Perception versus reality?: Virtual crossmatch--how to overcome some of the technical and logistic limitations

Mastering the risk of HLA antibodies in kidney transplantation: an algorithm based on pretransplant single-antigen flow bead techniques

The utilization of sensitive techniques of detection of HLA antibodies to define and measure sensitization has greatly evolved in recent years. We present here an approach to minimize the risk of HLA antibodies in kidney transplantation based on the evaluation of graft accessibility of sensitized patients by calculated PRA (cPRA) and estimation of potential matched donors (PMD) using a national simulation software program. This study included all registered patients on our waiting list (WL) for deceased donor (DD) kidney transplants. All patients were screened by single-antigen flow bead (SAFB) techniques. Of the 502 registered patients, 174 (34.7%) were sensitized. Among these, 48.3% (84 pts) had a cPRA>85%. For 75.3% of sensitized patients (90 pts with cPRA≤85% and 41 pts with cPRA>85%), the flow of PMD was considered sufficient to allow a transplant avoiding all unacceptable antigens. The 41 patients with a cPRA>85% (48.8%) had a satisfactory donor flow in the framework of the national prioritization program for highly sensitized patients. Finally, 43 sensitized patients (24.7%) were deemed eligible for a strategy of higher immunological risk through desensitization protocols or transplantation against HLA-DSAs. This approach provides a logical and systematic strategy to rationalize the access of sensitized patients to kidney transplantation minimizing the risk of HLA antibodies.

The complexity of human leukocyte antigen (HLA)-DQ antibodies and its effect on virtual crossmatching

BACKGROUND

We previously reported that in patients possessing human leukocyte antigen (HLA)-DQ-directed antibodies, the target molecule may include the patient's own DQβ chain if it is paired with non-self-DQα chain, thus forming a different DQ target. Herein, we sought to assess the breadth of this phenomenon.

METHODS

Serum samples from 104 patients awaiting kidney transplantation, known to have DQ antibodies, were studied. Antibody identification was performed using luminex-based HLA class II single-antigen bead assays from two vendors; DQA1/DQB1 typing was performed using luminex polymerase chain reaction - sequence specific oligo prob hybridization (PCR-SSO) technology.

RESULTS

A total of 71% of the 104 serum samples studied contained antibodies reactive against test beads coated with the patient's own DQα- or β-chain components. Of those, 35 patients (34%) exhibited antibodies to their own DQβ chain when in combination with non-self-DQα chains; and 64 patients (62%) had antibodies to their own DQα chain when in combination with non-self-DQβ chains. This is a striking observation.

CONCLUSIONS

To the best of our knowledge, this is the first systematic, high-resolution evaluation of DQ antibody repertoire. With the expansion of virtual crossmatching, particularly in the context of a national registry, the need for more detailed DQ antibody or antigen evaluation is critical to improve operational efficiency and patient outcomes.

The evolution and clinical impact of human leukocyte antigen technology

PURPOSE OF REVIEW

As human leukocyte (HLA) antigen and antibody testing has evolved, typing, screening, and crossmatching assays have become exquisitely specific and sensitive. How best to use this information is a focal point of the transplant community and of this review.

RECENT FINDINGS

Driven by advances in technology, two major changes in the transplant community recently occurred. Firstly, the United Network for Organ Sharing mandated the use of cPRA to replace the more subjective panel reactive antibody value. Secondly, the virtual crossmatch (vXM) became a reality. The vXM categorizes sensitized patients as compatible or incompatible with their donors by predicting the likelihood of a positive crossmatch. Although the use of cPRA and vXM represents major progress in organ allocation algorithms, there are compelling issues that must be addressed. These include defining appropriate threshold values for antibody assignment and recognizing the relevance of antibodies to HLA-Cw*, HLA-DQ*, and HLA-DP*.

SUMMARY

Undoubtedly, molecular HLA-typing and solid-phase antibody-detection assays have revolutionized clinical histocompatibility testing. These new testing methodologies have had a major impact in both allocation and outcome of transplanted kidneys. However, even these new methodologies are not flawless and pose new challenges to the transplant community.

Surveillance of alloantibodies after transplantation identifies the risk of chronic rejection

The monitoring of the levels of alloantibodies following transplantation might facilitate early diagnosis of chronic rejection (CR), the leading cause of renal allograft failure. Here, we used serial alloantibody surveillance to monitor patients with preoperative positive flow cytometric crossmatch (FCXM). Sixty-nine of 308 renal transplant patients in our center had preoperative positive FCXM. Blood was collected quarterly during the first postoperative year and tested by FCXM and single antigen bead luminometry, more sensitive techniques than complement-dependent cytotoxic crossmatching. Distinct post-transplant profiles emerged and were associated with different clinical outcomes. Two-thirds of patients showed complete elimination of FCXM and solid-phase assay reactions within 1 year, had few adverse events, and a 95% 3-year graft survival. In contrast, the remaining third failed to eliminate flow FCXM or solid-phase reactions directed against HLA class I or II antibodies. The inferior graft survival (67%) with loss in this latter group was primarily due to CR. Thus, systematic assessment of longitudinal changes in alloantibody levels, either by FCXM or solid-phase assay, can help identify patients at greater risk of developing CR.

Preexisting donor-specific HLA antibodies predict outcome in kidney transplantation

The clinical importance of preexisting HLA antibodies at the time of transplantation, identified by contemporary techniques, is not well understood. We conducted an observational study analyzing the association between preexisting donor-specific HLA antibodies (HLA-DSA) and incidence of acute antibody-mediated rejection (AMR) and survival of patients and grafts among 402 consecutive deceased-donor kidney transplant recipients. We detected HLA-DSA using Luminex single-antigen assays on the peak reactive and current sera. All patients had a negative lymphocytotoxic cross-match test on the day of transplantation. We found that 8-year graft survival was significantly worse (61%) among patients with preexisting HLA-DSA compared with both sensitized patients without HLA-DSA (93%) and nonsensitized patients (84%). Peak HLA-DSA Luminex mean fluorescence intensity (MFI) predicted AMR better than current HLA-DSA MFI (P = 0.028). As MFI of the highest ranked HLA-DSA detected on peak serum increased, graft survival decreased and the relative risk for AMR increased: Patients with MFI >6000 had >100-fold higher risk for AMR than patients with MFI <465 (relative risk 113; 95% confidence interval 31 to 414). The presence of HLA-DSA did not associate with patient survival. In conclusion, the risk for both AMR and graft loss directly correlates with peak HLA-DSA strength. Quantification of HLA antibodies allows stratification of immunologic risk, which should help guide selection of acceptable grafts for sensitized patients.

C4d-fixing capability of low-level donor-specific HLA antibodies is not predictive for early antibody-mediated rejection

BACKGROUND

Recent studies indicate that not all low-level donor-specific human leukocyte antigen (HLA) antibodies (HLA-DSA) (i.e., positive by solid-phase assays, negative by complement-dependent cytotoxic-crossmatch) have a detrimental clinical impact. The aim of this study was to investigate whether the pretransplant C4d-fixing capability allows distinguishing harmful from presumably clinically irrelevant HLA-DSA.

METHODS

We retrospectively investigated 64 patients with low-level HLA-DSA detected by single-antigen flow beads (SAFB). Thirty-four of 64 patients (53%) experienced early antibody-mediated rejection (AMR), whereas 30 patients (47%) did not. HLA-DSA characteristics (i.e., number, class, and strength), frequency of retransplants, and immunosuppressive regimens were not different in these two groups. Pretransplant sera were reanalyzed using a modified SAFB assay measuring C4d fixation induced by HLA antibodies.

RESULTS

C4d-fixing HLA-DSA were observed in 11 of 64 patients (17%). AMR occurred in 6 of 11 patients (55%) with C4d-fixing HLA-DSA and in 28 of 53 patients (53%) without C4d-fixing HLA-DSA (P=1.0). Positive C4d staining in peritubular capillaries was detected in 6 of 11 patients (55%) with C4d-fixing HLA-DSA and in 21 of 53 patients (40%) without C4d-fixing HLA-DSA (P=0.50).

CONCLUSION

The pretransplant capability of low-level HLA-DSA to trigger C4d fixation in vitro is not predictive for early AMR or C4d deposition in peritubular capillaries in vivo. This argues against pretransplant C4d SAFB testing to define the clinical relevance of low-level HLA-DSA.

Efficacy of induction therapy with ATG and intravenous immunoglobulins in patients with low-level donor-specific HLA-antibodies

Low-level donor-specific HLA-antibodies (HLA-DSA) (i.e. detectable by single-antigen flow beads, but negative by complement-dependent cytotoxicity crossmatch) represent a risk factor for early allograft rejection. The short-term efficacy of an induction regimen consisting of polyclonal anti-T-lymphocyte globulin (ATG) and intravenous immunoglobulins (IvIg) in patients with low-level HLA-DSA is unknown. In this study, we compared 67 patients with low-level HLA-DSA not having received ATG/IvIg induction (historic control) with 37 patients, who received ATG/IvIg induction. The two groups were equal regarding retransplants, HLA-matches, number and class of HLA-DSA. The overall incidence of clinical/subclinical antibody-mediated rejection (AMR) was lower in the ATG/IvIg than in the historic control group (38% vs. 55%; p = 0.03). This was driven by a significantly lower rate of clinical AMR (11% vs. 46%; p = 0.0002). Clinical T-cell-mediated rejection (TCR) was significantly lower in the ATG/IvIg than in the historic control group (0% vs. 50%; p < 0.0001). Within the first year, allograft loss due to AMR occurred in 7.5% in the historic control and in 0% in the ATG/IvIg group. We conclude that in patients with low-level HLA-DSA, ATG/IvIg induction significantly reduces TCR and the severity of AMR, but the high rate of subclinical AMR suggests an insufficient control of the humoral immune response.

Calculated PRA (CPRA): the new measure of sensitization for transplant candidates

The ways we measure whether a patient is sensitized to HLA antigens and to what extent sensitization affects access to transplantation have changed remarkably during the past decade. What we mean by sensitized and broadly sensitized today is heavily dependent upon the sensitivity of the test that is used to measure antibodies. Because we provide additional allocation points for broadly sensitized patients in the United States kidney allocation system in an effort to compensate for their biological disadvantage, some consistency and accountability are required. The calculated panel-reactive antibody, which provides an estimate of the percentage of deceased organ donors that will be crossmatch incompatible for a candidate provides both consistency and accountability.

Sharing kidneys across donor-service area boundaries with sensitized candidates can be influenced by HLA C

The United Network for Organ Sharing (UNOS) implemented the virtual crossmatch system in UNet as a way to improve the likelihood of a negative crossmatch when kidneys are shared with HLA-sensitized candidates across donor service area (DSA) boundaries. The role of HLA C in that process is not universally appreciated. We recently experienced an unexpected positive flow T and B cell crossmatch for an imported, HLA zero-mismatched kidney because of donor-specific HLA C antibodies and transplanted it into the backup candidate. HLA C locus antigens were not typed by the OPO's laboratory that sent the kidney so the UNet virtual crossmatch could not "strike" our candidate from the UNOS match run. HLA C locus typing data of donors for kidneys our DSA imported from other DSAs revealed that C typing was not performed in 23% (14/60) and was discrepant with our molecular type for 10% (6/60) and was concordant in 67% (40/60) of cases. The rate of positive donor-specific crossmatches was higher (83%) for HLA C discrepantly typed donors than for concordantly typed donors (44%). Sensitization for HLA C (42%) is less frequent than for A (80%) or B (83%) locus antigens but the immunogenicity of C locus antigens in patients who make C locus antibodies is equivalent in black and white patients. Finally, the transplant rate of imported kidneys into class I-sensitized candidates was 24%, and C locus-sensitized candidates comprised 55% of those transplanted.