This study showed in a cohort of children and adolescents previously presumed as primary ITP that 23.3% could be reassessed as having underlying IEI namely, IgA deficiency, IgM deficiency, C4 deficiency, low B cells, low T cells, and unclassified IEI. In addition, 86.7% of the ITP patients presented at least one altered immunoglobulin or complement level or lymphocyte subset proportion. Moreover, 6.7% of ITP patients had an additional autoimmune disease, 56.7% had an allergy diagnosis, 20.0% had a positive family history of autoimmune disease, and 70.0% had a positive family history of allergy all considered main clues to underlying IEI.
As suggested by current guidelines, it is essential to point out that the enrolled ITP patients had already been evaluated for IEI at diagnosis and when they evolved to persistent or chronic disease. Even so, during this study, we were able to diagnose additional IEI, reinforcing the need for a systematic periodical investigation of the etiological disease cause [2, 5, 18].
IEI diagnosis may be important clues to unveil ITP pathogenesis since 1- both loss and gain of function disorders may present with thrombocytopenia, 2- not only complete but also partial lesions of genes related to immune functions may give rise to autoimmune diseases, and 3- substantial clinical heterogeneity exists within patients with the same mutation [2, 3, 6, 12, 13, 22, 23].
IgA has anti-inflammatory roles, participates in the tolerance, and its levels go increasing through life. Thereby, its lack or maintained or decreasing values associated or not with other autoimmune diseases and/or positive autoantibodies in ITP patients may suggest for clinicians underlying IEI [8, 13, 15]. In addition, since the lack of IgA may impair mucosal antigens' clearance, it has also been linked to abnormalities in the gastrointestinal-associated mucosal immune system [15]. Interestingly, both the ITP patient diagnosed with IgA deficiency and the patient with a low IgA level had personal histories of diarrhea. ITP associated with low IgA appears in 19 IEI in the application developed to help clinicians' diagnosis [12].
IgM modulates both innate and adaptive immune responses, and its lack may impair apoptotic cell clearance and consequently lead to autoantibodies production, including those related to ITP [20, 24, 25]. Of note, IgM deficient patients frequently have high IgE levels, as observed in our patients [6, 13, 20]. Moreover, IgM deficiency has been observed in other autoimmune diseases including SLE [8, 16]. Notably, ITP patients with low IgM levels underwent significantly more splenectomy, suggesting worse treatment response and longer disease persistence as previously observed for SLE [16].
Complement system activation is important for platelet destruction [3]. Otherwise, complement deficient patients have an increased risk for autoimmune diseases, and it is described that 30% of ITP patients may evolve to SLE [4, 8, 9, 24–26]. Noteworthy, although the patient with C4 deficiency had negative autoantibodies, she had two family members with SLE. This finding puts in check the current perception that complement levels assessment has an uncertain benefit on ITP diagnosis, and reinforces the idea of reassessing not only immunoglobulin but also complement levels during the follow-up [5].
IEI that affect T and B lymphocytes also develop autoimmune manifestations, and low B cell number appears in at least 32 IEI in the developed application to help clinicians' diagnosis [3, 6, 9, 12]. Of note, most ITP patients with low B cells had positive autoantibodies, including one diagnosed with thyroiditis. Low B cell number is a peculiar feature of CVID [6, 13]. Genetic analysis will be fundamental to revealing underlying IEI like CTLA4 haploinsufficiency and LRBA deficiency [6]. Yet, although this study did not analyze regulatory or double-negative T cells or T lymphocyte functions, a previous study regarding low T-cell receptor excision circles (TREC) in ITP [27] and the current findings reiterate the need to periodically reassess T and B lymphocytes during ITP follow-up [2, 5, 18].
Low IgG is characteristic of CVID but also appears in STAT3-GOF and LRBA deficiency in which diverse genetic defects lead to various forms of non-organ and organ-specific autoimmunity, including ITP [10, 12, 13, 17, 23]. Interestingly, of the ITP patients with low IgG, 35.7% (5/14) had positive autoantibodies, including a patient diagnosed with thyroiditis. Yet, five ITP patients, including one with a low C4 level, had a positive family history of autoimmune diseases namely, vitiligo, SLE, rheumatoid arthritis, and ITP. Moreover, CVID is usually diagnosed over 20 years of age, and the ITP patients with low IgG may have this diagnosis in the future [13].
High IgG and IgM levels associated with ITP appear in a series of IEI including ALPS and Hyper-IgM syndrome [12, 14]. ALPS diagnosis was impaired in this study since double-negative T cell proportion was not analyzed. However, none of the enrolled ITP patients had splenomegaly and/or lymphadenopathy, making ALPS diagnosis unlikely [6, 7, 10, 12]. Hyper-IgM syndrome diagnosis was ruled out since the patients with high IgM had normal IgA and IgG levels. Again, genetic analysis will be important to reveal additional underlying IEI.
High IgE levels, allergic phenotypes, family history of allergy, and thrombocytopenia are frequently observed in partial and regulatory T cell defects, including IPEX [9, 10, 21]. High IgE, even without allergic or parasitic disease, was also observed in SLE [21]. Notably, SLE-like disease has been described in patients with Hyper-IgE syndromes [9, 10]. Interestingly, among ITP patients with high IgE levels, eight had disease onset before 6 years of age, and almost all had positive autoantibodies, including anti-dsDNA. Yet, one patient was diagnosed with T1D and another with thyroiditis. These findings corroborate with the idea that high IgE may not be directly related to disease pathogenesis, but it is a biomarker of immune dysregulation [21], and when associated with other altered immunoglobulin and/or an autoimmune disease and/or positive autoantibodies may be a warning sign for clinicians for underlying IEI.
Other limitations of this study were its cross-sectional design, the sample size, not measuring IgG subclasses, and not performing genetic analyses. Therefore, some IEI may have been missed. Otherwise, taking into account that genetic analysis is still expensive for most centers our findings may be clues for sorting ITP patients to be screened for molecular testing.
IEI diagnosis increases with the presence of positive family history [6, 10, 11]. However, no ITP patient had family members with IEI. In addition, this study showed that ITP patients may have IEI diagnosis regardless of the age at disease onset, disease classification, and disease outcome. Moreover, various ITP patients had a positive family history of autoimmune disease, including ITP and/or additional autoimmune disease and/or positive autoantibodies, strengthening the idea of a genetic cause [1, 3, 18, 22, 25, 28]. Furthermore, growing relevance has been given to combined biomarkers since bear better positive and negative predicted values for IEI diagnosis than genotype-phenotype correlation [6]. Beyond, studies point out thrombocytopenia improvement with the treatment of the primary disease [5].
Finally, as a whole, 86.7% of ITP patients had altered immunoglobulin or complement levels or lymphocyte subset proportion. This data is relevant and may have direct implications for physicians' care of children with ITP, especially for those with chronic disease, and particularly when splenectomy is considered to avoid additional infectious risk [5, 6].
In conclusion, underlying IEI could be diagnosed in children and adolescents previously presumed as primary ITP, reinforcing the idea that immunoglobulin and complement levels and lymphocyte subset proportions should periodically be reassessed during follow-up. Further genetic analyses will be a cornerstone for unveiling additional IEI related to ITP.