Regulation of T cell activation by CD222

T lymphocytes (T cells) play a key role in the adaptive immune system, the specific defense system of higher organisms. Efficient activation of T cells to execute the immune response requires concerted interactions of receptors on their plasma membrane with extracellular ligands and submembrane signaling molecules, which results in production of cytokines, clonal expansion and cell differentiation. The general goal of this project addresses the role the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222) in the regulation of T cell activation. The hypothesis that this receptor is involved in T cell activation has developed through a number of discoveries obtained during our previous FWF project where we have characterized successfully how the M6P/IGF2R negatively controls cell migration of cancer and endothelial cells (1-4). In the course of this project we have started to analyze also T cells and have found that the M6P/IGF2R expression is highly upregulated on the T cell surface upon activation. Further, we have discovered that M6P/IGF2R silencing by RNA interference leads to an altered CD3 surface expression and block of production of the T cell growth factor interleukin-2. Furthermore, we have got preliminary data that M6P/IGF2R interacts with Lck, the Src family tyrosine kinase indispensable for the early events of T cell activation, and that it functions as a receptor for lactoferrin, a potent immunomodulatory glycoprotein. Thus, we have evidence that the M6P/IGF2R is involved in T cell activation at several levels: as a receptor, as a regulator of the T cell surface expression status and finally as a component of intracellular signaling cascades leading to cytokine expression. In the course of the presented project we will study in detail the molecular mechanisms underlying the above- mentioned involvements of the M6P/IGF2R and their functional consequences. Basically, we will perform loss-of- function and gain-of-function experiments combined with immunobiochemical and cellular-functional assays as well as with mass spectrometry and ultrasensitive imaging methods. Because our preliminary data provide strong evidence that the M6P/IGF2R is an integral part of T cell activation processes, we are confident that our project will provide a deeper understanding of the molecular pathways and protein interactions involved in T cell signaling; the comprehensive knowledge of these mechanisms is prerequisite to develop specific therapeutic strategies to modulate unwanted immune responses occurring in allergy, autoimmunity, chronic inflammation or post-transplantation complications.

For the right functions of proteins, it is essential to control not only their synthesis but also distribution in the cell. Protein trafficking is of special importance in the context of the human immune system, when professional immune cells have to promptly react on a certain stimulus, e.g. a pathogen, but also, to terminate their reactions to avoid unwanted tissue disturbances. The FWF project entitled The integral role of CD222 in T cell activation (P22908) has revealed so far unknown mechanisms involved in protein transport controlling two central immune cell types, namely T cells and macrophages. First, we have found that the cation independent mannose 6-phospate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222), one of the major protein transporters, controls the distribution of the kinase Lck, a key signalling molecule of T cell activation. T cells represent a central branch of the adaptive immune system. CD222 via this novel function plays a crucial role in regulation of T cell responsiveness to the antigen and thus in controlling immunity. The second major finding yielded by the project regards macrophages, an important branch of the innate immune system. We have identified a novel mechanism in efferocytosis the clearance of apoptotic cells by macrophages which as well depends on intracellular transporting pathways. This is essential during inflammation and for homeostatic maintenance in general. We have illuminated how this process is controlled. Thus, our project not only deepens our understanding of how our immune system works but also provides potential diagnostic targets and tools for pharmacological modulation of immune responses through specific protein transport mechanisms.