Radioimmunotherapy in an Immunocompetent Tumor Model utilizing a 177Lu-mAb

Sammanfattning: Radioimmunotherapy (RIT) is a therapeutic strategy in which radionuclides are conjugated to monoclonal antibodies that bind to tumor-associated antigens in the tumor, so that the decay of the radionuclide takes place inside the tumor. The aim of the research presented in this thesis was to investigate the effects of RIT (utilizing the radioconjugated antibody 177Lu-BR96) on an inoculated tumor and the development of metastases. The main emphasis was on the immune response after RIT, since it has been shown that radiation can lead to reprogramming of tolerogenic immune cells to become immunogenic (promote rejection of the tumor cells). Several studies have shown that tumors can reprogram the immune cells of the host to be tolerogenic instead of rejecting tumor cells. The studies described here were carried out in an immunocompetent syngeneic rat colon carcinoma model with the potential to develop distant metastases. Thus, the model simulates the clinical situation of metastatic disease. Following RIT, the tumor cells of the inoculated tumor succumbed due to both caspase-3-dependent and caspase-3-independent cell death. Treatment with the unlabeled BR96 resulted only in caspase-3-independent cell death, indicating that unlabeled BR96 and 177Lu-BR96 induce different cell death mechanisms. Other studies have shown that cell death in which activated caspase-3 is expressed might result in an immunogenic type of cell death, although apoptosis has previously been considered to be immunogenic silent, in contrast to necrosis. The evaluation of immune cell markers showed that markers related to immune rejection were expressed to a higher degree than immune cell markers related to immune tolerance, in both untreated tumors and tumors treated with RIT. Both T cells and macrophages were present in untreated tumors of this model and decreased during RIT. The depletion of CD8-positive cells did not result in any delay in the rejection of the inoculated tumor after administration of RIT. The depletion of CD8-positive cells during RIT appeared to result in a higher frequency of animals developing metastases. This might indicate that RIT induces long-term immunity to the tumor cells. Metastases developing after RIT showed a reduced expression of the targeted antigen compared to untreated inoculated tumors in 17 of 23 metastases. However, none of the metastases or remaining primary tumors completely lacked expression of the targeted antigen, indicating the possibility of repeating RIT with BR96. It was concluded that the mechanisms of cell death in tumors were different when using RIT than with the unlabeled antibody. Immune cells are present in this syngeneic tumor model, although CD8-positive cells are not mainly responsible for the rejection of the primary tumor. However, CD8-positive cells seemed to prevent the development of metastases. Some of the metastases that developed after RIT exhibited reduced targeted antigen expression, but none of the metastases or remaining tumors lacked targeted antigen expression completely.