Transfer lipopolysaccharide activates monocytes through TLR4, which is thought to increase the risk of cardiovascular disease in HIV-infected individuals. Recently, in a study published in Communications Biology entitled “mTOR regulation of metabolic limits of LPS-induced monocyte inflammatory and procoagulant responses,” scientists from the State University of New York at Upstate Medical University found that therapeutic inhibition of a metabolic regulator in monocytes may unexpectedly increase the level of inflammatory signals in the body; this newly discovered mechanism may help develop novel therapies to treat chronic inflammatory diseases in humans more effectively.
Bacteria carrying lipopolysaccharide covers can colonize the gastrointestinal tract of the body, and when the function of the normal intestinal barrier to prevent bacterial infection is disrupted, these polysaccharide molecules detach from the bacterial surface and enter the bloodstream, which subsequently activates the innate immune response of host monocytes and induces inflammatory signals in the body, and the response produced by monocytes depends on the mechanical target of rapamycin (mTOR), a kinase that can form two complexes (mTORC1 and mTORC2) and regulate metabolic pathways.
Previous studies have shown that inhibition of mTOR in T cells may reduce the response to this stimulus, which causes the production of proinflammatory products such as cytokines, thereby maintaining the activation of other immune cells; the results of this study suggest that inhibition of mTOR may potentially prevent HIV infection of T cells at the early stages of their replication, which may indicate that this approach may reduce chronic inflammatory problems in HIV-infected individuals, who have higher levels of mTOR activation, which is thought to be caused by lipopolysaccharide translocation across the damaged intestinal barrier.
Given these findings, the researchers hypothesized that reduced levels of inflammation may also occur when mTOR, especially monocytes, is inhibited in other immune cells, but to their surprise, this may not be the case. Researcher Taylor said we thought we could use known anti-inflammatory therapies (which inhibit their activity in other cells) to reduce mTOR regulation in the body, but we saw the opposite and understood that mTOR may have a somewhat different role in these innate immune cells.
Using multi-omics analysis, the researchers found that pharmacological inhibition of mTOR function in monocytes actually increased the production of a variety of proinflammatory cytokines and also promoted the production of a tissue factor protein that increases hemagglutination; notably, inhibition of mTOR blocked intracellular NAD+ production, a metabolite necessary to turn off the transcriptional switch of the inflammatory gene promoter. Normally, when you use lipopolysaccharide to activate monocytes, mTOR basically stimulates NAD+, thereby slowing and restoring levels of transiently elevated inflammatory products to normal, but if mTOR is inhibited, feedback loops that allow everything to return to normal may become problematic, so inflammation continues and becomes worse.
These studies have found that they may provide relevant information for current treatments to better treat HIV-infected individuals and other viral infections, including COVID-19, among which known pro-inflammatory monocytes play an important role in inducing disease. Researcher Taylor said that we all know that another major symptom of severe COVID-19 patients is the problem of coagulation and coagulation of the entire body and tissues, and we believe that from the results of this study we may be able to see some potential associations in order to better understand how SARS-CoV-2 infection induces severe disease at a later stage.
In summary, the results of this study reveal the inhibitory effect mediated by mTOR on lipopolysaccharide-induced transcriptional responses in monocytes, and also elucidate a special metabolic mechanism, which can provide some information and reference for strategies to reverse the increased risk of coagulation diseases in proinflammatory states.