Background: CLL cells with del(17p) typically have loss of functional p53, rendering them refractory to chemotherapeutic agents. However, del(17p) CLL cells activated by CD40 ligand (CD154) are induced to express pro-apoptotic factors to overcome resistance to the cytotoxic activity of p53-dependent drugs, such as fludarabine. To examine whether a CD154-based therapeutic strategy can be developed in vivo for del(17p) and/or fludarabine-refractory CLL, a phase 1b clinical study evaluating an autologous cellular gene immunotherapy is being conducted. Autologous CLL cells transduced ex vivo with a replication-defective adenovirus vector encoding a membrane-stable, re-engineered form of CD154 (Ad-ISF35) are administered, followed by standard courses of FCR in subjects with fludarabine-refractory and/or del(17p) CLL. Methods: Subjects with fludarabine-refractory and/or del(17p) CLL receive three IV doses (one dose every two weeks) of 3x10⁸ autologous Ad-ISF35-transduced CLL cells. Two weeks following the third dose of Ad-ISF35-transduced cells, subjects receive up to six monthly cycles of FCR. Study endpoints include analysis of safety and efficacy. Results: Nine subjects have been enrolled and six have completed the study with an overall response rate (ORR) of 50%. This includes two subjects (33%) with an ongoing complete response (CR) after a median follow-up of 17 months, and no detectable minimal residual disease (MRD) in one subject. One subject (17%) had a partial response (PR) with resolution of lymphocytosis, lymphadenopathy and splenomegaly. One subject had stable disease after ISF35 and two cycles of FCR. Two subjects had progressive disease. Three subjects continue on study. Infusion of Ad-ISF35-transduced cells has been well-tolerated with transient fever, malaise and fatigue after ISF35 and cytopenias after FCR treatment. Conclusions: These encouraging data suggest the combination of Ad-ISF35 plus chemoimmunotherapy could offer a valuable treatment option for patients who otherwise would be resistant to standard forms of therapy.