Modulation of human monocyte functions by factor VIII-anti-factor VIII complexes present in an affinity-purified factor VIII product.

A recent review article in Blood’ dealt with the advantage of using highly purified factor VI11 (F VIII) concentrates for the treatment of hemophilia. These products have a high specific activity of F VIIIC and, if properly treated, do not contain contaminating viruses. However, despite a relatively high purity, therapeutic concentrations of conventionally prepared F VI11 concentrates still have been shown to exert immune-modulating properties, which led to an impairment of monocyte effector functions such as Fc receptor-mediated phagacytosis and intracellular killing of This phenomenon was due to the presence of immune complexes (IC) and immunoglobulin (Ig) aggregates that apparently copurify with F VI11 during the manufacturing process and cause a downmodulation of Fcy receptors (FcyR) in the monocyte membrane. Recently, we had the opportunity to test an early generation F VI11 concentrate purified by the use of anti-F VI11 monoclonal antibodies (MoAbs). This procedure is known to remove the majority of contaminating serum proteins4; however, we found, to our surprise, that with respect to its FcyR modulating capacity, the monoclonally purified product behaved comparably to a conventionally prepared F VI11 concentrate (Fig 1). This affinity (a-F VII1)purified product contained virtually no human IgG; but small amounts (478 ng/1,000 IU F VIII) of murine IgG could be detected. Logically, this murine IgG could only originate from the immobilized MoAb used for the purification process. Because this antibody is specific for F VIII, the generation of F VI11 anti-F VI11 IC would be a likely event, and the presence of these IC in the F VI11 preparation could account for its immunemodulating capacity. To test this hypothesis we prepared IC by mixing a mouse monoclonal anti-F VI11 antibody (anti-F VIII:CAg, ESH 5, Bioscot Ltd, Edinburgh, UK) with a conventionally purified F VI11 product free of the FcyR-modulating component due to treatment with a combination of molecular sieving and adsorption to immobilized