Defective TCR expression in transgenic mice constructed using cDNA-based alpha- and beta-chain genes under the control of heterologous regulatory elements

Central tolerance to tissue-specific antigens mediated by direct and indirect antigen presentation

Intrathymic expression of tissue-specific antigens (TSAs) by medullary thymic epithelial cells (Mtecs) leads to deletion of autoreactive T cells. However, because Mtecs are known to be poor antigen-presenting cells (APCs) for tolerance to ubiquitous antigens, and very few Mtecs express a given TSA, it was unclear if central tolerance to TSA was induced directly by Mtec antigen presentation or indirectly by thymic bone marrow (BM)-derived cells via cross-presentation. We show that professional BM-derived APCs acquire TSAs from Mtecs and delete autoreactive CD8 and CD4 T cells. Although direct antigen presentation by Mtecs did not delete the CD4 T cell population tested in this study, Mtec presentation efficiently deleted both monoclonal and polyclonal populations of CD8 T cells. For developing CD8 T cells, deletion by BM-derived APC and by Mtec presentation occurred abruptly at the transitional, CD4^high CD8^low TCR^intermediate stage, presumably as the cells transit from the cortex to the medulla. These studies reveal a cooperative relationship between Mtecs and BM-derived cells in thymic elimination of autoreactive T cells. Although Mtecs synthesize TSAs and delete a subset of autoreactive T cells, BM-derived cells extend the range of clonal deletion by cross-presenting antigen captured from Mtecs.

CD4+ T cells cross-compete for MHC class II-restricted peptide antigen complexes on the surface of antigen presenting cells

CD4(+) T cells are activated upon recognition of peptide antigen in the context of MHC class II molecules, expressed by specialized APC. In this study, we show that CD4(+) T cells cross-compete for antigenic complexes on the surface of APC, inhibiting activation of other potentially reactive T cells of the same and differing specificities. T cells with either a higher affinity receptor for antigen or which have undergone prior activation compete more efficiently than low affinity or resting T cells. This implies that T-cell avidity for the APC is primarily responsible for the competitive advantage. We also provide evidence that the mechanism for competition is steric hindrance of the surface of the APC, rather than T-cell-mediated sequestration or internalization of antigenic complexes. This is because removal of competing T cells restores the antigenic potential of the APC, and APC fixation does not abrogate competition. Demonstration that competition for access to APC can also occur in vivo suggests that this process may represent a physiologically important mechanism for influencing the quality and quantity of CD4(+) T-cell responses.

4-1BB and OX40 Act Independently to Facilitate Robust CD8 and CD4 Recall Responses

Mice deficient in OX40 or 4-1BB costimulatory pathways show defects in T cell recall responses, with predominant effects on CD4 vs CD8 T cells, respectively. However, OX40L can also stimulate CD8 T cells and 4-1BBL can influence CD4 T cells, raising the possibility of redundancy between the two TNFR family costimulators. To test this possibility, we generated mice deficient in both 4-1BBL and OX40L. In an adoptive transfer model, CD4 T cells expressed 4-1BB and OX40 sequentially in response to immunization, with little or no overlap in the timing of their expression. Under the same conditions, CD8 T cells expressed 4-1BB, but no detectable OX40. Thus, in vivo expression of 4-1BB and OX40 can be temporally and spatially segregated. In the absence of OX40L, there were decreased CD4 T cells late in the primary response and no detectable secondary expansion of adoptively transferred CD4 T cells under conditions in which primary expansion was unaffected. The 4-1BBL had a minor effect on the primary response of CD4 T cells in this model, but showed larger effects on the secondary response, although 4-1BBL(-/-) mice show less impairment in CD4 secondary responses than OX40L(-/-) mice. The 4-1BBL(-/-) and double knockout mice were similarly impaired in the CD8 T cell response, whereas OX40L(-/-) and double knockout mice were similarly impaired in the CD4 T cell response to both protein Ag and influenza virus. Thus, 4-1BB and OX40 act independently and nonredundantly to facilitate robust CD4 and CD8 recall responses.

B7-1 and B7-2 selectively recruit CTLA-4 and CD28 to the immunological synapse

The reported affinity differences between CD28 and CTLA-4 binding to B7-1 and B7-2 may serve to selectively regulate CD28 and CTLA-4 function by differentially recruiting and/or stabilizing these molecules at the immunological synapse. Here we show that ligand binding is important for the accumulation of both CD28 and CTLA-4 at the synapse. While CD28 is recruited to the synapse in the absence of B7-1 and B7-2 binding, it is not effectively stabilized there, as its localization can be disrupted by CTLA-4. In the case of CTLA-4, ligand binding is critical for its concentration at the synapse. We also demonstrate that the affinity and avidity differences in ligand binding translate into selective recruitment of CD28 or CTLA-4 to the immunological synapse--B7-1 is the major ligand mediating CTLA-4 localization, while B7-2 is the main ligand for CD28 concentration at the synapse.

Induction of antigen cross-presentation by Toll-like receptors

Cross-presentation is the pathway by which exogenous antigens are routed for presentation on MHC class I for activation of CD8(+) T cells. This pathway is important for the development of CD8(+) cytotoxic T lymphocyte responses against tumors and infectious pathogens that do not directly infect APC. We review studies showing that certain Toll-like receptors mediate cross-presentation by dendritic cells, initiating cytosolic processing of antigen after inducing dendritic cell maturation. The implications of these studies for understanding CD8(+) T cell activation and implementing novel vaccine strategies is considered.

Parasite-induced Th2 polarization is associated with down-regulated dendritic cell responsiveness to Th1 stimuli and a transient delay in T lymphocyte cycling

The nature of the signals that bias Th effector choice is still not completely understood. Using parasite extracts from pathogens known to induce polarized Th1 or Th2 responses and an in vitro experimental model for priming murine CD4(+) cells, we demonstrated that splenic dendritic cells (DC), but not B cells, promote Th1/Th2 differentiation of naive CD4(+) lymphocytes. Th polarization in this system was found not to depend on DC secretion of the polarizing cytokines IL-12/IL-4, but instead correlated with distinct states of DC activation induced by the different parasite preparations. As expected, conditioning of DC for Th1 development was associated with up-regulation of costimulatory molecules and enhanced chemokine production and required intact MyD88 signaling. In contrast, conditioning of DC for Th2 differentiation correlated with down-regulation of many of the same functions and was MyD88 independent. This dampened DC activation was accompanied in the cocultures by a reduction in the frequency of CD4(+) lymphocytes exiting the first division of the cell cycle. When the latter was mimicked by drug-induced arrest of peptide-primed CD4(+) cells after the S phase of the first cycle, a marked Th2 polarization was also observed. Together, these findings suggest that the emergence of IL-4-producing CD4(+) lymphocytes results from a suppression in DC function leading to a temporary delay in initial T cell cycling.

CCR7 governs skin dendritic cell migration under inflammatory and steady-state conditions

The CC chemokine receptor CCR7 has been identified as a key regulator of homeostatic B and T cell trafficking to secondary lymphoid organs. Data presented here demonstrate that CCR7 is also an essential mediator for entry of both dermal and epidermal dendritic cells (DC) into the lymphatic vessels within the dermis while this receptor is dispensable for the mobilization of Langerhans cells from the epidermis to the dermis. Moreover, a distinct population of CD11c(+)MHCII(high) DC showing low expression of the costimulatory molecules CD40, CD80, and CD86 in wild-type animals was virtually absent in skin-draining lymph nodes of CCR7-deficient mice under steady-state conditions. We provide evidence that these cells represent a semimature population of DC that is capable of initiating T cell proliferation under conditions known to induce tolerance. Thus, our data identify CCR7 as a key regulator that governs trafficking of skin DC under both inflammatory and steady-state conditions.

Tumor-infiltrating dendritic cell precursors recruited by a beta-defensin contribute to vasculogenesis under the influence of Vegf-A

The involvement of immune mechanisms in tumor angiogenesis is unclear. Here we describe a new mechanism of tumor vasculogenesis mediated by dendritic cell (DC) precursors through the cooperation of beta-defensins and vascular endothelial growth factor-A (Vegf-A). Expression of mouse beta-defensin-29 recruited DC precursors to tumors and enhanced tumor vascularization and growth in the presence of increased Vegf-A expression. A new leukocyte population expressing DC and endothelial markers was uncovered in mouse and human ovarian carcinomas coexpressing Vegf-A and beta-defensins. Tumor-infiltrating DCs migrated to tumor vessels and independently assembled neovasculature in vivo. Bone marrow-derived DCs underwent endothelial-like differentiation ex vivo, migrated to blood vessels and promoted the growth of tumors expressing high levels of Vegf-A. We show that beta-defensins and Vegf-A cooperate to promote tumor vasculogenesis by carrying out distinct tasks: beta-defensins chemoattract DC precursors through CCR6, whereas Vegf-A primarily induces their endothelial-like specialization and migration to vessels, which is mediated by Vegf receptor-2.

A tumor-associated glycoprotein that blocks MHC class II-dependent antigen presentation by dendritic cells

Tumors evade immune surveillance despite the frequent expression of tumor-associated Ags (TAA). Tumor cells escape recognition by CD8(+) T cells through several mechanisms, including down-regulation of MHC class I molecules and associated Ag-processing machinery. However, although it is well accepted that optimal anti-tumor immune responses require tumor-reactive CD4(+) T cells, few studies have addressed how tumor cells evade CD4(+) T cell recognition. In this study, we show that a common TAA, GA733-2, and its murine orthologue, mouse epithelial glycoprotein (mEGP), function in blocking MHC class II-restricted Ag presentation by dendritic cells. GA733-2 is a common TAA that is expressed normally at low levels by some epithelial tissues and a subset of dendritic cells, but at high levels on colon, breast, lung, and some nonepithelial tumors. We show that ectopic expression of mEGP or GA733-2, respectively, in dendritic cells derived from murine bone marrow or human monocytes results in a dose-dependent inability to stimulate proliferation of Ag-specific or alloreactive CD4(+) T cells. Dendritic cells exposed to cell debris from tumors expressing mEGP are similarly compromised. Furthermore, mice immunized with dendritic cells expressing mEGP from a recombinant adenovirus vector exhibited a muted anti-adenovirus immune response. The inhibitory effect of mEGP was not due to down-regulation of functional MHC class II molecules or active suppression of T cells, and did not extend to T cell responses to superantigen. These results demonstrate a novel mechanism by which tumors may evade CD4(+) T cell-dependent immune responses through expression of a TAA.

Fyn and PTP-PEST-mediated regulation of Wiskott-Aldrich syndrome protein (WASp) tyrosine phosphorylation is required for coupling T cell antigen receptor engagement to WASp effector function and T cell activation

Involvement of the Wiskott-Aldrich syndrome protein (WASp) in promoting cell activation requires its release from autoinhibitory structural constraints and has been attributed to WASp association with activated cdc42. Here, however, we show that T cell development and T cell receptor (TCR)-induced proliferation and actin polymerization proceed normally in WASp-/- mice expressing a WASp transgene lacking the cdc42 binding domain. By contrast, mutation of tyrosine residue Y291, identified here as the major site of TCR-induced WASp tyrosine phosphorylation, abrogated induction of WASp tyrosine phosphorylation and its effector activities, including nuclear factor of activated T cell transcriptional activity, actin polymerization, and immunological synapse formation. TCR-induced WASp tyrosine phosphorylation was also disrupted in T cells lacking Fyn, a kinase shown here to bind, colocalize with, and phosphorylate WASp. By contrast, WASp was tyrosine dephosphorylated by protein tyrosine phosphatase (PTP)-PEST, a tyrosine phosphatase shown here to interact with WASp via proline, serine, threonine phosphatase interacting protein (PSTPIP)1 binding. Although Fyn enhanced WASp-mediated Arp2/3 activation and was required for synapse formation, PTP-PEST combined with PSTPIP1 inhibited WASp-driven actin polymerization and synapse formation. These observations identify key roles for Fyn and PTP-PEST in regulating WASp and imply that inducible WASp tyrosine phosphorylation can occur independently of cdc42 binding, but unlike the cdc42 interaction, is absolutely required for WASp contributions to T cell activation.

MHC class II expression is differentially regulated in plasmacytoid and conventional dendritic cells

Major histocompatibility complex (MHC) class II-restricted antigen presentation is essential for the function of dendritic cells (DCs). We show here that plasmacytoid DCs (pDCs) differ from all other DC subsets with respect to expression of CIITA, the 'master regulator' of MHC class II genes. The gene encoding CIITA is controlled by three cell type-specific promoters: pI, pIII and pIV. With gene targeting in mice, we demonstrate that pDCs rely strictly on the B cell promoter pIII, whereas macrophages and all other DCs depend on pI. The molecular mechanisms driving MHC class II expression in pDCs are thus akin to those operating in lymphoid rather than myeloid cells.

Upregulation of the CLIP self peptide on mature dendritic cells antagonizes T helper type 1 polarization

Dendritic cells (DCs) initiate and regulate immunity against foreign and self antigens. Here we identified more than 200 individual major histocompatibility complex class II-associated peptides on human DCs and found that mature DCs selectively upregulated the self peptide CLIP. CLIP cosegregated together with foreign antigenic peptides in tetraspan microdomains on the surface and localized to DC-T cell synapses. The increased representation of CLIP-major histocompatibility complex class II complexes favored polarization of autologous naive T cells toward the nonpolarized and T helper type 2 (T(H)2) phenotype. There was also a considerably higher T(H)2/T(H)1 ratio in H2-DM-deficient mice, which have a CLIP(hi) phenotype, in contrast to wild-type, CLIP(lo) mice. Thus, the self peptide CLIP on DCs qualifies as an endogenous regulator in priming of T helper cells by antagonizing the polarization toward the T(H)1 phenotype.

An IL-12-independent role for CD40-CD154 in mediating effector responses: studies in cell-mediated glomerulonephritis and dermal delayed-type hypersensitivity

Crescentic glomerulonephritis (GN) results from IL-12-driven Th1-directed cell-mediated responses (akin to delayed-type hypersensitivity (DTH)) directed against glomerular Ags. CD40-CD154 interactions are critical for IL-12 production and Th1 polarization of immune responses. Crescentic anti-glomerular basement membrane GN was induced in C57BL/6 (wild-type (WT)) mice (sensitized to sheep globulin) by planting this Ag (as sheep anti-mouse glomerular basement membrane globulin) in their glomeruli. Crescentic GN did not develop in CD40(-/-) mice due to significantly reduced nephritogenic Th1 responses. IL-12 was administered to CD40(-/-) mice with GN to dissect interactions between IL-12 and CD40 in inducing nephritogenic immunity and injury. Administration of IL-12 to CD40(-/-) mice restored Th cell IFN-gamma production, and up-regulated intrarenal chemokines and glomerular T cell and macrophage accumulation compared with WT control mice. Despite this, renal macrophages were not activated and renal injury and dermal DTH were not restored. Thus, CD40-directed IL-12 drives Th1 generation and effector cell recruitment but CD40 is required for activation. To test this hypothesis, activated OT-II OVA-specific CD4(+) cells and OVA(323-339)-loaded nonresponsive APCs were transferred into footpads of WT, CD40(-/-), and macrophage-depleted WT mice. WT mice developed significant DTH compared with CD40(-/-) and macrophage-depleted WT mice. This study demonstrated that CD40-induced IL-12 is required for generation of systemic Th1 immunity to nephritogenic Ags, and that IL-12 enhances Th1 effector cell recruitment to peripheral sites of Ag presentation via generation of local chemokines. Effector cell activation, renal DTH-like injury, and dermal DTH require direct Th1 CD154/macrophage CD40 interactions.

Liver sinusoidal endothelial cells are insufficient to activate T cells

Liver sinusoidal endothelial cells (LSEC) have been reported to express MHC class II, CD80, CD86, and CD11c and effectively stimulate naive T cells. Because dendritic cells (DC) are known to possess these characteristics, we sought to directly compare the phenotype and function of murine LSEC and DC. Nonparenchymal cells from C57BL/6 mice were obtained by collagenase digestion of the liver followed by density gradient centrifugation. From the enriched nonparenchymal cell fraction, LSEC (CD45(-)) were then isolated to 99% purity using immunomagnetic beads. Flow cytometric analysis of LSEC demonstrated high expression of CD31, von Willebrand factor, and FcgammaRs. However, unlike DC, LSEC had low or absent expression of MHC class II, CD86, and CD11c. LSEC demonstrated a high capacity for Ag uptake in vitro and in vivo. Although acetylated low-density lipoprotein uptake has been purported to be a specific function of LSEC, we found DC captured acetylated low-density lipoprotein to a similar extent in vivo. Consistent with their phenotype, LSEC were poor stimulators of allogeneic T cells. Furthermore, in the absence of exogenous costimulation, LSEC induced negligible proliferation of CD4(+) or CD8(+) TCR-transgenic T cells. Thus, contrary to previous reports, our data indicate that LSEC alone are insufficient to activate naive T cells.

Lymphocyte activation gene-3 (CD223) regulates the size of the expanding T cell population following antigen activation in vivo

Lymphocyte activation gene-3 (LAG-3) is a CD4-related, activation-induced cell surface molecule that binds to MHC class II with high affinity. In this study, we used four experimental systems to reevaluate previous suggestions that LAG-3(-/-) mice had no T cell defect. First, LAG-3(-/-) T cells exhibited a delay in cell cycle arrest following in vivo stimulation with the superantigen staphylococcal enterotoxin B resulting in increased T cell expansion and splenomegaly. Second, increased T cell expansion was also observed in adoptive recipients of LAG-3(-/-) OT-II TCR transgenic T cells following in vivo Ag stimulation. Third, infection of LAG-3(-/-) mice with Sendai virus resulted in increased numbers of memory CD4(+) and CD8(+) T cells. Fourth, CD4(+) T cells exhibited a delayed expansion in LAG-3(-/-) mice infected with murine gammaherpesvirus. In summary, these data suggest that LAG-3 negatively regulates T cell expansion and controls the size of the memory T cell pool.

Massive number of antigen-specific CD4 T cells during vaccination with live attenuated Salmonella causes interclonal competition

The clonal burst size of CD4 T cells is predicted to be less than that of CD8 T cells. In this study, we demonstrate that massive numbers of Ag-specific CD4 T cells respond during vaccination of mice with live attenuated Salmonella, reaching a peak frequency of approximately 50% of CD4 T cells. Salmonella-specific T cells persisted at high frequency for several weeks and could be detected in the memory population for months after infection. Surprisingly, the expansion of endogenous Salmonella-specific CD4 T cells prevented the persistence of adoptively transferred Salmonella-specific T cells in vivo, demonstrating interclonal competition for access to the memory compartment.

Humoral immune responses to model antigen co-delivered with biomaterials used in tissue engineering

A model shed antigen, ovalbumin (OVA), was co-delivered with polymeric biomaterial carrier vehicles in C57BL6 mice to test whether the presence of the biomaterial acted as an adjuvant in the immune response towards the associated antigen. The biomaterials tested were non-biodegradable polystyrene microparticles and biodegradable 50:50 or 75:25 poly(lactic-co-glycolic acid) (PLGA) microparticles or scaffolds. For each biomaterial carrier vehicle, to assess the resulting time-dependent systemic humoral immune response towards the co-delivered OVA, the OVA-specific IgG concentration and isotypes (IgG2a or IgG1, indicating a predominant Th1 or Th2 response, respectively) were determined using ELISA. OVA co-delivered with biomaterial carrier vehicles supported a moderate humoral immune response that was maintained for the 18-week duration of the experiment. This humoral immune response was primarily Th2 helper T cell-dependent as indicated by the predominant IgG1 isotype. Furthermore, this humoral immune response was not material chemistry-dependent within the material set tested here. With the presence of the biomaterial resulting in an enhancement of the humoral immune response to co-delivered antigen, it appears that the biomaterial acts as an adjuvant in the development of an adaptive immune response to co-delivered antigen.

Essential Role of the E3 Ubiquitin Ligase Cbl-b in T Cell Anergy Induction

Antigen-specific immunotolerance limits the expansion of self-reactive T cells involved in autoimmune diseases. Here, we show that the E3 ubiquitin ligase Cbl-b is upregulated in T cells after tolerizing signals. Loss of Cbl-b in mice results in impaired induction of T cell tolerance both in vitro and in vivo. Importantly, rechallenge of Cbl-b mutant mice with the tolerizing antigen results in massive lethality. Moreover, ablation of Cbl-b resulted in exacerbated autoimmunity. Mechanistically, loss of Cbl-b rescues reduced calcium mobilization of anergic T cells, which was attributed to Cbl-b-mediated regulation of PLCgamma-1 phosphorylation. Our results show a critical role for Cbl-b in the regulation of peripheral tolerance and anergy of T cells.

A Toll-like receptor 2 ligand stimulates Th2 responses in vivo, via induction of extracellular signal-regulated kinase mitogen-activated protein kinase and c-Fos in dendritic cells

The adaptive immune system can generate distinct classes of responses, but the mechanisms that determine this are poorly understood. In this study, we demonstrate that different Toll-like receptor (TLR) ligands induce distinct dendritic cell (DC) activation and immune responses in vivo. Thus, Escherichia coli LPS (TLR-4 stimulus), activates DCs to produce abundant IL-12(p70), but little IL-10, and stimulates Th1 and Tc1 responses. In contrast, Pam-3-cys (TLR-2 stimulus) elicits less IL-12(p70), but abundant IL-10, and favors Th2 and T cytotoxic 2 (Tc2) responses. These distinct responses likely occur via differences in extracellular signal-regulated kinase signaling in DCs. Thus, Pam-3-cys induces enhanced extracellular signal-regulated kinase signaling, compared with LPS, resulting in suppressed IL-12(p70) and enhanced IL-10 production, as well as enhanced induction of the transcription factor, c-Fos. Interestingly, DCs from c-fos(-/-) mice produce more IL-12(p70), but less IL-10, compared with control DCs. Therefore, different TLR ligands induce distinct cytokines and signaling in DCs, and differentially bias Th responses in vivo.

Critical role for granulocyte colony-stimulating factor in inflammatory arthritis

Granulocyte colony-stimulating factor (G-CSF) is a well known regulator of granulopoiesis, but the role of endogenous G-CSF in inflammatory joint disease has not been explored. We studied the response of G-CSF-deficient mice in experimental models of joint inflammation. We show that G-CSF deficiency protects mice from acute and chronic arthritis. Reduced severity was associated with blunted mobilization of granulocytic cells from the bone marrow and less cellular infiltrate and cellular activation in inflamed joints. We also demonstrate that G-CSF blockade in established collagen-induced arthritis in WT mice markedly reduces disease manifestations and is as effective as tumor necrosis factor blockade. Our results reveal a critical role for G-CSF in driving joint inflammation and highlight G-CSF as a potential therapeutic target in inflammatory joint diseases, such as rheumatoid arthritis.

An optimized CD4 T-cell response can control productive and latent gammaherpesvirus infection

CD4 T cells are important for control of infection with murine gammaherpesvirus 68 (gamma HV68), but it is not known whether CD4 T cells function via provision of help to other lymphocyte subsets, such as B cells and CD8 T cells, or have an independent antiviral function. Moreover, under conditions of natural infection, the CD4 T-cell response is not sufficient to eliminate infection. To determine the functional capacities of CD4 T cells under optimal or near-optimal conditions and to determine whether CD4 T cells can control gamma HV68 infection in the absence of CD8 T cells or B cells, we studied the effect of ovalbumin (OVA)-specific CD4 T cells on infection with a recombinant gamma HV68 that expresses OVA. OVA-specific CD4 T cells limited acute gamma HV68 replication and prolonged the life of infected T-cell receptor-transgenic RAG (DO.11.10/RAG) mice, demonstrating CD4 T-cell antiviral activity, independent of CD8 T cells and B cells. Despite CD4 T-cell-mediated control of acute infection, latent infection was established in DO.11.10/RAG mice. However, OVA-specific CD4 T cells reduced the frequency of latently infected cells both early (16 days postinfection) and late (42 days postinfection) after infection of mice containing CD8 T cells and B cells (DO.11.10 mice). These results show that OVA-specific CD4 T cells have B-cell and CD8 T-cell-independent antiviral functions in the control of acute infection and can, in the absence of preexisting CD8 T-cell or B-cell immunity, inhibit the establishment of gammaherpesvirus latency.

GPI-anchor deficiency in myeloid cells causes impaired FcgammaR effector functions

Signaling by transmembrane immunoglobulin G (IgG)-Fc receptors (FcgammaRs) in response to ligand involves association with membrane microdomains that contain glycosyl phosphatidylinositol (GPI)-anchored proteins. Recent in vitro studies showed enhancement of FcgammaR signaling by forced monoclonal antibody-mediated cocrosslinking with various GPI-anchored proteins. Here, the possibility that GPI-anchored proteins are involved in normal physiologic FcgammaR effector functions in response to a model ligand was studied using myeloid-specific GPI-anchor-deficient mice, generated by Cre-loxP conditional targeting. GPI-anchor-deficient primary myeloid cells exhibited normal FcgammaR expression and binding or endocytosis of IgG-immune complexes (IgG-ICs). Strikingly, after stimulation with IgG-ICs, tumor necrosis factor-alpha release, dendritic cell maturation, and antigen presentation were strongly reduced by GPI-anchor deficiency. Tyrosine phosphorylation of the FcR gamma-chain in response to IgG-IC was impaired in GPI-anchor-deficient cells. Myeloid GPI-anchor deficiency resulted in attenuated in vivo inflammatory processes during IgG-IC-mediated alveolitis. This study provides the first genetic evidence for an essential role of GPI-anchored proteins in physiologic FcgammaR effector functions in vitro and in vivo.

Critical functions for c-Myb at three checkpoints during thymocyte development

The transcription factor c-Myb is expressed throughout T cell development in the thymus. However, little is understood about c-Myb function because of the embryonic lethality of traditional Myb-null mutations. Using tissue-specific deletion to abrogate c-Myb expression at distinct stages of T cell development, we identify three points at which c-Myb activity is required for normal T cell differentiation: transition through the double-negative 3 stage, survival of preselection CD4(+)CD8(+) thymocytes, and differentiation of CD4 thymocytes. Thus, c-Myb is essential at several stages during T cell development in the thymus.

Critical roles of interferon regulatory factor 4 in CD11bhighCD8alpha- dendritic cell development

IFN regulatory factors (IRFs) are a family of transcription factors that play an essential role in the homeostasis and function of immune systems. Recent studies indicated that IRF-8 is critical for the development of CD11b(low)CD8alpha(+) conventional dendritic cells (DCs) and plasmacytoid DCs. Here we show that IRF-4 is important for CD11b(high)CD8alpha(-) conventional DCs. The development of CD11b(high) DCs from bone marrow of IRF-4(-/-) mice was severely impaired in two culture systems supplemented with either GM-CSF or Flt3-ligand. In the IRF-4(-/-) spleen, the number of CD4(+)CD8alpha(-) DCs, a major subset of CD11b(high) DCs, was severely reduced. IRF-4 and IRF-8 were expressed in the majority of CD11b(high)CD4(+)CD8alpha(-) DCs and CD11b(low)CD8alpha(+) DCs, respectively, in a mutually exclusive manner. These results imply that IRF-4 and IRF-8 selectively play critical roles in the development of the DC subsets that express them.

A Bcl-2-dependent molecular timer regulates the lifespan and immunogenicity of dendritic cells

The lifespan of antigen-bearing dendritic cells (DCs) is determined by signals from pathogens and T cells. These signals regulate DC survival by modulating expression of Bcl-2 family proteins. Toll-like receptors and T cell costimulatory molecules both trigger a DC survival pathway that is dependent on Bcl-x(L). However, Toll-like receptors uniquely increase expression of Bim and trigger cell death by a pathway that is blocked by Bcl-2. This pathway serves as a molecular 'timer' that sets the lifespan of DCs and regulates the magnitude of T cell responses in vivo. Thus, signals derived from the innate and acquired immune systems control DC lifespan and immunogenicity by distinct molecular mechanisms.

Negative selection of semimature CD4(+)8(-)HSA+ thymocytes requires the BH3-only protein Bim but is independent of death receptor signaling

T cell receptor/CD3 ligation induces apoptosis in semimature CD4(+)8(-)HSA+ thymocytes, and this helps establish immunological tolerance and constitutes one of the safeguards against autoimmune disease. We analyzed several knockout and transgenic mouse lines and found that T cell receptor/CD3-ligation-induced killing of semimature thymocytes occurred independently of Fas and "death receptor" signaling in general but required the proapoptotic BH3-only protein Bim and could be inhibited by Bcl-2. Loss of Apaf-1 or caspase-9, which act downstream of the Bcl-2 family protein family, provided only minor protection, indicating that the "apoptosome" functions as an amplifier rather than as an essential initiator of this death program. These results reveal the mechanisms of apoptosis in negative selection of semimature thymocytes and have implications for immunological tolerance and autoimmunity.

Recently primed CD8+ T cells entering the liver induce hepatocytes to interact with naïve CD8+ T cells in the mouse

Large number of T cells traffic through the liver. In order to examine the effects of such traffic on the phenotype of hepatocytes, we vaccinated mice using DNA vaccines encoding antigens with MHC class I-binding epitopes. Small numbers of activated CD8(+) T blasts (10(5)-10(6)/liver) changed the surface phenotype and cytokine expression profile of hepatocytes (HCs). HCs upregulate surface expression of major histocompatibility complex (MHC) class I molecules and CD1d but not MHC class II molecules Qa-1, CD80, CD86, CD54, or CD95; in addition, they expressed/secreted interleukin (IL)-10 and IL-4 but not IL-1, IL-6, IL-13, interferon (IFN)-gamma, tumor necrosis factor (TNF), IL-4, or IL-27 (i.e., they acquire the HC* phenotype). HCs* (but not HCs) induced specific activation, proliferation, and IFN-gamma, TNF, and IL-13 release of cocultured naïve CD8(+) T cells. In contrast to the specific activation of naïve CD8(+) T cells by dendritic cells (DCs), specific CD8(+) T cell activation by HC* was not down-modulated by IFN-alphabeta. Only recently activated CD8(+) T blasts (but not recently activated CD4(+) T blasts or activated cells of the innate immune system, including natural killer T [NKT] cells) induced the HC* phenotype that is prominent from day 10 to day 20 postvaccination (i.e., time points at which peak numbers of recently primed CD8(+) T blasts are found in the liver). In conclusion, recently activated CD8(+) T blasts that enter the liver postimmunization in small numbers can transiently modulate the phenotype of HC, allowing them to activate naïve CD8(+) T cells with unrelated specificities.

Helper Requirements for Generation of Effector CTL to Islet β Cell Antigens

We have dissected the helper requirements for converting a tolerogenic CD8 T cell response into one capable of causing destruction of the pancreatic islets. Injection of naive OVA-specific CD8 T cells into transgenic mice expressing OVA in the pancreas only resulted in islet destruction when activated CD4 Th cells were coinjected. This requirement for activated CD4 T cell help for induction of primary CD8 T cell-mediated immunity to tissue Ags contrasts recent reports suggesting that help is only important for CTL memory. Our findings show that signaling of CD40 on the dendritic cell presenting to CD8 T cells is important, but not sufficient, for induction of diabetes. Furthermore, once helpers are activated, they need not recognize Ag on the dendritic cells they license. This provides insight into the helper requirements for adoptive transfer immunotherapy of tumors and suggests key points for inhibition of CTL-mediated autoimmunity.

Regulation of dendritic cell trafficking by the ADP-ribosyl cyclase CD38: impact on the development of humoral immunity

Mice lacking CD38, an ectoenzyme that generates the calcium-mobilizing metabolite cADPR, make reduced T cell-dependent antibody responses. Despite the predicted role for CD38 in B cell activation, we find that CD38 regulates the migration of dendritic cell (DC) precursors from the blood to peripheral sites and controls the migration of mature DCs from sites of inflammation to lymph nodes. Thus, T cells are inefficiently primed in Cd38(-/-) mice, leading to poor humoral immune responses. We also show that CD38 and cADPR modulate calcium mobilization in chemokine-stimulated DCs and are required for the chemotaxis of immature and mature DCs to CCL2, CCL19, CCL21, and CXCL12. Therefore, CD38 regulates adaptive immunity by controlling chemokine receptor signaling in DCs.

Ectopic T cell receptor expression causes B cell immunodeficiency in transgenic mice

Mice expressing transgenic T cell receptors (TCR) are used to explore important questions in immunity. However, transgene expression may have unexpected effects. We previously reported a B cell immunodeficiency, comprising decreased B cell numbers and diminished antibody responses, in mice that express a transgenic TCR specific for nicotinic acetylcholine receptor; the mice were generated using cassette vectors designed specifically for transgenic TCR expression [see Kouskoff et al. J. Immunol. Methods 1995. 180: 273-280]. We now show data suggesting that this defect is due to the expression and accumulation of TCR alpha and beta chains inside B cells and induction of an endoplasmic reticulum stress response, causing apoptosis at the pre B-I and later B cell stage. Thus, inappropriate transgene expression can profoundly affect B cells, leading to a previously undescribed mechanism of immunodeficiency.

Pinpointing IL-4-independent acquisition and IL-4-influenced maintenance of Th2 activity by CD4 T cells

Naive CD4 T cells develop Th2 activity early in primary responses to alum-precipitated proteins by producing IL-4 mRNA and inducing B cells to produce gamma1 and epsilon switch transcripts. Both IL-4-dependent and IL-4-independent pathways for IL-4 induction are recognized, but their relative contribution to the different phases of primary Th2 responses in vivo is uncertain. We show the primary induction of IL-4 synthesis in lymph nodes responding to alum-precipitated protein is overwhelmingly in antigen-specific CD4 T cells and is unimpaired in IL-4Ralpha(-/-) mice, which can produce but do not respond to IL-4 and IL-13. Ig class-switching in extra-follicular responses, reflecting Th2 activity, is also unimpaired in these mice. By contrast, 7 days after immunization--when T cells are selecting B cells in germinal centers and T cell priming has occurred--non-responsiveness to IL-4 is associated with smaller germinal centers, increased levels of T-bet and gamma2a switch transcripts and reduced gamma1 and epsilon transcripts. These data indicate that Th2 characteristics acquired during T cell priming and the initial CD4 T cell interaction with B cells are largely IL-4-independent, whereas IL-4 production induced during priming has a significant role in maintaining the Th2 phenotype as T cells select B cells in germinal centers.

Indirect Minor Histocompatibility Antigen Presentation by Allograft Recipient Cells in the Draining Lymph Node Leads to the Activation and Clonal Expansion of CD4+T Cells That Cause Obliterative Airways Disease

Ag recognition by OVA-reactive OT-II (I-Ab restricted) and DO11.10 (I-Ad restricted) TCR-Tg CD4+ T cells after heterotopic transplantation of OVA transgene-expressing tracheal grafts was examined as a model of minor histocompatibility Ag (mHAg)-induced chronic allograft rejection. In response to airway allotransplantation with grafts expressing the OVA transgene, these TCR-Tg CD4+ T cells expressed the activation markers CD69 and CD44, demonstrated evidence of blastogenesis, underwent multiple rounds of cell division leading to their clonal expansion in the draining lymph node, and proceeded to differentiate to a effector/memory T cell phenotype based on a reduction in the expression of CD45RB. These mHAg-specific TCR-Tg CD4+ T cells responded equally well to fully MHC-mismatched tracheas and to class II-deficient allografts, demonstrating that donor mHAg recognition by recipient CD4+ T cells does not rely on Ag presentation by donor-derived APC. The activation of mHAg-specific TCR-Tg CD4+ T cells after their adoptive transfer into recipient mice given MHC-matched, but mHAg-disparate, airway allografts was associated with their movement into the allograft and the near uniform destruction of the transplanted airway tissue secondary to the development of obliterative airways disease. These results demonstrate that an activation of mHAg-reactive CD4+ T cells in the draining lymph node by recipient APC that indirectly express graft mHAg-derived peptide/class II MHC complexes precedes responder T cell proliferation and differentiation, and leads to the eventual migration of these alloreactive T cells to the transplanted airway tissue and the promotion of chronic graft rejection.

T Cell-Intrinsic Expression of c-Rel Regulates Th1 Cell Responses Essential for Resistance to Toxoplasma gondii

The ability of many microbial and inflammatory stimuli to activate members of the Rel/NF-kappaB family of transcription factors is associated with the regulation of innate and adaptive responses required to control infection. Individual family members play distinct roles during different infectious and inflammatory responses. For example, c-Rel is essential for the production of IL-12 in response to LPS, but dispensable for IL-12 production in response to Toxoplasma Ag. To assess the role of c-Rel during immunity to the intracellular pathogen Toxoplasma gondii, wild-type (WT) and c-Rel(-/-) mice were infected with Toxoplasma and the immune response was analyzed. c-Rel(-/-) mice developed severe toxoplasmic encephalitis with increased numbers of parasites compared with WT controls and succumbed to infection within 5-8 wk. Although increased susceptibility of c-Rel(-/-) mice was associated with decreased T cell activation, proliferation, and production of IFN-gamma, these mice were able to generate Th1 effector cells that were present in the brain during chronic infection. In vitro mixing studies using WT and c-Rel(-/-) dendritic cells and WT and c-Rel(-/-) TCR transgenic T cells indicated that c-Rel(-/-) dendritic cells are defective in their ability to stimulate T cell responses. However, when c-Rel(-/-) T cells were transferred into T cell-deficient hosts, early defects in T cell activation, proliferation, and IFN-gamma production persisted, and these mice remained susceptible to infection. Together, these studies indicate that although c-Rel is an important regulator of innate immune responses, it also plays an important role in optimization and maintenance of adaptive T cell responses during infection.

Expression and function of 4-1BB during CD4 versus CD8 T cell responsesin vivo

4-1BBL(-/-) mice have a defect in recall CD8+ T cell responses to viruses, whereas CD4+ T cell responses to virus are unimpaired in these mice. In contrast, both CD4+ and CD8+ T cells respond to 4-1BB ligand (4-1BBL) in vitro. To clarify the role of 4-1BB/4-1BBL in CD4+ versus CD8+ T cell responses in vivo, we compared CD4 (OT-II) and CD8 (OT-I) TCR transgenic T cells responding to the same antigen in an in vivo adoptive transfer model in 4-1BBL(+/+) versus 4-1BBL(-/-) mice. During primary and secondary responses, expression of 4-1BB on in vivo-activated TCR transgenic T cells was earlier and more transient than previously observed in vitro, correlating with expression of the early activation antigen CD69 and preceding the transition to the CD44hi state. Although 4-1BB is expressed early in the primary response, there was no effect of 4-1BBL deficiency on initial CD8 T cell expansion and only a minor effect on initial CD4 T cell expansion. The major effect of 4-1BB/4-1BBL interaction is on the T cell recall response. This is due to effects of 4-1BBL on maintenance of T cell numbers at the end of the primary response with additional effects of 4-1BBL on secondary expansion of T cells.

CD8alpha+ and CD11b+ dendritic cell-restricted MHC class II controls Th1 CD4+ T cell immunity

The activation, proliferation, differentiation, and trafficking of CD4 T cells is central to the development of type I immune responses. MHC class II (MHCII)-bearing dendritic cells (DCs) initiate CD4(+) T cell priming, but the relative contributions of other MHCII(+) APCs to the complete Th1 immune response is less clear. To address this question, we examined Th1 immunity in a mouse model in which I-A(beta)(b) expression was targeted specifically to the DCs of I-A(beta)b-/- mice. MHCII expression is reconstituted in CD11b(+) and CD8alpha(+) DCs, but other DC subtypes, macrophages, B cells, and parenchymal cells lack of expression of the I-A(beta)(b) chain. Presentation of both peptide and protein Ags by these DC subsets is sufficient for Th1 differentiation of Ag-specific CD4(+) T cells in vivo. Thus, Ag-specific CD4(+) T cells are primed to produce Th1 cytokines IL-2 and IFN-gamma. Additionally, proliferation, migration out of lymphoid organs, and the number of effector CD4(+) T cells are appropriately regulated. However, class II-negative B cells cannot receive help and Ag-specific IgG is not produced, confirming the critical MHCII requirement at this stage. These findings indicate that DCs are not only key initiators of the primary response, but provide all of the necessary cognate interactions to control CD4(+) T cell fate during the primary immune response.

Ternary complex factor SAP-1 is required for Erk-mediated thymocyte positive selection

Thymocyte selection and differentiation requires extracellular signal-regulated kinase (Erk) signaling, but transcription factor substrates of Erk in thymocytes are unknown. We have characterized the function of SAP-1 (Elk4), an Erk-regulated transcription factor, in thymocyte development. Early thymocyte development was normal, but single-positive thymocyte and peripheral T cell numbers were reduced, reflecting a T cell-autonomous defect. T cell receptor-induced activation of SAP-1 target genes such as Egr1 was substantially impaired in double-positive thymocytes, although Erk activation was normal. Analysis of T cell receptor transgenes showed that positive selection was reduced by 80-90% in SAP-1-deficient mice; heterozygous mice showed a moderate defect. Negative selection was unimpaired. SAP-1 thus directly links Erk signaling to the transcriptional events required for thymocyte positive selection.

The costimulation-regulated duration of PKB activation controls T cell longevity

A brief antigenic stimulus can promote T cell proliferation, but the duration and nature of intracellular signals required for survival are unclear. Here we show that in the absence of OX40 costimulation, antigen-activated CD4+ cells are short-lived because the activity of protein kinase B (PKB; also known as Akt) is not maintained over time. Activated T cells that express a dominant-negative variant of PKB also undergo apoptosis, reproducing the OX40-deficient phenotype. In contrast, an active form of PKB prevents downregulation of antiapoptotic proteins in OX40-deficient T cells, rescues antigen-induced cell survival in vivo, and controls inflammation in recall responses. Thus, sustained and periodic PKB signaling has an integral role in regulating T cell longevity.

Complex and dynamic redistribution of NF-kappaB signaling intermediates in response to T cell receptor stimulation

The central zone of the supramolecular activation cluster (c-SMAC) is a zone of T cell receptor (TCR) enrichment that forms at a T cell/antigen-presenting cell (APC) junction in response to antigen stimulation. We demonstrate that there is a surprisingly complex relocalization process that brings PKC and Bcl10, two intermediates in TCR activation of NF-kappaB, to the cytoplasmic face of the c-SMAC. TCR activation causes enrichment of PKC at the c-SMAC, followed by Bcl10 relocalization to punctate cytoplasmic structures, often at sites distant from the c-SMAC. These Bcl10 structures then undergo further relocalization, becoming enriched at the c-SMAC. TCR activation of NF-kappaB therefore involves the dynamic relocalization of multiple signaling intermediates, with distinct phases proximal to and distant from the c-SMAC.

Cutting Edge: NF-κB2 Is a Negative Regulator of Dendritic Cell Function

RelB, a member of the NF-kappaB family of transcription factors, is essential for dendritic cell (DC) maturation. Recent findings indicate that RelB is exclusively regulated through its interaction with cytoplasmic NF-kappaB2/p100. The studies presented in this report show that DCs lacking NF-kappaB2 have dramatically enhanced RelB activity, associated with increased MHC class II and costimulatory molecule expression and an enhanced ability to induce CD4(+) T cell responses. These studies identify a novel role for NF-kappaB2 in the negative regulation of RelB-induced DC maturation, with critical consequences for the regulation of adaptive immune responses.

Interleukin 7 regulates the survival and generation of memory CD4 cells

Cytokines, particularly those of the common gamma chain receptor family, provide extrinsic signals that regulate naive CD4 cell survival. Whether these cytokines are required for the maintenance of memory CD4 cells has not been rigorously assessed. In this paper, we examined the contribution of interleukin (IL) 7, a constitutively produced common gamma chain receptor cytokine, to the survival of resting T cell receptor transgenic memory CD4 cells that were generated in vivo. IL-7 mediated the survival and up-regulation of Bcl-2 by resting memory CD4 cells in vitro in the absence of proliferation. Memory CD4 cells persisted for extended periods upon adoptive transfer into intact or lymphopenic recipients, but not in IL-7- mice or in recipients that were rendered deficient in IL-7 by antibody blocking. Both central (CD62L+) and effector (CD62L-) memory phenotype CD4 cells required IL-7 for survival and, in vivo, memory cells were comparable to naive CD4 cells in this regard. Although the generation of primary effector cells from naive CD4 cells and their dissemination to nonlymphoid tissues were not affected by IL-7 deficiency, memory cells failed to subsequently develop in either the lymphoid or nonlymphoid compartments. The results demonstrate that IL-7 can have previously unrecognized roles in the maintenance of memory in the CD4 cell population and in the survival of CD4 cells with a capacity to become memory cells.

MHC Class II-Peptide Complexes in Dendritic Cell Lipid Microdomains Initiate the CD4 Th1 Phenotype

We investigated differentiation of CD4 T cells responding to Ag presented by bone marrow-derived dendritic cells (DC) in association with MHC class II (MHC II) molecules. Peptides encapsulated in liposomes opsonized by IgG were taken up by endocytosis. MHC II-peptide-specific T cells responding to this Ag were polarized to a Th1 cytokine profile in a CD40-, CD28-, MyD88-, and IL-12-dependent manner. Th2 responses were obtained from the same transgenic T cell population exposed to the same DC on which MHC-peptide complexes had dispersed for 48 h following uptake of FcR-targeted liposomes. DC that took up the same FcR-targeted liposomes and then were exposed to methyl-beta-cyclodextrin, which chelates cholesterol and dissociates lipid microdomains, also stimulated Th2 differentiation. Incubation of T cells with DC incubated with peptides directly binding to MHC II resulted in Th2 responses, whether or not the DC were coincubated with opsonized liposomes as a maturation stimulus. CD4 Th1 polarization thus appears to depend on MHC II-peptide complex clustering in DC lipid microdomains and the time between peptide loading and T cell encounter.

Requirements for T Cell-Polarized Tubulation of Class II+ Compartments in Dendritic Cells

Activation of naive CD4 T cells by dendritic cells requires the sequential interaction of many TCR molecules with peptide-class II complexes of the appropriate specificity. Such interaction results in morphological transformation of class II MHC-containing endosomal compartments. In this study, we analyze the requirements for long tubular endosomal structures that polarize toward T cell contact sites using dendritic cells from I-A(b) class II -enhanced green fluorescent protein knock-in mice and I-A(b)-restricted CD4 T cells specific for OVA. Clustering of membrane proteins and ligation of T cell adhesion molecules LFA-1 and CD2 are involved in induction of endosomal tubulation. Activation of T cells increases their ability to induce class II-enhanced green fluorescent protein-positive tubules in dendritic cells, in part through up-regulation of CD40 ligand. Remarkably, and in stark contrast with the result obtained with dendritic cells loaded with intact OVA, OVA peptide added to dendritic cells failed to evoke T cell-polarized endosomal tubulation even though both conditions allowed T cell stimulation. These results suggest the existence of microdomains on the membrane of dendritic cells that allow Ag-specific T cells to evoke tubulation in the dendritic cell.

CD40, but Not CD154, Expression on B Cells Is Necessary for Optimal Primary B Cell Responses

CD40 is an important costimulatory molecule for B cells as well as dendritic cells, monocytes, and other APCs. The ligand for CD40, CD154, is expressed on activated T cells, NK cells, mast cells, basophils, and even activated B cells. Although both CD40(-/-) and CD154(-/-) mice have impaired ability to isotype switch, form germinal centers, make memory B cells, and produce Ab, it is not entirely clear whether these defects are intrinsic to B cells, to other APCs, or to T cells. Using bone marrow chimeric mice, we investigated whether CD40 or CD154 must be expressed on B cells for optimal B cell responses in vivo. We demonstrate that CD40 expression on B cells is required for the generation of germinal centers, isotype switching, and sustained Ab production, even when other APCs express CD40. In contrast, the expression of CD154 on B cells is not required for the generation of germinal centers, isotype switching, or sustained Ab production. In fact, B cell responses are completely normal when CD154 expression is limited exclusively to Ag-specific T cells. These results suggest that the interaction of CD154 expressed by activated CD4 T cells with CD40 expressed by B cells is the primary pathway necessary to achieve B cell activation and differentiation and that CD154 expression on B cells does not noticeably facilitate B cell activation and differentiation.

Development of a novel transgenic mouse for the study of interactions between CD4 and CD8 T cells during graft rejection

The goal of this study was the development of a system in which the cooperative interactions between CD4 and CD8 T cells specific for defined peptides from a single minor histocompatibility antigen could be studied. A transgenic mouse strain that expresses chicken ovalbumin (Act-mOVA) on the surface of all cells in the body was produced as a source of tissues containing such an antigen. Skin grafts from Act-mOVA donors were rapidly and completely rejected by wild-type recipients, but only when both CD4 and CD8 T cells were present. CD4 T cells by themselves caused an incomplete form of rejection characterized by rapid but partial contraction of Act-mOVA grafts. CD8 T cells alone caused complete rejection of Act-mOVA skin grafts but only after a long delay. Adoptively transferred ovalbumin-specific TCR-transgenic CD4 and CD8 T cells were stimulated by Act-mOVA graft antigens and CD8 T-cell accumulation in the grafts was enhanced by specific CD4 T cells. These findings, together with the fact that the ligand for ovalbumin peptide-specific CD8 T cells can be detected in Act-mOVA tissues with an MHC-restricted antibody, make this an ideal system for the study of cooperation between CD4 and CD8 T cells.

TRAF6 is a critical factor for dendritic cell maturation and development

IL-1 receptor (IL-1R)/Toll-like receptor (TLR) family and TNF receptor (TNFR) superfamily members are critical for regulating multiple aspects of dendritic cell (DC) biology. Several signaling pathways associated with each family utilize the adapter molecule, TRAF6, but its role in DCs is unclear. By examining TRAF6-deficient mice and bone marrow (BM) chimeras reconstituted with TRAF6-deficient fetal liver cells, we show that proper DC maturation requires TRAF6. In response to either microbial components or CD40L, TRAF6-deficient DCs fail to upregulate surface expression of MHCII and B7.2, or produce inflammatory cytokines. Moreover, LPS-treated TRAF6-deficient DCs do not exhibit an enhanced capacity to stimulate naive T cells. Interestingly, a major population of splenic DCs, the CD4(+)CD8alpha(-) subset, is nearly absent in both TRAF6-deficient mice and BM chimeras. Together these results indicate that TRAF6 regulates the critical processes required for maturation, activation, and development of DCs, the primary cellular bridge between innate and adaptive immunity.

Leukotriene B4 receptor BLT1 mediates early effector T cell recruitment

Leukotriene B4 (LTB4) was originally described as a potent lipid myeloid cell chemoattractant, rapidly generated from innate immune cells, that activates leukocytes through the G protein-coupled receptor BLT1. We report here that BLT1 is expressed on effector CD4+ T cells generated in vitro as well as in vivo when effector T cells migrate out of the lymphoid compartment and are recruited into peripheral tissues. BLT1 mediated LTB4-induced T helper type 1 (T(H)1) and T(H)2 cell chemotaxis and firm adhesion to endothelial cells under flow, as well as early CD4+ and CD8+ T cell recruitment into the airway in an asthma model. Our findings show that the LTB4-BLT1 pathway is involved in linking early immune system activation and early effector T cell recruitment.

Most lymphoid organ dendritic cell types are phenotypically and functionally immature

Dendritic cells (DCs) have been thought to follow a life history, typified by Langerhans cells (LCs), with 2 major developmental stages: an immature stage that captures antigens in the periphery and a mature stage that presents those antigens in the lymphoid organs. However, a systematic assessment of the maturity of lymphoid organ DCs has been lacking. We have analyzed the maturity of the DC types found in the steady state in the spleen, lymph nodes (LNs), and thymus. The DCs that migrate into the iliac, mesenteric, mediastinal, or subcutaneous LNs from peripheral tissues were mature and therefore could not process and present newly encountered antigens. However, all the other DC types were phenotypically and functionally immature: they expressed low levels of surface major histocompatibility complex class II (MHC II) and CD86, accumulated MHC II in their endosomes, and could present newly encountered antigens. These immature DCs could be induced to mature by culture in vitro or by inoculation of inflammatory stimuli in vivo. Therefore, the lymphoid organs contain a large cohort of immature DCs, most likely for the maintenance of peripheral tolerance, which can respond to infections reaching those organs and mature in situ.

A Critical Role for Stat3 Signaling in Immune Tolerance

Antigen-presenting cells (APCs) can induce T cell activation as well as T cell tolerance. The molecular mechanisms by which APCs regulate this critical decision of the immune system are not well understood. Here we show that Stat3 signaling plays a critical role in the induction of antigen-specific T cell tolerance. Targeted disruption of Stat3 signaling in APCs resulted in priming of antigen-specific CD4(+) T cells in response to an otherwise tolerogenic stimulus in vivo. Furthermore, APCs devoid of Stat3 effectively break antigen-specific T cell anergy in vitro. Conversely, increased Stat3 activity in APCs led to impaired antigen-specific T cell responses. Stat3 signaling provides, therefore, a novel molecular target for manipulation of immune activation/tolerance, a central decision with profound implications in autoimmunity, transplantation, and cancer immunotherapy.

Genetic tagging shows increased frequency and longevity of antigen-presenting, skin-derived dendritic cells in vivo

Dendritic cells (DCs) are key regulators of immune responses that activate naive antigen-specific T lymphocytes. In draining lymph nodes, antigen-bearing DCs are reported to be rare and short-lived. How such small numbers of short-lived DCs can activate rare antigen-specific T cells is unclear. Here we show that after immunization of mouse skins by gene gun, the number of antigen-bearing DCs that migrate to draining lymph node is 100-fold higher than previously estimated and that they persist for approximately 2 weeks. The substantial frequency and longevity of DCs in situ ensures ample antigen presentation and stimulation for the rare antigen-specific T cells in draining lymph nodes.