Neuroimmunomodulatory balance between pro and anti‐inflammatory cytokines regulate m‐CoV induced alteration of gap junction protein Cx43

Mouse hepatitis virus (MHV), a murine β-CoV, is an experimental model used to understand the viral-induced acute neuroinflammation and chronic progressive demyelination, which are the characteristic hallmarks of the human neurological disease Multiple Sclerosis. MHV induced neuroinflammation provides an excellent cause-effective platform to understand how the virus can initiate neuroinflammation by causing direct neuroglial cell dystrophy, leading to chronic progressive myelin damage with or without concurrent axonal loss. Previous studies have postulated that a significant nexus between host proteins is also involved in regulating host response to viral replication. One such protein is a transmembrane protein connexin 43 (Cx43) belonging to the gap junction family. Cx43 is one of the most abundant gap junction proteins in astrocytes responsible for metabolic coupling with other CNS cells to maintain homeostasis. MHV infection in mice results in a significant decrease in the expression of Cx43 during the acute stage (day 5-6) of MHV infection when viral titer reaches its peak and the brain presents with acute encephalitis characterized by perivascular cuffing and microglial nodule formation. Surprisingly, Cx43 returns to normal levels as infectious viral particles go below the detection limit by day 10 p.i. which marks the transition of innate immune responses to adaptive immunity. Interestingly, this reduced expression of Cx43 at Day 5 accords with increased pro-inflammatory cytokines, oxidative stress, and activation of UPR pathway proteins. This study investigated the transcriptional regulation of TNF-α, Il-6, IL-10, IL-1β and TGF-β in the alteration of Cx43 expression and trafficking during day 5/6 and day 10 post MHV infection. TNF-α, IL-10, IL-6,IL-1β are the major inflammatory cytokines upregulated during the acute stage of MHV infection (5-6 p.i.). Additionally, TNF-α and IL-1β are known to be negative regulators of Cx43 expression and could be attributed to the decrease in Cx43 observed in our studies. Furthermore, our studies also revealed that in MHV infection, apart from the very high expression of IL-10 another anti-inflammatory cytokine TGF-β expression increases when proinflammatory milieu of cytokines shifts towards the anti-inflammatory condition in the inflamed brain. As TGF-β is a positive regulator of Cx43, it may be responsible for the restoration of Cx43 expression and its trafficking to the cell surface resulting in functional gap junctional communication. This re-establishment of intercellular communication maybe essential to bring back the homeostasis in the inflamed brain. The balance between pro-inflammatory cytokines and the anti-inflammatory mediators may regulate the host response to counteract the viral infection by altering the gap junctional communication. This study thus presents crucial evidence to support the nexus between immune inflammatory mediators and host regulatory responders in MHV induced neuroinflammation.