Chronic stress is a risk factor for the development of multiple psychiatric disorders for which current treatments are inadequate. Evidence from our laboratory and others suggest that chronic stress can also provoke elevated inflammation and exaggerated inflammatory responses in both humans and animal models. However, the underlying mechanisms are not well understood. Mitochondria become damaged and dysfunctional following chronic stress conditions raising the question of whether neuroinflammation associated with chronic stress-related neuropsychiatric conditions is due to mitochondria-induced inflammation. By removing damaged mitochondria, mitophagy plays a central role in preventing inflammation. When this process is impaired, mtDNA is released and the extracellular cell free mtDNA (cf-mtDNA) promotes Toll-like receptor 9 (TLR9) signaling to activate immune system. Our recent study found that mice exposed to chronic restraint stress (RS) exhibit neuroinflammation and social behavior deficits. Our preliminary data show that depletion of cf-mtDNA with DNase I treatment attenuates RS-induced deficits in social behavior and increases in proinflammatory markers in the prefrontal cortex (PFC). Furthermore, we found that mtDNA-induced deficits in social behavior are TLR9-dependent. Also, RS induced significant increases in TLR9 expression in microglia and neurons. Among the various mitophagy-related molecules, activation of Mitochondrial antiviral-signaling protein (MAVS, a mitochondrial adaptor protein)signaling results in proinflammatory signaling. Our preliminary findings showed that RS-induced increase in cf-mtDNA is attenuated in MAVS KO mice. Also, we found impaired mitophagy in CD4+, but not CDS+ T cells following RS. We hypothesize that increased levels of mtDNA from CD4• T cells contribute to RS-induced neuroinflammation and social behavior deficits via TLR9 activation. Aim 1 will determine whether RS-induced mtDNA release drives neuroinflammation and reduced social behavior. Aim 2 will examine whether mtDNA-mediated activation of TLR9 on neurons promotes social behavior deficits following RS. Aim 3 will examine whether mtDNA release induced by RS is dependent on mitophagy. If successful, our project will create new developments in understanding the mechanism mediating stress-induced neuroinflammation social behavior deficits, and thereby allow the development of pharmacological approaches to inhibit mtDNA release, neutralize extracellular cf-mtDNA, or inhibit TLR9 activation in stress-related mental health disorders.