Oncolytic bovine herpesvirus type 1 induces immune microenvironment remodeling and enhances treatment responses in multiple myeloma.

Despite therapeutic advances, multiple myeloma (MM) remains incurable due to the development of drug resistance by malignant plasma cells (PCs) and a severe immunosuppressive bone marrow (BM) microenvironment. Oncolytic virotherapy offers the dual benefit of tumor cell lysis and immune activation, but the efficacy of human viruses is often hampered by pre-existing antiviral immunity. Here, we demonstrated that bovine herpesvirus type 1 (BoHV-1), a virus that is nonpathogenic to humans, efficiently infected MM cells, inducing mitochondrial apoptosis and suppressing pro-survival programs, including MYC targets, oxidative phosphorylation, and the unfolded protein response. Infected tumor cells upregulated NK-activating ligands and downregulated MHC class I, enhancing susceptibility to NK-mediated cytotoxicity. In patientderived BM mononuclear cells (BMMCs), BoHV-1 selectively reduced malignant PCs and immunosuppressive myeloid subsets, while sparing lymphoid populations and hematopoietic progenitors. The infection promoted activation of CD8⁺ T cells, NK cells, and monocytes, driving a shift toward a pro-inflammatory M1-like polarization. Monocyte depletion in BMMCs attenuated the BoHV-1 anti-MM effect, confirming their functional contribution. This pronounced immune remodeling was accompanied by an inflammatory cytokine storm dominated by type I/II interferons and key innate immune mediators. Co-treatment of BoHV-1 with either bortezomib or lenalidomide increased anti-MM cytotoxicity. Finally, BoHV-1 upregulated CD38 on both MM cells and immune effectors, thereby increasing sensitivity to the anti-CD38 daratumumab. These findings establish BoHV-1 as a promising immunovirotherapy agent, effective as a single agent and in combination strategies, by coupling direct oncolysis with broad immune remodeling of the BM microenvironment.