Virtual crossmatch by identification of donor-specific anti-human leukocyte antigen antibodies by solid-phase immunoassay: a 30-month analysis in living donor kidney transplantation

Impact of a positive crossmatch on pediatric heart transplant outcomes

BACKGROUND

Pediatric heart transplant (HT) candidates experience high waitlist mortality due to a limited donor pool that is constrained in part by anti-HLA sensitization. We evaluated the impact of CDC and Flow donor-specific crossmatch (XM) results on pediatric HT outcomes.

METHODS

All pediatric HTs between 1999 and 2019 in the OPTN database were included. Donor-specific XM results were sub-categorized based on CDC and Flow results. Primary outcomes were treated rejection in the first year and time to death or allograft loss. Propensity scores were utilized to adjust for differences in baseline characteristics.

RESULTS

A total of 4,695 pediatric HT patients with T-cell XM data were included. After propensity score adjustment, a positive T-cell CDC-XM was associated with 2 times higher odds of treated rejection (OR 2.29 (1.56, 3.37)) and shorter time to death/allograft loss (HR 1.50 (1.19, 1.88)) compared to a negative Flow-XM. HT recipients who were Flow-XM positive with negative/unknown CDC-XM did not have higher odds of rejection or shorter time to death/allograft loss. An isolated positive B-cell XM was also not associated with worse outcomes. Over the study period XM testing shifted from CDC- to Flow-based assays.

CONCLUSIONS

A positive donor-specific T-cell CDC-XM was associated with rejection and death/allograft loss following pediatric HT. This association was not observed with a positive T-cell Flow-XM or B-cell XM result alone. The shift away from performing the CDC-XM may result in loss of important prognostic information unless the clinical relevance of quantitative Flow-XM results on heart transplant outcomes is systematically studied.

BSHI and BTS UK guideline on the detection of alloantibodies in solid organ (and islet) transplantation

Solid organ transplantation represents the best (and in many cases only) treatment option for patients with end-stage organ failure. The effectiveness and functioning life of these transplants has improved each decade due to surgical and clinical advances, and accurate histocompatibility assessment. Patient exposure to alloantigen from another individual is a common occurrence and takes place through pregnancies, blood transfusions or previous transplantation. Such exposure to alloantigen's can lead to the formation of circulating alloreactive antibodies which can be deleterious to solid organ transplant outcome. The purpose of these guidelines is to update to the previous BSHI/BTS guidelines 2016 on the relevance, assessment, and management of alloantibodies within solid organ transplantation.

Detection of donor-specific HLA antibodies: A retrospective observation in 350 renal transplant cases

BACKGROUND

The main objective of this study was to determine the results of the cell-based assay (CDC-XM and FC-XM), and correlate with the results of solid phase assay (L-SAB).

METHODS

In this retrospective study, 350 prospective renal transplant recipients were tested for the presence of HLA antibodies by CDC-XM, FC-XM and L-SAB screening with their corresponding donor.

RESULTS

T-cell-FC-XM showed a sensitivity of 71.43% and a specificity of 91.50% for detecting class I L-SAB (+), while B-cell-FCXM showed a sensitivity of 94.94% and a specificity of 61.99% for detecting class II L-SAB (+). On the other hand, T-CDC-XM showed a sensitivity of 32.14% and a specificity of 98.64% for detecting class I L-SAB (+), while B-CDC-XM showed a sensitivity of 44.30% and a specificity of 94.83% for detecting class II L-SAB (+). In this study, the results indicated that DSA class I MFI value of 2845 and above significantly (p ≤0.001) correlated with T-cell-FC-XM positivity, while MFI value of 4585 and above (p ≤0.001) showed strong predictive accuracy of a positive T-cell-CDC-XM. However, DSA class II MFI cut-off of 1988 and above significantly (p ≤0.001) correlated with B-cell-FC-XM positivity, while MFI value of 5986 and above (p ≤0.001) showed strong predictive accuracy of a positive B-cell-CDC-XM.

CONCLUSIONS

Our study showed that CDC-XM has poor sensitivity, while FC-XM has poor specificity to detect DSA. L-SAB has good correlation with T-cell-FC-XM (p < 0.0001) but not with B-cell-FC-XM (P = 0.31). DSA strength >2845 and > 1988 significantly correlated with T-cell-FC-XM positivity and B-cell-FC-XM positivity, respectively. While, a MFI value of >4585 and > 5986 significantly correlated with T-cell-CDC-XM positivity and B-cell-CDC-XM positivity, respectively. These MFI cut-off values could serve as a surrogate marker for CDC-XM and FC-XM tests and may help in resolving the limitations of cell-based techniques. In conclusion, we found that L-SAB is more sensitive and specific than CDC-XM and FC-XM and therefore may be used as a test of choice.

Aspectos técnicos y clínicos de la prueba cruzada de histocompatibilidad en el trasplante de órganos sólidos

La presencia de anticuerpos dirigidos contra los antígenos leucocitarios humanos (Human Leukocyte Antigens, HLA) que se expresan en las células del donante, es uno de los factores de riesgo más importantes asociados con las complicaciones clínicas después del trasplante. La prueba cruzada es una de las pruebas de histocompatibilidad más eficaces para la detección de anticuerpos específicos contra el donante en los receptores de injertos. En los primeros métodos de la prueba cruzada, se utilizaba la citotoxicidad dependiente del complemento, que es útil para detectar dichos anticuerpos responsables del rechazo hiperagudo del injerto, pero carece de la sensibilidad adecuada. Por ello, se desarrollaron métodos de pruebas cruzadas más sensibles, entre ellas, la prueba cruzada por citometría de flujo que hoy se considera el método preferido.

En este artículo se revisa la evolución de la prueba cruzada y los factores más importantes que deben tenerse en cuenta al realizarla y al interpretar los resultados de esta prueba fundamental para la supervivencia a largo plazo del injerto.

Incidence and impact of allele-specific anti-HLA antibodies and high-resolution HLA genotyping on assessing immunologic compatibility

The ability to identify specific HLA molecules against which a patient has alloantibodies has revolutionized assessment of immunologic compatibility. Anti-HLA antibodies are typically evaluated as reactive against well-defined serologic antigen groups. Thus, donor HLA genotyping is aimed at defining HLA at the serologic split-antigen level to avoid incompatible antigen-antibody combinations. However, anti-HLA antibodies can have reactivities not accurately described by well-defined serologic antigens. While existence of these antibodies is acknowledged, their precise impact on clinical practice is not clear. We performed a single-center review of 2 years of pre-and post-transplant anti-HLA antibody testing data combined with high-resolution HLA genotyping data for living and deceased organ donors to evaluate the clinical impact of anti-HLA antibodies with reactivities outside of commonly defined serologic antigen groups. We find approximately 15% of patients awaiting transplantation have alloantibodies with differential reactivity for HLA proteins encoded by specific alleles within a serologic antigen group. Allele-specific antibodies are associated with positive cellular crossmatches not accurately predicted by standard donor HLA genotyping and can manifest as post-transplant donor-specific antibodies. Our data highlights the importance of evaluating anti-HLA antibodies at the allele-level and provides evidence supporting utility for high-resolution HLA genotyping in solid organ transplantation.

Virtual Crossmatching in Kidney Transplantation: The Wait Is Over

BACKGROUND

Flow cytometric crossmatching is currently the method of choice for most transplantation programs before kidney transplantation. In July of 2017, our program implemented the virtual crossmatch, without a prospective physical crossmatch, for the majority of patients in the setting of a new kidney allocation system implemented by the United Network for Organ Sharing.

STUDY DESIGN

A retrospective review was conducted to determine whether virtual crossmatching could reduce cold ischemia time (CIT). Secondary outcomes included the incidence of delayed graft function and 1-year patient and allograft failure.

RESULTS

A total of 825 patients received a kidney transplant between December 1, 2014 and July 1, 2018; 505 were in the pre-implementation group and 227 were in the post-implementation group. The CIT decreased between the pre-implementation era to post implementation era from 16.67 ± 8.7 hours to 14.5 ± 8.2 hours (p = 0.002). On univariate analysis, delayed graft function (DGF) rates were similar between the 2 eras (19% vs 17%; p = 0.415), despite having more donations after cardiac death and higher Kidney Donor Profile Index donors in the post-implementation era. There was no difference in biopsy-proven acute rejection (n = 28 [5.6%] vs n = 8 [3.5%]; p = 0.226), 1-year graft loss (4% vs 3%; p = 0.304), or patient death (2% vs 1%; p = 0.567) rate between groups. On multivariable modeling for mean CIT and incidence of DGF, patients receiving transplants in the post-implementation era had an adjusted reduction in CIT of an estimated 2.35 hours (95% CI, 1.15 to 3.55; p < 0.001). Patients in the post-implementation era also had 26% lower odds of DGF developing (odds ratio 0.74; 95% CI, 0.48 to 1.14; p = 0.170), after adjusting for covariates.

CONCLUSIONS

Kidney transplantation can be safely performed with virtual crossmatching, without a prospective physical crossmatch with improved CIT and potentially reduced DGF rate without increased risk of rejection.

Donor-specific antibodies detected by single antigen beads alone can help risk stratify patients undergoing retransplantation across a repeat HLA mismatch

Whether reexposure to mismatched HLA antigens (RMM) in the setting of a negative crossmatch is associated with increased immunological risk remains an area of uncertainty. This is due to evidence derived predominantly from registry data, which lacks comprehensive information on alloantibody and rejection. In this study, we analyze the impact of low-level preformed donor-specific antibodies (DSA) against an RMM on transplant outcomes. From 1988 consecutive renal transplant recipients, we analyzed 179 patients undergoing retransplantation, of whom 55 had a RMM. All patients were crossmatch negative and preformed DSA were detected by single antigen beads alone. Multivariate analysis revealed that patients with preformed DSA against an RMM were independently at risk of antibody-mediated rejection (HR 8.70 [3.42-22.10], P < .0001) and death-censored allograft loss (HR 3.08 [1.17-8.14], P = .023). In addition, prior transplant nephrectomy (HR 2.04 [1.00-4.17], P = .0495) was also associated with allograft failure, whereas receiving a retransplant that was matched at HLA class II was associated with a favorable outcome (HR 0.37 [0.14-0.99], P = .047). In the absence of preformed DSA, an RMM was not associated with de novo DSA development, rejection, or allograft loss. In conclusion, an RMM portends increased immunological risk only in the presence of a preformed DSA in patients undergoing retransplantation.

WNT pathway signaling is associated with microvascular injury and predicts kidney transplant failure

Microvascular injury is associated with accelerated kidney transplant dysfunction and allograft failure. Molecular pathology can identify new mechanisms of microvascular injury while improving on the diagnostic and prognostic capabilities of traditional histology. We conducted a case-control study of archived kidney biopsy specimens stored up to 10 years with microvascular injury (n = 50) compared with biopsy specimens without histologic injury (n = 45) from patients of similar age, race, and sex. We measured WNT gene expression with a multiplex quantification platform by using digital barcoding, given the importance of WNT reactivation to the response to wounding in the kidney microvasculature and other compartments. Of 210 genes from a commercial WNT panel, 71 were associated with microvascular injury and 79 were associated with allograft failure, with considerable overlap of genes between each set. Molecular pathology identified 46 biopsy specimens with molecular evidence of microvascular injury; 18 (39%) were either C4d negative, donor-specific antibody negative, or had no microvascular injury by histology. The majority of cases with molecular evidence of microvascular injury had poor long-term outcomes. We identified novel WNT pathway genes associated with microvascular injury and allograft failure in residual clinical biopsy specimens obtained up to 10 years earlier. Further mechanistic studies may identify the WNT pathway as a new diagnostic and therapeutic target.

Pretransplant Single Antigen Bead-Detected HLA Antibodies in Kidney Transplant Long-term Outcome: A Single-Center Cohort Experience

Single-antigen bead (SAB) platform permits the identification of antibodies not detectable by complement-dependent lymphocytotoxicity test, but their clinical significance is not completely understood. The aim of this study was to evaluate whether the presence of pretransplant SAB-detected antibodies is associated with the development of allograft failure. This is a single-center cohort study with 10-year follow-up in which 573 kidney recipients with negative pretransplant complement-dependent lymphocytotoxicity crossmatch who received transplants at the Kidney Transplant Center of Policlinico, Milan, from deceased donors between 1996 and 2005 were evaluated. Pretransplant plasma samples were retrospectively analyzed by SAB assay. Survival analyses were performed to assess the risk of allograft failures by SAB-detected antibodies. Pretransplant antibodies were found in 160 (28.0%) recipients, of whom 42 subsequently developed an allograft failure for a survival rate of 70.9% (95% confidence interval [CI), 63.5-78.4). Among those without antibodies, 58 (14.0%) returned to dialysis with a survival rate of 84.7% (95% CI, 81.0-88.4). In Cox regression analyses, patients with SAB-positivity had 2-fold higher risk of allograft failure than those who were SAB-negative (hazard ratio, 2.07; 95% CI, 1.39-2.79). Results did not change after adjustment for putative confounders. In conclusion, in this single-center cohort, 10-year allograft survival rate was significantly influenced by the presence of SAB-detected antibodies.

Positive Luminex and negative flow cytometry in kidney transplantation: a systematic review and meta-analysis

The presence of pre-formed donor-specific antibodies (DSAs) in kidney transplantation is associated with worse overall outcomes compared with DSA-negative transplantation. A positive complement-dependant cytotoxic crossmatch presents a high immunological risk, while a negative flow cytometry crossmatch is at the lower end of the risk spectrum. Yet, the presence of low-level DSA detected by Luminex alone, that is, positive Luminex and negative flow (PLNF) cytometry crossmatch lacks robust scientific exploration. In this systematic review and pooled analysis, we investigate the glomerular filtration rate, acute rejection (AR), graft survival and patient survival of PLNF transplants compared with DSA-negative transplants. Our analysis identified seven retrospective studies consisting of 429 PLNF transplants and 10 677 DSA-negative transplants. Pooled analysis identified no significant difference in the incidence of AR at 1 year [relative risk (RR) = 1.35, 95% confidence interval (CI) 0.90–2.02, Z = 1.46, P = 0.14, I2 = 0%], graft failure at 1 year (RR = 1.66, 95% CI 0.94–2.94, Z = 1.75, P = 0.08, I2 = 23%), graft failure at 5 years (RR = 1.29, 95% CI 0.90–1.87, Z = 1.38, P = 0.17, I2 = 0%), patient mortality at 1 year (RR = 0.89, 95% CI 0.31–2.56, Z = 0.22, P = 0.82, I2 = 0%) and patient mortality at 5 years (RR = 1.76, 95% CI 0.48–6.48, Z = 0.85, P = 0.39, I2 = 61%). Pooled analysis of graft function was not possible due to insufficient data. Current evidence suggests that low-level DSA detected by Luminex alone does not pose significant risk at least in the short–medium term. Considering the shortage of kidney transplants and the ever-increasing waiting time, the avoidance of PLNF transplants may be unwarranted especially in patients who have been enlisted for a long time.

Anti-perlecan antibodies and acute humoral rejection in hypersensitized patients without forbidden HLA specificities after kidney transplantation

BACKGROUND

The improvement in the definition of serum anti-HLA antibodies (HLA-Abs) profiles after Luminex-assay implementation in transplant patients follow-up is clear. This success has permitted the development of hypersensitized-recipient allocation and donor-paired exchange programs improving the access to transplantation. However, non-HLA Abs have been described in transplanted patients but their effect in hypersensitized transplanted recipients is unclear.

METHODS

Twenty-seven HLA hypersensitized patients awaiting for kidney transplantation (KT) were studied and 11 of them were followed after KT. The HLA Abs profile was confirmed in serum by Single Antigen Luminex assay and panel reactive of antigens >98% was achieved in all patients. Subsequently, the ability to fix complement by C1q test was also assessed. Serum non-HLA Abs before and 1 month after transplantation were measured in the 11 hypersensitized recipients.

RESULTS

95.2% of the hypersensitized on waiting list had concomitant serum anti-HLA and non-HLA Abs. The more frequent specificity in non-HLA Abs were found against Glutathione S-transferase theta-1 (GSST-1) (in 62%) and C-terminal fragment of perlecan (LG3) (in 52%). Four out of 11 transplanted patients presented early antibody-mediated rejection (ABMR) confirmed by biopsy and had serum anti-LG3 antibodies, two of them with concomitant anti-anti-angiotensin II type I receptor. Only one patient developed de novo-donor specific HLA antibodies.

CONCLUSIONS

The incidence of non-HLA antibodies in patients in the waiting list is largely underestimated. The concomitance anti-HLA and non-HLA Abs in hypersensitized patients is very common and the detection of non-HLA Abs in this population could allow to identify patients with an increased risk of humoral rejection.

Pediatric heart transplantation across a positive crossmatch: First year results from the CTOTC-04 multi-institutional study

Sensitization is common in pediatric heart transplant candidates and waitlist mortality is high. Transplantation across a positive crossmatch may reduce wait time, but is considered high risk. We prospectively recruited consecutive candidates at eight North American centers. At transplantation, subjects were categorized as nonsensitized or sensitized (presence of ≥1 HLA antibody with MFI ≥1000 using single antigen beads). Sensitized subjects were further classified as complement-dependent cytotoxicity crossmatch (CDC-crossmatch) positive or negative and as donor-specific antibodies (DSA) positive or negative. Immunosuppression was standardized. CDC-crossmatch-positive subjects also received perioperative antibody removal, maintenance corticosteroids, and intravenous immunoglobulin. The primary endpoint was the 1 year incidence rate of a composite of death, retransplantation, or rejection with hemodynamic compromise. 317 subjects were screened, 290 enrolled and 240 transplanted (51 with pretransplant DSA, 11 with positive CDC-crossmatch). The incidence rates of the primary endpoint did not differ statistically between groups; nonsensitized 6.7% (CI: 2.7%, 13.3%), sensitized crossmatch positive 18.2% (CI: 2.3%, 51.8%), sensitized crossmatch negative 10.7% (CI: 5.7%, 18.0%), P = .2354. The primary endpoint also did not differ by DSA status. Freedom from antibody-mediated and cellular rejection was lower in the crossmatch positive group and/or in the presence of DSA. Follow-up will determine if acceptable outcomes can be achieved long-term.

(F)Utility of the physical crossmatch for living donor evaluations in the age of the virtual crossmatch

Flow cytometric crossmatches (FCXM) are routinely performed to support living-donor renal transplantation. While long a laboratory mainstay, a physical crossmatch is costly, time consuming, and frequently poses interpretative conundrums with both false-positive and false- negative results. Given the increased utilization of the virtual crossmatch (vXM) in the deceased donor setting, our aim was to assess its utility in living donor evaluations. We reviewed 100 living donor FCXMs and retrospectively performed a vXM for each pair. Seventy-five (75) cases were concordant, (i.e., FCXM-/vXM- or FCXM+/vXM+) while 25 cases were discordant; Five were vXM+/FCXM- and 20 were FCXM+/vXM-. Since donor-specific antibodies (DSA) were not detected in the 20 FCXM+/vXM- cases, these were interpreted as false-positive, i.e., due to non-HLA antibodies. Importantly, none of these patients, when transplanted across a positive FCXM, experienced early antibody mediated rejection or subsequently developed HLA DSA. These data reveal that, for the vast majority of living donor evaluations, a vXM is an acceptable vetting procedure.

Quantitative Evaluation of the Impact of Ethylenediaminetetraacetic Acid Pretreatment on Single-Antigen Bead Assay

Background

Ethylenediaminetetraacetic acid (EDTA) pretreatment has been shown to overcome complement interference in the single-antigen bead (SAB) assay. However, a quantitative evaluation of its impact on the assay for preemptive application to diverse clinical samples is still lacking.

Methods

Serum samples from 95 renal transplant candidates were tested with and without EDTA-pretreatment in parallel. Changes in mean fluorescence intensity (MFI) values were analyzed to determine the impact of EDTA-pretreatment and the characteristics of complement interference.

Results

MFI values from EDTA-treated and untreated sera showed good correlations (r = 0.99) and were linear after excluding outliers (slopes, 1; intercepts, −63.7 and −24.2 for class I and II, respectively). Using an assay cutoff of 2000 MFI, positive/negative assignments were concordant for 99% of the 9215 class I beads and 9025 class II beads tested. As defined by an MFI increment above 4000 after EDTA pretreatment, complement interference affected 172 class I beads in 12 samples (12.6%) and 60 class II beads in 7 samples (7.4%), and the findings were supported in 83% and 86% of these samples by dilution studies. In a case study, EDTA pretreatment prevented falsely low MFI values and facilitated the interpretation of titration curves. Finally, EDTA pretreatment reduced the coefficient of variance (CV) by 2.1% and 2.4% for class I and II beads respectively (P < 0.0001).

Conclusions

It is safe to preemptively treat all clinical samples with EDTA before SAB assay to prevent false negative results or falsely low MFI values. EDTA pretreatment has the added benefit of improved assay precision.

Improvement in the definition of anti-HLA antibody profile in highly sensitized patients

The definition of anti-HLA antibody profile in highly sensitized patients on a waiting list is crucial when virtual crossmatch is used in organ allocation systems, but also when used to identify the true deleterious anti-HLA antibodies. Here we propose different levels of risk based on the results of anti-HLA antibody testing in neat serum (N) and after sera dilution (DIL) and C1q test in 18 highly sensitized patients. This group was heterogeneous in terms of anti-HLA antibody titers and their ability to fix complement. After dilution, 15 out of 18 patients (83.3%) showed a reduction of positive bead counts whereas 4 patients showed a prozone effect and complement fixation was demonstrated. The high dilution of sera and ascertaining the complement fixation allow the accurate definition of risk anti-HLA antibody profiles in highly sensitized patients, as demonstrated in 5 of the sensitized patients who were transplanted.

Polyomavirus Reactivation and Immune Responses to Kidney-Specific Self-Antigens in Transplantation

Humoral immune responses against donor antigens are important determinants of long-term transplant outcomes. Reactivation of the polyomavirus BK has been associated with de novo antibodies against mismatched donor HLA antigens in kidney transplantation. The effect of polyomavirus reactivation (BK viremia or JC viruria) on antibodies to kidney-specific self-antigens is unknown. We previously reported excellent 5-year outcomes after minimization of immunosuppression for BK viremia and after no intervention for JC viruria. Here, we report the 10-year results of this trial (n=193) along with a nested case-control study (n=40) to explore associations between polyomavirus reactivation and immune responses to the self-antigens fibronectin (FN) and collagen type-IV (Col-IV). Consistent with 5-year findings, subjects taking tacrolimus, compared with those taking cyclosporin, had less acute rejection (11% versus 22%, P=0.05) and graft loss (9% versus 22%, P=0.01) along with better transplant function (eGFR 65±19 versus 50±24 ml/min per 1.73 m², P<0.001) at 10 years. Subjects undergoing immunosuppression reduction for BK viremia had 10-year outcomes similar to those without viremia. In the case-control study, antibodies to FN/Col-IV were more prevalent during year 1 in subjects with polyomavirus reactivation than in those without reactivation (48% versus 11%, P=0.04). Subjects with antibodies to FN/Col-IV had more acute rejection than did those without these antibodies (38% versus 8%, P=0.02). These data demonstrate the long-term safety and effectiveness of minimizing immunosuppression to treat BK viremia. Furthermore, these results indicate that polyomavirus reactivation associates with immune responses to kidney-specific self-antigens that may increase the risk for acute rejection through unclear mechanisms.

Transplanting Sensitized Kidney Transplant Patients With Equivalent Outcomes Utilizing Stringent HLA Crossmatching

OBJECIVES

Elevated panel reactive antibody levels have been traditionally associated with increased acute rejection rate and decreased long-term graft survival after kidney transplant. In this study, our objective was to determine patient and allograft outcomes in sensitized kidney transplant recipients with advanced HLA antibody detection and stringent protein sequence epitope analyses.

MATERIALS AND METHODS

This was a subanalysis of a prospective, risk-stratified randomized controlled trial that compared interleukin 2 receptor antagonist to rabbit antithymocyte globulin induction in 200 kidney transplant recipients, examining outcomes based on panel reactive antibody levels of < 20% (low) versus ≥ 20% (high, sensitized). The study was conducted between February 2009 and July 2011. All patients underwent solid-phase single antigen bead assays to detect HLA antibodies and stringent HLA epitope analyses with protein sequence alignment for virtual crossmatching. Delayed graft function, acute rejection rates, and graft loss were the main outcomes measured.

RESULTS

Both the low (134 patients) and high (66 patients) panel reactive antibody level cohorts had equivalent induction and maintenance immunosuppression. Patients in the high-level group were more likely to be female (P < .001), African American (P < .001), and received a kidney from a deceased donor (P = .004). Acute rejection rates were similar between the low (rate of 8%) and high (rate of 9%) panel reactive antibody groups (P = .783). Delayed graft function, borderline rejection, graft loss, and death were not different between groups. Multivariate analyses demonstrated delayed graft function to be the strongest predictor of acute rejection (odds ratio, 5.7; P = .005); panel reactive antibody level, as a continuous variable, had no significant correlation with acute rejection (C statistic, 0.48; P = .771).

CONCLUSIONS

Appropriate biologic matching with single antigen bead assays and stringent epitope analyses provided excellent outcomes in sensitized patients regardless of the induction therapy choice.

Virtual crossmatch in kidney transplantation

BACKGROUND

The Luminex Single-Antigen Beads (LSA) assay allows an accurate detection and characterization of preexisting donor-specific antibodies (DSA) in kidney transplant candidates. But the ability of LSA to detect quite low levels of antibodies makes it hard to correctly predict crossmatch results in donor selection. In this study we retrospectively analyzed the accuracy of our virtual crossmatch (v-XM) protocol, which was used for selection of potential kidney transplant recipients, in predicting the results of actual crossmatch (a-XM) in cadaver-donor renal transplantation. We also investigated correlation between negative a-XM results and strength/specificity of preformed DSA.

METHODS

The correlation between negative v-XMs and a-XMs performed in 2007-2012 at the Regional Transplant Center of the Lazio Region, Italy, was analyzed. In carrying out v-XM, the donor HLA molecules against which patients showed LSA-detected DSA with normalized mean fluorescence intensity (MFI)≥5,000 were considered to be "unacceptable DSA," and LSA-DSA showing MFI<5,000 were defined as "acceptable DSA." All cadaver donors had been typed for HLA-A, -B, -DR, and -DQB molecules by sequence-specific primer methods. On the basis of a negative v-XM, we performed 507 a-XMs between serum samples from 256 renal transplant candidates and T/B lymphocytes from 302 cadaver donors with the use of both complement-dependent cytotoxicity (CDC) and flow cytometry (FC) methods.

RESULTS

The v-XM negative results showed good correlation with both CDC and FC a-XMs (97% and 90%, respectively). The sensitivity of v-XM was 100%; this high value was related to the lack of false-negative DSA results. The limited specificity with both techniques (CDC-XM, 74%; FC-XM, 79%) was due to the presence of "acceptable" and/or anti-DQA/DPB DSA in some patient sera used to perform the a-XMs. During the study period, 171 (67%) of the 256 sensitized patients received a kidney transplant: 30% of these had "acceptable DSA" and/or anti-DQA/DPB DSA. No antibody-mediated rejection due to preformed HLA-DSA was observed.

CONCLUSIONS

Our v-XM protocol showed high sensitivity in predicting donor-recipient immunologic compatibility. The results of this study also demonstrated the importance of evaluating DSA strength for implementing v-XM results in the selection of kidney transplant recipients. Moreover, the finding of anti-DQA/DPB DSA, especially in serum samples that gave positive results with the use of both CDC and FC a-XMs, highlights the importance of defining all of the donor HLA molecules to perform an accurate v-XM.

Kidney paired donation: principles, protocols and programs

Due to the ongoing shortage of deceased-donor organs, novel strategies to augment kidney transplantation rates through expanded living donation strategies have become essential. These include desensitization in antibody-incompatible transplants and kidney paired donation (KPD) programs. KPD enables kidney transplant candidates with willing but incompatible living donors to join a registry of other incompatible pairs in order to find potentially compatible transplant solutions. Given the significant immunologic barriers with fewer donor options, single-center or small KPD programs may be less successful in transplanting the more sensitized patients; the optimal solution for the difficult-to-match patient is access to more potential donors and large multicenter or national registries are essential. Multicenter KPD programs have become common in the last decade, and now represent one of the most promising opportunities to improve transplant rates. To maximize donor-recipient matching, and minimize immunologic risk, these multicenter KPD programs use sophisticated algorithms to identify optimal match potential, with simultaneous two-, three- or more complex multiway exchanges. The article focuses on the recent progresses in KPD and it also reviews some of the differences and commonalities across four different national KPD programs.

Screening panel-reactive antibody negative, single-antigen positive: a case report

Sensitized patients remain a challenge for successful transplant. Virtual crossmatch is used to determine the presence or absence of donor-specific antibodies. A 60-year-old woman with a negative screening for panel-reactive antibodies (PRA) received an A*11, A*68 type kidney with a negative anti-human globulin/complement-dependent cytotoxicity (AHG-CDC) crossmatch. Her transplant course was complicated by delayed graft function, and she required hemodialysis. On day 8 after receiving the transplant, she had a kidney biopsy that showed features of antibody-mediated rejection/severe acute tubular necrosis, which was treated by plasmapheresis for 5 sessions and intravenous immunoglobulin (2 g/kg). Her serum level of creatinine decreased from 6.7 to 3.6 mg/dL (600-320 μmol/L). Panel-reactive antibody by Luminex was repeated and again was negative. Single-antigen detection was tried next. Surprisingly, A*11:02 came up positive with a mean fluorescence intensity of 9500. High-resolution donor HLA type was A*68:01 and A*11:01. A*11:02 is not part of the screening Luminex PRA whereas the 11:01 allele is. Serologically, HLA-A11 has 2 defined splits, A11.1 and A11.2, which encode A*11:01 and A*11:02, respectively. In this case, the A*11:02 antibody does not seem to be responsible for the increasing creatinine level. However, if the donor had been A*11:02, a humeral rejection would have occurred and been missed by a virtual crossmatch. Thus virtual crossmatch may not work at all times. Screening for PRA by single antigens is suggested even in PRA-negative cases, if only virtual crossmatch is to be used.

Shared molecular eplet stimulates acute antibody-mediated rejection in a kidney transplant recipient with low-level donor-specific antibodies: a case report

HLA antibodies usually recognize epitopes rather than antigens. This case report reveals that acute antibody-mediated rejection (AMR) that occurred in a kidney transplant recipient with low-level donor-specific antibodies (DSAs) could be explained by shared epitope. A 39-year-old woman received a first kidney transplant from a deceased donor (HLA-DRB1 11:06, 12:02, DRB3 02:02, 03:01). She developed acute AMR confirmed by kidney biopsy on day 4 after transplantation. Antibody testing with pretransplant serum showed anti-DR11 DSA below cutoff level (mean fluorescence intensity [MFI], 702; cutoff >1,000). However, high-level DSAs were detected on day 5 after transplantation (anti-DR11 MFI, 8,531; anti-DR12 MFI, 3,146). We hypothesized that the sharp rise in DSA levels was a result of anamnestic response with donor-antigen sensitization that occurred during pregnancy. High-resolution HLA-DR typing of her husband showed HLA-DRB1 03:01, 15:02:01, DRB3 02:02, DRB5 01:02. No sharing between donor HLAs eliciting reactive antibodies and her husband's HLAs was detected. Nevertheless, we speculated that shared epitope, not antigen, was the cause of allosensitization. To identify the shared epitope recognized by patient's antibodies, we used HLAmatchmaker, a computer algorithm that considers small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes for analysis. The results showed that 149H, which was the eplet shared by HLA-DRB1 03:01 (from her husband) and DRB1 11:06, DRB1 12:02, DRB3 03:01 (from donor), was the most prevalent eplet on DRB1 reactive alleles in Luminex assay. In conclusion, pretransplant low-level DSAs can induce AMR early after transplantation as a result of shared epitopes with a previous immunizer.

Concordance and discordance in anti-HLA antibody testing

BACKGROUND

Correct identification of the specificity of antibodies directed against HLA using single antigen Luminex beads (SALB) is essential in current HLA laboratory practice for transplantation. The aim of this study was to investigate the magnitude of concordance and discordance among laboratories in testing for anti-HLA antibodies using SALB.

METHOD

35 sera were distributed by the ASHI Proficiency Testing Program to HLA laboratories worldwide. We analyzed 4335 test results submitted between April 2010 and April 2013 by participating laboratories.

RESULTS

SALB was used by approximately 94% of the participating laboratories, yet concordant assignment of antibody specificity was imperfect. For each serum, the assignment of an average of 10 antibody specificities was discordant. Disagreement was observed for antibodies directed against common as well as uncommon antigens. The assignment of an average of 15 antibody specificities in each "positive" serum appeared to be influenced by vendor-dependent causes. Inter-vendor concordance was lower than intra-vendor concordance, indicating that vendor dependent factors may be a central cause for disagreement.

CONCLUSIONS

Our study illustrates the prevalence of concordance and discordance, also affected by unpremeditated causes, in reporting SALB antibody results. Insufficient concordance and standardization in antibody testing may have practical implications for organ allocation and organ sharing programs.

Human leukocyte antigens and alloimmunization in heart transplantation: an open debate

Considerable advances in heart transplantation outcome have been achieved through the improvement of donor-recipient selection, better organ preservation, lower rates of perioperative mortality and the use of innovative immunosuppressive protocols. Nevertheless, long-term survival is still influenced by late complications. We support the introduction of HLA matching as an additional criterion in the heart allocation. Indeed, allosensitization is an important factor affecting heart transplantation and the presence of anti-HLA antibodies causes an increased risk of antibody-mediated rejection and graft failure. On the other hand, the rate of heart-immunized patients awaiting transplantation is steadily increasing due to the limited availability of organs and an increased use of ventricular assist devices. Significant benefits may result from virtual crossmatch approach that prevents transplantation in the presence of unacceptable donor antigens. A combination of both virtual crossmatch and a tailored desensitization therapy could be a good compromise for a favorable outcome in highly sensitized patients. Here, we discuss the unresolved issue on the clinical immunology of heart transplantation.

Immune responses to collagen-IV and fibronectin in renal transplant recipients with transplant glomerulopathy

Antibodies (Abs) to donor HLA (donor-specific antibodies [DSA]) have been associated with transplant glomerulopathy (TG) following kidney transplantation (KTx). Immune responses to tissue-restricted self-antigens (self-Ags) have been proposed to play a role in chronic rejection. We determined whether KTx with TG have immune responses to self-Ags, Collagen-IV (Col-IV) and fibronectin (FN). DSA were determined by solid phase assay, Abs against Col-IV and FN by enzyme-linked immunosorbent assay and CD4+ T cells secreting interferon gamma (IFN-γ), IL-17 or IL-10 by ELISPOT. Development of Abs to self-Ags following KTx increased the risk for TG with an odds ratio of 22 (p-value = 0.001). Abs to self-Ags were IgG and IgM isotypes. Pretransplant Abs to self-Ags increased the risk of TG (22% vs. 10%, p < 0.05). Abs to self-Ags were identified frequently in KTx with DSA. TG patients demonstrated increased Col-IV and FN specific CD4+ T cells secreting IFN-γ and IL-17 with reduction in IL-10. We conclude that development of Abs to self-Ags is a risk factor and having both DSA and Abs to self-Ags increases the risk for TG. The increased frequency of self-Ag-specific IFN-γ and IL-17 cells with reduction in IL-10 demonstrate tolerance breakdown to self-Ags which we propose play a role in the pathogenesis of TG.

Role of defensins in the pathogenesis of chronic lung allograft rejection

Chronic rejection predominantly manifested as bronchiolitis obliterans syndrome (BOS), still remains a major problem affecting long-term outcomes in human lung transplantation (LTx). Donor specific antibodies (DSA) and infiltration of neutrophils in the graft have been associated with the development of BOS. This study determines the role of defensins, produced by neutrophils, and its interaction with α-1-antitrypsin (AAT) towards induction of airway inflammation and fibrosis which are characteristic hallmarks of BOS. Bronchoalveolar lavage (BAL) and serum from LTx recipients, BOS+ (n=28), BOS- (n=26) and normal healthy controls (n=24) were analyzed. Our results show that BOS+ LTx recipients had higher α-defensins (HNP1-3) and β-defensin2 HBD2 concentration in BAL and serum compared to BOS-DSA-recipients and normal controls (p=0.03). BOS+ patients had significantly lower serum AAT along with higher circulating concentration of HNP-AAT complexes in BAL (p=0.05). Stimulation of primary small airway epithelial cells (SAECs) with HNPs induced expression of HBD2, adhesion molecules (ICAM and VCAM), cytokines (IL-6, IL-1β, IL-13, IL-8 and MCP-1) and growth-factor (VEGF and EGF). In contrast, anti-inflammatory cytokine, IL-10 expression decreased 2-fold (p=0.002). HNPs mediated SAEC activation was completely abrogated by AAT. In conclusion, our results demonstrates that neutrophil secretory product, α-defensins, stimulate β-defensin production by SAECs causing upregulation of pro-inflammatory and pro-fibrotic signaling molecules. Hence, chronic stimulation of airway epithelial cells by defensins can lead to inflammation and fibrosis the central events in the development of BOS following LTx.

Incidence and clinical significance of de novo donor specific antibodies after kidney transplantation

Kidney transplantation has evolved over more than half a century and remarkable progress has been made in patient and graft outcomes. Despite these advances, chronic allograft dysfunction remains a major problem. Among other reasons, de novo formation of antibodies against donor human leukocyte antigens has been recognized as one of the major risk factors for reduced allograft survival. The type of treatment in the presence of donor specific antibodies (DSA) posttransplantation is largely related to the clinical syndrome the patient presents with at the time of detection. There is no consensus regarding the treatment of stable renal transplant recipients with circulating de novo DSA. On the contrast, in acute or chronic allograft dysfunction transplant centers use various protocols in order to reduce the amount of circulating DSA and achieve long-term graft survival. These protocols include removal of the antibodies by plasmapheresis, intravenous administration of immunoglobulin, or depletion of B cells with anti-CD20 monoclonal antibodies along with tacrolimus and mycophenolate mofetil. This review aims at the comprehension of the clinical correlations of de novo DSA in kidney transplant recipients, assessment of their prognostic value, and providing insights into the management of these patients.

Pre-transplant antibodies to Kα1 tubulin and collagen-V in lung transplantation: clinical correlations

BACKGROUND

Immune responses to lung-associated self-antigens (SAgs) have been implicated in chronic lung allograft rejection. The goals of this study were to determine the prevalence of pre-existing antibodies (Abs) to the SAgs in pulmonary diseases and the association between pre-existing Abs to SAgs and the development of primary graft dysfunction (PGD), donor-specific antibodies (DSA), and chronic rejection.

METHODS

Pre- and post-transplant sera were analyzed from 317 lung transplant (LTx) recipients between 2000 and 2011 with diagnosis of chronic obstructive disease (n = 161), idiopathic pulmonary fibrosis (IPF; n = 50), cystic fibrosis (CF; n = 55), and others (n = 51). Samples were analyzed for Abs to SAgs by enzyme-linked immunosorbent assay, and DSA and cytokines by Luminex. The clinical diagnosis of PGD and bronchiolitis obliterans syndrome (BOS) was based on International Society for Heart and Lung Transplantation guidelines.

RESULTS

The overall prevalence of Abs to SAgs was 22.71%, including 18% in chronic obstructive pulmonary disease (p = 0.033), 34% in IPF (p = 0.0006), 29% in CF (p = 0.0023), and 19.6% in other diagnoses (p = 0.044). The incidence of PGD (88% vs 54%, p < 0.05), DSA (70% vs 45%, p < 0.01), and BOS (90% vs 38% (p < 0.001) after LTx was significantly higher in patients with pre-LTx Abs to SAgs than without. Pro-inflammatory cytokines (interleukin-1β, interleukin-17, and interferon-γ) were elevated in patients who had pre-LTx Abs to SAgs, along with a reduction in anti-inflammatory interleukin-10.

CONCLUSIONS

Patients with IPF and CF have the highest prevalence of Abs to SAgs. Patients with pre-existing Abs to SAgs are at increased risk for development of PGD, DSA, and BOS. Strategies to remove pre-existing Abs to SAgs should be considered to improve lung allograft outcome.

Anti-HLA-A, -B, -DR, -DQB1 and -DQA1 antibodies reactive epitope determination with HLAMatchmaker in multipare awaiting list for heart transplant

Human leukocyte antigen (HLA) antibodies represent a significant risk factor for transplant failure. It is very important to characterize anti-HLA antibodies as epitopes rather than antigens so that this knowledge can be applied clinically. The aim of the study was to investigate the extra reactivity patterns in sensitized multipare. Here, we have used the HLAMatchmaker program, a theoretical algorithm, to explain these unexpected antibody reactivity patterns in multipare awaiting for heart transplant. The patient was sensitized during pregnancy by alleles HLA-A(*)24:02, HLA-DRB1(*)07:01, HLA-DRB4(*)01:01, DQB1(*)02:02 and DQA1(*)02:01 mismatches with development of respective antibodies. However, the patient' sera were shown an unexpected reactivity not directed toward HLA mismatches of daughters: A(∗)23:01, A(*)24:03 and B(*)15:12 for class I and DRB4(*)01:03, DRB1(*)09:02, DRB1(*)09:01, DQB1(*)03:01, DQB1(*)03:03, DQB1(*)03:02, DQB1(*)04:02, DQB1(*)04:01 and DQB1(*)02:01 for class II. By HLAMatchmaker analysis we found that these antibodies reacted with eplet shared by antigens in single allele Luminex panels. These eplets were: 62EE, 66GKH, 70KAH, 71HS, 127K, 113YH, 144KR, 150AAH, 151AHV, 163TG and 167DG for class I and 4Q, 74RRAE, 71RRA, 98KN, 120N, and 135G, 25FT, 34HE, 41ER, 47EK2, 48LF for class II. Thus, HLAMatchmaker software together with to solid phase techniques could open new horizons for a more precise characterization of the HLA-antibodies.

Transnational validation of the Australian algorithm for virtual crossmatch allocation in kidney paired donation

An independent pool of 16 incompatible live donor-recipient pairs registered at the Vienna transplant unit was applied to test whether virtual crossmatch allocation used in the Australian kidney paired donation (KPD) program reliably predicts negative crossmatches. High resolution HLA data were entered into the computer-matching algorithm and allocation was performed excluding any DSA>2000MFI. CDC and flow crossmatch data of recipients against any of the donors were available for 112 crossmatch combinations. The computer program identified 19 possible pairings in 2-way or 3-way chains in multiple combinations. The top ranked combination included one 3-way and two 2-way ABO-compatible chains. Where crossmatches were available all recipients were CDC crossmatch negative with the computer-matched donor. Excluding allocation of KPD donors in the presence of DSA>2000MFI had a negative predictive of 99.9% for CDC and 96.4% for flow crossmatch. In the 12 pairings with ⩾1 DSA against crossmatched donors there was a negative CDC and flow crossmatch. These results show excellent correlation between matching using virtual crossmatch and actual crossmatch results. Using the 2000MFI cut-off the number of potentially unacceptable CDC and flow crossmatch positive pairings identified by virtual crossmatching is low, but some potential crossmatch negative pairings are missed.

Donor-specific antibodies adversely affect kidney allograft outcomes

The effect of low titers of donor-specific antibodies (DSAs) detected only by sensitive solid-phase assays (SPAs) on renal transplant outcomes is unclear. We report the results of a systematic review and meta-analysis of rejection rates and graft outcomes for renal transplant recipients with such preformed DSAs, defined by positive results on SPA but negative complement-dependent cytotoxicity and flow cytometry crossmatch results. Our search identified seven retrospective cohort studies comprising a total of 1119 patients, including 145 with isolated DSA-SPA. Together, these studies suggest that the presence of DSA-SPA, despite a negative flow cytometry crossmatch result, nearly doubles the risk for antibody-mediated rejection (relative risk [RR], 1.98; 95% confidence interval [CI], 1.36-2.89; P<0.001) and increases the risk for graft failure by 76% (RR, 1.76; 95% CI, 1.13-2.74; P=0.01). These results suggest that donor selection should consider the presence of antibodies in the recipient, identified by the SPA, even in the presence of a negative flow cytometry crossmatch result.

HLA antibody detection and characterization by solid phase immunoassays: methods and pitfalls

Solid phase immunoassays for the detection and characterization of HLA-specific antibodies provide greatly increased sensitivity, specificity, and time and reagent efficiency, compared to the traditionally used cell-based methods. Testing is performed using commercially available test kits. The assays are of two general types: enzyme-linked immunosorbent assays and multianalyte bead. The types vary in both sensitivity and equipment requirements.While these assays afford great improvement over the cell-based assays, they can be confounded by interference from substances within the serum that result in high background reactivity. The high sensitivity of the assays also makes them more susceptible to environmental factors and operator variability. The user must be aware of the capabilities of the various formats, the factors that can affect test results, and lot to lot variability of any single product. Knowledge of the characteristics of each product and thorough and accurate analysis of the results are essential to the utility of these assays.

An important role for autoimmunity in the immunopathogenesis of chronic allograft rejection

Organ transplantation is the treatment of choice for patients with end-stage organ dysfunction. In spite of advances in understanding of donor and recipient physiology, organ preservation, operative techniques and immunosuppression, long-term graft survival still remains a major problem primarily due to chronic rejection. Alloimmune responses to mismatched major histocompatibility antigens have been implicated as an important factor leading to rejection. However, there is increasing evidence pointing towards cross-talk between the alloimmune and autoimmune responses creating a local inflammatory milieu, which eventually leads to fibrosis and occlusion of the lumen in the transplanted organ i.e. chronic rejection. In this review, we will discuss recent studies and emerging concepts for the interdependence of alloimmune and autoimmune responses in the immunopathogenesis of chronic allograft rejection. The role of autoimmunity in the development of chronic rejection is an intriguing and exciting area of research in the field of solid-organ transplantation with a significant potential to develop novel therapeutic strategies towards preventing chronic allograft rejection.