The Dermody lab recently initiated studies of the Chikungunya virus (CHIKV), an emerging mosquito-borne alphavirus responsible for major outbreaks of polyarthralgia in countries of the Indian Ocean region and the Western hemisphere (image below). This important human pathogen causes an acute febrile illness characterized by a maculopapular rash and incapacitating arthralgia, which can evolve into chronic arthritis. CHIKV, like other alphaviruses, is transmitted through the bite of an infected mosquito. Little is known about how this virus interacts with its hosts and the factors that govern tissue tropism and pathogenesis. Using a multidisciplinary approach, we are investigating mechanisms of CHIKV cell entry, replication, and pathogenesis, with the aim of better understanding the viral and host factors that contribute to CHIKV virulence, which will inform strategies for the development of antiviral therapeutics and vaccines.
Global distribution of CHIKV and characteristic disease symptoms of CHIKV-infected individuals. (A) Countries and territories where chikungunya cases have been reported (as of April 22, 2016). (B) Acute phase-associated maculopapular lesions in a CHIKV-infected patient (top) and chronic stage symptoms of arthritis in a 55-year-old man infected 5 years earlier (bottom).
Cell entry of CHIKV
Engagement of the host cell surface by a virus is the initial step in viral infection and a critical determinant of viral tropism. Although CHIKV replicates in invertebrate and vertebrate cells, attachment receptors for either host have not been reported. Like other alphaviruses, CHIKV binds and infects many cell types, suggesting that CHIKV utilizes a highly conserved, ubiquitous receptor found in mosquitoes and mammals or that multiple receptors are utilized. To understand the host range, tissue tropism, and virulence of this virus, it is critical to elucidate molecular mechanisms of CHIKV attachment and cell entry. We have identified cell-surface glycosaminoglycans (GAGs) as important attachment factors for CHIKV infection. We are now conducting experiments to further define virus-GAG interactions by identifying glycans to which CHIKV binds and viral determinants required for GAG-binding. Future studies will investigate how virus-glycan interactions influence viral replication, tropism, and pathogenesis.
CHIKV replication cycle in mammalian cells. The replication cycle includes attachment, entry, fusion and uncoating, translation of nonstructural proteins, formation of replication compartments (spherules), negative-sense RNA synthesis, genomic and subgenomic RNA synthesis, translation of structural proteins, assembly, and budding.
All viruses, including alphaviruses, require host cell factors to enter, replicate in, and exit infected cells. Moreover, viruses must employ immune evasion strategies to allow replication in the host. In the case of CHIKV infection of humans, the immune response constitutes a major component of disease pathogenesis. Therefore, the identification of pro- and anti-viral host factors will reveal interactions essential for CHIKV replication and disease. Candidate cellular determinants of CHIKV virulence will be identified using high-throughput, genome-wide RNAi screening. Candidate genes that are confirmed following a second screen will be further validated using dominant-negative mutants and pharmacological inhibitors. Steps in viral replication at which the host cell genes function, along with the biochemical activities that promote infection, will be identified. This work will illuminate new targets for CHIKV therapeutics
CHIKV virulence factors
The host and viral determinants that induce CHIKV pathogenesis are not fully understood. Importantly, we still do not know whether pathology observed in those experiencing CHIKV symptoms can be contributed to viral replication at sites of tissue injury or the inflammatory immune response. However, the extent of CHIKV replication in affected tissues of experimentally infected animals does not strictly correlate with the magnitude of acute disease or chronic persistence of viral RNA. A number of specific cytokines and chemokines are elicited during infection and are critical mediators of CHIKV pathogenesis.
Model of acute and chronic CHIKV pathogenesis. CHIKV is transmitted by the bite of an infected mosquito and results in replication of CHIKV at primary and subsequent secondary sites within the body, which causes acute inflammation and polyarthralgia. Chronic CHIKV disease can persist for months or years after acute infection and is likely mediated by a persistent virus at discrete sites and inflammatory immune responses.
Although CHIKV is able to infect numerous tissues within the infected host, the specific cellular targets of infection that influence the induction of the proinflammatory immune response in vivo remain undefined. We are currently working to identify the discrete cell types targeted by CHIKV in infected tissues in the host that are responsible for eliciting immunopathology during acute and persistent phases of infection. These experiments will elucidate specific cellular mediators of CHIKV disease and provide an enhanced understanding of mechanisms of CHIKV pathogenesis and persistence, which will ultimately inform rational vaccine design and immunomodulatory therapeutics for CHIKV.