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Phagocytosis of apoptotic cells is fundamentally important throughout life, because non-cleared cells become secondarily necrotic and release intracellular contents, thus instigating inflammatory and autoimmune responses. Secreted "find-me" and exposed "eat-me" signals displayed by the dying cell in concert with the phagocyte receptors comprise the phagocytic synapse of apoptotic cell clearance. In this scenario, lysophospholipids (lysoPLs) are assumed to act as find-me signals for the attraction of phagocytes. However, both the identity of the lyso-PLs released from apoptotic cells and the nature of the phagocyte receptor are largely unknown. By a detailed analysis of the structural requirements we show here that lysophosphatidylcholine (lysoPC), but none of the lysoPC metabolites or other lysoPLs, represents the essential apoptotic attraction signal able to trigger a phagocyte chemotactic response. Furthermore, using RNA interference and expression studies, we demonstrate that the G-protein-coupled receptor G2A, unlike its relative GPR4, is involved in the chemotaxis of monocytic cells. Thus, our study identifies lysoPC and G2A as the crucial receptor/ligand system for the attraction of phagocytes to apoptotic cells and the prevention of autoimmunity. [ABSTRACT FROM AUTHOR]

Positron emission tomography (PET) reporter gene imaging can be used to non-invasively monitor cell-based therapies. Therapeutic cells engineered to express a PET reporter gene (PRG) specifically accumulate a PET reporter probe (PRP) and can be detected by PET imaging. Expanding the utility of this technology requires the development of new non-immunogenic PRGs. Here we describe a new PRG-PRP system that employs, as the PRG, a mutated form of human thymidine kinase 2 (TK2) and 2'-deoxy-2'-18F-5-methyl-1-β-L-arabinofuranosyluracil (L-18F-FMAU) as the PRP. We identified L-18F-FMAU as a candidate PRP and determined its biodistribution in mice and humans. Using structure-guided enzyme engineering, we generated a TK2 double mutant (TK2-N93D/L109F) that efficiently phosphorylates L-18F-FMAU. The N93D/L109F TK2 mutant has lower activity for the endogenous nucleosides thymidine and deoxycytidine than wild type TK2, and its ectopic expression in therapeutic cells is not expected to alter nucleotide metabolism. Imaging studies in mice indicate that the sensitivity of the new human TK2-N93D/L109F PRG is comparable with that of a widely used PRG based on the herpes simplex virus 1 thymidine kinase. These findings suggest that the TK2-N93D/ L109F/L-18F-FMAU PRG-PRP system warrants further evaluation in preclinical and clinical applications of cell-based therapies. [ABSTRACT FROM AUTHOR]

Cancer cells and immune cells modulate their metabolism according to specific needs during cancer progression and immune responses. The ability to measure cellular metabolic function in vivo would enable the evaluation of tumors and their response to therapy and also the effectiveness of cellular immune responses to cancer. Positron emission tomography (PET) is a highly sensitive clinical imaging modality that enables whole-body, quantitative measurements of tissue biochemical function. Here, we review work using PET probes for specific metabolic pathways to measure cell function in cancer and immunity. We focus on the use of probes for glycolysis and nucleoside salvage and then discuss the development of new metabolic probes that visualize distinct parameters of cell function during disease. [Copyright &y& Elsevier]