Mridu Acharya

Preferential Expression of Integrin αvβ8 Promotes Generation of Regulatory T Cells by Mouse CD103+ Dendritic Cells

BACKGROUND & AIMS Immune responses in the intestine are controlled by regulatory T cells (Tregs), which prevent inflammation in response to commensal bacteria. A specific population of intestinal dendritic cells (DCs), marked by expression of CD103, generate Tregs more efficiently than other DC populations through mechanisms that involve retinoic acid and transforming growth factor (TGF)-β. However, it is not clear how CD103(+) DCs are specialized for this function. We investigated the ability of CD103(+) DCs to promote Treg generation through activation of TGF-β and the role of integrins with the αv subunit in this process. METHODS Naïve T cells were cultured with purified DCs from mesenteric lymph nodes (MLNs) or intestines of wild-type and αv conditional knockout mice to assess generation of Tregs. Antigens were administered orally to mice, and antigen-specific generation of Tregs was measured in intestinal tissues. Expression of the integrin αv subunit was measured in purified subpopulations of DCs by quantitative polymerase chain reaction and immunoblot analyses. RESULTS In vitro, CD103(+) DCs generated more Tregs in the presence of latent TGF-β than other MLN DCs. Efficient generation of Tregs required expression of the integrin αv subunit by DCs; mice that lacked αv in immune cells did not convert naïve T cells to intestinal Tregs in response to oral antigen. CD103(+) DCs derived from the MLNs selectively expressed high levels of integrin αvβ8 compared with other populations of DCs. CONCLUSIONS Expression of αvβ8 is required for CD103(+) DCs to become specialized and activate latent TGF-β and generate Tregs during the induction of tolerance to intestinal antigens in mice.

Inflammation-induced effector CD4+ T cell interstitial migration is alpha-v integrin dependent

Leukocytes must traverse inflamed tissues to effectively control local infection. Although motility in dense tissues appears to be integrin-independent actin-myosin based, during inflammation changes to the extracellular matrix (ECM) may necessitate distinct motility requirements. Indeed, we found that T cell interstitial motility was critically dependent on RGD-binding integrins in the inflamed dermis. Inflammation-induced deposition of fibronectin was functionally linked to increased αv integrin expression on effector CD4+ T cells. Using intravital multi-photon imaging, we found that CD4+ T cell motility was dependent on αv expression. Selective αv blockade or knockdown arrested TH1 motility in the inflamed tissue and attenuated local effector function. These data show a context-dependent specificity of lymphocyte movement in inflamed tissues that is essential for protective immunity.Leukocytes must traverse inflamed tissues to effectively control local infection. Although motility in dense tissues seems to be integrin independent and based on actomyosin-mediated protrusion and contraction, during inflammation, changes to the extracellular matrix (ECM) may necessitate distinct motility requirements. Indeed, we found that the interstitial motility of T cells was critically dependent on Arg-Gly-Asp (RGD)-binding integrins in the inflamed dermis. Inflammation-induced deposition of fibronectin was functionally linked to higher expression of integrin αV on effector CD4+ T cells. By intravital multiphoton imaging, we found that the motility of CD4+ T cells was dependent on αV expression. Selective blockade or knockdown of αV arrested T helper type 1 (TH1) cells in the inflamed tissue and attenuated local effector function. Our data demonstrate context-dependent specificity of lymphocyte movement in inflamed tissues that is essential for protective immunity.

αv Integrin expression by DCs is required for Th17 cell differentiation and development of experimental autoimmune encephalomyelitis in mice

Th17 cells are a distinct lineage of T helper cells that protect the body from bacterial and fungal infection. However, Th17 cells also contribute to inflammatory and autoimmune disorders such as multiple sclerosis. Th17 cell generation requires exposure of naive T cells to the cytokine TGF-β in combination with proinflammatory cytokines. Here we show that differentiation of Th17 cells is also critically dependent on αv integrins. In mice, lack of integrin αv in the immune system resulted in loss of Th17 cells in the intestine and lymphoid tissues. It also led to protection from experimental autoimmune encephalomyelitis (EAE). Further analysis indicated that αv integrins on DCs activated latent TGF-β during T cell stimulation and thereby promoted differentiation of Th17 cells. Furthermore, pharmacologic inhibition of αv integrins using cyclic RGD peptides blocked TGF-β activation and Th17 cell generation in vitro and protected mice from EAE. These data demonstrate that activation of TGF-β by αv-expressing myeloid cells may be a critical step in the generation of Th17 cells and suggest that αv integrins could be therapeutic targets in autoimmune disease.

Dectin-1–Dependent LC3 Recruitment to Phagosomes Enhances Fungicidal Activity in Macrophages

Autophagy has been postulated to play role in mammalian host defense against fungal pathogens, although the molecular details remain unclear. Here, we show that primary macrophages deficient in the autophagic factor LC3 demonstrate diminished fungicidal activity but increased cytokine production in response to Candida albicans stimulation. LC3 recruitment to fungal phagosomes requires activation of the fungal pattern receptor dectin-1. LC3 recruitment to the phagosome also requires Syk signaling but is independent of all activity by Toll-like receptors and does not require the presence of the adaptor protein Card9. We further demonstrate that reactive oxygen species generation by NADPH oxidase is required for LC3 recruitment to the fungal phagosome. These observations directly link LC3 to the inflammatory pathway against C. albicans in macrophages.

The Role of Autophagy-Related Proteins in Candida albicans Infections

Autophagy plays an important role in maintaining cell homeostasis by providing nutrients during periods of starvation and removing damaged organelles from the cytoplasm. A marker in the autophagic process is the reversible conjugation of LC3, a membrane scaffolding protein, to double membrane autophagosomes. Recently, a role for LC3 in the elimination of pathogenic bacteria and fungi, including Candida albicans (C. albicans), was demonstrated, but these organisms reside in single membrane phagosomes. This process is distinct from autophagy and is termed LC3-associated phagocytosis (LAP). This review will detail the hallmarks of LAP that distinguish it from classical autophagy and review the role of autophagy proteins in host response to C. albicans and other pathogenic fungi.

CLEARANCE OF DYING CELLS IN HEALTHY AND DISEASED IMMUNE SYSTEMS

Defects in apoptotic cell clearance are thought to contribute to autoimmunity by failure to induce tolerance, coupled with accumulation of immunogenic material. However, little is known about the contribution of apoptosis to immune responses at mucosal sites, where regulatory T cells (Treg cells) and other immune cells play an essential active role in maintaining tolerance to self‐associated antigens. In recent studies, we have found that αv integrins have an important role in apoptotic cell phagocytosis and induction of Treg cells in the intestine, and deletion of αv from myeloid cells causes colitis associated with failed apoptotic cell removal and loss of Treg cells. Our data show that activation of transforming growth factor (TGF)‐β by αvβ8 on dendritic cells (DCs) is essential for generating Treg cells and inducing mucosal tolerance. These results provide a mechanism by which tolerance to apoptotic cell–derived and –associated antigens is maintained by DC “licensing” at sites of high TGF‐β expression.

Alpha (v) integrins license regulatory T cells to apoptotic cells and self‐associated antigens

Defects in apoptotic cell clearance are thought to contribute to autoimmunity by failure to induce tolerance, coupled with accumulation of immunogenic material. However, little is known about the contribution of apoptosis to immune responses at mucosal sites, where regulatory T cells (Treg cells) and other immune cells play an essential active role in maintaining tolerance to self‐associated antigens. In recent studies, we have found that αv integrins have an important role in apoptotic cell phagocytosis and induction of Treg cells in the intestine, and deletion of αv from myeloid cells causes colitis associated with failed apoptotic cell removal and loss of Treg cells. Our data show that activation of transforming growth factor (TGF)‐β by αvβ8 on dendritic cells (DCs) is essential for generating Treg cells and inducing mucosal tolerance. These results provide a mechanism by which tolerance to apoptotic cell–derived and –associated antigens is maintained by DC “licensing” at sites of high TGF‐β expression.

αv Integrins regulate germinal center B cell responses through noncanonical autophagy

Germinal centers (GCs) are major sites of clonal B cell expansion and generation of long-lived, high-affinity antibody responses to pathogens. Signaling through TLRs on B cells promotes many aspects of GC B cell responses, including affinity maturation, class switching, and differentiation into long-lived memory and plasma cells. A major challenge for effective vaccination is identifying strategies to specifically promote GC B cell responses. Here, we have identified a mechanism of regulation of GC B cell TLR signaling, mediated by &agr;v integrins and noncanonical autophagy. Using B cell–specific &agr;v-KO mice, we show that loss of &agr;v-mediated TLR regulation increased GC B cell expansion, somatic hypermutation, class switching, and generation of long-lived plasma cells after immunization with virus-like particles (VLPs) or antigens associated with TLR ligand adjuvants. Furthermore, targeting &agr;v-mediated regulation increased the magnitude and breadth of antibody responses to influenza virus vaccination. These data therefore identify a mechanism of regulation of GC B cells that can be targeted to enhance antibody responses to vaccination.