Development of acquired resistance to targeted cancer therapy is one of the most significant clinical challenges. Acquiring resistance under drug selection pressure is a result of evolutionary adaptation to a complex and dynamic tumor microenvironment (TME). New therapy regimens combining CDK4/6inhibitor are under active investigation in clinical trials to treat HER2+ breast cancer patients. In parallel with clinical trial settings, in this study, we sought to prospectively model the tumor evolution in response to a targeted therapy regimen in vivo and identify a clinically actionable strategy to combat potential acquired resistance. Notably, despite a promising initial response, acquired resistance emerged rapidly to the anti-Her2/Neu antibody plus CDK4/6 inhibitor Palbociclib combination treatment. By leveraging high-throughput single-cell analyses of the evolving tumors over the course of treatments, we revealed a distinct immunosuppressive immature myeloid cell (IMC) population infiltrated in the resistant TME.Guided by single-cell transcriptome analysis, we demonstrated a combinatorial immunotherapy of IMCtargeting tyrosine kinase inhibitor cabozantinib and immune checkpoint blockades enhanced anti-tumor immunity, and overcame the resistance. Further, sequential combinatorial immunotherapy enabled a sustained control of the rapidly evolving CDK4/6 inhibitor-resistant tumors. Our study demonstrates a translational framework for treating rapidly evolving tumors through preclinical modeling and single-cell analyses. Our findings provide a rationale for an immediate clinical proposition of combinatorial immunotherapy for HER2+ breast cancer as a strategy to mitigate the emergence of resistance. 6 results at the single-cell transcriptome level indicated that CDK4/6 inhibitor treatment elicits antigen presentation and stimulate IFN signaling, supporting and extending previous observations 33 . Given that increased antigen presentation and IFN signaling, which suggested an elevated tumor immunogenicity in Ab+Pal resistant (APR) tumors, we next sought to combine immune checkpoint blockades (ICB, anti-CTLA4 and anti-PD-1 antibodies) to overcome or prevent the resistance to Ab+Pal treatment. However, the addition of ICB to the rebound APR tumors showed only modest effect (Fig. 1H, Ab+Pal+ICB), suggesting neither CTLA4 nor PD-1/L1 axis was the major mediator for the resistance. There were likely other factors contributing to the resistant phenotype.
Single-cell RNA-sequencing reveals distinct immune milieu among different phenotypes and immature myeloid cells are enriched in resistant tumorsWe next investigated the TME factors that could potentially mediate the development of resistance.The observation that more CD45 + leukocytes in both APP and APR tumors compared to Ctrl (Supplemental Fig. 3) led us to focus on the immune compartment. CD45 + tumor infiltrated leukocytes (TILs) were isolated by magnetic-activated cell sorting (MACS) from single-cell suspensions of tumors and single-cell RNA sequencing was performed ( ...