Danita H. Schuurhuis

Reorganization of multivesicular bodies regulates MHC class II antigen presentation by dendritic cells

Immature dendritic cells (DCs) sample their environment for antigens and after stimulation present peptide associated with major histocompatibility complex class II (MHC II) to naive T cells. We have studied the intracellular trafficking of MHC II in cultured DCs. In immature cells, the majority of MHC II was stored intracellularly at the internal vesicles of multivesicular bodies (MVBs). In contrast, DM, an accessory molecule required for peptide loading, was located predominantly at the limiting membrane of MVBs. After stimulation, the internal vesicles carrying MHC II were transferred to the limiting membrane of the MVB, bringing MHC II and DM to the same membrane domain. Concomitantly, the MVBs transformed into long tubular organelles that extended into the periphery of the cells. Vesicles that were formed at the tips of these tubules nonselectively incorporated MHC II and DM and presumably mediated transport to the plasma membrane. We propose that in maturing DCs, the reorganization of MVBs is fundamental for the timing of MHC II antigen loading and transport to the plasma membrane.

Antigen-Antibody Immune Complexes Empower Dendritic Cells to Efficiently Prime Specific CD8+ CTL Responses In Vivo

Dendritic cells (DCs) require a maturation signal to acquire efficient CTL-priming capacity. In vitro FcγR-mediated internalization of Ag-Ab immune complexes (ICs) can induce maturation of DCs. In this study, we show that IC-induced DC maturation in vitro enables DCs to prime peptide-specific CD8+ CTLs in vivo, independently of CD4+ Th cells. Importantly, OVA/anti-OVA IC-treated DCs not only primed CD8+ CTLs to an exogenously loaded peptide nonrelated to OVA, but also efficiently primed CTLs against the dominant CTL epitope derived from the OVA Ag present in the ICs. Our studies show that ICs fulfill a dual role in priming of CD8+ CTL responses to exogenous Ags: enhancement of Ag uptake by DCs and activation of DCs, resulting in “license to kill.” These findings indicate that the presence of specific Abs can crucially affect the induction of cytotoxic cellular responses.

Expression of the murine CD27 ligand CD70 in vitro and in vivo

The interaction between TNFR family member CD27 and its ligand CD70 promotes lymphocyte expansion and effector cell formation. In humans, control of CD27 function is partly regulated by the restricted expression of CD70. We used newly developed mAbs to characterize murine (m) CD70 expression in vitro and in vivo. On resting lymphocytes and immature dendritic cells (DC), mCD70 is absent. In vitro, Ag receptor triggering induced mCD70 mRNA in T cells, but cell surface protein expression was very low. Activated B cells synthesized much higher levels of mCD70 mRNA than activated T cells and clearly expressed mCD70 at the cell surface. mCD70 cell surface expression could also be induced on the DC line D1 and on in vitro-generated murine DC upon maturation. In lymphoid organs of naive mice, virtually no mCD70-expressing cells were found, with exception of cells in the thymic medulla, which may be epithelial in origin. However, after intranasal infection with influenza virus, lung-infiltrating T cells and T and B cells in draining lymph nodes expressed mCD70 according to immunohistology. In such activated lymphocytes, mCD70 protein is largely retained intracellularly. Plasma membrane expression of mCD70 was only detectable by flow cytometry on a small proportion of lung-infiltrating T cells and peaked at the height of the primary response. Thus, expression of CD70 in the mouse is highly regulated at the transcriptional and posttranslational level. This most likely serves to limit excessive effector cell formation after antigenic stimulation.

Immune Complex-Loaded Dendritic Cells Are Superior to Soluble Immune Complexes as Antitumor Vaccine

Dendritic cells (DCs) play an important role in the induction of T cell responses. FcγRs, expressed on DCs, facilitate the uptake of complexed Ag, resulting in efficient MHC class I and MHC class II Ag presentation and DC maturation. In the present study, we show that prophylactic immunization with DCs loaded with Ag-IgG immune complexes (ICs) leads to efficient induction of tumor protection in mice. Therapeutic vaccinations strongly delay tumor growth or even prevent tumors from growing out. By depleting CD4+ and CD8+ cell populations before tumor challenge, we identify CD8+ cells as the main effector cells involved in tumor eradication. Importantly, we show that DCs that are preloaded in vitro with ICs are at least 1000-fold more potent than ICs injected directly into mice or DCs loaded with the same amount of noncomplexed protein. The contribution of individual FcγRs to Ag presentation, T cell response induction, and induction of tumor protection was assessed. We show that FcγRI and FcγRIII are capable of enhancing MHC class I-restricted Ag presentation to CD8+ T cells in vitro and that these activating FcγRs on DCs are required for efficient priming of Ag-specific CD8+ cells in vivo and induction of tumor protection. These findings show that targeting ICs via the activating FcγRs to DCs in vitro is superior to direct IC vaccination to induce protective tumor immunity in vivo.

Prolongation of skin graft survival by modulation of the alloimmune response with alternatively activated dendritic cells.

Background. Activation of immature dendritic cells (DC) in the presence of the glucocorticoid hormone dexamethasone (DEX) results in alternatively matured DC that present antigen in the absence of a proper co-stimulatory context. This maturation process is irreversible, making these cells an attractive potential tool for the induction of antigen-specific T-cell tolerance in vivo. The authors explored the possibility of using these DC for the induction of transplantation tolerance in a fully allogeneic setting in mice. Methods. Immature dendritic cells (D1, an immature splenic DC line derived from B6 mice) were pretreated with DEX for 24 hr, after which lipopolysaccharide or nothing was added to the culture for another 48 hr. These cells were analyzed for their in vitro and in vivo stimulating or tolerizing capacities. Results. In line with their phenotype, including decreased interleukin (IL)-12 production, in vitro co-culture of alternatively matured D1 (B6 origin; H-2b) with completely allogeneic T cells of BALB/c origin led to a significant decrease in the alloreactive T-cell response. A single injection of 1×106 alternatively matured H-2b DC into BALB/c mice induced a different alloimmune response compared with mature DC. The responding T cells showed a lower proliferation rate and a lower interferon-&ggr; production, whereas a significantly higher proportion of the cells produced IL-10 as measured ex vivo by enzyme-linked immunospot assay. Furthermore, injection with alternatively matured DC, followed by transplantation of fully mismatched skin grafts (C57BL/6), led to a significantly prolonged survival compared with that of mature DC-pretreated mice or untreated mice. The immunomodulatory effect was antigen specific, as third-party reactive alloresponses were not affected. Conclusions. The authors’ data constitute the first direct demonstration that DC alternatively matured in the presence of glucocorticoid hormones can be exploited for the specific suppression of the alloreactive Th1 response, resulting in a delayed skin graft rejection in a complete major histocompatibility complex-incompatible strain combination.

Dendritic cells, but not macrophages or B cells, activate major histocompatibility complex class II-restricted CD4 + T cells upon immune-complex uptake in vivo

Professional antigen‐presenting cells (APC) are able to process and present exogenous antigen leading to the activation of T cells. Antigen–immunoglobulin (Ig)G complexes (IC) are much more efficiently processed and presented than soluble antigen. Dendritic cells (DC) are known for their ability to take up and process immune complex (IC) via FcγR, and they have been shown to play a crucial role in IC‐processing onto major histocompatibility complex (MHC) class I as they contain a specialized cross‐presenting transport system required for MHC class I antigen‐processing. However, the MHC class II‐antigen‐processing pathway is distinct. Therefore various other professional APC, like macrophages and B cells, all displaying FcγR, are thought to present IC‐delivered antigen in MHC class II. Nonetheless, the relative contribution of these APC in IC‐facilitated antigen‐presentation for MHC class II in vivo is not known. Here we show that, in mice, both macrophages and DC, but not B cells, efficiently capture IC. However, only DC, but not macrophages, efficiently activate antigen‐specific MHC class II restricted CD4+ T cells. These results indicate that mainly DC and not other professional APC, despite expressing FcγR and MHC class II, contribute significantly to IC‐facilitated T cell activation in vivo under steady‐state conditions.

A Novel Role of Complement Factor C1q in Augmenting the Presentation of Antigen Captured in Immune Complexes to CD8+ T Lymphocytes

Ag-IgG immune complexes (IC) are efficiently taken up, and Ag-derived peptides are subsequently processed and presented by APC. In vitro experiments indicate that IgG Fc Receptors (FcγR) facilitate the efficient uptake of IC by dendritic cells. Previous experiments showed that the cross-presentation of Ag-derived peptides after s.c. administration of IC is FcγR-dependent. To study the role of different FcγR and complement in MHC class I Ag presentation after i.v. administration, we used mice deficient for FcγRs and complement components. These mice were injected with CFSE-labeled OVA-specific CD8+ T cells followed by administration of IC composed of OVA and rabbit anti-OVA IgG i.v. to measure MHC class I presentation of OVA-derived peptides. The Ag presentation was partly reduced in FcRγ-chain-deficient mice, but not affected in FcγRI/II/III-deficient mice, complement factor C3-deficient mice, or FcγRI/II/III × C3-deficient mice. Importantly, CD8+ T cell proliferation was significantly reduced in mice deficient for C1q. This proliferation could be restored when IC were incubated with purified human C1q before injection. Likewise, purified C1q could strongly enhance the uptake and presentation of IC by dendritic cells in vitro. Heat inactivation abrogated the C1q-mediated uptake of IC. In addition, in vivo uptake of OVA-IC in the spleen was significantly reduced in C1q-deficient mice compared with wild-type mice. Together, these results indicate a novel function of C1q, which is present in high levels in the bloodstream, by directly enhancing the uptake and MHC class I presentation of Ag captured in IC by APC to CD8+ T cells.

DC‐induced CD8+ T‐cell response is inhibited by MHC class II‐dependent DX5+CD4+ Treg

CD4+ T cells are important for CD8+ T‐cell priming by providing cognate signals for DC maturation. We analyzed the capacity of CD4+ T cells to influence CD8+ T‐cell responses induced by activated DC. Surprisingly, mice depleted for CD4+ cells were able to generate stronger antigen‐specific CD8+ T‐cell responses after DC vaccination than non‐depleted mice. The same observation was made when mice were vaccinated with MHC class II−/− DC, indicating the presence of a MHC class II‐dependent CD4+ T‐cell population inhibiting CD8+ T‐cell responses. Recently we described the expansion of DX5+CD4+ T cells, a T‐cell population displaying immune regulatory properties, upon vaccination with DC. Intriguingly, we now observe an inverse correlation between CD8+ T‐cell induction and expansion of DX5+CD4+ T cells as the latter cells did not expand after vaccination with MHC class II−/− DC. In vitro, DX5+CD4+ T cells were able to limit proliferation, modulate cytokine production and induce Foxp3+ expression in OVA‐specific CD8+ T cells. Together, our data show an inhibitory role of CD4+ T cells on the induction of CD8+ T‐cell responses by activated DC and indicate the involvement of DX5+CD4+, but not CD4+CD25+, T cells in this process.

Processing of Exogenous Protein Antigen by Murine Dendritic Cells for Presentation to Cytotoxic T Lymphocytes

Generation of cytotoxic T lymphocytes (CTL) from quiescent CD8+ precursors can be achieved by stimulation with professional antigen-presenting cells (APC), dendritic cells (DC). With the elucidation of CTL epitopes that can be manufactured synthetically, in vitro CTL responses specific for these peptides can be established with DC (l–3). In these studies, DC of supposedly mature phenotype were used, i.e. derived from murine spleens.