Human Herpesvirus 8 K1 Blocks Fas in Cells and Mice by Binding and Blocking Fas.

Background: Kaposi’s sarcoma and primary effusion lymphoma are linked to human herpesvirus 8 (HHV-8 or Kaposi’s sarcoma-associated virus) infection. Key to the pathogenesis of these cancers is the HHV-8 transmembrane oncoprotein K1. This viral protein, which immortalizes cells, produces lymphoproliferation and lymphomas through unknown mechanisms when expressed in mice. Results: We show here that K1 blocks Fas (CD95)-mediated apoptosis by binding the ectodomain of K1 to Fas in hematopoietic cancer cells (BJAB, U937 and THP-1). K1 bound to Fas in K1-transfected BJAB cells. To assess whether K1 and Fas formed complexes in cells, we chemically crosslinked proteins in transfected cells using 3,3′-dithiobissulfosuccinimidyl propionate in the presence of N-ethyl maleimide. Stabled K1-Fas complexes were present in intact cells despite blocking of free reactive cysteines. The selective ability of K1 to antagonize Fas-mediated apoptosis was strictly dependent on having its immunoreceptor tyrosine-based activation motif (ITAM). Using activating anti-Fas (CH-11) and nonactivating anti-Fas (B10) antibodies in a serial manner, we showed that K1 associated with Fas that was not activated, strongly suggesting that K1 prevents Fas from being activated. Mice transfected with K1 were protected from the apoptosis inducing effects of agonistic Jo2 anti-Fas antibody and tissues from these mice contained K1-Fas complexes. K1-transfected compared to vector-transfected mice had fewer terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling positive cells (13 ± 3% vs. 60 ± 7%, P=0.008), fewer activated caspase 3 positive cells (24 ± 5% vs. 88 ± 6%, P=0.006). Tissues had less hemorrhaging, and necrosis. Conclusion: These data support a mechanism of K1 direct binding to and blockage by K1 to Fas in suppression of apoptosis in an ITAM-dependent manner, which may be a key step in lymphoma development.