SAN DIEGO—Despite a global slowdown in Zika infections, researchers are still working to better understand the complicated mechanisms of the virus. Researchers at the University of California, San Diego (UC San Diego)—using a technique to label, or tag, infected and non-infected cells in a culture—have revealed the key mechanisms the Zika virus (ZIKV) uses to outsmart the immune system. Their findings, recently published in the journal Proceedings of the National Academy of Sciences, illuminate how Zika is able to stop macrophages from performing their key functions for immune cell recruitment and antiviral defense.
Zika has proven to be much more effective at penetrating the body’s natural barriers against infections than other viruses, primarily because of its effect on an immune cell type called a macrophage. Researchers knew that macrophages are a key part of the innate immune system system that protects us from viral infections, but wanted to better understand how Zika successfully infects those cells that normally kill viruses.
“How Zika virus (ZIKV) is able to infect different cell types and cause human disease/ pathology is not well understood,” according to Dr. Aaron Carlin, an associate physician at the UC San Diego School of Medicine. “Flaviviruses, like ZIKV, are known to infect innate immune cells (macrophages and dendritic cells) during infection. Human and animal studies of ZIKV show that ZIKV infects macrophages in brain and placenta that are potentially important for human disease. We wanted to know how ZIKV infects these cells and how this might contribute to ZIKV disease.”
Derived from the Greek term for “big eaters,” macrophages are a type of white blood cell that engulfs and digests cellular debris, foreign substances, microbes, cancer cells and anything else that doesn’t make the proper proteins consistent with healthy body cells. Macrophages typically float throughout the bloodstream and when a virus invades, they flock to the site of the disease to fight it. This is not, however, what happens when Zika enters the body, researchers found.
“Our method of FACS isolation of ZIKV-positive and ZIKV-negative bystanders cells for transcriptome and epigenetic analysis was novel,” says Carlin. “Bioinformatically comparing the two groups allowed us to quickly identify the key genes and pathways that are regulated by the virus.”
By tagging infected and non-infected cells, and sorting them to study their behavior, they found that ZIKV uses two mechanisms for suppressing gene production in cells. According to the published paper, ZIKV infection causes both targeted suppression of type I interferon responses (commonly known to trigger immune response) and general suppression by reducing RNA polymerase II protein levels and DNA occupancy.
“Our data clearly showed that in human macrophages the dominant mechanism is ZIKV degradation of STAT2 consistent with work from Adolfo Garcia-Sastre and others,” adds Carlin. “What was unexpected was that we found that ZIKV infection also leads to a suppression of RNApol2 genome occupancy, or general gene transcription. This loss was most notable at genes involved in maintaining or defining the core identity of a macrophage.”
The study will allow researchers to explore the design of inhibitors that could block STAT2 degradation, thereby restoring antiviral interferon responses that would clear the virus. They are also currently determining if ZIKV causes a loss in cellular identity in other cell types including neural stem cells, and if so, how? If the mechanism allows ZIKV to deprogram neural stem cells, perhaps this is the reason ZIKV causes microcephaly. If that is the case, then therapeutics that block this deprogramming could stop the devastating effects of Zika on the brain. Researchers are optimistic that their mechanism of separating cells, and the uncovered disruption tactics of ZIKV, will be of therapeutic value for many other diseases in the future.
“Our technique and analysis approach should be broadly applicable to the study of host-pathogen interactions,” asserts Carlin. “Other viruses, like influenza, chikungunya and herpes simplex virus type I, target RNApol2 in order to disrupt transcription. It is unknown if interference of RNApol2 by these viruses leads to preferential loss of expression of core transcription factors/genes. In addition, some studies have used ZIKV as a therapeutic to target and destroy glioblastoma. It is possible that this same mechanism of loss of expression of critical core genes may also be important for why ZIKV can kill these cancer cells.”