Desensitization protocols improving access and outcome in transplantation

Antibody testing strategies for deceased donor kidney transplantation after immunomodulatory therapy

BACKGROUND

Immunomodulatory protocols including intravenous immunoglobulin/rituximab (IVIG/R) are employed to decrease anti-human leukocyte antigen (HLA) antibody levels for patients broadly sensitized to HLA and increase chances for transplantation with a compatible deceased donor (DD). The aim of our study was to identify the optimal antibody levels allowing for selection of compatible DDs for these sensitized patients.

METHODS

From January 2006 to December 2009, 108 patients broadly sensitized to HLA who had reached the top of the DD waitlist were treated with IVIG/R. Antibody levels were monitored monthly by Luminex-based single-antigen bead assay. The antigens identified to produce positive complement-dependent cytotoxicity crossmatches (XMs; >200,000 standard fluorescence intensity [SFI]/10,000 median fluorescence intensity [MFI]) were determined to be unacceptable and entered into the United Network for Organ Sharing database generating the calculated panel reactive antibody (CPRA). The mean CPRA (mCPRA) for this group was more than 80. DDs were selected based on T-cell flow XMs (FXMs) less than 250 MCS and B-cell FXMs less than 300 mean channel shifts (MCS).

RESULTS

Monthly Luminex-based single-antigen assays showed that the IVIG/R therapy decreased antibody levels for a period of 30 to 120 days. Of the 108 patients treated, 80 (74%) were transplanted and 28 (26%) were not (mCPRA 96 ± 11). Forty-two (53%) patients were transplanted with a positive FXM; 28 (35%) patients (mCPRA 84 ± 25) were transplanted with a negative FXM; and 10 patients (12%; mCPRA 90 ± 19) received zero HLA ABDR mismatched grafts.

CONCLUSIONS

After therapy, careful selection of acceptable DD involves the antibody profiling strength and XM results. These approaches provide patients broadly sensitized to HLA with an opportunity for compatible DD transplantation.

Desensitization: achieving immune detente

Antibodies to donor HLA (human leukocyte antigen) and/or ABO antigens were a contraindication to transplantation of most organs for decades. Desensitization protocols have shown the ability to produce reduction of such antibodies sufficient to achieve a successful transplantation. The two major protocols in use are high-dose IVIg or plasmapheresis with low-dose IVIg. The protocols differ in the basic treatment and, to some degree, in their application, but both use standard immunosuppressive agents as well as more recently developed adjunctive agents such as cell-depleting antibodies. Graft and patient survival with both types of protocol are comparable to that of non-sensitized patients, although desensitized patients do have a higher incidence of antibody-mediated rejection (AMR). Antibodies to donor antigens may persist after transplantation, and while the initial antibody titer represents the level of difficulty for successful desensitization, the strength of antibodies that persist after transplantation reflects the risk of AMR. Current protocols do not eliminate B cell clones specific for donor HLA; therefore, desensitized patients remain at an increased risk of antibody rebound if patients experience pro-inflammatory events. Therefore, ongoing antibody monitoring is crucial for early detection of antibody-mediated graft injury. Importantly, the results of numerous programs show that ABOi- and HLA-positive crossmatch renal transplantation, with proper desensitization, can be performed successfully. Further, in addition to increasing the rate of transplantation among sensitized patients, desensitization is providing insight into immunoregulatory processes and may provide information useful in diseases involving immune dysfunction.

The strength of donor-specific antibody is a more reliable predictor of antibody-mediated rejection than flow cytometry crossmatch analysis in desensitized kidney recipients

The aim of this study was to evaluate the utility of donor-specific antibodies (DSA) and flow cytometry crossmatch (FCCM) as tools for predicting antibody-mediated rejection (AMR) in desensitized kidney recipients. Sera from 44 patients with DSA at the time of transplant were reviewed. Strength of DSA was determined by single antigen Luminex bead assay and expressed as mean fluorescence intensity (MFI). T- and B-cell FCCM results were expressed as mean channel shift (MCS). AMR was diagnosed by C4d deposition on biopsy. Incidence of early AMR was 31%. Significant differences in the number of DSAs (p = 0.0002), cumulative median MFI in DSA class I (p = 0.0004), and total (class I + class II) DSA (p < 0.0001) were found in patients with and without AMR. No significant difference was seen in MCS of T and B FCCM (p = 0.095 and p = 0.307, respectively). The three-yr graft survival in desensitized patients with DSA having total MFI < 9500 was 100% compared to 76% with those having total MFI > 9500 (p = 0.022). Desensitized kidney transplant recipients having higher levels of class I and total DSA MFI are at high risk for AMR and poor graft survival. Recipient DSA MFI appears to be a more reliable predictor of AMR than MCS of FCCM.

HLA-specific B cells: I. A method for their detection, quantification, and isolation using HLA tetramers

BACKGROUND

The development of highly sensitive and specific assays for detecting and characterizing HLA-specific antibodies has contributed to an appreciation of the extensive involvement of those antibodies in graft injury and dysfunction. However, understanding the regulatory processes of the humoral response to transplantation and the mechanisms underlying therapeutic agents and protocols for preventing and treating sensitization requires a way to study HLA-specific B cells.

METHODS

Lymphocyte preparations enriched for B cells were stained with one or more of three different HLA tetramers. Tetramer-positive (tet+) B cells were enumerated and evaluated for an association of their frequencies with known sensitization. In some cases, tet+ B cells were isolated and placed in culture with supplements known to activate B cells in a nonspecific fashion.

RESULTS

For all tetramers used, the frequencies of tet+ B cells were significantly higher (4.1%-5.5%) among sensitized patients than among nonsensitized patients (1.6%-3.2%, P<0.001). Binding of the tetramers occurred by the surface immunoglobulin antigen receptor with little or no binding to antibody captured in the Fc receptor. Cultured tet+ B cells produced antibodies specific for epitopes of the tetramer antigen. There appeared to be a certain amount of crossreactivity in the binding of tetramers. The frequency of CD27+ cells among tet+ B cells was higher, on average (34.4%-38.8%) than among all B cells (26.2%) whereas the frequencies of CD38 were comparable in the two groups.

CONCLUSIONS

Staining with HLA tetramers provides a means for identifying, quantifying, and isolating HLA-specific B cells.