Increasing the survival of dendritic cells in vivo does not replace the requirement for CD4+ T cell help during primary CD8+ T cell responses

The survival of dendritic cells (DC) in vivo determines the duration of Ag presentation and is critical in determining the strength and magnitude of the resulting T cell response. We used a mouse model to show that Ag-loaded C57BL/6 DC (MHC class II(+/+) (MHC II(+/+))) that reach the lymph node survived longer than Ag-loaded MHC II(-/-) DC, with the numbers of C57BL/6 DC being approximately 2.5-fold the number of the MHC II(-/-) DC by day 4 and approximately 5-fold by day 7. The differential survival of DC in vivo was not affected by low doses of LPS, but in vitro pretreatment with CD40L or with high doses of LPS increased the numbers of MHC II(-/-) DC to levels approaching those of C57BL/6 DC. Regardless of their numbers and relative survival in lymph nodes, MHC II(-/-) DC were profoundly defective in their ability to induce CTL responses against the gp33 peptide epitope, and were unable to induce expansion and optimal cytotoxic activity of CD8(+) T cells specific for the male Ag UTY. We conclude that CD4(+) T cell help for CD8(+) responses involves mechanisms other than the increased survival of Ag-presenting DC in the lymph node.     

HIV gag mRNA transfection of dendritic cells (DC) delivers encoded antigen to MHC class I and II molecules, causes DC maturation, and induces a potent human in vitro primary immune response

Dendritic cells (DC) are the major APCs involved in naive T cell activation making them prime targets of vaccine research. We observed that mRNA was efficiently transfected, resulting in superior translation in DC compared with other professional APCs. A single stimulation of T cells by HIV gag-encoded mRNA-transfected DC in vitro resulted in primary CD4(+) and CD8(+) T cell immune responses at frequencies of Ag-specific cells (5-12.5%) similar to primary immune responses observed in vivo in murine models. Additionally, mRNA transfection also delivered a maturation signal to DC. Our results demonstrated that mRNA-mediated delivery of encoded Ag to DC induced potent primary T cell responses in vitro. mRNA transfection of DC, which mediated efficient delivery of antigenic peptides to MHC class I and II molecules, as well as delivering a maturation signal to DC, has the potential to be a potent and effective anti-HIV T cell-activating vaccine.     

Simultaneous induction of CD4 T cell tolerance and CD8 T cell immunity by semimature dendritic cells

Previous studies suggested that depending on their maturation state, dendritic cells (DC) could either induce T cell tolerance (immature and semimature DC) or T cell activation (mature DC). Pretreatment of C57BL/6 mice with encephalitogenic myelin oligodendrocyte glycoprotein (MOG)(35-55) peptide-loaded semimature DC protected from MOG-induced autoimmune encephalomyelitis. This protection was mediated by IL-10-producing CD4 T cells specific for the self Ag. Here we show that semimature DC loaded with the MHC class II-restricted nonself peptide Ag (OVA) induce an identical regulatory T cell cytokine pattern. However, semimature DC loaded simultaneously with MHC class II- and MHC class I-restricted peptides, could efficiently initiate CD8 T cell responses leading to autoimmune diabetes in a TCR-transgenic adoptive transfer model. Double-peptide-loaded semimature DC also induced simultaneously in the same animal partially activated CD8 T cells with cytolytic function as well as protection from MOG-induced autoimmune encephalomyelitis. Our study suggests that the decision between tolerance and immunity not only depends on the DC, but also on the type and activation requirements of the responding T cell.     

Dendritic cells cross-dressed with peptide MHC class I complexes prime CD8+ T cells

The activation of naive CD8+ T cells has been attributed to two mechanisms: cross-priming and direct priming. Cross-priming and direct priming differ in the source of Ag and in the cell that presents the Ag to the responding CD8+ T cells. In cross-priming, exogenous Ag is acquired by professional APCs, such as dendritic cells (DC), which process the Ag into peptides that are subsequently presented. In direct priming, the APCs, which may or may not be DC, synthesize and process the Ag and present it themselves to CD8+ T cells. In this study, we demonstrate that naive CD8+ T cells are activated by a third mechanism, called cross-dressing. In cross-dressing, DC directly acquire MHC class I-peptide complexes from dead, but not live, donor cells by a cell contact-mediated mechanism, and present the intact complexes to naive CD8+ T cells. Such DC are cross-dressed because they are wearing peptide-MHC complexes generated by other cells. CD8+ T cells activated by cross-dressing are restricted to the MHC class I genotype of the donor cells and are specific for peptides generated by the donor cells. In vivo studies demonstrate that optimal priming of CD8+ T cells requires both cross-priming and cross-dressing. Thus, cross-dressing may be an important mechanism by which DC prime naive CD8+ T cells and may explain how CD8+ T cells are primed to Ags that are inefficiently cross-presented.     

Newly activated T cells promote maturation of bystander dendritic cells but not IL-12 production

The activation of dendritic cells (DC) leads to increased costimulatory activity (termed DC maturation) and, in some instances, production of immunomodulatory cytokines such as IL-12. Both innate and T cell-derived signals can promote DC activation but it is unclear to what extent the two classes of stimuli are interchangeable or regulate distinct aspects of DC function. In this study, we show that signals from newly activated CD4(+) T cells cannot initiate IL-12 synthesis although they can amplify secretion of bioactive IL-12 p70 by DC exposed to an appropriate innate stimulus. This occurs exclusively in cis and does not influence IL-12 synthesis by bystander DC that do not present Ag. In marked contrast, signals from newly activated CD4(+) T cells can induce an increase in DC costimulatory activity in the absence of any innate priming. This occurs both in cis and in trans, affecting all DC in the microenvironment, including those that do not bear specific Ag. Consistent with the latter, we show that newly activated CD4(+) T cells in vivo can deliver "help" in trans, effectively lowering the number of MHC/peptide complexes required for proliferation of third-party naive CD4(+) T cells recognizing Ag on bystander DC. These results demonstrate that DC maturation and cytokine production are regulated distinctly by innate stimuli vs signals from CD4(+) T cells and reveal a process of trans activation of DC without secretion of polarizing cytokines that takes place during T cell priming and may be involved in amplifying immune responses.     

Acquisition of MHC:Peptide Complexes by Dendritic Cells Contributes to the Generation of Antiviral CD8+ T Cell Immunity In Vivo

There is an increasing body of evidence suggesting that the transfer of preformed MHC class I:peptide complexes between a virus-infected cell and an uninfected APC, termed cross-dressing, represents an important mechanism of Ag presentation to CD8(+) T cells in host defense. However, although it has been shown that memory CD8(+) T cells can be activated by uninfected dendritic cells (DCs) cross-dressed by Ag from virus-infected parenchymal cells, it is unknown whether conditions exist during virus infection in which naive CD8(+) T cells are primed and differentiate to cytolytic effectors through cross-dressing, and indeed which DC subset would be responsible. In this study, we determine whether the transfer of MHC class I:peptide complexes between infected and uninfected murine DC plays a role in CD8(+) T cell priming to viral Ags in vivo. We show that MHC class I:peptide complexes from peptide-pulsed or virus-infected DCs are indeed acquired by splenic CD8α(-) DCs in vivo. Furthermore, the acquired MHC class I:peptide complexes are functional in that they induced Ag-specific CD8(+) T cell effectors with cytolytic function. As CD8α(-) DCs are poor cross-presenters, this may represent the main mechanism by which CD8α(-) DCs present exogenously encountered Ag to CD8(+) T cells. The sharing of Ag as preformed MHC class I:peptide complexes between infected and uninfected DCs without the restraints of Ag processing may have evolved to accurately amplify the response and also engage multiple DC subsets critical in the generation of strong antiviral immunity.     

TNF receptor 1 mediates dendritic cell maturation and CD8 T cell response through two distinct mechanisms

TNF-α and its two receptors (TNFR1 and 2) are known to stimulate dendritic cell (DC) maturation and T cell response. However, the specific receptor and mechanisms involved in vivo are still controversial. In this study, we show that in response to an attenuated mouse hepatitis virus infection, DCs fail to mobilize and up-regulate CD40, CD80, CD86, and MHC class I in TNFR1(-/-) mice as compared with the wild-type and TNFR2(-/-) mice. Correspondingly, virus-specific CD8 T cell response was dramatically diminished in TNFR1(-/-) mice. Adoptive transfer of TNFR1-expressing DCs into TNFR1(-/-) mice rescues CD8 T cell response. Interestingly, adoptive transfer of TNFR1-expressing naive T cells also restores DC mobilization and maturation and endogenous CD8 T cell response. These results show that TNFR1, not TNFR2, mediates TNF-α stimulation of DC maturation and T cell response to mouse hepatitis virus in vivo. They also suggest two mechanisms by which TNFR1 mediates TNF-α-driven DC maturation, as follows: a direct effect through TNFR1 expressed on immature DCs and an indirect effect through TNFR1 expressed on naive T cells.     

Cutting edge: dendritic cell actin cytoskeletal polarization during immunological synapse formation is highly antigen-dependent

Dendritic cells (DC) actively rearrange their actin cytoskeleton to participate in formation of the immunological synapse (IS). In this study, we evaluated the requirements for DC participation in the IS. DC rearrange their actin cytoskeleton toward naive CD4(+) T cells only in the presence of specific MHC-peptide complexes. In contrast, naive CD4(+) T cells polarized their cytoskeletal proteins in the absence of Ag. DC cytoskeletal rearrangement occurred at the same threshold of peptide-MHC complexes as that required for T cell activation. Furthermore, T cell activation was inhibited by specific blockade of DC cytoskeletal rearrangement. When TCR-MHC interaction was bypassed by using Con A-activated T cells, DC polarization was abrogated. In addition, directional ligation of MHC class II resulted in DC cytoskeletal polarization. Our findings suggest that a high Ag specificity is required for DC IS formation and that MHC class II signaling plays a central role in this process.     

The adjuvanticity of a mannosylated antigen reveals TLR4 functionality essential for subset specialization and functional maturation of mouse dendritic cells

The evidence that dendritic cell (DC) subsets produce differential cytokines in response to specific TLR stimulation is robust. However, the role of TLR stimulation in Ag presentation and phenotypic maturation among DC subsets is not clear. Through the adjuvanticity of a novel mannosylated Ag, mannosylated dendrimer OVA (MDO), as a pathogen-associated molecular pattern Ag, we characterized the functionality of GM-CSF/IL-4-cultured bone marrow DC and Flt3 ligand (Flt3-L) DC subsets by Ag presentation and maturation assays. It was demonstrated that both bone marrow DCs and Flt3-L DCs bound, processed, and presented MDO effectively. However, while Flt3-L CD24(high) (conventional CD8(+) equivalent) and CD11b(high) (CD8(-) equivalent) DCs were adept at MDO processing by MHC class I and II pathways, respectively, CD45RA(+) plasmacytoid DCs presented MDO poorly to T cells. Successful MDO presentation was largely dependent on competent TLR4 for Ag localization and morphological/phenotypic maturation of DC subsets, despite the indirect interaction of MDO with TLR4. Furthermore, Toll/IL-1 receptor-domain-containing adaptor-inducing IFN-beta, but not MyD88, as a TLR4 signaling modulator was indispensable for MDO-induced DC maturation and Ag presentation. Taken together, our findings suggest that DC subsets differentially respond to a pathogen-associated molecular pattern-associated Ag depending on the intrinsic programming and TLRs expressed. Optimal functionality of DC subsets in Ag presentation necessitates concomitant TLR signaling critical for efficient Ag localization and processing.     

Aspirin inhibits in vitro maturation and in vivo immunostimulatory function of murine myeloid dendritic cells

Aspirin is the most commonly used analgesic and antiinflammatory agent. In this study, at physiological concentrations, it profoundly inhibited CD40, CD80, CD86, and MHC class II expression on murine, GM-CSF + IL-4 stimulated, bone marrow-derived myeloid dendritic cells (DC). CD11c and MHC class I expression were unaffected. The inhibitory action was dose dependent and was evident at concentrations higher than those necessary to inhibit PG synthesis. Experiments with indomethacin revealed that the effects of aspirin on DC maturation were cyclooxygenase independent. Nuclear extracts of purified, aspirin-treated DC revealed a decreased NF-kappaB DNA-binding activity, whereas Ab supershift analysis indicated that aspirin targeted primarily NF-kappaB p50. Unexpectedly, aspirin promoted the generation of CD11c+ DC, due to apparent suppression of granulocyte development. The morphological and ultrastructural appearance of aspirin-treated cells was consistent with immaturity. Aspirin-treated DC were highly efficient at Ag capture, via both mannose receptor-mediated endocytosis and macropinocytosis. By contrast, they were poor stimulators of naive allogeneic T cell proliferation and induced lower levels of IL-2 in responding T cells. They also exhibited impaired IL-12 expression and did not produce IL-10 after LPS stimulation. Assessment of the in vivo function of aspirin-treated DC, pulsed with the hapten trinitrobenzenesulfonic acid, revealed an inability to induce normal cell-mediated contact hypersensitivity, despite the ability of the cells to migrate to T cell areas of draining lymphoid tissue. These data provide new insight into the immunopharmacology of aspirin and suggest a novel approach to the manipulation of DC for therapeutic application.     

CD1 molecules efficiently present antigen in immature dendritic cells and traffic independently of MHC class II during dendritic cell maturation

Upon exposure to Ag and inflammatory stimuli, dendritic cells (DCs) undergo a series of dynamic cellular events, referred to as DC maturation, that involve facilitated peptide Ag loading onto MHC class II molecules and their subsequent transport to the cell surface. Besides MHC molecules, human DCs prominently express molecules of the CD1 family (CD1a, -b, -c, and -d) and mediate CD1-dependent presentation of lipid and glycolipid Ags to T cells, but the impact of DC maturation upon CD1 trafficking and Ag presentation is unknown. Using monocyte-derived immature DCs and those stimulated with TNF-alpha for maturation, we observed that none of the CD1 isoforms underwent changes in intracellular trafficking that mimicked MHC class II molecules during DC maturation. In contrast to the striking increase in surface expression of MHC class II on mature DCs, the surface expression of CD1 molecules was either increased only slightly (for CD1b and CD1c) or decreased (for CD1a). In addition, unlike MHC class II, DC maturation-associated transport from lysosomes to the plasma membrane was not readily detected for CD1b despite the fact that both molecules were prominently expressed in the same MIIC lysosomal compartments before maturation. Consistent with this, DCs efficiently presented CD1b-restricted lipid Ags to specific T cells similarly in immature and mature DCs. Thus, DC maturation-independent pathways for lipid Ag presentation by CD1 may play a crucial role in host defense, even before DCs are able to induce maximum activation of peptide Ag-specific T cells.     

Comparison of the functional properties of murine dendritic cells generated in vivo with Flt3 ligand, GM-CSF and Flt3 ligand plus GM-SCF

Flt3 ligand (FL) and granulocyte-macrophage colony-stimulating factor (GM-CSF) are important growth factors for dendritic cells (DC). Substantial numbers of DC can be generated in vivo following the administration of either factor. We sought to extend our knowledge of the functional properties of these cells including their ability to prime na?ve CD8(+) T cells. In addition, we compared the nature of the DC generated in vivo with the single cytokines to those generated with the combination of FL+polyethylene glycol-modified GM-CSF (pGM-CSF). Treatment with FL+pGM-CSF yielded greater numbers of both CD11b(low) and CD11b(high) DC than with either cytokine alone, and these DC were more efficient at antigen (Ag) capture. The FL+pGM-CSF-generated CD11b(low) DC lacked expression of CD8alpha. Following treatment with LPS in vivo, all DC subsets upregulated CD40, CD80, CD86, and MHC class II expression, but surprisingly Ag capture was not downregulated and some DC subsets retained expression of intracellular MHC class II vesicles. Thus, even after activation in vivo with LPS, DC retained Ag capture properties of immature DC, and Ag presentation/costimulation properties of mature DC. Though all DC subsets stimulated CD4(+) T cell proliferation equivalently, FL-generated DC were more efficient at priming Ag-specific CD8(+) cytolytic T cells than DC generated with either pGM-CSF alone or FL+pGM-CSF, and CD11b(high) DC were more efficient at priming CD8(+) T cells than CD11b(low) DC.     

Efficient in vivo priming of specific cytotoxic T cell responses by neonatal dendritic cells

In early life, a high susceptibility to infectious diseases as well as a poor capacity to respond to vaccines are generally observed as compared with observations in adults. The mechanisms underlying immune immaturity have not been fully elucidated and could be due to the immaturity of the T/B cell responses and/or to a defect in the nature and quality of Ag presentation by the APC. This prompted us to phenotypically and functionally characterize early life murine dendritic cells (DC) purified from spleens of 7-day-old mice. We showed that neonatal CD11c(+) DC express levels of costimulatory molecules and MHC molecules similar to those of adult DC and are able to fully maturate after LPS activation. Furthermore, we demonstrated that neonatal DC can efficiently take up, process, and present Ag to T cells in vitro and induce specific CTL responses in vivo. Although a reduced number of these cells was observed in the spleen of neonatal mice as compared with adults, this study clearly shows that neonatal DC have full functional capacity and may well prime Ag-specific naive T cells in vivo.     

Partial activation of neonatal CD11c+ dendritic cells and induction of adult-like CD8+ cytotoxic T cell responses by synthetic microspheres

Neonatal cytotoxic T cell responses have only been elicited to date with immunogens or delivery systems inducing potent direct APC activation. To define the minimal activation requirements for the induction of neonatal CD8(+) cytotoxic responses, we used synthetic microspheres (MS) coated with a single CD8(+) T cell peptide from lymphocytic choriomeningitis virus (LCMV) or HIV-1. Unexpectedly, a single injection of peptide-conjugated MS without added adjuvant induced CD4-dependent Ag-specific neonatal murine cytotoxic responses with adult-like CTL precursor frequency, avidity for Ag, and frequency of IFN-gamma-secreting CD8(+) splenocytes. Neonatal CD8(+) T cell responses to MS-LCMV were elicited within 2 wk of a single immunization and, upon challenge, provided similar protection from viral replication as adult CTLs, demonstrating their in vivo competence. As previously reported, peptide-coated MS elicited no detectable activation of adult CD11c(+) dendritic cells (DC). In contrast, CTL responses were associated with a partial activation of neonatal CD11c(+) DC, reflected by the up-regulation of CD80 and CD86 expression but no concurrent changes in MHC class II or CD40 expression. However, this partial activation of neonatal DC was not sufficient to circumvent the requirement for CD4(+) T cell help. The effective induction of neonatal CD8(+) T cell responses by this minimal Ag delivery system demonstrates that neonatal CD11c(+) DC may mature sufficiently to stimulate naive CD8(+) neonatal T cells, even in the absence of strong maturation signals.     

Induction of MHC class I presentation of exogenous antigen by dendritic cells is controlled by CD4+ T cells engaging class II molecules in cholesterol-rich domains.

We investigated interactions between CD4+ T cells and dendritic cells (DC) necessary for presentation of exogenous Ag by DC to CD8+ T cells. CD4+ T cells responding to their cognate Ag presented by MHC class II molecules of DC were necessary for induction of CD8+ T cell responses to MHC class I-associated Ag, but their ability to do so depended on the manner in which class II-peptide complexes were formed. DC derived from short-term mouse bone marrow culture efficiently took up Ag encapsulated in IgG FcR-targeted liposomes and stimulated CD4+ T cell responses to Ag-derived peptides associated with class II molecules. This CD4+ T cell-DC interaction resulted in expression by the DC of complexes of class I molecules and peptides from the Ag delivered in liposomes and permitted expression of the activation marker CD69 and cytotoxic responses by naive CD8+ T cells. However, while free peptides in solution loaded onto DC class II molecules could stimulate IL-2 production by CD4+ T cells as efficiently as peptides derived from endocytosed Ag, they could not stimulate induction of cytotoxic responses by CD8+ T cells to Ag delivered in liposomes into the same DC. Signals requiring class II molecules loaded with endocytosed Ag, but not free peptide, were inhibited by methyl-beta-cyclodextrin, which depletes cell membrane cholesterol. CD4+ T cell signals thus require class II molecules in cholesterol-rich domains of DC for induction of CD8+ T cell responses to exogenous Ag by inducing DC to process this Ag for class I presentation.     

Priming of CTLs by lymphocytic choriomeningitis virus depends on dendritic cells

Appropriate activation of naive CD8(+) T cells depends on the coordinated interaction of these cells with professional APC that present antigenic peptides in the context of MHC class I molecules. It is accepted that dendritic cells (DC) are efficient in activating naive T cells and are unique in their capacity to prime CD8(+) T cell responses against exogenous cell-associated Ags. Nevertheless, it is unclear whether epitopes, derived from endogenously synthesized proteins and presented by MHC class I molecules on the surface of other APC including B cells and macrophages, can activate naive CD8(+) T cells in vivo. By infecting transgenic CD11c-DTR/GFP mice that allow conditional depletion of DC with lymphocytic choriomeningitis virus (LCMV), which infects all types of APC and elicits a vigorous CTL response, we unambiguously show that priming of LCMV-specific CD8(+) T cells is crucially dependent on DC, despite ample presence of LCMV-infected macrophages and B cells in secondary lymphoid organs.     

T cells signaled by NF-kappa B- dendritic cells are sensitized not anergic to subsequent activation

Paradoxically, while peripheral self-tolerance exists for constitutively presented somatic self Ag, self-peptide recognized in the context of MHC class II has been shown to sensitize T cells for subsequent activation. We have shown that MHC class II(+)CD86(+)CD40(-) DC, which can be generated from bone marrow in the presence of an NF-kappa B inhibitor, and which constitutively populate peripheral tissues and lymphoid organs in naive animals, can induce Ag-specific tolerance. In this study, we show that CD40(-) human monocyte-derived dendritic cells (DC), generated in the presence of an NF-kappa B inhibitor, signal phosphorylation of TCR zeta, but little proliferation or IFN-gamma in vitro. Proliferation is arrested in the G(1)/G(0) phase of the cell cycle. Surprisingly, responding T cells are neither anergic nor regulatory, but are sensitized for subsequent IFN-gamma production. The data indicate that signaling through NF-kappa B determines the capacity of DC to stimulate T cell proliferation. Functionally, NF-kappa B(-)CD40(-)class II(+) DC may either tolerize or sensitize T cells. Thus, while CD40(-) DC appear to "prime" or prepare T cells, the data imply that signals derived from other cells drive the generation either of Ag-specific regulatory or effector cells in vivo.     

Impaired ability of MHC class II-/- dendritic cells to provide tumor protection is rescued by CD40 ligation.

The contribution of CD4+ T cells to dendritic cell (DC) activation and to the induction of CD8+ T cell responses in vivo was investigated using a model of antitumor immune responses. Immunization with peptide-loaded MHC class II-deficient (MHC class II-/-) DC induced the activation of Ag-specific CD8+ T cells and their accumulation in the lymph nodes and spleens of immunized mice. The accumulation induced by MHC class II-/- DC immunization was lower than the accumulation observed after immunization with MHC class II+/+ DC. Similarly, immunization with peptide-loaded, MHC class II-/- DC induced some degree of protection against tumor challenge, but this protection was lower than the protection achieved after immunization with MHC class II+/+ DC. Incubation with a membrane-associated form of CD40 ligand resulted in the up-regulation of costimulatory molecules on MHC class II-/- DC and fully rescued their ability to induce antitumor immunity. We conclude that CD4+ T cells play a critical role in the generation of antitumor immune responses through their capacity to induce the activation of DC via CD40/CD40 ligand interaction, and thus maximize CD8+ T cell responses.     

Low TLR4 expression by liver dendritic cells correlates with reduced capacity to activate allogeneic T cells in response to endotoxin

Signaling via TLRs results in dendritic cell (DC) activation/maturation and plays a critical role in the outcome of primary immune responses. So far, no data exist concerning TLR expression by liver DC, generally regarded as less immunostimulatory than secondary lymphoid tissue DC. Because the liver lies directly downstream from the gut, it is constantly exposed to bacterial LPS, a TLR4 ligand. We examined TLR4 expression by freshly isolated, flow-sorted C57BL/10 mouse liver DC compared with spleen DC. Real-time PCR revealed that liver CD11c+CD8alpha- (myeloid) and CD11c+CD8alpha+ ("lymphoid-related") DC expressed lower TLR4 mRNA compared with their splenic counterparts. Lower TLR4 expression correlated with reduced capacity of LPS (10 ng/ml) but not anti-CD40-stimulated liver DC to induce naive allogeneic (C3H/HeJ) T cell proliferation. By contrast to LPS-stimulated splenic DC, these LPS-activated hepatic DC induced alloantigen-specific T cell hyporesponsiveness in vitro, correlated with deficient Th1 (IFN-gamma) and Th2 (IL-4) responses. When higher LPS concentrations (> or =100 ng/ml) were tested, the capacity of liver DC to induce proliferation of T cells and Th1-type responses was enhanced, but remained inferior to that of splenic DC. Hepatic DC activated by LPS in vivo were inferior allogeneic T cell stimulators compared with splenic DC, whereas adoptive transfer of LPS-stimulated (10 ng/ml) liver DC induced skewing toward Th2 responses. These data suggest that comparatively low expression of TLR4 by liver DC may limit their response to specific ligands, resulting in reduced or altered activation of hepatic adaptive immune responses.     

Superantigen enhancement of specific immunity: antibody production and signaling pathways

Superantigens are microbial proteins that induce massive activation, proliferation, and cytokine production by CD4+ T cells via specific Vbeta elements on the TCR. In this study we examine superantigen enhancement of Ag-specific CD4+ T cell activity for humoral B cell responses to T-dependent Ags BSA and HIV gp120 envelope, type I T-independent Ag LPS, and type II T-independent Ag pneumococcal polysaccharides. Injection of BSA followed by a combination of superantigens staphylococcal enterotoxin A and staphylococcal enterotoxin B (SEB) 7 days later enhanced the anti-BSA Ab response in mice approximately 4-fold as compared with mice given BSA alone. The anti-gp120 response was enhanced approximately 3-fold by superantigens. The type II T-independent Ag pneumococcal polysaccharide response was enhanced approximately 2.3-fold by superantigens, whereas no effect was observed on the response to the type I T-independent Ag LPS. The superantigen effect was completely blocked by the CD4+ T cell inhibitory cytokine IL-10. SEB-stimulated human CD4+ T cells were examined to determine the role of the mitogen-activated protein (MAP) kinase signal transduction pathway in superantigen activation of T cells. Inhibitors of the mitogen pathway of MAP kinase blocked SEB-induced proliferation and IFN-gamma production, while an inhibitor of the p38 stress pathway had no effect. Consistent with this, SEB activated extracellular signal-regulated kinase/MAP kinase as well as MAP kinase-interacting kinase, a kinase that phosphorylates eIF4E, which is an important component of the eukaryotic protein synthesis initiation complex. Both kinases were inhibited by IL-10. Thus, superantigens enhance humoral immunity via Ag-specific CD4+ T cells involving the stress-independent pathway of MAP kinase.