CR3 is the dominant phagocytotic complement receptor on human dendritic cells

Area of Science:

• Immunology
• Cell Biology
• Microbiology

Background:

• Dendritic cells (DCs) are crucial immune cells that interact with pathogens via pattern recognition and opsonophagocytotic receptors.
• β2-integrins, such as complement receptors CR3 (CD11b/CD18) and CR4 (CD11c/CD18), are involved in immune cell functions like migration, adherence, and phagocytosis.
• Human monocyte-derived dendritic cells (MDCs) express both CR3 and CR4, but their distinct roles in microbial uptake were previously unaddressed.

Purpose of the Study:

• To investigate potential differences in the roles of CR3 and CR4 in the binding and uptake of complement-opsonized microorganisms by MDCs.
• To elucidate the specific contribution of CR3 versus CR4 in the phagocytosis of microbes by MDCs.

Main Methods:

• Studied the expression of CR3 (CD11b) and CR4 (CD11c) during MDC differentiation.
• Assessed the role of CR3 and CR4 in phagocytosis using blocking antibodies and small interfering RNA (siRNA) to downregulate receptor expression.
• Measured inflammatory cytokine production and cell maturation markers.

Main Results:

• MDC differentiation led to decreased CD11b and increased CD11c expression, with both receptors present in an active conformation.
• Ligation of CD11b (CR3) enhanced MDC phagocytosis of complement-opsonized microbes without affecting cell maturation or cytokine production.
• Blocking or downregulating CD11c (CR4) did not significantly alter microbial uptake, whereas blocking/downregulating CD11b (CR3) markedly inhibited phagocytosis.

Conclusions:

• Complement C3-dependent phagocytosis by MDCs is predominantly mediated by CR3 (CD11b/CD18).
• CR4 (CD11c/CD18) plays a minor role in the uptake of complement-opsonized yeast and bacteria by MDCs.
• These findings highlight the specific role of CR3 in enhancing the phagocytic capacity of dendritic cells for microbial clearance.