In the present study, we demonstrate that macrophage-tropic HIV-1 opsonized by complement and limited amounts of anti-HIV-IgG causes up to 10-fold higher productive infection of human monocyte-derived dendritic cells than HIV treated with medium or HIV opsonized by Ab only. Enhanced infection is completely abolished by a mAb specific for the ligand-binding site of CD11b (i.e., α-chain of complement receptor 3, receptor for iC3b), proving the importance of complement receptor 3 in this process. Inhibition of complement activation by EDTA also prevents enhanced infection, further demonstrating the role of complement in virus uptake and productive infection. Since HIV is, even in the absence of Abs, regularly opsonized by complement, most probably the above-described mechanism plays a role during in vivo primary infection.

CR3 and CR4 belong to the family of β 2 -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 β 2 -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 thatCR4 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. Keywordsphagocytosis, podosomes, human phagocytes, β 2 integrins, CR3 (CD11b/CD18), CR4 (CD11c/CD18) Highlights -CR3 is the dominant receptor mediating the phagocytosis of iC3b opsonized S. aureus -surface bound and digested bacteria should be distinguished -MDDCs have a unique podosome pattern compared to monocytes, MDMs and neutrophils -both CR3 and CR4 are located in the adhesion ring of podosomes

Integrins are cell membrane receptors that are involved in essential physiological and serious pathological processes. Their main role is to ensure a closely regulated link between the extracellular matrix and the intracellular cytoskeletal network enabling cells to react to environmental stimuli. Complement receptor type 3 (CR3, αMβ, CD11b/CD18) and type 4 (CR4, αXβ, CD11c/CD18) are members of the β-integrin family expressed on most white blood cells. Both receptors bind multiple ligands like iC3b, ICAM, fibrinogen or LPS. β-integrins are accepted to play important roles in cellular adhesion, migration, phagocytosis, ECM rearrangement and inflammation. Several pathological conditions are linked to the impaired functions of these receptors. CR3 and CR4 are generally thought to mediate overlapping functions in monocytes, macrophages and dendritic cells, therefore the potential distinctive role of these receptors has not been investigated so far in satisfactory details. Lately it has become clear that a functional segregation has evolved between the two receptors regarding phagocytosis, cellular adhesion and podosome formation. In addition to their tasks on myeloid cells, the expression and function of CR3 and CR4 on lymphocytes have also gained interest recently. The picture is further complicated by the fact that while these β-integrins are expressed by immune cells both in mice and humans, there are significant differences in their expression level, functions and the pathological consequences of genetic defects. Here we aim to summarize our current knowledge on CR3 and CR4 and highlight the functional differences between these receptors, involving their expression in myeloid and lymphoid cells of both men and mice.

Complement receptors CR3 (CD11b/CD18) and CR4 (CD11c/CD18) belong to the family of beta2 integrins and are expressed mainly by myeloid cell types in humans. Previously, we proved that CR3 rather than CR4 plays a key role in phagocytosis. Here we analysed how CD11b and CD11c participate in cell adhesion to fibrinogen, a common ligand of CR3 and CR4, employing human monocytes, monocyte-derived macrophages (MDMs) and monocyte-derived dendritic cells (MDDCs) highly expressing CD11b as well as CD11c. We determined the exact numbers of CD11b and CD11c on these cell types by a bead-based technique, and found that the ratio of CD11b/CD11c is 1.2 for MDDCs, 1.7 for MDMs and 7.1 for monocytes, suggesting that the function of CD11c is preponderant in MDDCs and less pronounced in monocytes. Applying state-of-the-art biophysical techniques, we proved that cellular adherence to fibrinogen is dominated by CD11c. Furthermore, we found that blocking CD11b significantly enhances the attachment of MDDCs and MDMs to fibrinogen, demonstrating a competition between CD11b and CD11c for this ligand. On the basis of the cell surface receptor numbers and the measured adhesion strength we set up a model, which explains the different behavior of the three cell types.