Induction Of Allograft Tolerance Research Articles

Mechanism by which additional monoclonal antibody (mAB) injections overcome the requirement for thymic irradiation to achieve mixed chimerism in mice receiving bone marrow transplantation after conditioning with anti-T cell mABs and 3-Gy whole body irradiation.

A relatively nontoxic method of conditioning mice has been developed recently that allows allogeneic bone marrow engraftment and specific skin allograft tolerance induction. This regiment included anti-CD4 and anti-CD8 mAbs administered on day -5, followed by 3-Gy whole body irradiation (WBI) and 7-Gy thymic irradiation (TI) on day 0. We have recently shown that the potential toxicity of this regimen can be further reduced by replacing TI with additional anti-T cell mAb injections before and after bone marrow transplantation. Mixed chimerism and prolonged donor-specific skin graft acceptance are induced in 90% of B10 mice conditioned with anti-CD4 and anti-CD8 mAbs on days -6 and -1 and 3-Gy WBI on day 0 without TI, but only in a small fraction of mice receiving a similar regimen, except that mAbs are given on day -5 only. To determine the mechanism of tolerance induction in the former group, we compared the two groups for the extent of thymocyte depletion, for the timing of development of intrathymic and extrathymic chimerism, and for clonal deletion of host-type thymocytes with TCR recognizing superantigens presented by donor class II molecules. The results suggest that administration of a second mAb injection depletes or inactivates residual host thymocytes that are capable of causing intrathymic rejection of donor hematopoietic cells even when peripheral engraftment is achieved. The presence of donor class II+ hematopoietic cells in the thymus on day 14 correlated with marked deletion of mature host-type V beta 11+ thymocytes that recognize donor I-E plus endogenous superantigen. This suggests that tolerance is achieved primarily through a central deletional mechanism when peripheral and intrathymic host T cells are adequately inactivated or depleted by mAbs and 3-Gy WBI. In addition, the higher incidence of early failure of peripheral chimerism in mice conditioned with a single injection rather than than two mAb injections prior to bone marrow transplantation suggests that nontolerant residual host thymocytes can also induce early peripheral rejection after mAbs have cleared from the circulation. This early rejection is prevented by the longer persistence of anti-T cell mAbs observed in mice receiving two pretransplant mAb injections. Thus, administration of sufficient depleting anti-T cells mAbs followed by 3-Gy WBI allows the induction of central deletional tolerance while minimizing the toxicity of the conditioning regimen.

INDUCTION OF TOLERANCE TO SKIN ALLOGRAFTS BY INTRATHYMIC INJECTION OF DONOR SPLENOCYTES

The effect of donor-recipient strain combination and supplemental rapamycin (Rapa) on tolerance induction by intrathymic (IT) injection of donor splenocytes was examined in a mouse skin allograft model. In an MHC class I-mismatched C3H/He skin to (C57BL/6 x A)F1 (B6AF1) mouse combination, IT injection of 50 x 10(6) donor splenocytes with transient immunosuppression by rabbit anti-mouse lymphocyte serum (ALS) induced significant prolongation of skin allograft survival with a median survival time (MST) of 115 days versus an MST of 24.5 days in controls given ALS alone. With an additional short course of supplemental Rapa treatment at a dose of 1.5 mg/kg i.p. every other day from day 0 to 12, all C3H/He skin allografts survived indefinitely (> 350 days) in ALS-treated, donor splenocyte intrathymically injected B6AF1 recipient mice. Tolerance was antigen-specific, since the second donor-type skin allografts were accepted while third-party skin allografts were acutely rejected in these mice bearing long-term C3H/He skin allografts. In MHC class I- and II-disparate (DBA/2 to B6AF1) and fully MHC-incompatible (AKR to B6) strain combinations, IT injection of donor splenocytes and ALS treatment failed to prolong skin allograft survival over ALS controls. When supplemental Rapa was used, long-term skin allograft acceptance was observed with an MST of 127 days for the DBA/2 to B6AF1 combination and 70 days for the AKR to B6 combination. In contrast, supplemental treatment with cyclosporine was not effective in these combinations, which suggests that supplemental Rapa may have a unique effect in augmenting IT tolerance induction. Thymectomy within 7 days after IT injection significantly shortened the allograft survival, which suggests that interaction of the host thymus and the injected donor splenocytes, which takes place early after IT injection, plays an important role in the induction of allograft tolerance in this model.

Graft-infiltrating T helper cells, CD45RC phenotype, and Th1/Th2-related cytokines in donor-specific transfusion-induced tolerance in adult rats.

Specific tolerance to LEW.1W (RT1u) heart allografts can be induced in adult LEW.1A (RT1a) rats by donor-specific blood transfusion (DST). We have previously shown that both rejected and tolerated grafts are heavily infiltrated by T lymphocytes, and that in both cases these T cells are capable of developing similar cytotoxic responses against donor cells in vitro; tolerance is therefore not due to the deletion of alloreactive T cells. At the same time, we found that the accumulation of IL-2 and IFN-gamma mRNA was decreased in tolerated grafts compared with rejected grafts. These results suggested that the induction of allograft tolerance in DST-treated animals could be mediated by anergy or suppression of graft-infiltrating Th1 cells. Although Th1 and Th2 clones have not yet been characterized in the rat, peripheral CD4+ rat T cells can be divided into two populations, based on their expression of the isoform RC of the CD45 molecule. Upon activation, CD45RChigh CD4+ T cells produce IL-2 and IFN-gamma and responsible for the induction of the graft-versus-host reaction, whereas CD45RClow CD4+ T cells produce IL-4 in vitro and provide B cell help. In the present study, we show that heart allografts from both DST-treated and untreated rats were infiltrated by equivalent numbers of leukocytes, of which CD4+ T cells also made up similar percentages. Among these CD4+ T cells, we observed that in allografts from DST-treated recipients the CD45RChigh population on day 5 was very significantly smaller (P = 0.004) than in the untreated group, while CD45RClow populations remained comparable. Moreover, using a new quantitative RT-PCR method, we found a dramatic reduction in the accumulation of IL-2, IFN-gamma, IL-10, IL-4, and IL-13 mRNA in hearts from DST-treated recipients compared with those of untreated recipients during the week following transplantation. These results show that in heart allografts from DST-treated recipients, despite phenotypic changes suggesting Th1 inhibition by Th2 imbalance, T helper function was inhibited as a whole, and that in vivo the phenotype CD4+ CD45RClow does not always correlate with Th2-related cytokine-producing cells.

Early, complete burn wound excision partially restores cytotoxic T lymphocyte function

Cytotoxic lymphocytes (CTLs) are an important component of immune function, involved in antigen recognition and resistance to viral infection. Burn injury suppresses cell-mediated immunity, induces allograft tolerance, and increases the risk of viral infection, but the mechanisms are not well understood. This study analyzes the effect of burn size and burn wound excision on CTL activity. Anesthetized CBA mice (n = 12) received a 0%, 20%, or 40% body surface area contact burn. Additional mice (n = 16) received a 40% burn that was totally, partially, or not excised 72 hours after burn. Excised areas were covered with normal, syngeneic skin. Two weeks later harvested splenocytes were cocultured with allogeneic stimulators. CTL activity was determined by a 51Cr release assay, in which CTL effectors were tested on allogeneic, radiolabeled targets. Dilution curves of CTL activity were compared by ANOVA: Both 20% and 40% burns significantly inhibited CTL activity (p < 0.05). Total but not partial excision of a 40% burn restored CTL activity (p < 0.01). Both total and partial wound excision also improved survival (p < 0.05). Burn injury inhibits CTL activity in a size-dependent manner, and total wound excision significantly improves both CTL function and survival after injury. This study suggests a mechanism for the immunosuppressive effects of burn injury and provides an immunologic rationale for early, complete burn wound excision.

Induction of allograft tolerance in rats by an HLA class-I-derived peptide and cyclosporine A.

T cell recognition of MHC molecules initiates a cascade of events resulting in allograft rejection. CTLs damage the graft by targeting nonself-MHC class I molecules. We and others have previously shown that small synthetic peptides corresponding to regions of certain MHC class I molecules can inhibit the CTL response against MHC class I alloantigens in vitro. Here we report that rat heart allografts survived survived indefinitely when transplanted into recipients treated with a synthetic peptide corresponding to residues 75-84 of (B7.75-84) in combination with a subtherapeutic dose of cyclosporine A. Furthermore, this treatment induced long-term donor-specific tolerance that was mediated by anergic cells, indicating that such peptides may have potential as therapeutics for human organ transplantation.

The influence of intravenously introduced allogeneic lymphocytes on fetal rats

Semi-allogeneic or allogeneic bone marrow cells, or allogeneic spleen cells, were injected in utero into PVG, DA and Fischer rat fetuses at 15–18 days of gestation via the vitelline vein. Prenatal (PVG × DA) F 1 bone marrow cell inoculation produced allograft tolerance far more frequently in DA recipients than in PVG recipients. Inoculation of allogeneic or semi-allogeneic spleen cells into rat fetuses failed to induce allograft tolerance but sensitized the recipients. The observation that Fischer recipients could be rendered tolerant to skin allografts by prenatal inoculation of PVG, but not DA, bone marrow cells indicated that variation in susceptibility to tolerance induction among different donor-recipient strain combinations is unlikely to be explicable on the basis of strain differences in developmental maturity of the host immune system.

Pretransplantation Blood Transfusion Revisited

Blood transfusion before organ transplantation has a beneficial effect on allograft survival; the mechanism of this effect has remained a mystery. In murine models, the presence of common histocompatibility antigens in the blood donor and the recipient favors the induction of allograft tolerance. To investigate the effect of HLA compatibility between blood donor and recipient on the induction of allograft tolerance, we determined the relative frequency of cytotoxic T-lymphocyte precursors specific for donor cells before and at several times after blood transfusion in 23 patients awaiting a first renal transplant. We correlated the results with the presence of shared HLA antigens. T-cell nonresponsiveness against donor cells developed after blood transfusion in 10 of the 23 patients. Tolerance developed only if the blood donor and the recipient had one HLA haplotype or at least one HLA-B and one HLA-DR antigen in common (as was observed in 9 of these 10 patients). Tolerance developed relatively late after blood transfusion (one to two months) and was long-lasting. No decline in the T-cell response against donor alloantigens was observed in any of the 13 patients who received transfusions without having HLA-antigen compatibility with the donor. Blood transfusion in which there is a common HLA haplotype or shared HLA-B and HLA-DR antigens induces tolerance to donor antigens. This finding may lead to the development of new strategies with which to induce tolerance for transplantation after blood transfusion. Perhaps transplant donors will be selected not only by HLA-antigen matching, but also on the basis of acceptable HLA-antigen mismatches associated with T-cell non-responsiveness induced by selected blood transfusion.

The effect of intrathymic injection of donor blood on the graft versus host reaction and cardiac allograft survival in the rat

Our previous paper reported that it was difficult to induce tolerance using donor specific transfusion (DST) in Lewis rats, while in DA and PVG rats it was possible to induce tolerance using DST. In this paper we investigated the effects of DST in a local graft versus host reaction (GVHR) model using these rat strains. The following results were obtained: (i) DST suppressed the GVHR in the DA rat but not in the Lewis rat; (ii) DST did not suppress GVHR in PVG rat strain, but lymphocytes, prepared from the PVG rats that were tolerant of Lewis heart grafts induced by DST, could suppress the the GVHR; and (iii) in Lewis rats, intrathymic injection of whole blood suppressed the GVHR, suggesting this route of antigen delivery might be effective in prolonging allograft survival. Cardiac allograft survival in this strain was prolonged in some cases after pretreatment with intrathymic donor whole blood. These results suggest the value of the GVHR in the assessment of new protocols to induce allograft tolerance in rats.

Induction of allograft tolerance to the H-Y antigen in adult C57BL/6 mice: Differential effects on delayed-type hypersensitivity and cytolytic T-lymphocyte activity

This study describes the induction of allograft tolerance to the “male-specific,” minor histocompatibility antigen, H-Y, in adult C57BL/6 female mice, and the effects of this tolerance induction on two immune parameters associated with graft rejection: delayed-type hypersensitivity (DTH) and cytolytic T-lymphocytes (CTL). B6 females tolerized to H-Y, by a single iv injection of C57BL/6 male lymphocytes, exhibited prolonged or permanent survival of B6 male tail skin grafts. Graft-induced DTH against H-Y antigen was reduced or abrogated in tolerized females. Delayed onset of graft rejection in partially tolerant females correlated with delayed onset of DTH, and eventual rejection of grafts was accompanied by an increase in H-Y-specific DTH. In contrast, H-Y-specific CTL activity was not consistent with graft status. These data demonstrate a correlation between H-Y-specific DTH and rejection of male skin grafts by B6 female mice and are most consistent with a major effector role for DTH in chronic graft rejection.

Difference in allograft tolerance induction in X-irradiated and cyclophosphamide-treated chickens.

Les cellules-souches de la bourse de Fabricius de poussins de 4 j, transferees chez des receveurs traites par cyclosporine, produisent un chimerisme stable des cellules B peripheriques et une tolerance aux greffes avec antigenes du donneur. Cette tolerance est transmissible par des cellules spleniques aux receveurs d'une 2eme greffe, irradies. Si les cellules-souches sont transferees directement a des receveurs irradies, le chimerisme est transitoire

Resistance of fetal PVG rats to induction of allograft tolerance.

Artificially induced immunological tolerance is often envisaged as experimental mimicry of events which occur naturally during the acquisition of self tolerance. Thus, it has been inferred that similar circumstances will facilitate the induction of both types of unresponsiveness. Since tolerance to most self determinants appears to have been established by the time of birth and allograft tolerance can be experimentally induced more readily, if not exclusively, in very young animals, it is commonly assumed that susceptibility to induction of tolerance to foreign antigens will continue to increase as progressively younger animals are tested. The experiments reported in this study extend the range of circumstances under which younger animals may be found to be less susceptible, or even completely resistant, to tolerance induction. While neonatal PVG rats were highly susceptible to the induction of tolerance to (PVG x DA)F1 hybrid determinants, fetal PVG rats were only partially susceptible at 19 days and were completely resistant at 17 and 18 days of gestation. Furthermore, only 39% of neonatal PVG rats that had been inoculated with F1 hybrid bone marrow cells at 18 days of gestation could be rendered tolerant by procedures which were effective in 100% of previously untreated neonates. These observations were interpreted to indicate that a certain stage of maturity must be attained before fetal rats become susceptible to tolerance induction. Asymmetry in susceptibility to tolerance induction in response to the inoculation of F1 hybrid bone marrow cells was observed when PVG and DA fetuses, but not neonates, were compared. This difference in the response of the two strains recalls the divergence in immune reactivity of offspring, previously reported when PVG and DA embryos were transferred to surrogate mothers of the other strain. It is also in agreement with earlier reports of substantial differences in the ease of tolerance induction between different strain combinations. The asymmetry between PVG and DA rats in the present experiments is speculatively attributed to differences in rates of immunological maturation of the two strains.

Induction of Tolerance across Major Barriers Using a Two-Step Method with Genetic Analysis of Tolerance Induction

Using a murine skin allograft tolerance induction system that consists of intravenousinjection of 1 × 10 8 allogeneic spleen cells followed by intraperitoneal (i.p.) injection of 200 mg/kg cyclophosphamide (CP) 2 days later, sensitivity to tolerance induction was examined across various histocompatibility (H) barriers. Although each group of class I, class II or multiminor H antigens was not by itself a prohibitively strong barrier, resistance to tolerance induction increased when the three types of barriers were combined in various ways. When the donor-recipient combinations were disparate at the entire spectrum of both H-2 plus non H-2 antigens (fully allogeneic), profound tolerance to skin allografts was not induced by this method in any of the combinations examined. Based on these results, induction of tolerance across fully allogeneic barriers was attemptedin C57BL/10SnJ (B10; H-2 b) mice against C3H/HeSnj (C3H; H-2 k) strain by addressing the H barriers as two separate challenges. B10 mice were first given B 10.BR/SgSnJ (B 10.BR; H-2 k) spleen cells plus CP to make them tolerant to the H-2 k component represented among C3H antigens, and then later were given C3H spleen cells plus CP to establish a tolerant state to the remainder of the disparate antigens of the C3H donors. After these two separate manipulations, C3H skin was accepted in the B10 mice, and normal hair growth was observed in the grafted C3H skin. By contrast, B10 mice given C3H spleen cells plus CP and then again another injection of C3H spleen cells plus CP were not rendered tolerant to C3H skin. In B10 mice, tolerance to C3H induced with B10.BR spleen cells plus CP and then C3Hspleen cells plus CP was specific to C3H, and the tolerant B10 mice rejected third-party skin from DBA/2J (DBA; H-2 d) strain in a normal fashion. In transfer experiments, the mechanism of tolerance was found to be based largely on reduction of the effector cells rather than on a mechanism involving active suppression. Assays for chimerism revealed that maintaining the tolerant state required persistence of cells of donor origin. These data indicate that in a primary immune response to a certain dose of allogeneic cells(tolerogen), the existence of a relatively large proportion of potentially reactive clones in the host may trigger proliferation of only a part of the population and some of the potentially reactive cells may differentiate rapidly without a prolonged period of proliferation. The cells which are not responding by proliferation induced by the antigenic stimulation may then escape destruction by CP and ultimately survive to cause rejection of donor-type hematopoietic cells and skin grafts.

Induction of donor-specific unresponsiveness to rat cardiac allograft by donor leukocytes and cyclosporine.

Many recent reports have emphasized the importance of donor antigens in the induction of allograft tolerance. This study examines the effect of pretransplant infusion of 10(8) donor leukocytes (DL) combined with peritransplant cyclosporine (CsA) on W/F cardiac allograft survival in Lewis rats. Peritransplant recipient treatment consisted of CsA 20 mg/kg given i.m. on days 0, +1, and +2 relative to heart transplantation. Lewis recipients, 5-8 per group, were pretreated with 10(8) DL with or without peritransplant CsA. A single DL transfusion on day -3 or day -7 prior to transplantation significantly prolonged the mean survival time (MST) of W/F hearts from 7.0 +/- 0.9 days in controls to 12.2 +/- 4.5 days and 12.4 +/- 1.0 days (P less than 0.01), respectively. Two DL infusions on days -7 and -3 or on days -14 and -7 prolonged the MST to 10.6 +/- 1.3 days (P less than 0.02) and 16.4 +/- 2.8 days (P less than 0.001), respectively. The administration of peritransplant CsA alone significantly prolonged W/F heart allograft survival to 43.1 +/- 2.7 days. When pretransplant DL transfusion on day -3 was combined with CsA treatment, 4/8 animals maintained their grafts indefinitely (greater than 100 days). Similarly, DL infusion on day -7 with peritransplant CsA led to indefinite graft survival in 3/5 animals. Administration of DL on days -7 and -3 combined with CsA resulted in indefinite graft survival (greater than 100 days) in 4/6 animals. Transfusion of DL on day -3 alone or in combination with peritransplant CsA, had no effect on a third-party (ACI) heart allograft survival prolongation compared with appropriate controls. To define the underlying mechanisms responsible for donor-specific unresponsiveness in this model, pooled sera and unseparated spleen cells were passively transferred from recipients of long-term cardiac allografts to syngeneic rats receiving donor-type (W/F) or third-party (ACI) cardiac allografts. Transfer of serum (1 ml on days 0, and 1, 0.5 ml on days +2, +3, and +4) from ungrafted recipients of DL on days -14 and -7 led to significant donor graft survival of 9.8 +/- 0.4 days (P less than 0.02) in unmodified hosts. Similarly, passive transfer of serum obtained at 20 and 100 days after transplantation significantly prolonged the MST of donor-type hearts in syngeneic untreated hosts to 11.3 +/- 0.8 and 10.0 +/- 1.1 days, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

What is known about blocking factors in renal allograft recipients.

A variety of substances present in serum or plasma, either at the time of renal transplant or during stable graft function in long-term recipients, may interfere with cell-mediated immune functions. In several cases the presence of serum blocking factors has been correlated with decreased graft vulnerability to acute rejection. The question is: are serum blocking factors important for the induction or maintenance of allograft tolerance or, alternatively, are they merely by-products of the tolerant state? Perhaps the most compelling case for an essential role of serum blocking factors can be made in instances where vigorous cell-mediated immune responses can be demonstrated in vitro, but anergy (for example, absence of DTH response) is seen in vivo. However, in renal transplant recipients the majority of studies that show the presence of receptor blocking antibodies or other immunoregulatory serum factors also found a decreased cell-mediated immune response in vitro. Thus, allograft tolerance would appear to involve multiple mechanisms as suggested elsewhere. Due to the recent discovery of the molecular structure of the T cell receptor 33,34, it is now possible to identify determinants associated with the binding of T cell receptor blocking antibodies in renal transplant recipients; for example, one should be able to determine if families of V beta genes are involved. Alternatively, it may be possible to identify polymorphic structures on T cells other than the alpha, beta receptor complex as ligands for blocking antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional clonal deletion of class I-specific cytotoxic T lymphocytes by veto cells that express antigen.

Intravenous injection of class I incompatible spleen cells into mice results in a drastic reduction of the recipient's cytotoxic T lymphocyte (CTL) response against the injected, but not against third party, class I antigens when measured in bulk cultures initiated 5 to 6 days after the injection. This specific suppressive effect is partly due to T cells but can also be seen when high numbers of anti-Thy-1 and complement-treated spleen cells of nude mice are injected. Such cells suppressing CTL responses against self histocompatibility antigens are called "veto cells." The precursor frequency of CTL specific for the injected class I antigen is found to be reduced greater than 200-fold at days 5 to 6 after the injection, whereas the frequencies of CTL specific for third party class I antigens are not significantly changed. These results indicate that there is a functional clonal deletion of the CTL recognizing class I incompatible veto cells in vivo. The role of such a veto phenomenon in the induction and maintenance of self tolerance and allograft tolerance is discussed.

Induction of Allograft Tolerance after Total Lymphoid Irradiation (TLI): Development of Suppressor Cells of the Mixed Leukocyte Reaction (MLR)

We searched for the presence of suppressor cells of the MLR in C57BL/Ka leads to BALB/c chimeras. The chimeras were made with total lymphoid irradiation (TLI) and marrow transplantation. Spleen cells from the old chimeras inhibited the MLR of BALB/c responder cells against C57BL/Ka stimulator cells. Inhibition was specific for the stimulator cells, since no effect on the MLR was observed with C3H or BALB.C3H stimulator cells. Maximal inhibition was achieved when the responder cells in the MLR shared the H-2 haplotype of the chimeric recipient. Spleen cells obtained from chimeras young 30 to 40 days after BM transplantation inhibited the MLR nonspecifically, since similar marked inhibition was observed regardless of the H-2 haplotype of the responder or stimulator cells. The finding of antigen-specific and nonspecific suppressor cells is similar to that observed in mice rendered tolerant to bovine serum albumin after treatment with TLI.

THE INVOLVEMENT OF A SUPPRESSOR MECHANISM IN NEONATALLY INDUCED ALLOGRAFT TOLERANCE IN MICE

The possible involvement of a suppressor cell mechanism in neonatally induced allograft tolerance was tested in two different mouse-strain combinations. In an H-2 incompatible donor-recipient combination presented by the congeneic strains B10.D2 and B10.D2(M504), adoptive transfer of tolerance with 5 X 10(7) spleen cells to syngeneic sublethally (400 R) irradiated recipients was successful resulting in a markedly prolonged test graft survival or even take. The result of tests for negative tolerance (which should be suceptible to abolition by means of cells from normally reactive donors) did not indicate that neonatally induced tolerance across the given barrier rested on this principle. The suppressor mechanism seemed to be involved in neonatally induced allograft tolerance also in the other strain combination (B10-40NX).

Characteristics of specific unresponsiveness toward kidney and skin allografts in adult rats inoculated at birth with allogenic bone marrow or kidney cells across a strong H-1 barrier.

Specific unresponsiveness inducible in newborn Lewis (Le) rats by injection of bone marrow or kidney cells from (Bu x Le)F1 hybrids was tested in adults by allografting of skin or kidneys from strongly incompatible Buffalo (Bu) strain donors. High doses of bone marrow cells (10(7) or more) proved quite effective in procuring indefinite survival of subsequent Bu kidney allografts, but not Bu skin allografts. Kidney allografts remained fully functional without modulation of their inherent immunogenicity regardless whether skin allografts were acutely rejected before or after kidney grafting on bone marrow-tolerant recipients. Lower doses of bone marrow cells were either near threshold for induction of specific Bu kidney allograft tolerance (5 x 10(6)) or, conversely, led to increased immunity(1-5 x 106). A strikingly different pattern of reactivity was found in adult Le recipients of Bu skin allografts as shown by only a moderate, but stepwise increase in survival times as a function of increasing allogeneic cell dosage at birth. Allogeneic kidney cells at all doses were surprisingly ineffective in inducing tolerance of either kidney or skin allografts, so the existence of kidney-specific transplantation antigens remains problematical. Lymphocytes from kidney allograft-tolerant recipients showed much reduced but still significant antidonor activity in several tests of cell-mediated immunity in vitro and in vivo. Blocking antibody activity was also demonstrable in these animals. Acquired tolerance (i.e., essential nonreactivity) may not only exist in degrees in either T or B cell pathways but may coexist with specific immunoblocking reactions in a dynamic equilibrium. Instead of tolerance versus enhancement, a new concept of selective specific immunoregulation emerges.

Induction of skin allograft tolerance during metamorphosis of the toad Xenopus laevis: a possible model for studying generation of self tolerance to histocompatibility antigens.

AbstractFrom 15 days before to 60 days after metamorphosis, an immunologically specific tolerance to skin allografts between individuals of Xenopus can be induced by skin allografts. Genetic inheritance of histocompatibility antigens determines the degree of tolerance during metamorphosis: in individuals from the first generation, 25–50 % of the siblings were tolerant. In second generation individuals, the percentage of tolerance is 75–100 % when the parents show mutual tolerance to skin allografts at metamorphosis and only 25 % when the parents are mutually incompatible. In view of the characteristics of this tolerance it is suggested that it concerns mainly weak histocompatibility antigens and that it occurs preferentially at metamorphosis, as at this stage the animal becomes tolerant to many new self‐antigens.

Induction of T cell tolerance in agammaglobulinemic chickens.

AbstractImmunological tolerance to histocompatibility antigens as assessed by skin grafts and graft‐versus‐host reactions, was induced in hormonally bursectomized chickens that could not form antibody responses or even synthesize immunoglobulins. This demonstrates that the induction of allograft tolerance does not require the essential participation of a suppressive humoral antibody.