Innate Immune cGAS/STING Signaling is Both Necessary and Pathological
Chronic, unresolved inflammation is a feature of aging. When the immune system is constantly active in this way, the consequent altered cell behavior throughout the body becomes disruptive to tissue and organ function, harmful to the individual. Chronic inflammation accelerates the onset and progression of all of the common fatal age-related conditions. This unwanted inflammatory signaling arises from many different roots, including the growing presence of senescent cells, but also the interaction of innate immune sensors with other forms of age-related dysfunction. For example, damage-associated molecular patterns such as mislocalized fragments of mitochondrial DNA leaking from dysfunctional mitochondria into the cell cytosol can trigger cGAS/STING signaling. This mechanism evolved to detect the presence of bacterial DNA, and unfortunately runs awry with age.
The challenge inherent in dampening age-related chronic inflammation is that, so far, it appears to use exactly the same pathways that are involved in the normal, necessary, short-term inflammatory response to injury, pathogens, potentially cancerous cells, and so forth. All of the approaches developed to date to suppress the overactivity of an immune system also suppress necessary functions, and that produces unpleasant long-term consequences. This is well established via the use of immunosuppressant drugs in patients with autoimmune disease. There was some hope that targeting aspects of cGAS/STING function would prove to be a better option, but as researchers note in today's open access paper, an operating cGAS/STING pathway appears to be necessary for long-term health.
STING promotes homeostatic maintenance of tissues and confers longevity with aging
Local immune processes within aging tissues are a significant driver of aging associated dysfunction, but tissue-autonomous pathways and cell types that modulate these responses remain poorly characterized. The cytosolic DNA sensing pathway, acting through cyclic GMP-AMP synthase (cGAS) and Stimulator of Interferon Genes (STING), is broadly expressed in tissues, and is poised to regulate local type I interferon (IFN-I)-dependent and independent inflammatory processes within tissues. Recent studies suggest that the cGAS/STING pathway may drive pathology in various in vitro and in vivo models of accelerated aging.
To date, however, the role of the cGAS/STING pathway in physiological aging processes, in the absence of genetic drivers, has remained unexplored. This remains a relevant gap, as STING is ubiquitously expressed, implicated in multitudinous disorders, and loss of function polymorphisms of STING are highly prevalent in the human population (an incidence of more than 50%). Here we reveal that, during physiological aging, STING-deficiency leads to a significant shortening of murine lifespan, increased pro-inflammatory serum cytokines and tissue infiltrates, as well as salient changes in histological composition and organization.
We note that aging hearts, livers, and kidneys express distinct subsets of inflammatory, interferon-stimulated gene (ISG), and senescence genes, collectively comprising an immune fingerprint for each tissue. These distinctive patterns are largely imprinted by tissue-specific stromal and myeloid cells. Using cellular interaction network analyses, immunofluorescence, and histopathology data, we show that these immune fingerprints shape the tissue architecture and the landscape of cell-cell interactions in aging tissues. These age-associated immune fingerprints are grossly dysregulated with STING-deficiency, with key genes that define aging STING-sufficient tissues greatly diminished in the absence of STING. This altered homeostasis in aging STING-deficient tissues is associated with a cross-tissue loss of homeostatic tissue-resident macrophage (TRM) populations in these tissues. Ex vivo analyses reveal that basal STING- signaling limits the susceptibility of TRMs to death-inducing stimuli and determines their in situ localization in tissue niches, thereby promoting tissue homeostasis.
Collectively, these data upend the paradigm that cGAS/STING signaling is primarily pathological in aging and instead indicate that basal STING signaling sustains tissue function and supports organismal longevity. Critically, our study urges caution in the indiscriminate targeting of these pathways, which may result in unpredictable and pathological consequences for health during aging.