Periodic Reporting for period 3 - ENVISION (Novel mechanisms of early defense against virus infections)
Periodo di rendicontazione: 2021-12-01 al 2023-05-31
The innate immune system represents the first line of defense against infections, and in the case of virus infections, it is believed that the type I interferon (IFN) represents the initial response. However, IFN can also be pathological, and it can seem paradoxical, to have a potentially damaging system as the one activated most frequently. Therefore, the purpose of the project is to explore whether there are innate immune mechanisms which work indepoendet of the IFN system to enable host defense reactions that control viruses but have less inflammatory activity. Identification of such novel immune mechanisms would potentially provide a knowledge basis for novel antiviral therapies. The project is divided into four sections, each exploring one aspect of early innate antiviral defense: (i) identification of mechanisms of immune sensing of viruses at epithelial surfaces; (ii) ixploration of the importance and mechanisms of action of tonic IFN signaling, (iii) identification of novel restriction factors; (iv) identification of mechanisms initiating IFN exprssion during viral infection, including the possible interaction with constitutive immune mechanisms.
The purpose of ENVISION is to explore the existence of novel innate antiviral mechanisms working infependent of pattern recognition receptors and the type I interferon system. In the first 2.5 years of the project we have been able to both identify novel innate immune mechanisms, and to propose that these mechanisms in fact represent what may be a new layer of the immune system. Some of the identified mechanisms have been published, and several more are currently being characterized, including their mechanism of action and physiological importance.
Aim 1. We have demonstrated that Viral replication at epithelial surfaces (HSV2 and SARS-CoV2) activates autophagic responses as well ac activationo the stress-sensing Nrf2 pathway, and that both of these homeostasis-maintaining systems have potent antiviral activity independent of type I IFN. These discoveries represent the first indication that the underlying hypothersis in ENVISION is correct.
Aim 2. This part of the project is still very much in the development stage, but significant progress has been made. For instance, from our work on exosomes in driving interferon expression, we have found that cellular exposure to bacteria, including commensal bacteria leads to secretion of exosomes containing DNA wihch instigate a low-grade interferon response in adjacent cells. We propose this contributes to the mechanism underlying the phenomenon of tonic IFN expression. At the level of action, we have now strong data showing that epithelial cells constitutively express high levels of the transcription factor IRF1, which drives expression of a subset of IFN-stimulated genes. Importantly, depletion of IRF1 decreases expression of many of these IFN-stimulated genes, and increases susceptibility to infections with a panel of viruses.
Aim 3. Through genome-wide CRISPR screens, we have identified a number of candidate restriction factors against HSV. For one of these, we have also identified HSV encephalitis patients with loss-of-function mutations. We have now identified the mechanisms of antiviral action of this restriction factor. Mice deficient in the restriction factor are highly susceptible to HSV1 infection in the brain. These data provides a strong support of the idea of constitutive immune mechanisms being important for defense against infections.
Aim 4. This part of the project seeks to identify mechanisms through which epithelial initially are restricted in their prodution of type I IFN, and how this regulation is lifted upon infection. We have focused on the cGAS-STING pathway; first, because HSV1 potently activated this pathway, and second because we have generated significant new knowledge on how the pathway works.
Aim 1. We have demonstrated that Viral replication at epithelial surfaces (HSV2 and SARS-CoV2) activates autophagic responses as well ac activationo the stress-sensing Nrf2 pathway, and that both of these homeostasis-maintaining systems have potent antiviral activity independent of type I IFN. These discoveries represent the first indication that the underlying hypothersis in ENVISION is correct.
Aim 2. This part of the project is still very much in the development stage, but significant progress has been made. For instance, from our work on exosomes in driving interferon expression, we have found that cellular exposure to bacteria, including commensal bacteria leads to secretion of exosomes containing DNA wihch instigate a low-grade interferon response in adjacent cells. We propose this contributes to the mechanism underlying the phenomenon of tonic IFN expression. At the level of action, we have now strong data showing that epithelial cells constitutively express high levels of the transcription factor IRF1, which drives expression of a subset of IFN-stimulated genes. Importantly, depletion of IRF1 decreases expression of many of these IFN-stimulated genes, and increases susceptibility to infections with a panel of viruses.
Aim 3. Through genome-wide CRISPR screens, we have identified a number of candidate restriction factors against HSV. For one of these, we have also identified HSV encephalitis patients with loss-of-function mutations. We have now identified the mechanisms of antiviral action of this restriction factor. Mice deficient in the restriction factor are highly susceptible to HSV1 infection in the brain. These data provides a strong support of the idea of constitutive immune mechanisms being important for defense against infections.
Aim 4. This part of the project seeks to identify mechanisms through which epithelial initially are restricted in their prodution of type I IFN, and how this regulation is lifted upon infection. We have focused on the cGAS-STING pathway; first, because HSV1 potently activated this pathway, and second because we have generated significant new knowledge on how the pathway works.
The results obtained so far support the hypothesis that the interferon system is in fact not the first line of defense, but rather there exists a layer of constitutive and latent immune mechanisms, which exert the initial defense actions. These mechanisms are specific, and do not induce inflammatory activity. Based on the ongoing activities in the ENVISION project, I expect that we will be able to obtain further results supporting this idea/concept.