The present study confirms that increased tumor-infiltrating cytotoxic T lymphocytes are associated with improved overall survival and also presents a new candidate gene which might have a role in increasing the number of the tumor infiltrating lymphocytes and therefore improve survival in serous ovarian cancer patients.
In our study, utilizing Kaplan-Meier analysis with a 75th percentile cutpoint, patients with marked numbers of tumor-infiltrating CD8 T lymphocytes had a 20 month improvement in overall survival. The improvement in overall survival held in a multivariate analysis, indicating tumor-infiltrating CD8 T lymphocytes are an independent prognostic factor in advanced-stage serous ovarian cancer.
As demonstrated in this and previous studies, the presence of tumor-infiltrating T lymphocytes, specifically cytotoxic T lymphocytes (CD8), confers an improved overall survival in many different epithelial tumors.4,5,6,7,8
However, the underlying molecular mechanisms that promote or inhibit the infiltration of cytotoxic T lymphocytes is not fully understood. In our sample of 38 microdissected advanced-stage serous ovarian cancers, HLA-DMB was positively differentially expressed in the tumor epithelium of patients with an abundance of tumor-infiltrating cytotoxic T lymphocytes. Furthermore, a significant positive correlation between the differential expression of HLA-DMB mRNA in the tumor epithelium and the number of tumor-infiltrating CD8 T lymphocytes was noted both in the training set and in a separate validation set of specimens. This relationship held true at the protein level as well, implicating ovarian cancer epithelial
cells in the process of antigen presentation.
Normal adult epithelial cells are not thought to express MHC class II molecules, such as HLA-DR and HLA-DM, both of which are composed of α and β subunits. However, various epithelial tumor cells have been demonstrated to express HLA-DR molecules and their presence is associated with an improved prognosis.14,15,16
Additionally, in breast cancer, expression of HLA-DM and its cooperating molecules, HLA-DR and Ii, is associated with a helper CD4 T lymphocyte-associated response and improved survival.16
HLA-DM enhances antigen presentation by catalyzing the removal of class II-associated invariant chain peptide (CLIP) from the antigen-binding groove of HLA-DR molecules.17
CLIP is the product of cleavage of the invariant chain, which occupies and stabilizes HLA-DR molecules after their synthesis and assembly.18
By catalyzing the dissociation of CLIP from the binding groove of HLA-DR, HLA-DM allows efficient antigen peptide loading onto HLA-DR for presentation at the cell membrane to helper CD4 T lymphocytes.18,19
A robust and effective cytotoxic CD8 T lymphocyte immune response requires the aid of helper CD4 T lymphocytes.20
In vitro and in vivo experiments have demonstrated that tumor cells may be converted into efficient antigen presenting cells (APCs) by expression of MHC class II components and inhibition of the invariant chain.21
Consequently, they are able to present endogenous tumor antigen to CD4 helper T lymphocytes.21
The present study demonstrates that advanced serous ovarian cancers with an abundance of tumor-infiltrating cytotoxic CD8 T lymphocytes differentially express increased levels of HLA-DMB in the tumor epithelium
. HLA-DM serves to catalytically replace CLIP (cleavage product of Ii) with antigen peptide. Hence, increased expression of HLA-DM may be functionally similar to inhibition of the invariant chain and enhance antigen peptide presentation by HLA-DR. Additionally, we found that HLA-DMB and Ii mRNA expression both correlate positively with HLA-DRA mRNA expression in the tumor epithelium and HLA-DR localizes to the tumor epithelial cell membrane. Consequently, it may be hypothesized that certain serous ovarian cancer epithelial cells, coordinately expressing these three critical components of the MHC class II antigen presentation pathway, are able to present endogenous tumor antigen to helper CD4 T lymphocytes and, by extension, activate an efficient and robust CD8 cytotoxic immune response.
However, the present study cannot define whether the differential expression of HLA-DMB by the tumor epithelial cells leads, by extension, to a marked CD8 T lymphocyte response, or vice versa. It is plausible that tumor-infiltrating CD8 T lymphocytes may induce the expression of MHC class II components, such as HLA-DM, HLA-DR, and Ii in tumor epithelial cells by the production and secretion of IFN-γ.22
Therapeutic upregulation of the IFN-γ production of T lymphocytes might increase HLA-DM, HLA-DR and Ii expression in ovarian tumor cells which might contribute to better survival in selected patients.
In summary, the present study serves to begin to elucidate the molecular mechanisms that may underlie the differential numbers of tumor-infiltrating CD8 T lymphocytes in advanced-stage serous ovarian cancer. Further study will be necessary to evaluate the potential antigen presenting capabilities of serous ovarian cancer cells.