Evidence from both animal models and human studies suggest that the elevated female sex hormone levels and a Th2-biased immunological state in pregnancy play a major role in promoting the expansion of autoreactive B cells. In mouse models of human SLE, both oestrogen and prolactin can exacerbate and accelerate autoimmune conditions by exerting a positive influence on the survival, proliferation, maturation and autoantibody production of the mature B cell population [28, 67-70]. Such findings from animal models strongly reflect

the evidence in human clinical studies where Acalabrutinib female populations have a significantly higher ratio of autoantibody-mediated autoimmune conditions (including SLE, APS, Grave’s disease, myasthenia gravis, scleroderma GDC-973 and Sjögren’s syndrome) than males, and these conditions are often exacerbated during pregnancy, where elevated levels of the female sex hormones occur [70]. The Th2-biased state of pregnancy, which is influenced positively by

high levels of oestrogen during pregnancy, is also well known to promote B cell proliferation, activation and antibody production in experimental animal models [70]. Evidence from animal studies and human B cell models show that the expansion and activation of autoreactive B cells can be amplified by mutual positive regulatory feedback loops between the oestrogen-receptor alpha (ER-α) pathway and other autoimmune-promoting cytokines such as interferon (IFN)-α and B cell-activating factor (BAFF) to promote survival, maturation and expansion of autoreactive B cells [71, 72]. Data from animal models, in conjunction with evidence from human studies, suggest that these co-operative signalling pathways can also promote the antibody class-switching of polyreactive natural antibody IgM to a more pathogenic IgG autoantibody production by B1 cells [13, Amino acid 70-74]. The positive feedback loop and the production

of IFN-α and BAFF may be activated and amplified through the innate pathways mediated by endogenous ligands and Toll-like receptors (TLRs) on B cells, monocytes and dendritic cells. Such endogenous ligands may consist of self-antigens, including lipoproteins, glycoprotein, single-stranded RNA (ssRNA) and dsDNA materials that are generated as a by-product from placental tissue-shedding during pregnancy. These endogenous ligands also provide a readily available source of autoantigens for the positive selection and activation of autoreactive B cell clones through BCR signals as well as the activation of TLR-mediated innate responses that contribute further to the exacerbation of the maternal autoimmunity and expansion of pathogenic autoantibody production. Evidence from epidemiological, clinical and experimental studies has established that autoantibodies produced by maternal B cells contribute directly to adverse pregnancy outcomes [9, 10].