Complement opsonization is required for presentation of immune complexes by resting peripheral blood B cells

Area of Science:

• Immunology
• Cell Biology

Background:

• Complement receptor 2 (CD21, CR2) is a B cell receptor that binds complement degradation products on antigens or immune complexes.
• Its role in antigen presentation by resting B cells, particularly for soluble immune complexes, has been investigated.

Purpose of the Study:

• To investigate the role of CD21 in mediating antigen presentation of soluble immune complexes by resting B cells.
• To understand the mechanism by which CD21 facilitates antigen presentation and T cell activation.

Main Methods:

• Formation of complement-coated immune complexes using influenza virus and immune serum.
• Incubation of complexes with peripheral blood B cells and assessment of binding via CD21 and CR1.
• Pulsing B cells with immune complexes and evaluating T cell clone responses.
• Analysis of B cell activation markers (CD69, CD86) following CD21 ligation.

Main Results:

• Complement-coated immune complexes bound to B cells in a complement-dependent manner, requiring CD21 or CR1.
• B cells pulsed with complement-containing immune complexes elicited responses from influenza-specific T cell clones.
• CD21 ligation alone did not induce B cell activation markers (CD69, CD86).
• Up-regulation of activation markers required T cell factors, indicating antigen presentation independent of B cell activation.

Conclusions:

• CD21-mediated antigen presentation by B cells occurs independently of classical B cell activation pathways.
• Binding of antigens to CD21 allows antigen-nonspecific B cells to participate in activating antigen-specific T cells.
• This mechanism contributes to T cell activation, particularly in response to complement-associated antigens like influenza virus.