SummaryRegulatory T (Treg) cells play a pivotal role in suppressing self-harmful T cell responses, but how Treg cells mediate suppression to maintain immune homeostasis and limit responses during inflammation is unclear. Here we show that effector Treg cells express high amounts of the integrin αvβ8, which enables them to activate latent transforming growth factor-β (TGF-β). Treg-cell-specific deletion of integrin αvβ8 did not result in a spontaneous inflammatory phenotype, suggesting that this pathway is not important in Treg-cell-mediated maintenance of immune homeostasis. However, Treg cells lacking expression of integrin αvβ8 were unable to suppress pathogenic T cell responses during active inflammation. Thus, our results identify a mechanism by which Treg cells suppress exuberant immune responses, highlighting a key role for effector Treg-cell-mediated activation of latent TGF-β in suppression of self-harmful T cell responses during active inflammation.

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Background: Macrophages are pivotal in coordinating a range of important processes in the intestines, including controlling intracellular infections and limiting damaging inflammation against the microbiota. However, it is not clear how gut macrophages, relative to recruited blood monocytes and other myeloid cells, contribute to the intestinal inflammatory milieu, nor how macrophages and their monocyte precursors mediate recruitment of other immune cells to the inflamed intestine.Methods: Myeloid cell populations isolated from colonic inflammatory bowel disease (IBD) or murine dextran sulphate sodium (DSS) induced colitis were assessed using flow cytometry and compared to healthy controls. In addition, mRNA expression profiles in human and murine colon samples, and in macrophages and monocytes from healthy and inflamed murine colons, were analysed by quantitative PCR (qPCR) and mRNA microarray.Results: We show that the monocyte:macrophage balance is disrupted in colon inflammation to favour recruitment of CD14+HLA-DRInt cells in humans, and Ly6CHi monocytes in mice. In addition, we identify that murine blood monocytes receive systemic signals enabling increased release of IL-1β prior to egress from the blood into the colon. Further, once within the colon and relative to other myeloid cells, monocytes represent the dominant local source of both IL-1β and TNF. Finally, our data reveal that, independent of inflammation, murine colon macrophages act as a major source of Ccl7 and Ccl8 chemokines that trigger further recruitment of their pro-inflammatory monocyte precursors.Conclusions: Our work suggests that strategies targeting macrophage-mediated monocyte recruitment may represent a promising approach for limiting the chronic inflammation that characterises IBD.

TGFβ is a crucial immune regulator and attractive therapeutic target but needs to be activated to function. Kelly et al. show that human monocytes and macrophages activate TGFβ via expression of an integrin, αvβ8, which dampens pro-inflammatory cytokine production and is disrupted in inflammatory bowel disease.

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Regulation of intestinal T-cell responses is crucial for immune homeostasis and prevention of inflammatory bowel disease (IBD). A vital cytokine in regulating intestinal Tcells is transforming growth factor-β (TGFβ), which is secreted by cells as a latent complex that requires activation to function. However, how TGFβ activation is regulated in the human intestine, and how such pathways are altered in IBD is completely unknown. Here we show that a key activator of TGFβ, integrin αvβ8, is highly expressed on human intestinal dendritic cells (DCs), specifically on the CD1c+ but not the CD141+ intestinal DC subset. Expression was significantly upregulated on intestinal DC from IBD patients, indicating that inflammatory signals may upregulate expression of this key TGFβ-activating molecule. Indeed, we found that the Toll-like receptor 4 ligand lipopolysaccharide upregulates integrin αvβ8 expression and TGFβ activation by human DC. We also show that DC expression of integrin αvβ8 enhanced induction of FOXP3 in CD4+ Tcells, suggesting functional importance of integrin αvβ8 expression by human DC. These results show that microbial signals enhance the TGFβ-activating ability of human DC via regulation of integrin αvβ8 expression, and that intestinal inflammation may drive this pathway in patients with IBD.

The interferon lambda (IFN-l) cytokines have well-known antiviral properties, yet their contribution to immune regulation is not well understood. Epithelial cells represent the major target cell of IFN-l; peripheral blood mononuclear cells are generally considered nonresponsive, with the exception of plasmacytoid dendritic cells (pDCs). In this study we aimed to define the potential for discrete subpopulations of cells to directly respond to IFN-l. Analysis of peripheral blood leukocytes reveals that, while pDCs uniformly express the highest levels of IFN-l receptor, a small proportion of B cells and monocytes also express the receptor. Nevertheless, B cells and monocytes respond poorly to IFN-l stimulation in vitro, with minimal STAT phosphorylation and interferonstimulated gene (ISG) induction observed. We confirm that pDCs respond to IFN-l in vitro, upregulating their expression of pSTAT1, pSTAT3, and pSTAT5. However, we found that pDCs do not upregulate pSTAT6 in response to IFN-l treatment. Our results highlight unique aspects of the response to IFN-l and confirm that while the IFN-l receptor is expressed by a small proportion of several different circulating immune cell lineages, under normal conditions only pDCs respond to IFN-l stimulation with robust STAT phosphorylation and ISG induction. The difference in STAT6 responsiveness of pDCs to type I and type III interferons may help explain the divergence in their biological activities.

The study of human macrophages is often hampered by access to tissue and inability of this cell type to survive in vitro following isolation. The culture of human monocyte-derived macrophages (MDMs) represents a tool to study macrophages, with monocytes known to give rise to tissue macrophages influenced by certain environmental cues. Here we describe a method of culturing monocyte-derived macrophages from CD14+ blood monocytes and polarization toward different macrophage phenotypes.