Modulation of immune responses by opioids through toll-like and P2X receptor signaling in COVID-19

The pathogenesis of COVID-19 involves complex interactions between viral replication and host immune dysregulation, mediated by toll-like receptors (TLRs) and P2X receptors (P2XRs). These receptors detect pathogen- and damage-associated molecular patterns (PAMPs and DAMPs), triggering inflammatory cascades. Opioids, beyond their analgesic role, modulate innate and adaptive immune responses via opioid receptors and indirectly through TLR and P2X signaling. This narrative review integrates experimental, clinical, and bioinformatic evidence to explore the mechanistic crosstalk between opioid signaling, TLRs (notably TLR2, TLR4, TLR9), and P2XRs (notably P2X4 and P2X7) in COVID-19 immunopathology. Chronic opioid exposure may either enhance or suppress inflammation depending on dose, duration, and immune context. In COVID-19, the hyperactivation of TLR4, TLR7, and TLR9 drives cytokine storms, while the release of ATP from damaged cells activates P2X7, thereby amplifying the inflammatory response. ATP breakdown into adenosine further modulates immunity via A2A and A2B receptors. Targeting TLR4 and P2X7 offers a promising therapeutic strategy to mitigate hyperinflammation and improve outcomes in COVID-19 and related inflammatory diseases. In addition, we outline how the Contact System, the Kallikrein-Kinin System (KKS), the Renin-Angiotensin System (RAS), and the NLRP3 inflammasome provide the innate inflammatory backdrop through which opioids and P2 receptor signaling may shape immune dysregulation in COVID-19. Notably, direct clinical evidence for opioid-P2 receptor interactions in COVID-19 remains limited, highlighting the need for targeted translational studies.