The virtual crossmatch--a screening tool for sensitized pediatric heart transplant recipients

Cardiac transplant outcomes in pediatric patients with pre-formed anti-human leukocyte antigen antibodies and/or positive retrospective crossmatch

BACKGROUND

Children undergoing heart transplantation who have preformed anti-human leucocyte antigen (HLA) panel reactive antibodies (PRA) or positive retrospective crossmatch (XM) may be at increased risk for rejection and graft failure. We assessed outcomes of transplant recipients with either positive PRA before transplant or positive retrospective XM.

METHODS

A review of 148 heart transplant patients between 1990 and 2006 was undertaken, identifying transplants in patients with pre-transplant PRA > 1% and/or a positive XM. Demographic information and detailed post-transplant outcomes including episodes of rejection, infection, and graft failure were recorded.

RESULTS

There were 11 PRA positive (PRA+) transplants, 135 PRA negative (PRA-) transplants, and no PRA data on 2. There were 14 XM+ transplants, 115 XM- transplants, and no XM data on 19. Kaplan-Meier graft survival was better in XM- than XM+ patients (p < 0.015), but not different between PRA+ and PRA- Groups. Timing of first rejection and number of rejection episodes were not different between XM+ and XM- Groups or between PRA+ and PRA- Groups. Infections were not different between PRA or XM Groups. Four patients were PRA+/XM- (all PRA, 1%-10%), 7 were PRA-/XM+, and 7 were PRA+/XM+ (6 of 7 PRA >10%).

CONCLUSIONS

Pediatric heart transplant patients whose retrospective XM is positive are at significantly increased risk for graft failure. Elevated pre-transplant PRA may not predict increased risk of graft failure, although markedly positive PRA (>10%) predicts a positive retrospective XM. Improved treatment for pediatric transplant patients with a positive retrospective XM is needed.

Pediatric Heart Transplantation in Human Leukocyte Antigen–Sensitized Patients

Background— There is an elevated risk for poor outcomes after heart transplant (HTx) in patients sensitized to human leukocyte antigens including graft dysfunction, acute cellular and antibody-mediated (AMR) rejection, and cardiac allograft vasculopathy. We report our experience with human leukocyte antigens–sensitized pediatric HTx recipients.

Methods and Results— We identified pediatric HTx patients with elevated pre-HTx Panel Reactive Antibody (Class I/II; >10%), or a positive T- or B-cell crossmatch. Thirteen patients met criteria (5 female, 39%). The median age at HTx was 7 months (3.5 months to 15.5 years). Nine were infants who had prior palliation for congenital heart disease. Four were older patients (median 7.3 years; 4.8 to 15.5 years): 2 had congenital heart disease (Fontan), 2 were re-HTx. B-cell therapies were used in all patients, guided by assessment of CD19+ and CD20+ cells. Immunosuppression included thymoglobulin induction, and tacrolimus, mycophenolate mofetil, and steroids. Daily plasmapheresis ± intravenous immunoglobulin G was used if there was a positive crossmatch on day 1, with a gradual, biopsy-guided weaning schedule. Rituximab was used when AMR was detected on biopsy: more recently (n=3), used empirically perioperatively. AMR was confirmed in 9 patients within median 0.9 months post-HTx. Seven had early acute cellular rejection (≥ ISHLT Grade 2 R) with no hemodynamic compromise or graft dysfunction. There were 4 deaths post-HTx (range, 11 days to 9 months). The median follow-up of 9 survivors was 1.7 years (0.3 to 3.7 years). Of 7 patients >6 months post-HTx, no AMR or cardiac allograft vasculopathy was observed at a mean of 1.9+1.1 years post-HTx and no cardiac allograft vasculopathy.

Conclusions— Despite aggressive management, acute cellular rejection and AMR occurred frequently early post-HTx. An algorithm of B cell–directed strategies can be effective in managing these patients with reasonable intermediate-term outcomes.

Practical application of the virtual crossmatch

BACKGROUND

HLA antibody sensitization is a risk factor for morbidity and mortality following heart transplantation. We previously reported the results of our in vitro study demonstrating the predictive value of the Virtual Crossmatch (VXM) and have since applied it clinically for sensitized children listed for heart transplant at our center. The VXM utilizes the results of specific antibody screening via flow cytometry to predict acute incompatibility. This review examines the effect of the VXM on wait times and outcomes.

METHODS

The study population included all patients listed for heart transplantation at Children's Hospital of Wisconsin. Antibody sensitization was defined as PRA>10% and/or by the identification of HLA specific antibody using AHG-CDC or flow cytometry. Categorical data was analyzed via Fisher's exact test while continuous variables were compared via an unpaired t test.

RESULTS

There were a total of 111 listed patients between 7/91-9/06. The sensitization rate was 23% (25/111). 19 patients who went on to transplant, deterioration or death were divided into 3 groups depending on listing strategy; Group 1 were listed with a prospective crossmatch requirement, Group 2 with a VXM, and Group 3 with a retrospective cross match. 7/8 patients in group 1 died prior to transplant with median wait time of 119 days. 9/10 patients in group 2 were transplanted with 100% survival and median wait time of 65 days, Group 3 included 1 patient who received a graft after 54 days and died 3 months post transplant with humoral rejection. The VXM was highly concordant with the retrospective crossmatch.

CONCLUSIONS

Use of a VXM can lead to shorter wait times and better outcomes as a listing strategy for sensitized children requiring cardiac transplantation. The VXM allows transplant physicians to risk stratify patients and consider an anticipated positive crossmatch as one additional factor in the risk-benefit analysis inherent to any donor offer. This experience supports use of the VXM as an alternative to prospective crossmatch requirements. One should recognize that other era dependent improvements such as updated criteria for patient and donor selection along with newer therapies and bridging options may have contributed to superior outcomes seen in the more recent cohort of patients treated with the VXM approach.

Mortality and Morbidity in Pre-sensitized Pediatric Heart Transplant Recipients With a Positive Donor Crossmatch Utilizing Peri-operative Plasmapheresis and Cytolytic Therapy

BACKGROUND

The difficulty in obtaining a prospective negative donor/recipient crossmatch limits the ability to successfully transplant pediatric heart transplant candidates who show evidence of antibodies to multiple human leukocyte antigens (pre-sensitized patients).

METHODS

We utilized a protocol that included peri-operative plasmapheresis, thymoglobulin and cyclophosphamide in 17 pre-sensitized (panel-reactive antibodies [PRA] >10%) pediatric patients to accept donors for these patients without a prospective crossmatch between 1995 and 2005. A retrospective review of survival, rejection and infection was performed, comparing the frequency of rejection and infection in our patients who were transplanted with a complement-dependent cytotoxic (CDC)-positive donor/recipient crossmatch to those patients transplanted with a negative crossmatch.

RESULTS

Thirteen of 17 patients were found to have a CDC-positive crossmatch. Actuarial survival after transplantation was 85% at 1 year and 73% at 3 years. Twelve of 13 (92%) of these patients experienced rejection, and 5 of 13 (38%) had recurrent rejection, generally in the first 2 months after transplantation. Rejection was associated with hemodynamic compromise in 58% of first rejection episodes and 67% of episodes of recurrent rejection. The frequency of rejection in these patients was significantly greater than the frequency in patients with a negative crossmatch in the first 6 months after transplantation, but not afterward. The frequency of infection episodes was not significantly different between the groups.

CONCLUSIONS

Heart transplantation in pre-sensitized pediatric recipients with a CDC-positive donor/recipient crossmatch may result in reasonable short-term survival, but with a high frequency of early rejection, often with hemodynamic compromise.

Pretransplant risk assessment in renal allograft recipients using virtual crossmatching

Preformed donor-specific HLA-antibodies antibodies (DSA) are a major risk for early antibody-mediated rejection (AMR). This prospective study evaluated the accuracy of pretransplant risk assessment using virtual crossmatching (virtualXM) (i.e. comparing HLA-typing of the donor with the recipient's HLA-antibody specificities determined by flow-beads). Sixty-five consecutive patients were stratified according to virtualXM results: patients without DSA (n= 56) were considered low risk and received standard immunosuppression; patients with DSA (n= 9) were considered high risk and received additional induction with anti-T-lymphocyte-globulin (ATG) and intravenous immunoglobulins. Despite induction therapy 4 of 9 patients with DSA (44%) had clinical/subclinical AMR, whereas only 2 of 56 patients without DSA (4%) (p = 0.002). Notably, one of these two patients had early AMR likely induced by non-HLA-antibodies; the other had subclinical AMR at month 6 consistent with de novo DSA. The results of virtualXM and retrospectively obtained flow-cytometric crossmatches (FCXM) (n= 59) were concordant in 51 patients (86%), four patients (7%) were virtualXM-/FCXM+ and none had AMR, four patients (7%) were virtualXM+/FCXM- and one had AMR. VirtualXM can accurately define absence or presence of DSA and may become an invaluable tool for organ allocation and pretransplant risk assessment. However, further studies need to address whether all HLA-antibodies detected by flow-beads are clinically relevant.