Abstract

The need to better understand the tumour microenvironment (TME) dictates the characterization of the cell types involved, the roles they play and how they respond to treatment. To develop better cancer immunotherapies, in vitro primary immune cell bioassays offer an early assessment of their effects on the various players of the TME.
Exhausted T cells were first discovered in LCMV virus infections where chronic stimulation due to remaining virus antigens leads to T cells that stop responding to stimulation and lose their functionality (cytokine secretion and cytotoxic activity). The same phenotype was afterwards found in the tumor microenvironment where a lot of T cells were found to be “anergic”.
Reversing their loss of functionality as a therapeutic strategy has made the characterization of these exhausted T cells a priority.
Their phenotype is currently defined by an increase in the expression of a specific set of cell surface markers such as PD-1, TIM-3 and LAG-3. As exhaustion builds, the expression of these markers increases while T cell functionality decreases. The loss of functionality is defined by a reduction in cytokine secretion, proliferation, and cytotoxic activity in the presence of an antigen. To evaluate this phenotype in more depth, transcription factors can also be monitored. For example, BLIMP-1 and BATF, which play a role in PD-1 expression, are shown to increase following exhaustion.
New therapeutics that can reverse exhausted phenotype lead to a new population of T cells called reinvigorated T cells which possesses high effector activity such as cytotoxicity towards cancer cells.
Using primary immune cells, in vitro bioassays were developed to better screen the potential effect of new therapeutics on exhausted T cells. Their ability to reverse the exhausted phenotype and facilitate the anti-tumour immune response can as a result be assessed early in the drug development process.