Innate Immune Functions of Astrocytes are Dependent Upon Tumor Necrosis Factor-Alpha

Abstract

Acute inflammation is a key feature of innate immunity that initiates clearance and repair in infected or damaged tissues. Alternatively, chronic inflammation is implicated in numerous disease processes. The contribution of neuroinflammation to the pathogenesis of neurological conditions, including infection, traumatic brain injury, and neurodegenerative diseases, has become increasingly evident. Potential drivers of such neuroinflammation include toll-like receptors (TLRs). TLRs confer a wide array of functions on different cell types in the central nervous system (CNS). Importantly, how TLR activation affects astrocyte functioning is unclear. In the present study, we examined the role of TLR2/4 signaling on various astrocyte functions (i.e., proliferation, pro-inflammatory mediator production, regulatory mechanisms, etc) by stimulating astrocytes with potent exogenous TLR2/4 agonist, bacterial lipopolysaccharide (LPS). Newborn astrocytes were derived from WT, Tnfα −/− , Il1α −/− / Il1β −/− , and Tlr2 −/− /Tlr4 −/− mice as well as Sprague Dawley rats for all in vitro studies. LpS activated mRNA expression of different pro-inflammatory cytokines and chemokines in time-and concentrationdependent manners, and upregulated the proliferation of astrocytes based on increased 3 H-thymidine update. Following LPS-mediated TLR2/4 activation, TNF-α and IL-1β self-regulated and modulated the expression of pro-inflammatory cytokines and chemokines. Polyclonal antibodies against TNF-α suppressed TLR2/4-mediated upregulation of astrocyte proliferation, supporting an autocrine/paracrine role of tnf-α on astrocyte proliferation. Astrocytes perform classical innate immune functions, which contradict the current paradigm that microglia are the main immune effector cells of the CNS. TNF-α plays a pivotal role in the LpS-upregulated astrocyte activation and proliferation, supporting their critical roles in in cnS pathogenesis. Astrocytes are a specialized subtype of glia present throughout the central nervous system (CNS) 1. These star-shaped cells were initially described by German pathologist Rudolf Virchow as the "connective tissue of the brain". As a result of this dismissive analogy, astrocytes were long regarded as functionally insignificant 2. However, recent studies have revealed that astrocytes actually play diverse and critical roles in both CNS homeostasis and pathophysiology, many of which are currently being uncovered. Importantly, under normal circumstances, astrocytes are quiescent; however, in the presence of pathological stimuli, astrocytes re-enter the cell cycle, undergo morphological and biochemical changes, and produce pro-inflammatory cytokines 3,4. The molecular mechanisms underlying these changes remain unknown. One potential mechanism underlying this transition is the activation of toll-like receptors (TLRs). TLRs are a class of membrane-bound receptors called pattern recognition receptors (PRRs) that trigger innate immune responses. Innate immunity is a highly conserved system that is found in all animals an...

Role of astroglial toll-like receptors (TLRs) in central nervous system infections, injury and neurodegenerative diseases

“…Alternatively, it has been proposed that the cytokines originate from the contaminating microglia, whose cytokine secretion might be increased in the presence of astrocytes (Saura, 2007; Sola et al, 2002). However, recent investigations resolved this issue by using 99% pure astrocyte cultures and corroborated the finding that astrocytes release pro-inflammatory cytokines in response to stimulation of TLR (Li et al, 2020; Rodgers et al, 2020 ). Of note, even studies on microglia-depleted astrocyte cultures agree that astrocytes express functional TLRs, because they respond to TLR agonists by upregulating TLR expression ( Marinelli et al, 2015 ).…”

“…This was attributed to the upregulation of matrix metalloproteinase 9 (MMP-9), which degrades the extracellular matrix proteins and interacts with cell surface protein CD44 to facilitate the mobility of astrocytes ( Dufour et al, 2010 , Hsieh et al, 2010 ). Exposure of astrocyte cultures to the TLR2 and TLR4 agonist LPS promotes astrocyte proliferation via the release of TNF-α ( Rodgers et al, 2020 ). In contrast, TLR3 has opposite effects on astrocyte proliferation.…”

TNF Production and Release from Microglia via Extracellular Vesicles: Impact on Brain Functions

“…Similar to microglia, activated astrocytes themselves can produce and release TNF, giving rise to an autocrine/paracrine loop of activation and cytokine release [ 102 ]. However, it has been shown that astrocytes alone are not able to sustain an efficient inflammatory response, for which microglia-derived TNF and other factors are required [ 103 ].…”

“…Thus, it is likely that microglial cells, the first to be activated after injury, represent the main source of TNF in the CNS, which is released to trigger astroglial pro-inflammatory activation [ 72 , 97 ]. Reactive astrocytes, as final effector cells, gain the ability to self-maintain their pro-inflammatory state by further releasing TNF, leading to long-lasting detrimental effects [ 102 ]. To close the circle, reactive astrocytes are able to modulate microglial activity, through inhibition of microglial TNF expression and release [ 104 , 105 ].…”

Increased Expression of Granzyme B and Transforming Growth Factor-β in Intralesional Plasma of Oral Lichenoid Reactions: A Preliminary Study

“…Cytokines exert a transient immunomodulatory function at high local concentrations in autocrine or paracrine fashions [9][10][11]. They are also elevated in peripheral blood in endocrine fashion in some in ammatory diseases [12].…”