Gretchen E. Diehl

Interleukin 23 Production by Intestinal CD103+CD11b+ Dendritic Cells in Response to Bacterial Flagellin Enhances Mucosal Innate Immune Defense

Microbial penetration of the intestinal epithelial barrier triggers inflammatory responses that include induction of the bactericidal C-type lectin RegIIIγ. Systemic administration of flagellin, a bacterial protein that stimulates Toll-like receptor 5 (TLR5), induces epithelial expression of RegIIIγ and protects mice from intestinal colonization with antibiotic-resistant bacteria. Flagellin-induced RegIIIγ expression is IL-22 dependent, but how TLR signaling leads to IL-22 expression is incompletely defined. By using conditional depletion of lamina propria dendritic cell (LPDC) subsets, we demonstrated that CD103(+)CD11b(+) LPDCs, but not monocyte-derived CD103(-)CD11b(+) LPDCs, expressed high amounts of IL-23 after bacterial flagellin administration and drove IL-22-dependent RegIIIγ production. Maximal expression of IL-23 subunits IL-23p19 and IL-12p40 occurred within 60 min of exposure to flagellin. IL-23 subsequently induced a burst of IL-22 followed by sustained RegIIIγ expression. Thus, CD103(+)CD11b(+) LPDCs, in addition to promoting long-term tolerance to ingested antigens, also rapidly produce IL-23 in response to detection of flagellin in the lamina propria.

Microbiota restricts trafficking of bacteria to mesenteric lymph nodes by CX 3 CR1 hi cells

The intestinal microbiota has a critical role in immune system and metabolic homeostasis, but it must be tolerated by the host to avoid inflammatory responses that can damage the epithelial barrier separating the host from the luminal contents. Breakdown of this regulation and the resulting inappropriate immune response to commensals are thought to lead to the development of inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis. We proposed that the intestinal immune system is instructed by the microbiota to limit responses to luminal antigens. Here we demonstrate in mice that, at steady state, the microbiota inhibits the transport of both commensal and pathogenic bacteria from the lumen to a key immune inductive site, the mesenteric lymph nodes (MLNs). However, in the absence of Myd88 or under conditions of antibiotic-induced dysbiosis, non-invasive bacteria were trafficked to the MLNs in a CCR7-dependent manner, and induced both T-cell responses and IgA production. Trafficking was carried out by CX3CR1hi mononuclear phagocytes, an intestinal-cell population previously reported to be non-migratory. These findings define a central role for commensals in regulating the migration to the MLNs of CX3CR1hi mononuclear phagocytes endowed with the ability to capture luminal bacteria, thereby compartmentalizing the intestinal immune response to avoid inflammation.

FADD Is Required for DR4- and DR5-mediated Apoptosis LACK OF TRAIL-INDUCED APOPTOSIS IN FADD-DEFICIENT MOUSE EMBRYONIC FIBROBLASTS

TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) is a member of the tumor necrosis factor family that can kill a wide variety of tumor cells but not normal cells. TRAIL-induced apoptosis in humans is mediated by its receptors DR4 (TRAIL-R1) and DR5 (TRAIL-R2). What constitutes the signaling molecules downstream of these receptors, however, remains highly controversial. Using the FADD dominant negative molecule, several groups have reached different conclusions with respect to the role of FADD in TRAIL-induced apoptosis. More recently, usingFADD-deficient (−/−) mouse embryonic fibroblasts, Yehet al. (Yeh, W.-C., Pompa, J. L., McCurrach, M. E., Shu, H.-B., Elia, A. J., Shahinian, A., Ng, M., Wakeham, A., Khoo, W., Mitchell, K., El-Deiry, W. S., Lowe, S. W., Goeddel, D. V., and Mak, T. W. (1998) Science279, 1954–1958) concluded that DR4 utilizes a FADD-independent apoptotic pathway. The latter experiment, however, involved transient overexpression, which often leads to nonspecific aggregation of death domain-containing receptors. To address this issue in a more physiological setting, we stably transfected mouse DR4/5, human DR4, or human DR5 intoFADD −/− mouse embryonic fibroblast cells. We showed that FADD −/− MEF cells stably transfected with TRAIL receptors are resistant to TRAIL-mediated cell death. In contrast, TRAIL receptors stably transfected into heterozygous FADD +/− cells orFADD −/− cells reconstituted with a FADD retroviral construct are sensitive to the TRAIL cytotoxic effect. We conclude that FADD is required for DR4- and DR5-mediated apoptosis.

TRAIL-R as a Negative Regulator of Innate Immune Cell Responses

TRAIL receptor (TRAIL-R) signaling has been implicated in inducing apoptosis in tumor cells, but little is understood about its physiological function. Here, we report the generation and characterization of TRAIL-R(-/-) mice, which develop normal lymphocyte populations but possess enhanced innate immune responses. TRAIL-R(-/-) mice exhibited increased clearance of murine cytomegalovirus that correlated with increased levels of IL-12, IFN-alpha, and IFN-gamma. Stimulation of macrophages with Mycobacterium and Toll-like receptor (TLR)-2, -3, and -4, but not TLR9, ligands resulted in high levels of TRAIL upregulation and enhanced cytokine production in TRAIL-R(-/-) cells. The immediate-early TLR signaling events in TRAIL-R(-/-) macrophages and dendritic cells are normal, but I kappa B-alpha homeostatic regulation and NF-kappa B activity at later time points is perturbed. These data suggest that TRAIL-R negatively regulates innate immune responses.

CX3CR1+ mononuclear phagocytes support colitis-associated innate lymphoid cell production of IL-22

Intestinal CX3CR1+ mononuclear phagocytes regulate ILC3 in vivo in response to colitis associated microbial signals.

Fas-associated death domain (FADD) is a negative regulator of T-cell receptor–mediated necroptosis

Cell death is an important mechanism to limit uncontrolled T-cell expansion during immune responses. Given the role of death-receptor adapter protein Fas-associated death domain (FADD) in apoptosis, it is intriguing that T-cell receptor (TCR)–induced proliferation is blocked in FADD-defective T cells. Necroptosis is an alternate form of death that can be induced by death receptors and is linked to autophagy. It requires the death domain-containing kinase RIP1 and, in certain instances, RIP3. FADD and its apoptotic partner, Caspase-8, have also been implicated in necroptosis. To accurately assess the role of FADD in mature T-cell proliferation and death, we generated a conditional T-cell–specific FADD knockout mouse strain. The T cells of these mice develop normally, but lack FADD at the mature stage. FADD-deficient T cells respond poorly to TCR triggering, exhibit slow cell cycle entry, and fail to expand over time. We find that programmed necrosis occurs during the late stage of normal T-cell proliferation and that this process is greatly amplified in FADD-deficient T cells. Inhibition of necroptosis using an inhibitor of RIP1 kinase activity rescues the FADD knockout proliferative defect. However, TCR-induced necroptosis did not appear to require autophagy or involve RIP3. Consistent with their defective CD8 T-cell response, these mice succumb to Toxoplasma gondii infection more readily than wild-type mice. We conclude that FADD constitutes a mechanism to keep TCR-induced programmed necrotic signaling in check during early phases of T-cell clonal expansion.

Lymphoid Tissue Inducer Cells in Intestinal Immunity

During fetal development, lymphoid tissue inducer cells (LTis) seed the developing lymph node and Peyers patch anlagen and initiate the formation of both types of lymphoid organs. In the adult, a similar population of cells, termed lymphoid tissue inducer-like cells (LTi-like cells), supports the formation of organized gut-associated lymphoid tissue (GALT) in the intestine, including both isolated lymphoid follicles (ILFs) and cryptopatches (CPs). Both LTi and LTi-like cells require expression of the transcription factor RORgammat for their differentiation and function, and mice lacking RORgammat lack lymph nodes, Peyers patches, and other organized GALT. In ILFs and cryptopatches, LTi-like cells are in close contact with different populations of intestinal dendritic cells (DCs), including a subpopulation recently shown to extend dendrites and sample luminal microflora. This interaction may allow for communication between the intestinal lumen and the immune cells in the lamina propria, which is necessary for maintaining homeostasis between the commensal microflora and the intestinal immune system. The potential functional implications of the organization of LTi-like cells, DCs, and lymphocytes in the lamina propria are discussed in the context of maintenance of homeostasis and of infectious diseases, particularly HIV infection.

Serum amyloid A is a retinol binding protein that transports retinol during bacterial infection

Retinol plays a vital role in the immune response to infection, yet proteins that mediate retinol transport during infection have not been identified. Serum amyloid A (SAA) proteins are strongly induced in the liver by systemic infection and in the intestine by bacterial colonization, but their exact functions remain unclear. Here we show that mouse and human SAAs are retinol binding proteins. Mouse and human SAAs bound retinol with nanomolar affinity, were associated with retinol in vivo, and limited the bacterial burden in tissues after acute infection. We determined the crystal structure of mouse SAA3 at a resolution of 2 Å, finding that it forms a tetramer with a hydrophobic binding pocket that can accommodate retinol. Our results thus identify SAAs as a family of microbe-inducible retinol binding proteins, reveal a unique protein architecture involved in retinol binding, and suggest how retinol is circulated during infection. DOI: http://dx.doi.org/10.7554/eLife.03206.001

Bcl‐xl‐Specific antibody labels activated microglia associated with Alzheimer's disease and other pathological states

This report describes the production of a monoclonal antibody raised against Bcl‐xl, and includes an initial study of bcl‐xl expression in neuropathology including Alzheimers disease (AD). Bcl‐xl is a potent apoptotic inhibitor and is known to be the predominant Bcl‐x isoform in brain. To examine the expression of bcl‐xl in aged brain and neurodegenerative disease, we raised a Bcl‐xl‐specific monoclonal antibody. In aged human brain, the highest bcl‐xl expression was observed in cerebellum. By immunohistochemistry, significant bcl‐xl expression was detected in reactive microglia of patients with AD and other neurological diseases such as progressive supranuclear palsy. Bcl‐xl‐positive microglia frequently colocalized with β‐amyloid plaques in AD and with activated astrocytes in non‐AD and AD brains, suggesting a general role for Bcl‐xl in regions of pathology. High levels of Bcl‐xl protein might render microglia more resistant to cytotoxic environments such as areas of neurodegeneration and astrogliosis. J. Neurosci. Res. 47:98–108, 1997.

Loss of TRAIL-R does not affect thymic or intestinal tumor development in p53 and adenomatous polyposis coli mutant mice

Loss of TRAIL-R does not affect thymic or intestinal tumor development in p53 and adenomatous polyposis coli mutant mice