Spatial and phenotypic plasticity of B cells in remodeling the tumor microenvironment.

While cancer immunotherapy has traditionally centered on T cells, accumulating evidence highlights the dual roles of tumor-infiltrating B lymphocytes (TIL-Bs) in both promoting and restraining tumor progression. Within tertiary lymphoid structures (TLSs), TIL-Bs undergo germinal center (GC) maturation and generate tumor-reactive antibodies that enhance antitumor T-cell immunity. However, in defined spatial, inflammatory, or cytokine-rich niches, B cells can acquire immunosuppressive phenotypes, including interleukin (IL)-10, IL-35, TGF-β, programmed cell death protein 1/programmed death-ligand 1 subsets and IgA plasma cells. These populations suppress cytotoxic CD4/CD8 T cells, expand regulatory T cells, and impair dendritic cell function, thereby establishing an immunosuppressive tumor microenvironment. TIL-Bs also contribute to chronic inflammation, complement activation, and angiogenesis through polyclonal activation and immune complex formation. Tumor-infiltrating antibodies, particularly IgA, demonstrate context-dependent effects that are shaped by antigen specificity, isotype, and Fc-mediated interactions. Importantly, immunoregulatory B-cell subsets are increasingly implicated in resistance to immune checkpoint blockade and chemotherapy. This review synthesizes emerging insights into the tumor-promoting roles of TIL-Bs, emphasizing their spatially regulated phenotypic plasticity and immunosuppressive mechanisms, and highlights strategies to selectively target pathogenic B-cell subsets while preserving antitumor functions.