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

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 and, each critical to B-cell differentiation and fate. All- retinoic acid (ATRA) increased expression, restored the-to- ratio, abrogated BCR-NOTCH hyperactivation, and reduced NOTCH2 expression in cGVHD B cells without compromising viability. ATRA-treated cGVHD B cells had elevated and, but not (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.

B-cell targeting in chronic graft-versus-host disease

“…In that context it is important to understand which cell-intrinsic mechanisms enhance BCR responses. A novel observation here is that BCR responses to surrogate antigen were markedly increased when NOTCH2 was also activated 30 32 . Tregs are capable of selectively killing B-cells 33 and their deficiency would predispose to a failure to control pathogenic B-cells.…”

SYK inhibitor entospletinib prevents ocular and skin GVHD in mice

“…Furthermore, it is likely that these pathogenic immune cells emerge more rapidly in some HCT patients due to intrinsic signaling defects that are exacerbated due to continual exposure of developing cells to AlloAg and cytokines. Examples of this scenario in cGVHD include a paucity in Tregs and regulatory B cells (46, 60, 62, 67, 68), an increase in M2-like macrophages that express CSF-1R and invade the skin causing fibrosis (38), and B cells that have dysregulated expression of the key transcription factors IRF4 and IRF8, causing heightened signaling through NOTCH2, which synergizes with the BCR to induce abnormally high proliferative responses to limiting amounts of antigen (52). Emerging evidence suggests that some of these intrinsic defects that drive immune cell pathogenicity can be corrected pharmacologically, steering developing immune cells down either regulatory pathways or more mature functional pathways (3,9,45,47,52,69).…”

Developing role of B cells in the pathogenesis and treatment of chronic GVHD

“…NOTCH2 is a surface molecule and co‐stimulator of aberrant BCR responses in cGVHD. Poe et al () revealed increased NOTCH2‐BCR signalling and a decreased IRF4/IRF8 expression ratio in cGVHD. Increased NOTCH2 activation heightens BCR responsiveness and promotes the expression of the proximal BCR protein BLNK in cGVHD patients.…”