Human Single Domain Antibody-Based CAR-T Cells Targeting BAFF-R Demonstrate Promising Preclinical Activity in B Cell Malignancies

Background: B-cell activating factor-receptor (BAFF-R) is a potential B-cell specific target, highly expressed in B cell malignancies and regulates B cell proliferation and survival. Single-chain antibody-based CAR-T cells targeting BAFF-R (PMB101) has demonstrated antitumor effects against human B cell malignancies and can overcome CD19 antigen loss (Qin et al. Sci Transl Med. 2019;11(511)). However, CARs built on antigen-specific single chain antibody variable fragment (scFv) may have some immunological risks. Recently, single variable domain of heavy chain antibodies are becoming an alternative to scFv to construct CARs. To improve the function of BAFF-R CAR-T cells, we designed CARs with only a fully human heavy-chain variable domain.

Methods: A fully human antibody phage display library (IMARS, IASO Biotherapeutics) was used to select anti-BAFF-R clones using optimal protein/cell alternate panning. The enzyme-linked immunosorbent assay (ELISA) and flow cytometry were applied to evaluate the specificity of clones. The screened clones were grafted into a second‐generation CAR with CD8α hinge, 4‐1BB costimulatory and CD3ζ intracellular signaling domains to generate CAR-T cells. PMB101 and CAR-T cells constructed with VAY736 were used as positive controls. Non-transduced T cells from the same donor were used as a negative control.

Results: Four candidate clones including clone 1, 5, 77 and 80 were finally screened by in vitro functional evaluation, among which clone 5, 77 and 80 were variable domain heavy chain antibodies. The expression levels of the degranulation marker CD107a on CAR‐T cells constructed with the four candidate clones were significantly upregulated after coculture with BAFF-R positive tumor cells, while BAFF-R negative tumor cells could not effectively activate CAR-T cells (data not shown). The four clones exhibited similar cytotoxicity in CAR‐T cells compared with the PMB101 and VAY736 CAR‐T cells using the luciferase‐based cytotoxicity assay (data not shown). Xenografts were established in NPG mice following intravenous injection of 1 x 10 ⁶ jeko-1 cells expressing the firefly luciferase gene on day - 5, and a single dose of 4 x 10 ⁶ CAR T cells were infused IV on day 0. Compared to the other groups, no progressive tumor growth was discovered in mice treated with clone 5 CAR‐T cells until day 49. Clone 5 CAR-T cells showed excellent in vivo antitumor activity to PMB101 and conferred long-term survival (data not shown).

Conclusion: We developed a novel BAFF-R CAR-T cell product structured on single-domain antibody. This product demonstrated promising preclinical activity, and it may provide an alternative choice for patients with B cell malignancies.