SUBJECT: Ph.D. Dissertation Defense
BY: Jin Sung Hong
TIME: Monday, March 24, 2014, 1:00 p.m.
PLACE: ES&T, L1116
TITLE: 2D kinetics and force regulation study of T cell recognition and thymocyte selection
COMMITTEE: Dr. Cheng Zhu, Chair (BME/ME)
Dr. Andres Garcia (BME/ME)
Dr. Julia Babensee (BME)
Dr. Melissa Kemp (BME)
Dr. Brian Evavold (Emory)


T cell recognition and thymic selection are thought to be determined by the binding propensity (avidity or affinity) of the T cell receptor (TCR) to its ligands. However, binding propensity quantified by previous 3D TCRpMHC kinetics such as using tetramer staining or surface plasmon resonance (SPR) under estimate TCRpMHC interaction due to neglecting physiological conditions. Recent studies considering membrane contribution in TCRpMHC interaction reported 2D kinetics and force regulated bond dissociation kinetics have better prediction to biological responses in CD8+ T cells. In this study, we further tested the findings in CD4+ T cells and CD4+ CD8+ (double-positive, DP) thymocytes. We analyzed TCRpMHC interaction for a well-characterized panel of altered peptide ligands (APLs) on multiple transgenic mouse TCR systems. Using ultrasensitive 2D mechanical assays, in situ 2D kinetic measurements show better sensitivity than the SPR 3D kinetic measurements in gauging the ligand potency and thymic selection. Furthermore, force-regulated bond lifetime of TCRpMHC interaction amplifies the discrimination in recognition of APLs and thymic selection. When force was applied to TCRpMHCCD4/8 bonds, two distinct patterns emerged: agonist/negative selecting ligands formed CD4/8-dependent catch-slip bonds where lifetime first increased, reached a maximum, then decreased with increasing force, whereas antagonist/positive selecting ligands formed slip-only bonds where lifetime monotonically decreases with increasing force. Our results highlight an important role of mechanical force in ligand discrimination and suggest a new mechanism for T cell recognition and thymic selection that is distinct from previous models based on 3D measurements.