Tissue
Typing Education Primer
Part
5 of 6
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1(Intro), 2(Antigens), 3(Genetics), 4(Methods),
6(References)
CLINICAL
RELEVANCE OF THE HLA SYSTEM
Despite the temptation to think of them as "transplantation
antigens", HLA antigens are not present on tissues simply to confound
transplant surgeons.
A most important function of MHC molecules is in their induction and
regulation of immune responses. Tlymphocytes recognise foreign antigen
in combination with HLA molecules.
In an immune response, foreign antigen is processed by and presented
on the surface of a cell (eg. macrophage). The presentation is made
by way of an HLA molecule. The HLA molecule has a section, called its
antigen (or peptide) binding cleft, in which it has these antigens inserted.
Tlymphocytes interact with the foreign antigen/HLA complex and are
activated.
Upon activation, the T cells multiply and by the release of cytokines,
are able to set up an immune response that will recognise and destroy
cells with this same foreign antigen/HLA complex when next encountered.
The exact mode of action of HLA Class I and HLA Class II antigens is
different in this process.
HLA Class I molecules, by virtue of their presence on all nucleated
cells, present antigens which are peptides produced by invading viruses.
These are specifically presented to cytotoxic T cells (CD8) which will
then act directly to kill the virally infected cell.
HLA Class II molecules, have an intracellular chaperone network which
prevents endogenous peptide from being inserted into its antigen binding
cleft. They instead bind antigens(peptides) which are derived from outside
of the cell (and have been engulfed). Such peptides would be from a
bacterial infection.
The HLA Class II molecule presents this "exogenous" peptide
to helper T cells (CD4) which then set up a generalised immune response
to this bacterial invasion.
Thus it is apparent that MHC products are an integral part of immunological
health and therefore it is no surprise to see a wide variety of areas
of clinical and genetic implications. The following is a general overview
of some of the important functional aspects of HLA antigens.
HLA and Transfusion
The HLA Class I antigens are carried in high concentrations by leucocytes
and platelets, but only in trace amounts on erythrocytes. Each transfusion
of either platelets or leucocytes therefore carries a risk of immunising
the patient.
Patients, with an intact immune system, who require multiple transfusions
of whole blood, platelets or leucocyte concentrates will therefore usually
develop antibodies to HLA antigens. This risk can be minimised by washing
or filtering the red cell preparations and by reducing leucocyte contamination
as far as practicable.
In multi-transfused patients, such as those with leukaemia, antiHLA
antibodies may lead to two problems. Firstly, these patients become
refractory to platelet transfusions which they destroy rapidly, and
secondly nonhaemolytic transfusion reactions may occur in response
to HLA antigens. Both these problems can be circumvented with some difficulty.
Family donors, especially HLA identical siblings, provide one source
of platelets which may not be consumed. It is possible to use platelet
or lymphocyte crossmatching techniques to confirm the suitability of
an individual donor, but there is a limit to the frequency with which
a single individual can provide platelets.
An alternative source is to HLA phenotype a bank of potential platelet
volunteers for use in the appropriate patients. The disadvantage of
this approach is that the polymorphism of each of the HLA class I allelic
systems gives rise to low chances of finding HLA matched donors for
patients with any tissue type other than an extremely common one. The
volunteer banks thus have to be large to offer any chance of success.
HLA and Transplantation
Renal Transplants
HLA typing was applied to kidney transplantation very soon after
the first HLA determinants were characterised. The importance of reducing
mismatched antigens in donor kidneys was immediately apparent with superior
survival of grafts from HLA identical siblings compared to one haplotype
matches or unrelated donors.
It is apparent that the effect of HLA matching is significant, even
with the highly efficient immunosuppression used today.
In renal transplantation there are two major priorities that reduce
the (already low) chance of obtaining good HLA matching. These are the
need for ABO compatibility and the need for a negative Tlymphocyte
crossmatch (using cytotoxicity). AntiHLA Class I antibodies present
at the time of transplant will cause "hyperacute rejection"
of the graft (ie when the T cell crossmatch is positive).
Liver Transplantation
Patients awaiting liver transplantation can seldom afford to wait
for a well matched graft. Therefore, liver transplantation is more involved
with problems such as physical size rather than HLA. Also with the effects
of CyclosporinA and the action of the liver itself as a form of "immunological
sponge" (to mop up immune complexes) the effect of HLA matching
is difficult to determine.
The lymphocytotoxic T cell crossmatch is an important factor in liver
transplantaion. Transplants which are, through urgency, carried out
despite a positive T cell crossmatch have a significantly lower success
rate.
Heart Transplants
There has only recently been sufficient accumulated experience to show
the effect of HLA antigens on cardiac allograft survival.
It is now clear that HLADR antigens exert a powerful effect, analogous
to that seen in renal transplantation. However, the problem of applying
that knowledge to clinical practice is more analogous to liver transplantation.
Cardiac size match and availability at the right time, are of more pressing
importance than matching HLA antigens.
Corneal Transplantation
There is evidence that the cornea may survive slightly better if the
HLA class I antigens are matched. In the low risk, first corneal graft
recipient the efforts to HLA match and the hindrance to routines of
corneal procurement and transplantation do not appear to warrant tissue
typing.
The situation is however somewhat different when the recipient's cornea
has become vascularised from previous inflammation or they have rejected
one or more previous grafts. In those patients, matching for HLA class
I antigens does seem to be worthwhile.
Bone Marrow Transplantation
HLA matching of bone marrow donor and recipient is crucial to the
success of the graft. Incompatibility may not only lead to rejection
but also to the greater problem of graftversushostdisease (GVHD)
in which the immunologically compromised recipient is "attacked"
by the grafted bone marrow.
Most bone marrow transplants involve HLAidentical siblings with the
HLA identity confirmed by family study and MLC or highly definitive
molecular genetic techniques. The 5-year success rate for HLA identical
sibling bone marrow transplants for CML are shown in Figure 9.
Failing an HLA identical sibling being available, a close relative with
very similar (eg one HLA antigen mismatch) may be considered.
However since 60% to 70% of potential candidates do not have a suitable
family member to act as a donor, there has been interest in developing
lists of tissue typed volunteers prepared to donate bone marrow. There
are now a number of such registries established, which by international
collaboration now stand a reasonable chance of finding an HLA-A,B,C,DR,and
DQ matched donor for a further 30% or 40% of candidates. The success
rates of these transplants was initially dismal, but better conditioning
of both the patient and bone marrow are now resulting in much better
results.
HLA and Paternity Testing
The vast polymorphism of the HLA system makes it a most valuable
tool in the field of paternity testing.
There are two possible roles of paternity testing depending upon the
situation. Probability of exclusion of paternity, and probability of
paternity require different mathematical formulae. Excluding paternity
may in some cases be straightforward, for example when a putative father
does not have any of the HLA antigens that a child must have inherited
from its father, this is first order exclusion. When the father is homozygous
or the child appears to be homozygous it is possible that unidentified
antigens explain the differences.
The probability of paternity, on the other hand has to consider, even
when the HLA antigens are fully compatible with paternity, that an alternate
father may have possessed those same antigens.
Most laboratories now combine red cell and HLA testing of the mother,
child and putative father, together with further genetic analysis, such
as "DNA fingerprinting" where direct comparison of DNA fragments
yields excellent results.
HLA and Disease Susceptibility
In the 1960's, it was discovered that the mouse MHC (called H2)
controlled both the genetic susceptibility to certain leukaemias and
the immune response to certain antigens. Since then innumerable reports
have been published aimed at discovering the role of the human MHC in
the control of responsiveness and disease susceptibility.
There are two general explanations for HLA and disease associations.
Firstly, there may be linkage disequilibrium between alleles at a particular
disease associated locus and the HLA antigen associated with that disease
this is so for HLAA3 and Idiopathic Haemochromatosis.
Another possible explanation for these associations is that the HLA
antigen itself plays a role in disease, by a method similar to one of
the following models:-
a) by being a poor presenter of a certain viral or bacterial antigen
b) by providing a binding site on the surface of the cell for a disease
provoking virus or bacterium
c) by providing a transport piece for the virus to allow it to enter
the cell
d) by having a such a close molecular similarity to the pathogen that
the immune system fails to recognise the pathogen as foreign and so
fails to mount an immune response against it.
It is most likely that all these mechanisms are involved, but to a varying
extent in different diseases. In multiple sclerosis and ankylosing spondylitis,
cell mediated immunity is often depressed, not only in the patients
but also in their parents and siblings. Complement (C2) levels are known
to be low in systemic lupus erythematosus, a disease associated with
HLA DR2 and DR3. In gluten enteropathy, which shows a high association
with HLADR3, a specific gene product is thought to act as an abnormal
receptor for gliadin, the wheat protein, and present it as an imunogen
to the body.
Whatever the explanation for the long list of HLA and disease associations,
it is clear that the HLA system, collaborating with other nonlinked
genes have an influence on our response to environmental factors which
provoke disease.
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to Part 1(Intro), 3(Genetics),
4(Methods), 5(Relevance),
6(References)
