Activation of the extrinsic apoptotic pathway by TNF-α in Human Salivary Gland (HSG) cells in vitro, suggests a role for the TNF receptor (TNF-R) and Intercellular Adhesion Molecule-1 (ICAM-1) in Sjögren's Syndrome-associated autoimmune sialadenitis

Abstract 

Studies suggest that apoptosis plays a major role in destruction of salivary glands in Sjögren's Syndrome. We hypothesise that apoptosis results in the exposure of cryptic T cell epitopes and an autoimmune response which is pathway-specific.

Objective

To activate the extrinsic or intrinsic apoptotic pathway to examine morphology, adhesion molecules, markers of antigen processing, and autoantigens on apoptotic bodies in vitro.

Methods

Tumor necrosis factor-α (TNF-α) or staurosporine, a protein kinase inhibitor, was used to trigger the extrinsic or intrinsic apoptotic pathway in a Human Salivary Gland cell line (HSG), in vitro. Activated genes were profiled by cDNA array. Apoptotic bodies were visualised using light and SEM. Proteins were evaluated by immunofluorescence and confirmed by functional binding assays with Jurkat lymphocytes.

Results

TNF-α-triggered extrinsic apoptosis resulted in “sticky” aggregated, apoptotic bodies which displayed cleaved α-fodrin autoantigen. In contrast, intrinsic apoptosis induced by staurosporine, resulted in dispersed cell blebs which were α-fodrin-negative. cDNA arrays revealed that TNF-α, but not staurosporine, upregulated transcriptional expression of Intercellular Adhesion Molecule-1 (ICAM-1) and Macrophage Inflammatory Protein-3 (CCL20).

Conclusion

The apoptotic pathway controls morphological, structural and functional properties of apoptotic bodies. Collectively, TNF-α-dependent activation of the extrinsic apoptotic pathway leads to upregulation of ICAM-1 and CCL20 in HSG in vitro. This suggests that pathogenesis in Sjögren's Syndrome may involve a TNF-controlled cross-talk between apoptotic ductal and CCL20-attracted dendritic cells via the CD137/CD137L signalling pathway.

Keywords: Apoptosis, Salivary glands, Sjögren's Syndrome, Gene arrays, In vitro model HSG cells