Bispecific CAR-T Cells Targeting Both BCMA and CD24: A Potentially Treatment Approach for Multiple Myeloma

Introduction: Anti-myeloma BCMA-specific chimeric antigen receptor (CAR) T-cell therapies represent a promising new treatment strategy, with high response rates observed in the early stages of therapy. However, the responses are not durable. One known mechanism of relapse has been traced to the loss of BCMA expression following long-term CAR-T therapy. Another potential reason is that while BCMA CAR-T cells eliminate the bulk of BCMA-positive MM cells, a small subset of BCMA-negative, very drug-resistant MM cells, such as tumor-initiating cells (TICs) survive and seed relapses. There is a strong correlation between the presence of MM TICs with minimal residual disease, acquired drug resistance and relapse. This suggests that TIC-targeted therapies could improve outcomes. We have previously demonstrated that MM cells expressing CD24 also exhibit features of TICs, e.g. self-renewal, increased expression of embryonic stem cell genes and drug resistance. We have generated bispecific CAR-T cells which recognize both BCMA and CD24 antigens and have tested their therapeutic efficacy in MM cells in vitro and in vivo models.

Methods: We constructed a bispecific BCMA-CD24 CAR vector, with 2 complete CAR units: BCMA CAR and CD24 CAR. P2A was inserted between these two CARs. The BCMA CAR contains a safety switch in the hinge region, and a CD28 co-activation domain with CD3ζ. The CD24 CAR contained a 4-1BB co-activation domain with CD3ζ. To decrease the risk of severe immunological side effects, we integrated RQR8, an immunological safety switch with epitopes for CD34 and CD20 as a suicide molecule into the hinge region. Lentivirus particles were used to transduce primary human T cells. CAR-T cells were detected on day 7 by flow cytometry using antibodies to CD34. We performed co-culture killing assays, detected the T cell activation marker CD69 and measured the cytokines in the supernatant. We determined whether BCMA-CD24 CAR-T cells targeted the TIC population by flow cytometry and microscopy. The NOD. Cg-Prkdc ^scidIl2rg ^tm1Wjl/SzJ (NSG) xenograft mouse model was used for in vivo studies. 8-week-old NSG mice were administered 2 × 10 ⁶ MM cells by intravenous injection. On day 7 after MM cells injection, 1 × 10 ⁶ CAR-T cells were administered. Mice were weighed and monitored for signs of distress every two days. Bioluminescence images were acquired 10 min after D-luciferin injection. Myeloma progression was monitored every 7 days until the mice develop hind limb paralysis or the bioluminescence signal (ROI) is more than 2 × 10 ¹⁰.

Results: CAR-T cells were detected by flow cytometry using the RQR8-specific CD34 antibody. The BCMA-CD24 CAR was found to be expressed on roughly 13% of T-cells. To determine the selective lysis by the CAR-T cells, we performed co-culture killing assays in which MM cell lines over-expressing CD24 (ARP-1 ^CD24OE or OCI ^CD24OE cells) were incubated with CAR-T cells. When the CAR-T: MM ratio was 5:1, the lysis percentage of target cells was 99% (ARP-1 ^CD24OE) and 89% (OCI ^CD24OE). CAR-T cell activation was determined by increased CD69 expression and IL-2 production. As expected, exposure to CD24 ⁺ MM cells resulted in strong activation of CAR-T cells, and CAR-T cells did target and kill the TIC population. Bioluminescence imaging showed CAR-T mediated antitumor activity, yielding near-complete tumor clearance. Additionally, mice treated with CAR-T cells exhibited increased survival compared with mice in the control groups.

Conclusion and Significance: This study developed a BCMA-CD24 CAR-T, a novel MM immunotherapy. We have demonstrated strong cytotoxic activity and selectivity for MM cells in vitro and in vivo. Future studies will be aimed at determining if BCMA-CD24 CAR-T can target TIC-mediated relapses.