HLA typing in organ transplant part 8

In transplantation immunology, the major impact in graft loss comes from the effects of HLA-B and HLA-DR antigens.  The effect of HLA-DR mismatches are the most important in the first 6 moths after transplantation, the HLA-B effect emerges in the first 2 years, and HLA-A mismatches have a deleterious effect on long-term graft survival.

HLA typing in organ transplant part 7

HLA typing and its influence on organ transplantation

The selection of the optimal donor is based on high-resolution HLA typing. The MHC (Major Histocompatibility Complex) contains more than 200 genes which are situated on the short arm of chromosome 6 at 6p21.3 The role of HLA molecules is to present peptides to T cells (both CD4 and CD8 T cells), enabling them to recognize and eliminate "foreign" particles and also to prevent the recognition of "self" as foreign. HLA mismatches may occur at antigenic or allelic level; the first are characterized by amino acid substitutions in both peptide binding and  T cell recognition regions , whereas the latter are characterized by amino acid substitution in the peptide binding region only. When a human transplant is performed, HLA molecules from a donor are recognized by the recipient's  immune system by direct and indirect  methods of  allorecognition triggering an  alloimmune response. Matching of donor and recipient for MHC antigens showed a significant positive effect on graft acceptance. In organ transplantation, the adaptive immunity is the main response exerted to the transplanted tissue, since the main target of the immune response is the MHC molecules expressed on the surface  of donor cells. T cell activation leads to the production of cytokines and chemokines which in turn may recruit components of the innate immunity like natural killer cells or macrophages and complement. In addition to that, defensins and cathelicidin have chemoattractant properties on T lymphocyte.

HLA typing in organ transplant part 6

Variability

Since the biologic function of the HLA molecules is presenting endogenous and exogenous antigens, they manifested high structural polymorphism. HLA  presents the highest degree of polymorhism is due to gene conversion  between variable alleles and loci within each HLA gene. The number of HLA allele's increase at a regular rate, about 15% of the alleles in the high-expression loci (A,B,C and DRB1) and 30% in the low expression loci  (DRB3/4/5, DQB1, DPB1)are truly relevant in clinical typing. The number  of alleles will keep growing. The number of variant alleles at class I and class II loci  according to the IMGT-HLA database.

HLA typing in organ transplant part 5

Nomenclature

Two systems of nomenclature are applied to HLA. The first system is based on serological recognition. In this system, antigens were eventually assigned letters and numbers (eg HLA-B51 or, shortened, B51). Modern HLA nomenclature are begin with HLA- and the locus name, then * and even number of digits specifying the allele. The first two digits specify a group of alleles. Older typing methodologies often could not be completely distinguish alleles and so stopped at this level. The third through four digits specify a synonymous allele. Digits five through six denote any synonymous mutations within the coding frame of the gene. The seventh and eights digits distinguish mutation outside the coding region. Letters such as L,N,Q, or S may follow an allele's  designation to specify an expression level or other non-genomic data known about it. Thus, a completely described allele may be up to nine digits long, not including the HLA-prefix and locus notation (eg, HLA-A*24:02:01 NN=Null) to designate a specific allele at a given HLA locus. Every two years, a nomenclature is put forth to aid researchers in interpreting serotypes to alleles.

HLA typing in organ transplant part 4

Classification

Major histocompatibility complex class I proteins form a functional receptor on most nucleated cells of the body. There are 3 major and 3 minor  major histocompatibility complex (MHC) class I genes in HLA. Major genes are HLA-A, HLA-B and HLA-C. Minor genes are HLA-E, HLA-F and HLA-G. β2-microglobulin binds with major and minor gene subunits  to produce heterodimer.  There are 3 major and 2 minor major histocompatibility complex (MHC) class II proteins encoded by the HLA. The genes of the class II combine to form heterodimeric (αβ) protein receptors that are typically expressed on the surface of antigen-presenting cells. Major-major histocompatibility complex (MHC) class II HLA-DP:α-chain encoded by HLA-DPA1 locus  and β-chain encoded by HLA-DPB1 locus. HLA-DQ:α-chain encoded by HLA-DQA1 locus and β-chains encoded by HLA-DQB1 locus. The last one is  HLA-DR: α-chain encoded by HLA-DRA locus and four β-chains (only 3 possible per person), encoded by HLA-DRB1, DRB3, DRB4 and DRB5 loci. The other major histocompatibility (MHC) protein class II proteins, DM and DO, are used in the internal processing of antigens, loading the antigenic peptides generated from pathogens onto the HLA molecules of antigen-presenting cell.

HLA typing in organ transplant part 3

The HLA system is highly polymorphic

• The contribution of the allelic diversity of class I and II genes to immune recognition and alloreactivity can be analyzed by serological methods and molecular methods at the DNA level by different methods like sequence specific primer and oligotyping with locus and allele-specific oligonucleotide probes
• HLA class I and II matching is important in organ transplantation
• Especially in kidney and bone marrow transplantation. In heart and lung transplantation, HLA match at the DR locus is important but there is some difficulties like ischemic time, availability of donors and clinical need of recipients. Corneal grafts are not usually influenced by HLA matching, unless being transplanted into a vascularized bed
• Transplantation of foreign tissue induces both humoral and cellular immune responses in the recipient, which leads to graft rejection or, for bone marrow transplantation, graft versus host disease

HLA typing in organ transplant part 2

The HLA system includes a complex array of genes located on chromosome number 6 and their molecular products that are involved in immune regulation and cellular differentiation. Human leucocyte antigen (HLA) molecules are expressed on all nucleated cells, and they are the major molecules that initiate graft rejection. There are three classical loci at HLA class I(HLA-A, -B and -C) and five loci at class II (HLA-DR, -DQ, --DP, -DM, and -DO).