How tumour-cell killing by T cells is initiated

Background Receptor signaling underpins the new treatments transforming cancer medicine.

However, there is almost no agreement about how these receptors are triggered, even though a proper understanding could extend the reach of immunotherapy, increase its efficacy, or curb its toxicity.

We have proposed a mechanism in which receptor phosphorylation is kept low by an equilibrium between kinases and phosphatases, that is disturbed locally in favour of the kinases when the large receptor-type phosphatase, CD45, is sterically excluded from cellular contacts where receptors engage their ligands.

We and others have demonstrated local kinase/phosphatase segregation at contacts of lymphocytes with model surfaces, and we have developed a quantitative treatment of signaling that predicts the relative signaling potencies of well-characterised receptor ligands.

The important question now is: does the mechanism apply at authentic cell-cell contacts, especially those formed with tumours by T cells?

Aims In the last few months we have begun to observe local regions of phosphatase depletion at contacts that T cells make with the U-2 OS osteosarcoma cell line.

Our goal is to determine whether tumour-cell killing is initiated by the steric exclusion of the phosphatases and receptor dwell-time at these contacts.

Methods U-2 OS osteosarcoma cells are unusually flat (3 micron thick) cells, which means that we can interrogate their upper surfaces when they interact with T cells, using ‘highly inclined and laminated optical sheet’ imaging.

So that we can study primary T-cells, we will use bispecific T-cell ‘re-directors’ already in use in the clinic, called ImmTACs.

To demonstrate the important role that molecular dimensions play for these and other therapeutics, including immune checkpoint blockers, we will use protein engineering.

The mathematical model we have developed will be used to fit our measurements of triggering at cell-cell contacts, or refined to match the data.

All of our work will be done alongside tumour killing assays in vitro and in vivo, confirming the therapeutic relevance of our observations.

How the results of this research will be used A new understanding of receptor triggering could change how we approach immunotherapy, for example by placing more emphasis on the steric properties of drug-like molecules, or by suggesting new ways to move signaling molecules in and out of T-cell/tumour contacts.

We will illustrate this by testing whether the activity of the immune-checkpoint blocker, nivolumab, could be enhanced by altering its steric properties.