Discussion
Clinical primary immunodeficiency cases caused by TYK2 mutations are very rarely reported. To our best knowledge, there are 13 cases in total in English literature not including ours. Because of this, the exact roles of TYK2 in the development, function and regulation of immune cells in human remain largely unknown. In clinical setting, it is extremely difficult to raise a specific suspicion of TYK2 deficiency when there is no clear full picture of its clinical manifestations which are closely related to the roles TYK2 plays in various immunologic processes. A recent study showed the P1104A TYK2 common variant predisposes host to mycobacteria infection by selectively disrupting IL-23–dependent antimycobacterial IFN-γ immunity, which also contributed to our understanding of TYK2 function[19]. In this study, we presented five more cases of TYK2 deficiency and investigated many aspects of the effects TYK2 deficiency brings, which we think would further improve our understanding of TYK2 deficiency.
The first TYK2-deficient patient to be described was Japanese and had HIES as the main clinical feature, associated with numerous intracellular infections. Since then, TYK2 deficiency was considered a subset of AR-HIES. However, newly identified TYK2 deficient patients did not necessarily present with hyper-IgE. A comprehensive comparison of various immunologic defects between the first reported patient and seven patients without hyper-IgE was reported, which proposed that intact IL-6/STAT3 signaling in patients without hyper-IgE might be responsible for this phenotypic difference [4]. As defects in STAT3-mediated signaling play a key role in the development of HIES[20] and IL-6 signaling is TYK2 independent in mice [21]. Another TYK2 deficient patient without HIES was also reported to show normal IL-6 signaling[6] and a TYK2 deficient patient with HIES showing impaired IL-6 signaling was reported [5] (Table 1). Recently, autosomal recessive and dominant mutations inIL6ST, which encodes gp130, a subunit of cytokine receptor for IL-6, IL-11, IL-27 and some others, have been reported in patients with HIES [22,23]. Thus, this idea seems to be plausible according to these previously reported cases. Surprisingly, however, our data did not support this hypothesis. As shown in Fig. 3, PBMC from P1 with mildly elevated IgE and P3 with low IgE both responded to IL-6 treatment normally in terms of STAT3 activation. Furthermore, PBMC from P2 with normal IgE level showed impaired response to IL-6 treatment. These findings strongly suggest the relation between hyper-IgE and TYK2 deficiency cannot be explained by dysregulated IL-6/STAT3 signaling.
Actually, mechanisms underlying increased IgE production are variable and complicated [24]: a) enhanced class switching to IgE and the Th2 bias. B cell class switching to IgE is tightly regulated by interaction with CD4 T cells and the cytokine microenvironment. It is well established that IL-4 and IL-13, cytokines produced by Th2 cells and B cells, promote isotype class switching to IgE in B cells. Meanwhile, IL-10 and IL-21 act directly upon activated B cells to reduce the efficiency of class switching towards IgE. In addition, the engagement of CD40 on B cells by CD40L provides a second signal for IgE class switching [25]. Th bias towards a Th2 phenotype can indirectly promote a cytokine milieu capable of enhancing IgE production. b) defects in general tolerance, meaning the Th2 response is increased concurrently with Th1 and Th17 responses in cases of cataclysmic tolerance failure, which is seen in IPEX[26]. Intriguingly, P1, P2 and P3 in our cohort all showed Th1 bias instead, with variable IgE levels, indicating involvement of other mechanisms. c) defects in barrier. While the majority of patients with atopy begin with elevated IgE, a minority of patients develop atopic dermatitis with normal IgE levels but experience elevated IgE after prolonged disease, indicating increased IgE levels can be a result of disrupted barriers. So, it seems that in the case of TYK2 deficiency, disrupted cytokines signaling network is key to the normal, increased or decreased IgE levels.
The viral infections can probably be accounted for by defects of IFN-α/β/γ signaling. Similar to the first described TYK2 deficient patient, P1 in our cohort suffered repeated viral infection possibly due to abolished responses to IFN-α/β. Although impaired but not abolished IFN signaling does not necessarily lead to severe viral infection as the case of P2 (Table 1) and previously reported cases[4]. The explanation for this might be that TYK2 plays important roles in various immune cells. Mutation of P2 may affected innate immune cells but not adaptive immune cells such as B cells, which also play critical role in restricting viral infection.
IL-12 signaling induced IFN-γ production is believed to be responsible for the susceptibility to intracellular bacterial infection in TYK2 deficient patients, while IL-23 signaling induced IL-17 production for extracellular bacterial infection [3]. However, again our data present challenges to this notion as P3 showed no evidence of intracellular bacterial infection such as mycobacteria and salmonella in spite of impaired IL-12 signaling and P2 and P3 showed no evidence of extracellular bacterial infection such as S. aureusin spite of impaired IL-23 signaling. Similar to diverse responses to interferons, restricting extracellular or intracellular bacteria can be achieved by multiple cellular mechanisms.
Cytokine signaling not only affects the function of various immune cells but also plays essential roles in regulating homeostasis of immune system [18]. The effects of TYK2 deficiency on the homeostasis of lymphocytes compartment is unclear. In investigating this, we noticed that patients with elevated IgE (P1, P4 and P5) levels shared similar profile of disrupted T and B cell compartment and patient with normal IgE level (P2) shared similar profile of T cell compartment but distinct profile of B cell compartment with patient with low IgE level (P3).
The divergent cellular responses to cytokines, which is probably responsible for the divergent clinical phenotypes, might be resulted from residual expression of mutant TYK2. Another possibility is that the presence/absence of the cell population that should respond caused these divergent cellular responses as the experiments in our study are done on heterogeneous cell populations. The effects of these mutations on TYK2 protein stability could be due to disrupted post-translation modification of the protein.
In summary, we presented five more TYK2 deficiency cases caused by different novel mutations displaying divergent cellular defects and variable clinical phenotypes, which demonstrated that the correlation of cellular defects and clinical phenotypes is far more complicated than previously thought. In view of this, here we propose a new model that TYK2 works as a multi-tasker in orchestrating various cytokines signaling pathways, differentially combined defects of which account for the expressed clinical manifestations. Furthermore, we speculated that the discrepancies in cellular defects could at least partially be explained by different mutational effects to TYK2 protein, which needs to be investigated in the future. Nevertheless, our findings might lead us further towards more accurate understanding of TYK2 function in human immune system and a more timely diagnosis of TYK2 deficient patients.