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Cell death by apoptosis is very common during acute lung injury, sepsis and viral pneumonias, and is believed to contribute to the development of emphysema. Apoptotic cells must be cleared promptly and efficiently to prevent further lung damage and to limit development of auto-immunity by inappropriate presentation of self-antigens in the context of danger signals.
The principal cells involved in apoptotic cell clearance in most organs are tissue macrophages. Interactions with apoptotic cells typically causes macrophages to become anti-inflammatory, which may hasten resolution of inflammation. However, this process may also reduce the macrophage's ability to fight infection.
Our laboratory described and provided the most complete explanation for the very reduced ability of alveolar macrophages, the resident phagocyte of the gas-exchanging regions of the lungs, to ingest apoptotic cells. Our current studies focus on the interaction of receptor tyrosine kinases of the TAM (also known as the Tyro3) family with other macrophage molecules during apoptotic cell ingestion.Among these other molecules,
we are particularly interested in scavenger receptor A (SR-A, also known as CD204) and LRP (also known as CD91).
Model of how MerTK interacts sequentially to induce AMø to recognize, ingest, & induce novel genes in response to AC. (A) Resting AMø. MerTK is monomeric & mostly free from LRP & SR-A. (B) Early AC binding. We propose that SR-A (binding AC via oxidized phospholipids) accelerates MerTK dimerization & autophosphoryl-ation. MerTK also associates with LRP, which is likely to be the first receptor to capture AC (via calreticulin-collectin bridges). We propose that MerTK is responsible for protein modifications of LRP needed to induce AC ingestion. (C) ERK activation. We propose that MerTK activates the Ras/Raf-1/MEK1/2/ERK1/2 cascade, likely via the adapter Shc. ERK migrates to the nucleus to participate in induction of multiple genes, including specific chemokines. It is uncertain whether SR-A or LRP contribute to ERK activation, and thus to changes in AMø gene expression resulting from AC recognition.
These studies use murine and human alveolar macrophages and cell lines, gene-targeted mice, siRNA technology, and transfections, to investigate the molecular basis for apoptotic cell ingestion and to understand its consequences for immunoregulation of the lungs. These studies are funded by R01 HL056309 and RO1 082489 from NHLBI, and by the Biomedical & Laboratory Research Service, Department of Veterans Affairs.
This composite photomicrograph was produced by Bin Hu, M.D., Ph.D. while he was in the lab, as part of our ongoing studies to define the molecular basis for uptake of apoptotic cells by alveolar macrophages and its consequences for pulmonary immunity.
PLC inhibition blocks PKC βΙΙ translocation & phagosome formation. The Mø cell line J774 was preincubated in chamber slides with the inactive control reagent U73122 (A, B) or with the PLC inhibitor U73343 (C, D) for 30 min, and then, without washing, AC were added for 10 min. Slides were stained with DAPI (blue) and rabbit anti-PKC βΙI, secondary goat anti-rabbit Ig conjugated with Alexa Fluor 488 (green). Note early phagosome formation & increased intensity of PKC βΙI staining adjacent to contract with AC (A, B), both of which are abolished by the PLC inhibitor (C, D). E, thymocytes alone. F, J774 cells incubated with apoptotic thymocytes but stained with omission of primary antibody.
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