Immunologic effects of cancer therapy with oncolytic adenoviruses (2016)

In the history of oncology scientists have tried to treat cancer in numerous different ways. Traditionally surgery, chemotherapy and radiotherapy have been regarded as the three main treatment modalities. However, harnessing the power of the human immune system by cancer immunotherapy has recently attracted considerable interest, even though studies on immunotherapy started already more than hundred years ago. The crucial feature of immunotherapy is that it reshapes the immunosuppressive environment created by the tumor. Immunosuppression blocks the activity of body s own immune system, which would normally destroy the transformed cells, and it has been identified as one of the hallmarks of cancer. \n\nOncolytic viruses are a promising form of immunotherapy, and the first viruses have already been approved for clinical use. Oncolytic virotherapy is based on the concept of selective viral replication inside tumor cells. Replication causes tumor cell lysis, release of danger signals and immune cell activation. Viruses can also be modified to stimulate the immune system further, for example by inserting immunostimulatory cytokines into their genomes. One of the most popular viruses used as an oncolytic platform is adenovirus, which is a non-enveloped double-stranded DNA virus. Treatments with oncolytic adenoviruses have been previously shown to be safe and able to activate the anti-tumor immunity in pre-clinical and clinical settings. \n\nThis thesis investigates the influence of adenoviral immunotherapy on the human immune system. Additionally, predictive and prognostic potential of several immune-related biomarkers and clinical variables is evaluated. The patients included in these studies were treated as a part of the Advanced Therapy Access Program (ATAP), which was a clinical access program in contrast to a clinical trial. The thesis includes analyses from all of the 290 patients treated in ATAP. \n\nThe first part of the thesis includes findings from the T cell subsets in the peripheral blood and tumors of 50 patients treated with an oncolytic adenovirus. When comparing pre-and post-treatment samples, a significant shift in the ratio of CD4- and CD8-positive T cells in peripheral blood was identified. The proportion of CD4-positive T helper cells decreased in the majority patients, while the proportion CD8-positive cytotoxic T cells increased. Interestingly, the changes in peripheral blood T cell levels were correlated with T cell levels in the tumor biopsies. Moreover, this correlation was found to be inverse for CD4-positive T cells and positive for CD8-positive T cells. \n\nThe second study of the thesis concerned the pre-treatment tumor biopsies of 27 patients treated in the ATAP program. Several pathways related to immune responses, such as B cell receptor signaling, GM-CSF signaling and leukocyte extravasation signaling, were activated in patients surviving a shorter time than their matched controls. In the level of individual genes, several macrophage markers, complement components and complement receptors were also upregulated in this same patient population. To verify findings on a protein level, immunohistochemical stainings were performed for 19 biopsies. In these analyses, helper T cell marker CD4 and macrophage marker CD163 were significantly higher in patients with poor prognosis. Relating to infiltration of T helper cells, many T cell exhaustion markers, such as TIM-3, PD-L1, PD-L2, CTLA-4, were increased in patients with worse than expected survival, possibly suggesting a presence of exhausted T cell population. \n\nThird part focused on identifying predictive and prognostic factors for adenoviral immunotherapy. In this study, absence of pre-treatment neutralizing antibodies and low baseline neutrophil-to-lymphocyte ratio were found to correlate with a significantly longer overall survival, whereas absence of liver metastases was associated with an improved disease control rate. In multivariate Cox regression tumor type, gender and WHO performance status were linked to a lower hazard ratio for tumor related mortality. Interestingly, treatment with viruses that were armed with immunostimulatory cytokines was also associated with a lower hazard ratio. In predictive regression analysis intraperitoneal administration route of the viral treatment was correlated with a significantly higher odds ratio for disease control. \n\nIn addition to improving the understanding on immunologic phenomena related to adenoviral immunotherapy, these results offer interesting directions for the development of biomarkers and clinical criteria for selecting the right patients for the treatments. Moreover, the findings provide a rationale for combination studies with already approved and emerging immune checkpoint inhibitors. In the future this knowledge could possibly be utilized to improve also the design of the viruses by targeting them more specifically to the immunosuppressive pathways that are activated in cancer.