Kaitao Li

BME PhD Defense Presentation

Date: July 11th, 2016

Time: 11AM

Location: IBB 1128

Advisor:

Cheng Zhu, PhD (Georgia Institute of Technology)

Thesis Committee Members:

Simon J. Davis, PhD (Oxford University)

Arash Grakoui, PhD (Emory University)

Melissa L. Kemp, PhD (Georgia Institute of Technology)

Susan Thomas, PhD (Georgia Institute of Technology)

Title:

2D kinetic study of PD-1 interaction and its inhibition of T cell antigen recognition

Abstract

Programmed death-1 (PD-1) is an immune-checkpoint receptor with its primary function to maintain peripheral tolerance of the adaptive immune responses. The importance of PD-1 is evidenced by its deficiency leading to autoimmune disorders, its central role in the identification and restoration of the exhausted phenotypes of antigen-specific T cells, and the great success in targeting this pathway for cancer immunotherapy. To better understand the fundamental question as how PD-1 achieves the potent but well-controlled inhibition, we applied kinetic approaches focusing on its in situ ligand binding characteristics, and the early impact on antigen recognition by the T cell receptor (TCR) and coreceptor CD8. Different from the weak three-dimensional (3D) affinities measured in solution using purified PD-1 and ligands, the two-dimensional (2D) affinities of ligand binding to mouse and human PD-1 expressed on cell membrane span a range from middle to strong, whereas PD-L1–B7-1 binding is much weaker. Comparison of 2D and 3D affinities of TCR, costimulatory, and coinhibitory receptors reveals differential enhancement of in situ ligand binding for various receptors by the cellular environment. By integrating the 2D kinetic analysis of PD-1 with TCR and CD8, we probed an apparent “negative cooperativity” between these two axis, manifested as reduced molecular bond number and bond lifetime when respective ligands were co-presented. Examination with force spectroscopy suggests the “negative cooperativity” to be the net outcome of suppressed “positive cooperativity” between TCR and CD8. Moreover, the dependence of this suppression on Src homology region 2 domain-containing phosphatase-2 (SHP-2) and lymphocyte-specific protein tyrosine kinase (Lck) further identified it as a “binding-signaling-binding” feedback mechanism representing fine-tuning of antigen recognition by coinhibitory receptors via targeting the TCR-CD8 machinery. Overall, our results provide an in depth understanding of the in situ interaction and function of PD-1, highlighting the complexity and significance of costimulatory/coinhibitory molecules in modulating T cell responses.