Cancer immunotherapy
The body’s own immune system is the strongest ally in the fight against cancer. Our body cells are prone to mutations, which can lead to their malign transformation. This includes external stimuli, like smoking, radiation or pollution, but also due to the error prone replication process where mutations can occur during cell division (ref NCI). While most mutations are not harmful, representing the category of so called ‘passenger mutations’, some ‘driver mutations’ can affect key genes involved in control of cell growth and proliferation, which result in malignant cancer cells (Hanahan & Weinberg 2010) . Already at a very early stage of cell damage, intrinsic repair mechanisms are either reverting the mutation or actuating controlled cell death (apoptosis). In case these control mechanisms are overturned, immune cells are activated by the so called danger signals and these activated immune cells then eradicate the pre-malignant lesions. However, when more and more mutations are turning up, immune resistant tumor cells turn up, and soluble proteins, cytokines, are produced by the tumor which generate an immune suppressive environment (Swann & Smyth 2007).
It was William Coley in the late 19th century, who made use of the common observation, that in some cases people suffering from cancer recovered from the disease, at least temporarily, when they were affected by a strong bacterial, fungal or viral infection. Dr Coley started to inject a mixture of (killed) bacteria, including Streptococci, inducing a local infection (erysipelas). Remarkably, several of the patients recovered from cancer, although, some died due to the infection (JAMA 1893). The mixture, then called ‘Coley’s toxin’, was used until the 1960s, although with the upcoming radiation treatment and chemotherapy, its use was gradually discontinued.
With the increasing knowledge on the human immune system and its interplay with cancer, immunotherapy of cancer was more and more considered as a vital and important option in cancer treatment. Nowadays, protein and cell therapies are applied which can unleash the power of the immune system against cancer.
Our lab is participating in a collaboration project with the Institute for Cancer Research, (Medical University of Vienna, research Group Prof Petra Heffeter) and the Biotech-company BIRD-C, where empty bacterial shells, so called ‘bacterial ghosts (BGs) are combined with immunogenic chemotherapy. This combined treatment led to strong activation of the adaptive immune system (T-cells), and even led to complete tumor eradication with the occurrence of antitumoral immunity (Groza 2018). With the help of in vivo imaging, we were able to monitor the therapy response of individual tumor lesions. Currently, a research project is ongoing which applies this concept in a highly lethal and difficult to treat cancer type.
A key property of the immune system is to distinguish between ‘self’ and ‘non-self’. While not recognizing ‘non-self’ structures leads to infections and cancer, immune activity against ‘self’ results in autoimmune disease. CD47 is a protein expressed on most tissues in the body and on red blood cells, acting as a ‘don’t eat me’ signal. Its ligand SIRPα can be found on macrophages and once SIRPα recognizes CD47 on the cell surface, this stops a potential macrophage attack, i.e. phagocytosis (‘cell eating’) is inhibited. Unfortunately, most cancers overexpress CD47 on their cell surface making them invulnerable against macrophage attacks (Zhang 2020). Recently, antibodies have been developed and are applied in the clinics, which block CD47. To prevent excessive side effects (which would be due to CD47 blockage in body cells and red blood cells and subsequent macrophage attack on CD47-blocked tissues and red blood cells), antibodies either with a low affinity for CD47 are applied, or such which only weakly activate the immune cells with their so called Fc part, which is located at the distal end of the antibody. Both approaches weaken the overall antitumor activity. Our approach here is to combine ‘the best of both worlds’, i.e. having a protein which strongly blocks CD47 and giving maximized immune cell activation. To achieve a tumor restricted action of such a protein, we were pursuing a gene therapy approach. For this, a plasmid was cloned which encodes for a fusion protein consisting of a high-affinity SIRPα variant for maximized CD47 blocking and an engineered Fc moiety which maximizes immune response by strong activation of immune cells expressing Fc-receptor molecules.
In experiments with triple negative breast cancer, a very aggressive and difficult to treat tumor, we efficiently transferred the plasmid into tumor cells with nanoparticles based on linear polyethylenimine. Due to a built-in peptide sequence, the protein was secreted after its expression and blocked not only CD47 on the transfected cells, but also on tumor cells in the near vicinity pointing at a potent bystander effect. In an in vivo model, one third of all tumors were completely eradicated, as also shown by bioluminescence imaging. We could also observe migration of macrophages into the transfected tumor. Also, within an ex vivo assay it was observed that Natural killer (NK) cells, one of the most efficient immune cell types eradicating tumor cells, efficiently lysed and destroyed the transfected tumor cells. The CD47 project paper has recently been accepted in Molecular Therapy Oncolytics (Billerhart et al.)