The chemokine receptor CCR4 in vascular recognition by cutaneous but not intestinal memory T cells.

Lymphocytes that are responsible for regional (tissue-specific) immunity home from the blood to the intestines, inflamed skin or other sites through a multistep process involving recognition of vascular endothelial cells and extravasation. Chemoattractant cytokine molecules known as chemokines regulate this lymphocyte traffic, in part by triggering arrest (stopping) of lymphocytes rolling on endothelium. Here we show that many systemic memory T cells in blood carry the chemokine receptor CCR4 and therefore respond to its ligands, the chemokines TARC and MDC. These cells include essentially all skin-homing cells expressing the cutaneous lymphocyte antigen and a subset of other systemic memory lymphocytes; however, intestinal (alpha4beta7+) memory and naive T cells respond poorly. Immunohistochemistry reveals anti-TARC reactivity of venules and infiltration of many CCR4+ lymphocytes in chronically inflamed skin, but not in the gastrointestinal lamina propria. Moreover, TARC induces integrin-dependent adhesion of skin (but not intestinal) memory T cells to the cell-adhesion molecule ICAM-1, and causes their rapid arrest under physiological flow. Our results suggest that CCR4 and TARC are important in the recognition of skin vasculature by circulating T cells and in directing lymphocytes that are involved in systemic as opposed to intestinal immunity to their target tissues.     

Ribonucleotides in DNA newly synthesised in 3T6 cells in vivo.

Within the field of DNA replication, considerable interest has focused in recent years on the mechanism of initiation of synthesis of DNA molecules. In vitro replication systems from Escherichia coli have been instrumental in uncovering a priming function fo9r ribonucleotides on the earliest intermediates of DNA polymerisation in vitro and in identifying the proteins involved. In vitro replication systems from mammalian cells that permit the use of the phosphate-transfer method for detection of RNA-DNA junctions as well as direct labelling of the RNA moiety of the molecules have suggested a similar role for ribonucleotides in DNA synthesis in eukaryotes. However, the existence of this mechanism in mammalian cells in vivo has not been established. Here we report the first evidence that a significant proportion of the earliest intermediates in mammalian DNA polymerisation in vivo do, in fact, possess ribonucleotides, presumably because their synthesis was initiated with one or more ribonucleotides.     

Activation-independent binding of human memory T cells to adhesion molecule ELAM-1.

The induction of an ensemble of adhesion molecules on endothelial cells by inflammatory cytokines is likely to be crucial to the differential migration of T-lymphocyte subsets into inflammatory sites. Two molecular pathways involving the VLA-4 and LFA-1 integrins are known to mediate T-cell adhesion to activated endothelium. Here we show that a third pathway involving the rapidly inducible endothelial cell-surface adhesion molecule ELAM-1 contributes to the binding of resting CD4+ T cells to IL-1-induced human endothelial cells. All three pathways contribute to the greater adhesion to endothelium of memory T cells than naive T cells. There are two unique features of T-cell adhesion to purified ELAM-1: first, ELAM-1 exclusively mediates adhesion of memory T cells; second, memory T-cell binding to ELAM-1 is independent of acute activation events that regulate integrin-mediated adhesion. Thus, ELAM-1 may be of primary importance in the initial attachment of memory T cells to inflamed endothelium in vivo and to the preferential migration of memory T cells into tissue and inflammatory sites.     

Preferential expression of the T-cell receptor V beta 3 gene by Mlsc reactive T cells

The precursor frequency of T cells specific for any given foreign antigen is, in general, extremely low. Prominent exceptions to this rule are the T cells that are specific for foreign major histocompatibility complex (MHC) products or for products of the minor lymphocyte stimulatory (Mls) genes in the mouse which are present at high frequencies. Here, we report a striking overlap or cross-reactivity between the T cells specific for the protein antigen pigeon cytochrome c in association with Ek alpha Ek beta and the set of T cells specific for Mlsc products. In addition, we demonstrate that the basis for this overlap is the predominant expression of one T-cell receptor (TCR) V beta gene, V beta 3, by T cells that recognize Mlsc products. These results indicate the importance of specific TCR alpha beta dimers in the recognition of Mlsc products and that positive or negative selection of T cells specific for Mls self-determinants may selectively alter the repertoire of T cells available for MHC-restricted recognition of foreign antigens.     

Memory CD4 T cells emerge from effector T-cell progenitors

A hallmark of adaptive immunity is the generation of memory T cells that confer long-lived, antigen-specific protection against repeat challenges by pathogens. Understanding the mechanisms by which memory T cells arise is important for rational vaccination strategies and improved therapeutic interventions for chronic infections and autoimmune disorders. The large clonal expansion of CD8 T cells in response to some infections has made the development of CD8 T-cell memory more amenable to study, giving rise to a model of memory cell differentiation in which a fraction of fully competent effector T cells transition into long-lived memory T cells. Delineation of CD4 T-cell memory development has proved more difficult as a result of limitations on tracking the smaller populations of CD4 effector T cells generated during a pathogenic challenge, complicating efforts to determine whether CD4 memory T cells are direct descendants of effector T cells or whether they develop by alternative pathways. Here, using two complementary cytokine reporter mouse models to identify interferon (IFN)-gamma-positive effector T cells and track their fate, we show that the lineage relationship between effector and memory CD4 T cells resembles that for CD8 T cells responding to the same pathogen. We find that, in parallel with effector CD8 T cells, IFN-gamma-positive effector CD4 T cells give rise to long-lived memory T cells capable of anamnestic responses to antigenic rechallenge.     

Therapy with monoclonal antibodies by elimination of T-cell subsets in vivo

A major aim in immunology has been to understand how the immune system evokes characteristic responses to infection, foreign tissue grafts and tumours. The current view of immunoregulation is based mainly on studies of lymphocyte subsets, either in vitro or by adoptive transfer to irradiated recipients. Many reagents are available for defining T-cell subsets, but only recently have there been helper T-cell-specific antibodies against the mouse equivalent of the Leu3/T4 (man) and W3/25 (rat) antigens. It is clear that monoclonal antibodies will eventually replace antilymphocyte globulin for immunosuppression in organ grafting, but although there has been some clinical success, most monoclonal reagents cause only transient reductions in their target cells in vivo. This uncertainty in the potency of monoclonal antibodies has led some workers to consider them as targeting agents for such highly cytotoxic drugs as ricin A (ref. 21). We show here that unmodified monoclonal antibodies can be extremely effective at depleting cells in vivo and can be used for the selective manipulation of different aspects of the immune response.     

KIR expression on self-reactive CD8+ T cells is controlled by T-cell receptor engagement

Natural killer cell tolerance is maintained by the interaction of killer inhibitory receptors (KIRs) with self-major histocompatibility complex class I gene products. A subset of T cells also expresses inhibitory receptors, but the functional significance of these receptors on T cells is unclear. Here we show that, in the absence of T-cell receptor (TCR) engagement, KIRs expressed on CD8+ T cells are slowly downregulated by KIR ligands expressed on antigen-presenting cells. The resulting expression levels of KIR are no longer able to inhibit T-cell function. In contrast, TCR engagement sustains KIR expression, and re-induces functional levels of KIR expression after ligand-induced downregulation of KIR. Our data indicate that KIR expression on CD8+ T cells in vivo may be maintained through continuous encounters with antigen. As KIR-mediated inhibition of T-cell activation can be bypassed at high antigen concentrations, dynamic KIR expression may mediate T-cell tolerance to self-antigens by sparing self-reactive T cells, thus enabling them to mediate potentially useful immune functions to quantitatively or qualitatively different antigens.     

Induction of cytotoxic T-cell responses in vivo in the absence of CD4 helper cells

Cytotoxic T lymphocytes (CTL) seem to provide the major line of defence against many viruses. CTL effector functions are mediated primarily by cells carrying the CD8 (Ly-2) antigen (CD8+ cells) and are triggered by interactions of the T-cell receptor with an antigenic complex, often termed 'self plus X', composed of viral determinants in association with class I molecules of the major histocompatibility complex (MHC). The mechanism(s) of induction of virus-specific CTL in vivo is poorly understood, but data from in vitro experiments suggest that their generation is strictly dependent on functions provided by CD4+ helper T cells (also referred to as L3T4+; or TH) that respond to antigens in the context of class II (Ia) MHC determinants. The prevailing opinion that induction of most functions of CD8+ cells requires help provided by CD4+ cells has recently been challenged by the observation that CD8+ cells alone can mediate a variety of responses to alloantigens in vitro and in vivo; however, the possibility that CTL to self plus X could be generated in vivo in the absence of TH cells has not been evaluated. We report here that C57BL/6J (B6) and AKR/J mice, when functionally depleted of CD4+ cells by in vivo treatment with the CD4+-specific rat monoclonal antibody GK1.5 (refs 8-14) responded to ectromelia virus infection by developing an optimal in vivo virus-specific CTL response, and subsequently recovered from the disease (mousepox) that was lethal for similarly infected nude mice (CD4-, CD8-).     

Helper T cells regulate type-2 innate immunity in vivo

Type-2 immunity requires orchestration of innate and adaptive immune responses to protect mucosal sites from pathogens. Dysregulated type-2 responses result in allergy or asthma. T helper 2 (T(H)2) cells elaborate cytokines, such as interleukin (IL)-4, IL-5, IL-9 and IL-13, which work with toxic mediators of innate immune cells to establish environments that are inhospitable to helminth or arthropod invaders. The importance of T(H)2 cells in coordinating innate immune cells at sites of inflammation is not known. Here we show that polarized type-2 immune responses are initiated independently of adaptive immunity. In the absence of B and T cells, IL-4-expressing eosinophils were recruited to tissues of mice infected with the helminth Nippostrongylus brasiliensis, but eosinophils failed to degranulate. Reconstitution with CD4 T cells promoted accumulation of degranulated IL-4-expressing cells, but only if T cells were stimulated with cognate antigen. Degranulation correlated with tissue destruction, which was attenuated if eosinophils were depleted. Helper T cells confer antigen specificity on eosinophil cytotoxicity, but not cytokine responses, so defining a novel mechanism that focuses tissue injury at sites of immune challenge.     

Cross-dressed dendritic cells drive memory CD8+ T-cell activation after viral infection

After an infection, cytotoxic T lymphocyte precursors proliferate and become effector cells by recognizing foreign peptides in the groove of major histocompatibility complex (MHC) class I molecules expressed by antigen-presenting cells (APCs). Professional APCs specialized for T-cell activation acquire viral antigen either by becoming infected themselves (direct presentation) or by phagocytosis of infected cells, followed by transfer of antigen to the cytosol, processing and MHC class I loading in a process referred to as cross-presentation. An alternative way, referred to as 'cross-dressing', by which an uninfected APC could present antigen was postulated to be by the transfer of preformed peptide-MHC complexes from the surface of an infected cell to the APC without the need of further processing. Here we show that this mechanism exists and boosts the antiviral response of mouse memory CD8(+) T cells. A number of publications have demonstrated sharing of peptide-loaded MHC molecules in vitro. Our in vitro experiments demonstrate that cross-dressing APCs do not acquire peptide-MHC complexes in the form of exosomes released by donor cells. Rather, the APCs and donor cells have to contact each other for the transfer to occur. After a viral infection, we could isolate cross-dressed APCs able to present viral antigen in vitro. Furthermore, using the diphtheria toxin system to selectively eliminate APCs that could only acquire viral peptide-MHC complexes by cross-dressing, we show that such presentation can promote the expansion of resting memory T cells. Notably, naive T cells were excluded from taking part in the response. Cross-dressing is a mechanism of antigen presentation used by dendritic cells that may have a significant role in activating previously primed CD8(+) T cells.     

Viral escape by selection of cytotoxic T cell-resistant virus variants in vivo.

Viruses persist in an immune population, as in the case of influenza, or in an individual, as postulated for human immunodeficiency virus, when they are able to escape existent neutralizing antibody responses by changing their antigens. It is now shown that viruses can in principle escape the immunosurveillance of virus-specific cytotoxic T cells by mutations that alter the relevant T-cell epitope.