Reassessment of T Lymphocytes Crossmatches Results Prediction With Luminex Class I Single Antigen Flow Beads Assay

25-hydroxyvitamin D sufficiency is associated with lower de novo anti-HLA donor specific antibody and better kidney transplant outcomes

T-cell mediated rejection (TCMR), de novo anti-HLA donor-specific antibodies (dnDSAs) and ensuing antibody-mediated rejection (ABMR) reduce kidney transplantation (KT) survival. The immunomodulatory effects of 25-hydroxyvitamin D [25(OH)D] could be beneficial for KT outcomes. We aimed to evaluating the association between 25(OH)D levels, the development of dnDSAs, clinical TCMR and ABMR, and graft survival. This single center retrospective study included 253 KT recipients (KTRs) transplanted without preformed DSA between 2010 and 2013. We measured 25(OH)D in successive serum samples: at KT (M0) and M12 for the entire cohort, and additionally at M24 and/or M36 when sera were available. We assessed graft outcomes up to 5 years post-KT. The proportion of KTRs having sufficient 25(OH)D at KT (M0) was high (81.4%) and then dropped at M12 (71.1%). KTRs with sufficient 25(OH)D at M0 experienced less clinical TCMR (HR, 0.41; 95% CI, 0.19-0.88 in multivariate analysis). A sufficient 25(OH)D at M12 was independently associated with a longer dnDSA-free survival (HR, 0.34; 95% CI, 0.17-0.69). There was no association between 25(OH)D and clinical AMBR. Studying the KTRs with 25(OH)D measurements at M12, M24 and M36 (n = 203), we showed that 25(OH)D sufficiency over the 3 first-years post-KT was associated with a longer graft survival in multivariate analyses (HR, 0.39; 95% CI, 0.22-0.70). To our knowledge, this study is the first showing an association between 25(OH)D sufficiency post-KT and dnDSA occurrence in KTRs. Moreover, we reinforce previously published data showing an association between 25(OH)D, TCMR and graft survival in KT.

Imlifidase for Kidney Transplantation of Highly Sensitized Patients With a Positive Crossmatch: The French Consensus Guidelines

Imlifidase recently received early access authorization for highly sensitized adult kidney transplant candidates with a positive crossmatch against an ABO-compatible deceased donor. These French consensus guidelines have been generated by an expert working group, in order to homogenize patient selection, associated treatments and follow-up. This initiative is part of an international effort to analyze properly the benefits and tolerance of this new costly treatment in real-life. Eligible patients must meet the following screening criteria: cPRA ≥ 98%, ≤ 65-year of age, ≥ 3 years on the waiting list, and a low risk of biopsy-related complications. The final decision to use Imlifidase will be based on the two following criteria. First, the results of a virtual crossmatch on recent serum, which shall show a MFI for the immunodominant donor-specific antibodies (DSA) > 6,000 but the value of which does not exceed 5,000 after 1:10 dilution. Second, the post-Imlifidase complement-dependent cytotoxicity crossmatch must be negative. Patients treated with Imlifidase will receive an immunosuppressive regimen based on steroids, rATG, high dose IVIg, rituximab, tacrolimus and mycophenolic acid. Frequent post-transplant testing for DSA and systematic surveillance kidney biopsies are highly recommended to monitor post-transplant DSA rebound and subclinical rejection.

Stability of Anti-HLA Sensitization Profiles in Highly Sensitized Kidney Transplantation Candidates: Toward a Rational Serological Testing Strategy

BACKGROUND

Highly sensitized (HS) anti-HLA patients awaiting kidney transplantation benefit from specific allocation programs. Serological monitoring at 3-mo intervals is recommended to prevent unexpected positive crossmatch (XM), but this strategy is not evidence-based. Therefore, we assessed its relevance when using single-antigen flow bead (SAFB) and screening flow bead (SFB) assays.

METHODS

We included 166 HS patients awaiting a transplant and assessed their SAFB profile during the year preceding their inclusion. Anti-HLA antibodies were evaluated by SAFB assay and compared within patients as serum pairs at 3, 6, and 9 mo. We assessed the performance of SFB for detecting changes in SAFB profiles with 35 serum pairs.

RESULTS

On comparing 354, 218, and 107 serum pairs at 3, 6, and 9 mo, respectively, only 0.6%, 0.7%, and 1% of all antigens tested exceeded for the first time the unacceptable antigen threshold (mean fluorescence intensity ≥2000) in the most recent sample. Irrespective of the follow-up period, the calculated panel-reactive antibodies increased by a mean of 1%, and there was no significant increase in the proportion of donors at risk for positivity of flow- or complement-dependent cytotoxicity XM. The SFB did not accurately detect the variations of SAFB profiles.

CONCLUSIONS

Changes in HS patient profiles are anecdotal and show little association with transplant access or risk for positive XM. Less-frequent monitoring in HS patients should be considered to improve cost-effectiveness without affecting transplant safety.

The clinical utility and thresholds of Virtual and Halifaster Flow crossmatches in lung transplantation

INTRODUCTION

Immune sensitization, defined as the presence of alloreactive donor-specific antibodies (DSA), is associated with increased wait-times and inferior transplant outcomes. Identifying pre-transplant DSA with a physical cell-based assay is critical in defining immunological risk. However, improved solid phase antibody detection has provided the potential to forgo this physical assay. Here, we evaluated the association between DSA mean fluorescence intensity (MFI) and the recently introduced Halifaster Flow cytometry crossmatch (FXM) to determine if MFI could predict the outcome of FXM and whether a virtual crossmatch (VXM) would provide an accurate risk assessment.

METHODS

Sera from 134 waitlisted lung patients was retrospectively assessed by Halifaster FXM against lymphocytes preparations from 32 donors, resulting in 265 FXMs. HLA typing was performed to 2-field allelic level and Luminex single antigen beads (SAB) used to identify DSA. The association between FXM and Luminex MFI was calculated using ROC analysis. MFI threshold accuracy was confirmed using a separate validation cohort (174 recipient sera and 34 donors), whereby both virtual crossmatch (VXM) and FXMs were compared.

RESULTS

From the 265 FXM performed, 48 (18%) T-cell (TFXM) and 56 (21%) B-cell (BFXM) were positive. In the evaluation cohort, MFI thresholds of 2000 for HLA-A, B, DRB1 and >4000 for DQB1, were predictive of a positive FXM. The validation cohort of 233 paired FXM and VXM confirmed these MFI thresholds for both TFXM and BFXM with an accuracy of 91.4% and 89.3% respectively.

CONCLUSION

A positive VXM, defined with HLA-specific MFI thresholds predicts Halifaster FXM reactivity, and can potentially expedite organ allocation, by minimising the need for the more time-consuming flow cytometry crossmatch. This article is protected by copyright. All rights reserved.

Optimal donation of kidney transplants after controlled circulatory death

Controlled donation after circulatory death (cDCD) is used for "extended criteria" donors with poorer kidney transplant outcomes. The French cDCD program started in 2015 and is characterized by normothermic regional perfusion, hypothermic machine perfusion, and short cold ischemia time. We compared the outcomes of kidney transplantation from cDCD and brain-dead (DBD) donors, matching cDCD and DBD kidney transplants by propensity scoring for donor and recipient characteristics. The matching process retained 442 of 499 cDCD and 809 of 6185 DBD transplantations. The DGF rate was 20% in cDCD recipients compared with 28% in DBD recipients (adjusted relative risk [aRR], 1.43; 95% confidence interval [CI] 1.12-1.82). When DBD transplants were ranked by cold ischemia time and machine perfusion use and compared with cDCD transplants, the aRR of DGF was higher for DBD transplants without machine perfusion, regardless of the cold ischemia time (aRR with cold ischemia time <18 h, 1.57; 95% CI 1.20-2.03, vs aRR with cold ischemia time ≥18 h, 1.79; 95% CI 1.31-2.44). The 1-year graft survival rate was similar in both groups. Early outcome was better for kidney transplants from cDCD than from matched DBD transplants with this French protocol.

Intensity of de novo DSA detected by Immucor Lifecodes assay and C3d fixing antibodies are not predictive of subclinical ABMR after Kidney Transplantation

De novo donor-specific antibodies (dnDSA) are associated with antibody-mediated rejection (ABMR) and allograft loss. We tested Immucor* (IM) Luminex Single-antigen beads (LSAB) assay and C3d-fixing antibodies in the setting of dnDSA and subclinical (s) ABMR. This retrospective multicentric study included 123 patients biopsied because of the presence of subclinical de novo DSA detected by One Lamda* Labscreen (MFI > 1000). In 112 patients, sera of the day of the biopsy were available and tested in a central lab with IM Lifecodes LSAB and C3d fixing antibodies assays. In 16 patients (14.3%), no DSA was detected using Immucor test. In 96 patients, at least one DSA was determined with IM. Systematic biopsies showed active sABMR in 30 patients (31.2%), chronic active sABMR in 17 patients (17.7%) and no lesions of sABMR in 49 KT recipients (51%). Intensitity criteria (BCM, BCR and AD-BCR) of DSA were not statistically different between these 3 histological groups. The proportion of patients with C3d-fixing DSA was not statistically different between the 3 groups and did not offer any prognostic value regarding graft survival. Performing biopsy for dnDSA could not be guided by the intensity criteria of IM LSAB assay. C3d-fixing DSA do not offer added value.

Stratifying the humoral risk of candidates to a solid organ transplantation: a proposal of the ENGAGE working group

Detection of circulating antibodies directed against human leukocyte antigen (HLA) molecules, which corresponds to the current definition of 'sensitized patient', has been shown to have a severe impact on both access to transplantation and, if the anti-HLA antibodies are specific to the selected donor, survival of the graft. However, not all donor-specific antibodies (DSA) are equally harmful to the graft and progress in the understanding of humoral memory has led to the conclusion that absence of DSA at transplantation does not rule out the possibility that the patient has a preformed cellular humoral memory against the graft (thereby defining a category of DSA-negative sensitized recipients). Technological progress has led to the generation of new assays that offer unprecedented precision in exploring the different layers (serological and cellular) of alloimmune humoral memory. Based on this recent knowledge, the EuropeaN Guidelines for the mAnagement of Graft rEcipients (ENGAGE) working group to propose an updated definition of sensitization in candidates for solid organ transplantation - one that moves away from the current binary division towards a definition based on homogenous strata with similar humoral risk.

The incidence of post-transplant malignancies in kidney transplant recipients treated with Rituximab

BACKGROUND

Rituximab has been proposed as induction therapy in kidney transplant recipients (KTRs) with preformed donor-specific antibodies (DSA) or a positive flow cross-match. We here evaluated whether adding rituximab was associated with a higher incidence of post-transplant malignancies (PTM) due to greater immunosuppression.

PATIENTS AND METHODS

Forty-eight HLA-sensitized KTRs received induction therapy with anti-thymocyte globulin (ATG) and rituximab because of preformed DSA or a positive flow cross-match (RTX group). They were compared with a control group of 154 patients receiving ATG alone.

RESULTS

Thirty-nine of 202 (19.3%) patients developed PTM; the rate was similar in the RTX and no-RTX groups (14.6% vs. 20.8%, respectively, P = .3). The distributions of the types of cancer were similar between the two groups, with the majority being non-melanoma skin cancer (NMSC, n = 24). The risk factors for PTM were male gender, age, history of cancer, and azathioprine.

CONCLUSION

Our data do not indicate a higher rate of post-transplantation de novo malignancies after kidney transplantation in high-immunological risk patients who received induction therapy based on ATG and rituximab.

Efficacy of plasmapheresis and semi-selective immunoadsorption for removal of anti-HLA antibodies

BACKGROUND

In organ transplantation, apheresis is frequently used for removal of anti-HLA antibodies. However, it is unclear whether plasmapheresis (PP) or semi-selective immunoadsorption (IA) should be employed, and the optimal number of apheresis sessions required to reach post-treatment objectives is also unknown.

METHODS

We enrolled 43 patients from Bordeaux University Hospital who were treated with PP (n = 29) or IA (n = 14) for antibody-mediated rejection or pre-transplant desensitization. Using Luminex single-antigen flow beads, we assessed the initial mean fluorescence intensity (MFI) of 1416 positive beads with MFIs obtained after 7 to 8 apheresis sessions (extended protocol) and, if a serum was available, after the first four sessions (short protocol).

RESULTS

MFI reduction after extended apheresis protocol was stronger with IA [87% (61%-100%)] than with PP [73% (22%-100%)] (P < .001). Indeed, 59% of the beads had a final MFI < 2000 with IA, whereas only 38% with PP (P < .001). The efficacy of removal depended on initial MFI but not on HLA specificity. A short protocol of apheresis showed excellent results without superiority of IA over PP for antibodies with an initial MFI < 3000. For antibodies showing MFI ≥2000 after four sessions, the residual MFI predicted the effectiveness of four additional sessions.

CONCLUSION

Monitoring the MFI of anti-HLA antibodies before and during apheresis protocol can guide physicians in the selection of apheresis technique and the number of sessions to be performed.

Detection of antibodies to denatured human leucocyte antigen molecules by single antigen Luminex

The anti-HLA antibody detection has been improved in sensitivity and specificity with solid-phase antigen bead (SAB) assays based on Luminex. However, false positive results due to denatured HLA (dHLA) may arise after single antigen test. The aim of this study was to compare the performance of the two Luminex technology-based anti-HLA detection kits available in the market in showing undesired anti-HLA antibody results. A prospective cohort was assessed for anti-HLA antibodies with single antigen A manufacturer (AM) kit and a comparison cohort with single antigen B manufacturer (BM) kit. A total of 11 out of 90 patients in a prospective cohort presented monospecific HLA-I antibodies with AM, and 5 out of 11 confirmed monospecific reaction with BM. Despite the confirmation of monospecific reaction with both manufacturers, 80% were assigned as dHLA reaction by specific crossmatch. Further comparative cohorts detected four out of six monospecific reactions with BM that were confirmed as possible dHLA reactions. A positive SAB test should rule out a reaction against a dHLA molecule, thus avoidance of prolonged waitlist periods or misattribution of anti-HLA reactions after transplantation.

Clinical relevance of donor-specific antibodies directed at HLA-C: A long road to acceptance

In solid organ transplantation (SOT), the clinical relevance of donor-specific antibodies (DSA) directed at anti-HLA-A, -B, -DR and -DQ antigens is largely recognized while it is still a matter of debate for DSA directed at HLA-C. In this review, we summarize the peculiarities of HLA-C among class I HLA antigens as well as their immunogenicity, which underlie the clinical relevance of HLA-C locus and anti-HLA-C DSA in SOT. Many factors, both intrinsic and extrinsic to the HLA-C gene and HLA-C protein, explain its lower expression in comparison with HLA-A and -B. This lower expression can explain the apparent lower immunogenicity of HLA-C leading to a lower prevalence and strength of anti-HLA-C antibodies. Nevertheless, HLA-C antigens are truly immunogenic and preformed anti-HLA-C DSA are clinically relevant. Indeed, anti-HLA-C DSA are able to bind donor cells and to activate the complement pathway both ex vivo and in vivo. In line with this, numerous clinical studies now show that preformed DSA directed at native HLA-C molecules induce poorer graft outcomes. We then plead for the inclusion of HLA-C in all transplant allocation systems and we propose a strategy to cope with anti-HLA-C DSA in SOT. Beyond SOT, anti-HLA-C antibodies generate a growing interest in the allo-HCT, transfusion and obstetrics fields, while new concepts such as the role of the "missing-self" in solid organ rejection places HLA-C as an inescapable actor in transplant tolerance.

Monoclonal secondary reagents do not outperform polyclonal secondary reagents for detection of anti-HLA IgG using single antigen flow beads assays

Luminex single antigen flow beads (SAFB) assays are used to monitor anti-HLA antibodies, which are associated with allograft loss. The primary antibody binding level is evaluated by the mean fluorescence intensity (MFI) provided by the tracing secondary antibody. The aim of this study was to compare the MFI obtained with two secondary antibodies and to evaluate their sensitivity to detect newly developed de novo anti-HLA donor-specific antibodies (dnDSA). A total of 23 and 46 different sera from 20 HLA-sensitized kidney transplant recipients were tested with class I and II SAFB, respectively, as well as sera from 17 patients with dnDSA. The conventional secondary antibody (IgHPolyFab) was the phycoerythrin (PE)-conjugated goat anti-human IgG constant heavy chain (HC)-binding polyclonal-F(ab')2. The alternate secondary antibody was a PE-conjugated anti-human IgG Fc-specific monoclonal IgG (FcMonoIgG). MFI of negative control sera were drastically lower with the FcMonoIgG than with IgHPolyFab, requesting a unique positivity threshold to be defined for each secondary antibody. MFI obtained with positive control sera were also lower with FcMonoIgG. In kidney transplant recipient's sera, the number of antigenic specificities detected by each secondary antibody was comparable, for dnDSA as well as for nonDSA. In conclusion, decision thresholds must be refined when changing the secondary tracing reagent in SAFB assays. FcMonoIgG did not demonstrate any superiority over IgHPolyFab for monitoring HLA antibodies.

Reassessment of the clinical impact of preformed donor-specific anti-HLA-Cw antibodies in kidney transplantation

Anti-denatured HLA-Cw antibodies are highly prevalent, whereas anti-native HLA-Cw antibodies seem to lead to random flow cytometry crossmatch results. We aimed to reassess crossmatch prediction for anti-HLA-Cw using 2 types of single antigen flow beads (classical beads and beads with diminished expression of denatured HLA), and to compare the pathogenicity of preformed anti-denatured and anti-native HLA-Cw antibodies in kidney transplantation. We performed 135 crossmatches with sera reacting against donor HLA-Cw (classical beads fluorescence ≥500); only 20.6% were positive. Forty-three (31.6%) were anti-denatured HLA antibodies (beads with diminished expression of denatured HLA fluorescence <300); all were crossmatch negative. The correlation between classical beads fluorescence and the crossmatch ratio was low (ρ = 0.178), and slightly higher with beads with diminished expression of denatured HLA (ρ = 0.289). We studied 52 kidney recipients with preformed anti-HLA-Cw donor-specific antibodies. Those with anti-native HLA antibodies experienced more acute and chronic antibody-mediated rejections (P = .006 and .03, respectively), and displayed a lower graft survival (P = .04). Patients with anti-native HLA-Cw antibodies more frequently had previous sensitizing events (P < .000001) or plausibility of their antibody profile according to known anti-native HLA-Cw eplets (P = .0001). Anti-native but not anti-denatured HLA-Cw antibodies are deleterious, which underscores the need for reagents with diminished expression of denatured HLA.

HLA expression levels of unstimulated and cytokine stimulated human umbilical vein endothelial cells

Transplant rejection occurs following recipient recognition of mismatched HLA on donor tissue, but active rejection is dependent not only upon the severity of the T cell or alloantibody response, but also upon the cell surface expression of target HLA molecules. To investigate the variation in HLA expression using a model of endothelium, human umbilical vein endothelial cell (HUVEC) cultures were generated from 48 umbilical cords donated consecutively following planned caesarean section. HUVECs were stimulated using the cytokines tumour necrosis factor alpha and interferon gamma and HLA expression of unstimulated and stimulated cells determined using flow cytometry. HLA-A2, HLA-A3 and HLA-C antigens all showed a modest increase in expression for 12 hours post cell activation, followed by a more pronounced response over the next 24 to 36 hours. Each of these antigens increased by up to 40 times over unstimulated levels and in addition cells homozygous for specific HLA antigens on average had twice the amount of antigen expressed compared with cells heterozygous for that antigen, both when unstimulated and following cytokine stimulation. Cell activation is an important consideration in the assessment of transplant risk and may help progress towards understanding why rejection does not always occur in the presence of significant donor specific antibody. This data also confirms guidelines for transplantation, which recommend doubling the specific antibody level when considering immunological risk for homozygous donors.

Comparison of Single Antigen Bead-Based, Cell-Based, and Solid Phase-Based Crossmatch Methods

AIM

Antibody assessment during pretransplantation term is important to detect donor specific antibodies. These donor-specific antibodies are determined by various crossmatch methods (flow cytometric [FCXM], complement-dependent cytotoxic [CDCXM], and Luminex [LMXM]). Recently, single antigen bead (SAB) assays have been used for the assessment of hypersensitized patients. The aim of this study was to compare sensitivity and specificity of the 3 crossmatch methods in reference to the SAB method.

METHOD

In this study, 69 hypersensitized patients with high class I and/or class II panel reactive antibodies were tested using the flow cytometric SAB method. Serum samples were cross-matched by 3 crossmatch methods with the cells of a volunteer healthy individual, and the results were evaluated according to HLA and cross-reactive epitope groups (CREGs).

RESULTS

Sensitivity was found to be better with T FCXM (0.91) and class I LMXM (0.87). Specificity of peripheral blood lymphocyte CDCXM method (1.0) was found to be better than the other 2 methods (0.33 and 0.57, respectively). Sensitivity of class II LMXM (0.88) was found to be better than the others (0.42 for B CDCXM and 0.82 for B FCXM, respectively). The specificity of the B CDCXM, B FCXM, and class II LMXM was similar (0.44, 0.44, and 0.33, respectively). CREGs results were similar to HLA results.

CONCLUSION

Although CDCXM has high specificity for the detection of anti-HLA antibodies, it has low sensitivity. To increase sensitivity, FCXM or LMXM methods may be used with the CDCXM test. These results will be beneficial for laboratories and clinicians during graft survival and patient health assessment.

The Humoral Theory of Transplantation: Epitope Analysis and the Pathogenicity of HLA Antibodies

Central to the humoral theory of transplantation is production of antibodies by the recipient against mismatched HLA antigens in the donor organ. Not all mismatches result in antibody production, however, and not all antibodies are pathogenic. Serologic HLA matching has been the standard for solid organ allocation algorithms in current use. Antibodies do not recognize whole HLA molecules but rather polymorphic residues on the surface, called epitopes, which may be shared by multiple serologic HLA antigens. Data are accumulating that epitope analysis may be a better way to determine organ compatibility as well as the potential immunogenicity of given HLA mismatches. Determination of the pathogenicity of alloantibodies is evolving. Potential features include antibody strength (as assessed by antibody titer or, more commonly and inappropriately, mean fluorescence intensity) and ability to fix complement (in vitro by C1q or C3d assay or by IgG subclass analysis). Technical issues with the use of solid phase assays are also of prime importance, such as denaturation of HLA antigens and manufacturing and laboratory variability. Questions and controversies remain, and here we review new relevant data.