PBMC-reconstituted mouse models are critical for evaluating in vivo CAR-T therapies, but their utility is limited by rapid-onset Graft-versus-Host Disease (GVHD). GemPharmatech employed a novel NCG-MHC-dKO model to mitigate GVHD, enabling prolonged assessment of anti-tumor efficacy for different in vivo CAR-T modalities. The NCG-MHC-dKO model was reconstituted with human PBMCs, which confirmed delayed GVHD onset. Mice bearing Nalm-6 tumors were treated with a single dose of either a lentiviral vector or an LNP formulation delivering a CD19-targeting CAR. Tumor growth was monitored via bioluminescence imaging. The PBMC-NCG-MHC-dKO model demonstrated a significantly extended observation window due to postponed GVHD. In this model, both lentiviral and LNP-based in vivo CAR-T therapies potently suppressed Nalm-6 tumor growth. Functional CAR-T cells generated in vivo were detected and found to expand in treated mice. The PBMC-reconstituted NCG-MHC-dKO model provides a robust and durable platform for in vivo CAR-T evaluation by alleviating early GVHD.