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.

Binary Agrobacterium vectors for plant transformation

A vector molecule for the efficient transformation of higher plants has been constructed with several features that make it efficient to use. It utilizes the trans acting functions of the vir region of a co-resident Ti plasmid in Agrobacterium tumefaciens to transfer sequences bordered by left and right T-DNA border sequences into the nuclear genome of plants. The T-region contains a dominant selectable marker gene that confers high levels of resistance to kanamycin, and a lac alpha-complementing region from M13mp19 that contains several unique restriction sites for the positive selection of inserted DNA.

CD8(+) but not CD8(-) dendritic cells cross-prime cytotoxic T cells in vivo

Bone marrow-derived antigen-presenting cells (APCs) take up cell-associated antigens and present them in the context of major histocompatibility complex (MHC) class I molecules to CD8(+) T cells in a process referred to as cross-priming. Cross-priming is essential for the induction of CD8(+) T cell responses directed towards antigens not expressed in professional APCs. Although in vitro experiments have shown that dendritic cells (DCs) and macrophages are capable of presenting exogenous antigens in association with MHC class I, the cross-presenting cell in vivo has not been identified. We have isolated splenic DCs after in vivo priming with ovalbumin-loaded beta2-microglobulin-deficient splenocytes and show that they indeed present cell-associated antigens in the context of MHC class I molecules. This process is transporter associated with antigen presentation (TAP) dependent, suggesting an endosome to cytosol transport. To determine whether a specific subset of splenic DCs is involved in this cross-presentation, we negatively and positively selected for CD8(-) and CD8(+) DCs. Only the CD8(+), and not the CD8(-), DC subset demonstrates cross-priming ability. FACS((R)) studies after injection of splenocytes loaded with fluorescent beads showed that 1 and 0.6% of the CD8(+) and the CD8(-) DC subsets, respectively, had one or more associated beads. These results indicate that CD8(+) DCs play an important role in the generation of cytotoxic T lymphocyte responses specific for cell-associated antigens.

Defective CD8 T cell memory following acute infection without CD4 T cell help

The CD8+ cytotoxic T cell response to pathogens is thought to be CD4+ helper T cell independent because infectious agents provide their own inflammatory signals. Mice that lack CD4+ T cells mount a primary CD8 response to Listeria monocytogenes equal to that of wild-type mice and rapidly clear the infection. However, protective memory to a challenge is gradually lost in the former animals. Memory CD8+ T cells from normal mice can respond rapidly, but memory CD8+ T cells that are generated without CD4 help are defective in their ability to respond to secondary encounters with antigen. The results highlight a previously undescribed role for CD4 help in promoting protective CD8 memory development.

Introduction of soluble protein into the class I pathway of antigen processing and presentation

In order to investigate how peptides associate with class I major histocompatibility complex (MHC) glycoproteins intracellularly, we generated cytotoxic T lymphocytes (CTL) specific for a readily available soluble protein in association with class I. C57BL/6 (H-2b) mice immunized against a syngeneic tumor cell transfected with chicken ovalbumin (OVA) cDNA gave rise to H-2Kb-restricted CTL specific for the OVA258-276 peptide. This synthetic peptide and CNBr fragments of OVA (242-285 and 242-273) were able to target H-2b cells for lysis by the CTL in a 3 hr assay. Cells incubated with native OVA for up to 24 hr did not become sensitized for recognition and lysis. However, when OVA was introduced directly into the cytoplasm of cells by the osmotic lysis of pinosomes, the Kb restricted determinant formed readily.

CD8(+) T cells: foot soldiers of the immune system

Resting naive CD8(+) T cells have an astounding capacity to react to pathogens by massive expansion and differentiation into cytotoxic effector cells that migrate to all corners of the body to clear the infection. The initial interaction with antigen-presenting cells in the central lymphoid organs drives an orchestrated program of differentiation aimed at producing sufficient numbers of effectors to get the job done without resulting in clonal exhaustion. Interactions with antigen-presenting cells and other immune cells continue at the site of infection to regulate further on-site expansion and differentiation, all with the goal of protecting the host with minimal bystander tissue damage. Here we review recent advances in CD8(+) T cell recognition of antigen in lymphoid as well as in nonlymphoid tissues in the periphery, and how CD8(+) T cell expansion and differentiation are controlled in these contexts.

Effector and memory CTL differentiation

Technological advances in recent years have allowed for an ever-expanding ability to analyze and quantify in vivo immune responses. MHC tetramers, intracellular cytokine staining, an increasing repertoire of transgenic and "knockout" mice, and the detailed characterization of a variety of infectious models have all facilitated more precise and definitive analyses of the generation and function of cytotoxic T lymphocytes (CTL). Understanding the mechanisms behind the differentiation of effector and memory CTL is of increasing importance to develop vaccination strategies against a variety of established and emerging infectious diseases. This review focuses on recent advances in our understanding of how effector and memory CTL differentiate and survive in vivo in response to viral or bacterial infection.

Cross-priming for a secondary cytotoxic response to minor H antigens with H-2 congenic cells which do not cross-react in the cytotoxic assay

Cytotoxic effector T cells of F1 (BALB/c X BALB.B) (H-2d/b) mice immunized against the minor histocompatibility differences of C57BL/10 (H-2b) can lyse targets from C57BL/10, but cannot lyse B10.D2 (H-2d) targets. Despite this lack of cross-reaction in the cytotoxic assay, C57BL/10 cells do prime F1 (BALB/c X BALB.B) mice for a secondary cytotoxic response to B10.D2. C57BL/10-primed, B10.D2-boosted cytotoxic cells lyse B10.D2 targets but not C57BL/10 targets. DBA/2 (H-2d) spleen cells or thymocytes prime F1 mice for a secondary response to DBA/2, B10.D2, and C57BL/10 cells, but DBA/2 mastocytes, P815, do not prime for a response to C57BL/10. Whether H-2 congenic lymphoid cells express minor histocompatibility determinants which cross-react at the cytotoxic T-cell level or the helper T-cell level is discussed.

Selecting and maintaining a diverse T-cell repertoire

To provide a T-cell population that will respond promptly to foreign antigen, the immune system looks inward, using the variety of self-antigens to select and maintain a diverse repertoire of receptors. A protective immune system must include a T-lymphocyte population that is poised to respond to foreign antigenic peptides presented by self-major histocompatibility complex molecules. As the organism cannot predict the precise pathogen-derived antigens that will be encountered, the system uses the diverse array of self-peptides bound to self-major histocompatibility complex molecules, not only to select a receptor repertoire in the thymus, but also to keep naïve T cells alive and 'ready for action' in the periphery.

Massive expansion of antigen-specific CD8+ T cells during an acute virus infection

During LCMV infection, CD8+ T cells expand greatly. Bystander activation has been thought to play a role because few cells score as LCMV specific in limiting dilution analysis. In contrast, we find that at least a quarter of the CD8+ cells secrete IFNgamma specifically in response to LCMV peptides at the peak of the response. Moreover, by analyzing the expansion of adoptively transferred LCMV-specific, TCR-transgenic CD8+ T cells in congenic hosts, we have determined that most of the CD8+ cell expansion is virus specific. Analysis of the effect of the monospecific TCR-transgenic T cells on the host response to three LCMV epitopes suggests that CTL precursors compete for sites on the APC in an epitope-specific fashion and that this competition determines the specificity of the response.

Interleukin-2 signals during priming are required for secondary expansion of CD8+ memory T cells

Although interleukin-2 (IL-2) was initially characterized as the primary T-cell growth factor following in vitro activation, less is known about its role in shaping T-cell responses to acute infections in vivo. The use of IL-2- or IL-2-receptor-deficient mice is problematic owing to their early development of autoimmunity, attributable to the central role of IL-2 in the generation, maintenance and function of CD4+CD25+ regulatory T cells. To bypass these inherent difficulties, we have studied the effect of IL-2 on T-cell responses to acute infections by adopting a mixed chimaera strategy in which T cells lacking the high-affinity IL-2 receptor could be studied in an otherwise healthy mouse containing a full complement of regulatory T cells. Here we show that although IL-2 signalling to pathogen-specific CD8+ T cells affects the number of developing effector and memory cells very little, it is required for the generation of robust secondary responses. This is not due to an altered T-cell-receptor repertoire development or selection, and does not reflect an acute requirement for IL-2 during secondary activation and expansion. Rather, we demonstrate a previously unappreciated role for IL-2 during primary infection in programming the development of CD8+ memory T cells capable of full secondary expansion. These results have important implications for the development of vaccination or immunotherapeutic strategies aimed at boosting memory T-cell function.

Analysis of 1.9 Mb of contiguous sequence from chromosome 4 of Arabidopsis thaliana

The plant Arabidopsis thaliana (Arabidopsis) has become an important model species for the study of many aspects of plant biology. The relatively small size of the nuclear genome and the availability of extensive physical maps of the five chromosomes provide a feasible basis for initiating sequencing of the five chromosomes. The YAC (yeast artificial chromosome)-based physical map of chromosome 4 was used to construct a sequence-ready map of cosmid and BAC (bacterial artificial chromosome) clones covering a 1.9-megabase (Mb) contiguous region, and the sequence of this region is reported here. Analysis of the sequence revealed an average gene density of one gene every 4.8 kilobases (kb), and 54% of the predicted genes had significant similarity to known genes. Other interesting features were found, such as the sequence of a disease-resistance gene locus, the distribution of retroelements, the frequent occurrence of clustered gene families, and the sequence of several classes of genes not previously encountered in plants.

Complete but curtailed T-cell response to very low-affinity antigen

Following an infection, CD8⁺ T cells are activated and undergo a characteristic kinetic sequence of rapid expansion, subsequent contraction and formation of memory cells1–3. The pool of naïve T cell clones is diverse and contains cells bearing T cell antigen receptors (TCR) that differ in their affinity for the same antigen4,5. How these differences in affinity impact the function and the response kinetics of individual T cell clones was previously unknown. Here we show that during the in vivo response to microbial infection, even very weak TCR-ligand interactions are sufficient to activate naïve T cells, induce rapid initial proliferation and generate effector and memory cells. The strength of the TCR-ligand interaction critically impacts when expansion stops, when the cells exit lymphoid organs and when contraction begins, i.e. strongly stimulated T cells contract and exit lymphoid organs later than do weakly stimulated cells. Our data challenges the prevailing view that strong TCR ligation is a prerequisite for CD8⁺ T cell activation. Instead, very weak interactions are sufficient for activation, but strong TCR ligation is required to sustain T cell expansion. We propose that in response to microbial challenge, T cell clones with a broad range of avidities for foreign ligands are initially recruited, and that the pool of T cells subsequently matures in affinity due to the more prolonged expansion of high affinity T cell clones.

CD4+ T cells are required for the maintenance, not programming, of memory CD8+ T cells after acute infection

Immunization in the absence of CD4(+) T cell help results in defective CD8(+) T cell memory, deficient recall responses and diminished protective immunity. Here we investigated at what stage during the immune response to pathogen CD4(+) T cells are essential in the promotion of functional CD8(+) T cell memory. Memory CD8(+) T cell numbers decreased gradually in the absence of CD4(+) T cells despite the presence of similar numbers of memory cell precursors at the peak of the effector phase. Adoptive transfer of effector or memory CD8(+) T cells into wild-type or CD4(+) T cell-deficient mice demonstrated that the presence of CD4(+) T cells was important only after, not during, the early CD8(+) T cell programming phase. In the absence of CD4(+) T cells, memory CD8(+) T cells became functionally impaired and decreased in quantity over time. We conclude that in the context of an acute infection, CD4(+) T cells are required only during the maintenance phase of long-lived memory CD8(+) T cells.

Naive T cells transiently acquire a memory-like phenotype during homeostasis-driven proliferation

In a depleted lymphoid compartment, naive T cells begin a slow proliferation that is independent of cognate antigen yet requires recognition of major histocompatibility complex-bound self-peptides. We have followed the phenotypic and functional changes that occur when naive CD8(+) T cells undergo this type of expansion in a lymphopenic environment. Naive T cells undergoing homeostasis-driven proliferation convert to a phenotypic and functional state similar to that of memory T cells, yet distinct from antigen-activated effector T cells. Naive T cells dividing in a lymphopenic host upregulate CD44, CD122 (interleukin 2 receptor beta) and Ly6C expression, acquire the ability to rapidly secrete interferon gamma, and become cytotoxic effectors when stimulated with cognate antigen. The conversion of naive T cells to cells masquerading as memory cells in response to a homeostatic signal does not represent an irreversible differentiation. Once the cellularity of the lymphoid compartment is restored and the T cells cease their division, they regain the functional and phenotypic characteristics of naive T cells. Thus, homeostasis-driven proliferation provides a thymus-independent mechanism for restoration of the naive compartment after a loss of T cells.

Low-affinity ligands for the TCR drive proliferation of mature CD8+ T cells in lymphopenic hosts

In the absence of thymic emigration, the peripheral T cell pool is maintained by division of mature lymphocytes. We have examined the molecular interactions required for peripheral CD8+ T cell expansion in lymphopenic mice without conventional antigenic stimulation. Expansion of CD8+ T cells in lymphopenic hosts was found to be peptide specific. An antagonist peptide known to serve as a ligand for positive selection of these T cells promoted expansion; however, a control peptide that binds the same class I molecule did not. Surprisingly, the cells undergoing proliferation in lymphopenic hosts did not mature to cytotoxic effectors and displayed a partially activated surface phenotype. These data suggest that division of T cells in the periphery of lymphopenic hosts requires specific recognition of self-peptide/MHC complexes, similar to the signal for thymocyte maturation.

Cytokine requirements for acute and Basal homeostatic proliferation of naive and memory CD8+ T cells

Both naive and memory T cells undergo antigen-independent proliferation after transfer into a T cell-depleted environment (acute homeostatic proliferation), whereas only memory T cells slowly divide in a full T cell compartment (basal proliferation). We show, first, that naive and memory CD8+ T cells have different cytokine requirements for acute homeostatic proliferation. Interleukin (IL)-7 receptor(R)alpha-mediated signals were obligatory for proliferation of naive T cells in lymphopenic hosts, whereas IL-15 did not influence their division. Memory T cells, on the other hand, could use either IL-7Ralpha- or IL-15-mediated signals for acute homeostatic proliferation: their proliferation was delayed when either IL-7Ralpha was blocked or IL-15 removed, but only when both signals were absent was proliferation ablated. Second, the cytokine requirements for basal and acute homeostatic proliferation of CD8+ memory T cells differ, as basal division of memory T cells was blocked completely in IL-15-deficient hosts. These data suggest a possible mechanism for the dearth of memory CD8+ T cells in IL-15- and IL-15Ralpha-deficient mice is their impaired basal proliferation. Our results show that naive and memory T lymphocytes differ in their cytokine dependence for acute homeostatic proliferation and that memory T lymphocytes have distinct requirements for proliferation in full versus empty compartments.

Positive selection of thymocytes

Differentiation of alpha beta T cell receptor (TCR)-expressing T cells involves an obligatory interaction with self-major histocompatibility complex (MHC) molecules in the thymus. This process, called positive selection, both rescues thymocytes from programmed cell death and induces their differentiation into mature T cells. Another critical event in thymic development is to prevent maturation of hazardous autoreactive T cells; thus, mechanisms exist to eliminate T cells with self-reactive receptors (negative selection). How can these two pathways be distinguished? This question, which has long taxed immunologists, is more opposite because many features of the interactions in positive and negative selection are shared: Both processes are exquisitely MHC-allele specific, they involve MHC-bound peptide recognition, and employ at least some overlapping signal transduction pathways. However, resolution of this paradox has become much more feasible with the advent of powerful systems for withdrawing and reconstituting individual components involved in positive selection. This review describes recent advances in our understanding of the cells, receptors, ligands, and signaling pathways involved in this process. A pivotal part of this puzzle is the basis for discrimination between TCR ligands that induce positive vs negative selection. Recent work suggests that the peptide/MHC ligand for positive selection may bind with low avidity to the TCR. The implications of these data for the nature of T cell recognition during positive selection are discussed below.

Memory T cells persisting within the brain after local infection show functional adaptations to their tissue of residence

The brain is not routinely surveyed by lymphocytes and is defined as an immuno-privileged site. However, viral infection of the brain results in the infiltration and long-term persistence of pathogen-specific CD8(+) T cells. These cells survive without replenishment from the circulation and are referred to as resident memory T cells (Trm). Brain Trm selectively express the integrin CD103, the expression of which is dependent on antigen recognition within the tissue. After clearance of virus, CD8(+) T cells persist in tight clusters, presumably at prior infection hot spots. Antigen persistence is not a prerequisite for T-cell retention, as suggested by the failure to detect viral genomes in the T-cell clusters. Furthermore, we show that an intracranial dendritic cell immunization regimen, which allows the transient introduction of antigen, also results in the generation of memory T cells that persist long term in the brain. Brain Trm die rapidly on isolation from the tissue and fail to undergo recall expansion after adoptive transfer into the bloodstream of antigen-challenged recipients. These ex vivo defects imply a dependency on the local milieu for function and survival. Cumulatively, this work shows that Trm are a specialized population of memory T cells that can be deposited in tissues previously thought to be beyond routine immune surveillance.

The major histocompatibility complex determines susceptibility to cytotoxic T cells directed against minor histocompatibility antigens

Cytotoxic cells were generated by immunizing one strain of mouse with cells from an allogeneic strain which carries the same H-2 region. The effector cells assayed in a 4 h 51Cr release assay were shown to be T cells and indistinguishable, except in specificity, from cytotoxic T cells directed at H-2 alloantigens. Although the genetic differences between responder and stimulator cells responsible for the immunization did not code in H-2, the H-2 complex did restrict susceptibility of target cells. For example, BALB.B cytotoxic cells (H-2b) immunized against and capable of lysing C57BL/6 cells (H-2b) would not lyse B6.C/H-2d target cells. C57BL/6 and B6.C/H-2d are congenic and differ in the H-2 region. Two hypotheses are considered to explain the H-2 restriction of susceptibility to cytotoxic T cells generated by an H-2 identical alloimmunization. (a) The dual (self) recognition hypothesis states that the cytotoxic cell has two recognition units, one for H-2-coded structures and another clonally restricted receptor for the minor alloantigen. (b) The interaction antigen hypothesis states that all the surface alloantigenic determinants recognized by cytotoxic T cells are the result of interaction between H-2- and non-H-2-coded gene products. Two lines of evidence, one with F1 effector cells and the other a cold target competition experiment, are presented which argue strongly in favor of the interaction antigen hypothesis. The regions of H-2 required to be histocompatible were mapped to the D region and to the left of IC, probably the K region. These results, and recent work on the response to virus-infected and TNP-modified syngeneic cells, suggest that cytotoxic cells are restricted in specificity to preferentially recognizing alterations in structures that are coded in the major histocompatibility complex.

Towards functional characterisation of the members of the R2R3-MYB gene family from Arabidopsis thaliana

Transcription factors containing a conserved DNA-binding domain similar to that of the proto-oncogene c-myb have been identified in nearly all eukaryotes. MYB-related proteins from plants generally contain two related helix-turn-helix motifs, the R2 and R3 repeats. It was estimated that Arabidopsis thaliana contains more than 100 R2R3-MYB genes. The few cases where functional data are available suggest an important role of these genes in the regulation of secondary metabolism, the control of cell shape, disease resistance, and hormone responses. To determine the full regulatory potential of this large family of regulatory genes, a systematic search for the function of all genes of this family was initiated. Sequence data for more than 90 different A. thaliana R2R3-MYB genes have been obtained. Sequence comparison revealed conserved amino acid motifs shared by subgroups of R2R3-MYB genes in addition to the characteristic DNA-binding domain. No significant clustering of the genes was detected, although they are not uniformly distributed throughout the A. thaliana genome.

Hybrid antibodies can target sites for attack by T cells

It would be advantageous in the case of certain diseases to be able to focus a strong T-cell response at a chosen target, for example, in treating cancer or infections that have escaped the normal host response. At present, it seems inconceivable that we could use antigen-specific lines or clones of effector T cells for this purpose because of complications due to the major histocompatibility restriction of T-cell specificity and the problem of rejection of transplanted effector cells. Here we describe a novel technology which combines the power of T lymphocytes in eliminating unwanted cells and causing beneficial inflammatory reactions with the great advantages of monoclonal antibodies (their specificity and availability). We show that heteroconjugates of monoclonal antibodies (referred to hereafter as hybrid antibodies), in which one of the component binding sites is anti-T-cell receptor and the other component binding site is directed against any chosen target antigen, can focus T cells to act at the targeted site. Monoclonal antibodies directed against the T-cell receptor, such as the anti-allotype used here, are mitogenic for resting T cells and can be used to induce effector T cells carrying the T-cell receptor determinant which can then be directed against the target by a hybrid antibody.