A high frequency of cytotoxic precursor cells for a syngeneic tumour

Signal minus 1: a key factor in immunological tolerance to tissue-specific self antigens?

Recent data suggest that many autoreactive T cells, particularly to tissue-specific self antigens, can escape thymic deletion. The current dogma is that these autoreactive T cells are silenced by the failure of most tissues to provide co-stimulation (signal 2), antigen alone (signal 1) inducing T cell unresponsiveness. However, I propose that activation of autoreactive T cells frequently occurs but autodestruction by effector T cells is tightly regulated. This phenomenon is most evident with lymph node metastasizing tumour cells where the regional lymph node can mount a vigorous response to the invading tumour cells but tumour growth is unimpaired. I suggest that autodestruction is prevented by inhibitory receptors on T cells which recognize class I MHC structures on target cells. These receptors, which I propose deliver 'signal minus 1' to T cells, were recently described on NK cells and a subpopulation of peripheral T cells. They are also strikingly similar to a family of anti-self receptors that my laboratory described on murine T and B cells 15 years ago. In the 'signal minus 1' model, antigen-activated T cells acquire the inhibitory receptors when they become co-stimulation independent and gain the ability to exit lymphoid organs and enter non-lymphoid tissues. Thus, if autoreactive effector T cells encounter autoantigen in tissues they are functionally silenced by inhibitory receptor engagement and signal minus 1 delivery. In contrast, I propose that in response to intracellular infections, cells down-regulate expression of their ligands for inhibitory receptors. Such a model allows infected cells to be selectively eliminated by effector T cells. If correct, the model predicts that effector T cells, whether foreign-antigen- or autoantigen-specific, can selectively respond to infected cells. This apparent 'usefulness' of autoreactive T cells may explain their observed persistence even after an encounter with autoantigen. It is also suggested that signal minus 1 may silence autoreactive B cells specific for tissue-specific cell surface antigens and lack of signal minus 1 may partially explain the vigorous T cell response to allogeneic MHC. Finally, it is hypothesized that, in evolutionary terms, inhibition of autodestruction by the recognition of a 'self marker' and delivery of signal minus 1 is an ancient process which probably emerged in early metazoans.

In vivo cytotoxic responses induced by allogeneic normal and neoplastic cells in mice: relative lack of immunogenicity of B16 melanoma cells

It has been previously reported from this laboratory that the C57BL/6 melanoma B16 will grow in allogeneic mice provided that donor-derived passenger leukocytes are first removed by in vitro passage of the tumor cells. In this paper we report that allogeneic and syngeneic mice support similar survival of [125I]iododeoxyuridine-labeled B16 cells (125I-B16 cells) as indicated by whole-body retention of 125I. B16 failed to induce the early cytotoxic response which was induced by semiallogeneic peritoneal cells (PC) and by two other allogeneic tumors (EL4 lymphoma and MC-2 sarcoma). They were nevertheless rapidly destroyed by the response induced by PC or EL4. B16 did show some immunogenicity as indicated by regression of some primary B16 tumors (45% ip and the majority of footpad tumors) and by the slow cytotoxic response to secondary 125I-B16 challenge following preimmunization with live or irradiated B16 cells. An equivalent response was induced by preimmunization with a 100-fold lower dose of PC. These findings are discussed in relation to the requirement for inductive stimuli, in addition to antigen, for the stimulation of in vivo cytotoxic responses.

Regulation of cytotoxic lymphocyte precursors: I. Interactions between concanavalin A and T cell growth factors

The induction of cytotoxic T lymphocytes by concanavalin A has been analyzed under conditions of limit dilution. The dose-response curves deviate from linearity in a way that has been interpreted as revealing successive zones of suppression as the cell concentration was increased. The magnitude of suppression was influenced by both the concentration of concanavalin A and the amount of T cell growth factors added to the culture. These regulatory events involve the cytotoxic T cell clones produced by (CBA X DBA)F1 spleen cells which are detected by DBA mastocytoma (P815) targets at a maximum detectable frequency of 1 in 2000 cells. Similar multiphase dose-response data were also obtained with syngeneic and allogeneic combinations with the same target cell. It is suggested that the successive zones of suppression and activation are a consequence of the relative frequencies of CTL-P, suppressive and helper cells, and the ease with which the cells are activated in limit dilution cultures. The experimental approach illustrates how CTL production can be manipulated to study the balance of signals required to control effector cell production.

Stimulation of different pathways of T-cell functions by syngeneic tumor cells and soluble membrane proteins

Cells from the draining lymph nodes of DBA/2 mice bearing syngeneic intradermal P-815 tumors represent an excellent responding cell population for secondary stimulation in vitro, despite the virtual absence of response by spleen cells of the same animals. Cytotoxicity is a result of stimulation with intact mitomycin-treated tumor cells. Isolated tumor cell membranes, in the form of small vesicles, stimulated cytotoxicity to a very limited extent and inhibited the development of cytotoxic T lymphocytes over a wide range of concentrations. Membrane proteins solubilized with deoxycholate and papain had only a suppressive effect. Soluble proteins exerted their effect during the induction of cytotoxic T lymphocytes and were ineffective when present during the effector phase of cytotoxic T lymphocytes. The suppressive capacity was shown to reside in a cell population which was sensitive to treatment with monoclonal Lyt-2.1 antibody and complement but not with Lyt-1.1 antibody. This phenotype was compatible with a specific rather than a promiscuous type of suppressor effector cell.

Lymphokine production by peripheral blood leucocytes: quantitation of T-cell growth factor activity for assessment of immune response capability

Mitogen stimulation of as few as 200,000 human peripheral blood leucocytes (PBL) in 0.2 ml volumes produces highly active lymphokine supernatants. The lymphokine activity measured is T-cell growth factor. Under optimum conditions of PHA-P concentration, serum source and concentration, and time, activity is detectable to a dilution of between 1/250 and 1/700 for lymphokine supernatants from PBL of normal donors. Lymphokine supernatants prepared from PBL of donors previously treated with the immunosuppressive regimes of radiotherapy and/or chemotherapy had activities that were between 13% and 66% of normal controls. Lymphokine production as a quantitative measure of immune capability is discussed.

The generation of specific cytotoxic responses by human lymphocytes requires antigen and activities provided by a lymphokine supernatant

The requirements for the generation of specific cytotoxic responses by human lymphocytes have been investigated using as a model system the response to the ultraviolet light-killed mouse tumour P815. These requirements are antigen (P815UV) and activities provided by a human lymphokine (LK) supernatant. Cells that give rise to non-specific cytotoxicity can be separated on discontinuous Percoll density gradients after a 3-day incubation of peripheral blood leucocytes with the lymphokine supernatant. Cells from which specific cytotoxic responses can subsequently be generated in the presence of P815UV and LK sediment at a higher density.

Reduced tumor growth after low-dose irradiation or immunization against blastic suppressor T cells

Suppressor T cells have been shown to be much more radiosensitive than other lymphoïd cells, and we have tried to reduce tumor growth by low-dose irradiation. Syngeneic DBA/2 mice received whole-body irradiation (150 rads; 1 rad = 0.01 J/kg) 6 days after P815 tumor inoculation. Tumor growth is significantly reduced in mildly irradiated mice. We also attempted to reduce syngeneic tumor growth by raising immunity against suppressor T cells in two different systems. DBA/2 mice were immunized against splenic T cells collected after disappearance of cytotoxicity and then injected with P815 tumor cells. These mice develop a very high primary cytotoxicity against P815 cells. C57BL/6 mice were immunized against blastic suppressor T cells, before injection of T2 tumor cells. Some of these mice reject the tumor and other develop smaller tumors than control mice. These results could be explained by the induction of antiidiotypic activity directed against the immunological receptors of suppressor T lymphocytes, because immunization with blastic suppressor T cells from mice bearing the T2 tumor does not modify the growth of another tumor, T10.

A method for the production and quantitative assay of human lymphokine preparations

A procedure for the preparation and assay of a human lymphokine supernatant is described. Tonsillar lymphocytes at a concentration of 2 X 10(7)/ml in a serum-free medium are incubated for 2 h with PHA-P, washed free of unbound mitogen and incubated for a further 17 h. The supernatant is harvested and concentrated, and the active protein fraction is absorbed to and eluted from a column of hydroxylapatite. The eluate is desalted and sterile filtered. The preparations contain active material released from 10(8) cells in 1 ml and do not contain residual PHA-P. The most active preparations are obtained using PHA-P as the mitogen and at concentrations of 25-50 microgram/ml during the 2 h pulsing period. Activity is assessed by a quantitative assay based on the maintenance of mitogen activated blast cells in culture for 2 days.