iwCLL 2017 | Ibrutinib treatment results in a downregulatory membrane environment and an overall resting phenotype of CLL B-cells

On 14^th May 2017, the “Role of the B-Cell Receptor and Other Signaling Pathways in CLL” took place at iwCLL, and was co-chaired by Nicholas Chiorazzi (The Feinstein Institute for Medical Research, Manhasset, NY, USA) and Kostas Stamatopoulos (Center for Research and Technology Hellas, Thessaloniki, Greece).

Andrea Nicola Mazzarello, from The Feinstein Institute for Medical Research, New York, USA, gave a talk during this session titled “Differential CLL BCR/Co-Receptor density regulation in various intraclonal fractions before and during ibrutinib treatment.”

The group compared densities of IgM and IgD, as well as Ig associated stimulatory/inhibitory co-receptors, on the surface membranes of CLL B-cells. Conventional and imaging flow cytometry was performed on samples taken from 5 Unmutated IGHV CLL (U-CLL) and Mutated IGHV CLL (M-CLL) patients before and during ibrutinib treatment. Cell size was used as an indicator of metabolic activity in the cell subpopulations.

Density of immunoglobulin correlated directly with BCR activating (IgM > IgD) and inhibitory (IgM > IgD) co-receptors. This differential expression indicates different regulation for IgM and IgD. Immunoglobulin densities also correlated directly with CXCR4 expression (IgM = IgD). Decreased CXCR4 levels implies impaired ability of IgM^Bright and IgD^Bright cells to traffic.

Surface membrane IgM and IgD Immunoglobulins, as well as all other associated receptors/markers, were more highly expressed on recently-born (“proliferative fraction”) CLL cells. A higher level of activation was noted in cells of the proliferative fraction compared to resting fraction populations.

Clonal density analyses prior and during treatment imply that ibrutinib affects the expression of the majority of cell surface markers:

• Increase: IgM, CXCR4
• Decrease: IgD, CD5, CD20, HLA-DR, CD25, CD38
• Invariant: CD19, CD22, Siglec-10

It was further found that ibrutinib affects recently-divided cells more than others, leading to a general resting/quiescent phenotype for all subpopulations. IgM is the only receptor upregulated by ibrutinib, mainly occurring in intermediate and resting fractions. However, IgD is downregulated mainly in the proliferative fraction. All co-receptors display a trend similar to IgD.

The talk was concluded by Andrea Nicola Mazzarello with a concise summary slide: