Cocultures were seeded into 384 well plates (Corning) with the Multidrop Combi (Thermo Scientific). 800 stromal cells were seeded and incubated overnight. 17 hours later, 700 myeloma cells were added on top of the stromal cells. On the third day, 24 hours after the addition of myeloma cells, cocultures were treated with tofacitinib (LC Laboratories), ruxolitinib (Selleck Chemicals), JAK3i,20 (link) or IL-6 blocking antibody (R&D Systems). For drug combination studies, on the fourth day, melphalan (Sigma Aldrich), carfilzomib (Selleck Chemicals), or venetoclax (Selleck Chemicals) were additionally added to cocultures. On the fifth day, myeloma cell viability was detected with the addition of luciferin (Gold Biotechnology) and read for luminescence on Glomax Explorer plate reader (Promega) as previously described.21 (link) For monoculture studies cell viability was measured using CellTiter-Glo reagent (Promega). All measurements were performed in quadruplicate. All viability data are reported as normalized to dimethyl sulfoxide (DMSO)-treated cell line in monoculture.