Using an advanced microscopic technique to visualize how CAR T cells work in a mouse model of lymphoma, researchers found that these cells can induce tumor regression by directly targeting and killing cancer cells but there is significant diversity in how they behave in an animal.
The study, “Single-cell imaging of CAR T cell activity in vivo reveals extensive functional and anatomical heterogeneity,” was published in the Journal of Experimental Medicine.
CAR T-cell therapy is a type of treatment that involves collecting a patient’s own immune T-cells and modifying them to produce a chimeric antigen receptor, or CAR, that targets a specific cancer protein.
The two CAR T-cell therapies approved to date — Yescarta (axicabtagene ciloleucel) and Kymriah (tisagenlecleucel) — both target the CD19 protein, a cell surface marker that is present in healthy and malignant B cells.
However, despite their promising results in lymphoma and leukemias, the interactions that regulate anti-CD19 CAR T-cell activity and tumor regression are not completely understood.
Thus, researchers at France’s Institut Pasteur and INSERM sought to answer three major questions that remain regarding CAR T-cell function. Using a mouse model of lymphoma, they first aimed to understand how interactions between CAR T cells and healthy B cells affect T cell activity and persistence.
Next, they addressed how CAR T-cells induce tumor regression; do they directly kill cancer cells or activate other immune cells to do the job? Finally, they studied the emergence of CD19-negative cancer cells — known to cause treatment resistance — and the exact sites where these cells appear.
Using intravital two-photon imaging — a form of microscopy that helps tracking cells in a live animal — investigators followed the paths and interactions of anti-CD19 CAR T cells in a mouse model of B-cell lymphoma.
Interestingly, lymphoma cells died quickly upon direct contact with a CAR T cell, suggesting that these cells largely kill their targets directly and not through the activation of other immune cells.
“Computer simulations based on our experimental data supported the idea that CAR T cells rely on their direct cytotoxic activity rather than on the recruitment and activation of other cells to eliminate the bulk of the B-cell lymphoma in the bone marrow,” Philippe Bousso, the study’s lead investigator, said in a press release.
However, not all CAR T-cell interactions led to cancer cell death, which researchers attribute to the diversity in the killing potential of CAR T cells.
Furthermore, the team found that not all the injected CAR T cells were able to enter into the bone marrow because they were engaged outside it when they encountered other B cells (including lymphoma cells).
Because even small differences in the number of cells reaching the bone marrow significantly affected treatment efficacy, researchers believe that reducing circulating B-cell levels could enhance the ability of CAR T cells to enter the bone marrow and kill tumor cells.
“Purging both circulating tumor and normal B cells prior to CAR T-cell transfer may therefore offer a clinical benefit by improving CAR T-cell engraftment and persistence,” Bousso said.
Finally, researchers showed that tumor relapse and CD19-negative tumors emerge largely from the bone marrow, rather than other organs, because CAR T cells are mostly active in the bone marrow.
“In sum, our results reveal the large diversity in CAR T-cell behaviors in distinct anatomical sites impacting engraftment, anti-tumor activity, and tumor relapse,” Bousso said. “Understanding these differences is an important step toward developing strategies to optimize CAR T cell-based treatments.”