The formation of Factor VIII (FVIII) antibodies after FVIII treatment in hemophilic patients is currently the major treatment-related complication faced by hemophilic patients. It has been reported that the use of plasma-derived FVIII (pdFVIII) in patients and animal models of hemophilia A will result in reduced anti-FVIII antibody formation compared to recombinant FVIII (rFVIII). In an attempt to explain this phenomenon, an initial hypothesis relating to the uptake of FVIII by antigen presenting cells has been formulated. In contrast, here we examine the hypothesis that the cytokine microenvironment induced by pdFVIII has a critical influence on the reduction of anti-FVIII antibody titers in hemophilia A mice. Micro-array and confirmatory quantitative RT-PCR experiments performed on CD11c+ dendritic cells, isolated 24 hours after rFVIII or pdFVIII infusion, show that pdFVIII causes a different immune gene expression profile than rFVIII administration. Both treatments caused the up regulation of pro-inflammatory gene expression. However, the rFVIII and pdFVIII treatments caused the up regulation of genes that induce Th1 and Th2 responses, respectively. Moreover, after administering rFVIII or pdFVIII concentrates to mice, we observed that each treatment induced a distinct T cell splenic cytokine micro-environment. Recombinant FVIII induced the release of Th1 cytokines and IL10, while pdFVIII induced the release of Th2 cytokines and TGF-α. In this mouse model, we observed the formation of high titers of anti-human VWF antibodies in the pdFVIII-treated mice and we propose that in these animals, VWF competes with FVIII for antigenic presentation. We further investigated the potential of antigenic competition by treating mice with FVIII and increasing concentrations of another unrelated protein, recombinant human factor IX (FIX). Our studies have shown an inverse relationship between increasing concentrations of FIX and the production of anti-FVIII antibodies. In summary, these studies allude to new and additional mechanisms that may contribute to the reduction of anti-FVIII antibody development in hemophilia A mice treated with pdFVIII.

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