Identifying Cancer Tumor-Associated Antigen Peptides

William Hildebrand, PhD, professor at the University of Oklahoma Health Sciences Center and CSO at Pure Protein LLC, notes that there is a dearth of cell-surface targets of value in cancer, and his work focuses on how to detect valid cell-surface targets. During a talk at the Catalent Applied Drug Delivery Institute-run meeting, “Advances in Biologics Development and Delivery,” on March 24, 2016, Hildebrand presented information on providing a pipeline of tumor targets through peptide analysis.

Class I human leukocyte antigens (HLA) are available on tumor cell surfaces, and they mediate adaptive immune responses to cancer. Checkpoint/blockade inhibitors (such as PD-1/PD-L1) enable the immune system to more freely target the HLA of tumor cells. These HLA cell receptors display peptide bits of normal “self” proteins as well as bits of abnormal proteins. Hildebrand’s goal, he said at the meeting, is to identify and validate HLA/peptide targets that appear on virus-infected cells and on cancerous cells.

Hildebrand hypothesized that numerous unidentified HLA/peptide complexes distinguish ovarian cancer cells and can act as novel targets for immunotherapy with antibodies that mimic T-cell receptors (TCRm).  Specifically, for ovarian cancer immunotherapy, he hypothesized that a peptide bit of macrophage migration inhibitory factor (MIF) in complex with HLA-the MIF/HLA-A2 complex-could be developed as a future potential target for immune therapies.

To develop a TCRm monoclonal antibody (mAb) to target the HLA-A2/MIF complex on cancerous ovarian cells, Hildebrand first characterized the HLA-A*02:01 ligands on the surface of ovarian cancer cells and performed a deep mass spectrometric peptide characterization of the ovarian tumor-associated antigens. Both cell lines and tumor tissues were included in target identification. One of the challenges with this approach, however, is that after lysing tissue, a number of peptides still remain undetectable-and some of these undetectable peptides likely include many important tumor ligands, said Hildebrand.

Among four common ovarian cancer cell lines, Hildebrand looked at HLA/peptide proteins that were shared. He found that there were hundreds of HLA/peptide complexes shared between these four cell lines. Hildebrand noted that although a “vast majority of cancer immunotherapies focus on TAAs…very few TAA peptides have ever been directly eluted and sequenced from class I HLA.” Most of these targets, he said, are either unvalidated or predicted by drug developers-in other words, developers rely on educated guesses to figure out which peptides are at play. For example, ERBB2 (HER2), the target of the drug Herceptin (trastuzumab) and a favorite immunotherapy antigen target, was found to have 44 cancer TAA peptides-but Hildebrand pointed out there are currently no elution data for HER2 peptides. In the future, Hildebrand and his team plan to further test the cancer specificity of TAA peptides and process more tumor and normal tissues for expanded comparative analysis.