IgG-Immune Complexes Promote B Cell Memory by Inducing BAFF.

Area of Science:

• Immunology
• Cellular and Molecular Immunology
• Vaccinology

Background:

• Memory B cell responses are critical for adaptive immunity and preventing reinfection.
• Formation of high-affinity memory B cells requires T cell help, somatic hypermutation, and affinity maturation within germinal centers (GCs).
• The specific signals committing GC B cells to the memory pool are not fully understood.

Purpose of the Study:

• To investigate the role of IgG-immune complexes (ICs), Fc gamma receptors (FcγRs), and BAFF in memory B cell formation.
• To elucidate the signaling pathways involved in the commitment of GC B cells to the memory pool.

Main Methods:

• Immunization of mice with a T-dependent antigen.
• Analysis of B cell populations, including germinal center (GC) and memory B cells.
• In vivo manipulation involving genetic deletion (CD16) or blocking of IC:FcγR interactions.
• Assessment of Bcl-6 expression and antibody responses.

Main Results:

• Early IgG secretion formed immune complexes (ICs) that engaged FcγRs on dendritic cells.
• This interaction induced dendritic cell secretion of BAFF, which acted on B cells to promote Bcl-6 expression.
• Loss of FcγR signaling (CD16), BAFF, or IC:FcγR interactions impaired GC and memory B cell formation and secondary antibody responses.
• BAFF also supported the maintenance/expansion of follicular helper T cells.

Conclusions:

• Early antibody responses, mediated by IgG-ICs and FcγRs, are essential for optimal memory B cell formation.
• BAFF signaling, induced by IC-FcγR interactions, plays a key role in this process, acting upstream or at Bcl-6.
• This study identifies a novel role for FcγRs in regulating germinal center and memory B cell responses.