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An aberrant NOTCH2-BCR signaling axis in B cells from patients with chronic GVHD

Jonathan C. Poe, Wei Jia, Hsuan Su, Sarah Anand, Jeremy J. Rose, Prasanthi V. Tata, Amy N. Suthers, Corbin D. Jones, Pei Fen Kuan, Benjamin G. Vincent, Jonathan S. Serody, Mitchell E. Horwitz, Vincent T. Ho, Steven Z. Pavletic, Frances T. Hakim, Kouros Owzar, Dadong Zhang, Bruce R. Blazar, Christian W. Siebel, Nelson J. Chao, Ivan Maillard and Stefanie Sarantopoulos

Key Points

  • NOTCH2 activation confers a marked increase in BCR responsiveness by cGVHD patient B cells that associates with increased BLNK.

  • ATRA increases the IRF4-to-IRF8 ratio and blocks aberrant NOTCH2-BCR activation without affecting cGVHD patient B-cell viability/function.

Publisher's Note: There is an Inside Blood Commentary on this article in this issue.

Abstract

B-cell receptor (BCR)-activated B cells contribute to pathogenesis in chronic graft-versus-host disease (cGVHD), a condition manifested by both B-cell autoreactivity and immune deficiency. We hypothesized that constitutive BCR activation precluded functional B-cell maturation in cGVHD. To address this, we examined BCR-NOTCH2 synergy because NOTCH has been shown to increase BCR responsiveness in normal mouse B cells. We conducted ex vivo activation and signaling assays of 30 primary samples from hematopoietic stem cell transplantation patients with and without cGVHD. Consistent with a molecular link between pathways, we found that BCR-NOTCH activation significantly increased the proximal BCR adapter protein BLNK. BCR-NOTCH activation also enabled persistent NOTCH2 surface expression, suggesting a positive feedback loop. Specific NOTCH2 blockade eliminated NOTCH-BCR activation and significantly altered NOTCH downstream targets and B-cell maturation/effector molecules. Examination of the molecular underpinnings of this “NOTCH2-BCR axis” in cGVHD revealed imbalanced expression of the transcription factors IRF4 and IRF8, each critical to B-cell differentiation and fate. All-trans retinoic acid (ATRA) increased IRF4 expression, restored the IRF4-to-IRF8 ratio, abrogated BCR-NOTCH hyperactivation, and reduced NOTCH2 expression in cGVHD B cells without compromising viability. ATRA-treated cGVHD B cells had elevated TLR9 and PAX5, but not BLIMP1 (a gene-expression pattern associated with mature follicular B cells) and also attained increased cytosine guanine dinucleotide responsiveness. Together, we reveal a mechanistic link between NOTCH2 activation and robust BCR responses to otherwise suboptimal amounts of surrogate antigen. Our findings suggest that peripheral B cells in cGVHD patients can be pharmacologically directed from hyperactivation toward maturity.

  • Submitted May 4, 2017.
  • Accepted August 24, 2017.
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