by other factors (e.g., prematurity, delivery by sectio, postpartum antibiotic therapy). More characteristic symptom of granulocyte defect is omphalitis associated with delayed cord drop.

Manifestation of primary immunodeficiencies in adulthood: As congenital disorders, most primary immunodeficiencies manifest in childhood or adolescence. However, up to 50% of initial diagnoses affect patients over 25 years of age, making the prevalence of primary immunodeficiencies overall predominant in adulthood (Kobrynski L et al. 2014). In particular, the common CVID disease group does not manifest until a median of approximately 24 years of age. Almost any primary immunodeficiency can manifest late and then often with an atypical clinical presentation, particularly if the mutations in the affected genes are not null mutations but allow residual function of the affected gene product. More commonly reported examples are XLA, CGD, XLP or SCID variants such as ADA deficiency.

Basic diagnostics for suspected primary immunodeficiency

• Blood count with differentiation
• Determination of immunoglobulins (IgM, IgG, IgA, IgE).
• In the differential blood count, leukocytopenia, lymphocytopenia, neutropenia, monocytopenia or thrombocytopenia may be the first indication of an immunodeficiency and require further clarification. It is important to assess the absolute numbers taking into account age-dependent normal values. Decreased values may indicate decreased production (e.g., SCID, SCN, WAS, chronic bone marrow failure in CID) or reduced survival (e.g., ITP, AIHA, AIN, or HLH).
• Eosinophilia associated with susceptibility to infection or immune dysregulation may be indicative of a primary immunodeficiency (e.g., HIES, OS, IPEX syndrome). Furthermore, a morphological evaluation of the blood smear should be performed (e.g. Howell-Jolly bodies in asplenia, microthrombocytopenia in WAS, giant granules in Chediak-Higashi syndrome).
• Antibody deficiency disease (in about 50% of patients with primary immunodeficiency): this may occur in e.g. chronic enteric or renal loss, when taking drugs (e.g. some antiepileptics, antimalarial drugs, immunosuppressants or chemotherapies) or in malignant diseases, especially lymphomas and leukemias. When determining immunoglobulin concentrations in childhood, age-specific normal values must be observed. Diaplacentally transmitted maternal IgG antibodies are detectable up to the 6th-10th month of life, so that a reliable assessment of infant IgG production is only possible thereafter. Formerly premature infants often show lowered IgG levels, which moreover reach their nadir more rapidly. IgA production often does not mature until the first few years of life, so IgA deficiency cannot be reliably diagnosed until after 4 years of age. Increased immunoglobulin concentrations can also indicate an immunodeficiency, especially increased IgE (e.g. in HIES, OS, IPEX syndrome), increased IgM (e.g. in HIGM, some patients with NEMO defect) but also increased IgG (e.g. in ALPS).
• Importance of genetic diagnostics in primary immunodeficiencies: Most primary immunodeficiencies defined genetically so far are monogenetic. However, many show variable expressivity and penetrance (Boztug H et al. 2016). Patients with the same mutation can therefore show different phenotypes. This is especially true for "hypomorphic" variants, in which the affected gene product is not absent but retains residual function. Thus, for most defects, there is no constant genotype-phenotype correlation.
• Finally, a similar clinical phenotype may be due to mutations in different genes (Tsujita Y et al. 2016). Furthermore, many immunodeficiencies that have not yet been further defined appear to be influenced by multiple genes.
• In addition to classical targeted Sanger sequencing of single genes, parallel multi-gene analyses using next generation sequencing (NGS) are now commonly used. This includes gene panel diagnostics with the analysis of a group of known disease-associated genes, the sequencing of almost all protein-coding genes (whole exome sequencing, WES) or the entire genome (whole genome sequencing, WGS). In principle, new gene defects can also be identified by WES or WGS. However, the experimental validation of candidate genes is often very time-consuming, so that this rarely helps in acute diagnostics.
• Primary immunodeficiencies not detected by basic diagnostics: A number of primary immunodeficiencies are not detected by basic diagnostics. These include specific antibody deficiency diseases, many defects of innate immunity (e.g. complement defects, IRAK-4 or NEMO deficiency, neutrophil function defects, defects of the IL-12/IFN axis) as well as a number of immunodeficiencies in which immune dysregulation is in the foreground (e.g. in familial hemophagocytic lymphohistiocytosis (FHL) in the symptom-free interval, autoinflammatory diseases).

Abnormal laboratory findings as the first sign of a primary immunodeficiency. These include: neutropenia, lymphocytopenia and thrombocytopenia. They can be signs of an uncomplicated viral infection, but also the first indication of a primary immunodeficiency.

Antibody concentrations below the age-adapted normal range are an important indication of an immunodeficiency. Especially infants with SCID, agammaglobulinemia or SCN may develop life-threatening disease at the first infection. In case of reproducible abnormal laboratory findings, further immunological clarification as well as appropriate protective measures (e.g. antibiotic and antimycotic prophylaxis, isolation of the patient, refraining from live vaccinations) are therefore necessary.

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