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● ● ● HEMOSTASIS
Comment on Hausl et al, page 3415
Factor
VIII tolerance: is more better?
---------------------------------------------------------------------------------------------------------------Jay Lozier
FDA CENTER FOR BIOLOGICS EVALUATION AND RESEARCH
Immune tolerance protocols have been used to eliminate factor VIII inhibitors for
more than 20 years, but the mechanism for tolerance induction is not well understood. Hausl and colleagues provide mechanistic insights in this issue of Blood.
nhibitor antibodies to factor VIII complicate the treatment of hemophilia A in about
20% of patients with severe disease. Optimal
management of inhibitors consists of their
elimination to permit unfettered replacement
therapy with factor VIII. The strategy for
elimination of inhibitor antibodies traditionally has been to administer factor VIII for prolonged periods of time with the goal of deleting
reactive B-cell clones or inducing their unresponsiveness to factor VIII.
Clinicians can cite empiric examples of
success with high doses of factor VIII (the
“Bonn protocol”)1 or lower doses of factor
VIII to induce tolerance.2 Since factor VIII is
expensive and most immune tolerance induction protocols require indwelling catheters
(with the attendant risk for infection), insights
into how best to achieve tolerance in as little
time as possible are particularly valuable.
In this issue, Hausl and colleagues present
the results of their investigation of the mechanism for immune tolerance induction in mice.
They first immunized hemophilia A knockout mice with factor VIII and transferred
memory B cells to naive mice. They then
showed that a single high dose of intravenous
factor VIII delivered to the recipients prevented restimulation and differentiation of
specific memory B cells into antibody-secreting plasma cells. Conversely, low doses of factor VIII actually stimulated antibody
production.
In vitro experiments in this report indicated that the effect involved activation of
caspases and Fas/Fas ligand interaction, and
did not involve factor VIII–specific T cells.
The “threshold” effect observed in vivo suggests that higher doses of factor VIII may be
more effective than lower doses for immune
tolerance induction and may lead to apoptosis
of memory B cells.
It is always appropriate to be cautious when
extrapolating from animal models to human
diseases or conditions. For instance, the effect
I
3334
might be less clear in humans with hemophilia
due to different response of outbred human
populations compared with inbred mice (such
as the B6 strain that is less responsive to factor
VIII than other strains).3 Further, the in vivo
experiments were done in mice lacking antibodies to factor VIII that presumably would, if
present, neutralize some of the administered
factor VIII.
Regardless of these limitations, this study
provides mechanistic support for the highdose strategy of immune tolerance induction.
The doses of factor VIII that were most effective in inducing tolerance in this study corresponded to 200 to 2000 times normal factor
VIII levels. Theoretically, a short course of a
few extremely high doses of factor VIII might
have practical advantages over longer duration
regiments. However, this approach might pose
its own risk, since chronically elevated levels of
various coagulation factors (including factor
VIII) are associated with higher risk for
thrombosis. In all, this study suggests mechanistic explanations for clinical observations
and deserves consideration in the design and
interpretation of factor VIII tolerance induction trials.
This piece represents the opinion of the author
and does not constitute official US government
policy. ■
REFERENCES
1. Brackmann HH, Oldenburg J, Schwaab R. Immune
tolerance for the treatment of factor VIII inhibitors—
twenty years’ Bonn protocol. Vox Sang. 1996;70(suppl
1):30-35.
2. Ewing NP, Sanders NL, Dietrich SL, Kasper CK. Induction of immune tolerance to factor VIII in hemophiliacs
with inhibitors. JAMA. 1988;259:65-68.
3. Rawle FE, Shi CX, Brown B, et al. Heterogeneity of the
immune response to adenovirus-mediated factor VIII gene
therapy in different inbred hemophilic mouse strains.
J Gene Med. 2004;6:1358-1368.
● ● ● IMMUNOBIOLOGY
Comment on Palena et al, page 3515
MVA-TRICOM vaccine strategy makes CLL
cells
an attractive T-cell target
---------------------------------------------------------------------------------------------------------------John C. Byrd
THE OHIO STATE UNIVERSITY
In this issue of Blood, the paper by Palena and colleagues extends potential strategies for vaccine development in CLL.
o date, vaccine strategies in CLL have been
limited. This is due partly to CLL patients
having an impaired immune system that in part
relates to CLL cells not expressing many of the
necessary costimulatory cell surface antigens
required for effective T-cell recognition and
activation. In the absence of such cell surface
antigens, CLL cells are able to actively proliferate and remain untouched by native, anergic,
autologous T cells of patients with this disease.
Palena and colleagues in this issue of Blood have
provided an alternative strategy to increase expression of 3 essential costimulatory molecules
(CD80, lymphocyte function-associated antigen
3 [LFA-3], and intercellular adhesion molecule 1
T
[ICAM-1]) in primary CLL cells. Specifically,
the authors examined the ability of 3 types of
highly attenuated, nonreplicating engineered
pox virus constructs to infect and subsequently
express the 3 costimulatory molecules CD80,
LFA3, and ICAM1 in primary CLL cells. In
these studies, the modified vaccinia virus strain
Ankara (MVA)–triad of costimulatory molecules
(TRICOM) highly attenuated vaccinia virus was
best able to increase both the proportion of cells
expressing and antigen density of these specific
antigens. Both allogeneic T cells from healthy
donors and autologous T cells from patients with
CLL were activated as evidenced by proliferation and cytokine release when exposed to
15 NOVEMBER 2005 I VOLUME 106, NUMBER 10
blood
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MVA-TRICOM–infected cells. The authors
then carefully demonstrate the specificity of expression of costimulatory molecules in this process by blocking studies with antibodies directed
at CD80, LFA-3, and ICAM1. Cytotoxic T-cell
clones derived from these studies were also able
to mediate cytotoxicity toward CLL cells not
infected by MVA-TRICOM.
The significance of these in vitro studies using primary CLL cells is substantial for future
clinical development of vaccine strategies for this
disease. A construct similar to MVA TRICOM
described in this paper has been tested in immunocompromised HIV-infected patients without
significant consequence. Unlike some vaccine
therapies where the T-cell–specific target is
known, this therapeutic approach depends upon
unrecognized tumor cell antigens. Nonetheless,
this paper demonstrates several ex vivo assays
that can be used to follow T-cell response to native CLL cells not infected with this virus. Thus,
the authors have provided preclinical evidence
that this strategy might have therapeutic value
in CLL and pharmacodynamic assays that
could potentially be used to follow the effect of
therapy in vivo.
All of the preclinical data presented by
Palena and colleagues provide justification for
consideration of clinical trials with this reagent. One major question that remains is the
optimal clinical design of introducing MVATRICOM into CLL patients. Vaccine strategies generally work most effectively in the setting of minimal residual disease, which would
likely necessitate treatment with CLL therapy
prior to administering MVA-TRICOM. Unfortunately, CLL therapies such as fludarabine and alemtuzumab are immunosuppres-
sive and could impact the MVA-TRICOM
vaccine’s success. Assessment of autologous T-cell response to MVA-TRICOM–
infected CLL cells derived from patients
treated with fludarabine would be an additional preclinical step to pursue. Additionally,
the authors have offered that the MVATRICOM vaccine could occur via (1) ex vivo
infection of CLL cells with the modified vaccinia virus TRICOM followed by infusion of
these cells into the patient immediately after
this or (2) direct administration to the patient
as a vaccine. Identifying which of these schedules is optimal through preclinical animal
models of CLL or as part of early clinical trials
would be ideal. Palena and colleagues’ data
with the MVA- TRICOM vaccine clearly provide justification to pursue these additional
studies. ■
● ● ● CLINICAL OBSERVATIONS
Comment on Jelinek et al, page 3370, Kralovics et al, page 3374, and Levine et al,
page 3377
V617F
“JAKs” up myeloproliferative signal
---------------------------------------------------------------------------------------------------------------Ayalew Tefferi
MAYO CLINIC
JAK2V617F, an ostensibly myeloid lineage-specific mutation, also appears to be myeloid disease–specific according to 2 studies in the current issue of Blood, which
also features a third study that identifies altered gene expression in polycythemia
vera as a surrogate for JAK-STAT hyperactivation.
t all started with HOP, a JAK homolog in
Drosophila, where a dominant mutation in
either the Janus homology 4 (JH4) (HOPTum-1)
or JH2 (HOPT42) domain resulted in constitutive kinase activity, phosphorylation of Drosophila signal transducer and activator of transcription (STAT), and leukemia-like defects.1
A similar point mutation in murine Janus kinase 2 (JAK2; JAK2E665K) also resulted in an
activated protein, leading the authors to propose the leukemogenic potential of JAK2 mutations in mammals.1 Around the same time
(late 1990s), other investigators demonstrated
inhibition of acute lymphocytic leukemia
(ALL) cell growth by a JAK2 kinase inhibitor
(AG-490) and the association of TEL/JAK2
(t(9;12)(p24;p13)) with both T and pre-B
ALL as well as atypical myeloproliferative
disorder (MPD).2 Most recently, another
JAK2 fusion mutant, PCM1/JAK2 (t(8;
9)(p21-23;p23-24)), has been associated with
atypical MPD (AMPD), ALL, and acute myeloid leukemia (AML).3
I
(23%-57%), and MMM (35%-57%), 2subsequent studies disclosed the occurrence of the
same mutation in a spectrum of atypical
MPDs as well as in myelodysplastic syndrome
(MDS), albeit at a much lower mutational
frequency (3%-33%).4,5 In one of these latter
studies, JAK2V617F and other oncogenic kinase
mutations including BCR/ABL and FIP1L1PDGFRA were shown to be mutually exclusive events.5
In the current issue of Blood, 2 additional
studies from Levine and colleagues and Jelinek
and colleagues confirm the presence of
However, JAK2’s claim to fame came
about with the description of a novel somatic
point mutation (a G-C
to T-A transversion, at
nucleotide 1849 of
exon 12, resulting in
the substitution of valine to phenylalanine at
codon 617; JAK2V617F)
in classic, BCR/ABLnegative MPD including polycythemia vera
(PV), essential thrombocythemia (ET), and
myelofibrosis with
myeloid metaplasia
(MMM).2 However,
following the initial
wave of 5 studies that
reported a relatively
high incidence of
Phenotypic diversity associated with the JAK2 V617F tyrosine kinase mutation might
JAK2V617F in PV
arise from a combination of the particular myeloid progenitor compartment that is af(65%-97%), ET
fected and specific secondary mutations that occur during clonal evolution.
blood 1 5 N O V E M B E R 2 0 0 5 I V O L U M E 1 0 6 , N U M B E R 1 0
3335
From www.bloodjournal.org by guest on July 28, 2017. For personal use only.
2005 106: 3334-3335
doi:10.1182/blood-2005-09-3617
MVA-TRICOM vaccine strategy makes CLL cells an attractive T-cell
target
John C. Byrd
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