Abstract 250: Differential efficacy of immune checkpoint inhibitors on bone metastasis and its associated immune dysfunction
Authors: Kudo-Saito Chie , Fuwa Takafumi , Murakami Kouichi 
Published: August 2015

Abstract
Purposes:
We previously demonstrated that Snail expression in tumor cells promotes bone metastasis not only by enhancing motility and bone polarity of tumor cells, but also by hindering anti-tumor immunity via increase of immunoregulatory cells including CD4(+)Foxp3(+) Tregs, CD11c(+)MHC II(-/low)PDL1(+) DCregs and CD45(-)ALCAM(+) mesenchymal stem cells (MSCs), and the consequent CD8(low)Tim3(+)PD1(+) exhausted/impaired T cells. In this study, we examined whether immune checkpoint inhibitors targeting CTLA4, PD1 or PDL1 would rescue from the bone metastasis-associated immune dysfunction using a bone metastasis tumor model that we previously established.
Results:
C57BL/6 mice are implanted with snail-transduced B16-F10 melanoma cells both subcutaneously and intravenously, and were intraperitoneally injected with anti-CTLA4, anti-PD1, or anti-PDL1 mAb, or the isotype IgG as a control (10 mg/kg) on days 5 and 8 after tumor implantation. Treatment with any immune checkpoint inhibitors significantly inhibited increase of CD4(+)Foxp3(+) Tregs followed by increase of tumor-specific CD8(+) T cells in the subcutaneous tumors, resulting in significant suppression of the tumor growth as compared to that of the control group. Unexpectedly, however, bone metastasis was rather promoted in these treated mice. Nevertheless, anti-PDL1-treated mice survived in good condition for a long time, although anti-CTLA4 or anti-PD1-treated emaciated mice died earlier than the control mice. In the mice receiving anti-CTLA4 or anti-PD1 mAb, although CD4(+)Foxp3(+) Tregs systemically decreased, other immunoregulatory cells such as CD45(-)ALCAM(+) MSCs, CD11b(+)Gr1(+) MDSCs and CD8(+)Foxp3(+) Tregs, and impaired CD8(low)Tim3(+)PD1(+) T cells systemically increased more than those in the control group. In addition, tumor cells isolated from the bone marrow (BM) formed a lot of larger sphere colonies in the matrigel culture than those of the control group, and were resistant to chemotherapeutics and CTLs, indicating cancer stem-like features. In contrast, in the anti-PDL1-treated mice, tumor-specific CD8(+) T cells were systemically induced, although immunoregulatory or impaired cells still increased. Interestingly, the BM-isolated tumor cells were quite different from those of other groups: high adhesive and chemo/CTL-sensitive, suggesting tumor transition into a less malignant epithelial type. The molecular mechanisms underlying these effects are now being investigated in detail.
Conclusions:
These results suggest that anti-PDL1 mAb is effective in treating bone metastasis by modulating both host immunity and tumor cells. Our study, at least using this tumor model, also gives a caution in the application of anti-CTLA4 and anti-PD1 mAb for cancer patients with bone metastasis.
Citation Format: Chie Kudo-Saito, Takafumi Fuwa, Kouichi Murakami. Differential efficacy of immune checkpoint inhibitors on bone metastasis and its associated immune dysfunction. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 250. doi:10.1158/1538-7445.AM2015-250