Adoptive cell therapy with T cells genetically modified to express chimeric antigen receptors (CAR-T) is one of the most promising advanced therapies for cancer treatment, especially in some hematological tumors. However, clinical results demonstrate that more than 40% of patients with B-cell malignancies relapse after this treatment. Because of its pro-survival and proliferative properties, IL-15 was proposed for the 4th generation of CAR-T cells to improve their persistence. In this work, we have studied the antitumor activity of CD19 CART cells expressing a tethered IL15-IL15R protein in BALB/c immunocompetent murine model challenged with A20 tumor cells. Potential long-term toxicity was also studied.

Conventional anti-murine CD19 CAR-T cells showed low persistence and poor efficacy in immunocompetent mice treated with mild lymphodepletion regimens (total body irradiation (TBI) of 1 Gy). The efficacy was improved when mice were previously treated with a high TBI regimen (4 Gy). The murine IL15-IL15R fusion protein enhances the persistence and in vivo efficacy of CD19 CAR-T cells, as they are able to eradicate established A20 B cell lymphoma. However, CD19 IL15-IL15R CAR-T displays important long-term toxicities, with marked splenomegaly, weight loss, ALT elevations, and significant inflammatory findings in some tissues. Mice survival is highly compromised after CD19 IL15-IL15R CAR-T cell transfer, particularly if a high TBI regimen is applied before CAR-T cell transfer.

In conclusion, tethered IL-15-IL15R augments the antitumor activity of CD19 CAR-T cells but displays long-term toxicity in immunocompetent mice. Inducible systems to regulate IL15-IL15R expression should be considered to control this toxicity.