Graft Donors and Recipients Are Typed for RBC and MHC Antigens

Since differences in blood group and major histocompatibility antigens are responsible for the most intense graft-rejection reactions, various tissue-typing procedures to identify these antigens have been developed to screen potential donor and recipient cells. Initially, donor and recipient are screened for ABO blood-group compatibility. The blood-group antigens are expressed on RBCs, epithelial cells, and endothelial cells. Antibodies produced in the recipient to any of these antigens that are present on transplanted tissue will induce antibody-mediated complement lysis of the incompatible donor cells.

HLA typing of potential donors and a recipient can be accomplished with a microcytotoxicity test (Figure 21-4a, b). In this test, white blood cells from the potential donors and recipient are distributed into a series of wells on a microtiter plate, and then antibodies specific for various class I and class II MHC alleles are added to different wells. After incubation, complement is added to the wells, and cytotoxicity is assessed by the uptake or exclusion of various dyes (e.g., trypan blue or eosin Y) by the cells. If the white blood cells express the MHC allele for which a particular monoclonal antibody is specific, then the cells will be lysed upon addition of complement, and these dead cells will take up a dye such as trypan blue. HLA typing based on antibody-mediated microcyto-toxicity can thus indicate the presence or absence of various MHC alleles.

Even when a fully HLA-compatible donor is not available, transplantation may be successful. In this situation, a one-way mixed-lymphocyte reaction (MLR) can be used to quantify the degree of class II MHC compatibility between potential

HLA-A allele 2 HLA-A allele 1

Donor cell Recipient cell

Antibody to "HLA-A allele 2^

Complement

Cells become leaky

Dye (trypan blue or eosin Y)

No lysis

FIGURE 21-4

Typing procedures for HLA antigens. (a, b) HLA typing by microcytotoxicity. (a) White blood cells from potential donors and the recipient are added to separate wells of a microtiter plate. The example depicts the reaction of donor and recipient cells with a single antibody directed against an HLA-A antigen. The reaction sequence shows that if the antigen is present on the lymphocytes, addition of complement will cause them to become porous and unable to exclude the added dye. (b) Because cells express numerous HLA antigens, they are tested separately with a battery of antibodies specific for various HLA-A antigens. Here, donor 1 shares HLA-A antigens recognized by antisera in wells 1 and 7 with the recipient, whereas donor 2 has none of HLA-A antigens in common with the recipient. (c) Mixed lymphocyte reaction to determine identity of class II HLA antigens between a potential donor and recipient. Lymphocytes from the donor are irradiated or treated with mitomycin C to prevent cell division and then added to cells from the recipient. If the class II antigens on the two cell populations are different, the recipient cells will divide rapidly and take up large quantities of radioactive nucleotides into the newly synthesized nuclear DNA. The amount of radioactive nucleotide uptake is roughly proportionate to the MHC class II differences between the donor and recipient lymphocytes.

Dye taken up

Dye excluded

(b) Antibody to different HLA-A antigens

l

2

3

4

5

B

7

B

9

Recipient

o

o

o

o

o

o

o

Donor 1

o

o

o

o

o

o

o

Donor 2

O

o

o

o

o

o

o

Irradiation i x Allele A Donor cells

Activation and proliferation of

Allele B

class II MHC of donor

Recipient cells sharing class II MHC of donor

No reaction proliferation ot J^

Recipient cells lacking aXjp iSf

Incorporation of of radioactivity into cell nuclear DNA

donors and a recipient (Figure 21-4c). Lymphocytes from a potential donor that have been x-irradiated or treated with mitomycin C serve as the stimulator cells, and lymphocytes from the recipient serve as responder cells. Proliferation of the recipient T cells, which indicates T-cell activation, is measured by the uptake of [3H]thymidine into cell DNA. The greater the class II MHC differences between the donor and recipient cells, the more [3H]thymidine uptake will be observed in an MLR assay. Intense proliferation of the recipient lymphocytes indicates a poor prognosis for graft survival. The advantage of the MLR over microcytotoxicity typing is that it gives a better indication of the degree of TH-cell activation generated in response to the class II MHC antigens of the potential graft. The disadvantage of the MLR is that it takes several days to run the assay. If the potential donor is a cadaver, for example, it is not possible to wait for the results of the MLR, because the organ must be used soon after removal from the cadaver. In that case, the microcytotoxicity test, which can be performed within a few hours, must be relied on.

The importance of MHC matching for acceptance of allo-grafts is confirmed by data gathered from recipients of kidney transplants. The data in Figure 21-5 reveal that survival of kidney grafts depends primarily on donor-recipient matching of the HLA class II antigens. Matching or mismatching of the class I antigens has a lesser effect on graft survival unless there also is mismatching of the class II antigens. A two-year survival rate of 90% is seen for kidney transplants in which one or two class I HLA loci are mismatched, while transplanted kidneys with differences in the class II MHC have only a 70% chance of lasting for this period. Those with greater numbers of mismatches have a very low survival rate at one year after transplant. As described below, HLA matching is most important for kidney and bone-marrow transplants; liver and heart transplants may survive with greater mismatching.

Current understanding of the killer-inhibitory receptors (KIR) on the NK cell (see Chapter 14) suggests that absence of a class I antigen recognized by the KIR molecules could lead to killing of the foreign cell. Rejection was observed in experimental bone-marrow transplants where the class I molecule recognized by the recipient NK-inhibitory receptor is absent on donor cells. The effects of such class I mismatching on solid organ grafts may be less marked.

MHC identity of donor and host is not the sole factor determining tissue acceptance. When tissue is transplanted between genetically different individuals, even if their MHC antigens are identical, the transplanted tissue can be rejected because of differences at various minor histocompatibility loci. As described in Chapter 10, the major histocompatibility antigens are recognized directly by TH and TC cells, a phenomenon termed alloreactivity. In contrast, minor histocom-patibility antigens are recognized only when they are presented in the context of self-MHC molecules. The tissue rejection induced by minor histocompatibility differences

_L

Time after transplantation, months

HLA mismatches (no.)

HLA mismatches (no.)

Curve no.

Class I

Class II

1

0

0

2

1 or 2

0

3

3 or 4

0

4

0

1 or 2

5

1 or 2

1 or 2

6

3 or 4

The effect of HLA class I and class II antigen matching on survival of kidney grafts. Mismatching of one or two class I (HLA-A or HLA-B) antigens has little effect on graft survival. A single class II difference (line 4) has the same effect as 3 or 4 differences in class I antigens (line 3). When both class I and class II antigens are mismatched, rejection is accelerated. [Adapted from T. Moen et al, 1980, N. Engl. J. Med. 303:850.]

FIGURE 21-5

The effect of HLA class I and class II antigen matching on survival of kidney grafts. Mismatching of one or two class I (HLA-A or HLA-B) antigens has little effect on graft survival. A single class II difference (line 4) has the same effect as 3 or 4 differences in class I antigens (line 3). When both class I and class II antigens are mismatched, rejection is accelerated. [Adapted from T. Moen et al, 1980, N. Engl. J. Med. 303:850.]

is usually less vigorous than that induced by major histocompatibility differences. Still, reaction to these minor tissue differences often results in graft rejection. For this reason, successful transplantation even between HLA-identical individuals requires some degree of immune suppression.

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  • phillipp busch
    Why is it important that graft recipients share mhc alleles?
    8 years ago

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