Intrathymic deletion of MHC class I-restricted cytotoxic T cell precursors by constitutive cross-presentation of exogenous antigen

Cross-presentation by the others

The critical role of conventional dendritic cells in physiological cross-priming of immune responses to tumors and pathogens is widely documented and beyond doubt. However, there is ample evidence that a wide range of other cell types can also acquire the capacity to cross-present. These include not only other myeloid cells such as plasmacytoid dendritic cells, macrophages and neutrophils, but also lymphoid populations, endothelial and epithelial cells and stromal cells including fibroblasts. The aim of this review is to provide an overview of the relevant literature that analyzes each report cited for the antigens and readouts used, mechanistic insight and in vivo experimentation addressing physiological relevance. As this analysis shows, many reports rely on the exceptionally sensitive recognition of an ovalbumin peptide by a transgenic T cell receptor, with results that therefore cannot always be extrapolated to physiological settings. Mechanistic studies remain basic in most cases but reveal that the cytosolic pathway is dominant across many cell types, while vacuolar processing is most encountered in macrophages. Studies addressing physiological relevance rigorously remain exceptional but suggest that cross-presentation by non-dendritic cells may have significant impact in anti-tumor immunity and autoimmunity.

The T Cell Repertoire-Diversifying Enzyme TSSP Contributes to Thymic Selection of Diabetogenic CD4 T Cell Specificities Reactive to ChgA and IAPP Autoantigens

Multiple studies highlighted the overtly self-reactive T cell repertoire in the diabetes-prone NOD mouse. This autoreactivity has primarily been linked to defects in apoptosis induction during central tolerance. Previous studies suggested that thymus-specific serine protease (TSSP), a putative serine protease expressed by cortical thymic epithelial cells and thymic dendritic cells, may edit the repertoire of self-peptides presented by MHC class II molecules and shapes the self-reactive CD4 T cell repertoire. To gain further insight into the role of TSSP in the selection of self-reactive CD4 T cells by endogenous self-Ags, we examined the development of thymocytes expressing distinct diabetogenic TCRs sharing common specificity in a thymic environment lacking TSSP. Using mixed bone marrow chimeras, we evaluated the effect of TSSP deficiency confined to different thymic stromal cells on the differentiation of thymocytes expressing the chromogranin A-reactive BDC-2.5 and BDC-10.1 TCRs or the islet amyloid polypeptide-reactive TCR BDC-6.9 and BDC-5.2.9. We found that TSSP deficiency resulted in deficient positive selection and induced deletion of the BDC-6.9 and BDC-10.1 TCRs, but it did not affect the differentiation of the BDC-2.5 and BDC-5.2.9 TCRs. Hence, TSSP has a subtle role in the generation of self-peptide ligands directing diabetogenic CD4 T cell development. These results provide additional evidence for TSSP activity as a novel mechanism promoting autoreactive CD4 T cell development/accumulation in the NOD mouse.

Central tolerance spares the private high-avidity CD4(+) T-cell repertoire specific for an islet antigen in NOD mice

Although central tolerance induces the deletion of most autoreactive T cells, some autoreactive T cells escape thymic censorship. Whether potentially harmful autoreactive T cells present distinct TCRαβ features remains unclear. Here, we analyzed the TCRαβ repertoire of CD4(+) T cells specific for the S100β protein, an islet antigen associated with type 1 diabetes. We found that diabetes-resistant NOD mice deficient for thymus specific serine protease (TSSP), a protease that impairs class II antigen presentation by thymic stromal cells, were hyporesponsive to the immunodominant S100β1-15 epitope, as compared to wild-type NOD mice, due to intrathymic negative selection. In both TSSP-deficient and wild-type NOD mice, the TCRαβ repertoire of S100β-specific CD4(+) T cells though diverse showed a specific bias for dominant TCRα rearrangements with limited CDR3α diversity. These dominant TCRα chains were public since they were found in all mice. They were of intermediate- to low-avidity. In contrast, high-avidity T cells expressed unique TCRs specific to each individual (private TCRs) and were only found in wild-type NOD mice. Hence, in NOD mice, the autoreactive CD4(+) T-cell compartment has two major components, a dominant and public low-avidity TCRα repertoire and a private high-avidity CD4(+) T-cell repertoire; the latter is deleted by re-enforced negative selection.

Steady-state antigen scavenging, cross-presentation, and CD8+ T cell priming: a new role for lymphatic endothelial cells

Until recently, the known roles of lymphatic endothelial cells (LECs) in immune modulation were limited to directing immune cell trafficking and passively transporting peripheral Ags to lymph nodes. Recent studies demonstrated that LECs can directly suppress dendritic cell maturation and present peripheral tissue and tumor Ags for autoreactive T cell deletion. We asked whether LECs play a constitutive role in T cell deletion under homeostatic conditions. In this study, we demonstrate that murine LECs under noninflamed conditions actively scavenge and cross-present foreign exogenous Ags to cognate CD8(+) T cells. This cross-presentation was sensitive to inhibitors of lysosomal acidification and endoplasmic reticulum-golgi transport and was TAP1 dependent. Furthermore, LECs upregulated MHC class I and the PD-1 ligand PD-L1, but not the costimulatory molecules CD40, CD80, or CD86, upon Ag-specific interactions with CD8(+) T cells. Finally, Ag-specific CD8(+) T cells that were activated by LECs underwent proliferation, with early-generation apoptosis and dysfunctionally activated phenotypes that could not be reversed by exogenous IL-2. These findings help to establish LECs as APCs that are capable of scavenging and cross-presenting exogenous Ags, in turn causing dysfunctional activation of CD8(+) T cells under homeostatic conditions. Thus, we suggest that steady-state lymphatic drainage may contribute to peripheral tolerance by delivering self-Ags to lymph node-resident leukocytes, as well as by providing constant exposure of draining peripheral Ags to LECs, which maintain tolerogenic cross-presentation of such Ags.

Antigen cross-presentation: extending recent laboratory findings to therapeutic intervention

The initiation of adaptive immune responses requires antigen presentation to lymphocytes. In particular, dendritic cells (DCs) are equipped with specialized machinery that promote effective display of peptide/major histocompatibility complexes (MHC), rendering them the most potent stimulators of naive T lymphocytes. Antigen cross-presentation to CD8(+) T cells is an important mechanism for the development of specific cytotoxic T lymphocyte (CTL) responses against tumours and viruses that do not infect antigen-presenting cells. Here, we review recent findings concerning antigen cross-presentation to CD8(+) T lymphocytes. Specific subtypes of DCs in the mouse have been defined as being especially endowed for antigen cross-presentation, and a human homologue of these DCs has recently been described. DC vaccination strategies for the prevention and treatment of human diseases have been under investigation in recent years, but have not generally reached satisfying results. We here provide an overview of new findings in antigen cross-presentation research and how they can be used for development of the next generation of human DC vaccines.

Peripheral T cells re-enter the thymus and interfere with central tolerance induction

The thymus mainly contains developing thymocytes that undergo thymic selection. In addition, some mature activated peripheral T cells can re-enter the thymus. We demonstrated in this study that adoptively transferred syngeneic Ag-specific T cells can enter the thymus of lymphopenic mice, where they delete thymic dendritic cells and medullary thymic epithelial cells in an Ag-specific fashion, without altering general thymic functions. This induced sustained thymic release of autoreactive self-Ag-specific T cells suggested that adoptively transferred activated T cells can specifically alter the endogenous T cell repertoire by erasing negative selection of their own specificities. Especially in clinical settings in which adoptively transferred T cells cause graft-versus-host disease or graft-versus-leukemia, as well as in adoptive tumor therapies, these findings might be of importance, because the endogenous T cell repertoire might be skewed to contribute to both manifestations.

Constitutive ablation of dendritic cells breaks self-tolerance of CD4 T cells and results in spontaneous fatal autoimmunity

Lack of immunological tolerance against self-antigens results in autoimmune disorders. During onset of autoimmunity, dendritic cells (DCs) are thought to be critical for priming of self-reactive T cells that have escaped tolerance induction. However, because DCs can also induce T cell tolerance, it remains unclear whether DCs are required under steady-state conditions to prevent autoimmunity. To address this question, we crossed CD11c-Cre mice with mice that express diphtheria toxin A (DTA) under the control of a loxP-flanked neomycin resistance (neo^R) cassette from the ROSA26 locus. Cre-mediated removal of the neo^R cassette leads to DTA expression and constitutive loss of conventional DCs, plasmacytoid DCs, and Langerhans cells. These DC-depleted (ΔDC) mice showed increased frequencies of CD4 single-positive thymocytes and infiltration of CD4 T cells into peripheral tissues. They developed spontaneous autoimmunity characterized by reduced body weight, splenomegaly, autoantibody formation, neutrophilia, high numbers of Th1 and Th17 cells, and inflammatory bowel disease. Pathology could be induced by reconstitution of wild-type (WT) mice with bone marrow (BM) from ΔDC mice, whereas mixed BM chimeras that received BM from ΔDC and WT mice remained healthy. This demonstrates that DCs play an essential role to protect against fatal autoimmunity under steady-state conditions.

The role of antigen-presenting cells in triggering graft-versus-host disease and graft-versus-leukemia

After allogeneic blood or bone marrow transplantation, donor T cells interact with a distorted antigen-presenting cell (APC) environment in which some, but not all, host APCs are replaced by APCs from the donor. Significantly, host APCs are required for the priming of acute graft-versus-host disease (GVHD). Donor APCs play a lesser role in the induction of acute GVHD despite their predicted capacity to cross-present host antigens. In contrast, donor APCs may play a role in perpetuating the tissue injury observed in chronic GVHD. Host APCs are also required for maximal graft-versus-leukemia responses. Recent studies have suggested potential strategies by which the continued presence of host APCs can be exploited to prime strong donor immunity to tumors without the induction of GVHD.

Factors affecting the susceptibility of the mouse pituitary gland to CD8 T-cell-mediated autoimmunity

We have previously shown, in a transgenic mouse model, that the pituitary gland is susceptible to CD8 T-cell-mediated autoimmunity, triggered by a cell-specific model autoantigen, resulting in pan-anterior pituitary hypophysitis and dwarfism. In the present study, we now demonstrate that antigen dose, the T-cell precursor frequency, the degree of lymphopenia and the context of target antigen expression, are important parameters determining the time course and extent of the pathological consequences of CD8 T-cell-mediated autoimmunity. Furthermore, our data indicate that the pituitary gland is susceptible to CD8 autoimmunity following an inflammatory insult such as a viral infection. As lymphocytic hypophysitis may be manifest in other autoimmune conditions, and the pituitary gland may be susceptible to T-cell-mediated pathology after immunization with a virus expressing soluble pituitary antigen, it is important to consider that strategies based on vaccination against soluble pituitary gonadotrophins could have other unexpected endocrine consequences.

Induction of tumor cell apoptosis in vivo increases tumor antigen cross-presentation, cross-priming rather than cross-tolerizing host tumor-specific CD8 T cells

Cross-presentation of cell-bound Ags from established, solid tumors to CD8 cells is efficient and likely to have a role in determining host response to tumor. A number of investigators have predicted that when tumor Ags are derived from apoptotic cells either no response, due to Ag "sequestration," or CD8 cross-tolerance would ensue. Because the crucial issue of whether this happens in vivo has never been addressed, we induced apoptosis of established hemagglutinin (HA)-transfected AB1 tumors in BALB/c mice using the apoptosis-inducing reagent gemcitabine. This shrank the tumor by approximately 80%. This induction of apoptosis increased cross-presentation of HA to CD8 cells yet neither gross deletion nor functional tolerance of HA-specific CD8 cells were observed, based on tetramer analysis, proliferation of specific CD8 T cells, and in vivo CTL activity. Interestingly, apoptosis primed the host for a strong antitumor response to a second, virus-generated HA-specific signal in that administration of an HA-expressing virus after gemcitabine administration markedly decreased tumor growth compared with viral administration without gemcitabine. Thus tumor cell apoptosis in vivo neither sequesters tumor Ags nor cross-tolerizes tumor-specific CD8 cells. This observation has fundamental consequences for the development of tumor immunotherapy protocols and for understanding T cell reactivity to tumors and the in vivo immune responses to apoptotic cells.

CD36 is differentially expressed by CD8+ splenic dendritic cells but is not required for cross-presentation in vivo

Cross-presentation allows the processing of Ags from donor cells into the MHC class I presentation pathway of dendritic cells (DCs). This is important for the generation of cytotoxic T cell immunity and for induction of self tolerance. Apoptotic cells are reported to be efficient targets for cross-presentation, and in vitro studies using human DCs have implicated CD36 in their capture. In support of a role for CD36 in cross-presentation, we show that this molecule is differentially expressed by CD8(+) splenic DCs, which previously have been identified as responsible for cross-presentation in the mouse. Three different cross-presentation models were examined for their dependence on CD36. These included cross-priming to OVA-coated spleen cells and cross-tolerance to OVA transgenically expressed in the pancreatic islet beta cells under constitutive conditions or during beta cell destruction. In these models, CD36 knockout DCs were equivalent to wild-type DCs in their capacity to cross-present either foreign or self Ags, indicating that CD36 is not essential for cross-presentation of cellular Ags in vivo.

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.

Analysis of thymic stromal cell populations using flow cytometry

The complexity of the lymphostromal interplay that is essential to alphabetaT-cell development is reflected by the heterogeneity of both lymphocytes and thymic stromal cells. While panels of monoclonal antibodies have described many of the cellular components of these microenvironments, the means to quantify stromal cell subsets using flow cytometry remains poorly defined. This study refines and compares various stromal cell isolation procedures and determines the effects of various digestion enzymes on important surface molecules. Three- and four-color flow cytometry is used to correlate established and novel stromal cell markers to define thymic fibroblasts, epithelium and a unique subset of thymic endothelium that express MHC class II. This work provides a basis for the purification of thymic stromal cells for further phenotypic, functional and genetic analysis.

Noncovalent association with stress protein facilitates cross-priming of CD8+ T cells to tumor cell antigens by dendritic cells

A viral oncogene carrying well-defined K(b)/D(b)-restricted epitopes was expressed in a heat shock protein (hsp)-associated or nonassociated form in the murine tumor cells P815 and Meth-A. Wild-type SV40 large T-Ag (wtT-Ag) is expressed without stable hsp association; mutant (cytoplasmic cT-Ag) or chimeric (cT272-green fluorescent fusion protein) T-Ag is expressed in stable association with the constitutively expressed, cytosolic hsp73 (hsc70) protein. In vitro, remnants from apoptotic wtT-Ag- or cT-Ag-expressing tumor cells are taken up and processed by immature dendritic cells (DC), and the K(b)/D(b)-binding epitopes T1, T2/3, and T4 of the T-Ag are cross-presented to CTL in a TAP-independent way. DC pulsed with remnants of transfected, apoptotic tumor cells cross-presented the three T-Ag epitopes more efficiently when they processed ATP-sensitive hsp73/cT-Ag complexes than when they processed hsp-nonassociated (native) T-Ag. In vivo, more IFN-gamma-producing CD8+ T cells were elicited by a DNA vaccine that encoded hsp73-binding mutant T-Ag than by a DNA vaccine that encoded native, non-hsp-binding T-Ag. Three- to 5-fold higher numbers of T-Ag (T1-, T2/3-, or T4-) specific, D(b)/K(b)-restricted IFN-gamma-producing CD8+ T cells were primed during the growth of transfected H-2(d) Meth-A/cT tumors than during the growth of transfected Meth-A/T tumors in F(1)(b x d) hosts. Hence, the association of an oncogene with constitutively expressed, cytosolic hsp73 facilitates cross-priming in vitro and in vivo of CTL by DC that process material from apoptotic cells.

Cross-presentation in viral immunity and self-tolerance

T lymphocytes recognize peptide antigens presented by class I and class II molecules encoded by the major histocompatibility complex (MHC). Classical antigen-presentation studies showed that MHC class I molecules present peptides derived from proteins synthesized within the cell, whereas MHC class II molecules present exogenous proteins captured from the environment. Emerging evidence indicates, however, that dendritic cells have a specialized capacity to process exogenous antigens into the MHC class I pathway. This function, known as cross-presentation, provides the immune system with an important mechanism for generating immunity to viruses and tolerance to self.

Sampling of complementing self-antigen pools by thymic stromal cells maximizes the scope of central T cell tolerance

Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity, yet the identity of cells involved remains elusive. Here we show that antigen expression in a minor fraction of medullary thymic epithelial cells leads to deletion of specific CD4 T cells. Strikingly, this deletion is not dependent on cross-presentation by hemopoietic antigen-presenting cells, which have been ascribed a predominant role in negative selection. By contrast, when the same antigen enters the thymus via the blood stream, negative selection is strictly dependent on antigen presentation by hemopoietic cells. These findings imply that the (re)-presentation of "self" by thymic stromal cells is non-redundant, and that different thymic antigen-presenting cells instead cover complementing sets of self-antigens, thus maximizing the scope of central tolerance

Self-antigen presentation by thymic stromal cells: a subtle division of labor

Self-antigen-MHC complexes expressed by thymic stromal cells serve as ligands for TCR-mediated positive and negative selection, resulting in a self-MHC-restricted, self-tolerant T cell repertoire. It has recently become apparent that thymic stromal cells differ in their accessibility to antigen as well as their ability to process and present antigen. These differences result in the sampling by thymic stromal cells of largely nonoverlapping self-antigen pools and the display of self-peptide profiles specific for each cell type. In conjunction with single or serial cell-cell interactions between thymocytes and stromal cells, such differences in self-antigen display allow for maximal (re)presentation of 'self' in the thymus and optimize the efficacy of positive and negative selection.