Abstract

Inhibitory antibodies against factor VIII (FVIII) are the major complication experienced by hemophilia A patients treated with FVIII products. The most effective therapy to eradicate these antibodies is elevated doses of FVIII over a prolonged period. Despite clinical practice in using such protocols, nothing is known about the immunological mechanisms that cause the down-modulation of FVIII-specific immune responses and the induction of long-lasting immune tolerance against FVIII. Understanding the underlying mechanisms, however, would facilitate designing new therapeutic strategies.

The re-stimulation of FVIII-specific memory responses after each dose of FVIII is probably the most important event in the maintenance of FVIII inhibitors in patients. Therefore, the eradication of these memory responses should be an essential step in the down-modulation of inhibitory antibodies and the induction of immune tolerance. We used a murine model of hemophilia A to answer the question whether FVIII-specific memory responses are sensitive to increasing doses of FVIII. In particular, we were interested in the differential effects of FVIII on memory-B-cell and memory-T-cell responses. For the analysis of FVIII-specific memory responses, we re-stimulated FVIII-specific memory B- and T-cells obtained from spleens of hemophilic mice treated with four doses of human FVIII or eight doses of murine FVIII as described (

Sasgary et al.:
Thromb Haemost
2002
;
87
:
266
–72
;
Hausl et al.:
Blood
2004
;
104
:
115
–22
).

Our results show dose-dependent effects of FVIII on the re-stimulation of FVIII-specific memory B cells in vitro. Physiological concentrations of FVIII below 100 ng/ml re-stimulate memory B cells and induce their differentiation into anti-FVIII antibody-secreting plasma cells. Supra-physiological concentrations above 100 ng/ml, however, inhibit memory-B-cell re-stimulation. The inhibition of memory-B-cell re-stimulation is irreversible and seems to be due to an induction of apoptosis that is at least partly mediated by Fas-dependent mechanisms. Furthermore, the inhibition appears to be initiated by triggering the B-cell receptor (BCR) without the requirement of an excessive cross-linking of the BCR. The activation of FVIII-specific T cells is not affected by increasing doses of FVIII.

We conclude that the induction of apoptosis in FVIII-specific memory B cells might be the first step in the induction of immune tolerance in hemophilia A patients with FVIII inhibitors who receive high doses of FVIII. The eradication of memory B cells would prevent their differentiation into antibody-secreting plasma cells and, moreover, might lead to a deficiency of effective antigen-presenting cells required for the re-stimulation of FVIII-specific memory T cells. The induction of regulatory T cells rather than effector T cells could be the consequence of this deficiency.

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