C3b acceptors on human peripheral blood mononuclear cells; characterization and functional role

Area of Science:

• Immunology
• Cell Biology
• Complement System

Background:

• The complement system protein C3b (C3bx) covalently binds to cellular targets via its labile site.
• Conventional chemical groups like hydroxyl (OH-) and amino (NH-2) are known targets for C3bx binding.
• It was hypothesized that all cells could bind C3bx, making C3b acceptors (C3bAs) difficult to saturate.

Purpose of the Study:

• To investigate the presence and characteristics of C3b acceptors (C3bAs) on human peripheral blood mononuclear cells (PBMC).
• To determine the cell type distribution and saturation properties of C3bAs on human PBMC.
• To explore the functional consequences of C3bx binding to C3bAs on PBMC surface structures.

Main Methods:

• Immune adherence (IA) test to detect C3b acceptors (C3bAs).
• Analysis of C3b binding to different human PBMC subsets (B cells, T cells, Null cells).
• Investigation of C3bAs saturation, sensitivity to trypsin, and covalent interaction with C3bx.
• Assessment of the impact of C3bx-C3bA binding on Fc gamma receptor function and mitogen-stimulated blastogenesis.

Main Results:

• Human PBMC exhibit heterogeneous expression of C3bAs: 57% of B cells and 21% of Null cells possess C3bAs, while T cells lack them.
• C3bAs on PBMC are saturable and sensitive to trypsin treatment.
• The interaction between C3bx and C3bAs is covalent.
• Binding of C3bx to C3bAs inhibits Fc gamma receptor function and abolishes the enhancement of pokeweed mitogen-stimulated blastogenesis by immune complexes.

Conclusions:

• Human PBMC possess specific, saturable, and trypsin-sensitive C3b acceptors (C3bAs), predominantly on B and Null cells.
• The covalent binding of C3bx to C3bAs significantly modulates immune cell functions, including Fc gamma receptor activity and response to mitogens.
• These findings reveal a specific mechanism of complement-mediated regulation on human immune cells.