Gregory Holt, M.D., Ph.D.
Assistant Professor of Medicine
Description of Research
Dr. Holt’s research interests in lung cancer immunotherapy have focused on optimizing vaccination strategies and evaluation of the immunosuppressive nature of cancer cells with an emphasis on designing methods to counteract them. Successful immunotherapy will need to address both the finding of present, but inadequate tumor reactive T lymphocytes in the blood of cancer patients and discovery of tumor escape mechanisms used to elude immunosurveillance. Focusing on DNA-based vaccination strategies, evaluation of construct design determined that inclusion of the immunogenic, minimal epitopes of a tumor antigen, insulin-like growth factor binding protein-2 (IGFBP2) produced superior anti tumor immunity when compared to vaccines containing the peptide, protein or full length gene equivalents. This DNA based vaccine required the participation of both CD4+ helper T lymphocytes and natural killer cells as depletion of either completely abrogated the tumor response. This is an interesting discovery since the use of a vaccine consisting of the identical sequences in peptide form was only affected by depletion of CD8+ cytotoxic T lymphocytes (CTL). The mechanism responsible for this difference is currently unknown, however its elucidation may both delineate the underlying biology and describe mechanisms that may be exploited to improve vaccine efficacy. To further improve the DNA-based vaccines, immunogenic epitopes of IGFBP2 were screened for their ability to induce stimulatory interferon gamma responses versus immunosuppressive IL-10 responses. DNA-based vaccines encoding only the immunostimulatory regions of IGFBP2 were created and are currently undergoing preclinical analysis for IND submission with plans for use in vaccine trials for ovarian and breast cancer patients at the University of Washington.
Tumors are intrinsically hypoxic due to aberrant vasculature and proliferation that outgrows angiogenesis. The decreased oxygen tension induces regulatory T cells to produce adenosine and CTLs to upregulate hypoxia induced factor 1 alpha (HIF1a). Binding of adenosine to the adenosine 2A receptor (A2AR) on CTLs in conjunction with the increased HIF1a expression induces suppression of their effector function. The associated hypoxia and adenosine gradient around tumors creates a functional barrier to CTLs and potentially explains why reactive T cells are present in cancer patients’ peripheral blood, but do not cause tumor eradication. In collaboration with Dr. Eckhard Podack and Dr. Luis Raez of the University of Miami, and Dr. Michail Sitkovsky of Northeastern University in Boston, Dr. Holt is proposing a clinical trial of advanced NSCLC patients using Dr. Podack’s gp96 vaccine combined with exogenous oxygen to decrease tumor site hypoxia and the adenosine antagonist, theophylline.
- Creation of two DNA-based vaccines targeting insulin-like growth factor binding protein-2 and insulin-like-growth factor-1 receptor currently undergoing IND submission at the University of Washington
- Determination of an optimal construct for DNA-based vaccine targeting tumor antigens considered self-proteins