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

Abstract

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

Soluble CD93 is an apoptotic cell opsonin recognized by αxβ2

“…A). After filtering for high abundance proteins found in at least three independent experiments, eight proteins were identified as putative sCD93 receptor components (Table ), including three phagocytic integrin subunits (α x , β 2 , and β 1 ), CD44, and multiple integrin‐associated signaling molecules, indicating that the sCD93 receptor is likely an integrin . To determine which of these proteins functioned as the sCD93 receptor, α x , β 1 , β 2 , and CD44‐deficient THP‐1 cell lines were generated using small hairpin RNA (shRNA) (Fig.…”

“…F). α x β 2 is also a receptor for complement, suggesting that sCD93 and complement may act in concert . Although the addition of human serum to sCD93‐opsonized cells enhanced efferocytosis, depletion of complement with cobra venom or heat inactivation did not abrogate this effect, indicating that other serum‐borne opsonins were responsible for this increase in opsonization (Supporting Information ).…”

Increased ferritin levels in non‐transfusion‐dependent β°‐thalassaemia/HbE are associated with reduced CXCR2 expression and neutrophil migration

“…However, the lacking expression of these proteins on neutrophils can affect their function, as seen with β 2 integrin‐deficient expression such as LFA‐1 and Mac‐1 in patients with leukocytes adhesion deficiency type 1, a rare inherited autosomal recessive disorder resulting in defective leukocyte adherence to the endothelium, neutrophil aggregation and chemotaxis, phagocytosis and cytotoxicity mediated by neutrophils, NK cells and T cells (van de Vijver et al , ). Additionally, Mac‐1 deficiency in mice (Morrison et al ., ; Carter et al ., ) and in humans (van de Vijver et al ., ; Lukacsi et al ., ) was shown to be related to increased susceptibility to infections. Therefore, the possible association between CD11b expression levels and susceptibility to infection in NTD β°‐thalassaemia/HbE patients should be further studied.…”

A generalized quantitative antibody homeostasis model: maintenance of global antibody equilibrium by effector functions

“…Proteases attached to the recognition molecules can trigger proteolytic cleavage of further complement components, depending on the stability—as reflected by Δ G —of the complex. Covalent binding of C4b—a general feature of thioester‐containing proteins 43 —further stabilizes the complex and provides ligand for other homeostatic receptors like CR3 and CR4 44 . In the absence of inhibitory and regulatory molecules, numerous complement C3 breakdown products are generated, leading to the formation of enzymatically active C3 and C5 convertases (Figure 4).…”