| Summary: | Numerous studies have demonstrated that tumour cells have the ability to alter immune function to create an immune suppressed environment. This allows tumour cells to escape immune surveillance and consequently the tumour can progress. Dendritic and T cells have critical roles in immune activation and tolerance and are thus major targets of tumour-mediated immune suppression. Understanding the mechanism(s) by which tumour cells modulate the immune system will facilitate the development of immune system-based therapies for cancer treatments. In this study we sought to determine the nature of, and cellular and molecular mechanisms underlying, changes in immune status during tumour progression using mouse models of prostate cancer.
Detailed analysis of the immunological status in a mouse prostate dysplasia model (12T-7slow) revealed that immune suppression accompanied tumour progression. We found that T cells isolated from tumour-bearing hosts were hypo-responsive to antigen stimulation. Furthermore, we demonstrated that CD4+CD25+ regulatory T cells were responsible, at least in part, for this alteration. Anti-CD25 antibody treatment reduced, but did not prevent, tumour growth in either a transplanted prostate tumour model or a spontaneously developing prostate tumour model. In addition, an altered dendritic cell phenotype and an elevated frequency of CD4+CD25+ regulatory T cells were observed within the tumour mass. Similar alterations were observed in the prostate-specific Pten knockout mice which develop advanced prostate adenocarcinoma. Interestingly, evidence of immune activation, such as an increased frequency of activated T cells, was detected in the tumour microenvironment in both mouse prostate tumour models.
To identify factors that may play critical roles in the altered immune cell phenotype observed in the tumour microenvironment, a global gene expression profiling analysis was carried out to evaluate the changes in immune-related gene expression patterns. This analysis provided additional evidence for the co-existence of immune suppression and immune activation. Moreover, subsequent analyses suggested that one differentially expressed transcript, interferon regulatory factor 7, and its target genes might be involved in modulating immune cells and/or tumour progression.
Taken together, these studies have important implications for designing specific and effective anti-tumour immune therapy strategies that involve manipulation of tumour cells, dendritic cells and regulatory T cells.
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