Introduction

Common Variable Immunodeficiency (CVID) is the most common primary immunodeficiency disorder, which encompasses a clinically and immunologically heterogeneous group of antibody deficiency disorders with many, mostly unknown causes [13].

The diagnosis of CVID could be established according to the criteria of low serum IgG concentration with marked decrease in serum IgA or IgM; poor response to vaccination; onset of immune deficiency after four years of age; and exclusion of other causes of hypogammaglobulinemia [4]. However, there are several concerns in this regard. For instance, there are several patients with hypogammaglobulinemia that other monogenic defects are excluded, but they cannot fully fill the above-mentioned criteria and are classified as CVID.

Due to deficit in specific antibody response, CVID patients are susceptible to recurrent sinopulmonary and gastrointestinal infections as well as an enhanced risk of malignancy, autoimmune and inflammatory disorders.

The immunological work up regarding etiology of CVID has revealed B cell intrinsic defects as well as T cell dysfunction contributing to the immunodeficiency. Since hypogammaglobulinemia is the cardinal manifestation of the disease, a large body of evidence has indicated the primary impairment of B-cell lineage as an important cause of the disease [5].

The observation that B cells from some CVID patients can produce immunoglobulin (Ig), if appropriately stimulated by CpG oligonucleotides [6] or addition of IL-10 in vitro [7] or IL-21 [8] suggests pivotal role of B-cell-extrinsic factors in the pathogenesis of the immunodeficiency in these patients.

In this regard, a vast array of T-cell abnormalities has been described in CVID patients, including T cell activation defects [911], lymphopenia and anergy [12], impaired proliferation in response to mitogens [13, 14], apoptosis enhancement [9], impairment in cytokine production [1518], failure in Ag-primed T cells generation after prophylactic vaccination [19] and neoantigens [20] and reduced expression of CD40L on activated T cells [21] and low levels of IL-2 mRNA in a subset of CVID patients [22] which may contribute to the B-cell differentiation failure.

Impairment in innate immune compartment such as dendritic cells (DC) has also been previously described in patients with CVID [23]. In this regard, Banchereau et al. has shown that several functions of B and T lymphocytes are controlled by DCs [24]. DCs, as the sentinel cells of the immune system, initiate the immune response by processing antigen material and direct it by T and B cells through the expression of co-stimulatory molecules and secretion of diverse chemokines and cytokines [2426].

Thus the normal immune response is regulated by a local and temporal orchestration of cell-cell contact and cytokines produced by different cells, often referred to as the costimulatory environment of the activation of effector cells. Cytokines have been recognized as such crucial elements of the developing immune response that there are often characterized and divided by the cytokine profile of the dominant T cell population. Depending on the cytokine profile of the main T-helper (Th) CD4+ cells, the responses are divided in Th1, Th2, Th9, Th17 responses and T follicular helper cells (TFH) are critical for the humoral antibody response [27]. While each of these responses has its typical trigger, profile and function, they very rarely occur in an isolated fashion. Th1 cells secrete mainly IL-2, Interferon gamma (IFN-γ), and tumor necrosis factor alpha (TNF-α) with a key role in cell mediated immunity where as Th2 cells secrete IL-4, IL-5, IL-10, and IL-13, which are important in eosinophilic and IgE mediated responses [27, 28].

Both types of cytokine profiles are often involved and shape the general humoral immune response.

Th17 cells mainly produce IL-17A, IL-17 F, IL-21 and IL-22, and contribute to the local inflammatory response by recruiting neutrophils [29].

Th9 cells are characterized by the secretion of IL-9 which seems to be involved in pathogen directed immunity and especially inflammatory responses in the context of allergy [30]. TFH cells are the main T cell population in the germinal center supporting B cell memory and plasma blast formation by the production of IL-21 and other costimulatory molecules [31].

In addition to division of cytokines based in the type of T-helper cell response, cytokines can be classified by their ability to promote or inhibit inflammatory responses.

Pro-inflammatory cytokines are: IL-1β, IL-2, IL-6, IL-8, IL-12, IL-17, IFN-γ, and TNF-α where as the anti-inflammatory cytokines are: IL-4, IL-5, IL-10, IL-13, and transforming growth factor beta (TGF-β). Given the crucial role of cytokines in effective antibody response [8, 3235], immune regulation and tolerance, disturbed cytokine production may contribute importantly to the immunodeficiency and dysregulation in CVID patients. Therefore several studies have addressed the production of cytokines in CVID [7, 3639].

The purpose of this article is to review the current literature on cytokines and assess their potential role in CVID.

Cytokines in CVID

The role of the different cytokines in CVID is summarized in Tables I, II, III, IV and V. However, due to the retrospective collection of incomplete data regarding the composition of the examined cell types, the clinical presentation at the time of examination and other potential confounding factors in different studies, there are clear limitations in the comparison of cytokine profiles in patients. Common cytokine profiles might reflect similarities in the underlying immunodeficiency but also common manifestations of secondary events in specific subgroups of CVID [40]. The technique and condition of the investigation as well as the selected cytokines vary significantly between the studies. While some studies measured the serum level of cytokines, others reported in vitro cytokine production with different or without stimulation. Results are reported on RNA transcript and protein level. Few studies also investigated cytokine receptor expression or functional response to specific cytokines adding to the heterogeneity of the literature.

Table I The role of pro-inflammatory cytokines in common variable immunodeficiency
Table II The role of Th1 cytokines in common variable immunodeficiency
Table III The role of Th2 cytokines in common variable immunodeficiency
Table IV The role of T-regulatory and Th17 cytokines in common variable immunodeficiency
Table V The role of other cytokines in common variable immunodeficiency

IL-1

The IL-1 family, compromises a group of 11 beta trefoil cytokines where two members IL-1α and IL-1β, play critical roles in immune regulation and the inflammatory response. The pro-inflammatory cytokines, IL-1α and IL-1β, are produced by macrophages and DCs [41].

IL-1 has a natural antagonist IL-1Ra (IL-1 receptor antagonist), which regulates IL-1α and IL-1β proinflammatory activity [42]. In addition to IL-1α and IL-1β, other IL-1 family members including IL-18, IL-33, IL-36α, IL-36β and IL-36γ have proinflammatory properties [43].

Our own previous study did not demonstrate any association between IL-1 (IL-1α, IL-1β, IL-1R and IL-1RA) cytokine gene polymorphism and CVID [44]. Normal IL-1α serum levels were found in some CVID patients compared to controls [16]. One other study has investigated IL-1 in CVID and has revealed a decreased proliferative response to low dose anti CD2 in combination with IL-1 in the presence of monocytes [45]. For clarifying the underlying mechanism, levels of IL-1RA from monocytes after lipopolysaccharide (LPS) stimulation were measured which revealed no significant difference between patients with CVID and controls [45].

Trujillo et al. who measured protein level of diverse cytokines has also shown normal range of IL-1β production after stimulation of peripheral blood mononuclear cells (PBMC) with LPS and lipoteichoic acid [46].

In addition, Haveman et al. also revealed no significant difference in IL-1β production after 3 days stimulation of PBMC with concanavalin A (ConA) or diverse viral antigens including EBNA1, BZLF1 (EBV), HHV-6B, CMV, HSV1 and HAdV [47]. Thus, in summary no major alteration was detected in the production of IL-1 in CVID.

IL-2

IL-2, the first in a series of lymphocytotrophic cytokines to be recognized, plays a central role in the regulation of growth, differentiation and function of lymphocytes [48].

The critical role for IL-2 and its receptor, IL-2R in T cell survival, particularly in T regulatory biology has been pointed out by previous studies [49, 50].

Up to now, numerous studies have investigated the IL-2 serum level or its mRNA production by T cells of CVID patients; however, contrary results have been obtained.

Defective IL-2 production by CD4+ T cells [39] associated with impaired growth and differentiation capacity of progenitor cells [51] have been reported before. However, normal IL-2 production has been found in other studies [28, 52].

Pasterolli et al. who examined production of lymphokines by peripheral blood lymphocytes (PBL) of CVID patients revealed reduced production of IL-2 and IL-4 compared to PBL of healthy donors after stimulation with PHA and ConA. In contrast, Giovannetti et al. found no significant differences in IL-2 production after ionomycin and PMA (phorbol myristate acetate) stimulation of CD4+ and CD8+ T cells between CVID and controls [53].

Analyzing cytokines in CVID patients with inflammatory bowel disease, Agarwal et al. observed a trend toward reduction in IL-2 production after stimulation of T cells with anti CD3/CD28, but not following stimulation by PHA (phytohaemagglutinin)/PMA which highlighted that defective TCR signaling might be involved [54]. In contrast, Pons et al. described a trend toward increased IL-2 production after stimulation of T cells with CD3 or CD3/CD28 in CVID patients [55].

Given the immunostimulatory function of IL-2 and the data of reduced IL-2 production mentioned above, two groups have investigated the effects of in vivo IL-2 treatment [1, 3, 56]. Cunningham-Rundles et al., who treated 5 CVID patients with polyethylene glycol-conjugated human recombinant interleukin-2 (PEG- IL-2) intravenously, have shown improved T cell helper activity [57]. Thereafter, the same group has demonstrated enhancement in T cell proliferation, increased IL-2 production and renewed production of serum antibody after 12 subsequent weeks of treatment with subcutaneous PEG-IL-2 [1]. One patient of this study was selected randomly for continuous PEG-IL-2 treatment for 16 months to assess the long-term effect of this therapy [56]. This 50-year-old CVID woman had higher numbers of circulating mononuclear cells, IL-2 and IL-6 cellular secretion and immunoglobulin concentration [56] after 16 months of treatment. In addition, the therapy was accompanied by several clinical benefits such as ending chronic diarrhea and decreasing respiratory tract infections [56].

Several years later, Cunningham-Rundles et al. examined the role of long-term PEG-IL-2 in 15 additional randomly chosen CVID patients [58]. Also, this study revealed enhanced T cell function, increase of the antibody response and reduced duration of infections after 6–12 months and 12–18 months which indicated IL-2 as an adjuvant therapy in CVID patients [58].

Similarly, Rump et al. found in a crossover study with natural human IL-2 (nhuIL-2) in combination with intravenous gamma globulin (IVIG) in 10 CVID patients a significant reduction of severe infection in patients treated with nhu IL-2, especially in the first 6 months; however nhu IL-2 therapy was ineffective in eliciting spontaneous IgG synthesis [59].

In the two mentioned papers on in vivo IL-2 therapy, no severe side effects except local skin reactions of the therapy were reported [58, 59]. Only one patient with prior splenectomy had elevated liver function tests, in whom liver biopsy revealed possible (preexisting) nodular regenerative hyperplasia [59].

Taken together, dysregulation of IL-2 possibly plays an important role in the immunopathogenesis of some patients and therefore IL-2 therapy might be beneficial in the therapy of selected CVID patients, but further trials are necessary to account for the correct selection of patient and the benefit-risk ratio of this systemic therapy.

IL-4

Interleukin-4 is produced by T cells and some innate cells. It mainly serves as an inducer in the differentiation of naïve T cells to Th2 lymphocytes, it co-stimulates proliferation of activated B- and T-cells, and induces B-cell class switching to production of IgE and IgG [60] and is associated with the manifestation of allergy [61].

Several studies have shown high serum levels of IL-4 in CVID patients [16, 28, 37, 55] inducing a predominance of Th2 activity in some CVID patients [28, 62], however, other studies showed contrary results [63, 64]. Despite previous studies have revealed association of allergic asthma [65] and allergic rhinitis [66] with CVID, no clear link of IgE up-regulation or Th2 increase in CVID has been shown so far.

This overproduction may be explained by specific IL-4 gene polymorphisms enriched in CVID patients [71]. On the other hand, an impaired IL-4 secretion by CD4+ lymphocytes following TCR/anti-CD28 stimulation [63] or decreased synthesis of IL-4 due to increased susceptibility of lymphocytes to apoptosis [64] have been reported. One study analyzed the IL-4 receptor function in pediatric cases without detecting an abnormal function in the analyzed cases [67].

Taken together, contrary results probably reflect the heterogeneity of the CVID cohort regarding predominant Th1/Th2 differentiation and further studies are important to correlate these phenotypes to clinical and other immunological phenotypes.

IL-5

IL-5, a Th2 cytokine, stimulates B cell growth and increases immunoglobulin secretion [68].

Reduced IL-5 production by peripheral T cells of CVID patients may therefore contribute to defective antibody production in CVID patients [15, 69]. Eisenstein et al. revealed impaired IL-5 mRNA production in CVID patients [39].

Kokron et al. have proposed increased susceptibility to apoptosis in lymphocyte following activation also as a cause for decreased synthesis of IL-5 [64].

However, one report found a similar frequency of IL-5 producing mononuclear cells inhypogammaglobulinemia as compared to controls by insitu hybridization after pokeweed mitogen (PWM) stimulation [70]. In summary, most, but not all studies found reduced IL-5 production by T cells of CVID patients.

IL-6

IL-6 is a cytokine produced by different cell types including antigen presenting cells, B cells and Th2 cells. Albeit its well known proinflammatory role [71], IL-6 is a pleiotropic cytokine with multiple functions on multiple cell types including anti-inflammatory effects [72].

One of the mechanisms IL-6 exhibits its immunoregulatory functions is by directing the differentiation of regulatory versus Th17 cells [73]. In addition, in the context of humoral immunodeficiency the role of IL-6 as a plasma cell growth factor has to be mentioned [74]. Up regulation in IL-6 production has previously been implicated in autoimmune disorders like rheumatoid arthritis [75], systemic lupus erythematosus (SLE) [76] and others [77, 78] possibly inducing hypergammaglobulinemia in these disorders.

In CVID patients, results reaching from increased production [55, 79], normal range [46] to decreased production of IL-6 after mitogen stimulation [80] have been reported suggesting a heterogeneity in the cohort.

Pandolfi et al. has shown an increased production of IL-6 in vitro which did not correlate with specific lymphocyte subpopulations [81]. Previous data have indicated that the increase in IL-6 production does not lead to improved immunoglobulin production in CVID after in vitro stimulation [82].

Another study which evaluated genotype frequencies of a number polymorphic genes coding IL-6 in CVID patients and healthy individuals revealed an increased level of IL-6 which could be due to single nucleotide polymorphism in these patients, however the functional importance of these polymorphisms is still doubtful [44]. In contrast to mentioned studies, Hong et al. revealed Pneumovax-23-induced monocytes produce less IL-6 in CVID patients as compared with controls.

Taken together, majority of studies represent a higher production of IL- 6 in CVID patients.

IL-7

Interleukin 7 (IL-7), a T cell growth factor and a regulator of Th1 and Th2 cytokine production, stimulates differentiation of stem cells into lymphoid progenitor cells and promotes survival and expansion of lymphoid precursors [8385]. It has also been shown that IL-7 stimulates proliferation of in the lymphoid lineage cells including T cells and NK cells [86]. While IL-7 is crucial in murine early B cell development, normal B cell differentiation in IL7Ra deficiency in humans negates an important influence of IL-7 on early human B cell differentiation [87, 88].

Previous studies have shown that serum level of IL-7 is elevated in lymphopenia such as low CD4+ T-cell in HIV infection and the level declines as T-cell numbers recover [89, 90].

Moreover, IL-7 increases cell numbers and function of leukocytes by reducing activated T lymphocyte apoptosis and promote INF- γ and other cytokines [91].

Concerning CVID and IL-7, a brief report has reported elevated plasma levels of IL-7 in a subgroup of CVID patients [92], while, Isgro et al. revealed reduced IL-7 secretion from bone marrow mononuclear cells of CVID patients in vitro [51].

IL-7 plays an important role in T-cell homeostasis by inducing proliferation and differentiation of immature thymocytes and by enhancing the peripheral expansion T-cell subsets.

Regarding correlation between IL-7 levels and in vitro T-cell proliferation, it has also been demonstrated that T cells from CVID patients secrete less TGFβ1, a negative regulator of IL-7 secretion, suggesting a functional link [93].

In summary, studies regarding Il-7 and CVID are scarce and it remains to be clarified whether it has a role in immune dysregulation in a subgroup of CVID patients.

IL-10

IL-10, an anti-inflammatory cytokine, is secreted by different cells, especially T cells and monocytes. It is capable of inhibiting synthesis of pro-inflammatory cytokines such as IFN-γ, IL-2, IL-3, TNFα. In addition, IL-10 regulates differentiation of B cells, T helper cells and NK cells [33, 94] and plays a role in plasma cell differentiation [95]. IL-10 plays an important role in the control of inflammation by T regulatory (Treg) cells [96]. Association of altered Treg has been evaluated in different autoimmune disorders. In this regard, several papers have revealed a decreased number of Treg in CVID patients especially when presenting with splenomegaly and increased inflammatory markers [97]. The authors reported a significant correlation between IL-10 production in vitro with the number of Tregs [7] suggesting Tregs to be the major T cell source of IL-10.

Increased serum level of IL-10 have been reported in SLE and arthritis rheumatoid patients [33]. Interestingly, IL-10 deficient patients present with normal Ig serum levels and specific antibody responses excluding a non-redundant role of IL-10 in plasma cell differentiation [98].

In CVID patients two studies demonstrated reduced IL-10 production by T cells in vitro [7, 99], one study found no difference [21]. At the same time Zhou et al. found increased IL-10 production by monocytes [99]. Holm et al. associated low IL-10 production by T cells with altered cAMP/PKAI signaling, opening a potential therapeutic target in CVID [7]. The potential therapeutic use of IL-10 in CVID was suggested by in vitro experiments in which the addition of IL-10 to IL-2 and SAC activated PBMC supported IgG secretion in 6/7 tested children [35]. This was not supported by an Italian study on 17 patients where only one patient responded to IL-10 co-stimulation [21]. Finally, a recent study by Kasztalska et al. has detected higher plasma levels of IL-10 in 17 CVID patients [100], potentially due to a lower frequency of a genetic haplotype associated with low IL-10 production in CVID [101].

In summary, the dysregulation of IL-10 production in CVID is complex. While T cells might produce less IL-10, a higher production of IL-10 by monocytes and in serum has been detected in CVID patients. Given the availability of therapeutic IL-10 it is worth revisiting the question of the potential therapeutic use in selected patients.

IL-12

The best known function of IL-12 is the induction of IFN-γ and differentiation of naïve T cells to Th1 [102]. The administration of IL-12 aggravates autoimmune manifestations in different mouse and rat models [103, 104].

A higher proportion of intracellular IL-12 producing monocytes has been disclosed in CVID patients, which was associated with increased frequency of IFN-γ positive T cells [105]. Compatible with this finding one study described increased levels of plasma IL-12p40, which was not related to IL-12p40 gene or promoter polymorphism [106]. Another study detected an increased expression of IL-12Rbeta1 on CD45RA + CD4+ and CD45RA + CD8+ T cells of CVID patients as a sign of the Th1 predominance in the examined patients [107]. Based on these findings, one study recommended the use of an oral IL-12/IL-23 inhibitor named Apilimod for CVID as it was suggested for other immune disorders such as Crohn’s disease and rheumatoid arthritis [108]. Currently, no data are however available.

On the other hand, Cunningham Rundles et al. have demonstrated a lower rate of IL-12 secretion by DCs of CVID patients as compared to DCs of normal controls as a result of a functional DC defect [109]. The low IL-12 production upon CD40 signaling in DCs of CVID patients was confirmed by Bayry et al. [23]. In summary, IL-12 production might be regulated differently in different cellular compartments. The impact of the dysregulation in IL-12 production needs to be addressed separately in the different subgroups of CVID patients.

IL-21

IL-21, a recently identified CD4+ T cell human cytokine is mainly expressed in TFH cells, but also Th2 and Th17 cells [110, 111]. IL-21 is involved in multiple processes of the immune system including NK cell maturation, B cell and T cell proliferation and especially plasma cell differentiation [112, 113]. IL-21, which is related to IL-2 and IL-15, sharing a common receptor gamma chain, is the master regulator of Ig production and Ig isotype switching [114].

One study revealed the capacity of exogenous IL-21 to restore the antibody producing capacity in 32 CVID patients [8]. Since there was no mutation in the coding region of IL-21 or IL-21R and no difference in mRNA induction in stimulated T cells the underlying failure of sufficient IL-21 co-stimulation in vivo remains open [67]. Subsequently, monogenetic IL-21R deficiency has been identified in 4 children of two unrelated families resembling combined immunodeficiency, interestingly, not fulfilling the current diagnostic criteria for CVID in any of the patients [115]. The role and potential therapeutic use of IL-21 in CVID deserves more attention in the future.

IFN

Interferons (IFNs) are cytokines, which were first reported in the context of antiviral defense [116]. Three major types of IFNs, based on the receptor type through which they signal have been classified [117]. Type I IFNs can be produced by all cells especially upon activation of cytosolic receptors that recognize nucleic acid [118] with plasmacytoid dendritic cells being the most potent producers. They play an important role in antiviral defense among others by inducing MHC expression [119] and can enhance isotype switching, increase the amount and long-term production of Ig [120]. Increased type I IFN has been shown in diverse autoimmune disorders such as SLE and rheumatic arthritis [121]. In CVID, a previous report by Strannegård et al. has revealed over-production of type Ι IFN [122]. Studies by the group of C. Cunningham-Rundles have revealed reduced type I IFN release on TLR7 or TLR9 stimulation by B cells and plasmacytoid dendritic cells of CVID patients [6, 123].

Type II IFNs are represented by IFN-γ, a dimerized cytokine. IFN-γ is the key Th1 cytokine and therefore plays an important role in both innate and adaptive immune systems [124]. IFN-γ has been investigated in multiple CVID studies with variable results. Inoue et al. has shown a normal range of IFN-γ and IL-2 serum level in CVID patients [52]. In contrast, Hauber et al. have demonstrated a dramatic reduction of IL-2 and IFN-γ mRNA levels in CVIDs [125]. Fischer et al. corroborated this finding demonstrating a deficiency in IFN-γ mRNA expression after stimulation with antigen in a subgroup of CVID patients [38], while Serrano et al. has revealed increased production of IFN-γ in purified T cells of CVID patients upon CD3 or PMA/PMH stimulation after 24 h [126]. North et al. also showed an increase production of IFN-γ in CD4+ and CD8+ T cells of CVID patients upon PMA/ionomycin stimulation after 12 h [127]. Thus IFN-γ profile seems to vary strongly according to the patients examined. There is a subgroup of CVID patients with increased Th1 profile. Thus, the characterization of the exact profile of these patients needs to be addressed in more detail.

TNF-α

TNF-αá, an inflammatory cytokine secreted by macrophages, monocytes, neutrophils, T-cells and NK-cells, is involved in systemic inflammation and stimulates the acute phase reaction. TNF-á has been shown to play an essential role in regulating the production of other pro-inflammatory cytokines [128].

TNF-á is associated with a variety of human diseases including asthma [129], Alzheimer’s disease [130], major depression [131] and inflammatory bowel disease [132]. Moreover, the therapeutic role of anti-TNF-á has been demonstrated in several studies [128, 133, 134]. Aukrast et al. has shown a higher spontaneous TNF-á serum level in CVID patients despite its down regulation after stimulation of PBMC with LPS [135]. Isgro et al. also indicated elevated levels of TNF-á in CVID patients [51]. Years later, Trujillo et al. has observed a significant increase in TNF- á production by CVID derived PBMC upon stimulation with LPS in comparison with healthy controls which support the inflammatory state in CVID patients [46]. In contrast, T cells of CVID patients seem to produce less TNF-α after stimulation by anti-CD3 + CD28 [54].

We have also examined the role of gene polymorphism in TNF-á variability in CVID patients as compared to controls [44]. The study has revealed an over-representation of an uncommon TNF-á allele with A instead of G at position −308 in CVID patients as compared to healthy individuals. This genotype is associated with a higher production of TNF-á.

TNF-á targeted therapies have been suggested for specific indications in CVID. Chua et al. reported the beneficial effect of anti-TNF-á (infliximab) therapy in 3 CVID patients suffering from severe enteropathy [136]. In addition, single cases have been treated successfully for granulomatous disease especially of the skin [137, 138].

Therefore, majority of studies supported the increase level of TNF-α in CVID and suggested anti TNF-α therapy for specific secondary complications. However its final value needs to be determined in regard to benefit and risk of secondary infections.

IL-17

Interleukin-17 (IL-17), a pro-inflammatory cytokine produced by Th17 cells, has been reported to be involved in different autoimmune and inflammatory conditions [139]. The IL-17 family consists of 6 family members; including IL-17A, IL-17B, IL-17C, IL-17days, IL-17E and IL-17 F.

Among IL17 family members, only IL-17A and IL-17 F are produced by Th17. The mentioned two cytokines share patterns of expression and most of their biological function [140].

IL-17 induces the production of many cytokines including IL-6, IL-8, G-CSF, GM-CSF, IL-1â, TGF-â and TNF-á [141] and plays a major role in the defense against mucocutaneous Candida infection.

Th17 cells require IL-23 for maturation and cytokine secretion [142]. Increased expression of IL-17A has been detected in a variety of autoimmune disorders but no increase in the frequency of Th17 cells has been found in CVID patients [139]. Expansion of CD21lowCD38low B cells is accompanied by a reduction in Th17 cells suggesting an inverse correlation between both populations in CVID [139]. Importantly, Mannon et al. has revealed less IL-17A being produced by lamina propria mononuclear cells in CVID autoimmune enteropathy than in Crohn’s disease [143] suggesting a different pathogenesis. In summary, studies on IL-17A in CVID are rare and currently do not suggest a major disturbance in CVID.

APRIL and BAFF

APRIL (a proliferation inducing ligand) and BAFF (B cell activating factor) are members of the TNF family which are primarily produced by monocytes, neutrophils, macrophages and dendritic cells [144].

APRIL binds to B cell maturation antigen (BCMA) and transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI), while BAFF can bind to BAFF-Receptor, TACI and only poorly to BCMA. These members of the TNF family have a crucial role in the development and maintenance of humoral immunity [145]. They maintain B cell and plasma cell survival, can act as a potent B cell activator [146], increase immunoglobulin production and influence class switching [147, 148].

Overexpression of BAFF and APRIL has been found in number of autoimmune diseases, including rheumatoid arthritis, multiple sclerosis and SLE which indicated the role of TNF family member cytokines in autoimmunity states [149151].

Mutations in BAFF-receptor and TACI have been identified as predisposing factors in some CVID patients [152], while so far no mutations in BCMA, April and BAFF have been found in sporadic or familial CVID cases [145, 153].

Results of previous studies have shown that serum levels of BAFF, APRIL and also soluble TACI are noticeably elevated in CVID [154, 155]. While there has been no association of serum APRIL or BAFF levels to clinical manifestations, soluble BAFF levels seem to be inversely correlated to peripheral B cell numbers [156]. This finding was corroborated by the high BAFF serum levels in X-linked agammaglobulinemia [155]. It remains to be determined whether elevated BAFF and APRIL serum levels might contribute to the autoimmune phenomena in CVID.

Conclusions

Patients suffering from CVID have diverse underlying molecular mechanisms leading to immunodeficiency and therefore present with different clinical manifestations [157].

The cytokine family has been extensively studied and different perturbations in cytokine pathways have been suggested to contribute to the development or different form and manifestations of CVID.

This overview over the different cytokines implicated in the pathophysiology of CVID revealed a very heterogeneous image of the underlying cytokine imbalance partly reflecting the heterogeneity of the syndrome and partly the limited comparability of the studies. Several studies could demonstrate that either the relative lack of cytokines like IL-2 or potentially IL-21 may contribute to the immunodeficiency in some patients while in others the overexpression for example of Th1-cytokines or TNF-α contributes to the local or global immune dysregulation.

From these findings already first therapeutic conclusions were drawn serving as proof of principle in single patients while failing in others.

Future studies on cytokine imbalance in CVID need to report the clinical and immunological phenotype of the patients, consider differences between various compartments (i.e., blood vs. tissue sites) in order to achieve a higher comparability between studies.

Only by this way, we will be able to dissect the complexity of the altered immune response in CVID and hopefully one day, we will be able to apply targeted therapies to selected patient groups as has been successfully implemented in different autoimmune diseases. Even more interesting, restoring immune function and regulation by supplementing missing cytokines without confining systemic side effects will be a challenge for the future.