Final Report Summary - RIMINCGD (Regulation of Inflammation by Macrophages in Chronic Granulomatous Diseases)
Chronic granulomatous disease (CGD) is a primary immunodeficiency caused by the genetic deficiency of NADPH oxidase. Patients with CGD suffer from recurrent life-threatening infections, consistent with the importance of NADPH oxidase for the induction of respiratory burst in macrophages and neutrophils. Another aspect of this disorder is a state of hyperinflammation, which leads to excessive persistent granulomatous responses. Granulomas are histologically defined areas of tissue inflammation which contain neutrophils and necrotic tissue surrounded by macrophages and multinucleated giant cells. The formation of granulomas is important for immune defence against pathogens, such as mycobacteria. However in CGD, granulomas can be sterile, destruct normal tissue and often lead to life-threatening complications such as granulomatous bowel obstruction. The mechanisms of hyperinflammatory granuloma formation in CGD as well as in other immune diseases with chronic granuloma formation, such as Crohn’s disease and sarcoidosis, are poorly understood. The goal of this project was to elucidate these mechanisms. Our work has been focused on the role of macrophages, as major components of granulomatous responses. Macrophages are known to be important for mycobacterial granuloma formation, since Pu.1 deficiency suppresses granuloma formation (Clay H, Ramakrishnan L, Cell Host Microbe 2007), and TNF-dependent macrophage survival is likewise significant for the maintenance and dissemination of granulomas (Clay H, Ramakrishnan L, Immunity 2008). In granulomas, macrophages appear enlarged and sometimes contain several nuclei, thus giving rise to the terms ‘epithelioid macrophages’ and ‘multinucleated giant cells’ respectively. Multinucleated giant cells express proteases such as MMP9 which are thought to be crucial for granuloma organization. How macrophages increase their size in the context of granuloma formation remains largely unknown. We have shown that macrophages can grow by polyploidization in response to mycobacteria or purified TLR ligands in vitro and in the context of Mycobacterium bovis granuloma formation in vivo. Using long term live cell imaging we found that in response to TLR2 ligands, multinucleation and polyploidization results from recurrent cytokinesis failure and re-programming of cell cycle regulators such as the transcription factors MafB/Myc.
In summary, we found a pathway of macrophage polyploidization and multinucleated giant cell formation which is distinct from the previously described cytokine-induced macrophage multinucleation by cell-to-cell fusion. We propose that macrophage growth by pathogen-mediated cell cycle regulation and polyploidization may be an important mechanism for the maintenance of granulomas during immunity to mycobacterial infection or autoimmune hyperinflammation. This work may thus lead to the development of new therapeutic strategies for hyperinflammatory granulomatous responses.
In summary, we found a pathway of macrophage polyploidization and multinucleated giant cell formation which is distinct from the previously described cytokine-induced macrophage multinucleation by cell-to-cell fusion. We propose that macrophage growth by pathogen-mediated cell cycle regulation and polyploidization may be an important mechanism for the maintenance of granulomas during immunity to mycobacterial infection or autoimmune hyperinflammation. This work may thus lead to the development of new therapeutic strategies for hyperinflammatory granulomatous responses.