Human Leukocyte Antigen Sensitization in Solid Organ Transplantation: A Primer on Terminology, Testing, and Clinical Significance for the Apheresis Practitioner

Abstract

The human leukocyte antigen (HLA) system is an important immunologic barrier that must be considered for successful solid organ transplantation. Formation of donor-specific HLA antibodies in solid organ transplantation is an important cause of allograft injury and may contribute to recipient morbidity and mortality. Therapeutic plasma exchange is often requested to lower HLA antibody levels prior to or after transplantation and for management of HLA antibodies in the context of organ rejection. In this review, we summarize the current terminology, laboratory testing, and clinical significance of HLA sensitization in the solid organ transplant population. Furthermore, to illustrate applications of HLA testing in clinical practice, we summarize our own lung and kidney institutional protocols for managing HLA antibodies in the peri-transplant setting.

Keywords: Human leukocyte antigen antibodies; Human leukocyte antigen sensitization; Solid organ transplantation; Therapeutic plasma exchange.

Figure 1.

Example of a FlowPRA® result. On the left, the Class I FlowPRA® shows that the majority of the cells are on the left (negative) side of the Flow graph; therefore, the PRA result is read as 0%. On the right, the class II FlowPRA® shows that about 54% of cells are on the right (positive) side of the graph; therefore, the PRA result is read as 54%.

Figure 2.

HLA class II antibody Luminex single antigen bead assay. The red bars on the left side of the graph show the detected HLA antibodies on the X axis and the mean fluorescent intensity (MFI) numbers on the Y axis. At the bottom of the figure, the HLA antibody signature of the patient is shown.

Figure 3.

Peri-transplant HLA testing protocol for the Duke University Medical Center lung transplant program. A. For the initial evaluation of a prospective lung transplant candidate, flow cytometry HLA antibody screening test (FlowPRA®) and Luminex® single antigen bead assay (LSA) are performed. For candidates with a negative HLA antibody (Ab) detection, serum samples are routinely re-tested until transplant. For candidates with a positive HLA Ab detection, depending on the calculated PRA (cPRA), the patient is listed for transplant with all unacceptable antigens reported in the United Network for Organ Sharing internet-based transplant information database, UNet^SM. For cPRA ≥ 80%, any antigens likely to result in positive crossmatch (XM) are also listed as unacceptable in UNet^SM. The gray dashed lines represent the pathway for some candidates with a cPRA ≥ 80 who are considered for a therapeutic plasma exchange (TPE)-based desensitization protocol. Prior to and after desensitization, FlowPRA®, LSA, and dilution test are performed. Changes in cPRA and MFI of the antibodies will be monitored and used to determine the success of desensitization. B. At transplant, A virtual crossmatch (VXM) is performed. If VXM is positive for a non-local deceased donor, the donor will be declined. If VXM is positive for a local donor, the transplant is put on hold pending results of the prospective crossmatch (XM). If the prospective XM is positive, then no transplant will occur. If, however, the initial VXM is negative, then we proceed with the transplant while a retrospective XM is in process. If the retrospective XM comes back negative, then the patient is maintained on a standard post-transplant monitoring protocol. If the retrospective XM unexpectedly comes back positive, then, the patient will be treated with a TPE-based protocol. * For patients with a negative XM, TPE is considered if there is evidence or high concern for early allograft injury.

Figure 4.

Peri-transplant HLA testing protocol for the Duke University Medical Center renal transplant program. A. For the initial evaluation of a prospective renal transplant candidate, flow cytometry HLA antibody screening test (FlowPRA®) is performed. For candidates with a negative HLA antibody (Ab) detection, serum samples are routinely re-tested (every 6 months or at the time of a sensitizing event, such as blood transfusion) until transplant. For candidates with a positive HLA Ab detection, the patient is listed for transplant with all unacceptable antigens reported in the United Network for Organ Sharing internet-based transplant information database, UNet^SM. B. At transplant, a virtual crossmatch (VXM) is performed. i) For a deceased donor, if either the virtual or actual crossmatch is positive, then the organ is declined. ii) For an ABO compatible living donor organ, if the actual crossmatch (XM) is positive, then class I and II T and B cell mean channel shift (MCS) is used to determine organ acceptability. If MCS > 250, then the organ is rejected. If MCS ≤ 250, then the organ will be transplanted after undergoing a peri-transplant TPE-based desensitization protocol. iii) For ABO incompatible organ transplants, a TPE-based desensitization protocol is used.