The incidence of Type 1 diabetes is increasing world-wide. The reasons for this remain unknown. One of the hypotheses is that decreased exposure to microbial infections in early life might predispose to Type 1 diabetes through its effects on the immune system. The intestinal epithelium is exposed to a large number of diverse microorganisms and to a variety of food antigens. An alteration in the gut microbiota during critical stages in the development of the immune system may therefore have profound and long-lasting immunomodulatory effects.
Evidence from animal studies
Bacterial modulation of Type 1 diabetes has been studied in animal models for a number of years. Satoh et al. showed that the cumulative incidence of Type 1 diabetes over 20 weeks in BioBreeding (BB) rats treated intraperitoneally with 0.2mg streptococcal preparation (OK-432) weekly from the fifth/sixth week until the 20th/30th week of age was 7.4% compared to 27.7% in the nontreated rats Citation[1]. The temporal association between bacterial exposure and the development of diabetes has been demonstrated in a study by Qin et al. BCG vaccination of non-obese diabetic (NOD) mice within 3 days of cyclophosphamide administration blocked cyclophosphamide-induced diabetes, with almost complete prevention of insulitis when BCG was administered within 1 day of cyclophosphamide treatment. On the other hand, BCG administration 3 days before or 7 days after cyclophosphamide had no effect Citation[2]. Furthermore, Alyanakian et al. found that bacterial extract (OM-85) administered orally delayed the onset of Type 1 diabetes, while intraperitoneal administration initiated at 3 or 6 weeks of age prevented it completely Citation[3]. The mechanism involved both transforming growth factor (TGF)-β and natural killer T (NKT) cells Citation[3].
Recently, King et al. demonstrated that there was no difference in the incidence of Type 1 diabetes in NOD mice reared under germ-free conditions and in those reared under specific pathogen-free conditions. However, Type 1 diabetes onset was delayed and incidence was reduced when a spontaneous contamination with Bacillus cereus occurred at week 16 Citation[4], providing support for the role of intestinal microbiota in the development of Type 1 diabetes. This is further supported by the finding that BB-diabetes-prone(DP) rats that eventually developed diabetes harbored more Bacteroides species before the development of clinical disease and that antibiotic treatment lowered diabetes risk Citation[5]. The role of intestinal microbiota is also highlighted by the finding that the administration of Lactobacillus johnsonii N6.2 (LjN6.2) isolated from BB-diabetes-resistant (DR) rats (but not L. reuteri) to postweaning BB-DP rats decreased the incidence of diabetes Citation[6].
Studies in animal models also provide insight into possible mechanisms whereby bacterial exposure can protect against Type 1 diabetes. The transfection of LjN6.2 referred to above was associated with an increased expression of tight junction proteins in intestinal mucosal cells Citation[6]; this may diminish the intestinal passage of environmental antigens thought to predispose to Type 1 diabetes. Infection of NOD mice at 4 weeks of age with wild-type Citrobacterrodentium resulted in intestinal barrier disruption, insulitis and enhanced activation of diabetogenic CD8+ T cells. However, insulitis did not occur in NOD mice infected with the non-barrier disrupting strain, highlighting the potential association of enteric bacteria with the intestinal barrier in modulating Type 1 diabetes Citation[7].
Lau et al Citation[8]. demonstrated that diabetes resistance in LjN6.2-fed BB-DP rodents was correlated to a Th17-cell bias within the mesenteric lymph nodes; Th17 bias was also observed in the spleen, but not in the non-gut-draining axillary lymph nodes. These findings suggest that the gut flora can induce Th17 differentiation, protecting against Type 1 diabetes. Changes in gut flora may also mediate the protective effect of a gluten-free diet Citation[9]. Oral administration of the probiotic compound VSL#3 to NOD mice three-times a week from 4 weeks of age prevented diabetes, decreased insulitis, decreased β-cell destruction and increased IL-10 production in Peyer’s patches, spleen and pancreas Citation[10].
Evidence from human studies
Data from human studies are more difficult to obtain because of the complex interplay of numerous genetic and envirnomental factors (which are difficult to control for), the relative rarity of the disease and the long time interval between the initial exposure to a factor of interest and the development of disease. Indeed, while most studies investigating risk of Type 1 diabetes in animal models involve exposure to a specific bacterial species, evidence in humans mostly comes from generalized infective exposure with a lot of data being collected retrospectively.
Young girls genetically predisposed to develop Type 1 diabetes have been reported to have a lower risk of islet autoimmunity if their mothers had symptom/s of infection during their pregnancy, suggesting that exposure to infections during pregnacy might confer some protection Citation[11]. Alternatively, maternal infections during pregnancy might be markers of environments more likely to expose the child to infections in the early postnatal period. A large study involving 900 Type 1 diabetic children aged less than 15 years and 2302 controls from seven centres in Europe found that perinatal infections were associated with an increased risk of diabetes, but attendance at preschool day-care (an environment which promotes microbial exposure) decreased diabetes risk Citation[12]. The authors conclude that the effect of infections may differ depending on the age of the immune system Citation[12].
A negative association between bacterial exposure in a community and its incidence of Type 1 diabetes has been demonstrated in an ecological study performed by our group. We found consistent and highly statistically significant negative correlations between the country incidence of Type 1 diabetes and country mortality from respiratory infections, tuberculosis, diarrheal diseases and country total mortality including mortality from infectious and parasitic diseases and respiratory infections Citation[13]. We believe that infectious disease mortality is a marker of a country’s infective burden.
As with animal studies, significant interest has focused on the intestinal microbiota. Comparing the intestinal microbiome of autoimmune children who eventually developed Type 1 diabetes with age-matched non-autoimmune controls of the same genotype revealed a larger proportion of unclassified microorganisms and bacterial diversity in healthy children than in the autoimmune group. Thus autoimmune children may be less capable of assimilating a variety of nutritional intake than healthy children in view of a less diverse and a less stable microbiome Citation[14]. A subsequent study by Brown et al. found that Type 1 diabetic children harbor more bacteria expressing genes involved in motility, adhesions and carbohydrate metabolism while healthy controls harbor a larger proportion of bacteria capable of maintaining intestinal epithelial integrity Citation[15].
Duration of exclusive breastfeeding and total duration of breastfeeding have been reported to be protective by many authors Citation[16]. Donnet-Hughes et al. demonstrated that during lactation, bacteria and bacterial rDNA sequences are present in human peripheral blood mononuclear cells and in breast milk cells suggesting modulation of the immune system of the neonate by bacterial genetic material Citation[17]. Delivery by cesarean section (C-section) has also been found in a meta-analysis to be associated with a 20% increased risk of childhood Type 1 diabetes independently of gestational age, birth weight, maternal age, birth order, breast feeding and maternal diabetes Citation[18]. Of particular interest is the observation that the infant’s microbiome differs in infants born vaginally from those born by C-section, with a preponderance to Lactobacillus, Prevotella, or Sneathia species (similar to maternal vaginal microbiome) in those born vaginally and of Staphylococcus, Corynebacterium, and Propionibacterium species (resembling skin flora) in those born by C-section Citation[19].
The major negative study was the failure to detect an association between infections in early life as reported in the General Practice Research Database in the United Kingdom and subsequent risk of childhood-onset Type 1 diabetes Citation[20]. It may be that the relationship between infections and Type 1 diabetes may be more difficult to detect in any single country study since there may be less variation in exposure to infections, that a substantial proportion of infections in this study were not during the critical time window, or that protection against autoimmunity may not be related so much to contracting infections serious enough to warrant seeking medical attention but to exposure to specific microbes which alter gut flora in a specific manner as suggested by animal studies.
Conclusion
Lack of bacterial exposure in early life probably contributes to the risk of Type 1 diabetes through its effects on intestinal microbiota and subsequent imunomodulatory effects. This depends on the timing of exposure, specific nature of exposure and antigen load with the greatest risk conferred when a number of these factors co-exist in the genetically predisposed individual. Animal studies have shown that exposure to a single bacterial agent at critical time may have profound protective effects. This opens up the possibility that probiotics or oral immunization may in future help protect against human Type 1 diabetes. This is difficult to prove but definitely merits further study. One way of doing this is to study effects of bacterial agents on the younger sibs of Type 1 diabetic patients.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
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