Xiaozheng (Mark) Xu

I started my scientific research in Dr. Jiahuai Han's lab at Xiamen University as an undergraduate student in the fall of 2007, studied the mechanisms underlying TNF mediated tumor cell necrosis/apoptosis. Firstly, I optimized and established a high SNR IP protocol, which allowed me to obtain much purer flag-TNFα signalosome as well as high-quality MS data. This work led to the development of a data-independent –acquisition (DIA) analyzing program, Group-DIA, by my colleagues (Li et al. Nature Methods 2014). Later, I purified all the main signalosomes in TNF signaling, TNF-R1, RIPK1, RIPK3, MLKL, and NEMO associated complexes, and identified most of the reported proteins involved in TNF signaling pathway as well as tens of promising candidates through quantitative Mass Spec (qMS). As a postdoc in Dr. Enfu Hui's lab at UCSD, I established a GST pull down-qMS system and a nanobody-based IP system, for the identification and study of downstream effectors recruited by numerous immune checkpoints in T cells, such as PD-1, BTLA, CTLA4, TIGIT. Combined with in vitro membrane reconstitution system, T Cell:supported lipid bilayer (SLB):TIRF microscopy imaging system, and APC:T cell coculture assays, I was able to investigate PD-1 and BTLA co-inhibitory signaling functionally and mechanistically in T cells. I found PD-1 recruits Shp2 but not Shp1 to inhibit CD28 signaling, while BTLA prefers Shp1 and inhibits both T cell receptor (TCR) signaling and CD28 signaling (Xu et al. JCB 2020). Subsequently, I started investigating how PD-1/BTLA distinguish Shp1/Shp2. Through the collaboration with my colleague, Dr. Takeya Masubuchi, we revealed the molecular features underlying the binding preferences between PD-1/BTLA and Shp1/Shp2. Then we further broadened the Shp1 binding specificities to other immune checkpoints by the finding of a hydrophobic pocket within Shp1-nSH2 that prefers medium-sized nonpolar residue at ITIM Y+1 position in a collaboration with Dr. Qixu Cai (HKUST) (Xu et al. eLife 2021). Next, in collaboration with Dr. Ira Mellman's lab at Genentech Inc., we found that PD-1 inhibits CD226 through its intracellular domain by recruiting Shp2 while TIGIT suppresses CD226 via its extracellular domain by competing their common ligand. This explained why PD-1 and TIGIT inhibit CD226 signaling synergistically in vivo. (Banta and Xu et al. Immunity 2022, Co-1st author). Other ongoing projects including mechanistic study of Themis functions and CTLA4 functions in T cells.