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Sustained Killing by Cytotoxic T cells: Mitochondrial Role

03/23/2022

Cytotoxic T lymphocytes (CTLs) can terminate both virally infected cells and cancer cells by secreting cytolytic proteins such as perforin and granzyme B. Lisci et al. have identified mitochondria as important regulators of CTL killing; mice lacking the deubiquitinase USP30 have CTLs acutely depleted of mitochondria, and these cells have reduced killing ability but normal motility, signaling, and secretion.

Although mitochondrial mass has been correlated with CTL antitumor activity, CTLs show an increased reliance on glycolysis, suggesting a decreased dependence on mitochondrial respiration. Whether, how, or why mitochondria contribute as CTLs seek, recognize, and kill their targets is not well understood.

In this research, Lisci et al., generated CTLs from USP30-deficient mice to study the nature of this defect and to understand how it affects killing.

 

Mitochondrial translation is required for sustained killing by cytotoxic T cells

 

They reported the following:

T cell development was unaffected in USP30-deficient mice. However, upon activation, CD8+ T cells generated CTLs with an acute loss of mitochondria and impaired killing.

The cytotoxicity of USP30-deficient CTLs diminished with time, indicating a defect in sustained killing.

Despite the loss of mitochondria and decreased oxidative phosphorylation in Usp30−/− CTLs, motility, signaling, and secretion, which are required for killing, were all intact. However, the secretory granule size was reduced in Usp30−/− CTLs, with a reduction in newly synthesized intermediates of key cytolytic proteins, perforin and granzyme B. This suggested an underlying defect in the de novo protein synthesis, which is required for sustained killing.

Monitoring translation revealed a marked reduction in protein synthesis in Usp30−/− CTLs.
 
Using mass spectrometry to ask whether the translational defect affected all proteins equally, only a subset of cytosolic proteins was found to be affected, including key mediators of the killing response [granzyme B, perforin, and the cytokines tumor necrosis factor–α (TNF-α) and interferon-γ (IFN-γ)]. CTLs showed no requirement for efficient oxidative phosphorylation to sustain cytotoxicity. However, selective inhibition of mitochondrial translation with doxycycline or chloramphenicol attenuated translation of cytolytic proteins, indicating an essential role for mitochondrial translation in sustained CTL killing.
 
How might mitochondrial translation selectively affect translation of cytolytic proteins and regulate the killing capacity of CTLs? Neither mTOR (mechanistic target of rapamycin) signaling nor activation of the integrated stress response caused loss of cytosolic translation and killing in Usp30−/− CTLs. However, there was an altered expression of metabolic enzymes that can moonlight as RNA-binding proteins (RBPs) in both USP30-depleted and doxycycline-treated CTLs. 
 
The findings suggest that posttranscriptional regulation by RBPs, a well-described phenomenon in CTLs, may mediate the selective down-regulation of protein synthesis detected upon loss of USP30.
 
This study highlighted a role for mitochondria as homeostatic regulators of CTL killing, with the production of new cytolytic proteins synchronized with mitochondrial translation. In this way, the energy requirements of protein synthesis can be fine-tuned to meet the demands of serial killing during immune challenges.
 
The World Mitochondria Society will dedicate a complete session on "Mitochondria, Immunity and Cancer" in this year's annual meeting so don't miss out.