Necrosis Driven Triglyceride Synthesis Primes Macrophages for Inflammation During Mycobacterium tuberculosis Infection

Abstract

Pulmonary tuberculosis (TB) exhibits granulomatous inflammation, a site of controlling bacterial dissemination at the cost of host tissue damage. Intrigued by the granuloma type-dependent expression of inflammatory markers in TB, we sought to investigate underlying metabolic changes that drive amplification of inflammation in TB. Here, we show an association of higher inflammation in necrotic granulomas with the presence of triglyceride (TG)-rich foamy macrophages. The conspicuous absence of these macrophages in solid granulomas identified a link between the ensuing pathology and the metabolic programming of foamy macrophages. Consistent with in vivo findings, in vitro infection of macrophages with Mycobacterium tuberculosis (Mtb) led to increase in TG synthesis only under conditions of ~60% necrosis. Genetic and pharmacologic intervention that reduced necrosis prevented this bystander response. We further demonstrate that necrosis independent of Mtb also elicits the same bystander response in human macrophages. We identified a role for the human enzyme involved in TG synthesis, diacylglycerol O-acyltransferase (DGAT1), in this phenomenon. The increased TG levels in necrosis-associated foamy macrophages promoted the pro-inflammatory state of macrophages to infection while silencing expression of diacylglycerol O-acyltransferase (DGAT1) suppressed expression of pro-inflammatory genes. Our data thus invoke a role for storage lipids in the heightened host inflammatory response during infection-associated necrosis. Our data provide a functional role to macrophage lipid droplets in host defense and open new avenues for developing host-directed therapies against TB.

Triacylglycerol synthesis enhances macrophage inflammatory function

“…S2g). These data are consistent with a previous report that DGATi diminished inflammatory cytokine production in an M. tuberculosis-infected human macrophage cell line 19 . Additionally, we observed marked reductions in mRNAs encoding the Macrophage Inflammatory Proteins 1a (Ccl3) and b (Ccl4), the inflammasome Nalp1 (Nlrp1b), and reductions in expression of additional genes involved in the inflammatory response including Ido1 and Il1a in inflammatory macrophages as a result of DGAT1 inhibition ( Fig.…”

Foam Cells Control Mycobacterium tuberculosis Infection

“…Only cells exposed to the fatty acids oleic and linoleic acid accumulated triglycerides. The profound TG accumulation observed upon fatty acid supplementation is presumably mediated via the action of the diacylglycerol O-acyltransferase, DGAT-1 (Guerrini et al, 2018;Jaisinghani et al, 2018;Greenwood et al, 2019). This is in accordance with other reports, where macrophages loaded with oleic acid (den Hartigh et al, 2010;Sarathy et al, 2016;Greenwood et al, 2019), linoleic acid or a mixture of both led to the accumulation of TG-rich lipid droplets.…”

“…Foam cells appear in a variety of conditions where macrophages need to dispose of excessive lipids, and these can be of different nature (Chistiakov et al, 2017;Guerrini and Gennaro, 2019). Atherosclerosis is not their only arena and foam cells are a particular feature of tuberculous infection and caseation, a hallmark of persistent mycobacterial immune reactions (Peyron et al, 2008;Jaisinghani et al, 2018). Foam cells are abundant in mycobacterial granulomata, and the general assumption is that these cells accumulate lipids and die, leading to caseation which contributes to chronic inflammation, tissue damage and dissemination of the pathogen (Hunter et al, 2007;Kim et al, 2010;Guerrini and Gennaro, 2019).…”

Fatty acid oxidation of alternatively activated macrophages prevents foam cell formation, but Mycobacterium tuberculosis counteracts this process via HIF-1α activation

“…In the in vitro context, both infected and uninfected macrophages exhibited a similar elevated lipid droplet content, implying that a paracrine signal may be associated with infection-induced lipid droplet accumulation [39]. Interestingly, the presence of FM in lungs is widely associated with the occurrence of tissue necrosis, thereby opening up the possibility that uninfected macrophages surrounding the necrotic centre within granulomas may differentiate to FM using lipids available from the necrotic cells [40]. All these findings support the relevance of studying the involvement of the TB microenvironment in the formation of FM.…”