The role of CR3 (CD11b/CD18) and CR4 (CD11c/CD18) in complement-mediated phagocytosis and podosome formation by human phagocytes

- CR3 is the dominant receptor mediating the phagocytosis of iC3b opsonized S. aureus.
- Surface bound and ingested bacteria should be discriminated in a phagocytosis assay.
- MDDCs have a unique podosome pattern compared to monocytes, MDMs and neutrophils.
- Both CR3 and CR4 are located in the adhesion ring of podosomes.

CR3 and CR4 belong to the family of β2-integrins and play an important role in phagocytosis, cellular adherence and migration. CR3 and CR4 are generally expected to mediate similar functions due to their structural homology, overlapping ligand specificity and parallel expression on human phagocytes. Although the different signalling pathways of these receptors suggest distinct functions, possible differences are just being revealed.Previously we proved that CR3 plays a key role in the uptake of iC3b-opsonized particles by human dendritic cells. Now, besides measuring the overall phagocytic capacity of cells including the assessment of surface bound as well as internalized particles, we extended our investigations and studied the digestion of the iC3b opsonized antigen by various human phagocytes. The participation of CR3 and CR4 was compared in the process of binding, internalization and digestion of iC3b opsonized Staphylococcus aureus by monocytes, monocyte derived macrophages (MDMs), monocyte derived dendritic cells (MDDCs), and neutrophils. Comparing the activity of the two β2-integrin type complement receptors we found that CR3 plays a dominant role in the phagocytosis of iC3b opsonized S. aureus by all of these cell types.Studying another important integrin-mediated function we demonstrated earlier that CR4 is dominant in the adhesion of monocytes, MDMs and MDDCs to fibrinogen. Here we studied the participation of CR3 and CR4 in podosome formation by human phagocytes, since these structures are known to play an essential role in cell migration. Our confocal microscopy analysis revealed that both CD11b and CD11c concentrate in the podosome adhesion ring.In summary our data highlight differences in the function of human CR3 and CR4 in the process of uptake and digestion of complement opsonized antigen, while in the process of podosome formation, connected to cellular motility, both receptors equally take part.