Extracellular mitochondria drive CD8 T cell dysfunction in trauma by upregulating CD39

Abstract

RationaleThe increased mortality and morbidity seen in critically injured patients appears associated with systemic inflammatory response syndrome (SIRS) and immune dysfunction, which ultimately predisposes to infection. Mitochondria released by injury could generate danger molecules, for example, ATP, which in turn would be rapidly scavenged by ectonucleotidases, expressed on regulatory immune cells.ObjectiveTo determine the association between circulating mitochondria, purinergic signalling and immune dysfunction after trauma.MethodsWe tested the impact of hepatocyte-derived free mitochondria on blood-derived and lung-derived CD8 T cells in vitro and in experimental mouse models in vivo. In parallel, immune phenotypic analyses were conducted on blood-derived CD8 T cells obtained from trauma patients.ResultsIsolated intact mitochondria are functional and generate ATP ex vivo. Extracellular mitochondria perturb CD8+ T cells in co-culture, inducing select features of immune exhaustion in vitro. These effects are modulated by scavenging ATP, modelled by addition of apyrase in vitro. Injection of intact mitochondria into recipient mice markedly upregulates the ectonucleotidase CD39, and other immune checkpoint markers in circulating CD8+ T cells. We note that mice injected with mitochondria, prior to instilling bacteria into the lung, exhibit more severe lung injury, characterised by elevated neutrophil influx and by changes in CD8+ T cell cytotoxic capacity. Importantly, the development of SIRS in injured humans, is likewise associated with disordered purinergic signalling and CD8 T cell dysfunction.ConclusionThese studies in experimental models and in a cohort of trauma patients reveal important associations between extracellular mitochondria, aberrant purinergic signalling and immune dysfunction. These pathogenic factors with immune exhaustion are linked to SIRS and could be targeted therapeutically.

Mitochondria in diseases: from cell power plants to DAMPs

“…Indeed, the proportion of exhausted T cells (CD39 + or PD-1/Tim3 + ) was increased in the peripheral blood of trauma patients compared with control subjects. Moreover, increased levels of mitochondrial DNA were detected in the plasma of trauma patients compared with control subjects and correlated with the proportion of peripheral CD39 + CD8 T cells 3. The proportion of exhausted CD8 T cells seems to be further increased in patients developing SIRS compared with trauma patients without SIRS.…”

“…In mice inoculated with mitochondria, the proportion of CD8 T cells positive for CD39 also increased and these cells were anti-inflammatory as seen by the increased proportion of IL-10 + cells and the decreased proportion of granzyme B + cells. Along with CD39, this subpopulation of CD8 T cells exhibits an increased expression of cell exhaustion markers, PD-1 and Tim-3 3. Thus, mitochondria, through purinergic signalling, may induce an immunoregulatory effect and CD8 T cells exhaustion.…”

“…Among DAMPs, mitochondria and mtDNA have generated scientific interest as they provoke a vigorous response and seem to trigger severe illnesses in humans, including SIRS. In their Thorax paper, Tiwari-Heckler et al have developed an elegant study to understand how this essential organelle participates in the immune response observed during SIRS 3…”

“…Hence, free mitochondria are defined as DAMPs and seem to act as producers of extracellular DAMPs. In the literature, the release of mitochondria through different forms (eg, free mtDNA, functional mitochondria or encapsulated within extracellular vesicles) has been shown to play an active role in health and diseases 3. Comprehending which forms of extracellular mitochondria induce proinflammatory responses or immune regulatory response is key to developing therapeutics for potential medical applications 4…”

“…Paradoxically, CD8 T cells, despite expressing exhaustion markers, were functional as they expressed granzyme B in mice treated with bacteria with or without mitochondria. However, in mice treated with mitochondria and bacteria, CD8 T cells seem to contribute to the proinflammatory environment in lung tissue by downregulating the anti-inflammatory cytokine IL-10 3. These interesting results raise questions about the effector functions of T cells.…”