H e m a t o lo g y E d u c a t io n - European Hematology Association

common variants in the regulatory regions of cytokines
(TNF, LTA, IL1RN, IL1A, IL1B, IL4, IL6, and IL10) and
structural variants of the low affinity Fcg receptors
(FCGR2A, FCGR3A, and FCGR3B). 33 Two combinations
of genotypes (TNF and FCGR3A; P = 0.0003, and LTA
and FCGR3B; P = 0.011) were significantly associated
with ITP compared with healthy controls. The
NA1/NA1 genotype of the FCGR3B locus was observed
in 30% of patients compared with 10% of controls and
may be particularly relevant to ITP, as NA1 has a higher
affinity than NA2 to IgG. The heterozygous V/F genotype
of the FCGR3A locus was also more frequent in
patients than in controls (62% vs. 41%). These results
suggest that immune complex handling may play a role
in the pathophysiology of ITP, and that variant FcgR
genes with decreased activity may provide partial protection
against ITP.
With regards to cytokines, the transcriptionally more
active allele of TNF (allele 2 of −308) and the closely
linked LTA allele 1 were both less common in children
with ITP than in healthy controls. No clear hypotheses
to account for these findings have been advanced.
In an earlier study, deletions of Humhv3005, a developmentally
regulated Ig variable (V) gene, and/or highly
homologous VH genes were found more frequently in
ITP patients (14 of 44, 31.8%) than in healthy controls
(7/88, 8%, p = 0.002). 34
Finally, associations with HLA-DRw2 35 and HLA-
DRB1*0410 36 have been reported, although the role
played by these MHC molecules remains obscure.
Mechanisms leading to thrombocytopenia
Ex vivo studies have shown that the spleen is the primary
site of antibody production, 37,38 whereas platelet
kinetic studies have shown that the spleen is also the
dominant organ for the clearance of IgG-coated
platelets. 39,40 In a minority of patients, hepatic clearance
predominates.
Human macrophages express several Fc receptors that
bind IgG specifically. 41 Functionally, there are two different
classes of Fc receptors: the activation and the
inhibitory receptors, which transmit their signals via
immunoreceptor tyrosine-based activation (ITAM) or
inhibitory motifs (ITIM), respectively. Clinical data,
along with information gained from animal models,
suggest that the FcgRI, the high affinity receptor, does
not play a relevant role in ITP. 42,43 On the other hand, evidence
has accumulated to indicate that the low-affinity
receptors FcgRIIA and FcgRIIIA are primarily responsible
for removal of opsonized platelets. 44 Engagement of
FcgRIIA on the surface of human macrophages by anti-
GPIIb/IIIa-coated platelets triggers intracellular signaling
through the tyrosine kinase Syk that leads to engulfment
of the opsonized platelets.
The presence of antibodies against GP Ib/IX has been
associated with resistance to intravenous immunoglobulin
therapy both in a mouse model 45 and in retrospective
series of ITP patients. 46 These findings suggest the
possibility of direct cytotoxicity or complement fixation
as a mechanism of platelet destruction rather than antibody-dependent,
Fc receptor-mediated phagocytosis by
macrophages.
Interestingly, platelet kinetic studies using indium-111
( 111 In)-labeled autologous platelets have shown consider-
London, United Kingdom, June 9-12, 2011
able heterogeneity in platelet turnover in patients with
chronic ITP. 39,40,47,48 While the platelet lifespan is often
markedly decreased in most patients, in some it is only
mildly reduced; furthermore, platelet turnover (a measure
of platelet production) is frequently subnormal.
Overall, approximately 40% of patients with ITP were
found to have a reduced platelet turnover. 39,40
If platelet destruction were the only mechanism to
cause thrombocytopenia, then platelet production
would be expected to increase and offset low platelet
counts. It, therefore, was proposed that thrombocytopenia
may result not only from platelet destruction,
but also from antibody-mediated damage to megakaryocytes.
Evidence to support this hypothesis has accumulated
over time.
McMillan et al. reported that IgG produced by cells
(grown in vitro) from the spleens of patients with ITP
would bind to megakaryocytes, whereas IgG produced
by cells from the spleens of healthy controls did not
bind to megakaryocytes. 49 A few years later, other investigators
demonstrated that antibodies against platelet
antigens would bind to megakaryocytes as well. 50,51
More recent in vitro experiments have further defined
the role of autoantibodies in patients with ITP. Two
studies in particular, by Chang et al. 52 and McMillan et
al. 53 support the view that autoantibodies in ITP suppress
megakaryocyte production and maturation and
platelet release.
Electron microscopy studies have clarified some
aspects of the autoantibody-induced damage in bone
marrow megakaryocytes from patients with ITP.
Extensive megakaryocytic abnormalities were consistently
present in a significant percentage of all stages of
ITP megakaryocyte. 54,55 In the most recent of these studies,
Houwerzijl et al. described the features characteristic
of nonclassic apoptosis, including mitochondrial
swelling with cytoplasmic vacuolization, distention of
demarcation membranes, and condensation of nuclear
chromatin. 55 Para-apoptotic changes could be induced in
megakaryocytes derived from CD34+ cells grown in
ITP plasma, suggesting that autoantibodies may initiate
the cascade of programmed cell death. In addition,
megakaryocytes may be surrounded by neutrophils and
macrophages, suggesting an inflammatory response
against these cells.
A role for direct T cell–mediated cytotoxicity against
platelets has been demonstrated in vitro. 18 Whether this
effect occurs in vivo and its relative importance in determining
platelet destruction has not been elucidated.
There is also evidence that ITP is associated with accumulation
and activation of T cells in the bone marrow
that occurs through increased VLA-4 and CX3CR1
expression. 56 It has been advanced that these activated T
cells may mediate the destruction of platelets in the
bone marrow. 56
Thrombocytopenia associated with infectious diseases
is characterized by antibody-mediated platelet
destruction. However, platelet production may be
impaired by infection of megakaryocytes (HCV and
HIV), decreased production of thrombopoietin (HCV),
and splenic sequestration of platelets secondary to portal
hypertension (HCV). 26
A unique feature of antibodies specific for GP49-66,
frequently found in patients with HIV and HCV infec-
Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1) | 175 |