Mitogen-induced B-cell proliferation activates Chk2-dependent G1/S cell cycle arrest

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• B-cell activation and proliferation are crucial for adaptive immunity.
• The DNA damage response (DDR) acts as an intrinsic sensor in B cells, particularly during Epstein-Barr virus (EBV) infection.
• The role of DDR in limiting proliferation in response to non-viral mitogens is not fully understood.

Purpose of the Study:

• To investigate the role of the DDR as a limiting factor in the proliferative response of B cells to non-viral mitogens.
• To compare the DDR activation and subsequent cell cycle arrest induced by different B-cell activation pathways.

Main Methods:

• Primary human B cells were stimulated with various mitogens: CpG-rich oligonucleotides (TLR9 agonist), CD40 pathway activation with IL-4, and Epstein-Barr virus (EBV).
• Activation of the ATM/Chk2 signaling pathway (part of the DDR) was assessed.
• Cell cycle progression (G1/S phase arrest) was monitored, and the effect of Chk2 inhibition on proliferation was evaluated.
• Intrinsic apoptosis independent of DDR was also measured.

Main Results:

• TLR9 activation induced robust B-cell hyper-proliferation and significant ATM/Chk2 signaling.
• CD40/IL-4 stimulation resulted in less proliferation and only modest DDR activation.
• All three mitogens (TLR9, CD40/IL-4, EBV) triggered a DDR-dependent G1/S phase cell cycle arrest, limiting B-cell proliferation.
• The degree of G1/S arrest, reversed by Chk2 inhibition, correlated with proliferation rates.

Conclusions:

• The DDR acts as a critical brake on B-cell proliferation induced by various mitogens, primarily through G1/S phase cell cycle arrest.
• The extent of this DDR-mediated arrest dictates the proliferative capacity of activated B cells.
• These findings shed light on the regulation of extra-follicular B-cell activation and have implications for autoimmune diseases and lymphoma.