A TCR-like CAR T Cell Therapy for the Treatment of MZB1 Positive Multiple Myeloma and Other B-Cell Malignancies

Chimeric antigen receptor (CAR) T cell therapy has revolutionized the therapeutic landscape of hematologic cancers. In multiple myeloma (MM), BCMA- and GPR5CD-directed CAR T cell therapies have significantly improved the outcome of MM patients; however, CAR T cell exhaustion and tumor antigen loss or downregulation may lead to relapse. The limited repertoire of tumor-specific surface proteins makes novel target identification challenging, hampering the widespread application of immunotherapy. We evaluated the possibility of targeting an intracellular protein presented in the context of cell surface HLA. Such ability to immunologically target intracellular protein using TCR-like antibodies would significantly enhance targetable antigen repertoire.

To identify potential antigens, we performed an integrated analysis of transcriptomic and proteomic data from MM cell lines, patient MM cells and healthy tissues. We identified the intracellular MZB1 protein as universally and specifically expressed in myeloma cells. By computational prediction, we identified putative peptides of MZB1 that can be presented in the context of HLA-A*02:01, the most frequent HLA allele worldwide.

To select monoclonal antibody clones specific to the MZB1:HLA-A*02 complex, we used a human scFv antibody yeast surface display library. The binding affinity of the clones was evaluated by flow cytometry against the MZB1^pos/HLA-A2^pos U266 MM cell line, using as a negative control the MZB1^pos/HLA-A2^neg AMO1 MM cell line. Two clones (10 and C4) showed higher binding affinity to the HLA-A2+ MZB1+ U266 cell line. Further validation confirmed its HLA-A2 and MZB1 restricted binding both by flow cytometry and immunostaining. Moreover, an MZB1:HLA-A*02 (MZB1-A2) antibody drug conjugate specifically caused cell death in HLA-A2 restricted manner in MZB1 expressing cells.

We further utilized these antibodies for CAR design. CAR cells containing the MZB1-A2 scFv, a 41BB co-stimulatory domain and CD3z signaling domain were generated. These MZB1-A2 CAR T cells showed significant cytotoxicity against MZB1 positive and HLA-A2 positive MM cells lines but not against HLA-A2 negative MM cell lines or MM cell lines engineered to be MZB1 negative. These CAR T cells also demonstrated high level of activity against a panel of Waldenström Macroglobulinemia (WM) cell lines with differing expression of MZB1 and HLA-A2. We are currently testing the tumor-killing activity of the CAR T cells against primary MM cells. Importantly, our data suggest the presence of peptide cross-reactivity, with MZB1 also being recognized by other HLA-A alleles, such as HLA-A*24:02 and HLA-A*23:01, which are more common in African Americans and Asian Americans, respectively, suggesting broad applicability of our therapy across patient groups.

In conclusion, we have generated a TCR-like CAR T cell therapy that shows potent and specific antitumor efficacy against MZB1 bound to the HLA-A*02:01 complex on the surface of MM and WM cells. The selected scFvs can also be engineered into CAR-natural killer cells, bi-specific antibodies and/or antibody drug conjugate for clinical application.

Our study will provide the logic and framework for similar therapies to be developed against other antigens in MM and other cancers.