Introduction
Selective IgA deficiency (SIgAD) is the most prevalent primary immunodeficiency disorder (PID) or inborn errors of immunity (IEI), identified by serum concentration of IgA below 7 mg/dL and normal concentrations of IgG and IgM in patients over four years of age. The majority of SIgAD patients are asymptomatic, although some of them manifest different clinical manifestations, including gastrointestinal and respiratory tract infections, allergic diseases and autoimmune disorders. The disease progression to common variable immunodeficiency (CVID) in a selected group of SIgAD patients with IgG subclasses deficiency or autoimmune disorders has been reported [
1].
No specific causes for the pathogenesis of SIgAD have not been reported yet. However, defects in the process of IgA class switch recombination (CSR), IgA production and secretion, as well as the long-term survival of IgA, switched memory B cells and plasma cells of SIgAD patients have been identified in unsolved cases [
2]. Defects in these immunologic processes are associated with abnormalities in the lymphocytes of SIgAD patients. Hence, the assessment of the lymphocytes, especially B cell and T cell subsets, could be valuable and helpful. Several studies have demonstrated B cell and T cell abnormalities in some groups of SIgAD patients [
3,
4]. Regarding B cell subsets, a decrease in the number of switched memory B cells, IgA plasma cells and transitional IL-10
+ regulatory B cells of SIgAD patients has been reported [
5‐
8]. On the other hand, defect in some T cell subsets of SIgAD cases have been reported that is linked to insufficient IgA-producing B cells [
5,
8,
9].
Flow cytometric immunophenotyping can play an important role in the diagnosis, prognosis, classification and management of patients with SIgAD. Hence, for the first time, we aimed to investigate the main subpopulations of B and T lymphocytes along with an evaluation of T cell function, to clarify the correlation between immunological characteristics and clinical manifestations in symptomatic patients with SIgAD.
Discussion
SIgAD is the most prevalent IEI with various clinical manifestations. These patients have a different spectrum of clinical manifestations. Accordingly, immunologic investigations in patients with a different spectrum of clinical manifestations are helpful. The most prevalent clinical manifestation in IEIs, especially in SIgAD, is recurrent respiratory infections [
18‐
20]. We found pneumonia as the most frequent complication in our registered symptomatic patients. Recurrent respiratory infections commonly manifest in the form of upper respiratory tract infection and may remain undiagnosed for several years; however, some SIgAD patients manifest more severe phenotypes such as bronchiectasis or obliterate bronchiolitis which force immunological investigation in these patients [
21]. Given that recurrent respiratory infections have been reported as the most important cause of morbidity and death in children with IEIs, especially primary antibody deficiencies [
22,
23], early diagnosis and management of respiratory disorders associated with SIgAD is very important [
24,
25].
It has been indicated that abnormalities in B cell subsets are observed in some SIgAD patients [
3,
4]. Our results indicated a significant increase in naïve and transitional B cells and a strong decrease in marginal zone-like and switched memory B-cells. This abnormal B cell pattern suggests defects in the terminal stages of B-cells differentiation, similar to CVID patients [
26]. Given that CVID and SIgAD share almost similar genetic backgrounds and may accumulate as multiple cases within a family, this resemblance is predictable. In general, more than half of IEI patients registered in the Iranian registry have parental consanguinity, but this rate still is less common in SIgAD patients. In SIgAD Iranian patients compared to Western patients, consanguinity is more prevalent. Although monogenetic causes may exist, it has not been identified yet despite next-generation sequencing in several patients [
27].
We detected a reduction in marginal zone-like and switched memory B-cells, especially in severe SIgAD patients, as has been previously reported [
28]. Recently we reported similar reduction in marginal zone-like and switched memory B-cells in CVID patients [
14]
. On the other although we observed a significant decrease in switched memory in Ataxia Telangiectasia (AT) patients but a sharp increase in the marginal zone-like B cells was observed [
29]
. SIgAD patients, especially a group of patients with severe clinical manifestations (recurrent and intensive infection, and autoimmunity), have lower switched memory B-cells [
28,
30,
31]. It has been suggested that the decrease in switched memory B-cell subpopulation is due to defects in the level of antibody class-switching recombination (CSR) process, caused by enzymatic deficiency, or abnormalities in the cytokine networks and their receptors [
28]. Some SIgAD patients with severe phenotype progress to CVID, which reflects this subgroup of SIgAD may share with CVID common immune pathogenesis, particularly in the development of CSR step. Accordingly, switched memory B-cells are considered a diagnostic biomarker in patients [
28]. However, the frequency of switched memory B-cells is normal among children in our study population, and the reduction was observed more in adult patients; suggesting that aging probably leads to the progression of SIgAD to CVID, especially in patients with severe clinical manifestations (data not shown). On the other hand, marginal zone B cells are a specialized population of B cells that produce IgM for the protection against infections, especially encapsulated bacteria [
32]. Although previous studies have shown that the number of marginal zone-like B cells in SIgAD patients was not different compared to normal controls [
33], nevertheless, we obtained a significant reduction in marginal zone-like B cells in our cases, similar to a previous report in CVID patients [
34]. Reducing marginal B cell subsets in other patients with antibody production defects could be associated with an increased risk of infection such as pneumonia and a decrease in serum IgM levels, similar to CVID patients [
35].
We found increased CD21
low B cells compared to control, mainly in severe SIgAD patients. Previous studies have reported an increase in CD21
low B cells in both SIgAD [
3] and CVID patients [
36], and other autoimmune diseases [
37]. Recently also has been reported increase in the CD21
low in AT patients [
29]. An increase in the number of CD21
low cells is directly related not only to autoimmunity but also to infection [
36]. On the other hand, chronic exposure to viral infection may lead to the conversion of antigen-reactive B cells to unresponsiveness CD21
low B cells [
38]. To clarify the cause of expanded CD21
low B cells; it is necessary to make further investigations for this B cell subpopulation. Given the high subpopulation of CD21
low B cells in CVID patients and the progression of some patients with SIgAD to CVID, the severe group of SIgAD patients with increased CD21
low B cells will more likely develop CVID. Therefore, they need a more regular follow-up to assess the course of the disease.
Transitional B cells are at an intermediate stage in the development between bone marrow immature cells and mature B cells in the spleen [
39]. In the present study, we observed significantly increased transitional B cells in our SIgAD patients, especially in severe SIgAD patients, although the number of transitional B cells in children with SIgAD was normal (data not shown). We recently observed significantly decreased of the transitional B cells AT [
29]. In contrast to previous studies that showed decreased transitional B cells [
8,
28,
40], adult patients indicated slightly increased transitional B cells. Moreover, Lemarquis et al. showed a decrease in the functional activity of transitional B cells based on IL-10 production and CpG stimulation [
40]. Given the defect in terminal stages of B-cells in SIgAD, it seems that an increase in transitional B cells and naïve B cells of our patients is due to a compensatory mechanism that augment early B cell development. Regarding different results between our study and others, it seems that this difference is due to different selection processes, as all of our patients were symptomatic, while others studied heterogeneously asymptomatic and symptomatic SIgAD patients.
Regarding T cell subsets, we observed decreased total CD4
+ T cells, Th1, Th2, Treg cells and increased T
EMRA in both CD4
+ and CD8
+ cells. Consistent with our results, previous studies have shown an increase and reduction in CD8
+ and CD4
+ T lymphocytes population, respectively [
4]. Also, we found that central memory in both CD4
+ and CD8
+ T cells and effector memory CD8
+ T lymphocytes were decreased in SIgAD patients compared to HCs. We observed a significant increase in the T
EMRA cell subset in both CD4
+ and CD8
+ lymphocytes population, especially in severe SIgAD patients. T
EMRA is a third T cell memory subset in peripheral inflammatory tissues that express CD45RA but lack expression of CCR7 or CD27. In humans, T
EMRA cells accumulation is affected by chronic infections, such as CMV [
41,
42]. An increase in these terminated T cell subsets might be due to chronic cellular response to infections in these patients; however further studies need to be performed regarding this phenomenon. Consistent with our results, Nechvatalova et al. demonstrated expanded CD4
+ and CD8
+ T
EMRA cells in SIgAD patients that were related to CMV infection [
43]. We did not examine CMV infection in SIgAD patients, but an increase in the number of TEMRA cells subset in our patients could be related to chronic infections.
We recently evaluated specific antibody responses to PPSV-23 in patients with SIgAD and AT and revealed that 18.6% of the SIgAD patients and 81.3% of AT patients had an inadequate response. The number of plasmablasts, marginal zone B cells, transitional B cells, naïve CD8
+ T cells, and percentage of CD8
+ T cells, IgM memory B cells and switched memory B cells in SIgAD patients were significantly lower in non-responder group than responder group. Although specific antibody deficiency is more frequent in AT patients than SIgAD patients [
44].
Regulatory T cells play an important role in the production of IgA antibodies by transforming growth factor-beta (TGF-β) secretion [
45‐
47]. We found significantly decreased Tregs in our patients consistent with previous studies published [
48], although one study reported increased Tregs in SIgAD patients [
43]. It has also been reported a correlation between reduced Treg cells and the severity of SIgAD disease, especially in individuals with autoimmunity, and IgA CSR deficiency in patients with severe clinical manifestations [
30,
48]. The low frequency of Treg cells and other T cell subsets, including Th1 and Th2 in our patients, may be due to low thymic emigrants caused by defective thymopoiesis and or increased apoptosis of these cells [
49].
T cell functional assay by mitogenic or antigenic stimulation is an important feature in the diagnosis of various immune disorders and immunodeficiencies [
50]. Traditionally, there is one protocol for evaluating the function of T cells based on uptake of [3H] thymidine following PHA stimulation using radioactive components that needs specific laboratory conditions and also it is not T cell-specific as it can stimulate several other immune cells as well. On the other hand, the most important weakness is that no information about specific cell subsets could be obtained. CFSE proliferation assay is a practical choice for evaluating T cell responses to an antigen or mitogen in IEI patients, especially SIgAD for targeting further potential T cell defects analyses in these patients [
51]. So far, there are few reports of T-cell response defects in SIgAD patients. As expected, our study does not reveal any significant difference in T cell response between patients and controls. However, when we categorized patients into two groups based on severe and mild phenotypes, severe patients indicated decreased T cell proliferation compared to mild patients. This result could be an important finding for categorizing SIgAD patients for knowing prognosis of the patient. We recently reported that T cell proliferation was markedly impaired compared to the healthy controls in CVID patients and AT patients [
29,
52]. Moreover, this indicates that SIgAD patients with defective T cell proliferation should be followed further for precise medical management. We recommend further studies for evaluation of T cell function for SIgAD patients based on severe and mild phenotypes in other studies. Limitations of the experiment included the small number of symptomatic patients, the unavailability of many of them and even the improvement of some patients.
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