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Clinical Transplantation |




* Department of Nephrology and Intensive Care, Campus Virchow Clinic, and
Institute of Medical Immunology and
HLA-Laboratory, Institute of Transfusion Medicine; Charité University Medicine, Berlin, Germany
Address correspondence to: Dr. Petra Reinke, Department of Nephrology and Intensive Care, Charité Campus Virchow, Augustenburger Platz 1, Berlin D-13353, Germany. Phone: +49-30-450-524062; Fax: +49-30-450-524932; E-mail: petra.reinke{at}charite.de
Received for publication March 18, 2005. Accepted for publication November 20, 2005.
| Abstract |
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enzyme-linked immunosorbent spot memory T cell frequencies were determined in 10 healthy volunteers and 41 hemodialysis patients using for stimulation an allogeneic cell bank (ACB) from 17 healthy individuals who represented the most frequent white HLA antigens. Positive responses to ACB were analogous to PRA defined as percentage of positive assays of the ACB sets. Hemodialysis patients expressed higher PRT levels compared with healthy volunteers. Five of 10 PRT++ patients were PRA negative, and only four of 10 PRA++ patients exhibited PRT reactivity, suggesting independence of humoral and cellular sensitization. Pretransplantation PRT testing of recipients might improve individual risk assessment to make individualized therapy decisions. | Introduction |
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With the recent availability of new cellular technologies, measurement of T cell sensitization gained more attention. The IFN-
enzyme-linked immunosorbent spot (ELISpot) assay for determination of alloreactive memory T cells represents a highly sensitive and easily applicable technique for cellular alloimmunity (5). Using ELISpot, we and others demonstrated recently that kidney transplant recipients with high frequencies of donor-reactive memory T cells before transplantation are at risk for severe acute rejection episodes during the early posttransplantation period (6,7).
Blood transfusions, gravidities, and previous transplantations increase the risk for allosensitization, but allospecific memory does not necessarily result from alloantigen exposure. It is widely accepted that cross-reactivity is an essential feature of the human T cell repertoire and that environmentally primed T cells may cross-react with donor MHC-peptide complexes, leading to rapid T cell activation and early graft injury in individuals who have not been immunized directly by alloantigens. We could demonstrate that the presence of high frequencies of donor-reactive memory T cells is independent of established risk factors for sensitization, such as high levels of panel-reactive antibody (PRA) testing (7).
For further improvement of individual therapy decisions, early identification of T cellsensitized individuals before transplantation might be useful. Severe graft injury could be avoided in these individuals by using more appropriate immunosuppressive regimen targeting memory T cells.
On the basis of on the ELISpot technique for the detection of alloreactive memory T cells, we developed an assay for measurement of panel-reactive memory T cell (PRT) responsiveness by using a panel of stimulator cells that represent most HLA antigens encountered in the German population (8), analogous to PRA testing. Our data suggest that high levels of T cell sensitization are more frequent in dialysis patients compared with healthy individuals but are not routinely associated with high levels of PRA, previous transplants, blood transfusions, or gravidities, supporting the concept of heterogenicity. This assay might add significant new information for the assessment of the individual risk and selection of the appropriate immunosuppressive regimen for transplant patients.
| Materials and Methods |
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PRA Determination
All patients were on the waiting list for renal transplantation and were screened routinely for the presence of PRA every 3 mo by a complement-dependent cytotoxicity test according to the National Institutes of Health as described in detail elsewhere (14).
Responder Cells for IFN-
ELISpot Assay
Peripheral blood samples of hemodialysis patients were T cell enriched by rosetting technologies (RosetteSep T cell Enrichment Cocktail) using tetrameric antibodies cross-linking unwanted CD16+, CD19+, CD36+, and CD56+ cells to glycophorin A on erythrocyte cell surface, which were removed along with the pellet after Ficoll preparation. Responder cells were co-incubated directly with freshly thawed stimulator cells of each of the 17 ACB donor individuals in a 96-well ELISpot plate.
ELISpot Assay
To investigate effector/memory T cell reactivity against stimulator cells of the 17 healthy blood donors, a modification of the ELISpot assay as described by Valujskikh et al. (15) was used. Briefly, 96-well plates (Millipore, Eschborn, Germany) were coated with primary IFN-
antibody (Perbio Science, Bonn, Germany). Responder T cells and donor stimulator cells, respectively, were placed in the 96-well plates at a concentration of 1 x 105/well. Wells that contained responder and stimulator cells plus medium alone, respectively, were used as negative controls. Next, the plates were incubated for 24 h at 37°C. After repeated washing, biotinylated secondary IFN-
antibody (Perbio Science) was added for 4 h at room temperature. After an incubation with streptavidinhorseradish peroxidase for 90 min, the plates were developed using 3-amino-9-ethylcarbazol (Sigma-Aldrich, Munich, Germany). The resulting spots were counted using a computer-assisted ELISpot reader (BIO-SYS, Karben, Germany). Frequencies of IFN-
producing donor-reactive T cells were calculated by subtracting both the responder alone and the stimulator alone control wells from the wells that contained recipient plus stimulator cells.
Statistical Analyses
Mann-Whitney U test was used for comparison of T cell frequencies or other parameters between subgroups of patients. Spearman rank correlation was used to calculate bivariate correlations. Cross-table analysis was performed by
2 test. Data were analyzed using the statistical software SPSS (SPSS GmbH Software, München, Germany).
| Results |
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80%, whereas two of them had recent PRA
80%. Eighteen of 41 patients had received at least one previous kidney transplant (range 1 to 3). Twenty-two of 41 patients received blood transfusions in the past (range 1 to 69), and six of 17 female patients had gravidities (range 1 to 6).
PRT
ELISpot frequencies of IFN-
producing alloreactive memory T cell frequencies of 10 healthy volunteers and 41 HD patients were determined using a panel of stimulator cells from 17 healthy blood donors of our ACB. On the basis of previous studies correlating heightened pretransplantation ELISpot donor-reactive memory T cells with early acute rejection, we arbitrarily defined positive assays as >50 spots/100,000 T cells and calculated PRT levels as percentage of positive responses to the 17 ACB blood donors (7).
Figure 1 summarizes the results of our experiments. We observed a wide range of responses against the allogeneic stimulators. Approximately half of the HD patients showed no signs of T cell sensitization, whereas 19 of 41 patients exhibited positive responses (PRT > 0%) against at least one of the ACB donors, approximately 15% even a very strong panel reactivity (PRT > 25%). The median PRT level among the 19 PRT+ patients was 17.7% (mean 20.4%). The peak PRT level was 58.8% in a patient who had a positive ELISpot response to 10 of 17 ACB stimulators.
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producing T cells to ACB stimulators showed a slight but statistically significant correlation (Figure 2) with the cumulative mismatch of HLA A, B, and DR antigens (r = 0.139, P < 0.001, n = 697).
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0,
20%) did also not reveal statistical significance in
2 cross-table analysis. Figure 3 summarizes the distribution of PRT+ and PRA+ patients of our population. Next, we analyzed whether high cellular (PRT) and humoral (PRA) allosensitization might be closely related or occur independent of each other. Table 3 shows PRA levels and other parameters of the 10 highly T cellsensitized HD patients of our study cohort (>100 spots/100,000 T cells against at least one ACB stimulator). Most important, five of 10 highly T cellsensitized individuals had both negative recent and historic PRA and no history of classic allosensitization, clearly indicating that high cellular alloreactivity does not routinely imply humoral allosensitization. The other five of 10 patients with >100 spots/100,000 T cells against at least one healthy blood donor all had significant historic PRA of >40% and a history of allosensitization by transplants, transfusions, or gravidities. It is interesting that the patient with the highest ACB response in our study cohort of 529 spots/100,000 T cells was the liver transplant patient.
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Independent Association of PRA and PRT with the Number of Previous Transplants, Blood Transfusions, and Duration of ESRD
Next, we investigated whether PRT testing would correlate with well-established risk factors for allosensitization. Both recent and historic PRA correlated significantly with number of previous kidney transplants (n = 41; r = 0.529, P < 0.001/r = 0.690, P < 0.001), blood transfusions (n = 40; r = 0.553, P < 0.001/r = 0.759, P < 0.001), duration of ESRD (n = 41; r = 0.478, P = 0.002/r = 0.792, P < 0.001), cumulative duration of dialysis (n = 41; r = 0.376, P = 0.015/r = 0.784, P < 0.001), and cumulative duration of previous transplantations (n = 41; r = 0.463, P = 0.002/r = 0.634, P < 0.001). Both recent and historic PRA testing did not correlate with number of previous gravidities and patient age.
Regarding the parameters median ELISpot response against the ACB stimulators as well as PRT level, a significant correlation was found between median (r = 0.391, P = 0.012) ELISpot responses and numbers of previous transplants. Moreover, median (P = 0.006) ELISpot responses were significantly higher in HD patients with previous transplantations compared with HD patients without previous transplantations (Figure 5). Furthermore, both HD patients with and without previous transplantations showed significantly heightened median (P = 0.001/P = 0.014) ELISpot alloreactive T cells compared with control subjects, whereas patients with former transplants also exhibited heightened PRT levels compared with control subjects (P = 0.010/P = 0.001; Figure 5).
Median ELISpot responses (r = 0.363, P = 0.020) as well as PRT level (r = 0.333, P = 0.034) correlated with duration of dialysis. Synthetic dialyzer membranes were used in most of the HD patients in our study. We found no difference in alloreactive T cell frequencies between HD patients on polysulfone high-flux versus polyamide low-flux membranes. No ELISpot parameter correlated with age, numbers of transfusions, or gravidities.
Other Demographic Parameters
It is interesting that female HD patients had significantly higher PRT levels (P = 0.003) and median (P = 0.003) ELISpot responses compared with male HD patients (Figure 6), whereas they showed no statistically significant difference regarding recent or historic PRA. In contrast, nullipara and multipara groups could not be distinguished by PRT frequencies. We found no difference in any of the PRT parameters regarding patients who were younger or older than 55 yr. Moreover, there was no difference between cytomegalovirus IgGpositive and cytomegalovirus IgGnegative individuals.
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| Discussion |
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According to the principle of PRA screening, we established a test system for measuring cellular sensitization of patients who are on the transplant waiting list, called PRT assay. As previous studies have shown that highly elevated pretransplantation donor-reactive IFN-
ELISpot frequencies were associated with severe early acute rejection episodes, we focused on the direct pathway of allorecognition, which was suggested to dominate the early phase after transplantation, whereas the indirect pathway of alloreactivity has been associated with later forms of alloresponses, chronic allograft nephropathy, and antibody production.
Our definition of a positive PRT assays being >50 spots/100,000 T cells is yet arbitrary and should be reevaluated in future studies that analyze the clinical relevance of the PRT assay for transplant outcome.
The alloreactive IFN-
ELISpot frequencies measured in this study are slightly lower compared with previously published pretransplantation donor-reactive IFN-
ELISpot frequencies in kidney transplant recipients, which were generally in the range of <10 to 1000 spots/300,000 peripheral blood mononuclear cells (6,7). This can be explained by different responder and stimulator cell preparation. We now use a standardized number of 100,000 enriched T cells/well (95% purity) instead of 300,000 peripheral blood mononuclear cells/well. Furthermore, previous studies used unseparated donor cells (containing also donor T cells) as stimulators, which have been shown to be capable of IFN-
production despite irradiation or treatment with mitomycin C. Another difference to North American studies might be that they included black patients who are more sensitized (Heeger et al., Cleveland Clinic Foundation; personal communication, April 2005).
Whereas only few healthy donors expressed alloreactive T memory responses, HD patients showed enhanced cellular alloreactivity. It is widely known that uremic toxins, dialytic devices, and water contamination in chronic HD exhibit complex immunologic effects such as chronic inflammation, complement activation, and upregulation of adhesion molecules on leukocytes. Dialysis patients may also experience chronic infections (16). Moreover, a predominance of Th1 CD4+ T cells has been reported in dialysis patients, probably related to an overproduction of monocyte-derived IL-12 (17). Thus, repeated antigenic stimulation and bystander activation by chronic inflammation might explain heightened frequencies of cross-reactive alloreactive T cells in HD patients without a history of allosensitization. We found, however, no differences between patients using distinct dialysis membranes.
In patients with a history of allosensitization, we found further enhanced T cell alloreactivity. Most important, we could demonstrate that approximately 50% of highly PRT+ patients exhibit negative recent and historic PRA and have no history of allosensitization, supporting the concept of heterogenicity and cross-reactivity for the generation of alloreactive memory T cells. Hence, PRT testing allows the identification of PRA negative but clearly memory T cellalloreactive patients irrespective of known risk factors for allosensitization, thereby providing important new informations for the assessment of the pretransplantation immune status in transplantation candidates. Further clinical studies that analyze the relevance of cellular allosensitization in PRT screening for posttransplantation outcome are now warranted. In accordance with our results, a significant number of transplant patients experience early acute rejection and poor 1-yr graft function despite negative testing for humoral sensitization in PRA and final cross-match testing (7). These early acute rejections might be related to preformed alloreactive memory T cells, as we and others recently showed that heightened frequencies of pretransplantation donor-reactive memory T cells predict acute rejection in kidney transplant recipients irrespective of humoral reactivity (6,7).
High levels of humoral sensitization (PRA++) increase the probability of positive pre/posttransplantation cross-match associated with poor outcome, but also in the absence of donor-reactive alloantibodies, PRA++ patients have a poorer prognosis compared with PRA recipients, suggesting that broad humoral sensitization might be a marker of "high immune reactivity." Similarly, high levels of pretransplantation PRT levels may result in higher probability of donor-reactive memory T cells associated with poorer outcome (6,7). However, it also might be possible that broad T cell sensitization simply indicates general cellular hyperreactivity. Further studies have to address this issue.
Conclusions
We have established an easily applicable technique for assessing pretransplantation T cell allosensitization status to a broad panel of allogeneic stimulators in patients who are on the waiting list for transplantation. Future studies that analyze the predictive value of PRT testing for posttransplantation outcome are required. Considering the effects of preformed donor-reactive T cells on rejection frequencies in transplant patients irrespective of PRA and cross-match testing, it is expected that PRT testing will provide useful additional information for identifying transplant candidates who are at high immunologic risk to improve individualized immunotherapy.
Pretransplantation ELISpot analyses by our HLA-typed ACB identified PRA-negative but T cellpresensitized HD patients before transplantation. We suggest introducing PRT testing and memory T cell cross-match for an early identification of presensitized transplant patients.
| Acknowledgments |
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We thank Dr. Guido Heymann for assistance. Also, we express our sincere thanks to the healthy volunteers for blood donation.
| Footnotes |
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Published online ahead of print. Publication date available at www.jasn.org.
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