Background: Existing CD19-targeted autologous chimeric antigen receptor (CAR)-T cells introduce patient-to-patient response variability, require lengthy manufacturing, and are expensive. Thus, there is a need for novel allogeneic cell therapies. Invariant natural killer T (NKT) cells are promising alternative candidates for CAR-redirected allogeneic immunotherapy. Our ongoing phase I trial of allogeneic CD19.CAR.IL15.NKTs has yielded objective responses, but limited therapeutic cell persistence likely due to host rejection reduces response durability. Methods: We engineered CAR-NKTs to express 4-1BB-specific alloimmune defense receptors (ADRs) that eliminate alloreactive T and NK cells. We evaluated an established ADR with a 4-1BB ligand fragment (ADR1) and two new ADRs (ADR2/3) with a 4-1BB-specific scFv sequence to prevent NKT allo-rejection. We performed in vitro mixed lymphocyte reactions (MLRs) and cytotoxicity assays to test the ability of 4-1BB ADRs to protect donor NKTs/CAR.NKTs from allo-rejection. We also engineered the ADRs to include the iCaspase9 (iC9) safety switch for clinical application. Results: We found that ADRs are expressed effectively by NKTs (>90% ADR+) and CD19.CAR.NKTs (>60% CAR+ADR+). NKTs expressing ADRs 2 and 3 expanded faster than ADR1-transduced cells, suggesting lower fratricide levels in NKTs expressing ADRs 2/3. The MLRs showed that ADR+ NKTs/CD19.CAR.NKTs evade allo-rejection by eliminating alloreactive host T cells. The iC9 switch did not significantly decrease ADR+NKT expansion and mediated effective apoptosis of ADR+ cells within 24hrs of rimiducid administration. In vitro MLRs and tumor killing assays showed that iC9.ADR+CAR+ NKTs mediate 4-1BB- and CD19-specific cytotoxicity. Conclusions: Our results suggest that co-expressing 4-1BB-specific iC9.ADRs and a CD19.CAR in NKTs protects against host-mediated allo-rejection and generates robust antitumor activity in vitro. Future experiments will use MHC-KO lymphoma mice to identify CAR+ADR.NKTs with optimal in vivo persistence and resistance to allo-rejection. Results will inform the development of off-the-shelf NKT products for immunotherapy of B-cell malignancies and other types of cancer.