Introduction: Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors have demonstrated significant efficacy in clinical trials in a number of solid tumors, including breast cancer. In some cases, tumor regression has been observed. CDK4/6 inhibition induces G1 cell cycle arrest and causes cytostasis, but does not directly induce breast cancer cell apoptosis. Consequently, the mechanisms by which these drugs cause tumor regression are not fully understood. We therefore hypothesized that tumor extrinsic factors may play a role in the response to CDK4/6 inhibition.

Results: Consistent with clinical experience, treatment with the CDK4/6 inhibitor, abemaciclib, resulted in significant regression of bulky tumors in mice with Her2+ mammary carcinoma (MMTV-rtTA/tetO-Her2 transgenic mice). To explore mechanisms underlying this response, we evaluated the effects of abemaciclib therapy on intratumoral immune cell populations by flow cytometry. Abemaciclib treatment resulted in increased CD3+ T cells in these tumors, while decreasing the proportion of immunosuppressive regulatory T cells (Tregs). Reductions in infiltrating Tregs were also observed in MMTV-PyMT mammary tumors and CT-26 colon tumors from mice treated with abemaciclib. Interestingly, Treg depletion was also seen in spleens and lymph nodes of tumor-free mice treated with abemaciclib and was the result of selective inhibition of Treg proliferation.

Transcriptomic analysis of bulk tumor tissues revealed increased expression of antigen processing and presentation-related genes. In vitro treatment of cancer cell lines confirmed that CDK4/6 inhibition directly causes increased antigen presentation by tumor cells via MHC Class I, and these tumor cells were capable of stimulating T cell proliferation and IFNγ and TNFα production.

Tumor regression was no longer observed in response to abemaciclib when we depleted CD8+ T cells with a neutralizing antibody in the MMTV-rtTA/tetO-HER2 mice, indicating that cytotoxic T cells play a necessary role in the response of these tumors to CDK4/6 inhibitors. Furthermore, addition of anti-PDL1 immune checkpoint blockade further enhanced the depth and duration of response to CDK4/6 inhibition in both the MMTV-rtTA/tetO-HER2 and CT-26 models.

To confirm the clinical applicability of our findings, we analyzed transcriptomic data from serial biopsies obtained as part of a clinical trial of a CDK4/6 inhibitor in breast cancer patients. In concordance with our data from the MMTV-rtTA/tetO-HER2 mice, top-ranked GSEA signatures after 12 weeks of CDK4/6 inhibitor treatment included allograft rejection, inflammatory response, and interferon gamma response.

Conclusions: Our study has revealed a new, unexpected effect of CDK4/6 inhibition: the promotion of an anti-tumor immune response. This occurs through two mechanisms: selective depletion of regulatory T cells and increased tumor immunogenicity. These findings provide strong rationale for further explorations into combining CDK4/6 inhibition with immunotherapy as a treatment strategy for patients with solid tumors. Such combinations are of particular interest in breast oncology, where the benefits of immunotherapy have been limited thus far.

Citation Format: Molly J. DeCristo, Shom Goel, April C. Watt, Haley BrinJones, Jaclyn Sceneay, Ben B. Li, Naveed Khan, Jessalyn M. Ubellacker, Shaozhen Xie, Otto Metzger-Filho, Jeremy Hoog, Matthew J. Ellis, Cynthia Ma, Susanne Ramm, Ian E. Krop, Eric P. Winer, Thomas M. Roberts, Hye-Jung Kim, Jean J. Zhao, Sandra S. McAllister. CDK4/6 inhibition triggers an antitumor immune response [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr B04.