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

16 th Congress of the European Hematology Association
increased Th1/Th2 ratio in ITP patients compared with
controls. 15 In keeping with these findings, investigation
of whole blood gene expression profile using DNA
microarrays identified an ITP-specific signature, which
also included interferon (IFN)-induced genes, such as
GBP2 and IFIT2. 16 Pathway analysis demonstrated that
IFN signaling, death receptor, and protein ubiquitination
pathways were associated with ITP.
Other studies have shown that patients with chronic
ITP often exhibit expansion of oligoclonal T-cells 15,17 and
the presence of cytotoxic T cells against autologous
platelets. 18 In fact, T cells from patients with ITP show
increased expression of cytotoxic genes, such as tumor
necrosis factor- alpha, perforin, and granzyme A and
granzyme B. 18,19 Interestingly, several members of the
killer cell immunoglobulin-like receptor (KIR) family
were upregulated in patients with active disease, and
CD3+ lymphocytes expressing KIRs were greater in
number in ITP patients in remission than in patients
with active ITP or normal controls patients. 18 Since KIRs
downregulate cytotoxic T-lymphocyte (CTL) and natural
killer cell responses by binding to MHC class I molecules,
thereby preventing lysis of target cells, it may be
speculated that cytotoxic T cells play a part in at least
some patients with ITP.
The emergence of antiplatelet autoantibodies and
antiplatelet cytotoxic T cells is a consequence of a loss
of the immunological tolerance for self antigens. Filion
et al. have shown that autoreactive T cells directed
against GPIIb/IIIa are present in the peripheral blood of
all healthy individuals, 20 implying that peripheral tolerance
mechanisms are crucial to prevent autoreactive T
cells from becoming activated. Several other T cell
abnormalities have emerged from the investigation of
immune regulation in ITP patients. Among these,
CD4+CD25+ regulatory T cells have an impaired suppressive
activity when compared with healthy subjects.
21 Also, CD3+ T lymphocytes from patients with
active ITP present an altered expression of genes associated
with apoptosis and are significantly more resistant
to dexamethasone-induced suppression compared with
normal lymphocytes. 18,22
As far as B cells are concerned, the expansion of
autoreactive clones is suppressed in the bone marrow. If
some B cells escape this suppression or deletion, peripheral
mechanisms, most importantly the functional balance
between activating and inhibitory Fc receptors
(FcR), may also be launched to maintain tolerance. 23
Antigen-presenting cells in ITP
Like all immunoglobulin (Ig) isotype-switched IgG
antibody responses, autoreactive IgG against a protein
antigen is initiated by activated T helper cells that recognize
specific peptide sequences on major histocompatibility
complex class II-positive antigen-presenting
cells (APCs). The role of APCs for the loss of tolerance
in ITP remains unclear, but dendritic cells from patients
with ITP have upregulated costimulatory molecules
enhancing autoreactive T- and B-cell responses against
platelets. 24 A model has been advanced in which APCs
are crucial in generating a number of new or cryptic epitopes
from platelet glycoproteins. 25 In this model, APCs
expressing these novel peptides, and along with costimulatory
molecules, induce the activation of T cells
that recognize these additional platelet antigens. Thus,
this acquired recognition of new self-determinants, or
epitope spreading, may play an important role in the initiation
and perpetuation of ITP. T-cell clones that react
with cryptic epitopes may escape the negative selection
in the thymus when self-determinants are present at a
sub-threshold concentration.
Infection-associated ITP and the role of molecular mimicry
Thrombocytopenia may accompany or follow a variety
of infections from which ITP must be differentiated.
In adults, the most prevalent infections associated with
thrombocytopenia are those from hepatitis C virus
(HCV), human immunodeficiency virus (HIV), and
Helicobacter pylori (H. pylori). 26 In typical cases, the thrombocytopenia
presents with an insidious onset, has no
tendency to remit spontaneously (although its severity
may parallel the stage of the infectious disease), and
may closely mimic chronic ITP. Response to infection
may generate antibodies that cross-react with platelet
antigens, most notably GP IIb-IIIa or immune complexes
that bind to platelet Fc receptors. 27–30
Many patients with HIV-associated thrombocytopenia
have autoantibodies that recognize a restricted peptide
sequence (GPIIIa49-66) in platelet membrane
GPIIIa, and can be recovered from patient plasma in the
form of immune complexes consisting of autoantibody
and platelet fragments. 27,28 Recently, Zhang et al. found
that sera from patients coinfected with HCV and HIV
reacted with four peptides present in nonconserved
regions of the HCV core envelope 1 protein. 29 Antibodies
raised against one of these peptides (PHC09) caused
severe thrombocytopenia when injected into wild-type
mice whose GPIIIa is more than 80% identical to that of
humans. Immunization of wild-type mice with HCV
core envelope protein 1 had no effect on platelet count.
However, NZB/W F1 mice, a strain in which immune
surveillance is defective, produced antibodies specific
for PHC09 and became thrombocytopenic. The titer of
PHC09-specific antibody in patients coinfected with
HCV, and HIV correlated with both the incidence of
thrombocytopenia and its severity. The authors conclude
that humans and immunodeficient mice immunized
with HCV core envelope protein 1 often produce
antibodies that recognize GPIIIa49-66 through molecular
mimicry and are capable of causing clinically significant
thrombocytopenia.
With regards to ITP and H. pylori infection, plateletcross-reactivity
has been shown between platelet-associated
immunoglobulins and bacterial components, in
particular cytotoxin-associated gene (Cag) A protein 30
and urease B. 31
In support of the molecular mimicry theory, microbial
reduction/eradication leads to remission in a substantial
fraction of infected patients. In the case of H. pylori,
questions remain regarding why there appears to be
marked variation in response rates among patients from
different geographic areas. 32
Genetic factors
Little is known of the genetic factors that may contribute
to the predisposition to develop ITP or influence
the natural history of ITP and response to treatment. A
pilot study in 37 children with chronic ITP investigated
| 174 | Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1)