David Nachtigal

The role of specific suppressor t cells in immune tolerance.

The mechanism of immune tolerance is both one of the most studied and one of the most controversial fields of immunological research. The first attempt at proposing a coherent explanation for this phenomenon is to be credited to Bumet (1959), who formulated his hypothesis as an offshoot of the clonal selection theory of acquired immunity, claiming that selective elimination of antigen-oriented clones may explain all types of specific unresponsiveness. Burnets hypothesis provides a very elegant and relatively simple explanation for the phenomenon of immune tolerance, and since it was formulated during the heyday of molecular genetics, its basically genetical orientation captured the imagination of immunologists at that time. However, his interpretation has so far found no direct experimental support, and up till now it remains mainly of a speculative nature.

Immune response to polypeptidyl proteins in rabbit tolerant to the protein carriers

Abstract : A study was made of the immune responses to polypeptidyl proteins in rabbits which were either naturally tolerant or made experimentally unresponsive to the protein carriers. Experimentally acquired tolerance to HSA could be terminated by immunization of the unresponsive rabbits with poly-Ltyrosyl derivatives of HSA. Two parameters determined the breakdown of tolerance: (1) the chemical nature of the peptides attached: polytyrosyl HSA was effective in the termination of tolerance to HSA, whereas polyalanyl HSA was not; (2) the degree of enrichment was tyrosine. There appears to be an optimal degree of enrichment, i.e. of molecular alteration of the HSA, which will confer on the altered antigen the maximal potency to terminate tolerance: a slight alteration (3 per cent enrichment of HSA with Tyr) and a severe alteration (13 per cent enrichment) were less effective in tolerance breakdown than an intermediate degree of alteration (7 per cent enrichment with Tyr). The level of tolerance breakdown obtained by polytyrosyl HSA, as measured by the ratio of anti-HSA/ anti-polytyrosyl HSA, was greater than the level obtained previously by other chemically altered antigens. The significance of this ratio was discussed in relation to the cellular basis of immunological tolerance. Antibodies were produced to the peptides per se, when attached to proteins towards which the animal is naturally tolerant (RSA), or to proteins to which the animal has acquired tolerance (HSA). In both systems, there was a similar pattern of antibody formation, which may reflect a similarity in mechanism between natural and actively acquired tolerance.

Mechanisms in immune tolerance. II. Specific immunosuppression by t lymphocytes of b memory cells in mice made tolerant to hsa.

The specific suppressor cells which had been demonstrated by us in the spleens of mice tolerant to human serum albumin (HSA) could be shown to originate in the thymus of these animals. They were not depleted by passing such spleen cell suspensions on columns with an adsorbed antigen-antibody complex, a device which retains B cells, while they could be inactivated by treatment with anti θ serum and complement. These suppressor T cells were found to survive the pretreatment of their donors with hydrocortisone acetate. Nevertheless, indications were found that their precursors are apparently cortisone-sensitive since in a suitably designed experiment cortisone could be shown to interfere with tolerance induction. The suppressor cells of tolerant mice block the anamnestic response to the tolerizing antigen which can be activated after differential irradiation of tolerant animals. Analysis by means of cell transfer experiments indicated that this memory of tolerance induction was associated with a B cell population which remained functional after gamma irradiation.

Induction of allospecific suppressor T Cells in vitro

Abstract Incubation of mouse thymic lymphocytes with irradiated allogeneic spleen cells gave rise to suppressor cells. The suppressor activity was assayed by adding the incubated cell mixture to a mixed lymphocyte culture (MLC) in which the responder cells were syngeneic with the sensitized thymocytes and the stimulator cells were syngeneic with the sensitizing spleen cells. Such addition suppressed significantly thymidine incorporation in the mixed lymphocyte reaction (MLR). The suppressor cells were found to carry the θ antigen and to function allospecifically, as shown by cross-testing in three allogeneic combinations. Our data suggest that these cells may originate from immature cortisone-sensitive thymic lymphocytes and also provide some preliminary information concerning their mode of action.

Mechanisms in immune tolerance: I. A specific block of immunological memory in HSA-tolerant mice

Abstract Evidence is provided indicating that the transient immune response detected during the induction of tolerance to HSA in mice, results in immunological memory which persists in an immunosuppressed state in the tolerant animals. The mechanism which blocks this memory is antigen-specific and can be selectively inactivated by total body irradiation in the range of 650–900 R. Consequently tolerant mice, irradiated within this range and restored with normal syngeneic spleen cells, respond better to the tolerizing antigens than do similarly treated normal mice. The tolerizing block can be transmitted from “tolerant” to normal spleen cells when a mixture of both is transferred to normal irradiated recipients. Since similar inhibitory activity is also demonstrated by fresh sera of tolerant mice, it is suggested that the blocking mechanism depends on inhibitory cells which act via a humoral inhibitor.

Mechanisms in immune tolerance: III. Immunosuppression and the induction of tolerance—Synergism and antagonism

Several immunosuppressive procedures were examined for their facilitation of inducing immune tolerance to protein antigens in rabbits. Thus, gamma irradiation, 6-mercaptopurine, and cyclophosphamide treatments were found to be mutually potentiating in their effect of predisposing to the induction of tolerance. Hydrocortisone acetate, on the other hand, was found to interfere in the induction of tolerance following irradiation. An attempt is made to interpret this observation on a cellular basis.

The mechanism of breakdown of immune tolerance to a protein antigen in rabbits

Experimental termination of the state of immune tolerance to protein antigens has been extensively studied on the assumption that it could yield information relevant to the mechanism of unresponsiveness in general. One of the main points under discussion concerned the problem whether the abolition of tolerance to an antigen represents an authentic restoration of the previously deleted immune capacities or whether it results from an induction of a new type of antibody which cross-reacts with the tolerogen. Studies conducted on these lines demonstrated that the abolition of tolerance could elicit an antibody which displays the same specificity as that of an antibody formed by normal non-tolerant animals (1). Nevertheless, this observation can by no means serve as a proof of a genuine reversal of tolerance, since the reasoning based on it might be obsolete in the light of recent evidence. According to available reports (2, 3), immune tolerance may result from the functional elimination of specific antigen-reactive cells (ARC) and does not affect the antibody forming cells (AFC).

Studies on suppressor T cells in tolerance.

(1) Evidence is provided that tolerance induced in mice to HSA depends on a suppressive mechanism which inhibits the responsiveness of potentially competent cells. (2) the suppression is apparently the function of suppressor T cells which are comparatively radiosensitive and are resistant to cortisone. (3) The cortisone resistant suppressor cells are apparently the progeny of cortisone-sensitive precursors, the differentiation of which into mature suppressors seems to be signalled by antigen. (4) The precursor lymphocytes are comparatively short functioning cells (about 14 days), while the mature suppressor T cells function for at least two months.

THE GENERATION OF AN ALLOSPECIFIC SUPPRESSOR FACTOR IN CULTURE

When mouse spleen cells were stimulated by irradiated allogeneic cells in a mixed lymphocyte culture for 96 hours, allospecific suppressor cells were generated that could inhibit the replicative processes when transferred to another MLR. When the primed cells were purified on Ficoll and incubated alone for another 24 hours, they released into the medium a subcellular factor which could also suppress MLR allospecifically. Evidence is provided that the cells which generate this factor arise by cooperation of at least two types of T-lymphocytes, one of which is the progenitor of the suppressor cell and the other an essential accessory cell. Adherent cells were shown to be also essential for the priming of suppressor lymphocytes.

Studies of allospecific suppressor cells in culture

Allospecific suppressor T cells can be generated by allostimulating thymus, spleen and lymph node cells in culture. Bone marrow cells do not yield suppressor lymphocytes. While the efficiency of thymocytes for suppressor cell generation decreases with the age of thymus donors, the efficiency of spleen increases with age. Generation of suppressor lymphocytes depends on cell replication and protein synthesis. The suppressor function as such does not require cell replication. Induction of suppressor T cells in culture requires cell to cell interaction of cortical type and medullar type T cells.