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Co-stimulation provided by the B7 family of proteins underpins the development of protective immunity. There are three identified members of this family: CD80, its splice variant IgV-CD80 and CD86. It has hitherto been difficult to analyze the expression and function of IgV-CD80 since there are no appropriate reagents capable of distinguishing it from CD80. We have generated mice, by gene targeting, the lack CD80 whilst maintaining expression of IgV-CD80. Mutant animals did not delete T cells bearing mammary tumor virus-reactive TCR as efficiently as wild-type animals. We also demonstrate the importance of IgV-CD80 in the responses of recently activated cells and reveal a role for CD80 in sustaining T cell responses. CD86, whilst critical to primary T cell activation, made only a minor contribution to re-activation of normal cells.

Background: Peripheral tolerance is required to maintain balance within the immune system. A feature of peripheral tolerance is linked suppression, in which tolerance induced to a single T cell epitope inhibits the response to all epitopes in the same protein. It is suggested that this phenomenon is mediated by regulatory T cells through either the activity of immunopressive cytokines or direct cell contact. In previous experiments we failed to detect inhibitory cytokines when T cells from mice rendered tolerant by intranasal delivery of the immunodominant peptide of Der p 1 (p 1, 110-131) were restimulated with peptide in vitro. Therefore, the aim of this study was to determine if cognate interactions between T cells mediated by Notch/Delta signaling induce and maintain peripheral T cell tolerance., Methods: Using in situ hybridization and viral mediated gene transfer, the expression and function of Delta1 were investigated in a murine model of T cell tolerance to Der p 1 in vivo., Results: Delta1 expression is increased on peripheral T cells during the induction of tolerance with high-dose peptide delivered intranasally and when tolerant animals are rechallenged under immunogenic conditions. Peptide p 1, 110-131-specific CD4+ T cells transfected with Delta1 inhibited the response of antigen-primed T cells and induced linked suppression., Conclusions: High-dose peptide delivered intranasally induces transient expression of Delta 1 on inhibitory CD4+ T cells. Ligation of the Notch1 receptor on neighbouring T cells by Delta1+ regulatory T cells inhibits clonal expansion of the former and mediates linked suppression.

Background: Pretransplant blood transfusion has a well-known beneficial effect on posttransplant graft survival. Recently, it has been proposed that the clinical benefit of transfusion is due to HLA-DR antigen sharing between the blood donor(s) and the recipient. Immunological studies have suggested that this might result from a functional deletion of donor-reactive cytotoxic T lymphocytes., Methods: We investigated frequencies of alloreactive lymphocyte precursors with cytotoxic or interleukin-2-producing helper function by limiting dilution analysis in 10 renal dialysis patients before and after transfusion with fresh, allogeneic whole blood. Five patients received blood transfusions from donors matched for one HLA haplotype (or one HLA-B-DR antigen) and the other five patients received blood from fully HLA-mismatched donors., Results: Contrary to some previous reports, frequency analysis of cytotoxic T lymphocyte precursors revealed no significant differences between the two treatment groups in terms of development of blood donor-specific hyporesponsiveness after transfusion. Split-well analysis of cytotoxic T lymphocyte precursors reactive with single-mismatched HLA antigens demonstrated that the effects of transfusion on alloreactive specificity are complex and may vary depending on the particular antigens mismatched between the recipient and blood donor. Analysis of donor-specific helper T lymphocyte precursor frequencies revealed a significant decrease of interleukin-2-producing cells 3 months after transfusion in the total patient population. This effect was most prominent in the recipients of HLA-mismatched blood, but it also exhibited some degree of nonspecificity, as frequencies of third-party reactive helper T lymphocyte precursors were also significantly reduced., Conclusions: Our overall results suggest that the degree of HLA matching between blood donor and recipient does not greatly influence the effect of blood transfusion on the T lymphocyte allorepertoire. The apparent induced down-regulation of helper T lymphocyte activity may play a role in the reported immunosuppressive effects of allogeneic blood transfusion.

The success of transplantation is such that it is now the treatment of choice for many of those requiring renal replacement therapy. The use of other solid organs, including liver, pancreas, heart and lung, continues to progress. This article reviews some recent advances in our understanding of the immunological response to alloantigen and xenoantigen.

CD28 has been identified in man and mouse as a potent costimulatory receptor on T cells. We have generated a mAb, called JJ319, to rat CD28 and show that it is expressed on virtually all peripheral rat alpha beta and on most gamma delta T cells, and on about half of NK cells. In contrast to the mouse but as in humans, most immature CD4+8+TCRlow thymocytes express little or no CD28, whereas CD28 expression is high on TCRintermediate and TCRhigh cells. mAb JJ319 very effectively costimulates T cell proliferation and IL-2 secretion by resting rat T cells. In contrast to results obtained in mice and humans, phorbol ester did not synergize in T cell activation with CD28-specific mAb but even induced sensitivity to cyclosporin A in T cell cultures that were optimally costimulated by mAbs to the TCR and to CD28. This result points to a novel effect of protein kinase activation by phorbol ester on signal transduction by TCR plus CD28 costimulation which only becomes apparent if, as in the rat, the TCR-mediated signal cannot be replaced by phorbol ester.

Activated T cells bearing receptors for interleukin 2 (IL-2) play an important role in immunity and in immunopathological processes such as allograft rejection. In order to investigate the presence of activated T cells in lymphocytic infiltrates in transplanted kidneys, we investigated the uptake and retention of radioactivity after an intravenous injection of radioiodinated IL-2 in experimentally transplanted rats. IL-2 was enzyme radiolabelled with 123iodine using a glucose oxidase/lactoperoxidase method and shown to retain specific binding on an IL-2 receptor positive cell line, C58E6. To examine the kinetics of 123iodine-interleukin 2 (123I-IL-2) uptake in vivo, animals that had been transplanted five days previously with allogeneic or syngeneic grafts were injected with 123I-IL-2 and then imaged using an external gamma camera. Radioactivity was measured at time points up to 240 min after intravenous injection of 123I-IL-2. Four groups of animals were examined: allogeneic grafts (n = 7); syngeneic grafts (n = 6); ischaemic native kidneys (n = 5) all following injection with 123I-IL-2; and allogeneic transplants (n = 5) after injection of 123I-lactalbumin, an irrelevant molecule of similar molecular weight to IL-2. The peak radioactivity after injection was measured and the amount of radioactivity retained in the graft at increasing time intervals after injection was expressed as a function of initial peak radioactivity. At four hours after injection of 123I-IL-2, mean retention of activity by rejecting grafts was 77(+/- 2.68)% of peak activity, compared to 45(+/- 6.38)% in syngeneic controls (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance to alloantigen may be induced in rats by administration of blood followed by transplantation of a renal allograft. The mechanism of this tolerance was investigated by directly analyzing the functional activity of graft-infiltrating cells. We have previously shown cytotoxic T lymphocyte infiltration of, and major histocompatibility complex induction on, grafts of tolerant animals. We now report that cells isolated from the grafts of tolerant rats show a reduced expression of the p55 interleukin 2 receptor (IL-2R) chain on the cell surface compared with that seen on the cells of untreated animals. Scatchard analysis further reveals low expression of high affinity IL-2R. This is due to reduced transcription of both IL-2R alpha and beta chain mRNAs and results in a reduced ability of cells to proliferate in response to IL-2. Cells isolated from tolerant animals are unable to make biologically active IL-2 in culture, whereas cells from untreated animals make high levels. This is not reflected at the mRNA level as the IL-2 gene is induced in both tolerant and untreated animals to similar levels. The induction of tolerance is abrogated by administration of recombinant IL-2 to animals at the time of transplantation. Thus, we conclude that an altered regulation of the IL-2 pathway results in tolerance in these alloantigen-treated and transplanted animals.

A mouse monoclonal antibody, designated MRC OX-19, has been prepared that binds to virtually all rat thymocytes, T lymphocytes and a maximum of 2% B lymphocytes. Immunoperoxidase studies on tissue sections showed no reactivity with nonlymphoid cells in intestine, thymus, spleen and lymph node. The antigen recognized by MRC OX-19 antibody was identified by metabolic and cell surface labeling of thymocytes followed by immunoprecipitation and sodium dodecyl sulfate gel electrophoresis. It is a surface glycoprotein of Mr = 69 000. When included in in vitro assays for T cell functions, MRC OX-19 increased proliferation stimulated by allogeneic spleen cells or the lectins phytohemagglutinin and concanavalin A. The antibody itself was not mitogenic and its stimulatory effect could be correlated with an increase in interleukin 2 production. Taken together the data suggest that MRC OX-19 could be the equivalent of mouse Ly-1 antigen and human T1 antigen.

We studied the potential immunoenhancing effects of high doses of rIL-2 on murine T and B cell functions in vivo. Injection of rIL-2 caused a threefold or more increase in the frequencies of antigen-specific proliferative T cells, suggesting that rIL-2 initiated a polyclonal T cell response. In primary and secondary humoral immune responses, administration of rIL-2 in vivo selectively enhanced the production of IgM antibodies, whereas the IgG response was unaffected. Coadministration of rIL-2 with antigen failed to induce an isotype switch from IgM to IgG in genetically low-responding mice. Interestingly, in mice treated with rIL-2 alone (in the absence of exogenous antigen), polyclonal IgM production was induced. Polyclonal IgM production of lesser magnitude was found when mice were immunized with specific antigen in the absence of exogenous rIL-2, suggesting that local IL-2 concentrations in a primary immune response might be sufficient to elicit a polyclonal IgM response.