Investigation of molecular mechanisms underlying the role of ICAM-3 in the phagocytic clearance of apoptotic leukocytes

体内受损、感染、老化或不需要的细胞通过一种称为细胞凋亡的过程被诱导死亡。这一过程的最后一步是由健康的邻近细胞清除(吞噬)死亡的细胞，在安全和受控的环境中，细胞身体被吃掉并降解。尽管与死亡细胞上识别的分子有关的信息量有限，但一系列分子已被认为在这一过程中发挥了作用。有人提出，濒临死亡的细胞表面的变化会导致识别细胞的信号或标志暴露给存活的邻居，即濒临死亡并准备移除的细胞。目前，人们对“吃我”这类信号的性质知之甚少。众所周知，细胞表面的脂质在死亡过程中会发生变化，磷脂酰丝氨酸(PS)暴露在细胞表面，在存活的细胞中，磷脂酰丝氨酸通常保持在细胞膜的内叶内。一旦暴露，PS就会被活细胞上的受体识别，并介导死亡细胞的清除。死亡白细胞表面的另一种分子ICAM-3(细胞间黏附分子-3)的变化与专业吞噬细胞-巨噬细胞对死亡白细胞的识别和清除有关。ICAM-3是一种只在白细胞上发现的分子，当存在于活细胞上时，通过促进细胞间的相互作用在启动免疫反应中起着重要作用。ICAM-3通过与包括整合素LFA-1和非整合素DC-SIGN在内的受体结合来调节这种相互作用。先前的工作表明，死亡细胞上的ICAM-3失去了与其原型对抗性受体LFA-1结合的能力。这种与凋亡相关的功能丧失与另一种功能的获得有关，其中ICAM-3能够与另一种尚未确定的受体结合-这一功能在介导凋亡的白细胞吞噬清除中起重要作用。清除濒临死亡的白细胞对于消解炎症和控制免疫反应尤为重要。未能在适当的时间清除白细胞可能会导致慢性炎症和自身免疫性疾病。更好地了解濒临死亡的白细胞清除过程中涉及的分子和过程，将是未来改善人类健康和福祉的战略的核心。这个项目试图确定细胞间黏附分子-3发生的变化，这些变化是在细胞凋亡过程中报告的功能丧失(LFA-1结合丧失)和功能获得(结合其他受体(S)和调节垂死的白细胞清除的能力)的基础上发生的。细胞凋亡表面与ICAM-3相关的功能改变可能是由于ICAM-3的位置、配对分子或结构的改变(例如ICAM-3的糖含量)，而这些变化可能是由于ICAM-3在细胞凋亡过程中流动性的变化引起或促成的。在这个项目中，我们将识别和描述ICAM-3在细胞凋亡过程中结构和/或环境的变化，并将这些变化与死亡细胞清除联系起来。

Damaged, infected, aged or unwanted cells in the body are induced to die via a process known as apoptosis. The final step in this process is the removal (phagocytosis) of dying cells by healthy neighbouring cells where the cell corpses are eaten and degraded in a safe and controlled environment. A range of molecules have been suggested to play a role in this process though the amount of information relating to molecules recognised on dying cells is limited. It has been suggested that changes at the surface of dying cells result in the exposure of signals or flags that identify the cell, to viable neighbours, as dying and ready for removal. Relatively little is currently known about the nature of such 'eat me' signals. Lipids at the cell surface are known to change during death with phosphatidylserine (PS) becoming exposed at the cell surface where in viable cells it is usually maintained within the inner leaflet of the cell membrane. Once exposed PS is recognised by receptors on viable cells and mediates dying cell clearance. A change in another molecule, ICAM-3 (Intercellular Adhesion Molecule-3) on the surface of dying leukocytes have been implicated in the recognition and removal of dying leukocytes by professional phagocytes, macrophages. ICAM-3 is a molecule found only on leukocytes and, when present on viable cells, is important in the initiation of immune responses by facilitating cell-cell interactions. To mediate this interaction ICAM-3 binds to counter-receptors including the integrin LFA-1 and the non-integrin DC-SIGN. Previous work has indicated that ICAM-3 on dying cells loses its ability to bind to its prototypic counter-receptor LFA-1. This apoptosis-related loss of function is associated with a gain of alternate function where ICAM-3 becomes able to bind to another, as yet unidentified, receptor - a function that is important in mediating the phagocytic clearance of apoptotic leukocytes. The removal of dying leukocytes is especially important for the resolution of inflammation and the control of immune responses. Failure to remove leukocytes at the appropriate time may lead to chronic inflammatory conditions and autoimmune disease. A better understanding of the molecules and processes involved within the clearance of dying leukocytes will be central to future strategies for improving human health and well-being. This project seeks to identify the changes that occur in ICAM-3 that underlie the loss of function (loss of LFA-1 binding) and the gain of function (ability to bind other receptor(s) and mediate clearance of dying leukocytes) that is reported during apoptosis. The change of function associated with ICAM-3 at the apoptotic cell surface may be due to changes in ICAM-3 location, partner molecules or structure (e.g. sugar content of ICAM-3) and any such changes may arise from or contribute to changes in the mobility of ICAM-3 as cells undergo apoptosis. Within this project we will identify and characterise changes in the structure and/or environment of ICAM-3 occurring during apoptosis and will relate these changes to dead cell clearance.