Localization of cryptic tolerogenic epitopes in the alpha1-helical region of the RT1.Au alloantigen

Fifty years in the vineyard of transplantation: looking back

The past 5 decades have documented remarkable advances in basic knowledge and clinical expertise in transplantation. The first 12 years of this half century of my participation in the enterprise were consumed with the isolation, chemical characterization, and application of histocompatibility antigens purified from mouse, guinea pig, and human tissues, demonstrating that their specificity was based on unique amino acid sequences in protein structures. Initial unsuccessful attempts to use native molecules to induce tolerance in rat renal or heart transplantation models were followed by limited success when they were administered with a brief perioperative course of cyclosporine (CsA). Production of allochimeric constructs of class I major histocompatibility complex molecules bearing donor-type amino acid substitutions into the host-type C-terminal portion of the α1 helix yielded tolerogens whose activity was not dependent on conditioning with CsA or total lymphoid irradiation (TLI). The allochimeric molecules serve as altered peptide ligands that induce an aberrant T-cell signal 1 response producing transplantation tolerance. The potent activity of CsA in this experimental model was extended to clinical settings. Pharmacologic tools were employed to explore intra- and interindividual variations in drug exposure leading to the development of a better drug formulation. However, the intrinsic nephrotoxicity of CsA necessitated marked 80% reductions in de novo drug exposure as were achieved by exploiting the synergistic pharmacodynamic and pharmacokinetic interactions of CsA with sirolimus. The final decade in this 50-year experience includes editorship of this journal with marked changes in its direction. These experiences have afforded insights into future avenues for preclinical exploration and therapeutic drug development.

Allopeptides and the alloimmune response

The inherent ability of the host immune system to distinguish between self- and non-self forms the basis of allorecognition. T lymphocytes constitute the most important effector arm of allorecognition. Here we describe the fundamentals of direct and indirect pathways by which allopeptides are presented to effector T cells. The nature of allopeptides presented along with tolerogenic strategies like altered peptide ligands and intra- or extra-thymic allopeptide inoculation are discussed. In addition, we speculate on the potential of regulatory T cells to modulate alloimmune responses.

New concepts in organ transplantation

Long-term graft survival is the desired outcome of organ transplantation. The surrogate metric elimination of acute rejection episodes is not only inadequate but also deceptive, since this freedom does not promise long-term graft survival. Current clinical immunosuppressive agents have reduced acute rejection, but not prolonged graft survival. New paradigms in organ transplantation focus on adhesion-migration events using a selectin antagonist, an antisense oligonucleotide, and FTY 720; on peptide or allochimeric antigens on cytokine disruption, and on inhibition of costimulatory signals. Due to the array of adverse reactions to the available immunosuppressive drugs, these new approaches aim not only to augment long-term graft survival, but also minimize the associated toxicities.

Selection of lowly immunogenic and highly tolerogenic donor and recipient allochimeric class I major histocompatibility complex proteins

BACKGROUND

Ten different highly polymorphic amino acids (AAs) are located in the alpha1 (alpha1h) and alpha2 (alpha2h) helical regions of the class I major histocompatibility complex RT1. An rat alloantigen. We examined the potential of alpha1h-RT1. An versus alpha2h-RT1. An polymorphic AAs to induce accelerated rejection or tolerance of heart allografts.

METHODS

The allochimeric alpha1h52-90n-RT1.Ac and alpha2h148-179n-RT1.Ac cDNAs were produced by the substitution of nucleotides encoding recipient RT1.Ac AAs for donor RT1. An AAs. Allochimeric and wild-type (WT)-RT1. An proteins were generated in an Escherichia coli expression system.

RESULTS

A single portal vein administration of 100 mug alpha1h52-90n-RT1.Ac protein in combination with a 7-day course of oral cyclosporine A (4 mg/kg) induced tolerance to Brown Norway (BN) (RT1n) heart allografts in PVG (RT1c) recipients more effectively than did WT-RT1. An protein; alpha2h148-179n-RT1.Ac protein was ineffective. However, subcutaneous injection of 100 mug WT-RT1. An (but neither alpha1h52-90n-RT1.Ac nor alpha2h148-179n-RT1.Ac) protein induced accelerated rejection of BN heart allografts. Untreated PVG recipients of BN heart allografts displayed activation of both interleukin (IL)-2- and interferon-gamma-producing T helper (Th) 1 cells and IL-4- and IL-10-producing Th2 cells on days 5, 7, and 14 postgrafting, as measured by an enzyme-linked immunospot assay. In contrast, in comparison with rejectors, tolerant recipients showed down-regulation of Th1 cells and up-regulation of Th2 cells on days 5, 7, 14, and 200 postgrafting. Histology of heart allografts showed that tolerant BN heart allografts had no evidence of acute or chronic rejection when examined on day 100 after transplantation.

CONCLUSIONS

The poorly immunogenic alpha1h52-90n-RT1.Ac allochimeric protein induces tolerance by selective activation of regulatory Th2 cells.

Individuality: the barrier to optimal immunosuppression

Immunosuppressive therapy aims to protect transplanted organs from host responses. Individuals have unique repertoires of responses to foreign antigens and toxic reactions to immunosuppressants; the former determining the type or intensity of rejection reactions and the latter influencing the severity of iatrogenic effects. Because existing agents target molecules that are widely distributed in tissues, new strategies must selectively block lymphoid cells only, disrupt alloresponses but not innate immune responses, interact synergistically with other agents, facilitate the homeostatic process that naturally leads to graft acceptance and ideally only interrupt donor-specific responses. Approaches presently under investigation aim to alter cell trafficking, or selectively deviate the maturation of antigen-presenting cells or inhibit lymphocyte-activation cascades - events that are crucial to rejection responses.

The emerging matrix of immunosuppressive agents

In recent years, significant milestones have been reached in the field of transplantation through the development of immunosuppressive drugs that inhibit lymphocyte activation, cytokine signal transduction, and cellular proliferation. However, the widespread tissue distribution of the molecular targets exploited to date-calcineurin, mammalian target of rapamycin (mTOR), and inosine monophosphate dehydrogenase-produces an array of collateral toxicities. Avoiding these side effects requires new strategies that selectively block destructive immune responses: a fifth generation of immunosuppressants. These agents must target molecules that are critical for and specific to the adaptive immune response.

Posttransplant administration of allochimeric major histocompatibility complex class-I-molecules induces true transplantation tolerance

BACKGROUND

Allochimeric class-I major histocompatibility complex (MHC) molecules that contain donor-type immunogenic epitopes displayed on recipient-type sequences were shown to induce transplantation tolerance when administered at the time of transplantation. Here, we investigated the ability of posttransplant allochimeric administration to induce tolerance and concomitantly inhibit chronic rejection.

METHODS

Allochimeric (alpha1h(1/u))-RT1.Aa class-I MHC antigenic extracts were administered by way of the portal vein into ACI recipients of Wistar-Firth (WF) hearts at days +3, +7, and +10 posttransplantation in conjunction with subtherapeutic oral cyclosporine.

RESULTS

Delayed posttransplant allochimeric administration induced donor-specific transplantation tolerance to rat cardiac allografts. In contrast, delayed delivery of unaltered donor- or recipient-type MHC extracts failed to prolong allograft survival. In addition, histopathologic examination or estimation of transplant vascular sclerosis by neointimal index assessment, following delayed allochimeric therapy, revealed intact global architecture and minimal intimal thickening, respectively.

CONCLUSION

Allochimeric MHC class-I therapy is a unique and novel clinically applicable approach for induction of "true" transplantation tolerance where chronic rejection is concomitantly abrogated.

Allochimeric class I MHC protein-induced tolerance by partial TCR engagement requires activation of both CTL4- and common gamma-chain-dependent cytokine signals

BACKGROUND

The various toxicities associated with the general immune suppression resulting from current clinical immunosuppressive therapies continue to plague transplant recipients as well as jeopardize allograft survival.

METHODS

The present study utilized allochimeric class I MHC antigens (alpha1hu70-77-RT1.Aa) bearing only four donor RT1.Au polymorphic amino acids (a.a.; His70, Val73, Asn74, and Asn77) superimposed on the recipient RT1.Aa background to induce transplantation tolerance in the rat cardiac transplant model.

RESULTS

Oral delivery of alpha1hu70-77-RT1.Aa protein alone (days 0-6) induced tolerance, as evidenced by inhibition of both acute and chronic rejection processes. Delivery of alpha1hu70-77-RT1.Aa with therapeutic doses of cyclosporine (CsA) also prevented chronic rejection, otherwise readily developed after treatment with CsA alone. A polymerase chain reaction (PCR)-based analysis showed that tolerant recipients had reduced numbers of interleukin (IL)-2/interferon (IFN)-gamma-producing T helper (Th)1 cells and elevated numbers of IL-4/IL-10-producing Th2 cells. Adoptive transfer experiments revealed that potent regulatory T cells mediated tolerance. The same T cells displayed diminished T cell receptor (TCR)-driven signaling via extracellular regulated kinase, AP-1, and NF-kappaB, as well as the common gamma-chain (gammac) cytokine-receptor-induced signaling by Janus kinase 3 (Jak3)/stimulators and activators of transcription Stat/5 pathways. Tolerance induction was prevented in vivo by inhibition of signal 2 by CTL4Ig or of signal 3 by either rapamycin, which disrupts the mammalian target of rapamycin, or AG490, which inhibits Jak3. Finally, partial or complete tyrosine phosphorylation of Zap70 was observed in alloantigen-specific T cell clones in response to tolerogenic versus immunogenic peptides, respectively.

CONCLUSIONS

Tolerance induction by allochimeric proteins is achieved by partial TCR activation in the presence of signals 2 and 3, resulting in a skewed Th2 phenotype.

Allochimeric class I MHC molecules prevent chronic rejection and attenuate alloantibody responses

BACKGROUND

We have shown that treatment with molecularly engineered, allochimeric [alpha1 hl/u]-RT1.Aa class I MHC antigens bearing donor-type Wistar-Furth (WF, RT1.Au) amino acid substitutions for host-type ACI (RTI.Aa) sequences in the alpha1-helical region induces donor-specific tolerance to cardiac allografts in rat recipients. This study examined the effect of allochimeric molecules on the development of chronic rejection.

METHODS

Allochimeric [alpha1 hl/u]-RT1.Aa class I MHC antigenic extracts (1 mg) were administered via the portal vein into ACI recipients of WF hearts on the day of transplantation in conjunction with subtherapeutic oral cyclosporine (CsA, 10 mg/kg/day, days 0-2). Control groups included recipients of syngeneic grafts and ACI recipients of WF heart allografts treated with high-dose CsA (10 mg/kg/day, days 0-6).

RESULTS

WF hearts in ACI rats receiving 7 days of CsA exhibited myocardial fibrosis, perivascular inflammation, and intimal hyperplasia at day 80. At day 120, these grafts displayed severe chronic rejection with global architectural disorganization, ventricular fibrosis, intimal hyperplasia, and progressive luminal narrowing. In contrast, WF hearts in rats treated with [alpha1 hl/u]-RT1.Aa molecules revealed only mild perivascular fibrosis, minimal intimal thickening, and preserved myocardial architecture. Alloantibody analysis demonstrated no IgM alloantibodies in all groups. An attenuated, but detectable, anti-WF IgG response was present in recipients receiving allochimeric molecules, with IgG1 and IgG2a subclasses predominating. Immunohistochemical analysis of allografts demonstrated minimal T cell infiltration and IgG binding to vascular endothelium.

CONCLUSION

Treatment with allochimeric molecules prevents the development of chronic rejection. Such effect may be in part caused by deviation of host alloantibody responses.

HLA-derived peptides as novel immunomodulatory therapeutics

A growing body of experimental evidence demonstrates that synthetic peptides corresponding to linear sequences of MHC (HLA in humans) proteins have immunomodulatory effects in vitro and in vivo in animal models and in humans. Although the original concept was that these peptides inhibited antigen recognition at the MHC-T cell receptor interface via physical blockade, it is now clear that the mechanisms responsible for the myriad of functional effects are more complex. Recent findings show that some peptides affect signal transduction and cell cycle progression. Fragments of MHC molecules can dampen or downregulate immune responses via a variety of mechanisms. Some soluble MHC molecules or synthetic peptides are capable of inducing and maintaining immunologic tolerance in animals. This information suggests that synthetic peptides themselves or drugs mimicking their effects may represent a new class of immunotherapeutics.

Induction of tolerance toward rat cardiac allografts by treatment with allochimeric class I MHC antigen and FTY720

BACKGROUND

The combination of FTY 720, a novel immunosuppressant, and allochimeric class I MHC proteins bearing donor-type amino acid (aa) epitope substitutions for host-type sequences induces tolerance of Wistar Furth (WF; RT1.Au) heart allografts in ACI (RT1.Aa) recipients.

METHODS

Allochimeric alpha(1h)l58-80-RT1.Aa proteins were produced by substituting the allogeneic nucleotide sequence encoding 10 aa residues unique to the alpha1 helical (alpha1h) region of RT1.Al Lewis (Asp58, Arg62, Glu63, Gln65, Lys66, Gly69, Asn70, Asn73, Ser77, and Asn80) for native RT1.Aa residues. The RT1.Au and the RT1.Al haplotypes share four of these aa (Arg62, Glu63, Gln65, and Gly69). A baculovirus/Spodoptera frugiperda insect cell system was used to express the alpha(1h)l58-80-RT1.Aa proteins.

RESULTS

The addition of a 3-day oral gavage of 0.05 mg/kg/day FTY720 to a single portal vein injection of 10 microg alpha(1h)l58-80-RT1.Aa protein induced permanent acceptance of WF heart allografts in 16 of 26 ACI recipients (>100 days); the alpha(1h)l58-80-RT1.Aa protein alone only modestly prolonged WF heart survival (13.8+/-0.8 days). The same tolerogenic protocol did not prolong the survival of third-party Brown Norway (RT1.An) heart allografts (14.3+/-2.5 days) compared with FTY720 alone (14.0+/-2.3 days; NS). Tolerant ACI recipients bearing primary WF heart allografts for more than 100 days accepted second WF hearts, but promptly rejected third-party Brown Norway heart grafts (9.3+/-1.5 days). The tolerant state was transferred to irradiated ACI rats (400 rad) with either purified T cells (4-10 x 10[7]) or serum (1-2 ml) from tolerant hosts, and was not broken by daily intraperitoneal injections of interleukin-2 (1000 U/day; 7 days).

CONCLUSIONS

The combination of allochimeric protein with FTY720 induces transplantation tolerance, a state that may be associated with the appearance of donor-specific regulatory factors.

Peptide-mediated immunosuppression

New insights into the mechanisms of allorecognition and the interactions of the TCR with the MHC molecule-peptide complex on antigen presenting cells have focused attention on developing novel biological strategies to modify the alloimmune response. Peptides derived from various regions of MHC class I and II molecules and structure-based peptides have demonstrated immunomodulatory effects both in vitro and in vivo. Their binding sites and mechanisms of action are under active investigation. Trials in human transplant recipients, with an MHC class I peptide have already begun.