Myeloid Cell Targeting Strategies Show Limited Durable Activity in the Breast Cancer Tumor Microenvironment and Do Not Enhance the Activity of Thermally Ablative Focused Ultrasound

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer (BrCa), owing to its lack of targetable receptors and resistance to chemical and molecularly targeted therapeutic approaches. While chemotherapy and surgical resection remain the standard of care, these interventions have significant side effects and varying patient outcomes. Thermally ablative focused ultrasound (T-FUS)—a non-invasive and non-ionizing therapy that utilizes targeted acoustic energy to debulk tumors—has displayed immunomodulatory effects in BrCa. However, T-FUS as a monotherapy has had limited clinical efficacy in TNBC due to the presence of anti-inflammatory immunosuppressive myeloid cells (isMCs). We hypothesized that the elimination of isMCs or initiating tumoricidal activity from them would lead to augmented activity of T-FUS. Thus, we interrogated the ability of myeloablative chemotherapies and antibodies; myeloid recruiting chemokine receptor blockade; and TLR agonists to remodel the tumor myeloid populations. Consistent with our previous studies, we found that while myeloablative chemotherapies decreased circulating isMCs, they had little impact on intratumoral isMCs. In contrast, antibodies targeting Ly6C and Ly6G ablated intratumoral isMCs and systemic isMCs, yet their effect was transient and was accompanied by a surprising depletion of T cells. While targeting CCR2, the dominant chemokine receptor for intratumoral isMC diminished a large subset of immunosuppressive cells within the TME; it also depleted T cells and dendritic cells. Contrary to previous studies, TLR stimulation failed to repolarize myeloid cells into a pro-inflammatory, tumoricidal phenotype but did lead to their depletion from the tumor microenvironment (TME) and mobilization of conventional dendritic cells to the draining lymph nodes. We therefore hypothesized that combining isMC depletion and TLR-driven immune activation would enhance FUS efficacy; however, this combinatorial regimen did not enhance overall survival or control tumor volume after T-FUS treatment. Thus, the BrCa TME is highly resistant to approaches intended to remodel the myeloid cell component which fail to synergize with T-FUS-mediated tumor ablation.