Role of Nitric Oxide-Signalling in Hypoxia-Induced Immune Escape in Cancer

A key step in malignant progression is the acquired ability of tumour cells to escape immune-mediated lysis. A potential mechanism by which tumour cells avoid immune destruction involves the shedding of MHC Class I Chain-Related Protein A (MICA), a Natural Killer (NK) cell-activating ligand, from the tumour cell membrane. Hypoxia has been shown to cause increased MICA shedding; however, this hypoxia-induced effect can be attenuated by pharmacological activation of the cyclic guanosine monophosphate (cGMP)-dependent nitric oxide (NO)-signalling pathway in cancer cells. The primary objective of the present study was to determine whether treatment of tumour-bearing nude mice with the NO-mimetic glyceryl trinitrate (GTN) attenuates in vivo tumour growth and if so, whether this effect is dependent on the presence of an intact NK cell compartment. Results indicated that continuous transdermal administration of GTN (1.8 µg/h) can significantly attenuate the growth of transplanted human DU-145 prostate tumours but that this effect of GTN is lost in mice whose NK-cells have been depleted. Tumours and serum from the mice in this study were analysed to determine whether GTN treatment had any effect on the expression levels of proteins integral to the proposed MICA shedding mechanism; however, the results of these studies were inconclusive. As phosphodiesterase (PDE) inhibition represents a potential method to enhance NO-signalling, experiments were performed to determine whether treatment with the PDE5/6 inhibitor zaprinast could also attenuate hypoxia-induced MICA shedding and decrease in vivo growth of DU-145 tumours. Results demonstrated that treatment with zaprinast (10 mg/kg) significantly attenuates MICA shedding in DU-145 cancer cells and significantly decreases in vivo tumour growth. Taken together, the results of these experiments indicate that GTN attenuates tumour growth by sensitising tumour cells to innate immunity, likely by increasing membrane-associated tumour cell MICA levels through the reactivation of NO-signalling, and that zaprinast decreases tumour growth likely through a similar mechanism. These findings are important because they indicate that agents capable of reactivating NO-signalling, such as NO-mimetics and PDE inhibitors, can potentially be used as immunosensitisers in the treatment and/or prevention of cancer.

The PD-1/PD-L1 Axis: An Immune-Mediated Mechanism of Chemoresistance in Cancer

The ability of tumour cells to avoid immune destruction (immune escape) and their acquired resistance to anti-cancer drugs constitute important barriers to the successful management of cancer. The interaction between specific molecules on the surface of tumour cells with their corresponding receptors on immune effector cells can result in inhibition of these effector cells, consequently allowing tumour cells to evade the host’s anti-tumour immune response. The interaction of the Programmed Death Ligand 1 (PD-L1) on the surface of tumour cells with the Programmed Death-1 (PD-1) receptor on cytotoxic T lymphocytes leads to inactivation of these immune effectors, and is a specific example of an immune escape mechanism tumour cells use to avoid immune destruction. Clinically, antibodies capable of blocking the PD-1/PD-L1 interaction have demonstrated significant therapeutic benefit, and are currently being used to help bolster patients’ immune response against malignant cells in a variety of cancer types. Here we show that the PD-1/PD-L1 interaction also leads to tumour cell resistance to conventional chemotherapeutic agents. Incubation of PD-L1-expressing human and mouse tumour cells with PD-1-expressing Jurkat T cells or purified recombinant PD-1 resulted in tumour cell resistance to doxorubicin and docetaxel. Interference with the PD-1/PD-L1 interaction using blocking anti-PD-1 or anti-PD-L1 antibody or shRNA-mediated gene silencing resulted in attenuation of PD-1/PD-L1-mediated drug resistance. Moreover, inhibition of the PD-1/PD-L1 signalling axis using anti-PD-1 antibody enhanced the effect of doxorubicin chemotherapy to inhibit 4T1 tumour cell metastasis in an in vivo mouse model of mammary carcinoma. These findings indicate that blockade of the PD-1/PD-L1 axis may be a useful approach to immunosensitize and chemosensitize tumours in cancer patients and provide a rationale for the use of anti-PD-1/PD-L1 antibodies as adjuvants to chemotherapy.