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Primary biliary cholangitis (PBC) is an autoimmune liver disease with a striking female preponderance. The mechanisms behind this predominance are still to be elucidated, although multiple theories have been postulated and investigated. Among the proposed involved factors, sex hormones have been the first to be studied, but unfortunately data have been inconclusive or conflicting. Similarly, fetal microchimerism has received a huge attention in the past, but data in PBC have been unsatisfactory especially if compared to other autoimmune diseases like systemic lupus erythematosus. Studies focused on genetic factors have generated more intriguing and robust data, reporting a few abnormalities on the X chromosome in PBC patients. However, these data are able to explain only a part of the phenotypic variability attributed to the genetic component, and most importantly, need to be validated in larger series. More recently, a novel mice model of PBC, characterised by a constitutive expression of Interferon-γ (IFN-γ), has been developed and it is notable for being the first one with female predominance. At the same time, there has been a wide interest in the role of microbiome in health and disease, as well as in epigenetics, which have tried to explain differences in biological phenotypes not covered by genetics. The aim of this review is to outline established knowledge on the topic and try to provide novel perspectives on the potential future applications of newer techniques addressing microbiome and epigenome, in order to further understand the biology of sex divergence in PBC., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Unresolved low grade systemic inflammation represents the underlying pathological mechanism driving immune and metabolic pathways involved in autoimmune diseases (AID). Mechanistic studies in animal models of AID and observational studies in patients have found alterations in gut microbiota communities and their metabolites, suggesting a microbial contribution to the onset or progression of AID. The gut microbiota and its metabolites have been shown to influence immune functions and immune homeostasis both within the gut and systematically. Microbial derived-short chain fatty acid (SCFA) and bio-transformed bile acid (BA) have been shown to influence the immune system acting as ligands specific cell signaling receptors like GPRCs, TGR5 and FXR, or via epigenetic processes. Similarly, intestinal permeability (leaky gut) and bacterial translocation are important contributors to chronic systemic inflammation and, without repair of the intestinal barrier, might represent a continuous inflammatory stimulus capable of triggering autoimmune processes. Recent studies indicate gender-specific differences in immunity, with the gut microbiota shaping and being concomitantly shaped by the hormonal milieu governing differences between the sexes. A bi-directional cross-talk between microbiota and the endocrine system is emerging with bacteria being able to produce hormones (e.g. serotonin, dopamine and somatostatine), respond to host hormones (e.g. estrogens) and regulate host hormones' homeostasis (e.g by inhibiting gene prolactin transcription or converting glucocorticoids to androgens). We review herein how gut microbiota and its metabolites regulate immune function, intestinal permeability and possibly AID pathological processes. Further, we describe the dysbiosis within the gut microbiota observed in different AID and speculate how restoring gut microbiota composition and its regulatory metabolites by dietary intervention including prebiotics and probiotics could help in preventing or ameliorating AID. Finally, we suggest that, given consistent observations of microbiota dysbiosis associated with AID and the ability of SCFA and BA to regulate intestinal permeability and inflammation, further mechanistic studies, examining how dietary microbiota modulation can protect against AID, hold considerable potential to tackle increased incidence of AID at the population level., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Macrophages are innate immune cells that play an important role in activation of the immune response and wound healing. Pathogens that require T helper-type 2 (Th2) responses for effective clearance, such as parasitic worms, are strong inducers of alternatively activated or M2 macrophages. However, infections such as bacteria and viruses that require Th1-type responses may induce M2 as a strategy to evade the immune system. M2 are particularly efficient at scavenging self tissues following injury through receptors like the mannose receptor and scavenger receptor-A. Thus, M2 may increase autoimmune disease by presenting self tissue to T cells. M2 may also exacerbate immune complex (IC)-mediated pathology and fibrosis, a hallmark of autoimmune disease in women, due to the release of profibrotic factors such as interleukin-1beta, transforming growth factor-beta, fibronectin and matrix metalloproteinases. We have found that M2 comprise anywhere from 30% to 70% of the infiltrate during acute viral or experimental autoimmune myocarditis, and shifts in M2 populations correlate with increased IC deposition, fibrosis and chronic autoimmune pathology. Thus, women may be at an increased risk of M2-mediated autoimmunity due to estrogen's ability to increase Th2 responses.

Most autoimmune diseases occur significantly more frequently in women than men. This female preponderance for abnormal autoimmune function has largely gone unexplained. Many investigations have concentrated on the effects of female and male sex hormones on immune function, by suggesting that estrogens favor the antibody production-enhancing Th2 response and, by doing so, possibly, increase the risk towards abnormal autoimmune function. Others have suggested that women are genetically predisposed towards abnormal autoimmune function, possibly because the X chromosome may confer susceptibility towards tolerance breakdown. Recent developments have, however, opened new research avenues. The possible association between persistent fetal-maternal microchimerism and the development of autoimmune diseases has attracted special interest. Since, in analogy to allogeneic organ transplantation, fetal-maternal (and maternal-fetal) microchimerism may play an important role in the immunologic tolerance of the fetal semi-allograft, female preponderance for autoimmune diseases may be understood as a consequence of increased allogeneic cell traffic in females (in comparison to males), increased risk for long-term microchimerism and, therefore, as a consequence of the former two, the development of abnormal autoimmunity. Under an evolutionary view point the occurrence of autoimmune diseases, in general, can be seen as the price to be paid for successful reproduction. In view of increased exposure to cell traffic, women, of course, would be expected to pay a higher price, reflected in more autoimmunity.