VCKBP

Lalani et al reported later that M-T7 binds chemokines forming a complex that can be crosslinked and detected with specific antibodies. M-T7 was therefore designated vCKBP-1. This finding was unexpected and is a unique propierty of the IFN-yR encoded by MV since the IFN-yR homolog encoded by W does not bind chemokines. M-T7 and its closely related counterpart from SFV, S-T7, are the only members of the vCKBP-1 family. 

Poxvirus genomes encode a second class ofvCKBPs, known as vCKBP-2, that inhibit CC chemokines by a difierent mechanism from that of the MV M-T7 protein. Their identification and initial characterization was reported independently by three groups.Two different strategies were used to identify the chemokine binding activities and map them to a sprecific viral gene. 

This 35-kDa protein was confirmed as the vCKBP by pierforming the same assays with two recombinant Ws, W RPVA35 and W Lister A35 from which the fully functional gene had been inactivated by insertion of the LacZ gene. Neither of these viruses produced a secreted protein that bound to I-chemokine. Furthermore, when the W Lister 35-kDa ORF was expressed with a G-terminal 6xHis-tag in the baculovirus system, or as an Fc fusion protein from stably transfected mammalian cells, binding of the recombinant products to... 

I-CCL5 could be detected. The MV M-Tl and SFV ST-1 ORFs show 70% amino acid identity to each other and 40% identity to the W Lister and RPV 35-kDa gene products. Expression of the M-Tl ORF from W WR, which encodes no vCKBP activity, demonstrated that M-Tl encodes the leporipoxvims vCKBP-2. ... 

The high-affinity binding by vCKBP-2 of all CC, but not CXC, C or CX3C chemokines, is a unique property that distinguishes it from cellular CC chemokine receptors, which bind only a subset of CC chemokines with high affinity. The amino acid residues of human CCL2 that are required for high affinity interaction with the W 35 kDa vCKBP-2 have been recently... 

Disruption of either chemokine-receptor or chemokine-GAG complex formation might inhibit chemokine biological activity. It was of interest to determine whether the binding of CC chemokines by vCKBP-2 precluded their interaction with cellular receptors, GAGs or both. 

Preincubation of I-CCL3 with various concentrations of the GAGs heparin and heparan sulphate, up to 100 pg/ml, prior to binding to the W Lister vCKBP-2 revealed that GAGs did not interfere with the vCKBP-chemokine interaction. In contrast, pretreatment of human... 

RPV and camelpox virus inhibited the binding of chemokine to receptors on human U937 and THP-1 cells. The EV vCKBP-2 also inhibited the interaction of I-CCL5 with receptors on U937 cells.In contrast, the binding of I-CXCLl to receptors on U937 cells was not inhibited by vCKBP-2 from CPV BR, W Lister, RPV or camelpox vitus. ... 

The expression at early and late times post-infection of an abundantly secreted vCKBP-2 might be expected to make a significant contribution to virus virulence in vivo by inhibiting CC-chemokine-mediated host inflammatory responses. Surprisingly, the experimental evidence currently available indicates that vCKBP-2 has only marginal effects on poxvirus virulence. 

In the earliest study,intranasal inoculation of BALB/c mice with a mutant RPV A35 in which the vCKBP-2 gene has been replaced by the LacZ gene had little difference in disease progression or mortality as compared to wild type RPV. Inoculation of rabbits with 500 PFU... 

Figure 3- Structure of the CPV vCKBP-2. Ribbon diagrams. A and B are related by 90" rotation. Reproduced from reference 44 with permission.

The discovery of vCKBPs in the poxviruses raised an important question is the production of a secreted vCKBP an immune evasion strategy unique to the poxviruses, or is it also exploited by other mammalian viruses The herpesviruses are an obvious virus family in which to search for novel activities since they, like the poxviruses, have large DNA genomes encoding numerous genes involved in immune evasion. 

An interesting question derived from these studies is why viruses utilize different strategies to modulate chemokine activity. While vCKBPs have been identified mainly in poxviruses, herpesviruses frequently encode vCKs and vCKRs. This may reflect the need to modulate different aspects of chemokine biology as a result of diverse viral replication mechanisms. 

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