The role of human minor histocompatibility antigens in graft failure: a mini-review

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

A non-linear mathematical model using optical sensor to predict heart decellularization efficacy

One of the main problems of the decellularization technique is the subjectivity of the final evaluation of its efficacy in individual organs. This problem can result in restricted cell repopulation reproducibility and worse responses to transplant tissues. Our proposal is to analyze the optical profiles produced by hearts during perfusion decellularization, as an additional method for evaluating the decellularization process of each individual organ. An apparatus comprised of a structured LED source and photo detector on an adjustable base was developed to capture the relationship between transmitted light during the perfusion of murine hearts, and residual DNA content. Voltage-time graphic records were used to identify a nonlinear mathematical model to discriminate between decellularizations with remaining DNA above (Incomplete Decellularization) and below (Complete Decellularization) the standardized limits. The results indicate that temporal optical evaluation of the process enables inefficient cell removal to be predicted in the initial stages, regardless of the apparent transparency of the organ. Our open system also creates new possibilities to add distinct photo detectors, such as for specific wavelengths, image acquisition, and physical-chemical evaluation of the scaffold, in order to collect different kinds of information, from dozens of studies. These data, when compiled and submitted to machine learning techniques, have the potential to initiate an exponential advance in tissue bioengineering research.

Risk factors for graft failure after penetrating keratoplasty

The objective of our study was to define principal risk factors for graft failure in patients who underwent penetrating keratoplasty (PK).Retrospective data obtained from a cohort of 895 penetrating keratoplasties performed between 2001 and 2006 were analysed. Recipient related factors, graft characteristics, and surgical technique were assessed in a univariate analysis and with a multivariate proportional hazard model to detect principal risk factors for definitive graft failure.Multivariate analysis showed clear significance for diagnosis and number of previous grafts and border line significance for the oldest donor age group. Patients with keratoconus had the best 10-year survival estimate (95%), followed by endothelial and stromal dystrophies (both 55%), infectious leukomas (49%), trauma (33%) and chemical burns (14%). Primary PK grafts had a survival rate of 81%, second grafts of 33% and third or more grafts of 16%. Overall 10-year survival estimate based on univariate analysis was found to be 65%.In conclusion, we found that primary diagnosis and previous graft failures in the recipient are the most important risk factors of graft failure after a PK.

Effect of Histocompatibility Y Antigen Matching on Graft Survival in Primary Penetrating Keratoplasty

PURPOSE

To investigate the influence of histocompatibility Y (H-Y) antigen matching on corneal graft survival in primary penetrating keratoplasty (PK).

METHODS

Medical records of patients who underwent primary PK at Seoul National University Bundang Hospital between June 2005 and October 2015 were retrospectively analyzed. The eyes were classified into 2 groups: H-Y-compatible (115 eyes) and H-Y-incompatible (23 eyes). The H-Y-compatible group included donor/recipient combinations of male/male (57 eyes), female/male (44 eyes), and female/female (14 eyes). The H-Y-incompatible group included the male/female (23 eyes) combination alone. A subgroup analysis of low- and high-risk patients according to preoperative diagnoses was also performed. Survival analysis was conducted using the Kaplan-Meier method; differences between groups were assessed with a log-rank test.

RESULTS

A total of 138 eyes from 136 patients (age: 58 ± 18 years) were enrolled. Rejection-free graft survival and graft survival were not significantly different between H-Y-compatible and H-Y-incompatible groups (χ = 0.4, P = 0.548; χ = 1.9; P = 0.17, respectively). Preoperative diagnoses of high-risk cases included those with corneal perforation or thinning (8.7%) and infectious keratitis (7.2%). Low-risk cases included corneal opacity (50.0%), bullous keratopathy (25.4%), keratoconus (5.8%), and corneal dystrophy (2.9%). In the high-risk group, rejection-free graft survival rate was significantly higher in the H-Y-compatible group (χ = 3.9, P = 0.049).

CONCLUSIONS

H-Y antigen matching does not influence graft rejection and failure in cases of primary PK. However, matching the H-Y antigen could help reduce graft rejection, especially in preoperatively high-risk patients.

Matching for Human Leukocyte Antigens (HLA) in corneal transplantation - to do or not to do

As many patients with severe corneal disease are not even considered as candidates for a human graft due to their high risk of rejection, it is essential to find ways to reduce the chance of rejection. One of the options is proper matching of the cornea donor and recipient for the Human Leukocyte Antigens (HLA), a subject of much debate. Currently, patients receiving their first corneal allograft are hardly ever matched for HLA and even patients undergoing a regraft usually do not receive an HLA-matched graft. While anterior and posterior lamellar grafts are not immune to rejection, they are usually performed in low risk, non-vascularized cases. These are the cases in which the immune privilege due to the avascular status and active immune inhibition is still intact. Once broken due to infection, sensitization or trauma, rejection will occur. There is enough data to show that when proper DNA-based typing techniques are being used, even low risk perforating corneal transplantations benefit from matching for HLA Class I, and high risk cases from HLA Class I and probably Class II matching. Combining HLA class I and class II matching, or using the HLAMatchmaker could further improve the effect of HLA matching. However, new techniques could be applied to reduce the chance of rejection. Options are the local or systemic use of biologics, or gene therapy, aiming at preventing or suppressing immune responses. The goal of all these approaches should be to prevent a first rejection, as secondary grafts are usually at higher risk of complications including rejections than first grafts.

Immune recognition and rejection of allogeneic skin grafts

The transplantation of allogeneic skin grafts is associated with a potent inflammatory immune response leading to the destruction of donor cells and the rejection of the graft. Shortly after transplantation, skin dendritic cells (DCs) migrate out of the graft through lymphatic vessels and infiltrate the recipient's draining lymph nodes where they present donor antigens via two mechanisms: the direct pathway, in which T cells recognize intact donor MHC antigens on donor DCs; and the indirect pathway, involving T-cell recognition of donor peptides bound to self-MHC molecules on recipient DCs. Some recent studies have suggested that T cells can become activated via recognition of donor MHC molecules transferred on recipient antigen-presenting cells (semidirect pathway). Activation of T cells via direct or indirect allorecognition is sufficient to trigger acute rejection of allogeneic skin grafts. In addition, allospecific antibodies contribute to the rejection process either by killing allogeneic targets in a complement-dependent fashion or by opsonizing donor cells and forming immune complexes. Finally, several studies demonstrate that NK cells, activated due to missing self-MHC class I molecules on allogeneic cells, are involved in allogeneic skin graft rejection via direct killing of donor cells and through the production of proinflammatory cytokines including IFN-γ and TNF-α.

The relevance of minor histocompatibility antigens in solid organ transplantation

PURPOSE OF REVIEW

Disparities in minor histocompatibility antigens between HLA-matched organ and hematopoietic stem cell donors and recipients create the risks of graft failure and graft-versus-host disease (GvHD) respectively. A decade ago, technical advances combined with genomic information resulted in the identification of the chemical nature of the first series of minor histocompatibility antigens, facilitating their molecular typing. A new era of research had begun in exploring the role of minor histocompatibility antigens in physiological and nonphysiological settings. Here we summarize, to the best of our knowledge, human studies on the relevance of minor histocompatibility antigens in solid organ transplantation with a main focus on renal allografting.

RECENT FINDINGS

The minor histocompatibility antigen HY is associated with acute rejection, and male grafts in female recipients have reduced graft survival; both cellular and humoral responses are observed. Studies on autosomal minor histocompatibility antigens on graft rejection are less conclusive; their role in transplant tolerance, however, offers perspective.

SUMMARY

Information on the clinical relevance of minor histocompatibility antigen allo-immune responses in solid organ allografting is still scarce. The possible implications of the minor histocompatibility allo-immune responses for future clinical practice in solid organ transplantation are discussed in relation to their possible detrimental or beneficial effects on the host.

Matching von Minor-Transplantationsantigenen in der perforierenden Keratoplastik

Specific recognition of foreign tissue is a common feature in higher vertebrates. This capability has commonly been ascribed to the human leukocyte antigen (HLA) complex. Recent developments, however, point to an outstanding role of minor H antigens, especially in the context of corneal transplantation. It is likely that the matching of selected minor H antigens will further improve rejection free, clear graft survival following penetrating keratoplasty in the not so distant future.

Matching of the minor histocompatibility antigen HLA-A1/H-Y may improve prognosis in corneal transplantation

BACKGROUND

Minor histocompatibility (H) antigens are peptides of allelic intracellular proteins that play an important role in human leukocyte antigen (HLA) matched transplantations. In an animal model of keratoplasty, minor H antigens have even been reported to exceed the immunogenicity of major H antigens (MHC). This investigation is to assess any benefit of matching the broadly expressed gender (H-Y) and HA-3 antigens in HLA-A1 donor positive human keratoplasty.

METHODS

A total of 229 HLA-A1 donor positive keratoplasties were analyzed. A Cox proportional hazards model and Kaplan-Meier analysis were applied to estimate the effect of H-Y or HA-3 mismatches on rejection-free graft survival.

RESULTS

Eighty-one cases were mismatched for H-Y (male donor to female recipient). A mean follow up of two years showed graft survival as high as 88% in the H-Y compatible group compared to only 77% in the H-Y mismatched group (P = 0.02). Eight out of 62 cases were mismatched for HA-3. No statistically significant influence of HA-3 matching on rejection-free graft survival was observed (85% vs. 73%, P=0.52).

CONCLUSION

HLA-A1/H-Y matching and matching for other broadly expressed minor H antigens may further improve prognosis in keratoplasty.

Factors affecting rejection of second corneal transplants in rats1

BACKGROUND

Second and subsequent corneal transplants in the same eye are more prone to rejection reactions and failure than first grafts. This may be a result of local changes or systemic sensitization to antigen shared by the first and second donors. Because HLA typing is not routine in corneal transplantation, a clear correlation between accelerated rejection and specific sensitization has not been established.

METHODS

PVG (RT1), Lewis (LEW; RT1), or AO (RT1) strain corneas were transplanted to PVG strain rats, followed by a LEW strain cornea in the ipsilateral or contralateral eye 6 weeks later. Graft survival was evaluated by slit lamp biomicroscopy. Proliferation of recipient lymph node cells was tested against allogeneic, syngeneic and third-party stimulator cells after the second transplantation.

RESULTS

A second allograft in the ipsilateral or contralateral eye was rejected in an accelerated fashion that was not donor MHC specific. Rejection was not significantly accelerated in the ipsilateral eye compared with the contralateral eye. There was a secondary lymphocyte proliferation response to third party (AO strain) in animals previously exposed only to the LEW strain.

CONCLUSIONS

Systemic sensitization to donor antigens, rather than local changes induced by first transplantation, contributed to accelerated rejection of a second graft. Accelerated rejection is not dependent on MHC compatibility between the grafts. It could be caused by shared "public" MHC determinants, by minor antigens shared by the first and second donors, or by cross-reactivity of T cells to epitopes on AO and LEW grafts. HLA mismatching of first and second donors may not prolong second graft survival.

HLA class I/II matching and chronic endothelial cell loss in penetrating normal risk keratoplasty

OBJECTIVE

Chronic endothelial cell loss of the graft is very common after penetrating keratoplasty. The aetiology of this is unknown. Clinically, non-identifiable immune reactions have been suspected. Recently, we were able to demonstrate that proper human leucocyte antigen (HLA) matching is a suitable means to reduce classical immune reactions in normal risk keratoplasty patients. In this study, we therefore investigated whether HLA-matched grafts also experience less chronic endothelial cell loss.

METHODS

A homogenous group of 223 normal risk keratoplasty patients was divided into six groups with different degrees of HLA matching (group 1 with unknown HLA data, group 2 with up to two mismatches, group 3 with three mismatches, group 4 with four mismatches, group 5 with five mismatches and group 6 with six mismatches on the HLA A, B, DR loci). All serological HLA A, B, C and all moleculargenetic HLA DRB, DRQB typings of donors and recipients were performed in a single laboratory accredited by the American Society for Histocompatibility and Immunogenetics. Only patients with at least three postoperative endothelial cell density values were included in the study. The slopes of the regression lines for each individual scatterplot of endothelial cell density values plotted against postoperative time (linear regression, lost cells/mm2/day), and after logarithmic transformation (exponential regression, annual relative loss of cells) were evaluated, respectively.

RESULTS

There were no statistically significant differences between the six groups.

CONCLUSION

Whereas proper HLA matching at present standards is already a suitable means to reduce identifiable immune reactions and to prolong graft survival even in normal risk keratoplasty patients, the same HLA matching procedures are not effective in reducing the extent of chronic endothelial cell loss. For several reasons this does not yet exclude, however, the possibility that the underlying cause of chronic endothelial cell loss is immunological.

MHC matching and mechanisms of alloactivation in corneal transplantation

BACKGROUND

In human corneal transplantation the value of matching, particularly for MHC class II, is unclear and controversial. The contribution of the direct pathway to T cell activation is also uncertain. We have determined the relative contribution of class I, II and non-MHC antigens to graft rejection and of the direct and indirect pathways to T cell activation in a rat model mimicking human incompatibilities.

METHODS

DA (RT1a) strain recipients received fully mismatched PVG (RT1c) strain grafts or grafts from one of three recombinant strains bearing DA MHC genes on a PVG background. Graft survival was assessed and the specificity of T cells generated in the draining lymph nodes was determined in mixed lymphocyte (MLR) proliferation assays. To assess the contribution of the direct pathway, fully mismatched graft were performed and allospecific proliferation was measured after depletion of recipient APC from the MLR reaction.

RESULTS

There was no significant difference in survival of grafts between the four grades of mismatch, which ranged from a full mismatch to non-MHC mismatches alone (median survival 12.5, 11, 13 and 12.5 days respectively). In conformity with clinical results, strong secondary responses were generated against targets matched for MHC with the recipient. Depletion of recipient APC from a fully allogeneic secondary MLR did not fully abrogate donor-specific proliferation.

CONCLUSIONS

Class II matching is of no benefit in this model. Strong indirect responses to non-MHC mismatches are sufficient to induce the rapid rejection, but the small numbers of class II+ cells in the donor appear sufficient to generate a direct response.

Corneoscleroplasty with maintenance of the angle in two cases of extensive corneoscleral disease

PURPOSE

We report a 2 year follow-up in two patients after corneoscleroplasty.

METHOD

Using lamellar corneoscleral dissection to maintain the drainage angle and its function, we performed a 14 mm allograft corneoscleroplasty in 2 eyes. For surgical treatment in both cases a 9.5 mm corneal button was excised from the recipient after peritomy and scleral lamellar preparation up to 14 mm. A 14 mm donor button was inserted and held in place with multiple Prolene sutures. One eye presented with a large perforating corneal ulcer after herpetic keratitis in a patient with recurrent rheumatoid uveitis associated with rubeosis iridis. The second eye had had a penetrating keratoplasty for keratoconus 30 years previously and presented with decompensating keratoglobus. Immune suppression was performed with systemic cyclosporin A and additional steroids when required.

RESULTS

Both patients had a clear graft at the last follow-up visit and visual acuity was improved to a best corrected visual acuity of 0.6. Intraocular pressure in the keratoglobus eye was maintained at 6 mmHg without treatment, whereas the second case required continuing treatment with systemic acetazolamide because of neovascular glaucoma. The anterior chamber angles remained open in both patients. Contact lenses were helpful in the prevention of epithelial irregularities and defects. Both patients had an episode of immunological graft reaction which was reversed by immunosuppressive treatment. Phacoemulsification with intraocular lens implantation, which was performed 2 years after transplantation in the keratoglobus eye, did not affect the graft clarity or cause rejection episodes.

CONCLUSION

Our results using corneoscleroplasty have been encouraging in severe destructive corneal disease.

Histocompatibility Y antigen compatibility and allograft rejection in corneal transplantation

PURPOSE

To assess the effects of histocompatibility Y (H-Y) antigen matching on the rate of corneal allograft rejection after penetrating keratoplasty (PKP).

METHODS

We retrospectively investigated the graft survival rate and rejection-free graft survival rate after PKP in 396 eyes. The compatible combinations of H-Y antigen included male donors and male recipients (n = 135), female donors and male recipients (n = 107), and female donors and female recipients (n = 60). Incompatible combination was from male donors and female recipients (n = 94). The eyes were classified into two groups--high-risk (168 eyes) and low-risk (228 eyes)--depending on the degree of vascularisation in the recipient corneas or a history of previous allograft rejection. Data were analysed using the Kaplan-Meier life table method, the log-rank test and the Cox proportional hazards model.

RESULTS

In both the high-risk and low-risk groups, the graft survival and rejection-free graft survival rates were not affected by the H-Y compatibility. The graft survival (p < 0.001) and rejection-free graft survival (p < 0.001) rates were higher in the low-risk group than in the high-risk group. High-risk PKP was associated with greater risk of graft failure (risk ratio, 2.33) and rejection (risk ratio, 2.05) than low-risk PKP.

CONCLUSION

H-Y antigen matching does not influence the rate of allograft rejection after PKP.

Human minor histocompatibility antigens

Disparities in minor histocompatibility antigens between HLA-matched organ and bone marrow donors and recipients create a potential risk for graft failure and graft-versus-host disease. These conditions necessitate lifelong pharmacological immunosuppression of organ and bone marrow transplant recipients. Recent technical advances have resulted in the identification of the chemical nature of the first human minor histocompatibility antigens. A new era of research has begun to provide insights into the genetics of minor antigens and their putative role in transplantation.