Background T follicular helper (Tfh) cells underpin T-cell dependent humoral immunity and the success of most vaccines. Tfh cells also contribute to human immune disorders such as autoimmunity, immunodeficiency and malignancy. Understanding the molecular requirements for the generation and function of Tfh cells will provide strategies for targeting these cells to modulate their behavior in the setting of these immunological abnormalities. Objective To determine the signaling pathways and cellular interactions required for the development and function of Tfh cells in humans. Methods Human primary immunodeficiencies (PIDs) resulting from monogenic mutations provide a unique opportunity to assess the requirement for particular molecules in regulating human lymphocyte function. Circulating Tfh (cTfh) cell subsets, memory B cells and serum Ig levels were quantified and functionally assessed in healthy controls as well as patients with PIDs resulting from mutations in STAT3, STAT1, TYK2, IL21, IL21R, IL10R, IFNGR1/2, IL12RB1, CD40LG, NEMO, ICOS or BTK. Results Loss-of function (LOF) mutations in STAT3, IL10R, CD40LG, NEMO, ICOS or BTK reduced cTfh frequencies. STAT3, IL21/R LOF and STAT1 gain-of function mutations skewed cTfh differentiation towards a phenotype characterized by over-expression of IFNγ and programmed death -1 (PD-1). IFNγ inhibited cTfh function in vitro and in vivo, corroborated by hypergammaglobulinemia in patients with IFNGR1/2, STAT1 and IL12RB1 LOF mutations. Conclusion Specific mutations impact the quantity and quality of cTfh cells, highlighting the need to assess Tfh cells in patients by multiple criteria, including phenotype and function. Furthermore, IFNγ functions in vivo to restrain Tfh-induced B cell differentiation. These findings shed new light on Tfh biology and the integrated signaling pathways required for their generation, maintenance and effector function, and explain compromised humoral immunity in some PIDs.

Regulatory T (Treg) cell expansion is required for tolerance of the semi-allogeneic fetus in healthy pregnancy and impaired in preeclampsia in humans. However, the reasons remain unknown. Herein, we show that expansion of CD4(+)Helios(-)Foxp3(+) adaptive Treg (iTreg) cells, rather than CD4(+)Helios(+)Foxp3(+) natural Treg cells, accounts for this expansion in healthy pregnancy. This expansion is even more pronounced in the decidua, where there is an overrepresentation of iTreg cells. In preeclampsia, however, there is impaired systemic iTreg cell expansion, associated with a lack of iTreg cell overrepresentation in the decidua. Because decidual antigen-presenting cells (APCs) may be important for iTreg cell induction, we studied decidual CD14(+) APCs using immunohistochemistry and flow cytometry. We show that decidual CD14(+)DC-SIGN(+) APCs are closely associated with Foxp3(+) Treg cells. Furthermore, CD14(+)DC-SIGN(+) cells display a distinct phenotype compared with their CD14(+)DC-SIGN(-) counterparts. In particular, they have increased expression of tolerogenic molecules, HLA-G, and immunoglobulin-like transcript 4. In vitro, CD14(+)DC-SIGN(+) APCs from healthy pregnant women induced iTreg cells significantly more efficiently than CD14(+)DC-SIGN(-) APCs. Conversely, in preeclampsia, both CD14(+)DC-SIGN(+) and CD14(+)DC-SIGN(-) APCs induced iTreg cells poorly. These results suggest that decidual CD14(+)DC-SIGN(+) APCs may play important roles in iTreg cell induction, a process that is defective in preeclampsia and likely contributes to its pathogenesis.

Cholesterol 7α-hydroxylase (CYP7A1) is the rate-limiting enzyme in the bile acid biosynthetic pathway that converts cholesterol into bile acids in the liver. Recent studies have shown that bile acids may play an important role in maintaining lipid, glucose, and energy homeostasis. However, the role of CYP7A1 in the development of obesity and diabetes is currently unclear. In this study, we demonstrated that transgenic mice overexpressing Cyp7a1 in the liver [i.e., Cyp7a1 transgenic (Cyp7a1-tg) mice] were resistant to high-fat diet (HFD)–induced obesity, fatty liver, and insulin resistance. Cyp7a1-tg mice showed increased hepatic cholesterol catabolism and an increased bile acid pool. Cyp7a1-tg mice had increased secretion of hepatic very low density lipoprotein but maintained plasma triglyceride homeostasis. Gene expression analysis showed that the hepatic messenger RNA expression levels of several critical lipogenic and gluconeogenic genes were significantly decreased in HFD-fed Cyp7a1-tg mice. HFD-fed Cyp7a1-tg mice had increased whole body energy expenditure and induction of fatty acid oxidation genes in the brown adipose tissue. Conclusion This study shows that Cyp7a1 plays a critical role in maintaining whole body lipid, glucose, and energy homeostasis. The induction of CYP7A1 expression with the expansion of the hydrophobic bile acid pool may be a potential therapeutic strategy for treating metabolic disorders such as fatty liver diseases, obesity, and diabetes in humans.