Blood, 15 February 2003, Vol. 101, No. 4, pp. 1658-1659
CORRESPONDENCE
To the editor:
IVIG induces dose-dependent amelioration of ITP in
rodent models
We have read with interest the paper from Hansen and
Balthasar1 regarding the amelioration of thrombocytopenia
in a rat model of immune thrombocytopenia (ITP). The authors induce
thrombocytopenia with a monoclonal antiplatelet antibody and find that
pretreatment with IVIG (intravenous immunoglobulin) successfully
prevents thrombocytopenia. They also find that IVIG increases the
clearance rate of the antiplatelet antibody and suggest that this
occurs via IVIG saturation of FcRn, the salvage receptor for IgG, as
may be responsible for the protective effect of IVIG. Based upon their
work, the authors state that "...decreases in plasma antibody
levels would lead to decreases in the degree of platelet
opsonization."1(p2092) We have published, however, that
48 hours after IVIG injection, platelet-associated antiplatelet IgG
remained at the same levels as seen in mice receiving antiplatelet
antibody alone2(fig1); this suggests, at least
in the short term, that IgG catabolism does not result in a decrease in
the platelet-bound antibody, which is thought to mediate immune thrombocytopenia.
In an accompanying commentary, Dr John Kelton asked what the effect of
IVIG would be if the pathological antibody was produced under
steady-state conditions as it is in ITP.3 In fact, a recent study from a group in the Netherlands achieved a steady-state level of thrombocytopenia.4 We also find that preinjection of mice with an antiplatelet antibody induces a stable low platelet count (< 20% of normal levels) and that treatment with 2 g/kg body
weight of IVIG (Gamimune; Bayer, Elkhart, IN) successfully reverses the thrombocytopenia.2 This demonstrates that a
single dose of IVIG is effective in steady-state thrombocytopenia in an
animal model.
Hansen and Balthasar state that, to their knowledge, "no reports have
shown dose dependencies in IVIG effects"1(p2091) in
animal models of thrombocytopenia. In fact, we have
reported2 that in passive murine ITP, a dose response is
seen; for instance, IVIG treatment at 0.125 g/kg, 0.5 g/kg,
and 2.0 g/kg doses increased the platelet count 0%, 170%, and 306%, respectively.
Hansen and Balthazar also indicate that IVIG did not neutralize the 7E3
antibody. Both our work2 and that of Teeling et al4 reported in 2001 that preincubation of IVIG with the
antiplatelet antibody used to induce ITP does not diminish the
antibody's ability to bind to platelets in vitro or in vivo. We also
found that IVIG depleted of mouse IgG-reactive components and of
antibodies reactive with the antiplatelet antibody (ie, anti-idiotypic
antibodies) was able to prevent thrombocytopenia as successfully as
unmanipulated IVIG.
The publication by Hansen and Balthasar raises some interesting points
regarding the ability of IVIG to promote the clearance of antiplatelet
antibodies in murine ITP. Both their work1 and
ours2 indicate that anti-idiotypic antibody interference of antiplatelet antibody binding does not appear to be responsible for
the amelioration of ITP by IVIG (or at least is not solely responsible). We suggest that IVIG ameliorates immune thrombocytopenia via either competitive reticuloendothelial system blockade or reticuloendothelial system inhibition via an Fc
RIIB-dependent mechanism.5
Andrew R. Crow, Seng Song, John W. Semple, John Freedman, and Alan H. Lazarus
Correspondence: Alan H. Lazarus, Transfusion
Medicine Research, St Michael's Hospital, 30 Bond St, Toronto, ON,
Canada M5B 1W8; e-mail:
lazarusa{at}smh.toronto.on.ca
References
1.
Hansen RJ, Balthasar JP.
Effects of intravenous immunoglobulin on platelet count and antiplatelet antibody disposition in a rat model of immune thrombocytopenia.
Blood.
2002;100:2087-2093[Abstract/Free Full Text].
2.
Crow AR, Song S, Semple JW, Freedman J, Lazarus AH.
IVIg inhibits reticuloendothelial system function and ameliorates murine passive-immune thrombocytopenia independent of anti-idiotype reactivity.
Br J Haematol.
2001;115:679-686[CrossRef][Medline]
[Order article via Infotrieve].
3.
Kelton JG.
The mechanism of action of high-dose IgG in idiopathic thrombocytopenic purpura.
Blood.
2002;100:1933[Free Full Text].
4.
Teeling JL, Jansen-Hendriks T, Kuijpers TW, et al.
Therapeutic efficacy of intravenous immunoglobulin preparations depends on the immunoglobulin G dimers: studies in experimental immune thrombocytopenia.
Blood.
2001;98:1095-1099[Abstract/Free Full Text].
5.
Samuelsson A, Towers TL, Ravetch JV.
Anti-inflammatory activity of IVIG mediated through the inhibitory Fc receptor.
Science.
2001;291:484-486[Abstract/Free Full Text].
Response:
IVIG effects on antiplatelet antibody levels and on platelet
opsonization in ITP
Dr Crow and coworkers raise several interesting points in
discussing our recent paper.1 Our work showed that
intravenous immunoglobulin (IVIG) attenuated thrombocytopenia and led
to dose-dependent increases in the clearance of antiplatelet antibody
in a passive rat model of immune thrombocytopenia (ITP) (eg, antibody
clearance was increased by 140% following 2 g/kg of IVIG). Due to our
observation that IVIG therapy decreased antiplatelet antibody
concentrations, we had commented that this effect of IVIG
"would lead to decreases in the degree of platelet
opsonization"1(p2092) and, thus, may contribute to
the attenuation of thrombocytopenia observed following IVIG administration.
During the course of the review of our manuscript, 2 interesting papers
were published. Bleeker et al2 published a study demonstrating that administration of IVIG to mice could increase the
clearance of endogenous IgG and could also increase the clearance of a
murine monoclonal antibody (where the monoclonal antibody was not
reactive with murine antigens). Although Bleeker et al did not use a
model of autoimmunity, their results are consistent with our
pharmacokinetic data and support the hypothesis that IVIG
administration leads to a decrease in autoantibody concentrations.
Second, Crow and coworkers3 published an article that
demonstrated dose-dependent effects of IVIG in a passive murine model of ITP. Included within this study were flow cytometry results that
suggest that IVIG treatment did not alter the amount of antiplatelet antibody associated with platelets (assessed 48 hours after IVIG administration). Citing these results, Crow et al indicate here that
"in the short term... IgG catabolism does not result in a decrease in the platelet-bound antibody," thereby suggesting that IVIG effects
on antiplatelet antibody elimination may not contribute to the acute
effects of IVIG in ITP.
A few points are worth noting. First, it is well appreciated that
antibody binding to antigen is monotonic (ie, decreases in antibody
concentration lead to decreases in antibody binding). As such, our
comment regarding antiplatelet opsonization of platelets is well
grounded in theory. Nonetheless, antibody binding to antigen is
saturable and, thus, nonlinear. The impact of a change in antiplatelet antibody concentration on platelet-associated antiplatelet antibody may
be expected to differ from model to model and from patient to patient
because of differences in the determinants of antiplatelet antibody
binding (ie, antibody concentration, platelet count, the number of
antibody binding sites per platelet, the affinity of antiplatelet
antibody for platelet-binding sites, etc). As such, while Crow et al
find no difference in platelet-associated antiplatelet antibody
assessed 48 hours after IVIG administration, this may not be
representative of results observed in our model or of results observed
in patients (eg, where several studies have found that IVIG therapy
leads to reductions in platelet-associated antiplatelet
antibody4-6).
In addition, it is important to note that small changes in antiplatelet
antibody binding may lead to large changes in antiplatelet antibody-induced thrombocytopenia. Indeed, in our rat model, we predict steep relationships between antiplatelet antibody binding and
antibody-mediated platelet elimination (ie, inferred from previous work
where we measured 7E3 binding to rat platelets in vitro and assessed
7E3-induced thrombocytopenia in vivo7). As such, although
Crow et al failed to detect a change in platelet-associated antiplatelet antibody, this may be because their analytical method (flow cytometry) did not have sufficient power to detect small yet
significant changes in antibody binding to platelets.
Crow et al also comment that the results of our studies and their
studies indicate "...that anti-idiotypic antibody interference of
antiplatelet antibody binding does not appear to be responsible for the
amelioration of ITP by IVIG (at least not solely responsible)." Although our results are consistent with their statement, we feel that
it is important to caution readers that our work and the work of Crow
et al are performed in passive rodent models of ITP rather than in
patients. In the models studied, anti-idiotypic effects do not appear
to be important; however, much more work must be conducted before
conclusions may be made about anti-idiotypic mechanisms of IVIG action
in human ITP.
Ryan J. Hansen and Joseph P. Balthasar
Correspondence: Joseph P. Balthasar, Department of
Pharmaceutical Sciences, 521 Hochstetter Hall, University at Buffalo,
The State University of New York, Buffalo, NY 14260; e-mail:
jb{at}acsu.buffalo.edu
Acknowledgments
Supported by grant HL67347 from the National Heart, Lung, and
Blood Institute.
References
1.
Hansen RJ, Balthasar JP.
Effects of intravenous immunoglobulin on platelet count and antiplatelet antibody disposition in a rat model of immune thrombocytopenia.
Blood.
2002;100:2087[Abstract/Free Full Text].
2.
Bleeker WK, Teeling JL, Hack CE.
Accelerated autoantibody clearance by intravenous immunoglobulin therapy: studies in experimental models to determine the magnitude and time course of the effect.
Blood.
2001;98:3136[Abstract/Free Full Text].
3.
Crow AR, Song S, Semple JW, Freedman J, Lazarus AH.
IVIg inhibits reticuloendothelial system function and ameliorates murine passive-immune thrombocytopenia independent of anti-idiotype reactivity.
Br J Haematol.
2001;115:679[CrossRef][Medline]
[Order article via Infotrieve].
4.
Bussel JB, Kimberly RP, Inman RD, et al.
Intravenous gammaglobulin treatment of chronic idiopathic thrombocytopenic purpura.
Blood.
1983;62:480[Abstract/Free Full Text].
5.
Winiarski J, Kreuger A, Ejderhamn J, Holm G.
High dose intravenous IgG reduces platelet associated immunoglobulins and complement in idiopathic thrombocytopenic purpura.
Scand J Haematol.
1983;31:342[Medline]
[Order article via Infotrieve].
6.
Bussel JB.
Modulation of Fc receptor clearance and antiplatelet antibodies as a consequence of intravenous immune globulin infusion in patients with immune thrombocytopenic purpura.
J Allergy Clin Immunol.
1989;84:566[CrossRef][Medline]
[Order article via Infotrieve].
7.
Hansen RJ, Balthasar JP.
Pharmacokinetics, pharmacodynamics, and platelet binding of 7E3, a murine anti-glycoprotein IIb/IIIa monoclonal antibody in the rat.
J Pharm Exp Ther.
2001;298:165[Abstract/Free Full Text].
