Portada Simposios - Supplements - Haematologica

XLII Reunión Nacional de la AEHH y XVI Congreso de la SETH. Programa educacional
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and labelled reference/allele duplex) and three
bands for a heterozygous sample (labelled reference
homoduplex and two labelled reference/allele duplexes).
This method is simple and easy to use, in contrast
to the other DNA based methods described
previously, which require a large number of group
and/or allele specific probe or primer mixes for HLA
class I alleles and thus cannot achieve such high level
of resolution.
As these incompatibilities may be “invisible” in
routine matching techniques, cellular assays have
been developed in an attempt to confirm patient/
donor identity. Limiting dilution analysis has proved
to be a sensitive tool for the detection and investigation
of T lymphocytes of defined specificity. The
cytotoxic T lymphocyte and helper T lymphocyte
precursor (CTLp and HTLp) assays use limiting dilution
analysis to quantify the frequency of donor cytotoxic
T and helper T cell precursors capable of responding
to mismatched HLA antigens present on
the patient’s cells (Kaminski et al, 1989). High CTLp
frequencies correlate with class I mismatches usually
undetected by conventional typing, whereas HTLp
appears capable of detecting class II differences (O’
Shea et al, 1997).
How much do we have to match Many studies
have attempted to evaluate the clinical contribution
of 0, 1, 2 and multiple mismatches (Scott et al,
1998; Sasazuki et al, 1998), but by using high resolution
DNA typing it is becoming clear that once a
mismatch is detected for one locus, there is high
probability of an associated mismatch for another
locus. This is understandable given the strong linkage
disequilibria of HLA. We have observed a high
number of associated hidden mismatches between
HLA-C and either HLA-A or B mismatches and there
seems to be a clear additive effect of “additional
mismatches” in relation to GVHD and TRM. As HLA
types are increasingly defined to higher degrees of resolution,
so the probability of finding a completely
matched VUD is reduced. Until the “true” levels of
HLA identity od donor/ recipient pairs are resolved
by high resolution DNA typing, an understanding of
HLA matching and mismatching will be imprecise
and incompatibility will remain underestimated.
Recent publications have shown these undetected
mismatches to be important with regard to the outcome
of unrelated BMT (Scott et al, 1998; Sasazuki
et al, 1998; Petersdorf et al, 1998). Previous results
had indicated that class II mismatching was more
important than matching for HLA class I, however, it
is now clear that class I is just as important as
class II, and that class II mismatches probably reflected
mismatches at class I that were undetected (Sasazuki
et al, 1998). HLA-C mismatches are now
thought to be more important than ever before, with
results showing a C locus mismatch to a risk factor
for GVHD, and somewhat suprisingly, an HLA-C
match to be related to leukaemic relapse. This is
thought to be due to a graft-versus-leukaemia response
mediated by Killer Inhibitory Receptors (KIRs)
present on natural killer cells or T cells (Moretta et
al, 1997). However, KIR genotyping of HLA-C mismatched
pairs has shown that a the repertoire of
HLA-C ligands possessed by an individual does not
directly correspond to the KIRs expressed, therefore
KIRs may be present which not only recognise the
HLA-C alleles of that individual but others aswell
(A.L.Pay, unpublished data). A mismatch for HLA-A
at the allelic level has been shown to be a risk factor
for acute GVHD and death from all causes (Sasazuki
et al, 1998) and class II mismatching is still seen to
be important with regard to GVHD (Petersdorf et al,
1998). In all cases where a multiple mismatch with
class I and class II alleles was seen the survival rate
was low (Petersdorf et al, 1998).
In unrelated bone marrow transplantation, the
ideal situation would be to match donors and patients
to the highest resolution possible for all loci.
However, due to the polymorphism seen in HLA this
is mostly impossible, especially with loci such as
HLA-DP where there is a very low level of linkage disequilibrium.
Therefore, the route to take must be to
choose the donor which is most acceptable from
those matched to the allelic level, for HLA and other
polymorphic genes which can affect transplant outcome.
This decision will only be possible when a definitive
hierarchy of factors has been established with
respect to transplant outcome and many scientists
are currently working towards this goal.
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