467

Virus proteins that bind cytokines, chemokines or interferons Geoffrey L Smith

Some viruses express proteins to evade non-specific

host defences such as complement, interferons and the

inflammatory response. Recent notable discoveries are the

broad species specificity of orthopoxvirus interferon receptors,

herpesvirus and poxvirus proteins related to chemokine

receptors and proteins that bind more than one cytokine,

interferon or chemokine.

Addresses Sir William Dunn School of Pathology, University of Oxford, South

Parks Road, Oxford, OX1 3RE, UK; e-mail: glsmith@molbiol.ox.ac.uk

Current Opinion in Immunology 1996, 8:467-471

0 Current Biology Ltd ISSN 0952-7915

Abbreviations

GCR G-protein-coupled receptor

IgSF immunoglobulin superfamily

IFN interferon

IL interleukin

HCMV human cytomegalovirus MV myxoma virus ORF open reading frame R receptor RANTES regulated on activation, normal T-cell expressed and secreted SFV Shope fibroma virus TNF tumour necrosis factor W vaccinia virus

Introduction Viruses have coevolved with the host immune system

and consequently have developed countermeasures to

defend themselves in the hostile host environment.

This is particularly true for large DNA viruses such as

herpesviruses and poxviruses which, due to their extensive

coding capacity, express many proteins that are able to

inhibit components of the immune system. For instance,

these viruses inhibit the function of the complement

cascade and the interferons (IFNs) and modify the host

cytokine or chemokine activities. The reader is referred

to other recent reviews of these areas [l-6].

Viruses are able to interfere with the functions of cytokines

at several levels: they inhibit the synthesis or processing

of cytokines, block the binding of cytokines to host-cell

receptors, stop the intracellular events that cytokines

induce upon binding to the host cell and influence the

nature (Thl or ThZ) of the host response by expressing

their own cytokine(s). This review focuses on virus

proteins that directly bind cytokines, IFNs or chemokines,

either at the cell surface or in the extracellular space.

The pro-inflammatory cytokines tumour necrosis factor

(TNF) and interleukin (IL)-1 are important molecules that

either have direct antiviral action (e.g. TNF) or co-ordinate

the host inflammatory and immune responses to virus

infection. It is not surprising, therefore, that viruses

have developed countermeasures against the activities of

each cytokine. For example, adenoviruses have several

intracellular proteins that block the action of TNF [7],

whereas poxviruses express soluble proteins that directly

bind TNF or IL-1 (see below).

Interleukin-1 Poxviruses employ two mechanisms to block the pro-

duction or function of IL-l. One mechanism involves

the intracellular inhibition of ICE, the IL-lp converting

enzyme, and results in the inhibition of the processing

and release of IL-lfi [8]. The protein responsible for this

activity is crmA in cowpox virus (CPV) [S] or B13R in

vaccina virus (\‘V). Its importance to poxviruses probably

resides more in its ability to block apoptosis via inhibition

of ICE than in its ability to block IL-1 function directly,

and hence will not be considered further here. The second

mechanism involves the binding of IL-lfi. The protein

involved in this activity (BlSR) was discovered during the

sequencing of the \‘V genome [9] and is a member of

the immunoglobulin superfamily (IgSF); it has 25% amino

acid identity to the extracellular domain of the type I

IL-l receptor (IL-1R) [lo] but 33% identity to that of the

type II IL-1R [ll]. Tmro research groups characterized the

B15R protein and demonstrated that it binds IL-l [12,13].

Interestingly, BlSR binds IL-lp (&=234phI) but binds

neither IL-la nor the IL-1R antagonist protein (IL-lRa),

showing that binding is very specific [13]. Deletion of

the B15R gene from \‘V strain WR was reported to cause

contrary effects on virus virulence depending on the route

of inoculation; intracranial inoculation showed decreased

virus virulence [ 121, whilst intranasal infection resulted in

a slightly more severe infection as measured by weight

loss and signs of illness [13]. Consistent with the increased

virulence of BlSR-deficient virus, smallpox vaccine strains

lacking the IL-l PR were more virulent in man than strains

expressing the IL-1PR [13,14], and all strains of variola

major virus examined (which caused smallpox) also lacked

a functional IL-1PR gene [15-171. In both of those cases

virulence was likely to have been multifactorial.

Recently, the B15R protein was demonstrated to inhibit

the induction of fever in response to infection [18*]. hlice

infected intranasally with the BlSR deletion mutant, or

with other strains of W which naturally lack the IL-1PR

and which were used as smallpox vaccines (Copenhagen

and Tashkent strains), developed a fever for several

days after infection, whereas mice infected with strains

expressing the IL-1PR did not. Given the specificity of

B15R for IL-l& this result demonstrated that IL-lp is the

principal endogenous pyrogen controlling fever during this

live virus infection. This is the first example of a virus

468 Immunity to infection

controlling the fever of the infected host and it illustrates

that the study of virus cytokine receptors can teach us

about the physiological role of cytokines.

Tumour necrosis factor After the cloning and sequencing of the genes encoding

cellular type I and type II TNF receptors (TNFRs),

genes encoding related proteins were identified in Shope

fibroma virus (SFV), a leporipoxvirus, and in VV SFV

gene TZ encodes a secreted 58 kDa protein that binds

TNF-IX and TNF-P [19]; a similar TNFR from myxoma

virus (MV), a related leporipoxvirus that causes the rabbit

disease myxomatosis, contributes to virus virulence [ZO].

Interestingly, the MV TNFR inhibits TNF-a from rabbit

but not in man or mouse, reflecting the species specificity

of MV [Zl]. In VV strains Copenhagen and WR, there

are two TNFR-gene homologues (A53R and B28R/CZ2L), but each is broken into fragments by mutation and

is non-functional [20,22]. In CPV, one TNFR gene (a

B28R homologue encodes a 48 kDa early secretory protein

(crmB) that binds TNF-a and TNF-P [23], whilst another

TNFR gene (an A53R homologue) encodes crmC, a

partially characterized but active TNFR [24] (A Alcami,

GL Smith, unpublished data). In variola virus, one TNFR

gene (GZR/G4R) is predicted to encode an active TNFR,

whilst the other (an A53R homologue) has been deleted

[15-17,251. An interesting feature of the SFV T2 protein,

the equivalent protein in MV, CPV crmB and variola

virus GZ/G4, is the presence of a conserved, acidic,

carboxy-terminal region of 140-160 amino acids which is

absent from the cellular TNFRs.

lnterferons The IFNs are a collection of secreted proteins that bind

to cells bearing an appropriate IFN receptor (IFNR).

Binding to the receptor induces the expression of

several cellular proteins which prime the cell so that

upon subsequent virus infection protein synthesis and

hence virus replication are inhibited. Two important

proteins that mediate the antiviral activity are the double

stranded RNA dependent protein kinase (PKR) and the

Z’S’-oligoadenylate (Z’S’A) synthetase. The type I IFNs,

IFN-a (leukocyte) and IFN-0 (fibroblast), bind to a

common receptor, whilst type II IFN, IFN-y (T-cell),

binds to a distinct receptor. IFN-y is also important for the

induction of cell-mediated immunity. The importance of

the IFNs in combatting virus infections is illustrated firstly

by the many viruses which have evolved countermeasures

and secondly, by the enhanced sensitivity to virus infection

of mice lacking components of the IFN system. Many

virus defence mechanisms against IFN are intracellular

and directed against PKR [26]. In addition, poxviruses

have proteins which bind type I or type II IFNs at the

cell surface or in the extracellular medium.

Interferon-y

MV was the first poxvirus found to express an IFN-?/

binding protein. A 37 kDa protein with sequence similarity

to the extracellular domain of the human and murine

IFN-)I receptor (IFN-yR) was secreted by MV-infected

cells [27]. This protein (termed M-T7) neutralized the

antiviral activity of rabbit IFN-y. Further analysis showed

that 5 x 107 M-T7 molecules were synthesized per cell

and that the protein contained a single high affinity

binding site for rabbit IFN-y (& = 1.2 x 10-q M). However,

consistent with the natural host of MV, the protein was

shown to be specific for rabbit IFN-)I and was unable

to neutralize human or mouse IFN-y [ZB]. An MV strain

lacking the M-T7 gene is severely attenuated in viva,

but in addition to greatly diminished virus replication and

dissemination, infected animals also displayed increased

influx of leukocytes into the infected lesion and more ac-

tivation of lymphocytes in spleen and lymph nodes [29**].

In their discussion Mossman et al. [29**], mention that,

remarkably, the M-T7 protein binds several chemokines

in addition to IFN-y.

A VV gene (B8R) encodes a protein related to the SFV

T7 protein [ZZ] (which is equivalent to MV M-T7). The

vaccinia gene was shown by expression in recombinant

baculovirus to encode an IFN-yR [30’]. Unlike the

IFN-yR of MV, which has been shown to be specific

for rabbit IFN-y, the VV IFN-yR displayed a remarkably

broad species specificity that was without precedent in the

IFN system. Specifically, the protein bound and inhibited

human, cow, rabbit, rat but not mouse IFN-y [30*,31’].

An IFN-yR was expressed by all orthopoxviruses tested

and the CPV and camelpox virus IFN-yRs had a species

specificity comparable with that of vaccinia [30’,31’].

Likewise, the ectromelia virus IFN-yR had broad species

specificity, but this protein was also shown to efficiently

bind murine IFN-y, consistent with the murine tropism of

this virus [31*].

The expression of a soluble IFN-yR by orthopoxviruses

is an efficient way to inhibit both the directly antiviral

and pro-inflammatory roles of IFN-?/. Thus the deletion of

the IFN-yR would attenuate the virus, as was shown for

MV, but also increase virus immunogenicity. In this regard,

the modified virus Ankara (MVA) strain of VV, a safe and

attenuated smallpox vaccine strain, lacks an IFN-yR (T

Blanchard, A Alcami, GL Smith, unpublished data) and,

despite having a severely restricted replicative capacity

in human cells, is as immunogenic as a fully replication

competent VV strain [32]. MVA thus represents an

excellent candidate vector for new poxvirus recombinant

vaccines.

Another IFN-y binding protein is secreted from tanapoxvirus-

infected cells [33]. This 38 kDa protein comigrates with

an IL-Z and IL-S binding activity during two-dimensional

gel electrophoresis and it is quite possible that all three

cytokines are bound by the same protein. This would be

remarkable, but confirmation awaits the cloning of the

gene and its expression in a heterologous system.

Virus proteins that bind mediators Smith 469

Interferon-a/f3

In addition to a soluble IFN-yR, and two intracellular proteins (E3L and K3L) that block the antiviral affects of IFN, VV expresses a soluble IFN-a/BR. The gene encoding the IFN-CX/BR was identified as BZ8R [34*,35*] and the protein showed limited amino acid identity to the cellular type I IFNR [35*], but surprisingly belonged to a different protein superfamily: the cellular type I IFNRs are members of the class II cytokine receptor family, and contain fibronectin type III repeats, whereas the VV B18R protein is an IgSF member [lo]. Nonetheless, the B18R protein bound human IFN-a2 with high affinity (Kd = 174pM) and efficiently inhibited its biological ac- tivity [34*,35*]. The majority of orthopoxviruses express an IFN-a/BR, but notably W strain Lister lacked the receptor and strain Wyeth expressed a receptor with a greatly reduced affinity (&= 13.5nM) [34*]. These Ws were among the safest vaccines used for smallpox immunoprophylaxis [ 141.

Like the W IFN-yR, the W IFN-a/BR had a broad species specificity and bound and inhibited human, cow, rabbit, rat and mouse type I IFNs. However, it did this with different efficiencies, inhibiting the human and rabbit IFN most efficiently, cow and rat IFN less well and mouse IFN two orders of magnitude less well than human [34’]. The possession of IFNRs with such broad species specificity is consistent with the wide host range of VV and probably aided virus replication in multiple species during evolution. This range may be contrasted with the MV IFN-)IR, which only inhibits rabbit IFN-)I.

Another interesting feature of the VV B18R protein is that it is the orthopoxvirus S antigen. This antigen was first studied in the 1960s and was recently mapped to the B18R gene [36,37]. This protein is present on the cell surface and in the extracellular medium [13] and in each location is able to inhibit type I IFNs [34*,35’] (JA Symons, A Alcami, GL Smith, unpublished data). The protein is able to block transmembrane signalling otherwise induced by binding of type I IFN to its receptor [35’].

Chemokine-binding proteins The chemokines are a group of small structurally related proteins that fall into two groups: the a and B chemokines, which are exemplified by IL-8 and RANTES, respec- tively. These molecules are induced by pro-inflammatory cytokines such as IL-1 and TNF and function as chemoattractants for leukocytes. The chemokine recep- tors are G-protein-coupled receptors (GCRs) with seven transmembrane-spanning domains [38].

Several viruses encode proteins related to this GCR family and in some cases these proteins are known to bind chemokines. The first viral GCR-like proteins were identified in human cytomegalovirus (HCMV) [39], which had three genes, US27, LiS28 and US33, predicted to encode proteins related to the GCR family. Recently

another HCMV protein (UL78) was reported to be a member of this family [40*]. When the US28 protein was expressed in human kidney or erythroleukaemia cells it bound several B chemokines, such as human monocyte chemotactic protein 1 (MCP-l), macrophage inflammatory proteins la and 1B (MIP-la/B) and RANTES, with high affinity (&=2-6nM) [41,42]. In contrast, US28 did not bind ~1 chemokines such as IL-8 [41,42]. Additionally, US28 induced a transient increase in intracellular calcium in response to binding B chemokines, most notably RANTES [42]. Whether the US27, US33 and UL78 proteins bind chemokines and what roles are played by US28 and these other proteins in virus pathogenesis are unknown.

A chemokine receptor (ORF ECRF3) has also been identified and studied in herpes virus saimiri (HVS) [43]. Expression of ECRF3 in Xenopus oocytes resulted in mobilization of calcium in response to the binding of several cx chemokines, such as IL-B, GRO/melanoma growth stimulatory activity (GRO/MGSA) and neutrophil activating peptide- (NAP-Z), but not in response to binding of B chemokines [44]. The ligand specificity of the ECRF3 is closer to that of IL-8R type B than that of IL-8R type A, despite IL-8R type B sharing 77% identity with IL-8R type A but only 33% identity with ECRF3 [44].

The ability of the chemokine receptors from HCMV and HVS to signal in response to chemokine binding contrasts with the situation with poxvirus soluble cytokine recep- tors, which block cytokine binding and signalling. Possibly the increased sensitivity of HCMV- and HVS-infected cells to signalling in response to chemokine binding might enhance replication of these herpesviruses.

Several other viruses encode seven transmembrane GCRs. Equine herpesvirus 2 has three potential GCRs: one of these is very similar to the HVS ECRF3 (specific for o( chemokines), whereas a second is closely related to the HCMV US28 (specific for B chemokines) [45-l. Human herpes virus-6 (HHV-6) genes UZZ and ci51 are predicted to encode GCR-like proteins [40*]. Interestingly, HHV-6 also has ORFs related to the a (e.g. ORF UL83) and B (e.g. ORFs DRl and DR6) chemokines [40*]. Lastly seven transmembrane GCRs have been found in capripox virus [46] and swinepox virus [47], poxviruses from two different genera. The ligand-binding activity of all these molecules remains to be determined.

Finally, the MV M-T7 protein and the 38 kDa protein encoded by tanapoxvirus illustrate how viruses make very efficient use of their genetic information. The 38 kDa protein of tanapoxvirus potentially binds IFN-y, IL-Z and IL-S [33], while the M-T7 protein binds rabbit IFN-y and also a broad range of chemokines [29”]. Another notable feature of the M-T7 protein is that it is a soluble chemokine-binding protein, unlike the membrane-bound GCRs of cells and other viruses.

470 Immunity to infection

Conclusions The number of viruses known to have proteins which do, or may, interact with chemokines, cytokines or IFNs is rapidly increasing. The study of the ligand specificity and the mode of action of these virus proteins is providing new insights into virus pathogenesis and is also teaching us about the immune system itself. This is an exciting and expanding area of research that is very likely to identify new molecules that function in the immune system and new ways by which viruses modulate their function.

Acknowledgements The work carried out in the author’s laboratory is supported by the Medical Research Council and The Wellcome Trust. I thank Julian A Symons and Antonio Alcami for critical reading of the manuscript.

References and recommended reading 22.

Papers of particular interest, published within the annual period of review, have been highlighted as:

.

. .

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

of special interest of outstanding interest

McFadden G, Graham K: Modulation of cytokine networks by poxviruses: the myxoma virus model. Semin Viral 1994, 5:421-429.

Pickup DJ: Poxviral modifiers of cytokine responses to infection. infect Agents Dis 1994, 3:116-i 27.

Smith GL: Virus strategies for evasion of the host response to infection. fiends Microobiol 1994, 2:81-88.

Spriggs MK: Cytokine and cytokine receptor genes ‘captured’ by viruses. Curr Opin lmmunol 1994, 6:526-529.

Alcami A, Smith GL: Cytokine receptors encoded by poxviruses: a lesson in cytokine biology. lmmunol Today 1995, 16:474-478.

McFadden G (Ed): Viroceptors, Virokines and Related Immune Modulators Encoded by DNA Viruses. Austin: RG Landes; 1995.

Wold WS, Hermiston TW, Tollefson AE: Adenovirus proteins that subvert host defenses. Trends Microbial 1994, 2:437-443.

Ray CA, Black RA, Kronheim SR, Greenstreet TA, Sleath PR, Salvensen GS, Pickup DJ: Viral inhibition of inflammation: cawpox virus encodes an inhibitor of the interleukin-1 p converting enzyme. Cell 1992, 6g:597-604.

Smith GL, Chan YS, Howard ST: Nucleotide sequence of 42 kbp of vaccinia virus strain WR from near the right inverted terminal repeat J Gen Viral 1991, 72:1349-l 376.

Smith GL, Chan YS: Two vaccinia virus proteins structurally related to the interleukin-1 receptor and the immunoglobulin superfamily. J Gen Viral 1991, 72:51 l-51 8.

McMahan CJ, Slack JL, Mosley B, Cosman D, Lupton SD, Brunton LL, Grubin CE, Wignall JM, Jenkins NA, Branan Cl et a/.: A novel IL-I receptor, cloned from B cells by mammalian expression, is expressed in many cell types. EM60 J 1991, lo:2821 -2832.

Spriggs M, Hruby DE, Maliszewski CR, Pickup DJ, Sims JE, Buller RML, Vanslyke J: Vaccinia and cowpox viruses encode a novel secreted interleukin-1 binding protein. Cell 1992, 71 :145-l 52.

Alcami A, Smith GL: A soluble receptor for interleukin-I p encoded by vaccinia virus: a novel mechanism of virus modulation of the host response to infection. Cell 1992, 71 :153-l 67.

Fenner F, Anderson DA, Arita I, Jezek 2, Ladnyi ID: Smallpox and Its Eradication. Geneva: World Health Organization; 1988.

Alcami A, Smith GL: Two genes encoding poxvirus cytokine receptors are disrupted or deleted in variola virus. [Commentary on the paper by Shchelkunov et al., FEBS Letts 1993, 319:80-831. FEBS Letis 1993, 335:136-l 37.

Shchelkunov SN, Blinov VM, Sandakhchiev LS: Genes of variola and vaccinia viruses necessary to overcome the host protective mechanisms. FEBS Letts 1993, 319:80-83.

1 7. Massung RF, Liu LI, Qi J, Knight JC, Yuran TE, Kerlavage AR, Parsons JM, Venter JC, Esposito JJ: Analysis of the complete genome of smallpox variola major virus strain Bangladesh- 1975. Virology 1994, 201:215-240.

18. Alcami A, Smith GL: A mechanism for the inhibition of fever by . a virus. Proc Nat/ Acad Sci USA 1996, in press. Describes how the vaccinia virus IL-BlR suppresses the febrile reponse to infection, showing that IL-1 p is the principal endogenous pyrogen in re- sponse to this virus infection.

19.

20.

21.

23.

24.

25.

26.

27.

28.

29. . .

Smith CA, Davis T, Wignall JM, Din WS, Farrah T, Upton C, McFadden G, Goodwin RG: T2 open reading frame from Shope fibroma virus encodes a soluble form of the TNF receptor. Biochem Siophys Res Commun 1991, 1761335-342.

Upton C, Macen JL, Schreiber M, McFadden G: Myxoma virus expresses a secreted protein with homology to the tumor necrosis factor receptor gene family that contributes to viral virulence. Virology 1991, 184:370-382.

Schreiber M, McFadden G: The myxoma virus TNF-receptor homologue (T2) inhibits tumor necrosis factor-alpha in a species-specific fashion. Virology 1994, 204:692-705.

Howard ST, Chan YS, Smith GL: Vaccinia virus homologues of the Shope fibroma virus inverted terminal repeat proteins and a discontinuous ORF related to the tumour necrosis factor receptor family. Virology 1991, 180:633-647.

Hu F-Q Smith CA, Pickup DJ: Cowpox virus contains two copies of an early gene encoding a soluble secreted form of the type II TNF receptor. Virology 1994, 204:343-356.

Smith CA, Goodwin RG: Tumor necrosis factor receptors in the poxvirus family: biological and genetic implications. In Viroceptors, Virokines and Related immune Modulators Encoded by DNA Viruses. Edited by McFadden G Austin: RG Landes; 1995:29-40.

Aguado B, Selmes IP, Smith GL: Nucleotide sequence of 21.8 kbp of variola major virus strain Harvey and comparison with vaccinia virus. J Gen Viral 1992, 73:2887-2902.

Katze MG: Regulation of the interferon-induced PKR: can viruses cope? Trends Microbial 1995, 3:75-78.

Upton C, Mossman K, McFadden G: Encoding of a homolog of IFN-y receptor by myxoma virus. Science 1992, 258:1369-1372.

Mossman K, Upton C, McFadden G: The myxoma virus soluble interferon-y receptor homolog, M-T7, inhibits interferon-y in a species-specific manner. J Biol Chem 1995, 270:3031-3038.

Mossman K, Nation P, Macen J, Garbutt M, Lucas A, McFadden G: Myxoma virus M-T7, a secreted homolog of the intetferon- gamma receptor, is a critical virulence factor for the development of myxomatosis in European rabbits. Virology 1996, 215:17-30.

Shows that the host response to infection with MV differs depending on whether the M-T7 protein is expressed and mentions in the discussion that M-T7 binds chemokines as well as IFN-y.

30. Alcami A, Smith GL: Vaccinia, cowpox and camelpox viruses . encode soluble interferon-y receptors with novel broad species

specificity. J Viral 1995, 69:4633-4839. Reports the broad species specificity of the vaccinia, cowpox and camelpox virus IFN-yRs.

31. Mossman K, Upton C, Buller RM, McFadden G: Species . specificity of ectromelia virus and vaccinia virus interferon-

gamma binding proteins. Virology 1995, 208:762-769. Describes the broad species specificity of the vaccinia and ectromelia IFN-

Ps.

32. Sutter G, Wyatt LS, Foley PL, Bennink JR, Moss B: A recombinant vector derived from the host range-restricted and highly attenuated MVA strain of vaccinia virus stimulates protective immunity in mice to influenza virus. Vaccine 1994, 12:1032-l 040.

33. Essani K, Chalasani S, Eversole R, Beuving L, Birmingham L: Multiple anti-cytokine activities secreted from tanapox virus- infected cells. Microb Pathog 1994, 17:347-353.

34. Symons JA, Alcami A, Smith GL: Vaccinia virus encodes a . soluble type I interferon receptor of novel structure and broad

species specificity. Cell 1995, 81:551-560. Describes the identification, expression and characterization of a soluble type I IFN receptor which is conserved in otthopoxviruses and inhibits type I IFNs from man, cow, rabbit, rat and mouse.

Virus proteins that bind mediators Smith 471

35. Colamonici OR, Domanski P, Sweitzer SM, Lamer A, Buller RML: 42. Gao J-L, Murphy PM: Human cytomagelovirus open reading . Vaccinia virus B18R gene encodes a type I interferon-binding

protein that blocks interferon alpha transmembrane signaling. frame US28 encodes a functional beta chemokine receptor. J Biol Chem 1994. 269:28539-28542.

J Biol Chem 1995, 270:15974-l 5978. Describes a type I IFN from vaccinia virus which inhibits type I IFN signalling and is present in solution and on the cell surface.

36. Ueda Y, Ito M, Tagaya I: A specific surface antigen induced by poxvirus. Virology 1969, 38:i 60-l 82.

43. Nicholas J, Cameron KR, Honess RW: Herpesvirus saimiri encodes homologues of G protein-coupled receptors and cyclins. Nature 1992, 355:362-365.

37.

_. Ueda Y, Morikawa S, Matsuura Y: Identification and nucleotide sequence of the gene encoding a surface antigen induced by vaccinia virus. Virology 1990, 177:568-594.

44. Ahuja SK, Murphy PM: Molecular piracy of mammalian interleukin-8 receptor type B by herpesvirus saimiri. J Biol Chem 1993,268:20691-20694.

38. Murphy PM: The molecular biology of leukocyte 45. chemoattractant receptors. Annu Rev lmmunol 1994,

Telford EA, Watson MS, Aird HC, Perry J, Davison AJ: The .

12:593-633. DNA sequence of equine herpesvirus 2. J MO/ Biol 1995, 249:520-528.

39. Chee MS, Satchwell SC, Preddie E, Weston KM, Barrel1 BG: Describes the complete sequence of EHV-2 and identifies three ORFs re-

Human cytomegalovirus encodes three G protein-coupled lated to chemokine receptors.

receptor homologues. Nature 1990, 344:774-77X

40. Gompels UA, Nicholas J, Lawrence G, Jones M, Thomson BJ, 46. Cao JX, Gershon PD, Black DN: Sequence analysis of HindIll

. Martin ME, Efstathiou S, Craxton M, Macaulay HA: The DNA 02 fragment of capripoxvirus reveals a putative gene encoding

sequence of human herpesvirus-6: structure, coding content, a G-protein-coupled chemokine receptor homologue. Virology

and oenome evolution. Viroloov 1995. 209:29-51. 1995, 209:207-212.

Presents the-complete sequence of Hl%-6 and’identifies proteins related to chemokines and chemokine receptors.

Neote K, DiGregorio D, Mak JY, Horuk R, Schall TJ: Molecular 41. cloning, functional expression, and signaling characteristics of a C-C chemokina receptor. Cell 1993, 72:415-425.

47. Massung RF, Jayarama V, Moyer RW: DNA sequence analysis of conserved and unique regions of swinepox virus: identification of genetic elements supporting phenotypic observations including a novel G protein-coupled receptor homologue. virology 1993, 197:51 l-528.