1. Prof. Ileana Constantinescu
Immunogenetics and transplantation. Major Histocompatibility Complex. Rejection
Prof. Ileana Constantinescu

2. Nomenclature of HLA Alleles
Each HLA allele name has a unique number corresponding to up to four sets of digits separated by colons. The length of the allele designation is dependent on the sequence of the allele and that of its nearest relative. All alleles receive at least a four digit name, which corresponds to the first two sets of digits, longer names are only assigned when necessary.
The digits before the first colon describe the type, which often corresponds to the serological antigen carried by an allotype. The next set of digits are used to list the subtypes, numbers being assigned in the order in which DNA sequences have been determined. Alleles whose numbers differ in the two sets of digits must differ in one or more nucleotide substitutions that change the amino acid sequence of the encoded protein. Alleles that differ only by synonymous nucleotide substitutions (also called silent or non-coding substitutions) within the coding sequence are distinguished by the use of the third set of digits. Alleles that only differ by sequence polymorphisms in the introns, or in the 5' or 3' untranslated regions that flank the exons and introns, are distinguished by the use of the fourth set of digits.
In addition to the unique allele number, there are additional optional suffixes that may be added to an allele to indicate its expression status. Alleles that have been shown not to be expressed - 'Null' alleles - have been given the suffix 'N'. Alleles that have been shown to be alternatively expressed may have the suffix 'L', 'S', 'C', 'A' or 'Q'.
The suffix 'L' is used to indicate an allele which has been shown to have 'Low' cell surface expression when compared to normal levels. The 'S' suffix is used to denote an allele specifying a protein which is expressed as a soluble, 'Secreted' molecule but is not present on the cell surface. The 'C' suffix is assigned to alleles that produce proteins that are present in the 'Cytoplasm' and not on the cell surface. An 'A' suffix indicates an 'Aberrant' expression where there is some doubt as to whether a protein is actually expressed. A 'Q' suffix is used when the expression of an allele is 'Questionable', given that the mutation seen in the allele has been shown to affect normal expression levels in other alleles.

3. As of April 2016, no alleles have been named with the 'C' or 'A' suffixes.

6. Numbers of Confidential Alleles
Numbers of HLA Alleles
HLA Class I Alleles
10.730
HLA Class II Alleles
3.743
HLA Alleles
14.473
Other non-HLA Alleles
177
Numbers of Confidential Alleles 7

7. Immunogenetics – a part of genetics which studies the complexity of immune processes, concerning defense and integrity of the host.
MHC – encodes a group of highly diverse cell surface proteins. There are two types of MHC molecules: class I and class II molecules.
Polymorphism – each MHC locus can express any one of hundreds of different molecules.
Its genes are codominantly expressed.

8. TRANSPLANT REJECTION TISSUE COMPATIBILITY
Transplant rejection is graft failure resulting from recipient antibodies and cells directed against donor cells.
Transplantation, for the purpose of replacing a diseased organ with a healthy donor organ, represents an increasingly active field in modern medical practice, as our understanding of the immunological aspects of transplantation continues to grow.
In transplantation, we have to deal with adverse side effects of an immune system that has evolved to recognize and protect us from nonself, harmful pathogens. In transplantation we have the responsibility to make the nonself (the allograft) to be accepted and function normally within the recipient.
TISSUE COMPATIBILITY
The concept of tissue compatibility (the need for donor and recipient tissues to be "compatible" in order for a transplant to be accepted) was appreciated as early as the beginning of the 20th century. In a series of studies involving the transplantation of tumors and later skin grafts between mice, it became apparent that successful tissue transplantation depended upon genetic similarity of donor and recipient.
Tissues transplanted between genetically identical animals, also known as syngeneic transplants or isografts, are virtually always accepted.
In contrast, transplants performed between two subjects belonging to the same species who are genetically nonidentical, termed allogeneic transplants or allografts, invariably undergo rejection. Transplants performed between donor and recipient who belong to different species are known as xenogeneic transplants or xenografts and are promptly rejected.
Allogeneic transplants or allografts bear allogeneic or foreign major and minor histocompatibility antigens; whereas syngeneic transplants or isografts bear genetically identical or self major and minor histocompatibility antigens.

10. LAWS OF TRANSPLANTATION
The laws of transplantation are as follows:
Transplants are accepted between members of a highly inbred genetic strain (haplotype) or animals that are genetically identical (identical twins).
Transplants are rejected between members of different haplotype or animals that are genetically nonidentical.
Transplants are accepted from parental haplotype A or B to an F, (AXB) progeny, but transplants in the reverse direction are rejected.
Transplant outcome

11. HISTOCOMPATIBILITY ANTIGENS
Regarding the issue of genetic similarity between transplant donor and recipient (histocompatibility), early observations on transplant outcome led to the identification of a set of genes whose codominant expression elicits vigorous rejection responses in the case of allogeneic transplants. That set of genes has come to be characterized as the major histocompatibility complex or MHC.
It is important to realize that many target antigens exist in cases of graft rejection. The MHC represents the most critical set of genes encoding such cell surface antigens; however, several genetic loci or areas of the MHC have yet to be mapped and defined. Furthermore, another set of genes, encoding the minor histocompatibility antigens, may play a very important role in transplant outcome as well. These genes have been less well characterized compared with those encoded by the MHC and generally are believed to play a weaker role in graft rejection events.
In order for a graft to be accepted, the recipient must share an identical complement of donor genes. For example, donor strain A or B into a strain AXB recipient would result in graft acceptance, since both strain A and B genes would be shared by the AXB recipient. In the case of donor tissue from strain AXB transplanted into either strain A or B recipient, the outcome would be graft rejection, since recipient A has donor A genes but not B genes and recipient B has donor B but not A genes.

12. The structure of HLA class I molecules.

13. The structure of HLA class II molecules

16. HLA Genetics
HLA alleles set found on one chromosome represents a haplotype.
Each individual inherits two MHC haplotypes from each parent and thus he has two alleles for each gene.
These alleles are codominantly expressed.
The inheritence of MHC genes follows the Mendel segregation rules.
Father
Mother

17. 2 digits
HLA-DRB1*13
A group of alleles which encode the DR13 antigen, i.e. there 62 DRB1*13 alleles: 1301 to 1362

18. 4 digits
HLA-DRB1*1301
A specific allele

19. 8 digits
HLA-DRB1*
An allele which contains a mutation outside the coding region

20. 4 digits
HLA-DRB1*1301N
A null allele N

21. 8 digits
HLA-DRB1* N
A null allele N which contains a mutation outside the coding region

22. 4 digits
HLA-DRB1*1301L
L indicated that the allele is low expressed at the cell surface

23. 4 digits
HLA-DRB1*1301S
S indicated that the alleles is secreted but not stable at the cell surface, find at soluble form

24. 4 digits
HLA-DRB1*1301C
C indicated that the allele is present in the cytoplasm but not at the cell surface

25. Host immune response against allograft
Direct antigen presentation
The response of recipient T cells to intact MHC/peptide complexes on APCs from a graft is called direct allorecognition.
That is APCs in the graft directly present alloantigens (the foreign MHC molecules) for recognition by alloreactive T cells.
A recipient’s T cells can also react to donor MHC peptides presented on the recipient’s own APCs. This pathway is called indirect allorecognition.

29. MEDIATORS OF REJECTION
Several components of the immune system are known to mediate graft rejection.
First, we will consider antibody-mediated graft cell destruction, where antibodies specific for graft cell MHC antigens may be elicited with CD4+ T cell help generated in response to foreign graft MHC class II molecules.
In the presence of complement, these antigraft antibodies would be capable of lysing (killing) graft target cells.
In the case of cell-mediated graft destruction, at least two scenarios may be considered.
In the first, foreign graft MHC class II molecules stimulate host T-helper cells to provide "help" to host CD8+ T cytotoxic cells, which then may exert lytic action directly via recognition of foreign graft MHC class I molecules. Alternatively, stimulated host T helper cells may aid macrophages in an MHC-independent fashion to produce molecules capable of destroying graft cells.
Graft rejection has come to be regarded as being largely cell-mediated, with the T lymphocyte the primary effector cell. This fact is not surprising if one considers again and observes the central role occupied by the T cell.
In addition to the cytotoxic T cell, both antibody-mediated and macrophage mediated graft destruction rely upon T cell help. Antibody-mediated graft destruction, a feature of hyperacute rejection events, also plays an important role in a special situation termed second set rejection.
In this case, a transplant recipient is retransplanted due to primary graft failure, and upon encounter with the graft's foreign antigens or alloantigens to which the host was previously exposed, the host's immune system generates a hyperacute rejection response. This response consists of preformed cytotoxic antibodies formed during the host's first encounter with graft antigens.

30. Mediators of rejection

32. INFLUENCES ON TRANSPLANT SUCCESS
Several factors affect the outcome of tissue transplantation, notably the degree of histocompatibility between donor and recipient.
Fortunately, the availability of reagents specific for cell surface proteins encoded by the MHC has permitted a high degree of cross-matching of donor and recipient where possible prior to transplantation.
Tissue typing/cross-matching is the identification of the MHC type of transplant donor and recipient to optimize genetic similarity or match prior to transplant.
This method and consequently transplant outcome will continue to improve as more MHC loci are characterized and reagents recognizing the products of these gene loci are prepared. Equally important in cross-matching transplant donor and recipient is the knowledge of prior sensitization or foreign antigen encounter by a prospective transplant recipient. Events such as blood transfusion and pregnancy prior to transplant, as well as previous transplants, all may affect transplant success and result in responses similar to hyperacute or second set rejection, a rapid and vigorous reaction.
Of critical importance to transplant success and viability is the degree of tissue or organ preservation prior to grafting. Ischemia time, defined as the time during which the donor organ has been deprived of its proper blood supply, drastically affects transplant outcome and should be kept to a minimum. Ischemia time is particularly important for the proper functioning of kidney, heart, and liver transplants, where organ preservation is an issue. Indeed, considerations of organ preservation in the case of kidney transplants are compounded by the availability of cadaveric kidney donors, where ischemia time and longer term organ preservation affect transplant viability.
Interestingly, there exist a few sites in the body (central nervous system, reproductive tract) that are considered relatively "immune privileged' in terms of their vulnerability to an immune response. These sites generally lack lymphatic drainage and express few MHC antigens and are therefore weakly immunogenic.

33. Clinical signification
Successful transplantation relies upon the immunologic compatibility of recipients and their organ donor.
This compatibility depends on both, the extent of matching of the tissue types (HLA types) of donor and recipient and the absence of any pre-existing antibody reactivity of the recipient with the donor.
Both these factors influence the degree of immunosuppression required to prevent rejection of the graft by the recipient.
The key element to successful transplantation is the ability to correctly identify the tissue types of recipients and donors and to predict whether a graft is likely to be rejected ( host –versus graft disease) or in the case of a BMT whether the graft will attack the recipient ( graft –versus host disease GvHD)

35. HLA typing: Bone Marrow transplantation
Liver transplantation
Renal transplantation
Pancreatic transplantation
Heart transplantation
Lung transplantation
National Bone Marrow Donor Registry
Waiting list for - Bone Marrow transplantation
- Liver and Renal transplantation
- Lung transplantation
- Heart transplantation