Presentation of a self-peptide for in vivo tolerance induction of CD4+ T cells is governed by a processing factor that maps to the class II region of the major histocompatibility complex locus.

Self-proteins are regularly processed for presentation to autoreactive T cells in association with both class I and class II major histocompatibility complex (MHC) molecules. The presentation of self-peptides plays a crucial role in the acquisition of T cell repertoire during thymic selection. We previously reported that the self-MHC class I peptide Ld 61-80 was immunogenic in syngeneic B10.A mice (H-2a). We showed that despite its high affinity for self-MHC class II molecules, Ld 61-80 peptide failed to induce elimination of autoreactive CD4+ T cells, presumably due to incomplete processing and presentation in the B10.A's developing thymus (cryptic-self peptide). In this report, we showed that the cryptic phenotype was not an intrinsic property of the self-peptide Ld 61-80 since it was found to be naturally presented and subsequently tolerogenic in BALB/c mice (H-2d) (dominant self-peptide). In addition, the self-peptide Ld 61-80 was found to be immunogenic in different H-2a mice while it was invariably tolerogenic in H-2d mice regardless of their background genes. We observed that Ld 61-80 bound equally well to H-2d and H-2k MHC class II molecules. Also, no correlation was found between the quantity of self-Ld protein and the tolerogenicity of Ld 61-80. Surprisingly, Ld 61-80 was not naturally presented in (H-2d x H-2a) F1 mice, indicating that the H-2a MHC locus contained a gene that impaired the presentation of the self-peptide. Analyses of T cell responses to the self-peptide in several H-2 recombinant mice revealed that the presentation of Ld 61-80 was controlled by genes that mapped to a 170-kb portion of the MHC class II region. This study shows that (a) endogenously processed self-peptides presented by MHC class II molecules are involved in shaping the CD4+ T cell repertoire in the thymus; (b) The selection of self-peptides for presentation by MHC class II molecules to nascent autoreactive T cells is influenced by nonstructural MHC genes that map to a 170-kb portion of the MHC class II region; and (c) the MHC locus of H-2a mice encodes factors that prevent or abrogate the presentation by MHC class II molecules of the self-peptide Ld 61-80. These findings may have important implications for understanding the molecular mechanisms involved in T cell repertoire acquisition and self-tolerance induction.

The modulating influence of cyclic nucleotides upon lymphocyte-mediated cytotoxicity.

The capacity of allosensitized thymus-derived lymphocytes to destroy target cells bearing donor alloantigens is modulated by the cellular levels of cyclic AMP and cyclic GMP. Increases in the cyclic AMP levels of attacking lymphocytes by stimulation with prostaglandin E(1), isoproterenol, and cholera toxin inhibit lymphocyte-mediated cytotoxicity; whereas, depletion of cyclic AMP with imidazole enhances cytotoxicity. The augmentation of cytotoxicity produced by cholinergic stimulation with carbamylcholine is not associated with alterations in cyclic AMP levels and is duplicated by 8-bromo-cyclic GMP. The effects of activators of adenylate cyclase, cholinomimetic agents, and 8-bromocyclic GMP are upon the attacking and not the target cells and occur at the time of initial interaction of attacking and target cells. Indeed, the level of cyclic nucleotide (cyclic AMP and cyclic GMP) at the time of initial cell-to-cell interaction determines the extent of cytotoxicity.

Microtubule function in immune and nonimmune lymphocyte-mediated cytotoxicity.

Disaggregation of microtubular subunits in effector lymphocytes inhibits their ability to injure target cells. The inhibition is not reversed by denterium oxide, an agent known to stablize microtubular subunits.

Rapid mixed lymphocyte culture testing by analysis of the insulin receptor on alloactivated T lymphocytes: implications for human tissue typing.

Responses in the mixed lymphocyte culture (MLC) are traditionally evaluated by measurement of DNA synthesis or blast transformation. However, these events occur too late in the MLC to permit prospective matching for cadaveric renal transplantation. Presentation of allogeneic cells to the T lymphocyte within the MLC results in the emergence of an insulin receptor pharmacokinetically similar to that on other tissues such as fat, liver, and muscle. Intrafamilial MLC were studied by simultaneous assessment of DNA synthesis and insulin receptor binding. In 68 studies from seven families that provide examples of two haplotype identical matches, haplo-identical matches and total haplo mismatches, the presence of an insulin receptor correlated in every case with a positive MLC as estimated by [3H]thymidine incorporation. A quantitative relationship existed between the strength of the MLC and the amount of receptor binding. Based on analysis of cells from several families in which crossover events were known to have occurred, the appearance of an insulin receptor always corresponded with a mismatch at the portion of histocompatibility leukocyte antigen (HLA) chromosome bearing the D region. Finally, it was demonstrated in each of 30 cultures that insulin receptor emergence occurred significantly before detectable DNA synthesis, as early as 24 h after the initiation of the MLC, well within the time-constraint limitations for renal preservation. Appearance of the insulin receptor on activated lymphocytes may be a more rapid measure of mixed lymphocyte responses, and should permit prospective matching for cadaveric renal transplantation.

Early-onset pauciarticular juvenile rheumatoid arthritis associated with human leukocyte antigen-DRw5, iritis, and antinuclear antibody.

Evidence has been sought for a genetically determined predisposition among children with juvenile rheumatoid arthritis (JRA) who are also at particular risk for the development of inflammatory eye disease.45 unrelated Caucasian patients (41 female) with early-onset pauciarticular JRA were human leukocyte antigen (HLA) types. 28 of the study group were found to be HLA-DRw5 compared with 16 of 84 controls (X(2), 24.3, P = <0.001). 9 patients were HLA-DRw8 compared with 4 of 84 controls (X(2), 7.51, P = <0.01). Iritis developed in 24 of the 45 children studied, 17 of whom were typed as HLA-DRw5 when compared to controls (X(2), 26.76, P = <0.001) and 6 with iritis typed as HLA-DRw8 (X(2), 9.10, P = <0.01). Antinuclear antibody was found in the serum of 17 of the 28 patients typing as HLA-DRw5 compared with 4 of the 17 who did not have this antigen (X(2), 5.88, P = <0.02). No such association was seen in patients with HLA-DRw8. In a study of linked genes, a delta value of 0.090 was found for HLA-DRw5 with HLA-B12, of 0.070 for DRw5 with HLA-Cw4, and a value of 0.050 for DRw5 and HLA-Bw35. This suggests a linkage disequilibrium between HLA-DRw5 and these two B series alleles, a conclusion which was supported by haplotype analysis in families of 11 of the disease probands. HLA-DRw5 has not previously been reported to be increased in any rheumatic disease group. It is proposed that HLA-DRw5 is a genetic marker defining those at risk for early-onset pauciarticular JRA with iritis.

Muscle capillary basement membrane width and its relationship to diabetes mellitus in monozygotic twins.

Quadriceps (Q) and gastrocnemius (G) muscle capillary basement membrane width (CBMW) were measured in 18 pairs of monozygotic (MZ) twins. Thirteen of these twin pairs were discordant for insulin-dependent diabetes (IDD) and five pairs were concordant for either IDD (two pairs) or for non-insulin-dependent diabetes (NIDD). In 12 of the 13 nondiabetic (ND) twin mates of IDD, 50 oral glucose tolerance tests performed in the years before or after determination of CBMW revealed mean blood glucose levels in the 36-52 percentile range, compared with normal controls. The mean (+/-SD) age at the onset of IDD in discordant twins was 18.7 +/- 10.1 (range 8-37) yr and the mean duration of discordance at the time of biopsy was 13.6 +/- 8.3 (range 3-32) yr. CBMW data were compared within each twin (Q versus G) and between twin mates and age- and sex-matched controls. Overall, CBMW of IDD twins was greater than that of their ND twin mates. Differences between IDD and ND twins, however, were much more marked in gastrocnemius (1859 +/- 643 versus 1222 +/- 307 A, P less than 0.0003) than in quadriceps (1291 +/- 319 versus 1112 +/- 302 A; P less than 0.04). CBMW in gastrocnemius was significantly thicker than that in the quadriceps of IDD twins (t = 4.55, P less than 0.0008) but not in their ND twin mates (t = 1.15, P less than 0.27). CMBW was significantly thicker in IDD than in their ND twin mates (in quadriceps and/or gastrocnemius) in 10 of the 12 twin pairs.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of oligonucleotide uptake in human keratinocyte cultures.

Oligodeoxyribonucleotides have the potential to interfere selectively with cellular protein synthesis by sequence-specific hybridization to DNA or RNA molecules. We have investigated the properties of uptake and intracellular localization of fluorescently labeled oligonucleotides in cultured human keratinocytes using confocal laser scanning microscopy. Unlike many other cell types studied, keratinocytes can internalize oligonucleotides without apparent sequestration in endosomes or cell surface accumulation. Uptake is primarily nuclear and unaltered by sodium azide, monensin, or chloroquine pretreatment. We have verified our results with two different fluorophores, fluorescein and Bodipy, and found similar uptake and distribution patterns in both live and fixed cell populations. Surprisingly, we have found uptake to be heterogeneous within a population, with 15-30% of cells internalizing the oligonucleotides. This percentage is drastically increased to roughly 80% at cell population margins, and after release from M phase arrest. These results on uptake and intracellular localization suggest that keratinocytes may have increased sensitivity as target cells for oligonucleotide based gene regulation strategies.

Self determinant selection and acquisition of the autoimmune T cell repertoire.

Autologous proteins are continuously processed and presented in the form of peptides associated with self major histocompatibility (MHC) molecules at the surface of antigen-presenting cells for interaction with autoreactive T cells. During thymic selection, the presentation of self peptides is an essential element in the establishment of the T cell repertoire. Developing T cells which recognize self peptide/self MHC complexes with sufficient affinity are clonally deleted. However, we and others have recently demonstrated that a variety of self peptides, despite their high binding affinity to MHC molecules, never reach the threshold of presentation to ensure negative selection (cryptic self peptides). This mechanism may have been selected to avoid excessive purging of T cell repertoire during ontogeny. However, T cells directed to cryptic self determinants represent a continuous threat for the initiation of autoimmunity in adults. Supporting this view, recent studies have documented the involvement of cryptic self peptide presentation in different autoimmune diseases. In this article, we examine the factors that govern the selection of self peptides for presentation to autoreactive T cells in vivo and discuss their contribution to both the induction and the maintenance of self tolerance. In addition, we analyze the mechanisms by which the hierarchy of determinants on a self protein can be disrupted, thereby leading to the presentation of previously cryptic self peptides and the induction of an autoimmune T-cell-mediated process.

A flow cytometric method for the detection of the development of antibody to Orthoclone OKT3.

Orthoclone OKT3 is a monoclonal antibody used in the treatment of transplant rejection. The development of anti-OKT3 antibodies after therapy with this drug is a side effect which must be monitored. The current ELISA test to detect these antibodies is difficult to standardize between laboratories and does not lend itself to frequent monitoring of small numbers of samples. This prompted us to develop a new method employing flow cytometry to detect the development of anti-murine antibodies. Comparing samples tested by both techniques, we found the flow cytometric method to be more sensitive and as the ELISA methodology. This technique is adaptable to numerous serologic assays and could greatly expand the use of flow cytometry in the clinical laboratory.

Prediction of donor-specific transfusion sensitization. I. A linear logistic model.

Using linear logistic regression, six factors were identified as important predictors of risk of DST sensitization in a group of 195 patients. Factors increasing the risk were: percent panel reactive antibody (PRA), previous transplants, and pregnancy; those decreasing the risk were HLA antigens matched, third-party blood transfusions, and Imuran administration. From this analysis, the magnitude of the effect of each factor on the risk of sensitization was obtained. An equation was then obtained that can be used to compute an estimated probability of sensitization (PS) for each patient. As a test of predictive ability of the model, the PS was calculated for 66 patients in an independent patient group. These observations were arranged according to the estimated probability and then divided into intervals of risk. Overall, for each interval, a very high level of agreement was found between the predicted and actual number of sensitized patients. A total of 16.13 patients were predicted to become sensitized and 17 actually did.

Antibody-dependent lymphocyte-mediated cytotoxicity (Ab-LMC), definition of the "K cell" in the rat.

The nature and membrane characteristics of the "K cell" of antibody-dependent lymphocyte-mediated cytotoxicity (Ab-LMC) were investigated in a widely used rat model of transplantation. Treatment of sensitized effector cell populations with anti-immunoglobulin and complement eliminated K cell cytotoxicity without diminishing the component of T cell-mediated injury. EA and EAC depletion experiments, although demonstrating no loss of K cell cytotoxicity after removal of complement receptor-bearing lymphocytes, produced a marked abrogation of cytotoxicity following the removal of the Fc receptor-bearing lymphocyte pool. Studies on phagocytic properties showed K cell activity to be shared by an adherent as well as a nonadherent cell population. Thus, the Fc receptor emerged as the only constant surface marker of the rat K cell in Ab-LMC.

An oligonucleotide blocks interferon-gamma signal transduction.

Interferon (IFN)-gamma is an important mediator of transplant graft rejection. It induces endothelial cell expression of HLA-DR and intercellular adhesion molecule-1, which render transplant grafts more susceptible to rejection by the host. Oligonucleotide 5'-GGG GTT GGT TGT GTT GGG TGT TGT GT-RNH2 (oligo I) blocks multiple IFN-gamma effects in human K562 cell cultures. A systematic approach revealed that oligo I has a novel, and potentially important, mode of action--it blocks the binding of IFN-gamma to its receptor, thus preventing activation of the IFN-gamma signal transduction pathway. The results are consistent with an aptamer mechanism of action, because oligo I exerts its inhibitory effects by interacting with protein, not intracellular nucleic acid targets, such as mRNA or genomic DNA.

Two patterns of sensitization demonstrated by recipients of donor-specific transfusion. Limitations to control by Imuran.

Characteristics of the sensitization response to donor-specific transfusion (DST) have been studied in the context of the pretransfusion panel reactive antibody (PRA) status of the recipient. Two distinct patterns of response to DST and Imuran treatment have been found. In patients with one-haplotype-matched donors, the panel nonreactive patient (PRA less than 10%) has a 19% incidence of DST sensitization that is further reduced by Imuran treatment to 6%; antibodies are both anti-T cell and anti-B cells, are transient, and are specific to the mismatched HLA antigens of the blood donor. Panel-reactive patients (PRA greater than 10%) have a 56% incidence of DST sensitization; the antibodies appear within 2 weeks of the first transfusion, are anti-T cell, and are generally of broad specificity and persistent duration consistent with amplification of a previous antigenic exposure; Imuran seems to have little or no effect in reducing the incidence of sensitization in these panel-reactive patients. However, panel reactive patients whose PRA levels spontaneously fall to panel-nonreactive levels immediately prior to DST therapy have an exceedingly low (0-8%) incidence of sensitization with or without Imuran coverage.

A poorly differentiated lymphoma of donor origin in a renal allograft recipient.

Malignant lymphoma is a frequent complication of organ transplantation. It has been suggested that such tumors arise as a result of uncontrolled proliferation of Epstein-Barr virus-infected B lymphocytes in an immunosuppressed host. Although a few cases of posttransplant lymphomas in bone marrow transplantation have been shown to be of donor cell origin, no recipients of solid-organ transplants are known to have developed lymphomas arising from donor cells. In this report, a case of diffuse high-grade lymphoma that apparently arose in the allograft of a renal transplant recipient is described. DNA fingerprinting demonstrated the tumor to be of donor origin; Epstein-Barr sequences were absent. A therapeutic trial consisting of withdrawal of immunosuppressive agents and administration of acyclovir was unsuccessful. These data support the notion that donor cells can undergo malignant transformation in solid-organ transplant recipients, and such tumors need not carry EBV genetic material.

A phase I trial of humanized anti-interleukin 2 receptor antibody in renal transplantation.

The efficacy of murine monoclonal anti-interleukin 2 alpha chain receptor (Tac) antibodies is limited by a short half-life and the development of antibodies to the heterologous protein. The safety, pharmacokinetics-dynamics, and immunosuppressive effect of a humanized anti-Tac antibody (HAT) was evaluated in 12 renal transplant recipients. Ten patients received living related transplants (three HLA-identical matches and seven one-haplotype or zero-haplotype matches) and two patients received cadaver organs. The patients were divided into four HAT treatment arms: 0.5 mg/kg/week (n=4), 1 mg/kg/week (n=2), 0.5 mg/kg every other week (n=3), and 1 mg/kg every other week (n=3). The first dose of HAT was given within 12 hr before transplantation, and four additional doses were given after transplantation. Patients were also placed on cyclosporine, steroids, and azathioprine. Only one patient, a recipient of a cadaver kidney in the lowest HAT treatment arm, had a reversible rejection episode. The 10 recipients of living related transplants were compared with 17 historical controls treated with an identical immunosuppressive regimen except for HAT. Whereas none of the HAT-treated living related donor recipients had a rejection episode, 6 of 17 (41%) of the historical controls had a rejection episode in the first year after transplantation. There were no first-dose reactions after HAT therapy or other subsequent side effects. None of the patients experienced opportunistic infections or malignancies. One patient developed low-titer anti-HAT antibodies, although the patient maintained high serum HAT concentrations throughout the study. Immune monitoring showed that there were no changes in the percentage or absolute counts of CD3 cells or T-cell subsets after HAT therapy. However, there was a significant decrease in the number of circulating lymphocytes that expressed free Tac. The overall harmonic mean half-life of HAT was 273 hr. The results of this study indicate that HAT given at 1 mg/kg every other week for a total of five doses may provide therapeutic HAT concentration levels and result in good saturation of Tac receptors for at least 12 weeks after transplantation. In summary, HAT is safe and is well tolerated by patients. Its long half-life and lack of immunization could make it a very useful immunosuppressive drug.

Direct evidence for in vivo induction of CD8+ cytotoxic T cells directed to donor MHC class I peptides following mouse allotransplantation.

There is accumulating evidence indicating that the T cell response to donor major histocompatibility complex (MHC) peptides plays a crucial role in graft rejection. We and others previously demonstrated the involvement of MHC class-II-restricted recognition of donor MHC class I and II peptides by alloreactive CD4+ T helper cells in graft rejection. Here we studied the in vivo induction of CD8+ cytotoxic T lymphocytes (CTL) directed to donor MHC class I peptides following allotransplantation in the mouse. To address this question, BALB/c irradiated splenocytes (H-2d) (Kd, A(d), E(d), Ld, Dd) were injected into Ld-deficient BALB/c-dm2 (dm2) mutant mice (Kd, A(d), E(d), -, Dd). Nine days after allogeneic cell transplant, recipient lymph node T cells were tested for cytolytic activity using peritoneal macrophages as targets. We observed that in addition to BALB/c targets, dm2 macrophages could also be lysed but only when incubated with a dominant peptide on donor Ld molecule, Ld 61-80. This response was abolished by anti-CD8 but not anti-CD4 monoclonal antibodies. In addition, after immunization of dm2 mice with the peptide Ld 61-80, alloreactive CTL were generated in vivo and shown to destroy allogeneic donor BALB/c target cells in the absence of exogenously added peptide. We conclude that after allotransplantation, concomitant in vivo priming of alloreactive CD8+ CTL by donor MHC class I peptides occurs through both direct and indirect pathways of allorecognition.

Identification of high- and low-risk second kidney grafts.

The purpose of this study was to identify recipients who are at low or high risk of early cadaveric regraft failure by segregating results of the flow cytometric crossmatch (FCXM) test with previous graft survival time (PGST). Early immunologic kidney regraft failure was analyzed in 103 multicenter recipients by cross-stratifying FCXM negative/positive status with < or =3- and >3-month PGST. T cell and B cell cytotoxicity crossmatches were negative. All were tested retrospectively in the T cell FCXM and 60 of the 103 were also tested in the B cell FCXM. A positive T and B cell FCXM was defined as a mean channel shift of > or = 9 (256 channel log scale) or > or = 40 (1024 channel log scale) for pretransplant crossmatch serum above negative control serum. Recipients received triple immunosuppression therapy and limited-use antilymphocyte induction therapy. Early cadaveric regraft losses were biopsied. Comparably good rates of second kidney graft survival at 3 years were found among three ow risk subsets: 78% for 18 FCXM-positive patients with PGST >3 months, 78% for 49 FCXM-negative patients with PGST >3 months, and 84% for 19 FCXM-negative patients with PGST < or =3 months. in contrast, 53% 3-month and 44% 3-year regraft survival rates occurred in 17 high-risk FCXM-positive recipients with a PGST < or =3 months. The odds ratio for increased relative risk of early second graft loss was 4.5 (confidence interval: 1.32-1.67) for the high-risk versus low-risk subsets (P = 0.009). Within the high-risk subset, 56% (5 of 9) of those who were FCXM T negative B positive experienced early regraft loss. A positive B cell FCXM has an adverse clinical impact only for high-risk regraft recipients. Pretransplant panel reactive antibody levels, pregnancy, number of blood transfusions between grafts, repeat donor HLA mismatches, and regraft recipient HLA mismatches did not correlate with early regraft loss. We conclude that kidney regraft survival rates in low-risk recipients (PGST >3 months/FCXM negative or positive [T and/or B cell] and PGST < or = 3 months/FCXM negative) approach primary graft survival rates and justify retransplantation, but the rate in high-risk regraft candidates (PGST < or =3 months/FCXM positive T and/or B cell) suggests that retransplantation should be performed only with a negative FCXM.

Dramatic improvement in the success rate for renal transplants in diabetic recipients with donor-specific transfusions.

The chance of achieving successful kidney transplants in diabetic patients was previously limited because few of them had optimally-matched (2-haplotype) related donors. Hence, transplants were usually not carried out until renal failure had already occurred. The application of donor-specific transfusions (DSTs) prior to transplantation to poorly matched donor-recipient pairs (1-haplotype) has been associated with a high success rate for type-I diabetic recipients in our center. The rate of graft survival for 35 consecutive transplants in this category was 88%, 80%, and 73% at 1, 2, and 5 years, respectively. Furthermore, the rate of patient survival was 94%, 90%, and 90% at 1, 2, and 5 years. These patient and graft survival data were without significant difference when compared with the corresponding data for 142 optimally-matched (2-haplotype) related transplants performed without DSTs for nondiabetic recipients, and also when compared with the corresponding data for 130 poorly matched (1 or 0-haplotype) related transplants involving nondiabetic recipients who were prepared for transplantation with DSTs. These good results with DSTs in diabetic recipients emphasize that earlier transplantation utilizing poorly matched related donors should be seriously considered for diabetic patients even before the onset of renal failure, as long as the transplants are carried out in association with DSTs.

A seven-year experience with donor-specific blood transfusions. Results and considerations for maximum efficacy.

Two hundred thirty-nine transplants have been performed following donor-specific blood transfusions (DSTs) since 1978. Graft and patient survival in 1- and 0-haplotype-matched transplants with DST pretreatment is comparable to HLA-identical results through 4 years. Graft survival in 174 consecutive nondiabetic, non-HLA-identical DST recipients shows that the transfusion effect persists for at least 4 years, with graft survival of 88 +/- 3% at that time, compared with 83 +/- 4% in the concurrent HLA-identical group. Graft function, as determined by serum creatinine, was the same in both groups. Graft and patient survival in 20 0-haplotype matched pairs with DST pretreatment is 100% at 2 years. Low-dose Imuran coverage during DST administration (n = 91) was compared with a concurrent group with no Imuran (n = 93). Imuran had its maximum effect in patients undergoing their first transplant and with a pre-DST PRA less than 10% (12% vs. 21% sensitization rate in the no-Imuran group). Imuran did not appear to confer any beneficial effect in primary transplants with high PRAs and in patients undergoing a second or third transplant. The majority of patients formally excluded from transplantation because of a post-DST positive B-warm crossmatch can now be successfully transplanted with the use of flow cytometry analysis to rule out previously undetectable low levels of anti-T-lymphocyte antibodies. Of 62 patients with a positive B-warm crossmatch alone since 1982, 73% had a subsequent negative fluorescence-activated cell sorter (FACS) crossmatch permitting transplantation. Preliminary results of a DST and cyclosporine treatment study are described. In conclusion, a long-term immunologic effect of DST has been confirmed and the indications and considerations for optimum use of the DST protocol have been more clearly defined.

Modification of the rat alloimmune response by enhancing antibodies and the role of blocking factors in the survival of renal grafts.

Correlation of morphological and immunological events ocurring in control and passively enhanced rat renal allograft recipients has revealed an important role for vasculitis in rejection, whereas the tempo and severity of graft lymphocyte infiltration and tubular damage was comparable in both groups during the first 5 days. Thereafter, the degree of cellular infiltration in enhanced allografts progressed and actually exceeded that in control grafts. 2-Mercaptoethanol-sensitive lymphocytotoxic antibodies were present in both groups with comparable titers and appearance times; however, the presence of a severe IgG-containing necrotizing arteritis and glomerulitis in control, but not enhanced, grafts suggests that passive enhancement protects by interfereing with the cooperative T cell-dependent inductive response. Further support for this possibility comes from the fact that the development of cytotoxic lymphocytes against donor target cells was delayed for 48 hr in the enhanced group. While controls died at days 9-11, enhanced animals entered a period of prolonged survival with stable renal function. This state of "autoenhancement" was characterized by a low degree of cell-mediated cytotoxicity and the appearance of serum factors that blocked the in vitro cellular assay. Blocking factors have a low affinity for the attacking cell population, suggesting that they are immune complexes or anti-idiotypic antibodies, and not free alloantibody of high affinity.