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

Cell cycle block in anergic T cells during tolerance induction

The induction of anergy, or T cell unresponsiveness to antigen, is preceded by T cell activation and cell division in response to fed antigens. These events parallel the activation observed in T cells following sensitization with antigen and adjuvant. The events that distinguish eventual sensitization versus tolerance remain unclear. Using a T lymphocyte transfer model specific to OVA, we demonstrated previously that oral encounter with antigen leads to functional anergy. Antigen-specific CD4+ T cells nevertheless become activated and cycle briefly after encounter with antigen. In this study, we measured the extent of cell cycling of antigen-specific T cells after oral encounter with their antigen. Whereas T cells cycle on the average of eight times in 4 days after conventional immunization, an abortive proliferation was observed in the draining LN T cells after oral encounter with antigen; OVA-specific T cells divided fewer times after exposure to fed OVA, compared to T cells in mice immunized with OVA. This abortive proliferation is antigen specific and not due to bystander suppression, as coadministration of an unrelated antigen that was previously used as a tolerogen does not alter the degree of abortive proliferation. Measurement of BrdU incorporation in mice that were previously fed ovalbumin indicates that up to 3 days following feeding, OVA-specific cells are actively cycling in vivo. However, by day 4, they have stopped cycling while identical T cells in OVA-sensitized mice continue to cycle. Our results indicate either that tolerance is a default pathway and a secondary stimulus is required at day 3 to progress to sensitization, or that elements that limit cell cycle progression are provided for tolerance induction.

Constitutively active protein kinase B enhances Lck and Erk activities and influences thymocyte selection and activation

Protein kinase B (PKB), a serine threonine kinase is critically involved in cellular proliferation and survival. To characterize its role in T cell development in vivo, we have analyzed transgenic mice that express a membrane-targeted constitutively active version of PKB (myr PKB) in thymocytes and peripheral T cells. We report that myr PKB renders proliferative responses of thymocytes more sensitive to TCR signals by increased and sustained activation of Src kinase Lck and the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway. In addition, the proliferative response of myr PKB T cells is relatively independent of calcium mobilization and calcineurin activity. We also find that myr PKB enhances phosphorylation of glycogen synthase kinase 3, a negative regulator of NFAT and T cell activation, and the recruitment of the adapter protein Cbl-c. Interestingly, we demonstrate that upon TCR/CD3 stimulation of wild-type T cells PKB is translocated into lipid rafts, adding a new role for PKB in TCR-initiated signalosome formation in T cell activation. Localization of transgenic PKB in lipid rafts could contribute to the higher TCR sensitivity of myr PKB thymocytes which is reflected in an increase in positive selection toward the CD4 lineage and variable effects on negative selection depending on the model system analyzed. Thus, our observations clearly indicate a cross-talk between PKB and important signaling molecules downstream of TCR that modulate the thresholds of thymocyte selection and T cell activation.

OX40 (CD134) controls memory T helper 2 cells that drive lung inflammation

Asthma is caused by memory Th2 cells that often arise early in life and persist after repeated encounters with allergen. Although much is known regarding how Th2 cells develop, there is little information about the molecules that regulate memory Th2 cells after they have formed. Here we show that the costimulatory molecule OX40 is expressed on memory CD4 cells. In already sensitized animals, blocking OX40-OX40L interactions at the time of inhalation of aerosolized antigen suppressed memory effector accumulation in lung draining lymph nodes and lung, and prevented eosinophilia, airway hyperreactivity, mucus secretion, and Th2 cyto-kine production. Demonstrating that OX40 signals directly regulate memory T cells, antigen-experienced OX40-deficient T cells were found to divide initially but could not survive and accumulate in large numbers after antigen rechallenge. Thus, OX40-OX40L interactions are pivotal to the efficiency of recall responses regulated by memory Th2 cells.

Absence of preproenkephalin increases the threshold for T cell activation

Certain forms of the neuroendocrine hormone preproenkephalin (PPNK) are produced by T cells, B cells and macrophages. This hormone has been shown to be important in regulating a variety of immune responses; however, the basic mechanisms of this regulation are unknown. Here we examine the ability of CD8 and CD4 PPNK-deficient T lymphocytes to proliferate to antigenic and mitogenic stimuli. We found that lymphocyte activation and proliferation to suboptimal concentrations of both anti-CD3 and antigen was reduced in the absence of PPNK. Proliferation could be rescued by increasing antigen or by co-incubation of PPNK-deficient cells with wild-type cells. These data confirm the importance of neuroendocrine hormones such as PPNK in T cell activation and proliferation and provides a potential mechanism for the regulation of T cell responses by PPNK or its peptide derivatives.

The requirements for Fas-associated death domain signaling in mature T cell activation and survival

Fas-associated death domain (FADD) is a death domain containing cytoplasmic adapter molecule required for the induction of apoptosis by death receptors. Paradoxically, FADD also plays a crucial role in the development and proliferation of T cells. Using T cells from mice expressing a dominant negative form of FADD (FADDdd), activation with anti-TCR Ab and costimulation or exogenous cytokines is profoundly diminished. This is also seen in wild-type primary T cells transduced with the same transgene, demonstrating that FADD signaling is required in normally differentiated T cells. The defective proliferation does not appear to be related to the early events associated with TCR stimulation. Rather, with a block in FADD signaling, stimulated T cells exhibit a high rate of cell death corresponding to the initiation of cell division. Although CD4 T cells exhibit a moderate deficiency, this effect is most profound in CD8 T cells. In vivo, the extent of this defective accumulation is most apparent; lymphocytic choriomenigitis virus-infected FADDdd-expressing mice completely fail to mount an Ag-specific response. These results show that, in a highly regulated fashion, FADD, and most likely caspases, can transduce either a signal for survival or one that leads directly to apoptosis and that the balance between these opposing outcomes is crucial to adaptive immunity.

Visualization of the genesis and fate of isotype-switched B cells during a primary immune response

The life history of isotype-switched B cells is unclear, in part, because of an inability to detect rare antigen-specific B cells at early times during the immune response. To address this issue, a small population of B cells carrying targeted antibody transgenes capable of class switching was monitored in immunized mice. After contacting helper T cells, the first switched B cells appeared in follicles rather than in the red pulp, as was expected. Later, some of the switched B cells transiently occupied the red pulp and marginal zone, whereas others persisted in germinal centers (GCs). Antigen-experienced IgM B cells were rarely found in GCs, indicating that these cells switched rapidly after entering GCs or did not persist in this environment.

p53 deficiency and defective mitotic checkpoint in proliferating T lymphocytes increase chromosomal instability through aberrant exit from mitotic arrest

During the proliferation of T cells for successful immune responses against pathogens, the fine regulation of cell cycle is important to the maintenance of T cell homeostasis and the prevention of lymphoproliferative disorders. However, it remains to be elucidated how the cell cycle is controlled at the mitotic phase in proliferating T cells. Here, we show that during the proliferation of primary T cells, the disruption of the mitotic spindle leads to cell-cycle arrest at mitosis and that prolonged mitotic arrest results in not only apoptosis but also the form of chromosomal instability observed in human cancers. It is interesting that in response to spindle damage, the phosphorylation of BubR1, a mitotic checkpoint kinase, was significantly induced in proliferating T cells, and the expression of the dominant-negative mutant of BubR1 compromised mitotic arrest and subsequent apoptosis and thus led to the augmentation of polyploidy formation. We also show that in response to prolonged spindle damage, the expression of p53 but not of p73 was significantly induced. In addition, following sustained mitotic arrest, p53-deficient T cells were found to be more susceptible to polyploidy formation than the wild type. These results suggest that during flourishing immune response, mitotic checkpoint and p53 play important roles in the prevention of chromosomal instability and in the maintenance of the genomic integrity of proliferating T cells.

Responding naive T cells differ in their sensitivity to Fas engagement: early death of many T cells is compensated by costimulation of surviving T cells

Engagement of Fas (CD95) induces death of activated T cells but can also potentiate T-cell response to CD3 ligation. Yet, the effects of Fas-mediated signals on activation of naive T cells have remained controversial. We followed naive T cells responding under Fas ligation. Ligation of Fas simultaneously with activation by antigen-bearing dendritic cells promoted early death in half of the responding naive murine CD4 T cells. Surprisingly, it simultaneously accelerated cell division and interferon-gamma (IFN-gamma) production among surviving T cells. These cells developed quickly an activation-associated phenotype (CD44(hi), CD62L(lo)), responded vigorously to antigen rechallenge, were partially resistant to subsequent induction of cell death via Fas, and were long-lived in vivo. Compared with cells becoming apoptotic, the surviving cells expressed lower levels of Fas and higher levels of T-cell receptor (TCR), CD4, and interleukin-2 receptor (IL-2R). Their survival was associated with expression of antiapoptotic cellular FLICE-inhibitory protein (c-FLIP), Bcl-X(L), and Bcl-2. Thus, at the time of T-cell activation there is a subtle balance in the effects of Fas ligation that differs on a cell-to-cell basis. Factors that predict cell survival include expression levels of Fas, TCR, CD4, and IL-2R. Early death of some cells and a pronounced response of the surviving cells suggest that Fas ligation can both up- and down-regulate a primary T-cell response.

CD4(+)CD3(-) accessory cells costimulate primed CD4 T cells through OX40 and CD30 at sites where T cells collaborate with B cells

In this report we identify an accessory cell that interacts with primed and memory T cells at sites where they collaborate with B cells. These cells are distinguished from conventional dendritic cells by their lack of response to Flt3 ligand and their inability to process antigen. Unlike dendritic cells, the CD4(+)CD3(-) cells have little CD80 or CD86 expression but do express high levels of the TNF ligands, OX40 ligand and CD30 ligand. We show that Th2-primed cells express the receptors for these TNF ligands and preferentially survive when cocultured with these cells. Furthermore, we show that the preferential survival of OX40(+) T cells and support of memory T cell help for B cells are linked to their association with CD4(+)CD3(-) cells in vivo.

A subset of Toll-like receptor ligands induces cross-presentation by bone marrow-derived dendritic cells

Dendritic cells (DCs) are capable of cross-presenting exogenous Ag to CD8(+) CTLs. Detection of microbial products by Toll-like receptors (TLRs) leads to activation of DCs and subsequent orchestration of an adaptive immune response. We hypothesized that microbial TLR ligands could activate DCs to cross-present Ag to CTLs. Using DCs and CTLs in an in vitro cross-presentation system, we show that a subset of microbial TLR ligands, namely ligands of TLR3 (poly(inosinic-cytidylic) acid) and TLR9 (immunostimulatory CpG DNA), induces cross-presentation. In contrast to presentation of Ag to CD4(+) T cells by immature DCs, TLR-induced cross-presentation is mediated by mature DCs, is independent of endosomal acidification, and relies on cytosolic Ag processing machinery.

Costimulation via OX40L expressed by B cells is sufficient to determine the extent of primary CD4 cell expansion and Th2 cytokine secretion in vivo

The development of effector and memory CD4 cell populations depends upon both T cell receptor (TCR) engagement of peptide/major histocompatibility complex (MHC) class II complexes and ligation of costimulatory molecules with counter receptors on antigen-presenting cells (APCs). We showed previously that sustained interactions with APCs could be crucial for optimal expansion of CD4 cells and for development of effectors that secrete cytokines associated with Th2 cells. Using an adoptive transfer model with TCR transgenic CD4 cells, we now show that responses of CD4 cells primed in B cell-deficient mice become aborted, but are fully restored upon the transfer of activated B cells. Although B cells have the capacity to secrete multiple cytokines that could affect CD4 priming, including IL-4, we were unable to distinguish a role for cytokines that are secreted by B cells. However, B cell costimulation via the OX40L/OX40 pathway that has been implicated in CD4 cell expansion, survival, and Th2 development was required. Th2 but not Th1 responses were impaired in OX40L-deficient recipients and normal responses were restored with OX40L sufficient B cells. The results suggest that without engagement of OX40L on B cells, CD4 cell responses to many protein Ag would be dominated by Th1 cytokines. These data have important implications for strategies to achieve optimal priming of CD4 subsets.

Combined deficiency of p50 and cRel in CD4+ T cells reveals an essential requirement for nuclear factor kappaB in regulating mature T cell survival and in vivo function

Signaling pathways involved in regulating T cell proliferation and survival are not well understood. Here we have investigated a possible role of the nuclear factor (NF)-kappaB pathway in regulating mature T cell function by using CD4+ T cells from p50-/- cRel-/- mice, which exhibit virtually no inducible kappaB site binding activity. Studies with these mice indicate an essential role of T cell receptor (TCR)-induced NF-kappaB in regulating interleukin (IL)-2 expression, cell cycle entry, and survival of T cells. Our results further indicate that NF-kappaB regulates TCR-induced expression of antiapoptotic Bcl-2 family members. Strikingly, retroviral transduction of CD4+ T cells with the NF-kappaB-inducing IkappaB kinase beta showed that NF-kappaB activation is not only necessary but also sufficient for T cell survival. In contrast, our results indicate a lack of involvement of NF-kappaB in both IL-2 and Akt-induced survival pathways. In vivo, p50-/- cRel-/- mice showed impaired superantigen-induced T cell responses as well as decreased numbers of effector/memory and regulatory CD4+ T cells. These findings provide the first demonstration of a role for NF-kappaB proteins in regulating T cell function in vivo and establish a critically important function of NF-kappaB in TCR-induced regulation of survival.

Preferential accumulation of antigen-specific effector CD4 T cells at an antigen injection site involves CD62E-dependent migration but not local proliferation

The migration of antigen-specific T cells to nonlymphoid tissues is thought to be important for the elimination of foreign antigens from the body. However, recent results showing the migration of activated T cells into many nonlymphoid tissues raised the possibility that antigen-specific T cells do not migrate preferentially to nonlymphoid tissues containing antigen. We addressed this question by tracking antigen-specific CD4 T cells in the whole body after a localized subcutaneous antigen injection. Antigen-specific CD4 T cells proliferated in the skin-draining lymph nodes and the cells that underwent the most cell divisions acquired the ability to bind to CD62P. As time passed, CD62P-binding antigen-specific CD4 T cells with interferon gamma production potential accumulated preferentially at the site of antigen injection but only in recipients that expressed CD62E. Surprisingly, these T cells did not proliferate in the injection site despite showing evidence of more cell divisions than the T cells in the draining lymph nodes. The results suggest that the most divided effector CD4 T cells from the lymph nodes enter the site of antigen deposition via recognition of CD62E on blood vessels and are retained there in a nonproliferative state via recognition of peptide-major histocompatibility complex II molecules.

Role of the polymeric Ig receptor in mucosal B cell homeostasis

Secretory IgA (SIgA) is the most characteristic component of the mucosal immune system and has long been considered the major protective factor that prevents pathogens from invading hosts through the mucosae. Recent studies, however, have suggested that complete immunity against a range of mucosal bacterial and viral pathogens can be achieved in the absence of IgA. Therefore, to further dissect the role of SIgA, we generated mice deficient in the polymeric Ig receptor (pIgR(-/-) mice). As a result of an inability to transport dimeric IgA to the secretions, pIgR(-/-) mice are deficient in SIgA and accumulate circulating dimeric IgA, with serum levels 100-fold greater than those observed in normal mice. Examination of lamina propria mononuclear cells showed that pIgR(-/-) mice had approximately 3 times as many IgA-secreting cells as C57BL/6 mice. Further analysis showed that these cells displayed the differentiated IgA(+) B220(-) phenotype and accounted for a 2-fold increase in the number of lamina propria blast cells in the pIgR(-/-) mice. Subsequent experiments showed that OVA-specific CD4(+) T cell expansion following OVA feeding was not elevated in pIgR(-/-) mice. Furthermore, no differences in CD8(+) T cell tolerance or induction of influenza virus-specific CD8(+) T cells were detected in pIgR(-/-) mice compared with controls. Therefore, while SIgA is clearly involved in maintaining some parameters of mucosal homeostasis in the intestine, the mechanisms associated with its barrier function and the clinical consequences of its deficiency are yet to be identified.

Crystal structure of MHC class II I-Ab in complex with a human CLIP peptide: prediction of an I-Ab peptide-binding motif

Association between the class II major histocompatibility complex (MHC) and the class II invariant chain-associated peptide (CLIP) occurs naturally as an intermediate step in the MHC class II processing pathway. Here, we report the crystal structure of the murine class II MHC molecule I-A(b) in complex with human CLIP at 2.15A resolution. The structure of I-A(b) accounts, via the peptide-binding groove's unique physicochemistry, for the distinct peptide repertoire bound by this allele. CLIP adopts a similar conformation to peptides bound by other I-A alleles, reinforcing the notion that CLIP is presented as a conventional peptide antigen. When compared to the related HLA-DR3/CLIP complex structure, the CLIP peptide displays a slightly different conformation and distinct interaction pattern with residues in I-A(b). In addition, after examining the published sequences of peptides presented by I-A(b), we discuss the possibility of predicting peptide alignment in the I-A(b) binding groove using a simple scoring matrix.

Competition for self ligands restrains homeostatic proliferation of naive CD4 T cells

T cell antigen receptor (TCR) diversity is a critical feature of adaptive immunity. However, restriction of TCR diversity is a potential risk during immune reconstitution by homeostatic proliferation. What peripheral mechanisms are in place to maintain TCR diversity during recovery from lymphopenia? Here, we examine competition between several monoclonal CD4 T cell populations in RAG(-/-) and TCR Tg RAG(-/-) environments. The results suggest that specific self ligands constitute a critical limiting resource essential for homeostatic proliferation of naive CD4 T cells. In addition, T cells ignore large numbers of competitors as long as their TCR specificity is different and other non-MHC resources are not limiting. Therefore, the numbers of self ligands expressed in the periphery set the limits on TCR diversity.

Without peripheral interference, thymic deletion is mediated in a cohort of double-positive cells without classical activation

Peripheral activation can cause bystander thymocyte death by eliciting a "cytokine storm." This event complicates in vivo studies using exogenous ligand-induced models of negative selection. A stable transgenic model that selectively eliminates peripheral CD4 cells has allowed us to analyze negative selection as direct cognate events in two T cell receptor transgenic mice, OT-II and DO11. Whereas cognate peptide induced a massive deletion in double-positive (DP) cells in mice with peripheral CD4 cells, this DP deletion was modest in mice lacking peripheral CD4 cells. Using BrdUrd and annexin V staining, we found that negative selection primarily occurs in a cohort of DP cells and the absence of single-positive (SP) cells is largely caused by reduction in the cohort of DP precursors. Moreover, the fates of DP cells and SP cells after antigen exposure were vastly different. Whereas SP cells up-regulated uniformly their CD69 and CD44 levels, increased their cell size, and survived after antigen exposure, DP cells had less CD69 and CD44 up-regulation, no size change, and promptly died. Thus, negative selection represents an "abortive" activation different from activation-induced cell death of mature T cells.

Dynamic changes during the immune response in T cell-antigen-presenting cell clusters isolated from lymph nodes

Activation of antigen-specific T cells by mature dendritic cells in secondary lymphoid organs is a key control point of the adaptive immune response. Here we describe the ex vivo isolation of preformed multicellular clusters between T cells and antigen-presenting cells. Adoptively transferred, antigen-specific T cells segregated into individual clusters where their activation and proliferation was initiated in vivo. Transit of the T cell cohort through the cluster compartment required 32-36 h. The precise timing of the response to agonistic epitopes was remarkably invariant regardless of the T cell lineage, the major histocompatibility complex haplotype, and the antigen dose. Interestingly, initiation of cell division of T cells specific for a subdominant epitope and a weak agonist was delayed by 6 h. The results provide a basis for the analysis of short range, mutual cell-cell interactions within such confined microenvironments.

T cell accumulation in B cell follicles is regulated by dendritic cells and is independent of B cell activation

We investigated the mechanism of CD4 T cell accumulation in B cell follicles after immunization. Follicular T cell numbers were correlated with the number of B cells, indicating B cell control of the niche that T cells occupy. Despite this, we found no role for B cells in the follicular migration of T cells. Instead, T cells are induced to migrate into B cell follicles entirely as a result of interaction with dendritic cells (DCs). Migration relies on CD40-dependent maturation of DCs, as it did not occur in CD40-deficient mice but was reconstituted with CD40(+) DCs. Restoration was not achieved by the activation of DCs with bacterial activators (e.g., lipopolysaccharide, CpG), but was by the injection of OX40L-huIgG1 fusion protein. Crucially, the up-regulation of OX40L (on antigen-presenting cells) and CXCR-5 (on T cells) are CD40-dependent events and we show that T cells do not migrate to follicles in immunized OX40-deficient mice.

Suppressor of cytokine signaling-1 has IFN-gamma-independent actions in T cell homeostasis

Suppressor of cytokine signaling (SOCS)-1 is a member of a family of proteins that negatively regulate cytokine signaling pathways. We have previously established that SOCS-1 is a key regulator of IFN-gamma signaling and that IFN-gamma is responsible for the complex inflammatory disease that leads to the death of SOCS-1-deficient mice. In this study, we provide evidence that SOCS-1 is also a critical regulator of IFN-gamma-independent immunoregulatory factors. Mice lacking both SOCS-1 and IFN-gamma, although outwardly healthy, have clear abnormalities in their immune system, including a reduced ratio of CD4:CD8 T cells in lymphoid tissues and increased expression of T cell activation markers. To examine the contribution of TCR Ag specificity to these immune defects, we have generated two lines of SOCS-1-deficient mice expressing a transgenic TCR specific for an exogenous Ag, OVA (OT-I and OT-II). Although TCR transgenic SOCS-1(-/-) mice have a longer lifespan than nontransgenic SOCS-1(-/-) mice, they still die as young adults with inflammatory disease and the TCR transgenic SOCS-1(-/-) T cells appear activated despite the absence of OVA. This suggests that both Ag-dependent and -independent mechanisms contribute to the disease in SOCS-1-deficient mice. Thus, SOCS-1 is a critical regulator of T cell activation and homeostasis, and its influence extends beyond regulating IFN-gamma signaling.

The Wiskott-Aldrich syndrome protein acts downstream of CD2 and the CD2AP and PSTPIP1 adaptors to promote formation of the immunological synapse

The Wiskott-Aldrich syndrome protein (WASp) couples actin cytoskeletal rearrangement to T cell activation, but the mechanisms involved are unknown. Here, we show that antigen-induced formation of T cell:APC conjugates and synapses is abrogated in WASp-deficient T cells and that CD2 engagement evokes interactions between the proline-rich region required for WASp translocation to the synapse and the PSTPIP1 adaptor SH3 domain and between the PSTPIp1 coiled-coil domain and both CD2 and another CD2-binding adaptor, CD2AP. The induced colocalization of these proteins at the synapse is disrupted by expression of coiled-coil domain-deleted PSTPIP1. These data, together with the impairment in CD2-induced actin polymerization observed in WASp-deficient cells, suggest that PSTPIP1 acts downstream of CD2/CD2AP to link CD2 engagement to the WASp-evoked actin polymerization required for synapse formation and T cell activation.

Detection of low-avidity CD4+ T cells using recombinant artificial APC: following the antiovalbumin immune response

Subtle differences oppose CD4+ to CD8+ T cell physiologies that lead to different arrays of effector functions. Interestingly, this dichotomy has also unexpected practical consequences such as the inefficacy of many MHC class II tetramers in detecting specific CD4+ T cells. As a mean to study the CD4+ anti-OVA response in H-2(d) and H-2(b) genetic backgrounds, we developed I-A(d)- and I-A(b)-OVA recombinant MHC monomers and tetramers. We were able to show that in this particular system, despite normal biological activity, MHC class II tetramers failed to stain specific T cells. This failure was shown to be associated with a lack of cooperation between binding sites within the tetramer as measured by surface plasmon resonance. This limited cooperativeness translated into a low "functional avidity" and very transient binding of the tetramers to T cells. To overcome this biophysical barrier, recombinant artificial APC that display MHC molecules in a lipid bilayer were developed. The plasticity and size of the MHC-bearing fluorescent liposomes allowed binding to Ag-specific T cells and the detection of low numbers of anti-OVA T cells following immunization. The same liposomes were able, at 37 degrees C, to induce the full reorganization of the T cell signaling molecules and the formation of an immunological synapse. Artificial APC will allow T cell detection and the dissection of the molecular events of T cell activation and will help us understand the fundamental differences between CD4+ and CD8+ T cells.

Murine plasmacytoid pre-dendritic cells generated from Flt3 ligand-supplemented bone marrow cultures are immature APCs

The putative counterparts of human plasmacytoid pre-dendritic cells (pDCs) have been described in vivo in mouse models and very recently in an in vitro culture system. In this study, we report that large numbers of bone marrow-derived murine CD11c(+)B220(+) pDCs can be generated with Flt3 ligand (FL) as the sole exogenous differentiation/growth factor and that pDC generation is regulated in vivo by FL because FL-deficient mice showed a major reduction in splenic pDC numbers. We extensively analyzed bone marrow-derived CD11c(+)B220(+) pDCs and described their immature APC phenotype based on MHC class II, activation markers, and chemokine receptor level of expression. CD11c(+)B220(+) pDCs showed a nonoverlapping Toll-like receptor pattern of expression distinct from that of classical CD11c(+)B220(-) dendritic cells and were poor T cell stimulators. Stimulation of CD11c(+)B220(+) pDCs with oligodeoxynucleotides containing certain CpG motifs plus CD40 ligand plus GM-CSF led to increased MHC class II, CD80, CD86, and CD8alpha expression levels, to a switch in chemokine receptor expression that affected their migration, to IFN-alpha and IL-12 secretion, and to the acquisition of priming capacities for both CD4(+) and CD8(+) OVA-specific TCR-transgenic naive T cells. Thus, the in vitro generation of murine pDCs may serve as a useful tool to further investigate pDC biology as well as the potential role of these cells in viral immunity and other settings.

Nonlymphoid Fas ligand in peptide-induced peripheral lymphocyte deletion

Peripheral lymphocyte deletion is required for reduction of lymphocyte numbers after expansion in response to antigen. Peripheral deletion is mediated in part by the activation of apoptosis by engagement of the death receptor, Fas (CD95), by its ligand, Fas ligand (FasL; CD95L), among other mechanisms. Here we used T cell receptor (TCR) transgenic animals to examine the role of inducible expression of nonlymphoid FasL in response to peptide antigen. Antigenic challenge of TCR transgenic mice resulted in increased expression of FasL in a number of nonlymphoid tissues including the epithelium of the small intestine. Similar results were obtained in an adoptive transfer system in which TCR transgenic T cells were transferred into recipient animals. The functional relevance of nonlymphoid FasL in peripheral deletion is supported by the observation that FasL-deficient gld animals showed a significantly reduced rate of clearance of transferred antigen-specific lymphocytes, although the lymphocytes themselves were wild type for FasL. These observations were supported further by studies in a transgenic mouse model where lacZ was expressed under the control of the proximal promoter of the FasL gene. Using these transgenic mice, we observed induced activity of the FasL promoter in intestinal epithelial cells throughout the crypts and villi, where we also observed infiltration of activated T cells. These data demonstrate that nonlymphoid FasL is expressed in response to peripheral T cell activation and participates in the regulation of T cells that infiltrate peripheral tissues.

Inefficient response of T lymphocytes to glycosylphosphatidylinositol anchor-negative cells: implications for paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a hematopoietic stem cell disorder in which clonal cells defective in glycosylphosphatidylinositol (GPI) biosynthesis are expanded, leading to complement-mediated hemolysis. PNH is often associated with bone marrow suppressive conditions, such as aplastic anemia. One hypothetical mechanism for the clonal expansion of GPI(-) cells in PNH is that the mutant cells escape attack by autoreactive cytotoxic cells that are thought to be responsible for aplastic anemia. Here we studied 2 model systems. First, we made pairs of GPI(+) and GPI(-) EL4 cells that expressed major histocompatibility complex (MHC) class II molecules and various types of ovalbumin. When the GPI-anchored form of ovalbumin was expressed on GPI(+) and GPI(-) cells, only the GPI(+) cells presented ovalbumin to ovalbumin-specific CD4(+) T cells, indicating that if a putative autoantigen recognized by cytotoxic cells is a GPI-anchored protein, GPI(-) cells are less sensitive to cytotoxic cells. Second, antigen-specific as well as alloreactive CD4(+) T cells responded less efficiently to GPI(-) than GPI(+) cells in proliferation assays. In vivo, when GPI(-) and GPI(+) fetal liver cells, and CD4(+) T cells alloreactive to them, were cotransplanted into irradiated hosts, the contribution of GPI(-) cells in peripheral blood cells was significantly higher than that of GPI(+) cells. The results obtained with the second model suggest that certain GPI-anchored protein on target cells is important for recognition by T cells. These results provide the first experimental evidence for the hypothesis that GPI(-) cells escape from immunologic attack.

IgE cross-linking or lipopolysaccharide treatment induces recruitment of Th2 cells to the lung in the absence of specific antigen

We previously showed that Th1 cells can increase recruitment of Th2 cells to the lungs even in the absence of the Th2-specific Ag. The fact that Th2 recruitment is independent from the Th2 cell Ag suggested that Th1 cells may support Th2 cell recruitment using their Ag-nonspecific proinflammatory functions. To investigate the potential for inflammatory stimuli that are distinct from Ag-specific signals to affect the recruitment of T cells, we tested whether cross-linking of IgE or treatment with LPS modulated influx of Th2 cells into the airways in the presence or absence of inhaled Ag. When naive mice that had been treated with OVA-specific Th2 cells and passively sensitized with anti-DNP IgE were challenged by intranasal administration of either DNP-haptenated OVA or DNP-BSA, increased numbers of Th2 cells were recruited to the lung compared with mice challenged intranasally with OVA alone. Intranasal administration of LPS also increased recruitment of Th2 cells to the airways. These two distinct inflammatory stimuli increased the numbers of recruited Th2 cells equally with or without concurrent challenge using the cognate Th2 Ag. This Ag-independent recruitment of Th2 cells to the lung was not associated with localization of these cells to the regional lymph nodes and was independent of Th2 cell activation. Interestingly, P- or E-selectin contributed to Th2 cell recruitment to the lung. These data suggest that Th2 cells of the adaptive immune response are similar to cells of the innate immune response in their lack of requirement for protein Ag to initiate cell recruitment. They demonstrate further that recruitment can occur independently of Ag-dependent activation.

Constitutive versus activation-dependent cross-presentation of immune complexes by CD8(+) and CD8(-) dendritic cells in vivo

Murine splenic dendritic cells (DCs) can be divided into two subsets based on CD8alpha expression, but the specific role of each subset in stimulation of T cells is largely unknown. An important function of DCs is the ability to take up exogenous antigens and cross-present them in the context of major histocompatibility complex (MHC) class I molecules to CD8(+) T cells. We previously demonstrated that, when cell-associated ovalbumin (OVA) is injected into mice, only the CD8(+) DC subset cross-presents OVA in the context of MHC class I. In contrast to this selectivity with cell-associated antigen, we show here that both DC subsets isolated from mice injected with OVA/anti-OVA immune complexes (OVA-IC) cross-present OVA to CD8(+) T cells. The use of immunoglobulin G Fc receptor (Fc(gamma)R) common gamma-chain-deficient mice revealed that the cross-presentation by CD8(-) DCs depended on the expression of gamma-chain-containing activating FcgammaRs, whereas cross-presentation by CD8(+) DCs was not reduced in gamma-chain-deficient mice. These results suggest that although CD8(+) DCs constitutively cross-present exogenous antigens in the context of MHC class I molecules, CD8(-) DCs only do so after activation, such as via ligation of Fc(gamma)Rs. Cross-presentation of immune complexes may play an important role in autoimmune diseases and the therapeutic effect of antitumor antibodies.

Developmental kinetics and lifespan of dendritic cells in mouse lymphoid organs

The labeling kinetics of 5 dendritic cell (DC) subtypes within the lymphoid organs of healthy laboratory mice during continuous administration of bromodeoxyuridine (BrdU) was determined to investigate developmental relationships and determine turnover rates. Individual DC subtypes behaved as products of separate developmental streams, at least as far back as their dividing precursors. The rate of labeling varied with the lymphoid organ and the DC subtype. Labeling was faster overall in spleen and mesenteric lymph nodes (LNs) and slower in thymus and skin-draining LNs. The CD8+ DC subtype displayed the most rapid turnover, with a uniformly short (3-day) lifespan in spleen but with distinct short-lived and longer-lived subgroups in thymus. All the skin-derived DCs in LNs showed delayed and slow BrdU labeling, indicating a long overall lifespan; however, this was shown to reflect a long residence time in skin rather than a long-duration presenting antigen in the draining LN. Epidermal-derived Langerhans DCs displayed longer BrdU labeling lag and slower overall turnover than the dermal-derived DCs, and the movement of fluorescent Langerhans DC from skin to LN was slower than that of dermal DCs following skin painting with a fluorescent dye. However, once they arrived in lymphoid organs, all DCs present in healthy, uninfected mice displayed a rapid turnover, and this turnover was even faster after antigenic or microbial product stimulation.

T-cell engagement of dendritic cells rapidly rearranges MHC class II transport

Assembly of major histocompatibility complex (MHC) molecules, which present antigen in the form of short peptides to T lymphocytes, occurs in the endoplasmic reticulum; once assembled, these molecules travel from the endoplasmic reticulum to their final destination. MHC class II molecules follow a route that takes them by means of the endocytic pathway, where they acquire peptide, to the cell surface. The transport of MHC class II molecules in 'professional' antigen-presenting cells (APCs) is subject to tight control and responds to inflammatory stimuli such as lipopolysaccharide. To study class II transport in live APCs, we replaced the mouse MHC class II gene with a version that codes for a class II molecule tagged with enhanced green fluorescent protein (EGFP). The resulting mice are immunologically indistinguishable from wild type. In bone-marrow-derived dendritic cells, we observed class II molecules in late endocytic structures with transport patterns similar to those in Langerhans cells observed in situ. We show that tubular endosomes extend intracellularly and polarize towards the interacting T cell, but only when antigen-laden dendritic cells encounter T cells of the appropriate specificity. We propose that such tubulation serves to facilitate the ensuing T-cell response.

Generation of functional antigen-specific T cells in defined genetic backgrounds by retrovirus-mediated expression of TCR cDNAs in hematopoietic precursor cells

We have developed an alternative to transgenesis for producing antigen-specific T cells in vivo. In this system, clonal naive T cells with defined antigen specificity are generated by retrovirus-mediated expression of T cell antigen receptor cDNAs in RAG1-deficient murine hematopoietic precursor cells. These T cells can be stimulated to proliferate and produce cytokines by exposure to antigen in vitro, and they become activated and expand in vivo after immunization. IL-2-deficient T cells generated by this technique show decreased proliferation and cytokine production, both of which can be rescued by exogenous addition of this growth factor. Thus, retrovirus-mediated expression of T cell antigen receptor cDNAs in hematopoietic precursor cells permits the rapid and efficient analysis of the life history of antigen-specific T cells in different genetic backgrounds and may allow for the long-term production of antigen-specific T cells with different functional properties for prophylactic and therapeutic purposes.

Characterization of two TCR transgenic mouse lines specific for herpes simplex virus

To better understand the T cell-mediated processes involved in the immune response to herpes simplex virus type 1 (HSV-1)infection, two HSV-specific T cell receptor (TCR) transgenic mouse lines were produced. These mice (gBT-I.1 and gBT-I.3) are MHC class I-restricted and specific for the immunodominant peptide from HSV glycoprotein B (gB), gB498-505. Although derived from the same clone, the mice differ in the chromosomal location of the TCR transgenes and show marked differences in TCR alpha/beta expression on both CD4+ and CD8+ cells in the thymus. Despite this, peripheral CD8+ Tcells from both mice express equally high levels of the transgenic TCR and bind the KbgB498-505 tetramer to the same degree. In concordance with this, both were shown to respond equally well in vitro upon stimulation with the gB498-505 peptide or HSV-infected cells. These data show that selection of broadly equivalent peripheral T-cell subsets can occur in the presence of distinctly different thymic T-cell subsets.

The cross-priming APC requires a Rel-dependent signal to induce CTL

Induction of OVA-specific CTL by cross-priming requires help from CD4 T cells, which use CD154 to signal CD40 on the APC. To further dissect the molecular pathways involved in cross-priming, we examined the role of Rel, an NF-kappaB family member. c-rel(-/-) mice failed to generate OVA-specific CTL by cross-priming, but could induce CTL to HSV-1. Using chimeric mice, Rel expression was shown to be required by the APC, but not by the T cells. Notably, the deficiency in Rel could be overcome by triggering CD40, implying that the APC required Rel before receipt of the CD40 signal. These data suggest that the cross-priming APC must receive two signals before it can stimulate CTL. The first signal is Rel dependent and is required before activation of CD4 helper T cells, which then deliver the second signal using CD154 to trigger CD40.

CD53, a thymocyte selection marker whose induction requires a lower affinity TCR-MHC interaction than CD69, but is up-regulated with slower kinetics

The molecular mechanisms that govern the survival, maturation and export of thymocytes are the subject of intense study, and candidates for involvement in these processes might be identified by their differential expression during thymocyte selection. One such molecule is the tetraspanin CD53, which is not expressed on most CD4(+)CD8(+) double-positive (DP) cells in the normal mouse. We have examined CD53 expression on DP from several class I- and class II-restricted TCR transgenic (Tg) mice, and have found a strong correlation between CD53 expression and positive selection. CD53 expression in DP was formally demonstrated to be dependent upon MHC recognition as evidenced by studying DP from MHC-deficient mice which totally lack expression of this molecule. This link between selection and CD53 expression was reminiscent of CD69, and indeed the majority of selected DP from normal mice that express CD53 also express CD69. We compared CD53 and CD69 induction in vitro using pre-selected thymocytes from TCR-Tg mice that were stimulated either with mAb against TCR or with antigen-presenting cells (APC) pulsed with peptides. The data shows that with either stimulus, CD69 is induced rapidly on the thymocyte surface with expression detected in as little as 2 h. CD53 induction is slower with maximal expression taking up to 20 h. We also stimulated pre-selected thymocytes from the OT-1 TCR-Tg strain with APC pulsed with peptides of varying affinities for the TCR. Here low-affinity peptides which induce CD69 expression poorly were able to induce significant levels of CD53 expression. These data demonstrate that the induction of CD53 and CD69 upon selection is not identical. Thus a combination of the CD69 and CD53 selection markers may be a powerful tool to isolate thymocytes that have either been very recently selected or have arisen from differing MHC--TCR affinity interactions during selection.

BH3-only Bcl-2 family member Bim is required for apoptosis of autoreactive thymocytes

During lymphocyte development, the assembly of genes coding for antigen receptors occurs by the combinatorial linking of gene segments. The stochastic nature of this process gives rise to lymphocytes that can recognize self-antigens, thereby having the potential to induce autoimmune disease. Such autoreactive lymphocytes can be silenced by developmental arrest or unresponsiveness (anergy), or can be deleted from the repertoire by cell death. In the thymus, developing T lymphocytes (thymocytes) bearing a T-cell receptor (TCR)-CD3 complex that engages self-antigens are induced to undergo programmed cell death (apoptosis), but the mechanisms ensuring this 'negative selection' are unclear. We now report that thymocytes lacking the pro-apoptotic Bcl-2 family member Bim (also known as Bcl2l11) are refractory to apoptosis induced by TCR-CD3 stimulation. Moreover, in transgenic mice expressing autoreactive TCRs that provoke widespread deletion, Bim deficiency severely impaired thymocyte killing. TCR ligation upregulated Bim expression and promoted interaction of Bim with Bcl-XL, inhibiting its survival function. These findings identify Bim as an essential initiator of apoptosis in thymocyte-negative selection.

Cutting edge: CD4 and CD8 T cells are intrinsically different in their proliferative responses

In this study, we compared the proliferation and differentiation of Ag-specific CD4 and CD8 T cells following Listeria infection. Our results show that CD4 T cells responding to infection divide a limited number of times, with progeny exhibiting proliferative arrest in early divisions. Even with increased infectious doses, CD4 T cells display this restricted proliferative pattern and are not driven to undergo extensive clonal expansion. This is in striking contrast to CD8 T cells, which undergo extensive proliferation in response to infection. These differences are also evident when CD4 and CD8 T cells receive uniform anti-CD3 stimulation in vitro. Together, these results suggest that CD4 and CD8 T cells are programmed to undergo limited and extensive proliferation, respectively, to suit their function as regulator and effector cells.

Animal models of autoimmune liver disease

Autoimmune liver diseases in humans are characterized by chronic active hepatitis with serum autoantibodies, hypergammaglobulinemia and liver pathology showing necroinflammatory disease and fibrosis. There are an increasing number of autoantigens believed to be associated with various autoimmune liver diseases. This review will briefly outline human autoimmune hepatitis and the immunology of the liver. Various murine models of liver inflammation will be discussed, including transgenic and non-transgenic models, with emphasis on how these models aid in our knowledge of the mechanisms of disease development and chronicity. There are limitations with all of the models, including a preponderance of T-cell-focused responses. Murine models do not easily develop fibrosis, a hallmark of autoimmune hepatitis in humans. Different experimental models may not reach the same conclusions with differences between immune responses. However, this multiplicity of responses does not necessarily imply that these models are inappropriate for the study of liver immunology and autoimmune liver diseases, as different autoantigens may induce different liver responses. Knowledge of how the liver differs from other immune organs is essential to further our understanding of liver-specific autoimmunity. The plethora of antigens implicated in autoimmune hepatitis in humans predicts that multiple mechanisms may play a role in precipitating disease in the susceptible individual.

Roles for thrombin and fibrin(ogen) in cytokine/chemokine production and macrophage adhesion in vivo

Extravascular coagulation leading to fibrin deposition accompanies many immune and inflammatory responses. Although recognized by pathologists for decades, and probably pathologic under certain conditions, the physiologic functions of extravascular coagulation remain to be fully defined. This study demonstrates that thrombin can activate macrophage adhesion and prompt interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) production in vivo. Peritoneal macrophages were elicited with thioglycollate (TG) and then activated in situ, either by intraperitoneal injection of lipopolysaccharide (LPS) or by injection of antigen into mice bearing antigen-primed T cells. Others previously established that such treatments stimulate macrophage adhesion to the mesothelial lining of the peritoneal cavity. The present study demonstrates that thrombin functions in this process, as macrophage adhesion was suppressed by Refludan, a highly specific thrombin antagonist, and induced by direct peritoneal administration of purified thrombin. Although recent studies established that protease activated receptor 1 (PAR-1) mediates some of thrombin's proinflammatory activities macrophage adhesion occurred normally in PAR-1-deficient mice. However, adhesion was suppressed in fibrin(ogen)-deficient mice, suggesting that fibrin formation stimulates macrophage adhesion in vivo. This study also suggests that fibrin regulates chemokine/cytokine production in vivo, as direct injection of thrombin stimulated peritoneal accumulation of IL-6 and MCP-1 in a fibrin(ogen)-dependent manner. Given that prior studies have clearly established inflammatory roles for PAR-1, thrombin probably has pleiotropic functions during inflammation, stimulating vasodilation and mast cell degranulation via PAR-1, and activating cytokine/chemokine production and macrophage adhesion via fibrin(ogen).

Transient blockade of CD40 ligand dissociates pathogenic from protective mucosal immunity

Antigen administration via oral and other mucosal routes can suppress systemic immunity to the antigen and has been used to prevent experimental autoimmune disease. This approach may prove ineffective or even harmful if it leads to a concomitant induction of cytotoxic T lymphocytes (CTLs), and indeed, mucosal administration of the model antigen ovalbumin (OVA) has been shown to elicit CTL activation while simultaneously inducing oral tolerance. Here we show that induction by oral OVA of CTLs in wild-type mice, and of diabetes in mice expressing OVA transgenically in pancreatic beta cells, can be prevented by transiently blocking the CD40 ligand (CD40L). However, CD40L blockade did not diminish oral tolerance, as measured by suppression of systemic OVA-primed T cell proliferation, IFN-gamma secretion, and Ab production. Consistent with these findings, mice lacking CD40 expression could be orally tolerized to OVA. Transient CD40L blockade therefore dissociates pathogenic from protective immunity and should enhance the efficacy and safety of oral tolerance for preventing autoimmune disease.

Presented antigen from damaged pancreatic beta cells activates autoreactive T cells in virus-mediated autoimmune diabetes

The induction of autoimmunity by viruses has been attributed to numerous mechanisms. In mice, coxsackievirus B4 (CB4) induces insulin-dependent diabetes mellitus (IDDM) resembling the final step of disease progression in humans. The immune response following the viral insult clearly precipitates IDDM. However, the molecular pathway between viral infection and the subsequent activation of T cells specific for islet antigen has not been elucidated. These T cells could become activated through exposure to sequestered antigens released by damaged beta cells, or they could have responded to factors secreted by the inflammatory response itself. To distinguish between these possibilities, we treated mice harboring a diabetogenic T cell repertoire with either the islet-damaging agent streptozotocin (STZ) or poly I:C, which nonspecifically activates T cells. Significantly, only treatment of mice with STZ resulted in IDDM and mimicked the effects observed following CB4 infection. Furthermore, antigen-presenting cells from STZ-treated mice were shown to directly activate autoreactive T cells and induce diabetes. Therefore, the primary role of CB4 in the precipitation of IDDM is to damage tissue, causing release and presentation of sequestered islet antigen. These events stimulate autoreactive T cells and thereby initiate disease.

Homeostasis and anergy of CD4(+)CD25(+) suppressor T cells in vivo

CD4(+)CD25(+) suppressor T (TS) cells play a critical role in the maintenance of peripheral tolerance. We examined here proliferative and functional responses as well as differential gene expression in T(S) cells. We found that T(S) cells were hyporesponsive to antigenic stimuli in vivo and unable to flux Ca(2+) upon T cell receptor (TCR) engagement. However, T(S) cells were not impaired in their proliferative response to lymphopenia, which was dependent on major histocompatibility complex class II expression. Homeostatic proliferation did not abolish T(S) cell anergy; rather, it substantially augmented T(S) cell function. DNA array analyses identified genes that may inhibit responsiveness at a number of levels in multiple signaling cascades in T(S) cells, as well as several anti-apoptotic genes that may mediate their survival.

Lipopolysaccharides from distinct pathogens induce different classes of immune responses in vivo

The adaptive immune system has evolved distinct responses against different pathogens, but the mechanism(s) by which a particular response is initiated is poorly understood. In this study, we investigated the type of Ag-specific CD4(+) Th and CD8(+) T cell responses elicited in vivo, in response to soluble OVA, coinjected with LPS from two different pathogens. We used Escherichia coli LPS, which signals through Toll-like receptor 4 (TLR4) and LPS from the oral pathogen Porphyromonas gingivalis, which does not appear to require TLR4 for signaling. Coinjections of E. coli LPS + OVA or P. gingivalis LPS + OVA induced similar clonal expansions of OVA-specific CD4(+) and CD8(+) T cells, but strikingly different cytokine profiles. E. coli LPS induced a Th1-like response with abundant IFN-gamma, but little or no IL-4, IL-13, and IL-5. In contrast, P. gingivalis LPS induced Th and T cell responses characterized by significant levels of IL-13, IL-5, and IL-10, but lower levels of IFN-gamma. Consistent with these results, E. coli LPS induced IL-12(p70) in the CD8alpha(+) dendritic cell (DC) subset, while P. gingivalis LPS did not. Both LPS, however, activated the two DC subsets to up-regulate costimulatory molecules and produce IL-6 and TNF-alpha. Interestingly, these LPS appeared to have differences in their ability to signal through TLR4; proliferation of splenocytes and cytokine secretion by splenocytes or DCs from TLR4-deficient C3H/HeJ mice were greatly impaired in response to E. coli LPS, but not P. gingivalis LPS. Therefore, LPS from different bacteria activate DC subsets to produce different cytokines, and induce distinct types of adaptive immunity in vivo.

The enhanced effect of a hexameric deoxyriboguanosine run conjugation to CpG oligodeoxynucleotides on protection against allergic asthma

BACKGROUND

Oligodeoxynucleotides containing a CpG motif (CpG ODNs), as potent inducers of T(H)1 immunity, are considered promising candidates for immune modulation in asthma. We have previously demonstrated that conjugation of a hexameric deoxyriboguanosine run to the 3' terminus (3' dG(6)-run) of phosphodiester (PE) CpG ODNs enhanced their immuno-stimulatory activities in vitro.

OBJECTIVE

This study aimed to evaluate the effect of a 3' dG(6)-run conjugation to PE or phosphorothioate (PS) CpG ODNs on protection against murine allergic asthma in vivo.

METHODS

Balb/c mice were sensitized to ovalbumin by intraperitoneal injection with or without CpG ODNs (PS CpG ODNs, PE CpG ODNs, and those with 3' dG(6)-run) and subsequently challenged with ovalbumin. We evaluated airway hyperresponsiveness, eosinophil proportion in bronchoalveolar lavage fluid, airway inflammation, and ovalbumin-specific antibody responses.

RESULTS

The conjugation of a 3' dG(6)-run to PE CpG ODNs enhanced the production of IFN-gamma from ovalbumin-specific T(H) cells and prevented the development of asthma in terms of airway hyperresponsiveness, airway eosinophilia, and ovalbumin-specific IgE responses; these effects were comparable to those of PS CpG ODNs. Enhanced effects of the 3' dG(6)-run were also observed in PS CpG ODNs, though they were lower than those in PE CpG ODNs.

CONCLUSION

This study suggests that conjugation of a 3' dG(6)-run to CpG ODNs might provide an effective method for immune modulation of allergic asthma.

Control of the autoimmune response by type 2 nitric oxide synthase

Immune defense against pathogens often requires NO, synthesized by type 2 NO synthase (NOS2). To discern whether this axis could participate in an autoimmune response, we immunized NOS2-deficient mice with the autoantigen acetylcholine receptor, inducing muscle weakness characteristic of myasthenia gravis, a T cell-dependent Ab-mediated autoimmune disease. We found that the acetylcholine receptor-immunized NOS2-deficient mice developed an exacerbated form of myasthenia gravis, and demonstrated that NOS2 expression limits autoreactive T cell determinant spreading and diversification of the autoantibody repertoire, a process driven by macrophages. Thus, NOS2/NO is important for silencing autoreactive T cells and may restrict bystander autoimmune reactions following the innate immune response.

TNF-alpha receptor 1 (p55) on islets is necessary for the expression of LIGHT on diabetogenic T cells

Insulin-dependent diabetes mellitus results from T-cell-mediated destruction of pancreatic islet beta cells. Both CD4 and CD8 T cells have been shown to be independently capable of beta cell destruction. However, the mechanism of beta cell destruction has remained elusive. It has previously been shown that the absence of TNF-alpha receptor 1 (p55) on the islets protected islets from CD4 T-cell-mediated destruction as long as the T cells did not have access to wild-type islets in vivo. Wild-type and TNF-alpha receptor 1 (p55) deficient islets induce similar levels of proliferation of BDC2.5 T cells. In this study, we demonstrate that islet TNF-alpha receptor 1 (p55) influences the expression of LIGHT (TNFSF-14), a TNF family member with both cytolytic and costimulatory properties, on BDC2.5 T cells and the expression of its receptor HVEM (TNFRSF-14) by islets, indicating a role for LIGHT-HVEM interactions in autoimmune diabetes.

Signaling through OX40 (CD134) breaks peripheral T-cell tolerance

Peripheral T-cell tolerance is a mechanism to limit autoimmunity, but represents a major obstacle in diseases such as cancer. Tolerance is due to limited accumulation of antigen-specific T cells accompanied by functional hypo-responsiveness, and is induced by antigen encounter in a non-inflammatory environment. In contrast to advances in preventing induction of T-cell tolerance, there has been little progress in defining targets to reverse established tolerance. Here we show that signals from a single dose of an agonistic antibody against OX40 (CD134, a member of the tumor necrosis-factor family of receptors) can break an existing state of tolerance in the CD4+ T-cell compartment. OX40 signals promote T-cell expansion after the hypo-responsive phenotype is induced and restore normal functionality. These data highlight the potent costimulatory capacity of OX40, and indicate OX40 as a target for therapeutic intervention in a variety of related diseases.

Cell-associated ovalbumin is cross-presented much more efficiently than soluble ovalbumin in vivo

To better understand the antigenic requirements for cross-presentation, we compared the in vivo efficiency of presentation of cell-associated vs soluble OVA with the OT-I (CD8) and OT-II (CD4) TCR transgenic lines. Cross-presentation of cell-associated OVA was very efficient, requiring as little as 21 ng of OVA to activate OT-II cells and 100-fold less to activate OT-I cells. In contrast, soluble OVA was presented inefficiently, requiring at least 10,000 ng OVA for activation of either T cell subset. Thus, cell-associated OVA was presented 500-fold more efficiently than soluble OVA to CD4 T cells and 50,000-fold more efficiently to CD8 T cells. These data, which represent the first quantitative in vivo analysis of cross-presentation, show that cell-associated OVA is very efficiently presented via the class I pathway.

Cutting edge: intravenous soluble antigen is presented to CD4 T cells by CD8- dendritic cells, but cross-presented to CD8 T cells by CD8+ dendritic cells

Mouse spleen contains three distinct mature dendritic cell (DC) populations (CD4(+)8(-), CD4(-)8(-), and CD4(-)8(+)) which retain a capacity to take up particulate and soluble AGS: Although the three splenic DC subtypes showed similar uptake of injected soluble OVA, they differed markedly in their capacity to present this Ag and activate proliferation in OVA-specific CD4 or CD8 T cells. For class II MHC-restricted presentation to CD4 T cells, the CD8(-) DC subtypes were more efficient, but for class I MHC-restricted presentation to CD8 T cells, the CD8(+) DC subtype was far more effective. This differential persisted when the DC were activated with LPS. The CD8(+) DC are therefore specialized for in vivo cross-presentation of exogenous soluble Ags into the class I MHC presentation pathway.