Development and Evaluation of CART Targeting Bcma with Humanized Alpaca-Derived Single-Domain Antibody As Antigen Recognition Domain

Chimeric antigen receptor T cells (CART) targeting CD19 have shown substantial activity against leukemia and lymphoma, which motivated developing CART cell therapy for Multiple myeloma (MM). B cell maturation antigen (BCMA) is the target molecule in MM. Several kinds of CART targeting BCMA have been created from 2016. Among these, the Bluebird Bio uses the humanized murine BCMA scFv to make CART, the Nanjing Legend company uses the single-domain antibody targeting BCMA to make CART.

The single-domain antibody refers to the heavy chain antibody naturally present in camel species, which has no light chain. All VHh (Variable region of Heavy chain in heavy chain antibody) domains from the immunized alpaca were cloned into the phage-display library, then specific VHhs were screened out by solid-phase ELISA. Not like scFv using the linker region to connect the VH and VL to obtain the single chain region, the VHh itself is one single chain, which make it very suitable for making the recognition domain of CART. In order to increase the avidity, the Nanjing Legend company uses two tandem VHhs for binding BCMA. However, the single domain antibody has the capacity to bind the target molecule with very high affinity, one VHh should be able to bind BCMA tightly. To this aim, using our platform of single-domain antibody discovery and CART development, we successfully obtain the CART targeting BCMA (CART-BCMA) just using one VHh for recognizing the BCMA on tumor cells.

Firstly, we immunized the alpaca using the extracellular domain of BCMA, then the B cell RNA were extracted, and the VHh phage-display library were constructed and screened. Finally we got the 6 single-domain antibodies through solid-phase ELISA screening. After gene sequencing and humanization of VHh, we constructed 6 lentiviral vectors with CAR gene including different VHh sequence connecting with CD8 hinge and transmembrane region, CD137 intracellular region, and CD3ζ intracellular region. We used 293T to package the lentivirus, and infected the healthy T cells to make 6 kinds of CART-BCMA cells. With in vitro killing assay, we found the #3 CART-BCMA is the best (83% killing rate at the E:T ratio of 4:1). Simultaneously, we used the Biocore to test the binding affinity of single-domain antibodies, the #3 antibody's affinity is 1.14nM, which indicated only one #3 VHh domain already have enough binding ability for CART to recognize the BCMA positive tumor cells. Next, we did further experiment to demonstrate the function of #3 CART-BCMA.

We used the MM tumor cell line MM.1S (expressing high BCMA), myeloid tumor cell line K562 (no BCMA expression) as the target, the results show that CART-BCMA kill the MM.1S cells in 42% rate at the low E:T ratio of 0.5:1, but the CART-BCMA cells can't kill K562 cells even in high 4:1 E:T ratio. Accordingly, the cytokine level of IFN-γ or TNF-α is elevated dramatically in CART-BCMA wells. In animal experiment, the 1.5x10⁶ MM.1S-Luc were injected into one NSG mouse. 40 mice were separated into 5 groups for PBS, 10x10⁶ Mock T, 1.0x10⁶ CART, 5.0x10⁶ CART, 10x10⁶ CART treatment. The results show that three CART groups of 1.0x10⁶, 5.0x10⁶, 10x10⁶ CART are effective.

Using the purified lentivirus, we found the lentivirus can infect the T cells at the MOI of 3, the CAR positive rate is over 50% by FACS, this implied that the CAR gene can be easily expressed on T cell membrane. For T cell infection, CAR-BCMA lentivirus need less amount compared to traditional CAR virus derived from scFv.

We performed the clinical pilot study to investigate CART-BCMA's safety and effect on MM patients pre-conditioned with Fludarabine/cyclophosphamide. Until the time of abstract submission, 4 patients were enrolled, and got the dose of 5x10⁶/kg (Pt1), dose of 10x10⁶/kg (Pt2 and Pt3, split dose of 50% and 50% in two days), dose of 10x10⁶/kg (Pt4) CART-BCMA cells. Pt1 has the CR response, and has the fever after 14 days. Pt2 has the PR response with no CRS. Pt3 has the PR response with grade 2 CRS. Pt4 has the PR response with grade 3 CRS and neurotoxicity (see table).

In summary, we developed one CART-BCMA for MM treatment, which has the BCMA antigen recognition domain derived from the alpaca. Especially, the lentivirus carrying the CAR-BCMA gene can infect the T cells at the low MOI of 3, which will save the amount of lentivirus. The clinical pilot study showed that 4 patients received the CART-BCMA has the objective response (1 CR, 3 PR), further follow-up and enrollment is being performed.

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