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Background CD4 is expressed in several blood cancers, including T-cell malignancies (TCM), chronic myelomonocytic leukemia (CMML), and some acute myeloid leukemias (AML), but is absent from normal hematopoietic stem cells and non-blood tissues. CD4 is also present on immune-suppressive cells including regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), and monocytic leukemia stem cells (LSCs), especially those resistant to venetoclax. Therefore, CD4 is a promising but understudied treatment target.Methods We launched multi-center, FIH phase I trials testing CD4-directed CAR T cells in patients with relapsed or refractory CD4-positive blood cancers. The CD4CAR is a third-generation CAR using both CD28 and 4-1BB costimulatory domains and delivered via lentiviral vector into autologous T cells. Patients received conditioning with either fludarabine/cyclophosphamide, or bendamustine, followed by CD4CAR in a 3+3 dose-escalation design. Safety and immune effects were assessed, and adverse events graded by CTCAE v5.0.Results Fourteen patients were treated: CD4+ AML (n=2), CMML (n=3), and TCM (n=9). Median age was 65.5 years (range 18–78), and all had received a median of 2 prior treatments (range 1–5). CD4CAR was infused at doses ranging from 8.0×10 4 to 1.6×105 CAR+ cells/kg (Cohorts1&2). Products were successfully made for all patients without needing cell selection. CD4CAR therapy was well tolerated without dose-limiting toxicities. Grade ≥3 lymphopenia resolved by day 30 without infections. Grade 1 cytokine release syndrome (CRS) occurred in 2 patients and resolved without steroids or tocilizumab. No neurotoxicity was observed. The low CRS rates may be due to depletion of CD4+ cells in the final product, especially in TCM, to avoid infusing malignant cells. CD4CAR expanded early in blood and marrow. Median time to peak CAR count was 14.5 days(range 7–120), median peak percentage of CAR+/CD3+ T cells occurred at 9.5 days (range 7–150). Persistence was variable, from beyond day 90 in the majority and up to day 365 in one. Tregs and MDSCs, particularly monocytic subsets (CD45+/CD11B+/CD193-/CD84-/CD16+14+ and 16-) decreased after infusion, especially in responders. Early depletion of monocytic LSC-like cells was seen in AML and CMML. Among evaluable patients (N=7), response rate was 71.4%, including 57.1% complete responses. All complete responders remain in remission at last follow-up (median 5.7 months). Engaging in a necessary fratricide effect on cell exhaustion was minimal.Conclusions CD4CAR is safe and feasible, with signs of immune activation and clinical benefit. The trial supports further testing of CD4-targeted therapy and its combination with agents that enhance immune recovery.Acknowledgements These trials are funded by the FDA, the LLS and the Brown Center for ImmunotherapyTrial Registration NCT06071624 NCT03829540 NCT06197672Ethics Approval The study obtained ethics approval by Indiana University and University of Miami IRB and participants gave informed consent before taking part