B7-H1 expression is associated with expansion of regulatory T cells in colorectal carcinoma.

Aim: To investigate the expression of B7-H1 in human colorectal carcinoma (CRC) to define its regulating effects on T cells in tumor microenvironment.

Methods: One hundred and two paraffin blocks and 33 fresh samples of CRC tissues were subject to this study. Immunohistochemistry was performed for B7-H1 and CD3 staining in CRC tissues. Ficoll-Hypaque density gradient centrifugation was used to isolate peripheral blood mononuclear cells of fresh CRC tissues; flow cytometry and immunofluorescence staining were used for detection of regulatory T cells. Data was analyzed with statistical software.

Results: Costimulatory molecule B7-H1 was found strongly expressed in CRC tissues, localized in tumor cell membrane and cytoplasm, while weak or none expression of B7-H1 was detected in pared normal colorectal tissues. Meanwhile, CD3 positive T cells were found congregated in CRC tumor nest and stroma. Statistic analysis showed that B7-H1 expression level was negatively correlated to the total T cell density in tumor nest (P < 0.0001) and tumor stroma (P = 0.0200) of 102 cases of CRC tissues. Among the total T cells, a variable amount of regulatory T cells with a clear Foxp3⁺ (forkhead box P3) staining could be detected in CRC tissues and patients' blood. Interestingly, in the 33 samples (15 cases of B7-H1(high) CRC tissues and 18 cases of B7-H1(low) CRC tissues) of freshly isolated mononuclear cells from CRC tissues, the percentages of CD4⁺Foxp3⁺ and CD8⁺Foxp3⁺ regulatory T cells were found remarkably higher in B7-H1(high) CRC tissues than in B7-H1(low) CRC tissues (P = 0.0024, P = 0.0182), indicating that B7-H1 expression was involved in proliferation of regulatory T cell. No significant difference was found in CRC peripheral blood (P = 0.0863, P = 0.0678). PD-1 is the specific ligand for B7-H1 pathway transferring inhibitory signal to T cell, which is expressed by activated T cell. Our further analysis of PD-1 expression on T cells in CRC tissues showed that conventional T cells (CD4⁺Foxp3⁻/CD8⁺Foxp3⁻), which was thought to contribute to the anti-tumor immune response, highly expressed PD-1; while regulatory T cells (CD4⁺Foxp3⁺/CD8⁺Foxp3⁻) almost failed to express PD-1. The average percentage of PD-1 expression on regulatory T cells was significantly higher than the percentage of PD-1 on conventional T cells (CD4⁺Foxp3⁻ T cell, P < 0.0001; CD8⁺Foxp3⁻ T cell, P < 0.0001). The diverse expression of PD-1 might lead to different fate of T cell subsets in B7-H1 over-expression CRC tumor microenvironment.

Conclusion: B7-H1 expression in tumor cells can inhibit the conventional T cell proliferation in tumor microenvironment through the PD-1 expression on conventional T cells.