B-Cell Maturation Antigen Is a Promising Target for Genetically-Modified T-Cell Therapy of Multiple Myeloma

Abstract 937

Multiple myeloma (MM) is a usually incurable malignancy of plasma cells. New therapies are urgently needed for MM. Development of immunotherapies targeting antigens expressed by MM cells could improve the outcomes of patients with multiple myeloma (MM). Chimeric antigen receptors (CARs) are fusion proteins containing antigen-recognition moieties and T-cell activation domains. T cells genetically modified to express CARs can specifically recognize tumor antigens, and CAR-expressing T cells have potent in vivo activity against some types of lymphoma and leukemia. Unfortunately, development of CAR-expressing T-cell therapies for MM has been hindered because most proteins expressed on MM cells are also expressed on essential normal cells. We report here our work developing CARs that target B-cell maturation antigen (BCMA). BCMA is a protein that has been reported to be selectively expressed by B-lineage cells including MM cells. We hypothesized that BCMA would be a suitable target for CAR-expressing T cells. We evaluated BCMA expression in all major human organs, and we designed and tested the first anti-BCMA CARs.

We assessed BCMA RNA expression in all major human organs by quantitative PCR. Although BCMA RNA expression was mainly limited to hematologic tissues, low levels of BCMA RNA were detected in gastrointestinal organs, trachea, and testes. Next, we assessed BCMA protein expression in samples of 33 normal human organs by immunohistochemistry (IHC). BCMA-expressing plasma cells were detected in the lamina propria of several gastrointestinal organs by IHC, but BCMA protein was not detected in other gastrointestinal tissues. BCMA-expressing gastrointestinal plasma cells were probably the source of the low levels of BCMA RNA detected in normal gastrointestinal organs. Except for plasma cells, BCMA expression was not detected in any normal human tissues by IHC. Furthermore, BCMA was not detected on primary human CD34⁺hematopoietic cells by flow cytometry. In contrast, we detected uniform BCMA cell-surface expression by either flow cytometry or IHC on primary MM cells from 5 of 5 patients.

Because BCMA is expressed by MM cells but not by normal essential tissues, we reasoned that BCMA would be an appropriate target for CAR-expressing T cells. We designed anti-BCMA CARs that contained antigen receptor domains derived from anti-BCMA monoclonal antibodies. The CARs also contained the signaling moiety of the CD28 molecule and the signaling domains of the CD3-zeta molecule. DNA encoding these CARs was ligated into a self-inactivating lentiviral vector, and human T cells were transduced with the anti-BCMA CARs. After transduction, anti-BCMA CAR expression was routinely detected on more that 60% of the transduced T cells by flow cytometry. Notably, we were able to successfully culture and transduce T cells from heavily treated patients with advanced MM. T cells expressing anti-BCMA CARs produced IFN-gamma, IL-2, and tumor necrosis factor in a BCMA-specific manner. T cell degranulation, which is required for perforin-mediated cytotoxicity, is associated with upregulation of CD107a. Anti-BCMA-CAR-transduced T cells upregulated CD107a when stimulated with BCMA-expressing target cells but not when stimulated with BCMA-negative target cells. Anti-BCMA-CAR-transduced T cells proliferated in a BCMA-specific manner. Anti-BCMA-CAR-transduced T cells killed BCMA-expressing multiple myeloma cells lines but not BCMA-negative cell lines. Importantly, anti-BCMA-CAR-transduced T cells produced interferon-gamma when cultured with BCMA-expressing primary MM cells but not when cultured with BCMA-negative peripheral blood mononuclear cells. Anti-BCMA-CAR-transduced T cells killed 66% of autologous primary MM cells in a BCMA-specific manner at a T cell to MM cell ratio of 7:1 in a 4-hour cytotoxicity assay.

In summary, BCMA is expressed on MM cells of many patients, and BCMA is not expressed by normal essential tissues; therefore, BCMA is a suitable target for CAR-expressing T cells. We constructed and tested the first reported anti-BCMA CARs, and we showed that these T cells exhibited BCMA-specific functions including killing of primary MM cells. Adoptive transfer of anti-BCMA-CAR-transduced T cells is a promising new strategy for treating MM. These findings are the first steps toward clinical trials of anti-BCMA CAR-expressing T cells.