William R. Heath

T cell receptor antagonist peptides induce positive selection

We have used organ culture of fetal thymic lobes from T cell receptor (TCR) transgenic beta 2M(-/-) mice to study the role of peptides in positive selection. The TCR used was from a CD8+ T cell specific for ovalbumin 257-264 in the context of Kb. Several peptides with the ability to induce positive selection were identified. These peptide-selected thymocytes have the same phenotype as mature CD8+ T cells and can respond to antigen. Those peptides with the ability to induce positive selection were all variants of the antigenic peptide and were identified as TCR antagonist peptides for this receptor. One peptide tested, E1, induced positive selection on the beta 2M(-/-) background but negative selection on the beta 2M(+/-) background. These results show that the process of positive selection is exquisitely peptide specific and sensitive to extremely low ligand density and support the notion that low efficacy ligands mediate positive selection.

Help for cytotoxic-T-cell responses is mediated by CD40 signalling

Cytotoxic T lymphocytes (CTLs) which carry the CD8 antigen recognize antigens that are presented on target cells by the class I major histocompatibility complex. CTLs are responsible for the killing of antigen-bearing target cells, such as virus-infected cells. Although CTL effectors can act alone when killing target cells, their differentiation from naive CD8-positive T cells is often dependent on ‘help’ from CD4-positive helper T (TH) cells. Furthermore, for effective CTL priming, this help must be provided in a cognate manner, such that both the TH cell and the CTL recognize antigen on the same antigen-presenting cell,. One explanation for this requirement is that TH cells are needed to convert the antigen-presenting cell into a cell that is fully competent to prime CTL. Here we show that signalling through CD40 on the antigen-presenting cells can replace the requirement for TH cells, indicating that T-cell ‘help’, at least for generation of CTLs by cross-priming, is mediated by signalling through CD40 on the antigen-presenting cell.

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

We describe the generation of ovalbumin (OVA)‐specific, MHC class II‐restricted αβ T cell receptor (TCR) transgenic mice. Initial attempts at generating these transgenic mice utilized heterologous regulatory elements to drive the expression of cDNA genes encoding the separate α‐ and β‐chains of the TCR. Unexpectedly, T cells bearing the transgenic αβ TCR failed to emerge from the thymus in these mice, although the transgenes did modify endogenous TCR expression. However, subsequent modification of the approach which enabled expression of the TCR β‐chain under the control of its natural regulatory elements generated mice whose peripheral T cells expressed the transgenic TCR and were capable of antigen‐dependent proliferation. These results show that successful generation of MHC class II‐restricted, OVA‐specific αβTCR transgenic mice was dependent upon combining cDNA‐ and genomic DNA‐based constructs for expression of the respective α‐ and β‐chains of the TCR.

Cross-presentation, dendritic cell subsets, and the generation of immunity to cellular antigens

Summary:  Cross‐presentation involves the uptake and processing of exogenous antigens within the major histocompatibility complex (MHC) class I pathway. This process is primarily performed by dendritic cells (DCs), which are not a single cell type but may be divided into several distinct subsets. Those expressing CD8α together with CD205, found primarily in the T‐cell areas of the spleen and lymph nodes, are the major subset responsible for cross‐presenting cellular antigens. This ability is likely to be important for the generation of cytotoxic T‐cell immunity to a variety of antigens, particularly those associated with viral infection, tumorigenesis, and DNA vaccination. At present, it is unclear whether the CD8α‐expressing DC subset captures antigen directly from target cells or obtains it indirectly from intermediary DCs that traffic from peripheral sites. In this review, we examine the molecular basis for cross‐presentation, discuss the role of DC subsets, and examine the contribution of this process to immunity, with some emphasis on DNA vaccination.

Memory T cells in nonlymphoid tissue that provide enhanced local immunity during infection with herpes simplex virus

Effective immunity is dependent on long-surviving memory T cells. Various memory subsets make distinct contributions to immune protection, especially in peripheral infection. It has been suggested that T cells in nonlymphoid tissues are important during local infection, although their relationship with populations in the circulation remains poorly defined. Here we describe a unique memory T cell subset present after acute infection with herpes simplex virus that remained resident in the skin and in latently infected sensory ganglia. These T cells were in disequilibrium with the circulating lymphocyte pool and controlled new infection with this virus. Thus, these cells represent an example of tissue-resident memory T cells that can provide protective immunity at points of pathogen entry.

Cross-presentation of viral and self antigens by skin-derived CD103 + dendritic cells

Skin-derived dendritic cells (DCs) include Langerhans cells, classical dermal DCs and a langerin-positive CD103+ dermal subset. We examined their involvement in the presentation of skin-associated viral and self antigens. Only the CD103+ subset efficiently presented antigens of herpes simplex virus type 1 to naive CD8+ T cells, although all subsets presented these antigens to CD4+ T cells. This showed that CD103+ DCs were the migratory subset most efficient at processing viral antigens into the major histocompatibility complex class I pathway, potentially through cross-presentation. This was supported by data showing only CD103+ DCs efficiently cross-presented skin-derived self antigens. This indicates CD103+ DCs are the main migratory subtype able to cross-present viral and self antigens, which identifies another level of specialization for skin DCs.

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.

The CD8alpha(+) dendritic cell is responsible for inducing peripheral self-tolerance to tissue-associated antigens.

We previously described a mechanism for the maintenance of peripheral self-tolerance. This involves the cross-presentation of tissue-associated antigens by a bone marrow–derived cell type that stimulates the proliferation and ultimate deletion of self-reactive CD8 T cells. This process has been referred to as cross-tolerance. Here, we characterize the elusive cell type responsible for inducing cross-tolerance as a CD8α+ dendritic cell (DC). To achieve this aim, transgenic mice were generated expressing yellow fluorescent protein (YFP) linked to CTL epitopes for ovalbumin and glycoprotein B (gB) of herpes simplex virus under the rat insulin promoter (RIP). Although tracking of YFP was inconclusive, the use of a highly sensitive gB-specific hybridoma that produced β-galactosidase on encounter with antigen, enabled detection of antigen presentation by cells isolated from the pancreatic lymph node. This showed that a CD11c+CD8α+ cell was responsible for cross-tolerance, the same DC subset as previously implicated in cross-priming. These data indicate that CD8α+ DCs play a critical role in both tolerance and immunity to cell-associated antigens, providing a potential mechanism by which cytotoxic T lymphocyte can be immunized to viral antigens while maintaining tolerance to self.

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.

The CD8+ dendritic cell subset

Summary:  Mouse lymphoid tissues contain a subset of dendritic cells (DCs) expressing CD8α together with a pattern of other surface molecules that distinguishes them from other DCs. These molecules include particular Toll‐like receptor and C‐type lectin pattern recognition receptors. A similar DC subset, although lacking CD8 expression, exists in humans. The mouse CD8+ DCs are non‐migrating resident DCs derived from a precursor, distinct from monocytes, that continuously seeds the lymphoid organs from bone marrow. They differ in several key functions from their CD8− DC neighbors. They efficiently cross‐present exogenous cell‐bound and soluble antigens on major histocompatibility complex class I. On activation, they are major producers of interleukin‐12 and stimulate inflammatory responses. In steady state, they have immune regulatory properties and help maintain tolerance to self‐tissues. During infection with intracellular pathogens, they become major presenters of pathogen antigens, promoting CD8+ T‐cell responses to the invading pathogens. Targeting vaccine antigens to the CD8+ DCs has proved an effective way to induce cytotoxic T lymphocytes and antibody responses.