Peptide analogues as a strategy to induce tolerance in T cells with indirect allospecificity

Intragraft selection of the T cell receptor repertoire by class I MHC sequences in tolerant recipients

BACKGROUND

Allograft tolerance of ACI (RT1(a)) recipients to WF (RT1(u)) hearts can be induced by allochimeric class I MHC molecules containing donor-type (RT1A(u)) immunogenic epitopes displayed on recipient-type (RT1A(a)) sequences. Here, we sought the mechanisms by which allochimeric sequences may affect responding T cells through T cell receptor (TCA) repertoire restriction.

METHODOLOGY/PRINCIPAL FINDINGS

The soluble [alpha(1h) (u)]-RT1.A(a) allochimeric molecule was delivered into ACI recipients of WF hearts in the presence of sub-therapeutic dose of cyclosporine (CsA). The TCR Vbeta spectrotyping of the splenocytes and cardiac allografts showed that the Vbeta gene families were differentially expressed within the TCR repertoire in allochimeric- or high-dose CsA-treated tolerant recipients at day +5 and +7 of post-transplantation. However, at day 30 of post-transplantation the allochimeric molecule-treated rats showed the restriction of TCR repertoire with altered dominant size peaks representing preferential clonal expansion of Vbeta7, Vbeta11, Vbeta13, Vbeta 14, and Vbeta15 genes. Moreover, we found a positive correlation between the alteration of Vbeta profile, restriction of TCR repertoire, and the establishment of allograft tolerance.

CONCLUSIONS

Our findings indicate that presentation of allochimeric MHC class I sequences that partially mimic donor and recipient epitopes may induce unique tolerant state by selecting alloresponsive Vbeta genes.

Monitoring of indirect allorecognition: wishful thinking or solid data?

Monitoring of T cells involved in the alloimmune response after transplantation requires the availability of reliable in vitro assays for the detection of T cells with both direct and indirect allospecificity. While generally accepted assays exist to measure helper and cytotoxic T cells involved in direct allorecognition, consensus about an assay for monitoring indirect T-cell allorecognition in clinical transplantation is lacking. Many studies claim a relationship between the reactivity of T cells with indirect allospecificity and graft rejection, but different protocols are used and essential controls are often lacking. In this review, the disadvantages and pitfalls of the current approaches are discussed, in some cases supported by the results of our own in vitro experiments. We conclude that an international workshop is necessary to establish and validate a uniform, robust and reliable assay for the monitoring of transplant recipients and to study the actual role of indirect allorecognition in acute and chronic rejection.

Indirect recognition of T-cell epitopes derived from the alpha 3 and transmembrane domain of HLA-A2

Indirect allorecognition has been implicated in the mechanism of chronic rejection and alloantibody formation but precise definition of the epitopes involved has been limited. We have undertaken a detailed assessment of the antigenic properties of peptides derived from HLA-A2. Candidate epitopes were identified in vitro by assessment of MHC class II binding. The immune response to these epitopes was determined in patients awaiting a renal transplant by the assessment of PBMC activation using gamma-interferon ELISPOT. Twenty-two of fifty-five patients responded to peptides from HLA-A2 and this was associated with but not confined to those who had made antibody to HLA-A2 (14/18). Nineteen of twenty-two patients responded to peptides derived from the hypervariable alpha1 and alpha2 domains and 18/22 responded to peptides from the alpha 3 and transmembrane domain, the sequences of which show little polymorphism. In six patients, the sequence of these peptides was identical to self, that is, the response was autoimmune. The finding of indirect epitopes derived from regions of MHC class I that exhibit little polymorphism provides a novel perspective on the immune response to alloantigen and has potential implications for the development of specific therapies.

Self-peptide/MHC and TCR antagonism: physiological role and therapeutic potential

TCR antagonists are peptides that bind MHC molecules and can specifically inhibit T cell activation induced by antigens. Studying TCR antagonism has taken an important place in immunology for both theoretical and practical reasons. Deciphering the mechanism(s) of action of TCR antagonists can yield important information about interactions of the TCR with ligands, T cell development, and TCR signaling. Moreover, microorganisms may employ TCR antagonism to elude the attention of the immune system. Finally, specificity of inhibition makes TCR antagonists an ideal tool to seek antigen-specific immunomodulation. Present state of knowledge on these topics is reviewed.

Hypervariable region 1 variant acting as TCR antagonist affects hepatitis C virus-specific CD4+ T cell repertoire by favoring CD95-mediated apoptosis

We have described previously that hypervariable region 1 (HVR1) variants of hepatitis C virus (HCV) frequently act as T cell receptor (TCR) antagonists for HVR1-specific helper T cells. These naturally occurring HVR1-antagonistic sequences interfered with the effects of HVR1-agonistic sequences such as TCR down-regulation and early activatory signals. By taking advantage of these findings, in this paper, we have analyzed the fate of these HVR1-specific antagonized CD4+ T cells. We present the evidence that TCR antagonism renders agonist-activated T cells susceptible to bystander CD95-mediated killing by suppressing the expression of cellular Fas-associated death domain-like interleukin-1beta-converting enzyme-like inhibitor proteins. To verify whether the TCR repertoire of a HVR1-specific T cell population could be modified consequently, we used a HVR1-agonistic sequence to induce in vitro CD4+ T cells and another HVR1 sequence with antagonistic property to mediate suppressive phenomena. HVR1-specific T cells were cultured with the agonist alone or with the agonist plus the antagonist. HVR1 specificity and T cell repertoires were followed over time by analyzing TCR beta-variable gene segment by "spectratyping". The results showed that the specificity for the agonist was rapidly spoiled after culture in the presence of the antagonist, and the TCR repertoire was strongly modified as a result of CD95-mediated apoptosis of agonist-specific clonal expansions. These data support the hypothesis that in HCV infection, the generation of TCR antagonists may reshape the T cell repertoire, representing an efficacious immune evasion strategy of a highly mutant pathogen.

Prolongation of cardiac allograft survival by systemic administration of immature recipient dendritic cells deficient in NF-kappaB activity

OBJECTIVE

To develop a more applicable approach that uses recipient-derived dendritic cells (DC) for organ transplantation.

SUMMARY BACKGROUND DATA

Systemic administration of immature donor DC that are deficient in costimulatory molecules delays the onset of allograft rejection. However, this approach requires in vitro DC propagation and would not be applicable to deceased donor organ transplantation.

METHODS

DC were propagated from C3H (H2) mouse bone marrow with GM-CSF; their maturation was arrested by treatment with oligodeoxyribonucleotides (ODN) specifically against nuclear factor (NF)-kappaB. The DC were pulsed with B10 (H2) splenocyte lysate. DC phenotype was examined by flow cytometry. Their allostimulatory activity was assessed in vitro by MLR and CTL assays and in vivo by the influence on B10 cardiac allograft survival. Cytokine profiles were analyzed by ELISA and RNase protection assay. NF-kappaB activity in DC nuclear protein was detected by gel shifting assay.

RESULTS

Compared with mature DC, NF-kappaB ODN-treated immature DC pulsed with B10 (H2) spleen cell lysate elicited markedly lower proliferative responses and correlated with reduced IFN-gamma and increased IL-10 production. In contrast to administration of mature C3H DC pulsed with B10 antigen that accelerated rejection of B10 cardiac allografts, a single injection of NF-kappaB ODN DC pulsed with donor antigens significantly prolonged allograft survival in an antigen-specific manner. This was associated with induction of T-cell hyporesponsiveness and enhanced T-cell apoptosis.

CONCLUSIONS

An approach to use recipient DC as a "vaccine" strategy provides a more feasible approach for deceased-donor organ transplantation.

HLA-DRB1*1501 risk association in multiple sclerosis may not be related to presentation of myelin epitopes

Susceptibility to multiple sclerosis (MS) is associated genetically with human leucocyte antigen (HLA) class II alleles, including DRB1*1501, DRB5*0101, and DQB1*0602, and it is possible that these alleles contribute to MS through an enhanced ability to present encephalitogenic myelin peptides to pathogenic T cells. HLA-DRB1*1502, which contains glycine instead of valine at position 86 of the P1 peptide-binding pocket, is apparently not genetically associated with MS. To identify possible differences between these alleles in their antigen-presenting function, we determined if T-cell responses to known DRB1*1501-restricted myelin peptides might be diminished or absent in transgenic (Tg) DRB1*1502-expressing mice. We found that Tg DRB1*1502 mice had moderate to strong T-cell responses to several myelin peptides with favorable DRB1*1501 binding motifs, notably myelin oligodendrocyte glycoprotein (MOG)-35-55 (which was also encephalitogenic), proteolipid protein (PLP)-95-116, and MOG-194-208, as well as other PLP and MOG peptides. These peptides, with the exception of MOG-194-208, were also immunogenic in healthy human donors expressing either DRB1*1502 or DRB1*1501. In contrast, the DRB1*1502 mice had weak or absent responses to peptides with unfavorable DRB1*1501 binding motifs. Overall, none of the DRB1*1501-restricted myelin peptides tested selectively lacked immunogenicity in association with DRB1*1502. These results indicate that the difference in risk association with MS of DRB1*1501 versus DRB1*1502 is not due to a lack of antigen presentation by DRB1*1502, at least for this set of myelin peptides, and suggest that other mechanisms involving DRB1*1501 may account for increased susceptibility to MS.

Allogeneic core amino acids of an immunodominant allopeptide are important for MHC binding and TCR recognition

The indirect alloimmune response seems to be restricted to a few dominant major histocompatibility complex (MHC)-derived peptides responsible for T-cell activation in allograft rejection. The molecular mechanisms of indirect T-cell activation have been studied using peptide analogues derived from the dominant allopeptide in vitro, whereas the in vivo effects of peptide analogues have not been well characterized yet. In the present study, we generated allochimeric peptide analogues by replacing the three allogeneic amino acids 5L, 9L, and 10T in the sequence of the dominant MHC class I allopeptide P1. These allochimeric peptide analogues were used to define the allogeneic amino acids critical for the MHC binding and TCR recognition. We found that position 5 (5L) of the dominant allopeptide acts as an MHC-binding residue, while the other two allogeneic positions, 9 and 10, are important for the T-cell receptor (TCR) recognition. A peptide containing the MHC-binding residue 5L, as the only different amino acid between donor (RT1.A(u)) and recipient (RT1.A(l)) sequences, did not induce proliferation of lymph node cells primed with the dominant peptide and prevented dominant peptide-induced acceleration of allograft rejection. Identification of MHC and TCR contact residues should facilitate the development of antigen-specific therapies to inhibit or regulate the indirect alloimmune response.

Induction of allopeptide-specific human CD4+CD25+ regulatory T cells ex vivo

Although CD4+CD25+ regulatory T cells are pivotal in the prevention of autoimmunity and appear to mediate transplantation tolerance, little is known concerning their antigen specificity. Here we describe the induction of a human CD4+CD25+ regulatory T-cell line specific for a defined peptide alloantigen (human leukocyte antigen A2 [HLA-A2] 138-170) by priming purified CD4+CD25+ cells ex vivo. The regulatory cells were anergic and retained their ability to suppress antigen-driven responses of CD4+CD25- cells. They inhibited not only interleukin 2 (IL-2) secretion by CD4+CD25- T cells specific for the same peptide but also direct alloresponse of naive CD4+CD25- T cells stimulated by semiallogeneic dendritic cells (DCs) in the presence of the peptide ("linked suppression"). They also suppressed the response of CD4+ T cells specific for viral and bacterial antigens. The suppressive T-cell line showed sustained high CD25 expression. These findings suggest that peripheral CD4+CD25+ regulatory cells are a precommitted cell lineage from which cells with specificity for non-self-peptides can be selected. This may pave the way for inducing and expanding peptide antigen-specific regulatory T cells ex vivo for cell therapy in transplantation, allergy, and autoimmune disease.

Hierarchical immunogenicity of donor MHC class I peptides in allotransplantation

The recognition of major histocompatibility complex (MHC) allopeptides by recipient MHC class II-restricted CD4(+) T cells via indirect pathway is a prerequisite for the generation of an immune response to the allograft. We tested 13-mer to 24-mer peptides from the MHC class I molecule for their possible immunogenicity in a fully MHC-mismatched rat strain combination. Our results confirm the hierarchical distribution of the immunogenicity of donor MHC class I peptides in the T cell alloactivation via indirect pathway. In addition, we show that allopeptide-induced immune response is critical for acute rejection of heart allografts. Among the seven allopeptides tested, peptide P1 was identified as immunodominant; it induced the greatest T cell proliferation and cytokine production in vitro as well as a significant reduction in allograft survival time. The TCR repertoire of T cells involved in the in vitro and in vivo responses induced by the dominant allopeptide P1 was found to be limited to the Vbeta10 and Vbeta 19 gene families. The identification of dominant allopeptides should greatly facilitate characterization of the specific T cell population responsible for allograft rejection and may be used to modulate the alloimmune response through antigen-specific therapy.

TCR usage in naive and committed alloreactive cells: implications for the understanding of TCR biases in transplantation

The direct pathway of allorecognition is involved in acute allograft rejection and is characterised by TCR-mediated recognition of the MHC framework; this is thought to occur in a peptide-dependent but not peptide-specific manner. In contrast, the indirect pathway is restricted to the recipient's own MHC molecules and prevails in chronic rejection. In this pathway, the peptide has a major influence on the TCR recognition and selects alloreactive T cells with altered TCR Vbeta usage. However, qualitative analysis of Vbeta usage alone might limit our understanding of alloreactivity. The advantages of a combined quantitative assessment of Vbeta mRNA usage are discussed.

T-cell anergy and peripheral T-cell tolerance

The discovery that T-cell recognition of antigen can have distinct outcomes has advanced understanding of peripheral T-cell tolerance, and opened up new possibilities in immunotherapy. Anergy is one such outcome, and results from partial T-cell activation. This can arise either due to subtle alteration of the antigen, leading to a lower-affinity cognate interaction, or due to a lack of adequate co-stimulation. The signalling defects in anergic T cells are partially defined, and suggest that T-cell receptor (TCR) proximal, as well as downstream defects negatively regulate the anergic T cell's ability to be activated. Most importantly, the use of TCR-transgenic mice has provided compelling evidence that anergy is an in vivo phenomenon, and not merely an in vitro artefact. These findings raise the question as to whether anergic T cells have any biological function. Studies in rodents and in man suggest that anergic T cells acquire regulatory properties; the regulatory effects of anergic T cells require cell to cell contact, and appear to be mediated by inhibition of antigen-presenting cell immunogenicity. Close similarities exist between anergic T cells, and the recently defined CD4+ CD25+ population of spontaneously arising regulatory cells that serve to inhibit autoimmunity in mice. Taken together, these findings suggest that a spectrum of regulatory T cells exists. At one end of the spectrum are cells, such as anergic and CD4+ CD25+ T cells, which regulate via cell-to-cell contact. At the other end of the spectrum are cells which secrete antiinflammatory cytokines such as interleukin 10 and transforming growth factor-beta. The challenge is to devise strategies that reliably induce T-cell anergy in vivo, as a means of inhibiting immunity to allo- and autoantigens.