Characterization of an inhibitory allogeneic effect on humoral responsiveness in vitro

A biological context for the self-nonself discrimination and the regulation of effector class by the immune system

An effective immune response to an antigen requires two sets of decisions: Decision 1, the sorting of the repertoire, and Decision 2, the regulation of effector class. The repertoire, because it is somatically generated, large, and random, must be sorted by a somatic mechanism that subtracts those specificities (anti-self) that, if expressed, would debilitate the host, leaving a residue (anti-nonself) that, if not expressed, would result in the death of the host by infection. The self-nonself discrimination is the metaphor used to describe Decision 1, the sorting of the repertoire. In order to be functional, the sorted repertoire must be coupled to a set of biodestructive and ridding effector functions, such that the response to each antigen is treated in a coherent and independent manner. Although a reasonably complete framework for Decision 1 exists, Decision 2 lacks conceptualization. The questions that must be considered to arrive at a proper framework are posed. It should be emphasized that manipulation at the level of Decision 2 is where clinical applications are likely to be found.

Whither T-suppressors: if they didn’t exist would we have to invent them?

Arriving at an understanding of the role of suppressor T-cells (regulatory T-cells, CD4(+)CD25+) depends on whether their functional repertoire is somatically selected to be anti-Self or anti-Nonself. Immunologists are ambivalent; often publications espousing opposite views share an author. Here the arguments are detailed that the suppressor repertoire is not somatically selected to be anti-Self, but rather it is anti-Nonself. Therefore, suppression cannot regulate the Self-Nonself discrimination; its function is to regulate the magnitude and class of the anti-Nonself effector response.

Hypothesis: the MHC-restricted T-cell receptor as a structure with two multistate allosteric combining sites

This paper presents a dual-recognition model of the T-cell receptor that has been constructed to account for the phenomenon of MHC restriction as well as the paradoxical ability of T-cells to be both multispecific and precisely specific at the same time. In our model the combining sites for antigen and MHC are not independent as in classical dual-recognition models, but interact with each other by an allosteric mechanism. We envision a flexible receptor with combining sites for antigen and MHC that are capable of existing in a multitude of distinct complementarity states. MHC and antigen molecules act as allosteric effectors such that one ligand perturbs the conformation and therefore the specificity of the site for the other ligand. An essential feature of the model is that different MHC determinants induce different conformations at the anti-antigen site. In this way the receptor acquires multiple specificities. Within a particular complementarity state, precise recognition results from the requirement that antigen and MHC exhibit positive cooperativity in their binding to the T-cell receptor. Positive cooperativity is also the basis for MHC restriction. Reaction mechanisms are presented which describe the requirement that antigen and MHC both induce conformational changes in order to generate high-affinity binding to either ligand. As a precedent for the multistate allosteric receptor model, we discuss the properties of allosteric enzymes, especially ribonucleotide reductase, whose properties are analogous to those we have postulated for the T-cell receptor. Also discussed is the possibility that molecules such as Ly2, L3T4 and the Mls antigen, which have been found to play a role in antigen recognition, function as affinity-enhancing allosteric effectors that interact with the constant portion of the T-cell receptor.

Allosuppression: evidence for the involvement of both "noncytotoxic" and cytotoxic T cells

Allogeneic suppressor cells were generated by priming to a minor histocompatibility antigen. C57BL/10 female mice were primed to the male-specific minor transplantation antigen, H-Y. After boosting, anti-male-primed cells could potently suppress the secondary antibody response of male spleen cells in vitro. Anti-male suppressor cells were H-2-restricted, radiation-resistant T cells which could act on either T or B cells in the responding population. Mapping the restriction elements for anti-male suppressor cells revealed two distinct subpopulations of effector cells. The majority subpopulation was restricted to H-Y in the context of the H-2Db molecule. These cells were probably cytotoxic T cells as they were inhibited by culture of the suppressor cells in pyrilamide (a histamine receptor antagonist, which prevents the maturation of cytotoxic T cells from their precursors). The second subpopulation of suppressor cells was restricted to H-Y in the context of H-2K or I region-coded molecules. as neither H-2Kb nor H-2Ib molecules can act as restriction elements for anti-male-specific cytotoxic T cells, this subpopulations was almost certainly not composed of conventional cytotoxic cells. Furthermore, these cells were not affected by culture in pyrilamide. Together these two populations act to completely inhibit the anti-trinitrophenyl (TNP) plaque-forming cell response of male spleen cells in vitro.

Lymphocyte subpopulations in man: enhancement and inhibition in generation of cytotoxic T lymphocytes in vitro

The requirements for generation of allospecific CTLs in vitro have been studied by "three cell" experiments, with two allogeneic T cell suspensions as cocultured responders, stimulated by irradiated B cells. This study describes enhancement as well as inhibition of the response, functionally defining T amplifier and T depressor cells regulating the differentiation of CTLs. Enhancement is the result of an amplifying effect of cytotoxic precursor T cells. Amplification is due to HLA-D region incompatibility between the responding T cell donor and the stimulating donor resulting in strong proliferation. Inhibition is the result of a depressing effect on cytotoxic precursor T cells mediated by cocultured T cells. The depression seems to be due to HLA-A, -B, -D identity between one of the responder T cell donors and the stimulator cells. The induction of depression is radiosensitive, accompanied by strong proliferation and CTL generation with cytotoxic specificity against the cocultured and depressed donor target cells. It is suggested that the functionally defined T depressor cells are cytotoxic precursors mediating cytostatic functions before strong cytotoxicity is detectable.