The immune system’s correct functioning requires a sophisticated balance between responses to continuous microbial challenges and tolerance to harmless antigens, such as self-antigens, food antigens, commensal microbes, allergens, etc. When this equilibrium is altered, it can lead to inflammatory pathologies, tumor growth, autoimmune disorders, and allergy/asthma. The objective of this review is to show the existing data on the importance of regulatory T cells (Tregs) on this balance and to underline how intrauterine and postnatal environmental exposures influence the maturation of the immune system in humans. Genetic and environmental factors during embryo development and/or early life will result in a proper or, conversely, inadequate immune maturation with either beneficial or deleterious effects on health. We have focused herein on Tregs as a reflection of the maturity of the immune system. We explain the types, origins, and the mechanisms of action of these cells, discussing their role in allergy and asthma predisposition. Understanding the importance of Tregs in counteracting dysregulated immunity would provide approaches to diminish asthma and other related diseases in infants.

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a rare monogenic disease caused by mutations in the ABCB4 gene, resulting in a reduction in biliary phosphatidylcholine. Reduced biliary phosphatidylcholine cannot counteract the detergent effects of bile salts, leading to cholestasis, cholangitis, cirrhosis and ultimately liver failure. Here, we report results from treating two- or five-week-old Abcb4−/− mice with an AAV vector expressing human ABCB4, resulting in significant decreases of PFIC3 disease biomarkers. All male mice achieved a sustained therapeutic effect up through 12 weeks, but the effect was achieved in only 50% of females. However, two-week-old females receiving a second inoculation three weeks later maintained the therapeutic effect. Upon sacrifice, markers of PFIC3 disease such as, hepatosplenomegaly, biliary phosphatidylcholine and liver histology were significantly improved. Thus, AAV-mediated gene therapy successfully prevented PFIC3 symptoms in a clinically relevant mouse model, representing a step forward in improving potential therapy options for PFIC3 patients.

Cholestatic diseases can be caused by the dysfunction of transporters involved in hepatobiliary circulation. Although pharmacological treatments constitute the current standard of care for these diseases, none are curative, with liver transplantation being the only long-term solution for severe cholestasis, albeit with many disadvantages. Liver-directed gene therapy has shown promising results in clinical trials for genetic diseases, and it could constitute a potential new therapeutic approach for cholestatic diseases. Many preclinical gene therapy studies have shown positive results in animal models of both acquired and genetic cholestasis. The delivery of genes that reduce apoptosis or fibrosis or improve bile flow has shown therapeutic effects in rodents in which cholestasis was induced by drugs or bile duct ligation. Most studies targeting inherited cholestasis, such as progressive familial intrahepatic cholestasis (PFIC), have focused on supplementing a correct version of a mutated gene to the liver using viral or non-viral vectors in order to achieve expression of the therapeutic protein. These strategies have generated promising results in treating PFIC3 in mouse models of the disease. However, important challenges remain in translating this therapy to the clinic, as well as in developing gene therapy strategies for other types of acquired and genetic cholestasis.

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Background Bile acid (BA) homeostasis is mainly regulated by bile salt excretory pump (BSEP), a hepatocyte transporter that transfers BAs to the bile. BSEP expression is regulated by BA levels through activation of farnesoid X receptor transcription factor, which binds to the inverted repeat (IR-1) element in the BSEP promoter. Gene therapy of cholestatic diseases could benefit from using vectors carrying endogenous promoters physiologically regulated by BAs, however their large size limits this approach, especially when using adeno-associated viral vector (AAV) vectors. Results We evaluated the functionality and BA-mediated regulation of minimal versions of human and mouse BSEP promoters containing IR-1 using AAV vectors expressing luciferase. Unexpectedly, a minimal mouse BSEP promoter (imPr) showed higher BA-mediated expression and inducibility than a minimal human promoter (ihPr) or than full-length BSEP promoters in human hepatic cells. In addition, in mice receiving an AAV8 vector carrying imPr promoter-driven luciferase expression was efficiently regulated by administration of a BA-enriched diet. Interestingly, this vector also expressed significantly higher luciferase levels in Abcb4−/− mice, which have high levels of BAs, compared to wild type mice, or to mice receiving a vector containing the luciferase gene downstream of the constitutive alpha-1 antitrypsin promoter. In contrast, the AAV vector containing ihPr showed very low luciferase expression with no inducibility. Finally, we optimized imPr by adding three IR-1 repeats at its 5′ end. This new promoter provided higher levels of luciferase than imPr both in vitro and in vivo. Conclusions The imPr could represent a useful tool for gene therapy approaches in which physiological BA regulation is desired.