The Role of Microenvironment in Bispecific Antibodies Treatment in Multiple Myeloma

Multiple myeloma (MM) is a clonal plasma cell malignancy that remains largely incurable despite major therapeutic advances. T-cell-redirecting bispecific antibodies (BsAbs) and chimeric antigen receptor T (CAR-T) cells have recently emerged as highly effective therapies in relapsed/refractory MM, inducing deep responses even in heavily pretreated patients. However, disease relapse, limited durability of response, and treatment-related toxicities remain frequent, underscoring the need to better understand mechanisms of resistance. Accumulating evidence indicates that the tumor microenvironment (TME) plays a central role in shaping BsAb efficacy in MM. Immunosuppressive cellular components, including regulatory T-cells, myeloid-derived suppressor cells, and dysfunctional antigen-presenting cells, as well as inhibitory cytokines, hypoxia, and metabolic constraints within the TME, profoundly impair T-cell activation, expansion, and persistence following BsAb engagement. In addition, chronic CD3 stimulation within the TME may promote T-cell exhaustion, contributing to suboptimal responses and disease progression. This review focuses on the dynamic interplay between BsAbs and the MM TME, highlighting how microenvironment-driven immune suppression, antigen escape, and impaired T-cell fitness influence clinical outcomes. We further discuss emerging strategies designed to overcome these barriers, including rational combination approaches, immunomodulatory agents, and next-generation trispecific antibodies that enhance co-stimulation or dual-antigen targeting. Understanding and therapeutically modulating the TME represents a critical step toward improving the depth, durability, and safety of BsAb-based therapies in MM.