Cancer Immunotherapy Group Current Research
Project One: Regulation of the Immune Response by Perforin
One of the main obstacles to cancer immunotherapy is overcoming the mechanisms that limit current anti-tumour immune responses. We have observed that elimination of dendritic cells by the immune system prevents them from restimulating anti-tumour immune responses. Last year we determined that dendritic cell killing requires perforin, a protein produced and released by cytotoxic T lymphocytes (CTLs).
Early results indicate that while normal CTLs expand little after dendritic cell immunisation, do not increase their ability to secrete cytokines and inhibit the proliferation of other naïve CD8+ T cells, the opposite is true for CTLs that cannot make perforin. This suggests that perforin plays an important role in regulating the outcome of immune responses.
However, we also find that dendritic cells can escape killing in some situations. Dendritic cells that are immature do not present antigen efficiently, and can not be recognized by CTLs and be eliminated.
We are currently examining whether natural killer T (NKT) cells, which are also able to kill cells although they do not require perforin in order to do so, can kill dendritic cells in vivo.
Project Two: Tumour Vaccination by Epicutaneous Immunisation
The cancer vaccines used in our clinical trials are tailor-made in the laboratory from samples of the patient's blood and tumour tissue and are injected into the patient. Epicutaneous immunisation is a simpler, more cost-effective approach to the treatment of skin cancers such as melanoma that involves the direct application of tumour antigen in a cream to the skin.
In collaboration with Dr Patrizia Stoitzner and colleagues, we have shown that immunisation strategies through the skin are feasible and induce the activation of tumour-specific T cells that acquire cytotoxic activity.
The skin is the site of an especially dense network of dendritic cells called Langerhans cells. In 2007 we used mice in which the Langerhans cells can be depleted in vivo to confirm that it is this population of cells, and not some other skin antigen presenting cell that must be present for this form of immunisation to work.
We also demonstrated that Langerhans cells are required to transport antigen to the lymph node, where they present it directly to the T cells to initiate an anti-tumour immune response.
Project Three: Intratumoural Antigen Presenting Cells
Last year we began a new project to identify the immune cells present inside a tumour following an anti-tumour immune response.
Using FACS analysis and staining of melanoma tumour sections we found that most of the tumour-infiltrating immune cells were macrophages with a few dendritic cells, B cells and T cells. Interestingly, although dendritic cells isolated from the tumours were shown to be functional in antigen uptake, they were not able to activate tumour-specific CD4+ and CD8+ T cells in vitro.
We have now extended this study by examining the role of T regulatory cells in modulating the function of the intratumoural dendritic cells. We find that the inability of the tumour-infiltrating dendritic cells to activate T cells does not appear to be due to the presence of high numbers of suppressive T regulatory cells in the tumours because depletion of the T regulatory cells did not reverse this effect.
This finding suggests that the function of intratumoural dendritic cells is defective and might explain why tumours activate immune responses so inefficiently.
Clinical Relevance and Future Direction
Cancer immunotherapy is an effective means with which to harness the specificity and potency of the immune system to fight cancer. Unlike chemotherapy or radiotherapy, immunotherapy is a natural treatment that uses cells from a patient's own body, and is hence a more desirable therapy for most people.
The biggest bottleneck to cancer immunotherapy at present is overcoming the mechanisms that regulate the size of the anti-tumour immune response. This is because tumours seem to use strategies similar to those used by our normal tissues to turn off immune responses and avoid spontaneous or excessive immune-mediated damage.
In the future we hope to design effective cancer vaccines that are able to safely avoid the mechanisms responsible for limiting current anti-tumour immune responses. In doing so we hope to demonstrate that cancer immunotherapy should be used alongside current mainstream cancer treatments.
Collaborators
Prof Vincenzo Cerundolo, University of Oxford, UK
Dr Michelle Epstein, University of Vienna, Austria
Dr Ian Hermans, Malaghan Institute of Medical Research
Dr Bronwyn Kivell, Victoria University of Wellington, New Zealand
Prof Bernard Malissen, Centre d' Immunologie Marseille-Luminy, France
Prof Niki Romani, Innsbruck University, Austria
Other relevant progress
Discovery may increase dendritic cell survival
Once tumour-specific immunity is established, dendritic cells carrying tumour antigen are sometimes treated by the immune system as if they were tumour cells, and are quickly eliminated. We were able to demonstrate that dendritic cells are not eliminated in a strain of mice where the function of a specific population of immune cells (termed Cytotoxic T Lymphocytes or "CTL") is defective. We have used this strain of mice to test whether the elimination of dendritic cells may have an effect on the magnitude of the immune responses they induce. Indeed we find that, compared to normal mice, mice of this particular strain generate larger immune responses after they have been immunised with dendritic cells. This suggests that making dendritic cells able to resist the attack of immune cells is also likely to make the dendritic cells better at inducing immune responses.
Study of subpopulations and migration may enhance cell function
It is known that dendritic cells comprise of a diverse group of cells, but it is not clear whether all these cells have similar roles, or whether each population serves a specific function. By using genetic means, we are now trying to restrict antigen-presenting function to specific subpopulations of dendritic cells, so that their function can be studied directly. We are also attempting to generate models where the migration of dendritic cells, from peripheral tissues to immune organs, is altered. These studies should yield interesting information on the relationship between dendritic cell maturation and their capacity to elicit immune responses.
Overcoming obstacles in our fight against tumours
While dendritic cells are critical to the initiation of immune responses, the elimination of tumour cells is mediated by other immune cell populations such as T lymphocytes. The number of tumour-reactive T lymphocytes as well as their ability to survive, maintain effector function and deliver it to the tumour site are all critically important parameters in anti-tumour immune responses.
However, several mechanisms have been demonstrated which appear to limit each of these stages of the anti-tumour immune response. We are starting to examine in more detail some of these mechanisms. In particular, regulatory T cells have been reported to have an inhibitory role in many immune responses, including anti-tumour immune responses. We are trying to find out whether regulatory T cells also inhibit anti-tumour immune responses induced by dendritic cell vaccination, where in the body they carry out their inhibitory function, and the specific stages of the anti-tumour immune response that are affected by regulatory T cells.
