Characterization of canine monocytes in vitro: increased receptor activity for Fc, C3, and heterologous erythrocytes.

Abstract

These studies assess development in vitro of canine monocyte receptors for heterologous erythrocytes and erythrocytes opsonized with IgG or IgM and C3. A majority of monocytes demonstrates Fc receptor activity throughout a 2-month culture period. Surface receptors for C3-coated particles, initially present on relatively few monocytes (14%), are found on 85% of the cells after 2 weeks in culture. Binding of heterologous erythrocytes shows a distinct developmental pattern and increases from 50% to greater than 90% during cultivation in vitro. Surface structures mediating human erythrocytes (HuRBC) attachment to monocytes are distinct from Fc receptors since they remain on the monocyte surface after ingestion of EA(IgG). Furthermore, protease treatment of monocytes significantly reduces binding of HuRBC while attachment of EA(IgG) is unimpaired. A stable and persistent interaction is observed between canine monocytes and HuRBC which is energy independent and occurs in the absence of Ca2+ and Mg2+. Despite the fact that natural anti-HuRBC agglutinins of the IgM class are present in canine serum, it has been demonstrated that HuRBC bind to monocytes in the absence of ligands. Furthermore, monocyte binding of EA (IgM) is observed at frequencies (4%) far below those observed for HuRBC attachment, reflecting the absence of an IgM receptor on greater than 95% of these cells. We have used the direct binding of monocytes and HuRBC to investigate antigen presentation by monocytes. Evidence indicates that persistent HuRBC antigen localized at the monocyte surface may function as an inductive signal to initiate the afferent limb of the immune response.