2.1. Th1/Th2 Paradigm: Are Asthma and T1DM Mutually Exclusive?
Asthma and T1DM are chronic inflammatory diseases, although they classically involve opposite arms of the adaptive immune system: [
11].
Th1 cells defend against infections and tumors but are also involved in the development of autoimmune diseases (T1DM), producing cytokines such as interferon (INF)-γ, Tumor Necrosis Factor (TNF)-α, and interleukin (IL)-2 [
11].
Th2 cells protect against parasites but also promote the development of IgE-mediated atopic diseases (asthma), producing cytokines such as IL-4, IL-5 IL-6, IL-9, IL-10, and IL-13 [
11].
Several studies have sustained the so-called “Th1/Th2 paradigm” [
12,
13,
14,
15]. The theory, described three decades ago for the first time, explains that an expansion of the Th1 clones in individuals with T1DM would cause a reduction of the Th2 compartment, preventing the development of atopic diseases and vice versa [
16]. According to this latter concept, autoimmune (Th1-related) and allergic diseases (Th2-related) would be mutually exclusive.
Consistent with the Th1/Th2 paradigm, several observational studies have suggested that asthma and allergic respiratory symptoms are reduced in patients with T1DM [
12,
13,
14,
15]. In addition, this protection seems to extend to nondiabetic siblings [
15], suggesting an effect mediated by a shared genetic background or by the exposure to the same environmental factors during pregnancy or in early life. Tosca et al. [
17] evaluated the lung function of 20 T1DM children with allergic rhinitis compared to 59 controls affected by allergic rhinitis alone. The authors found that only children with allergic rhinitis had a significant increase of forced expiratory flow at 25% and 75% of forced vital capacity after bronchodilation compared to the T1DM group, suggesting a protective role of T1DM associated with allergic rhinitis on asthma development. In a meta-analysis of 25 studies from Europe and North America, Cardwell et al. [
18] described a reduction of frequency of asthma in children with T1DM (OR 0.82 CI 95% 0.68–0.99
p = 0.04). It is worth noting that this protective effect was only significant after the elimination of those studies with an inadequate design. Data also suggested an inverse association with eczema (OR 0.82 CI 95% 0.62–1.10
p = 0.18), although not statistically significant. A recent metanalysis of six studies suggested a potential protective role of diabetes against atopic dermatitis development (odds ratio, 0.69, 95% CI, 0.67–0.72) [
19]
To date, the epidemiologic data are not certain enough to confirm the Th1/Th2 paradigm as a potential theory explaining the relationship between asthma and T1DM.
2.2. The “Hygiene Hypothesis”: Could Asthma and T1DM Have a Common Environmental Background?
Environmental factors are also important in the development of atopic diseases and T1DM [
6]. The “Hygiene Hypothesis” is a theory suggesting that reduced or delayed exposure to infections at early ages may increase the risk of the onset of autoimmune disorders (atopic diseases or T1DM) [
20]. Several studies have shown an increased incidence of asthma and other atopic diseases in the general population, particularly in patients with T1DM [
21,
22,
23,
24,
25].
Klamt et al. [
22], in a prospective population-based case-control study, demonstrated that children with T1DM reported the presence of IgE-mediated allergies more frequently than controls. Another cross-sectional study in Brazil [
23] found that patients with T1DM had a higher prevalence of allergic diseases and atopic sensitization than expected. Hsiao YT et al. [
24] reported a higher incidence of asthma in young patients with T1DM than in the general population in Taiwan, showing a negative effect of poor glycemic control on the risk of asthma development. In the United States, Black et al. [
25] showed a higher prevalence of asthma (10.8%) in adolescents with T1DM compared to the general population (8.7%). However, these data should be interpreted with caution due a high percentage of obese and overweight participants in both T1DM and T1DM-asthma groups [
26]. In this latter study, being overweight, a noted risk factor for asthma, could have acted as a confounding factor, making conclusions unreliable [
27].
Fsadni et al. [
21] compared the reported country incidence of T1DM (from the Diabetes Mondial Project Group [
28]) to the prevalence of atopic diseases (from The ISAAC phase 1 study [
29,
30,
31]) and found that T1DM had a positive correlation with both wheezing and atopic eczema. Stene et al. [
32] showed a strong positive association between the occurrence of T1DM and asthma, analyzing epidemiological data extracted from three international studies (ISAAC 1998 [
31], EURODIAB ACE Study Group 2000 [
33], and Diabetes Mondial Project Group 1993 [
28]). In particular, the authors used the published data on the prevalence of asthma symptoms (>four episodes of wheezing in the last 12 months in 13–14-year-old age group) and on the incidence of T1DM (among children aged 0–15 years old), obtained from the average of all centers in each country, and calculated the Spearman correlation coefficient. The authors suggested that common environmental factors might influence the susceptibility to both disorders [
32].
The proposed molecular mechanisms underlying the “Hygiene Hypothesis” focus on the role of regulatory T lymphocytes (Treg) in the immune system response. The action and maturation of these cells can be modulated by several factors such as infectious agents, gut microbiome changes, and parasitic infections [
34,
35].
In the following subsections, the contribution of the aforementioned factors are discussed.
2.2.1. Infections
Protective effects of bacteria (
Listeria monocytogenes,
Propionibacterium acnes,
Chlamydia,
Lactococcus species) or bacterial products on asthma development have been well documented [
20,
36]. Viruses also seemed to have a protective role against asthma in murine models [
37]. Bacterial infections protect against asthma development by stimulating innate immune receptors, such as Toll-like receptors (TLR), and inducing Th1 responses [
20]. Viral infections exert their action against asthma development by induction of a natural killer (NK) cell subset or monocytes with a regulatory phenotype [
20,
37].
On the other hand, recurrent wheezing episodes in early life are known to be a major risk factor for asthma development, and viral infections have been linked to asthma development in 62–98% of cases [
38]. Respiratory Syncytial Virus- or Rhinovirus-induced wheezing in preschool children with family history of asthma were associated with an increased risk of asthma at 6 years of age [
20].
Children living with several older siblings or attending daycare centers before 1 year of age were at lower risk of developing allergic disorders than children attending from 2 years onward and children from small families [
39,
40].
Similar findings have also been reported for T1DM [
9]. Cardwell et al. [
41] demonstrated a reduced risk of T1DM in children living with siblings, sharing a bedroom, and moving to a new house more often. Furthermore, living in a farm environment seems to be a protective factor against T1DM due to early exposure to a larger number of microorganisms and different farm animals [
42]. These findings support the “Hygiene Hypothesis”, suggesting that the exposure to infections in early life may protect against the onset of T1DM.
2.2.2. Gut Microbiome
Early-onset autoimmune diseases are common in Finland and Estonia but less prevalent in Russia [
43]. Furthermore, sensitization to allergens and allergic symptoms are much more common in Finnish than in Russian schoolchildren [
44].
Interestingly, evaluating the intestinal colonization in Finnish, Estonian, and Russian populations, the presence of Bacteroides species in the intestinal microflora was more frequent in Finnish and Estonian infants than in Russian infants. Therefore, they were exposed primarily to Lipopolysaccharide (LPS) of Bacteroides than to LPS of
E. coli. The structure of the Bacteroides LPS, differently from
E. coli LPS, inhibits the innate immune activation and endotoxin tolerance [
43]. This could suggest the existence of a link between the composition of the intestinal microflora and the development of both diseases.
Gut microbiota composition in the first month of age is crucial for the development of a healthy immune system, and any alterations during this temporal window could affect its development irreversibly [
45].
Interestingly, gut microbiota analysis in children at high risk of developing T1DM or asthma showed common peculiar taxonomic changes. These children had a lower biodiversity [
46,
47], a higher Bacteroides/Firmicutes ratio, a relative abundance of Clostridia, and a relative deficit of Lactobacillus and Bifidobacterium [
46,
48]. The mechanism underlying this phenomenon is largely unknown, but one possibility is represented by the production of biomolecules capable of interaction even at great distance from the bowel. Among them, small-chain fatty acids (SCFAs) seem to be particularly promising. SCFAs, namely acetate, butyrate, and propionate, are produced by several different bacteria in the gastrointestinal tract through the fermentation of fibers [
49]. They contribute to the regulation of both the innate and adaptive immune system by the G-protein coupled receptor 43 (GPR43). Dietary supplementation with butyrate and acetate in non-obese diabetic (NOD) mice had a protective effect by reducing the incidence of autoimmune diabetes and delaying its onset [
50]. Similarly, mice with a defect of GPR43 or SCFAs production exhibited a stronger inflammatory response after exposure to common aeroallergens, showing higher production of Th2 proinflammatory cytokines [
51].
Acetate decreases autoreactive T cells while butyrate promotes Treg differentiation and function [
52]. In addition, they downregulate the major histocompatibility complex (MHC) class I and costimulatory proteins expression on B cells and promote differentiation of B cells into plasma cells and memory cells capable of producing specific IgG and IgA [
53]. Furthermore, acetate and propionate improve insulin sensitivity, whereas butyrate maintains the integrity of gut epithelium [
54].
2.2.3. Parasitic Infections
The improved living conditions in developed countries have also caused a decline of parasitic infections. This seems to be correlated to an increase of the incidence of immune-mediated disorders and atopic diseases [
55,
56]. According to a recent review, helminths could prevent the development of autoimmune and atopic diseases [
57]. Helminths seem to regulate both the innate and adaptive immune systems, promoting a typical Th2 response and modulating Th1/Th17 differentiation, causing an increase of Th2-related cytokines and a reduced secretion of Th1/Th17-related cytokines [
58].
In addition, parasite-derived proteins can also modulate the bacterial presence of the gut microbiota, leading to indirect regulation of the immune system [
59]. Immunomodulation induced by helminths may prevent diabetes mellitus and improve insulin sensitivity [
60,
61]. Products from helminths, such as
Fasciola hepatica helminth defense molecule (FhHDM) [
62] and Omega-1 of
S. mansoni [
63], play an important role against atopic diseases. However, the link of atopic and autoimmune disorders with helminth infection is still controversial. The aforementioned environmental factors may explain the common epidemiologic trend of asthma and T1DM, although they have immunological differences, as expressed by the Th1/Th2 paradigm.
2.3. Genetic Protective or Risk Factors for Asthma and Type 1 Diabetes
Both atopic diseases and T1DM have a multifactorial etiology due to complex gene-environmental interactions. Thus, one strategy to elucidate the relationship between these two disorders is to investigate the genetic protective and risk factors associated with both of them. The genes and related molecular pathways are shown in
Table 1.
Genetic analysis for
TLR2 indicated T allele in the single nucleotide polymorphism (SNP) rs3804100 as a susceptibility allele for both asthma and T1DM, and C allele as protective for both diseases [
64].
Other evidence of a common genetic predisposition to asthma and T1DM comes from the GABRIEL consortium asthma genome-wide association study (GWAS), which identified 9 regions with 10 SNPs associated with asthma [
65]. Among these regions,
ORLMD3/GSDMB was the only nonhuman leukocyte antigen (HLA) region shared between childhood-onset asthma and T1DM [
65,
66]. It was found that rs2305480 and rs3894194, the most atopy-associated SNPs in this region (
ORLMD3/GSDMB), were also associated with T1DM [
66]. Furthermore, these two SNPs are also in high-linkage disequilibrium with rs2290400, the most associated SNP with T1DM [
66]. In contrast, the minor T allele of two SNPs in
HLA-DQB1 (rs9273349 and rs1063355) seems to confer protection to both asthma and T1DM [
66].
Taleb et al. [
15] studied the genes implicated in the development of both asthma and diabetes, such as cytotoxic T-lymphocytes antigen 4 (
CTLA-4) and
HLA-DQB1*0201 and
DQB1*0302. They found an association between the G allele at the 49 (A/G) nucleotide of CTLA-4 gene and an increase in asthmatic symptoms, although not statistically significant [
15]. The same 49 G allele was associated with a higher risk of diabetes mellitus in a study carried out in a Lebanese population [
67].
CTLA-4 polymorphisms might represent a common genetic risk for both diseases, although further studies are needed to confirm this hypothesis. Regarding
HLA-DQB1 alleles, a cross-sectional study in the Chinese population showed a higher frequency of
HLA-DQB1*0201 in asthmatics and a higher frequency of
HLA-DQB1*0301 in healthy controls [
68]. Contrarily, Taleb et al. [
15] found that diabetic carriers of the
HLA-DQB1*0201 reported significantly fewer asthmatic symptoms compared to diabetic noncarriers. A trend of higher risk of asthma symptoms was observed in diabetic carriers of
HLA-DQB1*0302 [
15], although not statistically significant. The authors stated that these contrasting findings related to the development of the Th1 and Th2 immune responses were under control of HLA markers. Accordingly, the susceptible gene for one disease may become a resistant gene for the other [
15].
GTPase of the immunity-associated protein (GIMAP) family proteins are modulators and regulators of the immune cell homeostasis [
69]. They are highly expressed during Th1 differentiation and less during Th2 differentiation [
70]. Heinonen et al. [
70] carried out two Finnish population-based association studies, one for diabetes and the other for asthma and allergic sensitization, assessing the role of
GIMAP4 and
GIMAP5 polymorphisms [
70]. Interestingly, the authors found that a particular
GIMAP5 SNP (rs6965571) was associated with increased risk for both asthma and allergic sensitization but was inversely associated with T1DM [
70]. This association was significant only in participants from the Southwest Finland, who had a distinct genotypic structure compared to the Northeast ones. One polymorphism (rs13222905) in
GIMAP4 was only associated to both asthma and allergic sensitization [
70].
The genetic findings on the association of asthma and atopy with T1DM are currently inconclusive. However, in the literature, there are promising data on the role of protective or predisposing polymorphisms involved in the development of asthma and T1DM.
A common complex polygenetic basis might exist, encouraging future association studies to better characterize the potential link between them.
The potential factors which might play a crucial role in asthma and T1DM are reported in
Figure 1.