Master of Science
Microbiology and Immunology
Immunotherapy has improved patient outcomes in advanced or metastatic settings across a number of cancers. Patients with tumours deficient in the DNA mismatch repair (DNA-MMR) pathway often show high response rates to immune checkpoint inhibitors (ICIs) with a rise in immune surveillance. However, little is known about the immune sensitization effects of inducing DNA- MMR-deficiency in low tumour mutational burden (TMB) cancers, such as ICI refractory neuroblastoma. In addition, the dynamic T-cell profile that results from such a DNA-MMR inactivation, and whether this may confer a therapeutic benefit, is poorly understood. Here we used CRISPR/CAS9 genome editing technology to knock out (KO) MLH1, a crucial molecule in the DNA-MMR pathway, in mouse neuroblastoma (neuro-2a) cells – a low TMB pediatric cancer refractory to ICIs – to induce MMR deficiency. To analyze tumour growth inhibition in response to ICIs and immunophenotype the tumour-infiltrating lymphocytes (TILs), tumours with intact or induced MMR deficiency were injected subcutaneously into immunocompetent mice. Tumour growth were measured after treatment with anti-PD1 antibodies and TILs were analyzed for activation, exhaustion, and effector markers, allowing for an in-depth flow cytometric analysis of T-cell subsets in these tumours. Our study shows that inducing MMR-deficiency induces a robust anti-tumour response in a low TMB cancer. Moreover, this sensitization was accompanied by specific phenotypic changes of T-cells in response to anti-PD1 therapy in the tumour.
Summary for Lay Audience
Immunotherapy is a new form of cancer treatment that utilizes the immune system to recognize and kill cancer cells. In this thesis, I focus on immune checkpoint inhibitors (ICIs), which is a form of cancer immunotherapy that binds to “immune checkpoints” found on immune cells. These immune checkpoints typically serve to tamper down the immune response in order to prevent a harmful overactivation of the immune system. ICIs therefore bind to immune checkpoint molecules, blocking them from exerting their function, which then allows for immune cells to become re-invigorated and target cancer cells. Over the past few years, clinical trials have shown that a number of patients that are treated with immunotherapy have a long-term response to treatment. Moreover, responders to ICIs often have a very high number of mutations in their tumours. A subset of cancer patients that show a favorable response to immune checkpoint inhibitors are cancer patients that have a deficiency in their ability to repair DNA mistakes, which results in a large number of mutations in their tumour. Unfortunately, many cancer patients with a low number of tumour mutations do not respond to ICIs. In this thesis, I studied the effect of inducing a deficiency of DNA repair in a tumour with a low number of mutations and hypothesized that this would result in an increase in mutations in the tumour and a better response to ICIs.
El-Hajjar, Mikal, "Inducing DNA-Mismatch Repair Deficiency In Tumours: A Strategy To Enhance Anti-Tumour Immunity" (2020). Electronic Thesis and Dissertation Repository. 7432.
Available for download on Friday, November 11, 2022