Both Fcγ and complement receptors mediate transfer of immune complexes from erythrocytes to human macrophages under physiological flow conditions in vitro

Summary Abnormal clearance by the mononuclear phagocytic system of immune com- plexes (IC) is important in the pathogenesis of systemic lupus erythematosus (SLE). We have developed an in vitro model to investigate the cellular mech- anisms involved in the transfer of soluble IC from erythrocytes to human macrophages under physiological flow conditions. In this assay, erythrocytes bearing fluorescently labelled IC are perfused over monolayers of human monocytes or monocyte-derived macrophages in a parallel-plate flow cham- ber, and transfer quantified using confocal microscopy and flow cytometry. Using aggregated human IgG as a model IC, we have been able to demonstrate transfer of IC from erythrocytes to macrophages. Blocking studies with spe- cific neutralizing antibodies have shown that both complement and Fc γ γ γ recep- tors are required for IC transfer. Blockade of CR4 ( α x β 2 integrin), Fc γ RIIa or Fc γ RIII reduced transfer, while anti-CR3 ( α m β 2 integrin) had no effect. Block- ade of CR3, Fc γ RIIa or Fc γ γ γ RIII also reduced the number of adhesive interac- tions between fluorescently labelled IC-bearing erythrocytes and macrophage monolayers. Taken together with the transfer data, this suggests differing roles for these receptors in the human IC transfer reaction that includes an adhesive function which facilitates IC processing by mononuclear phagocytes. Finally, a functional effect of the Fc γ RIIa R131/H131 polymorphism, important in susceptibility to SLE, has also been demonstrated using this model. Uptake of IgG 2 but not IgG 1 -containing soluble IC was reduced by macrophages from individuals homozygous for the R131 allelic variant of the receptor.