Recent studies describe a heterogeneous population of cells of the myeloid lineage, termed myeloid derived suppressor cells (MDSC), which are observed with increased prevalence in the peripheral blood and tumor microenvironment of cancer patients, including pancreatic cancer. Accumulation of MDSC in the peripheral circulation has been related to extent of disease, and correlates with stage. MDSC have primarily been implicated in promoting tumor growth by suppressing antitumor immunity. There is also compelling evidence MDSC are also involved in angiogenesis and metastatic spread. Two main subsets of MDSC have been identified in cancer patients: a monocytic subset, characterized by expression of CD14, and a granulocytic subset characterized by expression of CD15. Both subsets of MDSC actively suppress host immunity through a variety of mechanisms including production of reactive oxygen species and arginase. Just as in humans, accumulation of monocytic and granulocytic MDSC has been noted in the bone marrow, spleen, peripheral circulation, and tumors of tumor bearing mice. Successful targeting of MDSC in mice is associated with improved immune responses, delayed tumor growth, improved survival, and increased efficacy of vaccine therapy. By further elucidating mechanisms of MDSC recruitment and maintenance in the tumor environment, strategies could be developed to reverse immune tolerance to tumor. We discuss here what is currently known about MDSC as well as some potential strategies targeting MDSC in the context of our work on pancreatic cancer and recent literature. Due to the number of new reports on MDSC, the most pertinent ones have been selected.

Purpose: Mesothelin is a glycoprotein expressed on normal mesothelial cells and is overexpressed in several histologic types of tumors including pancreatic adenocarcinomas. A soluble form of mesothelin has been detected in patients with ovarian cancer and malignant mesothelioma, and has prognostic value. Mesothelin has also been considered as a target for immune-based therapies. We conducted a study on the potential clinical utility of mesothelin as a biomarker for pancreatic disease and therapeutic target pancreatic cancer. Experimental Design: Tumor cell–bound and soluble mesothelin in patients was evaluated by immunohistochemistry and ELISA, respectively. The in vitro cellular immune response to mesothelin was evaluated by INFγ ELISA and intracellular cytokine staining for IFNγ in CD4⁺ and CD8⁺ T cells. The level of circulating antibodies to mesothelin was measured by ELISA. Results: All tumor tissue from patients with pancreatic adenocarcinoma expressed mesothelin (n = 10). Circulating mesothelin protein was detected in patients with pancreatic adenocarcinoma (73 of 74 patients) and benign pancreatic disease (5 of 5) but not in healthy individuals. Mesothelin-specific CD4⁺ and CD8⁺ T cells were generated from peripheral blood lymphocytes of patients with pancreatic cancer in 50% of patients compared with only 20% of healthy individuals. Antibodies reactive to mesothelin were detected in <3% of either patients or healthy individuals. Conclusions: Circulating mesothelin is a useful biomarker for pancreatic disease. Furthermore, mesothelin-specific T cells can be induced in patients with pancreatic cancer. This suggests that mesothelin is a potential target for immune-based intervention strategies in pancreatic cancer. (Clin Cancer Res 2009;15(21):6511–8)

Neoantigens are tumor-specific peptide sequences resulting from sources such as somatic DNA mutations. Upon loading onto major histocompatibility complex (MHC) molecules, they can trigger recognition by T cells. Accurate neoantigen identification is thus critical for both designing cancer vaccines and predicting response to immunotherapies. Neoantigen identification and prioritization relies on correctly predicting whether the presenting peptide sequence can successfully induce an immune response. Because most somatic mutations are single-nucleotide variants, changes between wild-type and mutated peptides are typically subtle and require cautious interpretation. A potentially underappreciated variable in neoantigen prediction pipelines is the mutation position within the peptide relative to its anchor positions for the patient’s specific MHC molecules. Whereas a subset of peptide positions are presented to the T cell receptor for recognition, others are responsible for anchoring to the MHC, making these positional considerations critical for predicting T cell responses. We computationally predicted anchor positions for different peptide lengths for 328 common HLA alleles and identified unique anchoring patterns among them. Analysis of 923 tumor samples shows that 6 to 38% of neoantigen candidates are potentially misclassified and can be rescued using allele-specific knowledge of anchor positions. A subset of anchor results were orthogonally validated using protein crystallography structures. Representative anchor trends were experimentally validated using peptide-MHC stability assays and competition binding assays. By incorporating our anchor prediction results into neoantigen prediction pipelines, we hope to formalize, streamline, and improve the identification process for relevant clinical studies.

Mammaglobin is a gene that is expressed almost exclusively in the normal breast epithelium and human breast cancer. It is a member of the secretoglobin gene family and forms a heterodimer with lipophilin B. We have focused on the tissue-specificity of mammaglobin as a potential mechanism for the specific killing of breast cancer cells. By elucidating the promoter region of mammaglobin, we hope to utilize this site as a method for turning on the apoptosis inducer gene, Bax, in breast cancer cells. The Bax gene will only be expressed at levels necessary to induce apoptosis in mammaglobin positive cells. This would include > 80% of all breast cancer cells and some normal breast epithelium. This type of targeted killing could be conceptualized as a biochemical mastectomy; that is, genetic ablation of breast tumor cells and perhaps non-malignant breast epithelium while preserving the adipose and stromal components of the breast. Work is also being done to address the binding specificity of the secreted mammaglobin protein. There is early evidence that the mammaglobin heterodimer may in fact bind to breast and breast cancer cells. If this finding is validated, this creates the possibility that mammaglobin can be tagged with a radioisotope or a toxin, so that binding of the tagged-mammaglobin complex results in the specific killing of that breast cancer cell. Finally, mammaglobin is being explored as a target for immune-based interventions. In vitro studies have demonstrated that T cellmediated immune responses can be induced against mammaglobin-derived peptides expressed by MHC molecules on tumor cells and antigen-presenting cells. In summary, mammaglobin displays several unique features that make it a promising target for intervention.