CDK4/6 inhibitors trigger anti-tumor immunity. CDK4/6 inhibitors induce cell cycle arrest in tumor cells and enhance anti-tumor immune responses. This is achieved through two mechanisms: activation of endogenous retroviral elements, increasing intracellular double-stranded RNA and stimulating type III interferon production, which enhances tumor antigen presentation; and suppression of regulatory T cells (Tregs) via reduced activity of the E2F target, DNA methyltransferase 1. These events promote cytotoxic T cell-mediated tumor cell clearance, which is further enhanced by immune checkpoint blockade. CDK4/6 inhibitors increase tumor immunogenicity and provide rationale for combination regimens with immunotherapies. In murine models of breast carcinoma and other solid tumors, CDK4/6 inhibitors caused tumor regression, evidenced by a 40% reduction in tumor volume. Transcriptomic analysis of clinical trial samples confirmed enhanced anti-tumor immune responses. CDK4/6 inhibitors upregulated genes related to antigen processing and presentation, including MHC class I molecules and peptide transporters. These effects were also observed in patient-derived breast cancer xenografts. Analysis of The Cancer Genome Atlas (TCGA) data revealed that breast cancers with cyclin D1 amplification had lower expression of HLA-A, HLA-B, and HLA-C. CDK4/6 inhibition increased interferon signaling, leading to upregulation of interferon-sensitive genes. This was associated with increased expression of type III interferons, which drive interferon-stimulated gene expression. CDK4/6 inhibition reduced DNA methyltransferase 1 (DNMT1) expression, leading to reduced DNA methylation of endogenous retroviral genes and increased expression of endogenous retroviral elements. This triggered a double-stranded RNA response, stimulating type III interferon production. CDK4/6 inhibition also reduced Treg numbers and the Treg:CD8 T cell ratio, favoring anti-tumor immunity. CDK4/6 inhibitors enhanced the activity of cytotoxic T cells, which were necessary for tumor regression. Combination therapy with CDK4/6 inhibitors and immune checkpoint blockade significantly enhanced tumor regression. These findings suggest that CDK4/6 inhibitors enhance tumor immunogenicity and provide a rationale for combination regimens with immunotherapies.CDK4/6 inhibitors trigger anti-tumor immunity. CDK4/6 inhibitors induce cell cycle arrest in tumor cells and enhance anti-tumor immune responses. This is achieved through two mechanisms: activation of endogenous retroviral elements, increasing intracellular double-stranded RNA and stimulating type III interferon production, which enhances tumor antigen presentation; and suppression of regulatory T cells (Tregs) via reduced activity of the E2F target, DNA methyltransferase 1. These events promote cytotoxic T cell-mediated tumor cell clearance, which is further enhanced by immune checkpoint blockade. CDK4/6 inhibitors increase tumor immunogenicity and provide rationale for combination regimens with immunotherapies. In murine models of breast carcinoma and other solid tumors, CDK4/6 inhibitors caused tumor regression, evidenced by a 40% reduction in tumor volume. Transcriptomic analysis of clinical trial samples confirmed enhanced anti-tumor immune responses. CDK4/6 inhibitors upregulated genes related to antigen processing and presentation, including MHC class I molecules and peptide transporters. These effects were also observed in patient-derived breast cancer xenografts. Analysis of The Cancer Genome Atlas (TCGA) data revealed that breast cancers with cyclin D1 amplification had lower expression of HLA-A, HLA-B, and HLA-C. CDK4/6 inhibition increased interferon signaling, leading to upregulation of interferon-sensitive genes. This was associated with increased expression of type III interferons, which drive interferon-stimulated gene expression. CDK4/6 inhibition reduced DNA methyltransferase 1 (DNMT1) expression, leading to reduced DNA methylation of endogenous retroviral genes and increased expression of endogenous retroviral elements. This triggered a double-stranded RNA response, stimulating type III interferon production. CDK4/6 inhibition also reduced Treg numbers and the Treg:CD8 T cell ratio, favoring anti-tumor immunity. CDK4/6 inhibitors enhanced the activity of cytotoxic T cells, which were necessary for tumor regression. Combination therapy with CDK4/6 inhibitors and immune checkpoint blockade significantly enhanced tumor regression. These findings suggest that CDK4/6 inhibitors enhance tumor immunogenicity and provide a rationale for combination regimens with immunotherapies.