Transplant compatibility testing

Abstract

Introduction

The laboratory that performs transplant compatibility testing may go by several names—human leukocyte antigen (HLA) laboratory, tissue typing laboratory, histocompatibility laboratory, or transplant immunology laboratory. Regardless of name, the testing performed in this laboratory supports multiple aspects of clinical practice. First, the primary focus of this laboratory is to provide an immunologic risk profile for the transplant team. Second, transfusion medicine relies on testing and expertise from this laboratory to select compatible platelet units for patients who are refractory to platelet transfusion and investigation of suspected transfusion-related acute lung injury (TRALI) cases. Third, physicians may order HLA typing to assess the risk of certain genetic diseases that have strong HLA associations. Fourth, the field of pharmacogenomics is developing a place in this laboratory to avoid certain adverse drug effects. The diversity of these clinical activities speaks to the important biologic roles of HLA.

Human leukocyte antigen structure and properties

The major histocompatibility complex (MHC) on the short arm of chromosome 6 is the genetic location of the HLA genes. HLA genes are inherited as a haplotype, co-dominantly expressed, extremely polymorphic, and highly immunogenic. When expressed, HLA proteins are “loaded” with peptides and trafficked to the cell surface, where the immune system is able to monitor the peptides displayed. There are several genes in this MHC region of the human genome; however, this chapter will focus on class I and class II HLA.

Classical class I genes includes the following HLA: A, B, and C. Classical class II genes includes the following HLA: DR, DQ, and DP. The structure of class I HLA consists of an α chain that is covalently bound to a β-2-microglobulin ( Fig. 97.1 A). In contrast, class II HLA is a haplo-dimer that consists of an α and β chain ( Fig. 97.1 B). Because the function of HLA is to present peptides to the immune system for surveillance, the most critical aspect of these structures is their peptide-binding groove. In HLA class I the peptide-binding groove is formed by two domains on the α chain. In HLA class II the peptide-binding groove is formed by the combination of domains on the α and β chains. The physical proximity of the HLA genes means that they are inherited together from each parent as a haplotype. This means that HLA genes occur in particular combinations far more frequently than would have been predicted (i.e., linkage disequilibrium). The experienced individual can use this linkage disequilibrium information to check for a potential error in typing. Inheriting HLA as a haplotype means that each child will inherit one full haplotype from each parent ( Fig. 97.2 ). Furthermore, any two full siblings have a 25% chance inheriting the same two HLA haplotypes (i.e., HLA identical match), 50% chance of inheriting one identical HLA haplotype (i.e., haploidentical), and 25% chance of inheriting two different HLA haplotypes (i.e., HLA mismatch). Beyond the immediate family, it is rare to identify HLA matches (unless consanguineous marriage is practiced in the community).

Any given HLA protein is able to bind and present a subset of potential peptides in its peptide-binding groove. Presumably for this evolutionary reason, HLA proteins demonstrate co-dominant expression to maximize the number of potential antigens that the immune system is able to present on HLA proteins. HLA genes are co-dominantly expressed, which equips the individual with the greatest number of HLA proteins for immune surveillance. HLA class I and class II proteins have different patterns of expression, based on cell type. Class I proteins are expressed on all nucleated cells (which includes platelets, since they are fragments of the nucleated megakaryocyte); whereas class II proteins are typically restricted to antigen presenting cells (APCs) such as: B lymphocytes, monocytes/macrophages, and dendritic cells. However, in the presence of inflammation, most cells can be induced to express these class II HLA proteins. Differential expression is important for understanding why support of the platelet refractory patient is focused on class I HLA and why solid organ transplant rejection can be mediated by antibodies to both class I and class II HLA. There are also differences in expression of the amount of HLA proteins on the cell surface by locus. The higher expressed HLA loci are HLA-A, HLA-B, and HLA-DRB1. The lesser expressed HLA loci are HLA-C, HLA-DRB3/4/5, HLA-DQB1, and HLA-DPB1. Understanding expression levels help to appreciate why HLA testing practices evolved as they did—beginning with the higher expressed proteins.

The highly polymorphic nature of HLA cannot be overstated. Because of its role in discriminating self from non-self, the HLA proteins function like a combination lock. This makes it nearly impossible for infectious organisms to perfectly mimic an individual’s HLA, maintaining the integrity of immune surveillance. While the complexity inherent in the HLA protein is beneficial for immune surveillance, this creates compatibility challenges for organ transplantation.

HLA molecules are highly immunogenic. During immune surveillance, HLA has the critical role of presenting antigens to T cells. In the case of HLA class I, the intracellular environment is continuously sampled and presented to CD8+ T cells. If the cell presents a fragment of virus in the peptide-binding groove, then the CD8+ T cell becomes activated and promotes a cell-mediated cytotoxic immune response. In the case of HLA class II, the intercellular environment is continuously sampled and presented to CD4+ T cells. If the cell presents a fragment of bacteria in the peptide-binding groove, then the CD4+ T cell becomes activated, releasing cytokines and differentiating into different cells to promote a humoral immune response.