The response of human monocytes to interaction with immobilized immune complexes

The interaction between human peripheral blood monocytes and immobilized immune complexes has been monitored by morphological and biochemical criteria using an established model system previously used with human eosinophils. The model consists of an agar layer containing immune complexes. Monocytes flatten extensively on these layers and make very close contact with the agar surface; cells incubated on control layers, without antibody, are more loosely attached. After 15 min incubation on immobilized immune complexes there is a 'pulse' of lysosomal enzyme release from the monocytes and electron micrographs indicate that extracellular secretion occurs. Simultaneously, two proteins with apparent molecular weights of 50 000 and 25 000 appear at the cell surface as a specific consequence of the interaction with immune complexes. These newly accessible proteins were detected by lactoperoxidase-catalysed iodination, and the major of the two (labelled M3) has similarities to a protein with an apparent molecular weight of 55 000 of human eosinophils (labelled protein 3), which has been shown to be closely associated with the eosonophil Fc receptor. It will be of interest to establish whether a similar association occurs in monocytes.