Functional Differences between IgM and IgD Signaling in Chronic Lymphocytic Leukemia

Abstract

B-cell receptor (BCR) signaling is a central pathogenetic pathway in chronic lymphocytic leukemia (CLL). Most CLL cells express BCRs of IgM and IgD isotypes, but the contribution of these isotypes to functional responses remains incompletely defined. We therefore investigated differences between IgM and IgD signaling in freshly isolated peripheral blood CLL cells and in CLL cells cultured with nurselike cells (NLC), a model that mimics the lymph node microenvironment. IgM signaling induced prolonged activation of ERK kinases, promoted CLL cell survival, CCL3 and CCL4 chemokine secretion, and down-regulation of BCL6, the transcriptional repressor of CCL3. In contrast, IgD signaling induced activation of the cytoskeletal protein HS1, along with F-actin polymerization, which resulted in rapid receptor internalization, and failure to support downstream responses including CLL cell survival and chemokine secretion. IgM and IgD receptor down-modulation, HS1 and ERK activation, chemokine secretion and BCL6 down-regulation were also observed when CLL cells were co-cultured with NLC. The BTK kinase inhibitor ibrutinib effectively inhibited both, IgM and IgD isotype signaling. In conclusion, through a variety of functional readouts, we demonstrate very distinct outcomes of IgM and IgD isotype activation in CLL cells, providing novel insight into the regulation of BCR signaling in CLL.

Targeting BTK through microRNA in chronic lymphocytic leukemia

“…Most CLL cells express IgM receptors that, upon stimulation, activate signaling via the BCR, BTK, ERK, and AKT kinases to favor survival. 29 Our results demonstrating the ability of HDAC inhibitors singly or in combination with ibrutinib to effectively target BTK protein and abrogate its downstream signaling under conditions of IgM stimulation highlight the utility of this combination to antagonize the BCR-mediated survival advantage. In addition, because HDAC inhibitors can effectively also target mutated BTK in vitro, trials of HDAC inhibitors in treating ibrutinib-resistant CLL would be a rational strategy for patients with ibrutinib-resistant disease because survival for this group of patients is extremely poor.…”

“…Similar to cells exposed to abexinostat alone, there were time-dependent losses in p-PLCg2, p-ERK, and p-AKT, indicating the inhibition of BTK-mediated signaling ( Figure 5C). CLL cells respond to stimulation with IgM by activating BCR and BTK signaling to favor survival, 29 which was measured by increased p-ERK and p-AKT compared with their levels in unstimulated cells (supplemental Figure 5B). Exposure to ibrutinib inhibited p-BTK and For personal use only.…”

Ibrutinib Therapy Releases Leukemic Surface IgM from Antigen Drive in Chronic Lymphocytic Leukemia Patients

“…When the recently egressed and the persistently circulating fractions were identified by means of these markers, we observed that sIgM was higher in the recently egressed CLL cells. Interactions of CLL cells with tissue environmental cells will affect levels of functional BCR IgM/D complex (47). We and others have speculated that the higher sIgM might be a consequence of expression enhancement by tissue-derived IL4 (48,49).…”

Hematopoietic cell-specific lyn substrate (HCLS1 or HS1): A versatile actin-binding protein in leukocytes

“…Interestingly, different BCR isotype stimulation played a role in B‐CLL. IgM signaling triggered B‐CLL cell survival via Erk activation, whereas IgD signaling induced phosphorylation of HS1 leading to BCR internalization and failure to induce B‐CLL cell survival . Thus, the type of BCR response may critically affect B‐CLL outcome which again involves HS1.…”