We have established a murine model system for exploring the ability of a CD4 subset-deficient host to cope with cytomegalovirus infection, and reported three findings. First, an antiviral response of the CD8 subset of T lymphocytes could be not only initiated but also maintained for a long period of time despite a continued absence of the CD4 subset, whereas the production of antiviral antibody proved strictly dependent upon help provided by the CD4 subset. Second, no function in the defense against infection could be ascribed as yet to CD4-CD8- T lymphocytes, which were seen to accumulate to a new subset as a result of depletion of the CD4 subset. This newly arising subset did not substitute for CD4+ T lymphocytes in providing help to B lymphocytes, and was also not effective in controlling the spread of virus in host tissues. As long as a function of these cells in the generation and maintenance of a CD8 subset-mediated response is not disproved, caution is indicated with concern to an autonomy of the CD8 subset. Third, even though with delay, the CD8+ effector cells raised in the CD4 subset-deficient host were able of clear vital tissues from productive infection and to restrict asymptomatic, persistent infection to acinar glandular epithelial cells in salivary gland tissue.

https://rupress.org/jem/article-pdf/169/4/1199/1098943/1199.pdf

Site-restricted persistent cytomegalovirus infection after selective long-term depletion of CD4+ T lymphocytes.

Perforin-deficient mice have been generated by homologous recombination to determine whether the effects of CD8+ cytolytic T cells and natural killer cells are mediated by pore formation involving perforin. These mice are viable and fertile and have normal numbers of CD8+ T cells and natural killer cells which do not lyse virus-infected or allogeneic fibroblasts or natural killer target cells in vitro. The mice fail to clear lymphocytic choriomeningitis virus and they eliminate fibrosarcoma tumour cells with reduced efficiency. Perforin is therefore a key effector molecule for T-cell- and natural killer-cell-mediated cytolysis.

Cytotoxicity mediated by T cells and natural killer cells is greatly impaired in perforin-deficient mice

THE receptor–ligand pairs CD28–B7 and CD40–gp39 are essential for the initiation and amplification of T-cell-dependent immune responses1,2 CD28–B7 interactions provide 'second signals' necessary for optimal T-cell activation and IL-2 production3–5, whereas CD40–gp39 signals co-stimulate B-cell, macrophage, endothelial cell and T-cell activation6–12 Nonetheless, blockade of either of these pathways alone is not sufficient to permit engraftment of highly immunogenic allografts13–15 Here we report that simultaneous but not independent blockade of the CD28 and CD40 pathways effectively aborts T-cell clonal expansion in vitro and in vivo, promotes long-term survival of fully allogeneic skin grafts, and inhibits the development of chronic vascular rejection of primarily vascularized cardiac allografts The requirement for simultaneous blockade of these pathways for effective inhibition of alloimmunity indicates that, although they are interrelated, the CD28 and CD40 pathways are critical independent regulators of T-cell-dependent immune responses

Long-term acceptance of skin and cardiac allografts after blocking CD40 and CD28 pathways.

The initiation of T-cell-mediated antitumor immune responses requires the uptake and processing of tumor antigens by dendritic cells and their presentation on MHC-I molecules. Here we show in a human in vitro model system that exosomes, a population of small membrane vesicles secreted by living tumor cells, contain and transfer tumor antigens to dendritic cells. After mouse tumor exosome uptake, dendritic cells induce potent CD8+ T-cell-dependent antitumor effects on syngeneic and allogeneic established mouse tumors. Therefore, exosomes represent a novel source of tumor-rejection antigens for T-cell cross priming, relevant for immunointerventions.

Tumor-derived exosomes are a source of shared tumor rejection antigens for CTL cross-priming.

A mAb J43 has been produced against the product of the mouse PD-1 gene, a member of the Ig gene superfamily, which was previously isolated from an apoptosis-induced T cell hybridoma (2B4.11) by using subtractive hybridization. Analyses by flow cytometry and immunoprecipitation using the J43 mAb revealed that the PD-1 gene product is a 50-55 kDa membrane protein expressed on the cell surface of several PD-1 cDNA transfectants and 2B4.11 cells. Since the molecular weight calculated from the amino acid sequence is 29, 310, the PD-1 protein appears to be heavily glycosylated. Normal murine lymphoid tissues such as thymus, spleen, lymph node and bone marrow contained very small numbers of PD-1(+) cells. However, a significant PD-1(+) population appeared in the thymocytes as well as T cells in spleen and lymph nodes by the in vivo anti-CD3 mAb treatment. Furthermore, the PD-1 antigen expression was strongly induced in distinct subsets of thymocytes and spleen T cells by in vitro stimulation with either anti-CD3 mAb or concanavalin A (Con A) which could lead T cells to both activation and cell death. Similarly, PD-1 expression was induced on spleen B cells by in vitro stimulation with anti-IgM antibody. By contrast, PD-1 was not significantly expressed on lymphocytes by treatment with growth factor deprivation, dexamethasone or lipopolysaccharide. These results suggest that the expression of the PD-1 antigen is tightly regulated and induced by signal transduction through the antigen receptor and do not exclude the possibility that the PD-1 antigen may play a role in clonal selection of lymphocytes although PD-1 expression is not required for the common pathway of apoptosis.

Expression of the PD-1 antigen on the surface of stimulated mouse T and B lymphocytes

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.

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

Summary
Rat monoclonal antibodies were used to deplete selectively Lyt 2 (cytotoxic) and L3T4 (helper) T cell populations in vivo. These antibodies produced >95% depletion of the respective T cell subset as determined by fluorescent antibody and cytofluoro-graphic analyses. Antibody-treated mice were infected in the ear pinna with herpes simplex virus (HSV) and the induction of virus-specific T cell and antibody responses were monitored during the acute infection. Lyt 2-deficient mice produced delayed hypersensitivity and HSV-specific antibodies comparable to those in untreated animals. However, major histocompatibility complex class I-restricted T cell killing was abolished. In contrast, L3T4-deficient animals failed to produce either primary delayed hypersensitivity response or specific antibodies to the virus, but cytotoxic T cells responses were induced and even augmented in comparison with infected, normal animals. This observation clearly demonstrates that Lyt 2 cytotoxic T cells can be induced in a helper T cell-deficient environment. The ability of T cell subset-deficient mice to clear infectious virus was investigated in the skin of the ear and the part of the nervous system innervating the site of infection. L3T4-deficient animals showed a markedly delayed clearance of virus from the ear and also had a more florid infection of the nervous system. However, Lyt 2-deficient mice cleared the infection in the ear normally, but a severe infection of the nervous system was still observed. The implication of these observations to the pathogenesis of this virus is discussed.

A Jayasuriya

Papers

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

Different roles for L3T4+ and Lyt 2+ T cell subsets in the control of an acute herpes simplex virus infection of the skin and nervous system.