HDAC6 Inhibition Increases CD20 Level and Improves The Efficacy Of Anti-CD20 Monoclonal Antibodies

CD20, an integrate membrane protein expressed on the surface of normal and malignant B-cells is widely used as a molecular target for monoclonal antibodies (mAbs) in the therapy of non-Hodgkin’s lymphomas and chronic lymphocytic leukemia (CLL). Accumulating evidence indicates that CD20 can be modulated at several levels, both transcriptional and posttranscriptional and its up-regulation would result in increased efficacy of anti-CD20 mAbs. CD20 antigen has been reported to be regulated epigenetically e.g. by histone deacetylases (HDACs). The results of our preliminary experiments show that use of non-selective HDAC inhibitors as well as blocking the activity of a single HDAC isoform - HDAC6 leads to up-regulation of CD20 protein in B-cell lymphoma cell lines and increase of the efficacy of therapy with anti-CD20 mAbs. Since HDAC6 is engaged mainly in the acetylation of non-histone substrates and the observed up-regulation of CD20 molecule does not seem to rely on transcriptional mechanisms we postulate that HDAC6 is engaged in processes of CD20 trafficking or/and degradation. CD20 being a membrane bound protein is most probably undergoing endocytosis. However, this process and the role of HDAC6 in its regulation has not been explored so far.

The aim of this study was to understand how the inhibition of HDAC6 activity influences CD20 level in normal and malignant B-cells. We wanted to determine the mechanisms underlying this phenomena.

This study required use of B-cell lymphoma cell lines as well as lymphocytes infected with Epstein-Barr virus and normal B- lymphocytes. Several HDAC pan-inhibitors and HDAC6 inhibitors were tested. To assess the membrane level of CD20 antigen, FITC-anti-CD20 staining was performed followed by cytometric analysis. The influence of HDACi on total level of CD20 protein was assessed in Western blotting. The complement-dependent cytotoxicity (CDC) assay was performed using rituximab and ofatumumab as well as human serum as a source of complement. Cell cytotoxicity was assessed by propidium iodide staining followed by cytometric analysis. The influence of HDAC inhibition on the transcription of CD20 was examined by qRT-PCR using SyBR Green and hydrolysis probes. The activity of CD20 promoter after inhibition of HDAC was assessed in Dual Luciferase Assay. The colocalisation of CD20 with other proteins that may influence its trafficking/degradation was assessed using immunocytochemistry with specific antibodies and observed under confocal microscope.

The results of our study strongly suggest that combining HDACi with anti-CD20 antibodies can be an effective therapeutic modality for patients suffering from B-cell malignancies. Extensive experiments aiming at determining what factors are engaged in the regulation of CD20 by HDAC6 are planned.

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