Preclinical evaluation of adoptive T cell therapy and liposomal tumour antigen vaccination as a combinatorial cancer immunotherapy

The immune system is designed to protect our bodies from infection by constantly surveying peripheral tissues for signs of infection. Immune effectors are activated and mobilized upon detection of components from pathogenic intruders, but the immune system can also recognize and eliminate transformed, cancerous cells if they appear foreign by expression of mutated antigens.

The ability of the immune system to recognize and eliminate cancer cells have motivated the development of cancer immunotherapies that aim at mobilizing a tumour reactive immune response. Adoptive T cell therapy (ACT) is a type of cancer immunotherapy that has undergone extensive development during the past decades and now offers a curative option for subgroups of patients with malignant melanoma and haematological malignancies.

ACT is based on treatment with autologous tumour-reactive, T cells that are isolated from the patient, expanded ex vivo and infused back to the patient. Despite impressive response rates, the majority of patients are not cured by ACT and currently the therapy is not effective for solid cancers other than melanoma.

It is therefore necessary to continuously advance the procedure by identifying factors that influence the therapeutic efficacy of ACT. The phenotype of the infused T cells is known to influence the persistence and ability of T cells to sustain long-term tumourreactivity, and studies have shown that using minimally differentiated T cells for ACT improves the therapeutic outcome.

Furthermore, providing post-infusion priming of T cells by tumour antigen vaccination has been found to enhance the persistence and functionality of the infused T cells. Motivated by these findings, this thesis provides a preclinical evaluation of a therapeutic strategy that aims at improving the therapeutic response of ACT by combining infusion of naïve, antigen-specific T cells with a liposomal tumour antigen vaccination consisting of tumour antigen and a toll-like receptor 7 (TLR7) agonist conjugated to a liposomal formulation.

The work behind manuscript 1 presented in this thesis demonstrate that adoptively transferred, naïve antigen-specific T cells can be primed and expand in vivo by post-infusion liposomal vaccination with their cognate antigen. The liposomal vaccine effectively activated and induced antigen-presentation by antigenpresenting cells in vitro and in vivo, which stimulated extensive expansion of adoptively transferred T cells.

The T cell expansion resulted in a high tumour infiltration of antigen-specific T cells and was associated with improved tumour control and prolonged survival of treated mice. Manuscript 2 describes an evaluation of the tumour-intrinsic mechanisms associated with tumour response to ACT and an eventual tumour relapse that was observed after an initial period of ACT-induced tumour regression.

A comparative transcriptional analysis of untreated control tumours, responding tumours and relapsing tumours demonstrated that response to ACT was associated with upregulation of genes indicative of tumour-infiltration of activated dendritic cells and cytotoxic effector functions of T cells and natural killer cells.

Furthermore, responding tumours had an upregulated expression of genes associated with tumour associated macrophage-mediated immune suppressive mechanisms. Contrasting this, relapsing tumours were characterized by a transcriptional profile associated with immune exclusion, as genes associated with both pro-inflammatory and immune suppressive mechanisms were significantly downregulated.

These results indicate that the ACT-induced immunogenic tumour reactivity is associated with induction of immune suppressive mechanisms and that relapsing tumours evolve to become immune excluded. Collectively, the work behind this thesis describes a preclinical evaluation of performing ACT using naïve, antigen-specific T cells that are primed post-infusion by a liposomal tumour antigen vaccination, and it elucidates tumour-intrinsic mechanisms associated with response to and eventual tumour escape from ACT and tumour antigen vaccination.

This work can have implications for the future design of ACT treatment regimens by offering insights to the mechanisms behind response to ACT and subsequent escape mechanisms.