Apoptosis of Hematopoietic Stem Cells Contributes to Bone Marrow Suppression Following Chimeric Antigen Receptor T Cell Therapy

Chimeric antigen receptor (CAR) T cell (CAR-T) therapy represents a revolutionary treatment for patients with relapsed/refractory hematologic malignancies. However, its use can result in significant toxicities, including cyto-kine release syndrome (CRS), a potentially life-threatening clinical syndrome resulting from the release of proin-flammatory cytokines upon T cell activation. In addition, patients who develop CRS often experience prolonged cytopenias, and those with the most severe CRS also have the longest delays in full marrow recovery. Although an association between CRS and delayed bone marrow recovery has been established, the precise mechanism under-lying this phenomenon remains unknown. This study was conducted to test our hypothesis that delayed bone marrow recovery following CAR-T therapy is caused by elevation of proinflammatory cytokines, leading to apo-ptosis and depletion of hematopoietic stem and progenitor cells (HSPCs). SCID-beige mice bearing intraperitoneal CD19+ Raji cell tumors were treated with injection of human CD19.28z CART cells. Bone marrow was then har-vested for analysis by flow cytometry, and HSPCs were isolated for whole-transcriptome analysis by RNA sequenc-ing. Complete blood counts and serum cytokine levels were measured as well. A second model was developed in which SCID-beige mice were treated with murine IFN-y (mIFN-y), murine IL-6 (mIL-6), or both. Bone marrow was harvested, and flow cytometry assays were conducted to evaluate the degree of apoptosis and proliferation on specific HSPC populations. SCID-beige mice bearing intraperitoneal Raji cell tumors that were treated with CAR-T therapy developed CRS, with elevations of several proinflammatory cytokines, including profound elevation of human IFN-y. Gene set enrichment analysis of RNA sequencing data revealed that genes associated with apoptosis were significantly upregulated in HSPCs from mice that developed CRS. Endothelial protein C receptor (EPCR)-negative HSCs, a subset of HSCs that is poised for terminal differentiation, was found to be specifically decreased in mice that were treated with CAR T cells. Furthermore, HSPCs were found to have increased levels of apoptosis upon treatment with mIFN-y and mIL-6, whereas short-term HSCs and multipotent progenitors exhibited increases in proliferation with mIFN-y treatment alone. The results from this study provide evidence that the ele-vation of proinflammatory cytokines following CAR-T therapy impacts the bone marrow through a combined mechanism: pluripotent HSCs that are exposed to elevated levels of IFN-y and IL-6 undergo increased cell death, while more committed progenitor cells become more proliferative in response to elevated IFN-y. These combined effects lead to depleted stores of repopulating HSCs and ultimately cytopenias. (c) 2023 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc. (c) 2022 The American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Automated summary

This study investigates the mechanism of delayed bone marrow recovery following CAR-T therapy. The results indicate that the elevation of inflammatory cytokines after CAR-T therapy leads to apoptosis and depletion of hematopoietic stem and progenitor cells, resulting in cytopenias. This finding has important implications for the safety and efficacy of CAR-T therapy.