Abstract 5020: Cooperative effects of programmed cell death-1 blockade and radiation in a model of a poorly immunogenic breast carcinoma
Authors: Pilones Karsten , Aryankalayil Joseph , Formenti Silvia , Demaria Sandra 
Published: May 2015

We have previously shown that radiation therapy (RT) converts poorly immunogenic tumors resistant to antibodies (mAbs) against the checkpoint receptor CTLA-4 into susceptible ones. Importantly, RT induced the upregulation of PDL-1 and PDL-2 on the tumor cells and infiltrating myeloid cells in the TSA mouse breast cancer, suggesting that PD-1 ligands may limit rejection of the irradiated tumor by T cells. PD-1 is a checkpoint receptor upregulated on T cells shortly after activation and expressed at high levels on exhausted T cells. To test the hypothesis PD-1 blockade can promote the ability of RT to induce effective anti-tumor immunity we employed BALB/c mice inoculated s.c. with the syngeneic poorly immunogenic TSA breast cancer cells. When tumors became palpable mice were randomly assigned to one of 4 treatment groups (N=6-7): (1) control, (2) RT, (3) anti-PD-1 mAb and (4) RT+anti-PD-1 mAb. RT was delivered exclusively to the primary tumor in 8 Gy fractions on days 13, 14 and 15 post tumor inoculation. PD-1 blocking mAb RMP1-14 was given on day 15 and every 4 days thereafter, and mice were followed for tumor growth. In a separate experiment, mice were euthanized on day 20 to characterize tumor-infiltrating lymphocytes (TILs) and development of CD8+ T cells specific for the tumor epitope AH1 by pentamer analysis.

TSA tumors were resistant to PD-1 blockade. RT significantly delayed tumor growth (p&lt;0.01), but tumor regression was seen only in 1 of 6 mice. In contrast, when RT was combined with anti-PD-1 mAb, all of the mice completely rejected tumors by day 25. AH1-specific CD8+ T-cells were markedly increased (5.5%) in the spleen of mice treated with RT+anti-PD-1 compared to control (1.7%) and anti-PD-1 (1.9%) or RT (3.7%) treated mice (p&lt;0.01). Analysis of TILs showed a significant increase in mice treated with RT compared to control in CD8+ T cells expressing high levels of PD-1 (CD8+PD-1hi) (67% versus 36%, p&lt;0.01), as detected by a mAb non-cross-reactive with RMP1-14. Interestingly, in mice receiving anti-PD-1 mAb there was a significant decrease in CD8+PD-1hi (14% RMP1-14 versus 36% control, p&lt;0.01; and 30% RT+RMP1-14 versus 67% RT, p&lt;0.01). In contrast, CD8+ T cells expressing activation markers CD69 and CD137 and low levels of PD-1 were increased significantly in the tumors of mice treated with RT (64%) compared to control (42%) (p&lt;0.001) irrespective of the treatment with anti-PD-1.

Overall, data suggest that the IgG2a anti-PD-1 mAb RMP1-14 selectively depletes CD8+PD-1hi TILs, possibly by ADCC, while enhancing the RT-elicited priming and activation of tumor-specific CD8+ T cells. We propose that this represents a novel mechanism of anti-tumor activity of anti-PD-1 mAb and supports testing its combination with radiotherapy.

Supported by the Breast Cancer Alliance (Exceptional Project Award).

Citation Format: Karsten Pilones, Joseph Aryankalayil, Silvia Formenti, Sandra Demaria. Cooperative effects of programmed cell death-1 blockade and radiation in a model of a poorly immunogenic breast carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5020. doi:10.1158/1538-7445.AM2014-5020