Genetic aspects of the tolerance to allografts induced at metamorphosis in the toadXenopus laevis

Genetic aspects of tolerance to allografts induced at metamorphosis in the toadXenopus laevis were studied in one sibship expressing four different major histocompatibility complex (MHC) haplotypes. Tolerance by skin grafting was induced during metamorphosis in thiourea-blocked individuals, a technique that allows prolonged observation of graft behavior at this stage. Four classes of mutually tolerant animals could be determined. The use of antisera recognizing red blood cell antigens segregating with mixed lymphocyte reaction (MLR) haplotypes revealed that the four abovementioned classes corresponded to the four MHC genotypes of the family. The tolerance is, therefore, preferentially induced to antigens not dependent on the MHC. Under certain circumstances tolerance can also be induced to MHC antigens, provided that the animals differ at the level of one MHC haplotype only. Study of MLR during ontogeny suggested that, between sibs, only the two MLR haplotype differences were stimulatory at metamorphosis, whereas in larval and adult animals a one-haplotype difference was enough for stimulation.     

Joint Report of the Fourth International Bovine Lymphocyte Antigen (BoLA) Workshop, East Lansing, Michigan, USA, 25 August 1990

Blood samples from 54 animals were exchanged between 15 laboratories in nine countries to improve and expand BoLA class I and class II typing. A total of 27 out of 33 (82%) of previously accepted BoLA-w specificities were represented within the cell panel. Seventeen new serum-defined BoLA specificities were accepted by the workshop participants, thus expanding the number of internationally recognized BoLA specificities to 50. The large number of new specificities detected resulted from the number of serological reagents used (n = 1139) and the genetic diversity of the cell panel. Confidence derived from the high percentage of agreement between the laboratories on antigen detection (97.3%; r = 0.84) permitted the removal of the workshop (w) notation from 23 BoLA-w specificities and their acceptance as full status BoLA-A antigens. Two new non-BoLA antigens were also detected, one completely included within the red blood cell factor S' (BoLy-S'), whereas a second (BoLy-w1) did not show any association with tested red blood cell factors. A comparison between serological, isoelectric focusing (IEF) and DNA typing for BoLA class II polymorphism was conducted with a subset of workshop cells. Correlation between the three methods was significant for three combinations of alleles. Three other serologically defined class II specificities were correlated with DR and/or DQ restriction fragment length polymorphism (RFLP) types, whereas six additional IEF types were correlated with DR and/or DQ RFLP types (r greater than or equal to 0.50). Several new IEF, DRB, DQA and DQB RFLP patterns were identified. In 46 animals that were typed for BoLA-DR and DQ genes by RFLP analysis, 46 different BoLA haplotypes were tentatively defined. These 46 haplotypes were distinguished by 31 serologically-defined BoLA-A alleles (and 2 'blanks'), 15 DRB RFLP types (plus up to 10 new DRB RFLP patterns) and 23 DQA-DQB haplotypes.     

Studies on the bovine major histocompatibility class I and class II antigens using homozygous typing cells and antigen-specific BoT4+ blast cells

Animals were identified from two sire lines as being homozygous for the class I bovine lymphocyte antigen (BoLA-A) w23. These animals were also shown to be homozygous for class II antigens (BoLA-D) which, however, differed between the two sire lines. Lymphocytes from these animals were then used either as stimulator cells in one-way mixed lymphocyte reactions (MLR) with all animals in the herd carrying the w23 antigen or as antigen presenting cells to bovine T4+ cell blasts. It was shown that, within each sire line, the genes encoding the MHC class I and class II antigens were closely linked. There were no detected recombinations between the MHC class I and class II regions nor within the BoLA-D region responsible for mixed lymphocyte reactivity. MLR typing of MHC class II antigens correlated with the results from T-lymphocyte proliferation studies. Cells from these cattle, which are homozygous at the class I and II MHC loci but differ in the class II antigen expressed, could be used to type the BoLA-D of other cattle.     

Restimulation properties of human alloreactive cloned t-cell lines. Dissection ofHLA-D-Region alleles in population studies and in family segregation analysis

Alloactivated human lymphocytes were cloned by limiting dilution. After 1 month in culture with T-cell growth factor several clones incorporated tritiated thymidine when stimulated with the appropriate allogeneic cells. Specificity of restimulation of two primed lymphocyte clones, designated 12-2 and 12-8, was studied in detail after varying periods of culture (up to 50 days). Clone 12-2 cells were stimulated only by cells expressing the HLA-Dw antigens of the original priming cells (Dw3); furthermore, this primed lymphocyte reagent specifically recognized antigens associated with only one of the three distinct Dw3-bearing haplotypes from an informative family (KOH). Clone 12-8 cells, on the other hand, failed to recognize Dw3 antigens in the random panel or on homozygous typing cells (including the original priming cell), but were strongly restimulated by certain cells expressing Dw4 antigens. In addition, within family KOH, these restimulating products segregated with another one of the three Dw3-bearing haplotypes but with none of the three Dw4-bearing haplotypes. These two clones exemplify a hitherto unknown precision in cellular typing of theHLA-D region. Clone 12-2 allows the discrimination of a probably rare and as yet undetected HLA-Dw3 subtypic specificity. Clone 12-8, on the other hand, apparently identifies an allelic system segregating withHLA but distinct from the HLA-D determinants definable by HTC-typing.Abbreviations used in this paper MHC major histocompatibility complex - HLA human leukocyte antigens - PBL peripheral blood leukocytes - HTC homozygous typing cells - MLC mixed leukocyte culture - PLT primed lymphocyte testing - TCGF T-cell growth factor - CTC cultured T cells - Tdr tritiated thymidine     

Effect of polyphenols extracted from tamarind (Tamarindus indica L.) seed coat on pathophysiological changes and red blood cell glutathione peroxidase activity in heat-stressed broilers

The purpose of this study was to determine the effect of polyphenols extracted from the tamarind seed coat (PETSC) on glutathione peroxidase (GPx) activity, red blood cell parameters and bilirubin in heat-stressed broilers. One hundred forty-seven broilers, 18-days old were divided into two groups. In group 1, broilers were maintained at an environmental temperature of 26?±?2 °C throughout the experimental period. In group 2, the broilers were maintained at 38?±?2 °C (cyclic temperature: 26?±?2 °C; ?38?±?2 °C; and ?26?±?2 °C, and broilers were maintained at 38?±?2 °C for 6 h/ day) and received PETSC at a concentration of 0, 100, 200, 300, 400 or 500 mg/kg in their diet ad libitum. Parameters were investigated on days 1, 7, 14 and 21 of the experimental period. Results showed that GPx activity of heat-stressed broilers that received 100 mg/kg of PETSC in their diet was lower (P?<?0.05) than that in broilers fed the other concentrations. The mean total red blood cell count and hemoglobin concentration of heat-stressed broilers that received 100 mg/kg PETSC was higher (P?<?0.05) than those in broilers in group 1 and those fed the other concentrations. The mean bilirubin level in the excreta of heat-stressed broilers that received 100 mg/kg of PETSC was lower (P?<?0.05) than that in broilers that received 0, 300, 400 and 500 mg/kg of PETSC. This showed that PETSC could reduce GPx activity and bilirubin in feces, and increase red blood cell parameters in heat-stressed broilers.     

Analysis of stimulating products involved in primary and secondary allogenic proliferation in man

HLA-D typing, primed lymphocyte test (PLT), and DR (Ia-like) serology were compared in a population and family study. A significant positive correlation was observed between theHLA-D region products detected by these three techniques. The strongest correlation observed was between PLT and DR serology, indicating a very close functional similarity between PL and DRw antigens. The DRw antigens and/or PL products appear to be mainly responsible for secondary proliferation. Data are presented which suggest that DRw and/or PL products could be distinct from the Dw products, involved in primary MLR. Nevertheless, a DRw disparity associated with a Dw incompatability is able to increase the intensity of a primary MLR, suggesting that DRw antigens also influence a primary proliferative response.     

Immunological factors in Peromyscus speciation

Reciprocal interspecific F1 hybrids of deermice (Peromyscus maniculatus) and oldfield mice (P. polionotus) differ significantly and substantially in fetal and placental, as well as adult, size and weight. Hybrid fetal mortality is associated with large conceptus size. Skin grafts were exchanged between and within the two species to ascertain whether any relationship exists between mean graft retention time and body size of fetuses and adults. P. maniculatus skin grafted to P. polionotus rejected significantly earlier than the reciprocal xenograft. All interspecific graft combinations rejected significantly earlier than intraspecific grafts. Pre-immunization of female P. maniculatus with con- and trans-specific paternal spleen cell antigens reduced fetal, placental, neonatal, and ten-day size compared with controls. Size, weight, fertility, and graft rejection data were compared with several theoretical models. The data were consistent with the hypothesis that immunological disparity between the species could produce marked size variations in reciprocal hybrids. Multiple minor histocompatibility factors can account for large placental size and fetal mortality in Peromyscus hybrids. Physiological reproductive isolation may result from immunological differences between closely allied species.     

Bovine monoclonal alloantibodies to blood group antigens prepared by murine x bovine or (murine x bovine) x bovine interspecies fusions.

In order to obtain monoclonal alloantibodies against bovine blood group antigens, lymph node cells from calves immunized with bovine red blood cells (RBC) were fused with either murine NSO/1 myeloma cells or a HAT sensitive murine x bovine heterohybridoma cell line. Both fusion partners resulted in heterohybridoma cell lines, producing monoclonal alloantibodies against bovine red blood cell antigens. Several clones produced antibodies against identical antigens and some of these clones have been further analysed. The antibodies produced by these selected cell lines have been compared with conventional polyclonal antisera used in bovine blood typing service. Thus extensive tests--including the ISAG Comparison Tests 1989/90 and 1991/92--have proved that monoclonal alloantibodies specific for the internationally recognized bovine red cell antigens A2, I1, O1, Q, A', B', Q', C1, R1, X1, S and Z have been produced. The Q, A', B', and C1 antibodies react weakly with certain phenogroups, whereas the A2, I1, O1, Q', R1, X1, S and Z antibodies have proved to be excellent blood typing reagents and have now substituted the polyclonal antisera in routine bovine blood typing in our laboratory.     

Morphology of glutaraldehyde-fixed preserved red blood cells and 24-hr post-transfusion survival

Liquid-stored red blood cells and washed, previously frozen red blood cells were studied to determine whether a correlation existed between morphology and post-transfusion survival. Red cell concentrates were stored at 4 °C in citrate-phosphate-dextrose (CPD) for 21 days or in CPD-adenine (CPDA-1, CPDA-2, or CPDA-3) for as long as 35 days as liquid-preserved red cells. Both nonrejuvenated and rejuvenated red blood cells were frozen with 40%$\text{w}\text{v}$ glycerol at ?80 °C and were washed prior to testing.Samples of fresh, liquid-stored, and washed, previously frozen red blood cells were fixed with a 2% veronal glutaraldehyde solution. Phase, light, and electron microscopy were used to measure the numbers of discocytes, discoechinocytes, echinocytes, echinospherocytes, and spherocytes in each sample. A morphology score was assigned, with 100 representing all discocytes and 500 all spherocytes. In all samples phase and light microscopy gave nearly identical scores (r = 0.94), and phase and electron microscopy gave highly similar scores (r = 0.83).The morphology score proved to be a good indicator of 24-hr post-transfusion survival in liquid-stored red blood cells but not in washed, previously frozen red blood cells. Red blood cells stored in the liquid state at 4 °C in CPD, CPDA-1, CPDA-2, or CPDA-3 showed a significant inverse correlation between morphology and 24-hr post-transfusion survival (r = ?0.611) and a significant correlation between red cell ATP and 24-hr post-transfusion survival (r = 0.742). We saw no significant correlation between morphology scores and 24-hr post-transfusion values or between ATP levels and post-transfusion survival values in nonrejuvenated or rejuvenated washed, previously frozen red blood cells.     

Predictive analytics and cord blood banking: toward utilization-based unit selection

Background aimsTotal nucleated cell (TNC) and CD34+ cell doses are considered among the most important parameters when assessing the suitability of a human leukocyte antigen-matched cord blood unit (CBU) for allogeneic hematopoietic stem cell transplantation (HSCT). Cord blood banks therefore frequently select CBUs for cryopreservation based on pre-process TNC content. However, cell loss during processing can lead to a significant quantity of CBUs that do not meet desired post-process quality criteria, and such grafts are less likely to be selected by transplant centers for HSCT. Here the authors present a multi-parameter linear regression (MLR) model capable of identifying CBUs that would process poorly, despite meeting established pre-process TNC and CD34+ quality thresholds.MethodsHistorically processed CBUs were graded from A+ to D depending on post-process cell content, and the utilization rate of each grade category was examined. Eight pre-process predictors of post-process cell content were used to train the MLR model, including red blood cell (RBC) content; CBU volume; age of CBU when received; and TNC constituent cell subsets. The selection efficacy of this model was then compared to that of methods conventionally used to select CBUs for processing, with receiver operating characteristic (ROC) and mean inventory quality analysis forming the basis of assessment.ResultsWithin the Anthony Nolan Cell Therapy Centre, CBUs graded ‘D’ accounted for 37% of processing expenditures despite providing only 11% of grafts shipped for HSCT. The MLR model significantly improved pre-process identification of 'D' grade CBUs relative to thresholds based primarily on CD34+ cell content (P < 0.0001) and TNC content (P < 0.0001). At a comparable financial investment, this translated to a banked graft inventory of significantly higher quality than that produced by CD34+ (+8.8% mean increase, P = 0.007) and TNC (+9.9% mean increase, P = 0.010) selection methods.ConclusionsA predictive modelling approach to pre-process CBU selection is a simple and effective means to increase graft inventory quality and potentially future graft utilization, at no additional financial investment.     

Histocompatibility studies in pigs from outbred and semi-inbred families

Skin graft experiments with pig siblings from semi-inbred and outbred families confirmed the existence of a pig main histocompatibility system (MHS). Grafts exchanged between serological (lymphocytotoxicity) MHS incompatible siblings from outbred matings survived 5.6 days and between compatible animals 9.8 days. In compatible pairs from semi-inbred matings they survived 22.4 days and in incompatible individuals 6.1 days. Skin grafts exchanged between individuals with E blood group compatibility led to the formation of haemagglutinating and lym-phocytotoxic antibodies reacting with some E antigens. The results indicate that the survival of skin grafts in semi-inbred pigs is apparently influenced also by differences in the E system.     

Relationship of B blood group locus to skin allograft in chickens

Skin grafts were exchanged among 21 genotypic pairs of B blood group locus in the non-inbred chicks of White Leghorn at 5–7 days of age. The mean percentages of B locus compatible pairs were 94.7, 84.2 and 56.8 at the 11th, 15th and 19th days after grafting, respectively. These percentages of survival grafts were significantly higher than those of incompatible pairs. The effects of three B alleles were investigated but the the differences of effects of them were not found in this experiment. Two of the prolonged survival grafts survived for 35 days after grafting and all of the incompatible grafts were rejected the 20th day after grafting. The results of skin graft provided evidence that the B blood group locus was a histocompatibility locus or closely linked to such a locus.     

Biochemical characterization of the feline AB blood group system

The biochemical nature of the feline AB blood group system was characterized by analysing red blood cells from homozygous (genotype A/A) and heterozygous (A/B) type A, type B (B/B), and type AB cats. High performance thin layer chromatography (HPTLC) of red cell gly-colipids revealed that specific neuraminic acids (NA) on gangliosides, containing ceramide dihexoside (CDH) as a backbone, correlated with the feline AB blood group antigens. Although disialogangliosides predominated, mono- and trisialogangliosides were also isolated. B cats expressed solely N-acetyl-NA (NeuNAc) on these gangliosides. In addition to expressing N-glycolyl-NA (NeuNGc) containing gangliosides, A red cells have gangliosides with only NeuNAc or mixtures of both NA. HPTLC profiles of disialogangliosides from homozygous and heterozygous A cats differed slightly in the quantity of disialogangliosides. Equal amounts of NeuNAc and NeuNGc containing disialogangliosides, as well as two intermediary forms, were recovered from AB erythrocytes. Analysing disialogangliosides from red cells belonging to 17 genetically related cats, we consistently obtained the expected disialoganglio-side profile, based on blood typing and pedigree information. SDS-PAGE of red cell membrane proteins and blotting with Triticum vulgaris, a lectin recognizing NeuNAc, revealed glycoproteins of approximately 51, 53, and 80 kD in B and AB cats but only a faint band of approximately 53 kD in A cats. By haemagglutination, Triticum vulgaris could also distinguish different blood types by specifically binding to B and AB cells. Flow cytometry showed that more anti-B bound to B cells than anti-A bound to A cells. Although AB cells had a broad range of fluorescence when compared to the profiles seen with A and B cells, the mean fluorescence with AB cells was half of that seen with A or B. These results further characterize the antigens determining the feline AB blood group system illustrating differences between A, B and AB cats, and between homozygous and heterozygous A cats.     

A possible role of initial cell death due to mechanical stretch in the regulation of subsequent cell proliferation in experimental vein grafts

 The proliferation of vascular cells contributes to the formation of neointima and hypertrophy of the blood vessel wall. Here we show that mechanical stretch possibly regulates the proliferation of vascular cells via the mediation of cell death in a rat vein graft model. The wall of vein grafts is subject to a suddenly increased mechanical stretch due to exposure to arterial blood pressure. Such a stretch induces rapid cell death with a reduction in cell density by ∼60% within the first day after surgery. The initial cell death was followed by an increase in the percentage of proliferating cells, as shown by a BrdU incorporation assay (1.55 ± 1.27%, 8.48 ± 2.27%, 11.93 ± 2.36%, 6.36 ± 1.77%, and 5.60 ± 1.46% at days 1, 5, 10, 20, and 30, respectively). When mechanical stretch was reduced by restraining the vein graft using a polytetrafluoroethylene sheath, the percentage of proliferating cells reduced significantly (0.76 ± 0.76%, 1.70 ± 0.46%, 1.29 ± 0.56%, 0.99 ± 0.83%, and 0.47±0.52% at days 1, 5, 10, 20, and 30, respectively). A further reduction in cell density, induced by local administration of a cell death inducer ceramide to experimental vein grafts (without sheath), enhanced subsequent cell proliferation. In contrast, a prevention of cell death, induced by local administration of a cell death inhibitor tetrapeptide-aldehyde DEVD-CHO to experimental vein grafts (without sheath), significantly reduced subsequent cell proliferation. These results suggest that mechanical stretch induces cell death, which possibly mediates subsequent cell proliferation in the present model. Received: 9 September 2001 / Accepted: 19 November 2001     

Blood groups of crab-eating macaques (Macaca fascicularis) demonstrated by isoimmune rhesus monkey (Macaca mulatta) sera.

Twenty-one isoimmune sera produced in rhesus monkey (Macaca mulatta) containing type-specific antibodies for simian-type red cell antigens were tested for their cross-reactivity with red cells from crab-eating macaques (M. fascicularis). The majority of the antisera gave cross-reactions determining polymorphisms in the red cells of crab-eating macaques, homologous to those of rhesus monkeys. These results attest to the close taxonomic realationship between the two species of macaques, and have the practical implication that isoimmune sera produced for blood typing can also be used for typing red cells from related species, as has been also observed in studies on apes.     

Analysis of a horse family with a crossing-over between the ELA complex and the A blood group system

A horse family in which a recombination occurred in the chromosome region coding for the serological specificities of the ELA complex and those of the A blood group system of a mare was further analysed by mixed lymphocyte reaction (MLR) and Southern blot hybridization. This family consisted of a stallion, a mare and five full sibs. The stallion and the mare were heterozygous for internationally recognized ELA specificities while only the mare was heterozygous for the A blood group system. MLR between all members of the family confirmed that the stallion possessed two different ELA haplotypes and suggested that recombination in the mare occurred outside the segment delimited by the ELA-A locus and the MLR region. DNA samples from all individuals were investigated by Southern blot analysis using three restriction enzymes (EcoRI, HindIII or TaqI), three human HLA probes (one of class I cDNA and two of class II probes), one cDNA (DR beta) and one genomic (DQ alpha). Class I and class II restriction fragments of the mare segregated in accordance to the ELA specificities and thus clearly confirming that the crossing-over did not occur between the ELA-A gene and the class I, class II region nor between DR beta and DQ alpha subsets. The A blood group genetic determinants would thus be situated outside the ELA region defined by class I and class II genes.     

Human alveolar macrophages: HLA-DR-positive macrophages that are poor stimulators of a primary mixed leukocyte reaction

Previous studies demonstrated that alveolar macrophages (AM) from most normal human volunteers failed to stimulate the antigen-induced proliferation of peripheral blood T lymphocytes although greater than 90% of AM expressed HLA-DR antigens. The current studies establish that AM also fail to induce allogeneic peripheral blood mononuclear cells to proliferate in a mixed leukocyte reaction (MLR). Suppressive activity by AM was not an explanation for their failure to induce an MLR. Indirect immunofluorescence established the presence of both HLA-DR and DQ antigens on the majority of AM and the persistence of these antigens on cells in culture for up to 6 days, the period of time required to observe a maximal MLR. Metabolic labeling experiments also demonstrated that HLA-DR antigens were synthesized by AM. It was recently reported that AM secrete relatively small amounts of IL 1, an important ancillary signal provided by accessory cells to enhance the stimulation of lymphocyte proliferation. However, addition of optimal concentrations of IL 1 to cultures containing AM failed to enhance the MLR. Thus, there is at least one additional, but as yet undefined, requirement for an accessory cell to induce an optimal MLR besides the display of HLA-D region antigens and the secretion of IL 1. In contrast, AM were effective in specifically stimulating proliferation of alloreactive T cell lines, suggesting that at least some cell lines do not require this nonspecific undefined second signal. We speculate that although AM may not initiate primary immune responses in the lung, they may be important in maintaining immune-mediated inflammatory responses by specifically restimulating already activated T cells.     

The influence of 2-mercaptoethanol (2-ME) treatment of IgG antibody on its ability to induce cytotoxicity in nonsensitized lymphocytes

Administering dextran 2 hr before giving antigen (sheep red blood cells) to X-irradiated mice repopulated with B and T cells caused alterations in antibody synthesis. Besides a slight increase in the number of cells making IgM antibodies, cells producing IgG antibodies were detected at a time when none are usually present in untreated control animals.Repopulating X-irradiated mice with B cells treated with dextran either in vivo or in vitro and immunizing with sheep red blood cells resulted in twice the background number of sheep red blood cell-specific IgM-plaque-forming cells at 4.5 days of immunization as well as a small number of IgG-plaque-forming cells at 8.5 days. At these times, control animals given only B cells and sheep red blood cells possessed a background number of IgM-plaque forming cells and no IgG plaque-forming cells.Incubating B cells with θ-specific antiserum and complement to remove residual T cells prior to transplantation obliterated dextran's stimulus. Dextran's alterations of immunological responses towards unrelated antigens therefore appears to be manifested through T cells. The latter must be in company with B cells at the time of exposure to dextran. Thus, T cells upon contact with dextran apparently release B cell stimulatory factor (s) responsible for increasing the number of IgM-forming cells and for triggering IgG-forming cells.     

Effect of localized cytokine dysregulation: accelerated rejection of IL-2-expressing skin grafts

Transgenic mice were created in which a sheep keratin promoter directed the expression of IL-2 into the dermis. These KIL-2 transgenic mice were used to investigate the effects of localized IL-2 dysregulation on immune responses. Peripheral tolerance to skin antigens was not broken by in situ IL-2 expression because syngeneic KIL-2 skin grafts were not rejected. However, MHC Class I-disparate skin grafts from KIL-2 donors were rejected faster (median survival time (MST) 12 days) than grafts of non-transgenic littermate skin (MST 18 days). In contrast, the kinetics of KIL-2 H-Y-disparate skin graft rejection (MST 14 days) did not differ significantly from controls (MST 16 days), suggesting that upregulation of IL-2 at the effector site could affect CD4+ T cell- independent, but not CD4+ T cell-dependent, responses. No effect on rejection kinetics was observed when wild type allogeneic skin was grafted onto transgenic mice that expressed bcl2 constitutively in their lymphocytes (MST of 14 days, both sets), indicating that this was not simply due to increased longevity of T cells within the IL-2 expressing graft. We therefore suggest that aberrant expression of IL-2 can accelerate helper-independent CD8+ T cell responses by increasing proliferation and/or differentiation of cytolytic T cells at the effector site.     

Split tolerance induced by chick embryo thymic epithelium allografted to embryonic recipients

To test the capacity of the epithelial component of the chick embryo thymus to induce tolerance to major histocompatibility complex (MHC) antigens, pre-colonized thymic rudiments were grafted into chick embryonic recipients. Semi-allogeneic or allogeneic transplantations were done between two lines of chickens histocompatible at the MHC locus. Approximately 10% of these thymic chimeras hatched and were studied 3 mo after hatching. Thymic grafts were not rejected by the allogeneic host. The tolerance of chimeric chickens to thymus donor MHC antigens was tested by using a skin graft rejection test and a graft-vs-host (GvH) assay. Chimeric chickens that received an MHC-incompatible thymic graft during the embryonic life tolerated skin graft with the MHC haplotype of the thymus donor. Nevertheless, the lymphocytes within the thymic graft, the host thymus, and the blood were tolerant to the host MHC antigens but were alloreactive in GvH reaction for the MHC antigens of the thymic graft type. These results suggest that the epithelial component of the thymus when taken before the starting of the colonization by hemopoietic precursors and grafted into an early chick embryonic host can induce a tolerance for the MHC determinants involved in allograft rejection but not in the GvH reaction.