Crimean-Congo hemorrhagic fever virus : interactions with host cell structures in viral replication

Abstract

Crimean-Congo hemorrhagic fever virus (CCHFV) is a human pathogen, and was first associated with its corresponding disease during an outbreak in the mid 1940s. At that time, outbreaks were mainly restricted to the former Soviet Union, Africa and Asia, but lately disease incidents have markedly risen and CCHFV is now also endemic in several countries in the Middle East and Europe. The prevalence of CCHFV is intimately associated with the geographic occurrence of its tick vector, and is the most widely spread tick-borne virus of medical importance. The disease caused by CCHFV is manifested by severe hemorrhage that occasionally causes fatalities. There is currently no cure for CCHF. Antivirals are routinely used but treatment is mainly supportive. Although research in the last decade has significantly contributed to our current understanding, much remains before we can fully elucidate the mechanisms underlying CCHF pathogenesis.

An important feature of a productive virus infection is to access the cell interior where viral replication occurs. This first step is accomplished by binding to a virus-specific receptor on the host cell surface. For CCHFV the receptor is presently not known. In the subsequent internalization process, our investigations show that CCHFV enters cells by clathrin-dependent endocytosis. We also found that CCHFV infection was impaired when cells were depleted of cholesterol. This suggests that CCHFV entry could also be mediated by specialized plasma membrane microdomains, enriched in cholesterol.

Following internalization viruses must reach the appropriate intracellular sites where replication can be initiated. Because the cytoplasm is dense and does not admit long-range diffusion, intracellular transport is mediated by microtubules or actin filaments and their associated motors. We showed that intact microtubules were important in the early events following CCHFV internalization and later, in viral replication. In the process of viral transcription, however, neither intact nor dynamic microtubules were required. In the late stages of CCHFV replication cycle progeny virus release was markedly reduced when microtubules were perturbed, suggesting that microtubules are important in viral assembly and/or egress. Besides their involvement in cargo trafficking, actin filaments are also important in the uptake and release of molecules into and out from cells. Our investigations of the role of actin filaments in the life cycle of CCHFV show that virus proteins interact directly with actin and that actin is important in their intracellular positioning. Moreover, we show that extracellular progeny virus is markedly reduced when actin filaments are depolymerized early in infection, suggesting that actin may be involved in several steps of CCHFV replication cycle.

Interaction with the cytoskeletal actin filaments and microtubules is also demonstrated for different nitric oxide synthases, the enzymes responsible for nitric oxide production. Nitric oxide is a biologically active molecule that functions as a potent signaling molecule and is critically involved in the unspecific host defense against pathogens. In virus infections, nitric oxide is known to inhibit replication. Accordingly we show that CCHFV viral RNA is reduced when infection is performed in the presence of a nitric oxide donor. Consequently, virus proteins and progeny virus titers are also reduced.

Although much remains to be explored, this thesis has provided data that will further our understanding of CCHFV interactions with host cell structures, which may be of importance for future antiviral and vaccine design.