PO-365 Dissecting the synergistic effect of chemotherapy and immunotherapy on anti-tumoral T cell functions in breast cancer

Introduction Immunotherapies that target immune checkpoint molecules proved effective for the treatment of several solid tumours. However, the majority of breast cancer patients does not respond to this kind of approach, underlining the need of new strategies able to elicit an effective anti-tumoral immune response. The combination of conventional and immuno-based therapies represents a promising strategy to elicit such response. Our aim is to evaluate the efficacy of immunotherapy and chemotherapy combination in breast cancer, to dissect its effect on the endogenous anti-tumoral cytotoxic CD8 +T cells, and finally to assess the impact of a spontaneously developing tumour on T-cell biology. Material and methods The experiments are performed in K14cre;Cdh1 F/F ;Trp53 F/F (KEP) mice, a spontaneous breast tumour model that closely resembles human invasive lobular carcinoma. Mice are treated with α-CTLA-4 and α-PD-1 antibodies, as a single treatment modality or in combination with MTD dosing Cisplatin or docetaxel. Therapy is initiated when the tumour has reached 50mm 2 and continues until it reached 225mm 2 or after a pre-defined time point, depending on the experiment. Results and discussions Mammary tumours in KEP mice show an altered T-cell balance compared to tumor-free mammary glands, with a reduced infiltration of CD8 + T cells and conventional CD4 + T cells, and higher percentage of T regulatory cells. Tumor-infiltrating CD8 + T cells (TILs) have an increased expression of multiple inhibitory receptors, and reduced IFNγ production, compared to peripheral CD8 +T cells of KEP mice. In vitro exposure of sorted CD8 +TILs to IL-15 or IL-2 augments their capacity to produce IFNγ, indicating that their suppressed state is reversible. Treatment of tumor-bearing KEP mice with a-CTLA-4 and a-PD-1 does not affect the tumour growth, but a synergistic therapeutic benefit is observed when α-CTLA-4 and α-PD-1 are combined with cisplatin, and not docetaxel, in a CD8 + T-cell dependent mechanism. Conclusion CD8 +TILs display some features of dysfunctional T cells. However, ex vivo cytokine stimulation can restore part of their effector functions, indicating that they are not terminally dysfunctional. Nonetheless, only the combination with chemotherapy and immunotherapy is able to elicit an efficient CD8 +T cell response, in a drug-dependent manner. We are currently dissecting the mechanisms underlying the synergistic therapy response, in order to ultimately contribute to the rational design of new immunomodulatory treatment strategies for breast cancer.