The Initial Events of T-Cell Activation in Realistic Model Systems

Abstract

T cells are part of the cellular component of the adaptive immune system and specifically recognise foreign antigens in the body. They use their membrane-bound T-cell receptor (TCR) to bind small peptide antigens presented within the major histocompatibility complex (pMHC) on the surface of antigen presenting cells (APCs). Therefore, T-cell activation is crucially cell-cell contact dependent. The exact molecular mechanism that allows T cells to detect antigens with unprecedented selectivity and sensitivity remains controversial. This work investigates the T-cell decision-making in response to antigens. It focuses on how T cells can discriminate between foreign- and self-peptides in the initial cell-cell contacts formed with a target cell. This analysis relied on two crucial steps: the development of more realistic model systems to study those interactions, and advanced fluorescence microscopy to image and quantify the interactions in live T cells. Firstly, supported lipid bilayers (SLBs) as model surfaces for the APC membrane were improved by presenting a glycocalyx barrier and small adhesion protein CD58 alongside pMHC and ICAM-1. It was found that T cells use small membrane protrusions (microvilli) to penetrate the glycocalyx barrier, which were stabilised by CD58 and initiated signaling. A new image analysis was then developed that provided a framework to quantify T cell-APC interactions. The interaction could be classified into four stages and highlighted the presence of small contact zones throughout, supporting the kinetic segregation model of receptor triggering. Lastly, these concepts were transferred to real cell-cell contacts. Three-dimensional imaging showed T cells also responded to ImmTAC, a bispecific immunotherapeutic, presented on APCs via small contact zones during all stages of interaction. The results presented show the importance of small contact zones in the initial events of T-cell activation and provide a way to image and quantify them. This will help unravel the basis of TCR triggering and the development of novel immunotherapeutics.