Poxvirus tropism

Abstract

Despite the success of the WHO-led smallpox eradication programme a quarter of a century ago, there remains considerable fear that variola virus, or other related pathogenic poxviruses such as monkeypox, could re-emerge and spread disease in the human population. Even today, we are still mostly ignorant about why most poxvirus infections of vertebrate hosts show strict species specificity, or how zoonotic poxvirus infections occur when poxviruses occasionally leap into novel host species. Poxvirus tropism at the cellular level seems to be regulated by intracellular events downstream of virus binding and entry, rather than at the level of specific host receptors as is the case for many other viruses. This review summarizes our current understanding of poxvirus tropism and host range, and discusses the prospects of exploiting host-restricted poxvirus vectors for vaccines, gene therapy or tissue-targeted oncolytic viral therapies for the treatment of human cancers.

Figure 1. Examples of host-restricted poxviruses.

Some poxviruses, like variola major (smallpox) of humans (a), ectromelia virus (mousepox) of mice (b) or camelpox virus of camels (c) remain largely restricted to one host species and rarely, if ever, cause zoonotic infections outside of that species. Other poxviruses (Table 1) can infect multiple zoonotic host species. Part a is reproduced with permission from the WHO web site (see the Online links box); part b is reproduced with permission from Ref. © (1982) Academic Press; part c was kindly provided by U. Wernery (United Arab Emirates) and H. Meyer (Germany).

Figure 2. All poxviruses are morphologically similar.

Electron microscopic images reveal that poxviruses share common features of size and shape. For example, vaccinia virus (a; image courtesy of CDC) can infect a broad range of hosts but is very similar in size, shape and morphology to poxviruses (intracellular mature virus (IMV) forms) that are highly host restricted, such as molluscum contagiosum virus (b; image courtesy of CDC/Fred Murphy/Sylvia Whitfield), which has only been shown to infect man and replicates exclusively in human basal keratinocytes.

Figure 3. Poxvirus replication cycle.

All poxviruses replicate in the cytoplasm of infected cells by a complex, but largely conserved, morphogenic pathway. Two distinct infectious virus particles ? the intracellular mature virus (IMV) and the extracellular enveloped virus (EEV) ? can initiate infection. The IMV and EEV virions differ in their surface glycoproteins and in the number of wrapping membranes^,,. The binding of the virion is determined by several virion proteins and by glycosaminoglycans (GAGs) on the surface of the target cell or by components of the extracellular matrix. Fully permissive viral replication is characterized by three waves of viral mRNA and protein synthesis (known as early, intermediate and late), which are followed by morphogenesis of infectious particles. The initial intracellular mature virus (IMV) is transported via microtubules (not shown in the figure) and is wrapped with Golgi-derived membrane, after which it is referred to as an intracellular enveloped virus (IEV). The IEV fuses to the cell surface membrane to form cell-associated enveloped virus (CEV; not shown), which is either extruded away from the cell by actin-tail polymerization (not shown) or is released to form free EEV. EEV might also form by direct budding of IMV, therefore bypassing the IEV form. Poxviruses also express a range of extracellular and intracellular modulators, some of which are defined as host-range factors that are required to complete the viral replication cycle. Poxviruses can be markedly diverse in their portfolio of specific modulators and host-range factors, which determine tropism and host range. Non-permissive poxvirus infections generally abort at a point downstream of the binding/fusion step.

Figure 4. Intracellular signalling events modulate poxvirus tropism.

A comparison of the infection of primary murine embryo fibroblasts (pMEFs) by two poxviruses, one of which (myxoma virus) is non-permissive because it is prematurely aborted by an induced type-I interferon response, whereas the other (vaccinia virus) is fully permissive. Both infections are characterized by the induced activation of MEK1,2, which then phosphorylates extracellular signal-regulated kinase (ERK)1,2. However, in the non-permissive infection by myxoma virus, phosphorylated ERK1,2 remains in the cytoplasm where it induces the activation of interferon regulatory factor 3 (IRF3), which then migrates to the nucleus where it initiates the transcriptional upregulation of β-interferon (IFN-β). In the case of the permissive infection by vaccinia virus, the activated ERK1,2 migrates to the nucleus where it activates ELK1 but does not activate IRF3 or the interferon genes. Inhibitors of ERK1,2 activation, such as U0126, render pMEFs permissive for replication of myxoma virus but, in contrast, inhibit the replication of vaccinia^,,.

Figure 5. Origin of modified vaccinia Ankara strain.

Modified vaccinia Ankara (MVA) was derived from chorioallantois vaccinia Ankara (CVA), which is a smallpox vaccine strain that was used originally in Turkey and that was adapted for growth in chicken cells. After more than 500 passages in chicken embryo fibroblasts (CEFs), the CVA strain lost more than 30 kb of viral sequences that mapped to six main sites (denoted I–VI) and lost the ability to replicate in almost all mammalian cells, including human and rabbit kidney (RK13) cells, but was permissive for chicken embryo fibroblasts and baby hamster kidney (BHK) cells. MVA that is engineered to express the vaccinia K1L host-range gene regained the ability to replicate in RK13 cells, but not in human cells.