Haematologica 2000;85:supplement to no. 10 - Supplements ...

Immune Tolerance and the Treatment of Hemophilacs with an Inhibitor
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large deletions of 5.6, respectively. Missense
mutations and small deletions were associated
with a low relative risk of 0.1 and 0.6, respectively.
Notably, genetic defects that were not
detected in our study showed a high relative risk
of 1.7. The results confirmed the earlier findings
that inhibitor formation mainly occurred in
intron 22 inversions, nonsense mutations and
large deletions. However, more data are needed,
especially for the less frequent mutation types.
MHC class I/II genes and risk of inhibitor
development
The immune response genes e.g. the genes of
the MHC (major histocompatibility complex)
classes represent another parameter that may
be responsible for a genetic predisposition to
inhibitor development. The genes of the MHC
classes are located within a small region of the
short arm of chromosome 6 and therefore are
inherited as a haplotype. The MHC class II genes
DQ, DR and DP are in a focus of interest
because their function is to present extracellular
antigens – such as substitute FVIII – to the
patients’ immune system. In the past, several
studies addressed the influence of these immune
response genes on inhibitor formation. 20-23 However,
the results were inconclusive and sometimes
even contradictory. One main problem of
these former studies was that they were not able
to consider the patients’ FVIII mutation type and
therefore the influence of specific immune
response genes on inhibitor formation might
have been masked by the strong influence of the
FVIII gene defect. Our group, therefore, conducted
a study in which the influence of the
MHC class I/II genotype on inhibitor formation
was exclusively investigated in patients with the
homogenous intron 22 inversion. 4 A distillate of
the results is shown in Table 5. The MHC class
I/II alleles A3, B7, C7, DQA0102, DQB0602 and
DR15 could be assigned as risk alleles (relative
risk 1.9 to 4.0), because they occurred more
often in inhibitor than in non-inhibitor patients.
In contrast the MHC class I/II alleles C2,
DQA0103, DQB0603 and DR13 could be
assigned as protective alleles (relative risk 0.1 to
0.2) because they occurred less often in inhibitor
than in non-inhibitor patients. These MHC class
I/II alleles belonged to extended haplotypes (A3-
B7-C7-DQA0102-DQB0602-DR15 and C2-
DQA0103-DQB0603-DR13) that were also frequent
and less frequent, respectively, in the normal
population. Therefore our number of
patients was too small to reach a clear statistical
significance. Moreover, inheritance as haplotypes
might mask those MHC class I/II alleles
that are decisive for the risk of or protection
from inhibitor formation. From the immunologic
point of view the MHC class I alleles should
be less important than the MHC class II alleles,
which are known to be necessary for the presentation
of extracellular antigens. In this context
Chics et al. 24 made an interesting finding of
a 16 amino residue peptide from the FVIII light
chain (amino acids 1706-1721) that could be
eluted from a DR15 cell line. The peptide was
located on the surface of the FVIII molecule and
bound by two functional cleavage sites.
Notably, Hay et al. 5 found the same MHC class
II alleles to be associated at similar frequencies
with inhibitor formation in patients with intron
22 inversions, thus supporting the concept that
the MHC class II alleles are of some significance
for the risk of inhibitor formation.
Two further studies provide indirect evidence
that genes belonging to the immune response system
may influence the risk of inhibitor development.
Scharrer et al. 25 made a meta-analysis of
three USA studies (Kogenate, 26 Recombinate 27
and US retrospective study 28 ) that clearly demonstrated
the influence of race on inhibitor formation.
In the ethnic group of Africo-Americans the
incidence of inhibitors in severe hemophiliacs was
double (51.9%, 14 of 27) that of in Caucasians
(25.8%, 51 of 191). Cox-Gill 29 compared the incidence
of inhibitor formation in hemophilic siblings
to that in more extended hemophilic relatives
and found a much higher incidence in sibs
(50%) than in extended hemophilic relatives (9%).
Since the genetic defects of the FVIII gene should
have been similar in both studies, the observed
difference in inhibitor incidence should be caused
by genetic variations of the immune system.
Inflammatory modulation of the immune
response
In a recent review Kaufman et al. 6 pointed to
the role of inflammatory processes in inhibitor
formation, if the inflammatory reaction coincides
with exposure to FVIII antigen. He gave two examples
from gene therapy studies on animal models.
In one study two Auburn dogs tolerated canine
FIX infusions but develop FIX antibodies when
the protein was synthesized after FIX gene delivery
by Lentivirus or AAV (both to liver). A similar
observation was made in Rhesus monkeys which
tolerated repeated infusions of human FIX, but
developed antibodies after delivery of human FIX
by an adenovirus to liver. It was suggested that
the liver inflammation induced by the infection
with the virus vector triggered the onset of
inhibitors. In a further study only one of seven
dogs in which canine FIX was delivered by AAV to
skeletal muscle developed an inhibitor. Notably,
this dog had a skin infection at the time of gene
delivery. From the experiences of these animal
models it may be speculated that inflammatory
Haematologica vol. 85(supplement to n. 10):October 2000