Fibroblast Growth Factor Inducible (Fn14)-specific Antibodies Concomitantly Display Signaling Pathway-specific Agonistic and Antagonistic Activity

The Fn14-specific monoclonal antibodies PDL192 and P4A8, which are under consideration in clinical trials, showed no agonistic activity with respect to IL8 production and cell death induction. However, oligomerization with protein G or binding to Fc gamma receptors converted both anti-Fn14 antibodies into potent agonists. TNF-like weak inducer of apoptosis (TWEAK), the ligand of Fn14, occurs naturally in two forms with partly different signaling capabilities, as a membrane-bound ligand and as a soluble trimeric molecule. Although membrane TWEAK strongly triggers all Fn14-associated pathways, soluble TWEAK predominately triggers the alternative nuclear factor kappa B (NF kappa B) pathway and enhances TNF-induced cell death but has only a poor effect on the classical NF kappa B pathway and chemokine production. Thus, the oligomerized and Fc gamma R-bound anti-Fn14 mAbs mimicked the activity of membrane TWEAK. Notably, both anti-Fn14 antibodies significantly triggered p100 processing, the hallmark of the alternative NF kappa B pathway, and therefore resembled soluble TWEAK. In contrast to the latter, however, the anti-Fn14s showed no effect on TNF receptor 1-induced cell death and P4A8 even blocked the corresponding TWEAK response. Thus, we showed that Fn14 antibodies display an alternative NF kappa B pathway-specific agonistic activity but fail to phenocopy other activities of soluble TWEAK, whereas oligomerized or Fc gamma R-bound Fn14 antibodies fully mimic the activity of membrane TWEAK. In view of the trivalent nature of the TWEAK-Fn14 interaction, this suggests that the alternative NF kappa B pathway is uniquely responsive already to Fn14 dimerization enabling antibodies to elicit an unnatural response pattern distinct from that of the naturally occurring Fn14 ligands.