In this study we investigated different aspects of monocyte functions following interaction of monocytes (Mo) with therapeutic concentrations of factor VIII (F VIII) concentrate. A short (one-hour) treatment of normal Mo with F VIII concentrates led to a significant (P less than 0.001) down modulation of Fc receptors expressed in the Mo plasma membrane. This down modulation was accompanied by a decrease of Mo effector functions that was expressed by a reduced capacity of F VIII- treated Mo to release O2 radicals (40% of controls) and to kill bacteria (% killing: control Mo, 65%; F VIII-treated Mo, 24% to 51%). Further studies showed that the modulating activity was due to a contaminant present in F VIII concentrates (immune complexes or IgG aggregates). Fractionation using molecular sieving revealed that the modulatory activity was confined to a high-molecular range fraction (Mr greater than 1,270,000 daltons), while the fraction containing monomeric IgG had no effect. Further fractionation by affinity chromatography on protein A-Sepharose separated the coagulation activity (effluent) from the Mo function-modulating activity (eluate). We conclude that treatment with F VIII concentrates might contribute to an immunocompromised state in some hemophiliacs and facilitate opportunistic infections in these patients.

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