RNA sensing via the RIG-I-like receptor LGP2 is essential for the induction of a type I IFN response in ADAR1 deficiency.

RNA editing by the adenosine deaminase ADAR1 prevents innate immune responses to endogenous RNAs. In ADAR1-deficient cells, unedited self RNAs form base-paired structures that resemble viral RNAs and inadvertently activate the cytosolic RIG-I-like receptor (RLR) MDA5, leading to an antiviral type I interferon (IFN) response. Mutations in ADAR1 cause Aicardi-Goutières Syndrome (AGS), an autoinflammatory syndrome characterized by chronic type I IFN production. Conversely, ADAR1 loss and the consequent type I IFN production restricts tumor growth and potentiates the activity of some chemotherapeutics. Here, we show that another RIG-I-like receptor, LGP2, also has an essential role in the induction of a type I IFN response in ADAR1-deficient human cells. This requires the canonical function of LGP2 as an RNA sensor and facilitator of MDA5-dependent signaling. Furthermore, we show that the sensitivity of tumor cells to ADAR1 loss requires LGP2 expression. Finally, type I IFN induction in tumor cells depleted of ADAR1 and treated with some chemotherapeutics fully depends on LGP2 expression. These findings highlight a central role for LGP2 in self RNA sensing with important clinical implications.

CD200R1L is a functional evolutionary conserved activating receptor in human neutrophils.

Inhibitory and activating immune receptors play a key role in modulating the amplitude and duration of immune responses during infection and in maintaining immune balance in homeostatic conditions. The CD200 Receptor (CD200R) gene family in humans encodes one inhibitory receptor, CD200R1, and one putative activating member, CD200R1 Like (CD200R1L). It is demonstrated that CD200R1L is endogenously expressed by human neutrophils and activates cellular functions such as reactive oxygen species (ROS) production via Syk, PI3Kß, PI3Kδ, and Rac GTPase signaling. Phylogenetic analysis shows that CD200R1L is present in many species among vertebrates, ranging from birds to primates, suggesting that evolutionary conservation of this receptor is critical for protection against co-evolving pathogens. The duplication event that generated CD200R1L from CD200R occurred several times throughout evolution, supporting convergent evolution of CD200R1L. In our phylogenetic trees, CD200R1L has longer branch lengths than CD200R1 in most species, suggesting that CD200R1L is evolving faster than CD200R1. It is proposed that CD200R1L represents a hitherto uncharacterized activating receptor on human neutrophils.

Self RNA Sensing by RIG-I-like Receptors in Viral Infection and Sterile Inflammation.

The innate immune system uses pattern recognition receptors to survey the intracellular and extracellular environment for signs of infection. Viral infection is detected through the presence of viral nucleic acids in infected cells. Pattern recognition receptor activation by viral nucleic acids induces the expression and secretion of type I IFNs (IFN-Is), important mediators of antiviral immunity. RIG-I-like receptors (RLRs) are RNA sensors that detect viral RNA in the cytosol and induce an IFN-I response. Viral RNAs contain features that set them apart from host RNAs, allowing RLRs to discriminate between cellular/self and viral/non-self RNA. The notion emerged that self RNAs can also engage RLRs and modulate the IFN-I response, indicating that the distinction between self and non-self RNA is not watertight. We review how self RNAs regulate RLR activation and the IFN-I response during viral infection and how recognition of self RNAs by RLRs is implicated in autoinflammatory disorders and cancer.

Can Patrolling Liver-Resident T Cells Control Human Malaria Parasite Development?

Recently, a population of non-recirculating, tissue-resident memory CD8+ T cells has been identified; cells that seems to act as key sentinels for invading microorganisms with enhanced effector functions. In malaria, the liver represents the first site for parasite development before a definite infection is established in circulating red blood cells. Here, we discuss the evidence obtained from animal models on several diseases and hypothesize that liver-resident memory CD8+ T cells (hepatic TRM) play a critical role in providing protective liver-stage immunity against Plasmodium malaria parasites. Although observations in human malaria trials are limited to peripheral blood, we propose recommendations for the translation of some of these findings to human malaria research.