Mechanisms of immune escape : implications for immunotherapy against cancer

Abstract

Tumor cells can be recognized and killed by cytotoxic T cells specific for certain tumor antigens. Immune mediated selection pressure along with genetic instability of tumor cells result in a growth advantage of tumor cells that acquire a less immunogenic phenotype. Increased understanding of immune escape mechanisms is crucial for the development of effective immunotherapy against cancer. This thesis deals with several aspects of immune escape and tumor induced immune suppression.

We demonstrate the paradoxical finding that IFN-gamma treatment of short-term ovarian cancer cell lines (OVAC) protected these from cytotoxic T Lymphocyte (CTL) lysis. This was dependent on enhanced signaling to inhibitory NK receptors (iNKR), CD94/NKG2A, which were expressed by the CTLs. The ligand for CD94/NKG2A is HLA-E, which is a non-classical HLA class I molecule. HLA-E expression depends on its association with leader sequence peptides derived from classical and non-classical HLA class I molecules. Furthermore, the signaling capacity of the HLA-E / peptide complex is influenced by specific properties of the peptide that is bound to the HLA-E molecule. The leader sequence peptide of another non-classical HLA class I molecule, HLA-G, provides a particularly strong inhibitory signal to CD94/NKG2A receptors when bound in the peptide groove of the HLA-E molecule. Ovarian carcinomas were found to frequently express HLA-G at the protein level and HLA-E on a transcriptional level. The expression of both these molecules was induced by IFN-gamma.

We speculated that the increased intracellular accessibility to HLA-G leader sequence peptides (Gsp), followed by increased surface expression of HLA-E / Gsp complexes, was the underlying mechanism behind the IFN-gamma mediated protection of tumor cells. In support of this possibility we were able to mimic the effect of IFN-gamma by exogenously adding synthetic HLA-G leader sequence peptides to untreated tumor cells. It is concluded that IFN-gamma may shift the balance towards inhibitory signaling to the CTLs, turning off the lysis of otherwise sensitive targets. The role of such ligand - iNKR interactions deserves further attention in future attempts of immunotherapy against ovarian carcinoma.

The importance of inhibitory NK receptors was further emphasized by the phenotypic analysis of ovarian tumor associated T and NK cells. There was a bias towards expression of inhibitory CD94/NKG2A receptors on the tumor infiltrating CD56+ T cells as compared to CD56+ T cells derived from PBL of patients and healthy donors. Further, there was an over-representation of regulatory, non-cytotoxic, CD56bright NK cells among the tumor associated lymphocytes. This was associated with high expression of inhibitory CD94/NKG2A receptors on this subset of NK cells. Expression of inhibitory receptors by a large number of tumor associated lymphocytes is likely to decrease their cytotoxic activity against autologous tumors.

Hydrogen peroxide, released by tumor associated macrophages severely impairs the cytokine production and cytolytic function of T and NK cells. It was demonstrated that the Th1 cytokine production of T cells with an activated/memory (CD45RO+) phenotype was more sensitive than naive T cells to the influence of H202. Furthermore, the reduced Th1 cytokine production was associated with a block of NF-kappaB activation. The majority of tumor infiltrating lymphocytes is of a CD45RW phenotype and our results may explain why T cells that reside in tumor lesions often display anergic properties.

Based on these results we speculated that exogenous supply of antioxidants may protect immune cells from the attack of free radicals. This hypothesis was tested on 12 patients with advanced colorectal cancer who were given high doses of dietary vitamin E during a period of two weeks. In a majority of patients the supplementation of vitamin E lead to enhanced IL-2 and IFN-gamma production by their T cells. Moreover, CD4/CD8 ratios were increased after treatment with vitamin E. It is concluded that the administration of dietary vitamin E may counter-act tumor induced immune suppression and form an effective supplement to more specific immunotherapy.