Rapid peptide turnover and inefficient presentation of exogenous antigen critically limit the activation of self-reactive CTL by dendritic cells

Regulation of MHC class I transport in human dendritic cells and the dendritic-like cell line KG-1

Dendritic cells (DCs) progress through distinct maturational phases; immature DCs capture Ag while mature DCs are optimized for Ag presentation. Proper control of immunity requires regulated compartmentalization of MHC class II molecules. We report that DCs also regulate MHC class I trafficking throughout maturation. Although mature human DCs express high levels of surface MHC class I, immature DCs exhibit lower surface levels while retaining MHC class I-peptide complexes in the Golgi. A cell line, KG-1, behaves similarly. We confirm the similarity of KG-1 to DCs by demonstrating its capacity to present exogenous Ags in an MHC class I-restricted fashion to CD8(+) T cell hybridomas, a phenomenon called cross-presentation. Biochemical characterization of MHC class I trafficking throughout maturation showed that, in early KG-1 dendritic-like cells, surface arrival of MHC class I-peptide complexes is delayed by their retention in the Golgi. In mature dendritic-like cells, these complexes relocate to the surface and their stability increases, concomitant with up-regulation of costimulatory molecules. Maturation induces qualitative changes in the MHC class I-associated peptide repertoire demonstrated by increased thermostability. The differential processing of MHC class I throughout maturation may prevent premature immune activation while promoting T cell responses in lymph nodes to Ags acquired at sites of inflammation.

Encapsulation of peptides in biodegradable microspheres prolongs their MHC class-I presentation by dendritic cells and macrophages in vitro

Biodegradable microspheres (MS) consisting of poly(D,L-lactide-co-glycolide) (PLGA) represent a promising alternative to conventional adjuvants. The adjustable pulsatile release of encapsulated material from such MS offers the potential to mimic the priming and boosting injections of conventional immunization regimens. In this paper, we demonstrate that MS can serve as antigen reservoirs in antigen presenting cells (APC), so that antigen is presented for extended periods of time (up to 9 days). In particular, we could show by measurement of IFN-gamma production that encapsulated peptides were presented to cytotoxic T lymphocytes (CTL) by mouse and human macrophages as well as by human dendritic cells in vitro for a longer time period as compared to soluble peptides. The extended antigen presentation may thus improve the CTL response in vivo. These results may be of paramount importance in cancer vaccination therapy since MS may serve as antigen reservoirs to extend the presentation time by APC used to boost the patient's immune response to tumor antigens.

Cutting edge: dissociation between autoimmune response and clinical disease after vaccination with dendritic cells

Autoimmunity represents a caveat to the use of dendritic cells (DCs) as adjuvant for human vaccines. We derived DCs from normal BALB/c mice or from mice prone to autoimmunity (NZB x NZW) F(1). We allowed DCs to phagocytose apoptotic thymocytes and vaccinated syngeneic animals. All mice developed anti-nuclear and anti-dsDNA Abs. Autoantibodies in normal mice were transient, without clinical or histological features of autoimmunity or tissue involvement. In contrast, autoimmunity was maintained in susceptible mice, which underwent renal failure and precociously died. The data suggest that DC vaccination consistently triggers autoimmune responses. However, clinical autoimmunity develops in susceptible subjects only.

Activation of CD8 T cells by antigen expressed in the pituitary gland

Ag expressed exclusively in the anterior pituitary gland and secreted locally by pituitary somatotrophs can gain access to the MHC class I presentation pathway and activate CD8 T cells. Influenza nucleoprotein (NP) was expressed as a transgene under the control of the human growth hormone (GH) locus control region. Activation of monoclonal F5 CD8 T cells specific for NP resulted in spontaneous autoimmune pathology of the pituitary gland in mice transgenic for both NP and the F5 TCR. Destruction of somatotrophs resulted in drastically reduced GH levels in adult mice and a dwarf phenotype. Adoptive transfer of F5 T cells into NP-transgenic hosts resulted in full T cell activation, first demonstrable in regional lymph nodes, followed by their migration to the pituitary gland. Despite the presence of activated, IFN-gamma-producing CD8 T cells in the pituitary gland and a slight reduction in pituitary GH levels, no effect on growth was observed. Thus, CD8 T cells have access to the neuroendocrine system and get fully activated in the absence of CD4 help, but Ag recognition in this location causes autoimmune pathology only in the presence of excessive CD8 T cell numbers.

RNA-transfected dendritic cells

Based on their unique ability to stimulate primary immune responses, dendritic cells are the most potent antigen-presenting cells known. This ability stems from the fact that they are very efficient at the uptake and processing of antigen and they express high levels of major histocompatibility complex class I and class II, as well as costimulatory molecules, which are required to prime naive cytotoxic T-cells. Many groups of investigators have tried to take advantage of these features by developing dendritic cell-based vaccines against tumors and infectious diseases. While the basic principle in these studies is the same--dendritic cells pulsed with antigen are used to elicit cytotoxic T-cell responses--the methods used are varied. This is particularly true with respect to the nature of the antigen used and the method of antigen delivery. In this article, we will focus on the use of RNA as a form of antigen with which to load dendritic cells. We will discuss the rationale behind using RNA as an antigen source and will review recent studies in both murine and human settings that use RNA-pulsed dendritic cells as vaccines.

Polybrene and interleukin-4: two opposing factors for retroviral transduction of bone-marrow-derived dendritic cells

BACKGROUND

Gene transfer using retroviral transduction offers the advantage of long-term transgene expression in developing strategies that use dendritic cells (DCs) for immunotherapy. The goal of this study was to infect DCs in an immature state in order to take advantage of their proliferating and tolerogenic potential.

METHODS

Immature DCs were generated from murine bone marrow (BM) using either GM-CSF alone or GM-CSF plus IL-4. The cells were transduced directly with retroviral supernatants or by co-culture with the GP + E-86 retroviral packaging cell line in the presence of two different cationic polymers: polybrene and protamine sulfate. Phenotypic and functional characterization of the transduced cells were then performed.

RESULTS

Our results show a low efficiency of retroviral infection of DCs in the presence of polybrene. This cationic polymer was found to be directly cytotoxic to murine DCs and thus favored the growth of contaminating macrophages. This effect was not observed using protamine sulfate. Furthermore, stimulation by IL-4 early in the culture increased DC differentiation, proliferation and transduction. However, we found that DCs generated in GM-CSF plus IL-4 presented a more mature phenotype with an enhanced allogeneic stimulating activity. Finally, we showed that DCs themselves down-regulated transgene expression in the co-cultured packaging cell line in a promoter-dependent manner.

CONCLUSIONS

We have defined optimal conditions to generate and transduce murine BM-derived DCs. This included: the use of protamine sulfate during exposure to retroviral infectious supernatant and the addition of IL-4 at an early stage of the culture. Nevertheless, this cytokine also induced DC maturation. These findings have potential implications in experimental gene therapy.

Experimental models linking dendritic cell lineage, phenotype and function

One of the important issues in dendritic cell (DC) biology today is how DC control the fate of T cells. Our data suggest that an important branch point in determining T cell fate is the decision between deletion and memory. We have previously hypothesized that this binary decision is determined by contact with DC derived from lymphoid- versus myeloid-restricted progenitors. However, the false attribution of CD8alpha expression as a reliable marker of lymphoid origin has underpinned a number of studies in which DC expressing CD8alpha did not induce deletion, thereby clouding the issue of whether deletion is indeed a function of lymphoid DC. By returning to basics, that is, functional testing of the progeny of lymphoid- and myeloid-restricted progenitors in vivo, we hope to provide clear evidence of the in vivo roles of lymphoid and myeloid DC subsets, independent of assumptions about the surface phenotypes they can assume.

Current methods for loading dendritic cells with tumor antigen for the induction of antitumor immunity

The immunotherapy of cancer is predicated on the belief that it is possible to generate a clinically meaningful antitumor response that provides patient benefit, such as improvement in the time to progression or survival. Indeed, immunotherapeutics with dendritic cells (DC) as antigen-presenting delivery vehicles for cell-based vaccines have already improved patient outcome against a wide range of tumor types (1-9). This approach stimulates the patient's own antitumor immunity through the induction or enhancement of T-cell immunity. It is generally believed that the activity of cytotoxic T lymphocytes (CTL), the cells directly responsible for killing the tumor cells in vivo, are directed by DC. Therefore, the goal of many current designs for DC-based vaccines is to induce strong tumor-specific CTL responses in patients with cancer. In practice, most studies for DC-based cancer vaccine development have focused on the development of methods that can effectively deliver exogenous tumor antigens to DC for cross-priming of CD8+ T cells through the endogenous MHC class I processing and presentation pathway (10). To date, many methods have been developed or evaluated for the delivery of defined and undefined tumor antigens to DC. This review provides a brief summary on these methods, the techniques used in these methods, as well as the advantages and disadvantages of each method.

Antigen presentation and T cell stimulation by dendritic cells

Dendritic cells take up antigens in peripheral tissues, process them into proteolytic peptides, and load these peptides onto major histocompatibility complex (MHC) class I and II molecules. Dendritic cells then migrate to secondary lymphoid organs and become competent to present antigens to T lymphocytes, thus initiating antigen-specific immune responses, or immunological tolerance. Antigen presentation in dendritic cells is finely regulated: antigen uptake, intracellular transport and degradation, and the traffic of MHC molecules are different in dendritic cells as compared to other antigen-presenting cells. These specializations account for dendritic cells' unique role in the initiation of immune responses and the induction of tolerance.

Homeostasis of naïve, effector and memory CD8 T cells

Homeostatic control of CD8 T cell populations is essential for defense against infectious pathogens. Our understanding of the mechanisms that control naïve, effector and memory T cell populations in the intact animal has increased significantly over the last several years. There have been some surprises. For example, peripheral tissues have been found to harbor unexpectedly large numbers of effector memory T cells. Also unexpected was the finding of programmed T cell proliferation following very brief exposure to antigen. These and other recent advances are summarized in the following review.

Dendritic cells pulsed with exogenous hepatitis B surface antigen particles efficiently present epitopes to MHC class I-restricted cytotoxic T cells

L(d)- and K(b)-binding epitopes processed by murine dendritic cells (DC) pulsed with exogenous, particulate hepatitis B surface antigen (HBsAg) are presented to cytotoxic T lymphocytes (CTL). The specific and dose-dependent induction of IFN-gamma release and cytotoxicity in CTL by metabolically active DC did not depend on antigenic peptides contaminating the particles, was cytochalasin D resistant, independent of the maturation state of DC, and blocked by primaquine, amiloride and NH(4)Cl (indicating involvement of acid proteolysis). The specific immunostimulatory phenotype of pulsed DC was maintained for about 3 h after the end of the pulse but rapidly decayed thereafter. Processing of L(d)- and K(b)-binding epitopes from exogenous HBsAg particles by pulsed DC for presentation was TAP independent. Surface-associated 'empty' (presentation-deficient) 64(+) L(d) molecules (defined by the mAb 64-3-7), but not trimeric (presentation-competent) 30(+) L(d) molecules (defined by the mAb 30-5-7) had to be available during the pulse of DC with exogenous HBsAg particles to generate 30(+) L(d)molecules that present the antigenic S(28-39) peptide. Exogenous beta2-microglobulin present during the pulse of DC with HBsAg particles facilitated presentation of L(d)- and K(b)-restricted epitopes. DC generated from bone marrow progenitors in vitro, as well as splenic and liver DC (generated in vivo) presented epitopes to specific CTL. HBsAg particles thus efficiently enter an alternative processing pathway in DC that leads to presentation of epitopes to MHC class I-restricted CTL.

Cell-based therapy approaches using dying cells: from tumour immunotherapy to transplantation tolerance induction

Cell-based therapies are promising approaches to treat uncontrolled pathologies, such as tumours. Apoptotic tumour cells have recently been proposed as a source of tumour-associated antigens to stimulate an efficient immune response. However, a complex relationship exists between apoptosis and the immune system. In this review, the different factors that may influence immune responses against apoptotic cells are detailed and discussed in the light of recent publications. These factors include the nature of the phagocytes and the receptors involved in apoptotic cell uptake, as well as the environment in which cells are dying. A possible distinction between apoptosis and necrosis by immune system sentinels adds a further level of complexity. The potential use of the immunomodulatory properties associated with apoptosis to favour engraftment and induce tolerance in transplantation is then discussed. In conclusion, this review will suggest appropriate conditions to efficiently and safely use apoptotic cells as a new cell therapy product.

CD40L blockade prevents autoimmune diabetes by induction of bitypic NK/DC regulatory cells

Systemic treatment with antibody to CD40 ligand (aCD40L) can prevent autoimmunity and transplant rejection in several animal models and is currently under evaluation in clinical trials. While it is known that aCD40L administration inhibits expansion and effector functions of aggressive T cells, it is still unclear whether additional regulatory mechanisms are operative. Here we demonstrate that a single episode of CD40L blockade during development of the autoaggressive immune response completely prevented autoimmune disease in the RIP-LCMV mouse model for virally induced type 1 diabetes. Interestingly, protection could be transferred by a highly potent, bitypic cell population sharing phenotypic and functional properties of both natural killer (NK) and dendritic cells (DC). Furthermore, protection of prediabetic recipients was autoantigen specific and did not result in generalized immunosuppression. The origin, function, and therapeutic potential of these bitypic NK/DC regulatory cells is discussed.

Peptide-pulsed splenic dendritic cells prime long-lasting CD8(+) T cell memory in the absence of cross-priming by host APC

Immunization with cells expressing endogenous antigens can stimulate long-lived CD8(+) T cell memory. In many cases, the response is also stimulated by host antigen-presenting cells (APC) that have processed antigen from internalized apoptotic cells or cell fragments. This study investigated whether immunization with peptide-pulsed dendritic cells (DC) could prime long-lasting, peptide-specific CD8(+) T cell immunity in the absence of cross-priming by host APC. C57BL / 6 female mice immunized with syngeneic male splenic DC pulsed with the H-2K(b)-restricted ovalbumin peptide OVA(257 - 264) made memory CD8(+) CD44(high) T cell responses to OVA(257 - 264) and the male antigen HY more than 1 year after immunization. Establishment and maintenance of peptide-specific CD8(+) T cell memory did not require antibody or B cells. Immunization of H-2(bxd) mice with OVA(257 - 264)-pulsed minor-incompatible H-2(b) or H-2(d) DC demonstrated that CD8(+) T cells were primed exclusively by the injected cells, and not by peptide transferred to host APC, even though there was very effective cross-priming for CD8(+) T cell responses to the minor antigens expressed by the DC. Thus peptide-pulsed DC can prime long-lasting CD8(+) memory responses without any requirement for cross-priming by other APC.

Dendritic cells in autoimmune diseases

Subclinical autoimmune responses can be frequently detected in healthy individuals. Sustained activation of autoreactive lymphocytes is, however, required for the development of autoimmune diseases associated with ongoing tissue destruction either in single organs or generalized with multiple manifestations. Clinical and experimental evidence suggests that prolonged presentation of self antigens by dendritic cells is crucial for the development of destructive autoimmune disease. We discuss here a simplified threshold model where the key parameters for the magnitude of the autoimmune response are the amount of previously ignored self peptides presented by dendritic cells and the duration of the antigen presentation in secondary lymphoid organs. Multiple factors influence the threshold for the conversion of an autoimmune response to overt autoimmune disease. Frequent or persistent viral infections of the target organ may favor autoimmune disease by increasing the amounts of released self antigens, generating cytokine-mediated bystander activation of self-reactive lymphocytes and/or sustaining a chronic response via neoformation of lymphoid structures in the target organ.

Dendritic cells: their significance in health and disease

Dendritic cells (DCs) represent a heterogeneous population of professional antigen-presenting cells. Precursor cells move via the blood to peripheral tissues. These immature DCs can take up invading pathogens and then rapidly migrate to the draining secondary lymphoid organs. Converted into antigen-presenting mature DCs, these cells are able to prime naive T cells and to initiate an adoptive immune response. The extraordinary functional profile suggests that, under certain preconditions, DCs may represent an ideal vector in the immunotherapy of cancer and infectious diseases