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Ann Thorac Surg 1998;66:2022-2026
© 1998 The Society of Thoracic Surgeons

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Original Articles

Immunogenic human leukocyte antigen class II antigens on human cardiac valves induce specific alloantibodies

^a Department of Internal Medicine I, University Hospital Rotterdam, Rotterdam, the Netherlands
^b Department of Thoracic Surgery, University Hospital Rotterdam, Rotterdam, the Netherlands
^c Heart Valve Bank, University Hospital Rotterdam, Rotterdam, the Netherlands
^d Department of Immunohematology and Blood Bank, University Hospital Leiden, Leiden, the Netherlands

Accepted for publication June 8, 1998.

Address reprint requests to Dr Hoekstra, Department of Internal Medicine I, University Hospital Rotterdam-Dijkzigt, Room Bd 293, Dr Molewaterplein 40, 3015 GD Rotterdam, the Netherlands
e-mail: (hoekstra{at}inw1a2r.nl)

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. The kinetics of panel reactive antibodies (PRA) and incidence of antibodies directed against human leukocyte antigen (HLA) class II were studied in patients who received a cryopreserved cardiac valve allograft.

Methods. A complement-dependent microlymphocytotoxicity test was used to determine the percentage of panel reactive antibodies. Anti–HLA class II antibodies were measured by two-color fluorescence assays.

Results. The panel reactive antibodies became positive in 25 (78%) of 32 recipients between 1 and 16 months after implantation. Antibodies against HLA class II antigens were detected in 11 (37%) of 30 patients. In 9 (82%) of 11 cases these antibodies were donor specific. The induction of antibodies against donor HLA class II antigens suggests that intact HLA class II antigens are expressed by viable cells within the graft. Dithiothreitol analysis showed that the antibodies were of the immunoglobulin G type. Apparently, the HLA class II antigens are expressed in an immunogeneic way, as activation of specific T-helper cells is essential for the switch from immunoglobulin M to immunoglobulin G antibodies.

Conclusions. Allogeneic valve transplantation is associated with the production of donor-specific anti–HLA class I and II antibodies that could contribute to graft failure. This possibly detrimental effect might be prevented by cross matching in sensitized patients.

	Introduction

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Human cardiac valve allografts have been used for years for the reconstruction of cardiovascular anomalies. In adult recipients good clinical results have been reported in a number of series [1, 2], although the durability of the grafts is limited. In young recipients early valve failure is more frequent [3, 4]. Basket and colleagues [5] reported a 46% valve failure (more than two grades’ regurgitation) within the first 50 months after implantation in an echocardiographic follow-up study of young children with a pulmonary valve allograft. At our center, the 10-year graft survival rate of allografts implanted into the right ventricular outflow tract was 70% at a mean patient age of 22 years [6].

Although valve failure is related to different extrinsic factors, including sizing, surgical techniques, and preservation methods, structural valve failure resulting from intrinsic abnormalities of the valve leaflets [7] may have an immunologic basis. Alloantigens, which stimulate the immune system of the recipient and are responsible for rejection in organ transplantations, could also be present on valve allografts. Indeed, after preservation procedures such as cryopreservation or fresh storage, valve allografts generally contain viable cells [8, 9] capable of expressing human leukocyte antigen (HLA) class I and II antigens [10, 11]. Although the leaflet matrix is important for the long-term function of the valve leaflets, and fibroblasts are responsible for the maintenance of this matrix [12], these viable cells could be responsible for the initiation of the activation of the immune system of valve recipients. A rejection process is therefore conceivable, especially because valve recipients do not routinely receive immunosuppressive drugs. In animal studies the antigenic properties of allogeneic valves have been clearly demonstrated [13].

To analyze the immunogenicity of fresh and cryopreserved human valves, we previously performed lymphocyte stimulation assays in vitro with valve pieces as stimulator, which resulted in high proliferative responses when responder lymphocytes were mismatched for HLA-A, HLA-B, and HLA-DR with the valve donors [14]. Endothelial cells cultured from fresh valves also induced significant proliferation of mismatched responder lymphocytes [15]. Graft-infiltrating cells could be cultured from explanted allografts, and their cytotoxicity was shown to be donor specific in a cell-mediated lympholysis assay [16]. The cultures obtained were cytotoxic against donor HLA class I or class II antigens, or both, in five of six explants [16]. These data show that the cellular immune system is activated by allogeneic valve transplantation. Antibodies directed against donor major histocompatibility complex antigens have been detected on explanted valve allografts [17]. In cross-sectional studies, the presence of antibodies directed against HLA class I antigens has been previously reported in children and adults who received a cryopreserved valve allograft and in adults with a homovital or antibiotic sterilized valve [18–20].

Because of the presence of HLA class II antigens on cryopreserved valves and the cytotoxic activity of graft-infiltrating cells against these antigens, we hypothesized that a humoral response against HLA class II antigens could also be present. In the present study, we determined the kinetics of panel reactive antibodies (PRA) and the incidence of anti–HLA class II antibodies in a group of 32 patients receiving cryopreserved aortic or pulmonary cardiac valve allografts during the first year after transplantation and sought to confirm the production of donor-specific anti–HLA class I and II antibodies.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Patients
Heparin-treated blood samples were taken at varying times after implantation from 32 patients (21 males and 11 females) who received a cryopreserved pulmonary or aortic valve allograft. The allografts had been implanted for various indications, including endocarditis and congenital anomalies. The median age of the recipients was 41 years (range, 3 to 69 years). Two patients received a second allogeneic valve. Five patients were treated with steroids perioperatively (eg, because of chronic obstructive pulmonary disease and sarcoidose). The medical ethics committee of the University Hospital Rotterdam and the medical faculty of the Erasmus University Rotterdam had approved the study, and patients gave written informed consent before participation.

Valve allografts
Aortic and pulmonary roots were obtained from heartbeating and nonheartbeating donors within 24 hours after death. Preparation took place under sterile conditions according to standard operation procedures. Subsequently the grafts were sterilized for 24 hours at 4°C in Medium 199 (Bio-Whittaker, Alkmaar, the Netherlands) containing a low-concentration antibiotic solution: vancomycin 0.012 mg/mL, flucytocin 0.03 mg/mL, amikacin 0.012 mg/mL, metronidazole 0.012 mg/mL, and ciprofloxacin 0.003 mg/mL. Thereafter they were cryopreserved according to the standard cryopreservation protocol (-1°C per minute) in Medium 199 containing 10% dimethylsulfoxide and stored in the vapor phase of liquid nitrogen (-160 to -180°C) at the Heart Valve Bank in Rotterdam, the Netherlands. Just before implantation, the selected valves were thawed in a 37°C bath, and the dimethylsulfoxide was removed by stepwise dilution in Medium 199.

Blood sampling
Blood samples were taken once after transplantation in 18 of 32 patients. From 14 of 32 patients, two or more samples were collected, one sample immediately before valve implantation (negative controls) and thereafter between 1 week and 3 months, 3 to 6 months, 6 to 12 months, or longer than 1 year after implantation.

Antibody screening
Panel reactive antibodies and anti–HLA class I antibody screening
Screening of patient plasma was performed by the standard National Institutes of Health complement-dependent microlymphocytotoxicity test against a panel of 50 selected donors carrying the majority of the serologically defined HLA-A and HLA-B specificities [21]. A positive reaction was defined as greater than 30% dead cells per well. Results are expressed as percentage PRA: the number of positive donors divided by the number of donors tested.

Anti–HLA class II antibody screening
Platelet-absorbed patient sera were tested for HLA class II specific antibody reactivity in a two-color fluorescence assay [22]. The specificity of the antibodies was determined on the basis of their reactivity against a panel of 60 HLA-DR and HLA-DQ typed donors.

Dithiothreitol
Dithiothreitol was used for the immunoglobulin class determination [23]. A negative antibody screening test after dithiothreitol reduction is indicative for the presence of immunoglobulin (Ig) M type antibodies only, and the detection of antibodies despite dithiothreitol treatment is an indication for IgG antibodies (with or without IgM antibodies).

	Results

Top Abstract Introduction Material and methods Results Comment References

 
Incidence of panel reactive antibodies
Panel reactive antibodies against HLA antigens were detected in 25 (78%) of 32 recipients. Seven patients samples showed no significant PRA, but in 5 of these patients the samples were taken within 4 weeks after implantation, which could be too early to detect antibodies. Antibodies against HLA class II antigens were detected in 11 (37%) of 30 valve recipients. Determination of the antibodies in the presence of dithiothreitol showed that these antibodies were IgG antibodies in all cases.

Kinetic aspects
The follow-up group included 14 patients (10 men and 4 women). Five patients received more than one blood transfusion. Four patients had received an aortic allograft, and the remaining patients received a pulmonary graft. In the 14 blood samples taken directly before implantation, no antibodies could be detected. The production of antibodies started between 1 and 3 months after transplantation in 6 of 14 patients. In 4 of 14 patients antibodies were present between 3 and 6 months after transplantation, but no blood samples were available for these patients for the period 0 to 3 months. The PRA remained positive for more than 6 months in 8 of 14 recipients (Fig 1).

View larger version (10K): [in this window] [in a new window]   Fig 1. Follow-up results for the panel reactive antibodies (PRA) in both the cross-sectional and follow-up groups.

	Comment

Top Abstract Introduction Material and methods Results Comment References

 
Implantation of cardiac valve allografts was found to be associated with the production of donor-specific anti–HLA class I and II antibodies in the majority of our recipients. These results support the assumption that cryopreserved human cardiac valves contain viable cells that are able to express HLA class I and II antigens. The immunogenicity of these antigens is reflected by the high PRA detected and the specificity of these antibodies for donor alloantigens. Antibody production in valve recipients started approximately 4 weeks after transplantation. In patients who received blood transfusions, antibodies were generally detected after 2 weeks. The delay in antibody production after valve transplantation could be caused by the late presence of HLA class I and II antigens, possibly because of a recovery phase of viable cells after thawing and implantation. Hogan and colleagues [24] detected anti–HLA class II antibodies in 4 of 11 patients who received a cryopreserved valve allograft. However, varying allograft preparation methods are used in surgical centers; for example, different concentrations of antibiotic agents in the nutrient medium, duration of the antibiotic bath, and varying ischemia times could lead to a different antigenic load of the valves before implantation. Compared with the humoral response after implantation of homovital valves [20], antibiotic sterilization for 24 hours and the cryopreservation procedure at our center did not lead to lower percentages PRA after 3 months to 1 year after transplantation.

The finding that the antibodies were IgG antibodies suggests the involvement of T-helper lymphocytes in the immune response because these cells are necessary for the switch of the production of IgM antibodies to IgG antibodies. This activation of T-helper cells is in agreement with earlier results of the detection of donor-specific cytotoxic T lymphocytes into explanted valve allografts, which also implies a role of T-helper cells [16]. For the initiation of an antibody response directed against class II antigens, direct evidence of the presence of intact donor class II antigens is necessary. The presence of HLA class II antigens in valve allografts could be attributed to dendritic cells, endothelial cells, and fibroblasts. The presence of dendritic cells in the valve leaflet has not been confirmed in the literature, but Bobryshev and colleagues [25] reported the presence of dendritic cells in aortic vessel walls. The vessel wall represents an important component of the valve allograft. Endothelial cells are not consistently present on the surface of cryopreserved valves. Lupinetti and coworkers [26] showed that cryopreservation of human donor valves is associated with loss of endothelial cells. Generally, in our center only a few endothelial cells are present on the vascular and valve leaflet surfaces of cryopreserved valves before implantation. Viable fibroblasts are found in the majority of cryopreserved valve leaflets before implantation [26], but it is not clear how long these cells survive after transplantation. Neves and associates [27] detected fibroblasts in explanted valve allografts on histologic examination. They also compared the number of fibroblasts in heart valve recipients and patients with a donor heart. In the heart transplantation group, which received treatment with immunosuppressive drugs, the number of fibroblasts was normal. In contrast, in the valve recipients, who do not receive immunosuppression, the number of fibroblasts was strongly reduced. The reduction in the amount of fibroblasts in transplanted valve allografts could be caused not only by the cryopreservation procedure but also by immunologic destruction [27]. Schoen and colleagues [28] described 20 explanted cryopreserved valve allografts that were morphologically nonviable and unlikely to have active metabolic functions.

In conclusion, these data show that viable cells capable of the expression of HLA class I and II antigens are present in cryopreserved human valve allografts. Alloantibodies induced by the graft could damage transplanted tissue and therefore may contribute to graft dysfunction. We recommend determination of the HLA type of valve recipients and donors to evaluate the effect of histoincompatibility between valve donor and recipient on valve function. Although there is still a shortage of human valve allografts, prospective cross matching may prevent acute antibody-mediated rejections in high-risk patients (ie, young recipients in need of retransplantation).

	References

Top Abstract Introduction Material and methods Results Comment References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Hoekstra, F. M.E. Articles by Claas, F. H.J. Search for Related Content PubMed PubMed Citation Articles by Hoekstra, F. M.E. Articles by Claas, F. H.J. Related Collections Related Article