Abstract: Background and purpose: Inducible costimulator (ICOS) is a member of B7-CD28 immunoglobulin superfamily and expressed on the surface of activated T cells, which plays an important role in T cell activation and effector functions. However, the expression pattern and the significance of ICOS and ICOS⁺ T cells in tumor tissue of hepatocellular carcinoma (HCC) are not defined. To this end, the current study was planned to quantitatively detect the expression of ICOS and ICOS⁺ T cells in the tumor tissue of HCC patients and evaluate their clinical significance. Methods: Tissue microarrays (TMAs) and multiplex immunohistochemistry (mIHC) were used to detect the expression levels of ICOS⁺ cells, ICOS+CD4⁺ and ICOS⁺CD8⁺ T cells in tumor and paracancerous tissues from HCC patients who received surgical treatment in Zhongshan Hospital, Fudan University from 2006 to 2007 (n=358). The clinical prognosis was evaluated by Kaplan-Meier analysis and COX regression analysis. Results: The densities of infiltrating ICOS⁺ and ICOS⁺CD4⁺ T cells were significantly higher in tumor tissue than in paracancerous tissue (P<0.000 1 and P=0.009 1). By contrast, the density of ICOS⁺CD8⁺ T cells was significantly lower in tumor tissue than in paracancerous tissue (P=0.033). Within the tumor tissue, the density of ICOS⁺CD4⁺ T cells was significantly higher compared with ICOS⁺CD8⁺ T cells (P<0.000 1). Moreover, the frequencies of ICOS⁺CD4⁺ and ICOS⁺CD8⁺ T cells in tumor tissue were significantly higher compared with their counterparts in paracancerous tissue (P<0.000 1). Multivariate COX regression analysis identified that ICOS⁺ cells, ICOS⁺CD4⁺ and ICOS⁺CD8⁺ T cells were independent favorable prognostic indices for overall survival (OS). Conclusion: Tumor infiltrating ICOS⁺ cells are greatly elevated in the tumor tissue of HCC, and their abundance is associated with prolonged OS. Thus, ICOS⁺ cells, ICOS⁺CD4⁺ and ICOS⁺CD8⁺ T cells might be used as novel prognostic immune biomarkers for HCC.

Key words: Hepatocellular carcinoma, Inducible costimulator, Multiplex immunohistochemistry, Prognosis