PROJECT SUMMARY/ ABSTRACT Multiple myeloma (MM) ranks as the second most common blood cancer and it remains incurable. Autologous stem cell transplantation (ASCT) remains a mainstay of therapy for eligible patients. Despite the routine use of novel agents as post-ASCT “maintenance” to delay or prevent relapse, most patients will succumb after transplant. There is considerable body of evidence to suggest that immunoregulatory mechanisms established in post-ASCT bone marrow (BM) microenvironment favor relapse and constitute attractive therapeutic targets. Post-ASCT relapses depend on tolerogenic IL10-producing myeloid cells (dendritic cells (DC) and macrophages, collectively referred to as tol-DC) and IL17 proposed to act on MM cells in a cell-autonomous manner. However, the upstream signals or microenvironmental triggers that elicit these processes are unclear. Tol-DC polarization and Th17 differentiation are promoted through Toll-like receptor (TLR)-2 signaling. We previously reported that MM-accessory cells secrete the TLR2-ligand matrix proteoglycan, versican (VCAN). VCAN promotes tol-DC polarization in carcinomas and therefore, it constitutes a prime suspect for triggering relapse-promoting, TLR2-dependent processes in MM. In the MM microenvironment, specifically post-ASCT, VCAN undergoes ADAMTS-mediated extracellular proteolysis to release an N-terminal fragment, versikine. Versikine acts as a matrikine (an extracellular matrix- derived fragment that regulates cell activity, often in a manner distinct from that of its parent macromolecule). Versikine is a weak IL6/IL10 trigger, therefore it is unlikely to be a potent tol-DC/Th17 inducer. Instead, versikine stimulates IRF8-dependent transcripts and promotes the IRF8-dependent Batf3-DC subset in vitro and in vivo. We hypothesize that the versikine-IRF8-Batf3-DC axis may engage the potent (and perhaps dominant) tolerogenic VCAN-TLR2 pathway in a dynamic crosstalk. We have delineated 2 specific Aims to investigate the mechanisms by which VCAN and versikine regulate anti- MM immunity post-ASCT: In Aim 1, we shall dissect VCAN-TLR2 signaling in anti-MM immunity and design novel post-ASCT treatment strategies based on targeting tolerogenic VCAN-TLR2 signaling. In Aim 2, we shall study in-depth the role of the matrikine, versikine, in anti-MM immunity. Success of our Aims will optimize MM treatment (maintenance) strategies to prolong post-ASCT survival. The experiments proposed here are facilitated by our recent generation of the first Ras-driven MM model, VQ. RAS pathway is the most commonly mutated pathway in human MM. In contrast to current state-of-art MM models, VQ is readily transducible by lentiviral vectors and engrafts in C57BL/6J recipients (facilitating mechanistic in vivo studies). Several of the studies proposed here have been impossible or impractical using existing MM models.