**5. The role of microbiota in autoimmune-mediated endocrine diseases**

The role of the microbiota in autoimmune pathology has been highlighted by experimental data collected from germ-free mice. The intestinal microbiota maintains the balance of protective reactions to pathogens and tolerance to commensals aimed at maintaining intestinal homeostasis [49, 50]. Alterations produced in the balance of the microbiota (that is dysbiosis) activate the proinflammatory immune response and favor the progression of autoimmune disorders, such as multiple sclerosis, inflammatory bowel disease, T1DM, rheumatoid arthritis, and other pathologies of the digestive tract and ancillary glands, including malignancies. However, the intimate mechanism of microbiota involvement in this pathogenesis remains unknown [51–53].

AIDS are caused primarily by predisposing genetic factors but also by other endogenous or environmental triggers. There is a permanent interaction of the local immune system with bacterial antigens in humans, and therefore dysbiosis of the microbiome is associated with autoimmune disorders and metabolic syndromes. Dysbiosis means, in fact, the numerical alteration, diversity, and physiology of the intestinal microbiota (the transcriptome, proteome, and metabolome change) [54].

Experimental results in germ-free or induced dysbiotic animals support either the microbiota's direct or indirect involvement in the pathogenesis of some AIDS. Hence, in patients with type 1 diabetes mellitus, rheumatoid arthritis, multiple sclerosis, or lupus, as in those suffering from inflammatory bowel disease (both Chron's disease and ulcerative colitis), Sjögren's syndrome, Behcet's disease, autoimmune skin diseases (such as vitiligo, psoriasis, atopic dermatitis), the digestive microbiota is altered in terms of diversity and numerical representation of some species [9, 18, 35]. Kriegel et al. consider that dysbiosis is an essential trigger of autoimmunity both at the mucosal and systemic levels [9]. The spread of autoimmune response seems to be generated either by disseminating bacterial antigens but mostly by cross-immune reactivity under homeostasis conditions [55]. Such a mechanism is supported by a rheumatic fever induced by M and SLO antigens of *Streptococcus pyogenes*, or Guillain-Barre syndrome induced by *Campylobacter jejuni* infection, both as transient autoimmune syndromes. Cross-reactivity of lipopolysaccharides, bacterial polysaccharides, or D amino acid polymers would be an important mechanism for initiating the autoimmune conflict. Patients with autoimmune disorders often have vitamin D deficiency; its administration in experimental settings to animals improves the course of the disease. Vitamin D deficiency is also associated with an increased risk of infectious diseases. Inflammatory cells convert vitamin D to its active form, which is calcitriol. Vitamin D is an essential factor for the activation and proliferation of inflammatory cells (macrophages, neutrophils) [56]. The probiotics could also reverse the chronic systemic inflammation associated with AIDS.

### **5.1 Type I diabetes mellitus**

Type 1 diabetes mellitus (T1DM) has a well-defined autoimmune component, characterized by selective immune aggression against β-cells that secrete insulin [57]. The genetic predisposition for T1DM is unanimously accepted, but the interaction of genetic factors with environmental ones explains the sudden increase in incidence in Western countries [58]. More than 50% of monozygotic twins who have a sibling with T1DM remain healthy, showing that environmental factors (such as infectious agents, consumption of cow's milk in early childhood, or ingestion of contaminated food) play a major role in triggering the disease. Hence, out of 50 individuals suffering from congenital rubella virus infection [59, 60], nine developed diabetes at an average age of 28 years. However, some infections (i.e., *M. tuberculosis*, viruses, or parasites) exert a nonspecific inhibition on the onset of T1DM, probably by stimulating regulatory T cells [61–64].

The pathological mechanisms leading to the autoimmune destruction of pancreatic beta-cells in T1DM are very complex and incompletely elucidated. The pancreatic beta-cells express MHC II and co-stimulatory molecules, suggesting their role as antigen-presenting cells to TCD4 cells. Auto-antigens that stimulate the specific immune reactivity against pancreatic beta-cell are represented by insulin, glutamic acid decarboxylase—isoform 2 of 65 kD from beta-cell cytoplasm, a Zn transporter protein (ZnT8) involved in active secretion of insulin from islet granules, insulinoma-associated antigen 2 (alpha and beta), and a membrane protein acting as tyrosine phosphatase. The presence of humoral autoimmunity defines the risk of T1DM; antibodies against insulin were identified in 40% of children with the overt disease [65].

In patients with T1DM, it has been shown by immunohistochemical staining that the islets are infiltrated with macrophages, dendritic cells, TCD4, TCD8, NK, and fewer B lymphocytes, which can act as antigen-presenting cells for TCD4 cells. The immune response against islet antigens is associated with an inflammatory one in which IL-1, TNFα, and IFNγ are released [66]. The immune and inflammatory process destroys the beta cells. When about 80% of the beta-cell mass has been destroyed, the disease overt. This silent period may last for several years, sometimes decades. Along with the progressive destruction of β cells, the humoral antibody response and decreased glucose tolerance are documented until the clinical onset of the disease. Immune effectors selectively lyse insular β cells, leaving the other cell types intact. After the onset of hyperglycemia, the degree of mononuclear infiltration decreases [67].

The inflammatory diseases of the pancreas (such as chronic pancreatitis, neoplasia) are characterized by mast cells infiltrates into the acinar parenchyma, which releases various proteases (chymase, tryptase), acting as direct destroyers on islet's beta cells. The B4 type of leukotrienes, which derives from mast cells, exerts a chemoattractant effect on T lymphocytes [68].

Loss of pancreatic beta cells leads to insulin secretion deficiency, while the glucagon secretion becomes excessive and disrupts metabolism, resulting (in the absence of insulin) in diabetic ketoacidosis [69].
