Induction of immunogenic cell stress through the inhibition of topoisomerase I

When normal tissue homeostasis cannot be maintained, cells initiate distinct pathways of regulated cell death (RCD) to preserve health of the organism. Exploiting RCD pathways to induce tumor cell death is a mainstay of cancer therapy. More recently it has become appreciated that the manner in which cancer cells die can have drastic impacts on local tissue microenvironments and ultimately disease progression. Emerging preclinical and clinical data suggest certain forms of RCD can cause the release of damage-associated molecular patterns (DAMPS) to not only activate innate immunity but also generate antigen-specific adaptive immune responses, a defining feature of immunogenic cell death (ICD). In addition to bona fide ICD, it is recognized that tumor cells paralyzed in a non-replicative senescent state are also highly immunogenic through the release of immunostimulatory soluble factors, upregulation of major histocompatibility complex (MHC) molecules, and altering the array of antigens presented to T cells. Clinical data with topoisomerase I inhibitor (TOP1i)-ADCs show responses in numerous human cancers with growing evidence that immune system engagement may contribute to their clinical activity. Therefore, we sought to determine the immunogenicity of topoisomerase I inhibition. TOP1i treatment in vitro induced a highly immunogenic state of murine tumor cells, which when injected to mice, acted as a cellular vaccine and resulted in the rejection of secondary live tumors. Vaccination with TOP1i treated cells required host type 1 conventional dendritic cells (cDC1) and CD8 + T cells for protection. Interestingly, characterization of cell death kinetics following TOP1 inhibition reveled that the immunogenicity of TOP1i treated cells is largely derived from stressed, living cells rather than dead cells. Treatment with highly immunogenic doses of TOP1i in vitro revealed that tumor cells rapidly alter their physiology to exhibit multiple hallmarks of cellular senescence. Following TOP1i treatment, tumor cells were durably arrested in cell cycle (G2M), upregulated p21, exhibited heightened beta-galactosidase activity and produced inflammatory senescence associated secretory phenotype (SASP) cytokines, as well as type I IFNs, and became sensitive to senolytic agents. Moreover, TOP1i treated cells were able to induce robust cDC1 activation and promote the acquisition of a mregDC like phenotype which was dependent on tumor expression of the stimulator of interferon genes (STING). Using a human antigen expressing syngeneic tumor model we demonstrated that TOP1i-ADC activity was greatly enhanced in the presence of adaptive immunity and synergizes with anti-PD1 therapy. Together these data highlight the immunogenic nature of topoisomerase I inhibition and provide new insights into the potential mechanism underlying the emerging clinical benefits of TOP1i-ADCs for the treatment of cancer.