Regulation of CD28 Signaling in T cells by Cytoplasmic Domain Membrane Binding

T细胞受体（TCR）是在基因组水平上由单个细胞的V、D和J段重排产生的。因此，TCR库可以集体识别任何抗原。由于这种随机重排过程，T细胞缺乏区分自身抗原和病原体相关抗原的能力。相反，T细胞必须被先天免疫抗原呈递细胞（APCs）激活，并被指示分化为效应细胞。这种激活信号需要抗原依赖的TCR刺激和CD28的共刺激，CD28是一种膜受体，在所有幼稚T细胞和记忆T细胞亚群上均有表达。与apc上表达的配体B7.1 （CD80）或B7.2 （CD86）结合，通过识别病原体相关分子，传递共刺激信号，与TCR信号协同作用，促进细胞存活、增殖和效应功能。

The T cell receptor (TCR) is generated by rearrangement of V, D and J segments in individual cells at the genomic level. Therefore, the TCR repertoire collectively can recognize any antigen. As a consequence of this random rearrangement process, T cells lack the ability to discriminate between self antigens and pathogen- associated antigens. Instead, T cells must be activated by innate immune antigen presenting cells (APCs) and instructed to differentiate into effector cells. This activating signal requires antigen-dependent stimulation of the TCR and costimulation through CD28, which is a membrane receptor constitutively expressed on all naove T cells and subsets of memory T cells. Binding to its ligands B7.1 (CD80) or B7.2 (CD86) expressed on APCs activated by recognition of pathogen-associated molecules delivers a costimulatory signal that synergizes with TCR signaling and promotes cell survival, proliferation and effector functions. The mechanism by which ligand engagement by TCR or CD28 leads to receptor triggering and signal transduction remains unknown; a general mechanism of receptor triggering remains to be elucidated for any phosphotyrosine-based immune receptor. Conversely, the mechanism preventing spontaneous and aberrant signaling of CD28 and other phosphotyrosine-based receptors is also poorly understood. Using a combination of fluorescence resonance energy transfer (FRET) microscopy, biochemical and cellular assays to assess T cell signaling and function, and an in vivo model of multiple sclerosis (murine experimental autoimmune encephalomyelitis), this research will examine: a) association of CD28 cytoplasmic domain (CD28CD) with the inner leaflet of the plasma membrane; b) the consequences of this association for regulation of signaling and T cell function; and c) the mechanism by which CD28CD is released from the plasma membrane under physiological conditions. Mechanistic understanding of CD28 regulation would provide insight into a more generalizable model of receptor triggering.