Our knowledge of how immune checkpoints shape anti-tumour immunity is continuously growing, creating new possibilities for cancer immunotherapy. Nevertheless, some fields in this research area remained quite unexplored, such is the time course for transition from functional to dysfunctional state of anti-tumour immune response. We created bioluminescence imaging model that allowed us to precisely monitor the behaviour of immunogenic tumour (B16OVA-Luc2) under the different immunological conditions, which in turn provided us with valuable information of exact time frame when tumour escape from immune control occurs.
In our model, immunization with tumour antigen ovalbumin (OVA-mice) resulted in time-limited immune control of tumour growth that we discovered to be far more dynamic than can be observed with standard calliper measurement. Anti-tumour immune response in OVA-mice was critically dependent on IFN-γ and CD8+ T cells, whereas in the absence of NK cells tumor growth suppression in OVA-mice was still conceivable, yet duration of immune control over tumor growth was significantly shorter. Although we found anti-PD1 mAb treatment to be partially effective in OVA-mice, PD-1 itself turned out not to be a reliable marker of exhausted OVA-specific CD8+ T cells: expression of PD-1 and its ligand PD-L1 on OVA-specific CD8+ T cells and B16OVA-Luc cells, respectively, were significantly higher in OVA-mice (in both wild type and IFN-γ-/- phenotype) compared to non-immunized mice, even during the time of evident immune control of tumour growth. In contrast, we found tumour escape to strongly correlate with both reduced clonality of OVA-specific CD8+ T cells and their ability to proliferate in tumour microenvironment. Ongoing studies are focused on further exploring time dependent changes in OVA-specific CD8+ T cells as well as in immunogenic tumour, in order to better define the events that precede tumour escape from immune control.