Experimental: Decidua Stroma Cells (DSC)

Placenta derived decidua stroma cells (DSC). In the first phase I part, two different dose levels will be used, 1x10\^6/kg and 3x10\^6/kg. Two doses, one week apart, will be given. The decision to proceed to the next dose level will depend on results observed at the previous dose level.~The dose in the randomized part will be based on the findings in the phase I part. In the Phase II study all patients will receive 2 doses, one week apart. Depending on response, up to 6 doses in total may be given. Additional doses (beyond the first 2 doses) may be given one week apart until response.

Active_comparator: Best Available Treatment (BAT)

The BAT in this study will freely be identified by the Investigator prior to patient randomization and may include treatments such as: anti-thymocyte globulin (ATG), extracorporeal photopheresis (ECP), low-dose methotrexate (MTX), mycophenolate mofetil (MMF), mTOR inhibitors (everolimus or sirolimus), etanercept, vedolizumab, ruxolitinib or infliximab. Dose and frequency will depend on label (where approved) and institutional guidelines for various BAT.