New CAR T cell design boosts persistence and reduces toxicity

CAR T-cell therapy has transformed the treatment of hematologic malignancies, yet many patients do not achieve lasting benefit – often because the infused cells fail to expand or persist. This study addresses these challenges by focusing on a more defined cell product: a highly homogeneous population of stem cell memory T (T_SCM) cells with strong self-renewal and proliferative capacity. Gattinoni and colleagues previously established a clinical-grade platform for generating CAR-modified T_SCM cells, which showed superior anti-leukemic activity in models of acute lymphoblastic leukemia (ALL), providing a rationale for clinical translation.

Today's CAR T-cell products are heterogeneous, and that variability is reflected in the range of clinical responses and toxicity profiles we see in patients. To address this, we developed a highly homogeneous CD8⁺ CAR T-cell product selectively enriched for T_SCM cells and compared its performance to conventional CAR T cells."

Gabriele Inchingolo, PhD student in Gattinoni's team and shared first author of the study

What the T_SCM platform revealed

In this first-in-human trial, the team found that CAR-modified T_SCM cells exhibited greater expansion and persistence than standard CAR T cells, enabling complete responses at low doses ­even without lymphodepleting chemotherapy, which is typically given before CAR T-cell infusion to help the cells engraft. "The T_SCM platform yielded higher CAR T-cell levels on a per cell basis – and across many CAR T-cell studies, high blood CAR T-cell levels have been one of the strongest predictors of clinical efficacy," confirms Dr. James Kochenderfer, Senior Investigator at the Surgery Branch of the National Cancer Institute (NCI) in Bethesda, USA and co-corresponding author of the study. "Seeing patients achieve complete responses at doses as low as 250,000 cells per kilogram, without chemotherapy preconditioning, validates years of preclinical work and opens a new chapter in CAR T-cell design," adds Prof. Luca Gattinoni, Head of the LIT Research Division "Functional Immune Cell Modulation" and first author of the study.

The T_SCM platform also demonstrated a favorable tolerability profile. "We also observed less cytokine-release syndrome on this study compared to most other CAR clinical trials that I have participated in," notes Kochenderfer. Cytokine release syndrome (CRS) is a common and potentially serious inflammatory reaction triggered when CAR T cells become active in the body. Remarkably, even at expansion levels that caused severe CRS in the standard cohort, T_SCM-treated patients experienced only mild side effects, suggesting that the T_SCM product may separate the beneficial effects of expansion from its toxic consequences.

Beyond clinical outcomes, the study shed light on how the two products behave differently once inside the body. Unlike conventional CAR T cells, which depleted their stem-like reserves, CAR T_SCM cells maintained a durable self-renewing pool throughout the response through clonal succession rather than self-renewal of individual clones. "Rather than differentiating all at once, T_SCM cells are recruited in small, sequential waves: each cohort of active clones succeeding the last while preserving the long-term reservoir of quiescent stem-like cells. For the first time, we are witnessing this fundamental biology play out directly in patients," explains Dr. Enrico Lugli, Principal Investigator in the Laboratory of Translational Immunology and Head of Flow Cytometry core facility at Humanitas Research Hospital, Rozzano, Italy, and coauthor of the study.

Study design

The phase 1 trial (NCT01087294), led by James N. Kochenderfer and colleagues at the U.S. National Institutes of Health, enrolled patients with relapsed or refractory CD19⁺ B-cell malignancies following allogeneic hematopoietic stem cell transplantation (HSCT), a population with severely limited therapeutic options. Patients were treated in two sequential cohorts: one receiving conventional donor-derived CD19 CAR T cells and the other receiving a T_SCM-enriched product. All infusions were administered without lymphodepleting preconditioning, enabling a direct assessment of each product's intrinsic capabilities.

Immunomonitoring was conducted and coordinated by the LIT researchers, in collaboration with scientists from Humanitas Research Hospital, who performed longitudinal patient follow-up using multidimensional flow cytometry and advanced bioinformatics to precisely track the expansion, persistence, and fate of CAR T cells. As with any early-phase trial, these findings will require validation in larger, randomized cohorts to establish the broader clinical significance of the T_SCM approach.

Looking ahead

Where the T_SCM product did not succeed, the reasons proved instructive. Treatment failure was driven not by any deficiency in the T cells themselves, but by external factors such as low levels of the target protein on tumor cells, immunosuppressive signals (notably IL-10), and immune responses directed against the CAR construct providing a clear roadmap for the design of next-generation trials.

"We have shown that a more defined, stem-like cell product can perform effectively at lower doses. By employing highly homogeneous T_SCM population, we can potentially achieve more consistent engraftment and persistence, paving the way for more predictable outcomes and more rationally designed clinical trials," concludes Gattinoni.

The full therapeutic potential of CAR T_SCM cells has yet to be realized. Future studies incorporating lymphodepleting preconditioning, fully humanized CAR constructs, and the potential addition of CD4⁺ T cells are expected to further improve outcomes. The biological principles established here are likely applicable far beyond the post-alloHSCT setting, including autologous CAR T-cell therapies and solid tumors, where limited T-cell persistence has historically been a barrier to efficacy.