Background Chimeric antigen receptor-T (CAR-T) cells that target B cell maturation antigen (BCMA-CARs) have emerged as a promising treatment for multiple myeloma (MM). Despite impressive initial responses to BCMA-CAR therapy in clinical trials, relapse is common, signifying a need to improve the in vivo efficacy and persistence of BCMA-CARs.¹ The development of unfavourable differentiation or T cell dysfunction, such as exhaustion and senescence, during the ex vivo expansion of the BCMA-CARs could be limiting their therapeutic potential. For CD19-directed CARs, reduced dysfunction and differentiation and improved anti-tumour responses were achieved by expanding the cells with IL-15 instead of IL-2.² Therefore, in this study, our aim was to determine whether expanding BCMA-CARs with IL-15 or IL-15/IL-7 instead of IL-2 alters their levels of exhaustion, senescence, differentiation and activity.

Methods T cells stimulated with anti-CD3/anti-CD28-coated beads were supplemented with IL-2, IL-15, IL-15 + IL-7 or no cytokine and transduced with ARI2h, a BCMA-CAR with a 4-1BB co-stimulatory domain produced at our institution.³ Expanded BCMA-CARs were analysed by flow cytometry for markers of T cell dysfunction, or challenged with MM cell line ARP-1 and then tested for cytokine production, cytotoxic ability and activation signals.

Results BCMA-CARs cultured in IL-15 or IL-15/IL-7 expanded similarly to those grown in IL-2, with comparable CAR transduction efficiencies, CD4:CD8 ratios and proliferation rates. BCMA-CARs grown in IL-15 had low expression of exhaustion marker LAG-3 and high expression of the costimulatory molecule CD27, which is important for T cell survival and persistence, when compared to BCMA-CARs cultured in IL-2. Moreover, BCMA-CARs grown solely in IL-15 were less differentiated than those supplemented with IL-7, and had higher expression of stem cell memory marker CXCR3 within the naïve population than those expanded with IL-2. When challenged with MM cell line ARP-1, IL-15-grown BCMA-CARs upregulated activation marker CD69, exhibited strong cytotoxicity and robust production of IFNγ and IL-2. However, in comparison to BCMA-CARs expanded in IL-2 or IL-15/IL-7, those grown with IL-15 had lower mTORC1 activity and p38 MAPK phosphorylation when activated by ARP-1 cells, suggesting differential regulation of key pathways for T cell metabolism and senescence, respectively.

Conclusions To summarise, BCMA-CARs expanded with IL-15 alone exhibited the most favourable phenotype for therapeutic use compared those grown with IL-2 or IL-15/IL-7. Future experiments using murine MM models will be critical in understanding the in vivo benefits or drawbacks of culturing BCMA-CARs in IL-15 compared to IL-2 or IL-15/IL-7.

Ethics Approval Research involving human material was approved by the Ethical Committee of Clinical Research (Hospital Clinic, Barcelona). Peripheral blood T cells were obtained from healthy donors after informed consent in accordance with the Declaration of Helsinki.

References

1. Roex G, Feys T, Beguin Y, Kerre T, Poiré X, Lewalle P, et al. Chimeric Antigen Receptor-T-Cell Therapy for B-Cell Hematological Malignancies: An Update of the Pivotal Clinical Trial Data. Pharmaceutics [Internet]. 2020;12:1–15. Available from: http://www.ncbi.nlm.nih.gov/pubmed/32102267

2. Alizadeh D, Wong RA, Yang X, Wang D, Pecoraro JR, Kuo CF, et al. IL15 enhances CAR-T cell antitumor activity by reducing mTORC1 activity and preserving their stem cell memory phenotype. Cancer Immunol Res 2019;7:759–72.

3. Perez-Amill L, Suñe G, Antoñana-Vildosola A, Castella M, Najjar A, Bonet J, et al. Preclinical development of a humanized chimeric antigen receptor against B cell maturation antigen for multiple myeloma. Haematologica [Internet]. 2020; Available from: http://www.ncbi.nlm.nih.gov/pubmed/31919085