A p53 AXIS REGULATES BCR-TRIGGERED, INNATE IMMUNE SYSTEM-DRIVEN B CELL CLONAL EXPANSION1

• Published in: The Journal of immunology (1950) Vol. 188; no. 12; pp. 6093 - 6108
• Main Authors: Lee, Hyunjoo, Haque, Shabirul, Nieto, Jennifer, Trott, Joshua, Inman, John K., McCormick, Steven, Chiorazzi, Nicholas, Mongini, Patricia K. A.
• Format: Journal Article
• Language: English
• Published: 18-05-2012
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.1103037

Resting mature human B cells undergo a dynamic process of clonal expansion, followed by clonal contraction, during an in vitro response to surrogate C3d-coated antigen and innate immune system cytokines, IL-4 and BAFF. We here explore the mechanism for clonal contraction through following the time- and division-influenced expression of several pro- and anti-apoptotic proteins within CFSE-labeled cultures. Several findings, involving both human and mouse B cells, show that a mitochondria-dependent apoptotic pathway involving p53 contributes to the high AICD susceptibility of replicating blasts. Activated B cell clones exhibit elevated p53 protein and elevated mRNA/protein of pro-apoptotic molecules known to be under direct p53 transcriptional control, Bax, Bad, Puma, Bid, and pro-caspase 6, accompanied by reduced anti-apoptotic Bcl-2. Under these conditions, Bim levels were not increased. Findings that full length Bid protein significantly declines in AICD-susceptible replicating blasts, while Bid mRNA does not, suggests that Bid is actively cleaved to short-lived, pro-apoptotic tBid. AICD was diminished, albeit not eliminated, by p53 siRNA transfection, genetic deletion of p53, or Bcl-2 overexpression. DNA damage is a likely trigger for p53-dependent AICD since susceptible lymphoblasts expressed significantly elevated levels of both phospho-ATM
ser1980
and phospho-H2AX
ser139
. Deficiency in activation-induced cytosine deaminase (AID) diminishes but does not ablate murine B cell AICD, indicating that AID-induced DNA damage is only in part responsible. Evidence for p53-influenced AICD during this route of TI clonal expansion raises the possibility that progeny bearing p53 mutations might undergo positive selection in peripherally inflamed tissues with elevated levels of IL-4 and BAFF.