1. ¹Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta, Canada
2. ²Biotechnology Research Center, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran
3. ³Department of Laboratory Medicine and Pathology, Cross Cancer Institute, University of Alberta, Edmonton, Alberta, Canada

Correspondence: Dr A Lavasanifar, Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, 4119 Dent/Pharm Centre, Edmonton, Alberta, Canada T6G 2N8. E-mail: alavasanifar@pharmacy.ualberta.ca

Dedication: This paper is dedicated to the memory of our mentor, colleague and friend, Professor John Samuel, who was a distinguished scientist in the field of cancer immunotherapy. Professor Samuel passed away on April 17, 2007.

Received 14 December 2007; Revised 3 March 2008; Accepted 6 March 2008; Published online 8 April 2008.

Abstract

One of the major limitations for cancer immunotherapy is related to the frequent existence of an intra-tumoral immunosuppressive environment, to which STAT3 (Signal transducer and activator of transcription-3) activation in tumor and dendritic cells (DCs) are believed to contribute. In this study, we tested the hypothesis that the combination of CpG (a DC activator) and JSI-124 (a STAT3 inhibitor) may generate synergistic antitumor effects compared to CpG or JSI-124 alone. B16-F10, a mouse melanoma cell line that has constitutively active STAT3, was grafted in C57BL/6 mice and then tumor-bearing mice treated intra-tumorally with (a) phosphate buffered saline, (b) 10 g CpG, (c) 1 mg kg^-1 JSI-124 or (d) 10 g CpG+1 mg kg^-1 JSI-124. The effects of treatments on tumor growth, survival and antitumor immune responses were evaluated. Although significant antitumor effects were detected with the single-agent treatments, the CpG+JSI-124 treatment resulted in synergistic antitumor effects compared to CpG or JSI-124 alone. Correlating with these findings, the combination therapy resulted in significantly higher intra-tumoral levels of several proinflammatory, T_H1-related cytokines (including IL-12, IFN-, TNF- and IL-2), increases in intra-tumoral CD8⁺ and CD4⁺ T cells expressing activation/memory markers and NK cells and increases in activated DCs in the tumors and regional lymph nodes (LNs). Concomitantly, the combination therapy led to a significantly decreased level of immunosuppression, as evidenced by lower intra-tumoral level of VEGF and TGF-, and decreased number of CD4⁺CD25⁺Foxp3⁺ regulatory T cells in the regional LNs. This study has provided the proof-of-principle for combining CpG and JSI-124 to enhance antitumor immune responses.