Tumor necrosis factor-α and CD40L modulate cell surface morphology and induce aggregation in Ramos Burkitt's lymphoma cells

Abstract

Interaction of CD40L and its cognate receptor is an essential component of B-lymphocyte signaling, affecting various aspectsof B-cell differentiation pathways and immunoglobulin gene expression. However, much less is known about the biologicalconsequences of B-cell signaling through tumor necrosis factor (TNF)-α and its cognate receptors TNF-R1 and 2. We usedRamos Burkitt's lymphoma cell line as a model system to study the direct effects of these cytokines on B cells. Treatment ofRamos cells with either TNF-α or CD40L, but not with interleukin (IL)- 4, interferon (IFN)-γ and transforming growthfactor (TGF)-β, resulted in enhanced cell aggregation and enhancement of adherence to glass cover-slips. Scanning electronmicroscopy showed that Ramos cells have a polarized cell surface morphology and exhibit at least 3 cell surfacemorphological domains: microvilli, filopodia and ruffled membranes. The cells adhered to the glass matrix through multiplefilopodia/podopodia-like cell processes and demonstrated distinct ruffled-like membrane projections on their opposite pole.Induction by TNF-α or CD40L, but not with IL-4, IFN-γ and TGF-β, resulted in increased number and complexity of bothtypes of membrane projections. TNF-α and CD40L upregulated the expression of the adhesion molecule intercellularadhesion molecule-1 and the Fas receptor on Ramos cells, without affecting the expression levels of membraneimmunoglobulin M or its secretion rate. Reverse transcriptase-polymerase chain reaction, and flow cytometry demonstratedthat Ramos cells expressed TNF-R1 but very little if any TNF-R2, indicating that TNF-α exerted its effects on Ramos cellsthrough the former receptor.