How tissue-resident macrophages (TRM) impact adaptive immune responses remains poorly understood. We report novel mechanisms by which TRMs regulate T cell activities at tissue sites. These mechanisms are mediated by the complement receptor of immunoglobulin family (CRIg). Using animal models for autoimmune type 1 diabetes (T1D), we found that CRIg⁺ TRMs formed a protective barrier surrounding pancreatic islets. Genetic ablation of CRIg exacerbated islet inflammation and local T cell activation. CRIg exhibited a dual function of attenuating early T cell activation and promoting the differentiation of Foxp3⁺ regulatory (Treg) cells. More importantly, CRIg stabilized the expression of Foxp3 in Treg cells, by enhancing their responsiveness to interleukin-2. The expression of CRIg in TRMs was postnatally regulated by gut microbial signals and metabolites. Thus, environmental cues instruct TRMs to express CRIg, which functions as an immune checkpoint molecule to regulate adaptive immunity and promote immune tolerance.

Liver cancer has become the second most deadly malignant disease, with no efficient targeted or immune therapeutic agents available yet. While dissecting the roles of cytoplasmic signaling molecules in hepatocarcinogenesis using an inducible mouse gene targeting system, Mx1-cre, we identified a potent liver tumor-inhibitory effect of synthetic double-stranded RNA (dsRNA), polyinosinic-polycytidylic acid (pIC), an inducer of the Mx1-cre system. Injection of pIC at the pre-cancer stage robustly suppressed liver tumorigenesis either induced by chemical carcinogens or by Pten loss and associated hepatosteatosis. The immunostimulatory dsRNA inhibited liver cancer initiation, apparently by boosting multiple anti-tumor activities of innate immunity, including induction of immunoregulatory cytokines, activation of NK cells and dendritic cells, and reprogramming of macrophage polarization. This study paves the way for the development of preventive and early interfering strategies for liver cancer to reduce the rapidly increasing incidences of liver cancer in an ever-growing population with chronic liver disorders.

miR-23∼27∼24 was recently implicated in restricting Th2 immunity, as well as the differentiation and function of other effector T cell lineages. Interestingly, miR-24, unlike other family members, actually promotes Th1 and Th17 responses. In this article, we show that miR-24 drives the production of IFN-γ and IL-17 in T cells at least in part through targeting TCF1, a transcription factor known for its role in limiting Th1 and Th17 immunity. Surprisingly, whereas TCF1 was previously shown to promote Th2 responses through inducing GATA3, enforced TCF1 expression in miR-24-overexpressing T cells led to further downregulation of IL-4 and GATA3 expression, suggesting miR-24-mediated inhibition of Th2 immunity cannot be attributed to TCF1 repression by miR-24. Together, our data demonstrate a novel miR-24-TCF1 pathway in controlling effector cytokine production by T cells and further suggest miR-24 could function as a key upstream molecule regulating TCF1-mediated immune responses.

CD4⁺ Foxp3⁺ T regulatory (Treg) cells are key players in preventing lethal autoimmunity. Tregs undertake differentiation processes and acquire diverse functional properties. However, how Treg's differentiation and functional specification are regulated remains incompletely understood. Here, we report that gradient expression of TCF1 and LEF1 distinguishes Tregs into three distinct subpopulations, particularly highlighting a subset of activated Treg (aTreg) cells. Treg-specific ablation of TCF1 and LEF1 renders the mice susceptible to systemic autoimmunity. TCF1 and LEF1 are dispensable for Treg's suppressive capacity but essential for maintaining a normal aTreg pool and promoting Treg's competitive survival. As a consequence, the development of T follicular regulatory (Tfr) cells, which are a subset of aTreg, is abolished in TCF1/LEF1-conditional knockout mice, leading to unrestrained T follicular helper (Tfh) and germinal center B cell responses. Thus, TCF1 and LEF1 act redundantly to control the maintenance and functional specification of Treg subsets to prevent autoimmunity.

The nature of obesity-associated islet inflammation and its impact on β cell abnormalities remains poorly defined. Here, we explore immune cell components of islet inflammation and define their roles in regulating β cell function and proliferation. Islet inflammation in obese mice is dominated by macrophages. We identify two islet-resident macrophage populations, characterized by their anatomical distributions, distinct phenotypes, and functional properties. Obesity induces the local expansion of resident intra-islet macrophages, independent of recruitment from circulating monocytes. Functionally, intra-islet macrophages impair β cell function in a cell-cell contact-dependent manner. Increased engulfment of β cell insulin secretory granules by intra-islet macrophages in obese mice may contribute to restricting insulin secretion. In contrast, both intra- and peri-islet macrophage populations from obese mice promote β cell proliferation in a PDGFR signaling-dependent manner. Together, these data define distinct roles and mechanisms for islet macrophages in the regulation of islet β cells.