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Determining how mitoDAMPs promote immunoregulatory functions of lymphocytes

Posted: October 05, 2022

Written by: Misaal Mehboob

Edited by: Elizabeth Balint and Ana Portillo

In their recent work published in Cell Reports, Lauren Westhaver and Dr. Jeannette Boudreau demonstrate that mitochondrial-derived damage-associated molecular patterns (mitoDAMPs) induce an immunoregulatory phenotype in T cells and natural killer (NK) cells in response to cellular damage.

MitoDAMPs are a heterogenous mixture of components such as mitochondrial DNA, free ATP, formyl peptides, and other mediators that stimulate the activation of inflammasomes and TLR pathways. Following tissue injury, mitoDAMPs initially trigger an inflammatory response among innate immune cells. To counterbalance the initial inflammation, a wave of immunoregulatory responses follow, in order to prevent excessive inflammation and clear debris from tissue damage. This is when the initially inflammatory T cells and NK cells can adopt immunoregulatory functions in response to their local microenvironment and metabolism. Prior to this research, the mechanism behind how mitoDAMPs regulate lymphocyte polarization was still incompletely understood.

To investigate the effects of mitoDAMPs on the function of lymphocytes, Westhaver et al. first measured IFN-γ production by cytokine-stimulated mouse splenic NK cells treated with mitoDAMPs isolated from syngeneic livers using their unique protocol published in STAR Protocols. MitoDAMPs blocked IFN-γ production by NK cells in a dose-dependent manner. MitoDAMPs also induced phenotypic changes in NK cells, where expression of the activating markers CD69 and NKG2D were downregulated, while the regulatory markers PD-L1 and CD73 were upregulated. Additionally, mitoDAMPs were found to have conserved immunoregulatory effects, as mitoDAMPs from the livers of other species similarly suppressed human NK cells. The authors also showed that exposure to mitoDAMPs dampened T cell activation by blocking IFN-γ production, impairing proliferation, and limiting anti-viral CD8+ T cell priming.

Since mitoDAMP-treated NK cells produced the anti-inflammatory cytokine IL-10, the researchers next examined whether mitoDAMP-induced immunoregulation could be attributed to IL-10 signaling. However, IL-10 blockade could not fully restore proliferation, and did not eliminate the mitoDAMP-induced phenotype in T cells and NK cells. Mass spectrometry and proteomic analysis demonstrated that many mitoDAMP components were designated to the arginine-ornithine biosynthesis pathway, and since this pathway is involved in the resolution of inflammation, the role of arginase was investigated. The authors found that arginase inhibition restored functional and phenotypic changes in T cells and NK cells, while arginine supplementation partially restored mitoDAMP-suppressed T cell proliferation.

Altogether, the results of this work demonstrate that the mitoDAMP-dependent immunoregulatory program triggered in lymphocytes is coordinated by arginase-mediated depletion of arginine. By regulating immune responses after tissue injury, this mechanism may serve to protect from developing pathologies caused by hyper-inflammation.

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