Antibody Responses to Minor Histocompatibility Antigens After Solid Organ Transplantation

Antibody-mediated rejection (AMR) is a major barrier to long-term graft survival following solid organ transplantation (SOT). Major histocompatibility antigens mismatched between donor and recipient are well-recognized targets of humoral alloimmunity in SOT and thought to drive most cases of AMR. In contrast, the implication of minor histocompatibility antigens (mHAs) in AMR has not been fully investigated, and their clinical relevance remains controversial. Recent technological advances, allowing for genome-wide comparisons between donors and recipients, have uncovered novel, polymorphic mHA targets with potential influence on the graft outcome following SOT. Here, we review these latest studies relating to mHAs and discuss their clinical significance.

Impact of SIRPα polymorphism on transplant outcomes in HLA-identical living donor kidney transplantation

Signal-regulatory protein α (SIRPα), a polymorphic inhibitory membrane-bound receptor, and its ligand CD47 have recently been implicated in the modulation of innate immune allorecognition in murine models. Here, we investigate the potential impact of SIRPα donor-recipient mismatches on graft outcomes in human kidney transplantation. To eliminate the specific role of HLA-matching in alloresponse, we genotyped the two most common variants of SIRPα in a cohort of 55 HLA-identical, biologically-related, donor-recipient pairs. 69% of pairs were SIRPα identical. No significant differences were found between donor-recipient SIRPα-mismatch status and T cell-mediated rejection/borderline changes (25.8% vs. 25%) or slow graft function (15.8% vs. 17.6%). A trend towards more graft failure (GF) (23.5% vs. 5.3%, P = .06), interstitial inflammation (50% vs. 23%, P = .06) and significant changes in peritubular capillaritis (ptc) (25% vs. 0%, P = .02) were observed in the SIRPα-mismatched group. Unexpectedly, graft-versus-host (GVH) SIRPα-mismatched pairs exhibited higher rates of GF and tubulitis (38% vs. 5%, P = .031 and .61 ± .88 vs. 0, P = .019; respectively). Whether the higher prevalence of ptc in SIRPα-mismatched recipients and the higher rates of GF in GVH SIRPα-mismatched pairs represent a potential role for SIRPα in linking innate immunity and alloimmune rejection requires further investigation in larger cohorts.

Non-HLA Abs in Solid Organ Transplantation

The role of anti-HLA antibodies in solid organ rejection is well established and these antibodies are routinely monitored both in patients in the waiting list and in the post-transplant setting. More recently, the presence of other antibodies directed towards non-HLA antigens, or the so-called minor histocompatibility antigens, has drawn the attention of the transplant community; however, their possible involvement in the graft outcome remains uncertain. These antibodies have been described to possibly have a role in rejection and allograft failure. This review focuses on the most studied non-HLA antibodies and their association with different clinical outcomes considered in solid organ transplantation with the aim of clarifying their clinical implication and potential relevance for routine testing.

Novel insights into non-HLA alloimmunity in kidney transplantation

Recognition of non‐self structures on donor cells represents the main immunological barrier in solid organ transplantation. The human leukocyte antigens (HLA) are considered the most important non‐self (allo)antigens in transplantation. Long‐term graft attrition is mainly caused by the formation of alloreactive antibodies that are directed against non‐self structures (i.e., epitopes) on cell surface proteins. Recently published data provided evidence for a similar importance of non‐HLA mismatches between donors and recipients in acute rejection as well as long‐term kidney allograft survival. These data suggest a broader concept of immunological non‐self that goes beyond HLA incompatibility and expands the current concept of polymorphic non‐self epitopes on cell surface molecules from HLA to non‐HLA targets. Amino acid substitutions caused by single nucleotide variants in protein‐coding genes or complete loss of gene expression represent the basis for polymorphic residues in both HLA and non‐HLA molecules. To better understand these novel insights in non‐HLA alloimmunity, we will first review basic principles of the alloimmune response with a focus on the HLA epitope concept in donor‐specific antibody formation before discussing key publications on non‐HLA antibodies.

Immunosuppression and Graft Rejection in Living-related HLA-identical Renal Transplantation: The RADOVFULL Study

BACKGROUND

We aimed to describe the immunosuppressive regimens and graft rejection rates in living-related HLA-identical (LR HLAid) renal transplantation.

METHODS

We performed a retrospective multicenter analysis of the French national database for LR HLAid renal transplantations performed between 2002 and 2012. Univariate and multivariate analysis were performed to determine risk factors for graft rejection in LR HLAid recipients.

RESULTS

A total of 27 218 renal transplantations were performed, of whom 163 had a LR HLAid donor. About immunosuppressive treatment, <60% of the cohort had induction therapy with polyclonal or monoclonal antibodies, 28% did not receive calcineurin inhibitors, and 36% did not receive steroids in maintenance. Biopsy-proven acute rejection was diagnosed in 21 patients (12.9%). Rejection occurred on an average of 24 months after transplantation, in 28.5% of the cases after minimization of immunosuppression. Factors associated with rejection were age of recipient (OR, 0.91 [0.84-0.96]; P = 0.003), the body mass index of donors (odds ratio [OR], 1.22 [1.04-1.46]; P = 0.01), and minimization of immunosuppression (OR, 26.2 [5.48-166.6]; P < 0.001). Overall and graft survival rates were not statistically different according to rejection at 1, 5, and 10 years posttransplantation.

CONCLUSIONS

Minimization of immunosuppression should be done with caution in LR HLAid renal transplantations.

Contribution of non-HLA incompatibility between donor and recipient to kidney allograft survival: genome-wide analysis in a prospective cohort

BACKGROUND

The introduction of HLA matching of donors and recipients was a breakthrough in kidney transplantation. However, half of all transplanted kidneys still fail within 15 years after transplantation. Epidemiological data suggest a fundamental role of non-HLA alloimmunity.

METHODS

We genotyped 477 pairs of deceased donors and first kidney transplant recipients with stable graft function at three months that were transplanted between Dec 1, 2005, and April 30, 2015. Genome-wide genetic mismatches in non-synonymous single nucleotide polymorphisms (nsSNPs) were calculated to identify incompatibilities in transmembrane and secreted proteins. We estimated the association between nsSNP mismatch and graft loss in a Cox proportional hazard model, adjusting for HLA mismatch and clinical covariates. Customised peptide arrays were generated to screen for antibodies against genotype-derived mismatched epitopes in 25 patients with biopsy-confirmed chronic antibody-mediated rejection.

FINDINGS

59 268 nsSNPs affecting a transmembrane or secreted protein were analysed. The median number of nsSNP mismatches in immune-accessible transmembrane and secreted proteins between donors and recipients was 1892 (IQR 1850-1936). The degree of nsSNP mismatch was independently associated with graft loss in a multivariable model adjusted for HLA eplet mismatch (HLA-A, HLA-B, HLA-C, HLA-DP, HLA-DQ, and HLA-DR). Each increase by a unit of one IQR had an HR of 1·68 (95% CI 1·17-2·41, p=0·005). 5-year death censored graft survival was 98% in the quartile with the lowest mismatch, 91% in the second quartile, 89% in the third quartile, and 82% in the highest quartile (p=0·003, log-rank test). Customised peptide arrays verified a donor-specific alloimmune response to genetically predicted mismatched epitopes.

INTERPRETATION

Genetic mismatch of non-HLA haplotypes coding for transmembrane or secreted proteins is associated with an increased risk of functional graft loss independently of HLA incompatibility. As in HLA alloimmunity, donor-specific alloantibodies can be identified against genotype derived non-HLA epitopes.

FUNDING

Austrian Science Fund, WWTF (Vienna Science and Technology Fund), and Ministry of Health of the Czech Republic.

Viral infection causes a shift in the self peptide repertoire presented by human MHC class I molecules

PURPOSE

MHC class I presentation of peptides allows T cells to survey the cytoplasmic protein milieu of host cells. During infection, presentation of self peptides is, in part, replaced by presentation of microbial peptides. However, little is known about the self peptides presented during infection, despite the fact that microbial infections alter host cell gene expression patterns and protein metabolism.

EXPERIMENTAL DESIGN

The self peptide repertoire presented by HLA-A*01;01, HLA-A*02;01, HLA-B*07;02, HLA-B*35;01, and HLA-B*45;01 (where HLA is human leukocyte antigen) was determined by tandem MS before and after vaccinia virus infection.

RESULTS

We observed a profound alteration in the self peptide repertoire with hundreds of self peptides uniquely presented after infection for which we have coined the term "self peptidome shift." The fraction of novel self peptides presented following infection varied for different HLA class I molecules. A large part (approximately 40%) of the self peptidome shift arose from peptides derived from type I interferon-inducible genes, consistent with cellular responses to viral infection. Interestingly, approximately 12% of self peptides presented after infection showed allelic variation when searched against approximately 300 human genomes.

CONCLUSION AND CLINICAL RELEVANCE

Self peptidome shift in a clinical transplant setting could result in alloreactivity by presenting new self peptides in the context of infection-induced inflammation.

Minor histocompatibility antigens: past, present, and future

Minor histocompatibility (H) antigens are key molecules driving allo-immune responses in both graft-versus-host-disease (GvHD) and in graft-versus-leukemia (GvL) reactivity in human leukocyte antigen (HLA)-matched hematopoietic stem-cell transplantation (HSCT). Dissection of the dual function of minor H antigens became evident through their different modes of tissue and cell expression, i.e. hematopoietic system-restricted or broad. Broadly expressed minor H antigens can cause both GvHD and GvL effects, while hematopoietic system-restricted minor H antigens are more prone to induce GvL responses. This phenomenon renders the latter group of minor H antigens as curative tools for HSCT-based immunotherapy of hematological malignancies and disorders, in which minor H antigen-specific responses are enhanced in order to eradicate the malignant cells. This article describes the immunogenetics of minor H antigens and methods that have been developed to identify them. Moreover, it summarizes the clinical relevance of minor H antigens in transplantation, with special regards to allogeneic HSCT and solid-organ transplantation.