This is a prospective, multicenter (with two performance sites under the auspices of Case Western Reserve University Clinical and Translational Science Award (CWRU CTSA) at University Hospitals and at MetroHealth Medical Center), double-blind, placebo-controlled, interventional study to evaluate the safety and efficacy of allogeneic mesenchymal stem cells (MSCs) infusion in 20 new onset Rheumatoid Arthritis (RA) patients with moderate to high disease activity despite adequate doses of methotrexate (MTX) for 12 weeks (at least 10 weeks of which has been at a dose of >/=15 mg per week). The study is a single dose, phase I clinical trial, and this is the first time that this product will be infused in RA patients. After a screening period, the baseline visit will be conducted. Patients will halt their concomitant MTX 3 days prior to the MSC infusion and may resume their methotrexate at 7-10 days following MSC infusion. This is a dose escalation study of a total of 20 patients with three groups of five patients each and five placebo patients. Patients will be randomized to receive MSC or placebo infusion using a computer-generated randomization scheme that takes into account that there are two sites. The first cohort will consist of a total of six patients. Of these six patients, five patients each will receive a single infusion of 2 million/kg MSCs and one patient will receive placebo infusion. The second cohort will include a total of seven patients, of these seven patients; five patients will receive 4 million/kg MSCs and two patients who will receive placebo infusion. The final cohort consists of a total of seven patients. Of these seven patients, five patients will receive 6 million/kg allogeneic MSCs and two patients will receive placebo infusion. Infusion will occur on Day 0. Post-infusion study visits will occur on Days 1 (Visit 3), 7 (Visit 4), 14 (Visit 5), 28 (Visit 6), 56 (Visit 7), and weeks 24 (Visit 8), 39 (Visit 9) and 52 (Visit 10). Phone calls will occur on Day 4, 21, and 72. Subject safety and tolerability of the single dose of MSCs will be evaluated at these study visits by reviewing interval histories, administering patient questionnaires (legacy [Routine Assessment of Patient Index Data 3 (RAPID 3) / Short Form 36 (SF36)] and Patient Reported Outcome Measurement Information System (PROMIS) computer adaptive technology CAT), performing physical exams and spirometry, and obtaining safety laboratories. Special attention will be given to exacerbation of RA or "flare". This study will also explore the efficacy measures: Disease Activity Score 28-C-Reactive Protein (DAS28-CRP) and American College of Rheumatology20/50/70 (ACR20/50/70 will be calculated. Since the primary hypothesis is that infusion of MSCs will induce a state of immune tolerance, various assays to detect post-infusion changes in cells subsets, function or protein biomarker will be repeated at Day 7 and/or 14 and compared to baseline values (with-in subject comparison). Biomarkers to look at cell-subsets will be drawn. Selected patient samples will be stored for exploratory studies using Mass Cytometry (Cytof 2). Lastly, for those subjects who are agreeable to undergo the procedure, a bone marrow aspiration will be performed prior to infusion with allogeneic MSCs. MSCs from RA bone marrow will be expanded, banked, and used for future translational studies.