Background: Multiple myeloma (MM) usually responds to induction therapy but relapses with therapy-resistant disease. CD28 expressed on MM cells is correlated with worse outcomes. Bone marrow stromal cells expressing CD28 ligands CD80 and/or CD86 are cellular partners in the MM niche transducing a pro-survival signal to MM cells contributing to therapy resistance in relapsed disease. We have previously shown in in vitro and in vivo preclinical studies that blocking the pro-survival CD28 activation on MM cells with abatacept (CTLA4 IgG binding to CD80/CD86 and blocking engagement to CD28) reverses chemotherapy resistance and re-sensitizes MM cells to drugs they previously were resistant to. Methods: We tested efficacy and safety of combining Abatacept with the proteasome inhibitor (PI) Ixazomib (Ixa) and Dexamethasone (Dex) in patients with MM who had relapsed (or primary refractory) disease following treatment with first line PI Bortezomib based regimen. Previous studies found that Ixa/Dex alone only had an 11% overall response rate (ORR) and 11% Clinical Benefit Rate (CBR) in patients with prior Bortezomib exposure. In our trial, patients with MM cells positive for CD28 or CD86 by flow cytometry or immunohistochemistry in any proportion were eligible. From September 11, 2018 to August 5, 2021, 15 patients received Abatacept loading dose cycle 1 day 1 followed by 125 mg subcutaneously on day 2 and then weekly. Patients received Ixa 4 mg on days 1, 8, and 15 and Dexamethasone 40 mg weekly of a 28-day cycle. The primary endpoint was ORR (partial response (PR) or better) according to International Myeloma Working Group criteria. Secondary end points were toxicity profile, progression-free (PFS), and overall survival (OS). Results: Median age was 62.8 years (range 50-83.9). Ten (66.6%) patients received prior autologous stem cell transplant. The ORR was 33.33% (90% CI 16.58%-54.46%) (one-sided Binomial exact test 0.4845). Complete remission (CR) was achieved in 6.7%, and 26.7% achieved PR with 53.3% of patients having stable disease (SD). Median time to best response among patients with CR and PR was 8.9 weeks (95% CI 4–21). Median time on treatment was 23 weeks (95% CI 12.143-48.143) for all patients and 52.5 weeks (95% CI 39.286-68) for patients with response. One-year PFS was 45% (90% CI 21%-66%), median PFS 12 months (90% CI 5.7-25.9) and 1-year OS 100%, median OS not reached. Two grade 3 treatment emergent adverse events (TEAE) (diarrhea and low platelets) occurred requiring hospitalization. Grade 1/2 GI TEAEs were most common. No grade 3/4 treatment related infections were observed and 2 grade 3 infections occurred. Conclusions: Despite prior exposure to Bortezomib, one third of patients had response to Ixa with Abatacept and Dex. Patients with response stayed on treatment for almost 1 year on average with limited AEs. Most patients on trial gained benefit with a clinical benefit rate (CR+PR+SD) of 86.66%. Clinical trial information: NCT03457142.