Every Component Matters: Extracellular matrix and its value in testing novel CAR-T cell products in Multiple Myeloma
Multiple myeloma (MM), a plasma cell malignancy that resides in the bone marrow (BM), is treatable, but at the moment still considered incurable. Chimeric antigen receptor (CAR) T cell therapy has significantly advanced in the last decade, however nearly all MM patients eventually relapse. One of the overlooked key players that plays a role in therapy resistance is the BM extracellular matrix (ECM). The ECM is a highly heterogenous and dynamic microenvironment composed of different structural and adhesive proteins which can regulate many cellular processes, such as growth, differentiation and survival. The BM ECM includes distinct endosteal (collagen I-rich) and vascular (collagen IV-rich) niches. The endosteal niche is especially essential in understanding disease progression and resistance to CAR T cell therapy, because of its bi-functional role: (1) T cells reside here in proximity of CXCL12-producing stromal cells, and (2) MM cells localize in this niche near osteoclasts that secrete necessary growth factors (e.g., IL-6, APRIL).
3D cell culture systems, such as hydrogels, are among the most effective models for studying the ECM. Currently, Matrigel is considered the golden standard for 3D cell culture, although it contains many undefined components. Its composition, mainly collagen IV (~30%) and laminins (~60%), does not reflect the extracellular BM environment and has been shown to influence T cell activation and proliferation. To predict success of novel CAR T products, we developed different hydrogels to better study how the individual ECM components influence resistance to CAR T therapy. These 3D culture models are made of collagen I or collagen I & IV and additional ECM components are added in a brick-by-brick manner to study their influence on CAR T cell proliferation, cytotoxicity and migration.
Our results show that BCMA CAR T cell killing is hampered in Matrigel and in collagen I hydrogels, in a dose-dependent manner. This reveals an inhibitory effect of these matrices on CAR T cell function. Moreover, BCMA CAR T cell proliferation and activation was decreased in presence of laminin 111, a key component of Matrigel.
We are optimizing our 3D collagen models by incorporating different BM components to improve its predictive value for immunotherapeutic approaches. Our next steps will be assessing proliferation and activation of CAR T cells in presence of laminin 521 and of Transforming Growth Factor β (TGF-β), characteristic components of human BM. Finally, we hope to get insights on how the 3D microenvironment contributes to the development of MM cell resistance and efficacy of immunotherapies.