Heparan sulfate serves as a receptor for several herpesviruses. For herpes simplex virus 1 (HSV-1), pseudorabies virus, and bovine herpesvirus 1, glycoprotein C homologues have been shown to mediate the binding to cell-surface heparan sulfate. It has been assumed that glycoprotein C of HSV-2 (gC-2) plays a similar role in HSV-2 entry, but this has not been established experimentally. We first determined, using heparin-affinity chromatography, that gC-2 is a heparin-binding glycoprotein. To examine the role of gC-2 in HSV-2 infection, we constructed a gC-2 deletion mutant, HSV-2(G)gC-. In contrast to results obtained for the other alpha herpesviruses, we found that the HSV-2(G)gC- virus showed no loss in specific binding activity (particles bound/cell) or specific infectivity (PFU/particle) compared to the parental wild-type virus. Moreover, while gC-1 mutants show a marked lag in the rate of viral penetration, the gC-2-deletion virus did not. We did find that gC-2, like gC-1, protects virus from complement-mediated neutralization. These results suggest that, in contrast to HSV-1, gC-2 does not play the key role in viral binding. The major role of gC-2 may be to protect virus from complement-mediated neutralization. We speculate that serotype differences in the contribution of gC to viral binding may contribute to serotype differences in cell tropism.

Binding of HSV-1 to cells is mediated by interactions of virion glycoproteins gC and/or gB with heparin sulfate (HS) glycosaminoglycans on cell surface proteoglycans. HS and the related glycosaminoglycan, heparin, comprise a family of heterogeneous carbohydrates composed of long, unbranched polysaccharides modified, for example, by sulfations and acetylations. To define the specific features of HS important for viral binding, we took advantage of the structural similarities between heparin and cell surface HS and compared the ability of chemically modified heparin compounds to inhibit the binding of viral particles to the cell surface and subsequent plaque formation. Because binding presumably involves multiple, complex interactions between both known heparin-binding glycoproteins, gC and gB, and cell surface HS, we compared the effects of modified heparin compounds on the binding and subsequent plaque formation of wild-type and gC-negative strains of HSV-1 and, in select cases, the binding of gB-negative virus to cells. We identified specific structural features of heparin essential for the inhibition of viral binding. For example, both N-sulfation and 6-O-sulfation must be important determinants since desulfation of heparin at these sites abolished or decreased the antiviral activity of heparin. Moreover, we found that the antiviral activity of heparin was independent of its anticoagulant activity. Carboxyl-reduced and 2,3-O-desulfated heparin selectively inhibited binding of gC-positive viruses (wild-type or a gB-negative strain) to cells, but had little or no inhibitory effect on binding and subsequent plaque formation for a gC-deletion virus. These results suggest that gC and gB interact with different structural features of HS.

Although heparan sulfate (HS) serves as an initial receptor for the binding of both herpes simplex virus type 1 (HSV-1) and HSV-2 to cell surfaces, the two serotypes differ in epidemiology, cell tropism, and ability to compete for viral receptors in vitro. These observations are not necessarily contradictory and can be explained if the two serotypes recognize different structural features of HS. To compare the specific features of HS important for the binding and infection of HSV-1 and HSV-2, we took advantage of structural similarities between heparin and cell surface HS and compared the abilities of chemically modified heparin compounds to inhibit plaque formation. We found that the antiviral activity of heparin for both serotypes was independent of anticoagulant activity. Moreover, specific negatively charged regions of the polysaccharide, including N sulfations and the carboxyl groups, are key structural features for interactions of both HSV-1 and HSV-2 with cell surfaces since N desulfation or carboxyl reduction abolished heparin's antiviral activity. In contrast, 6-O sulfations and 2-,3-O sulfations are important determinants primarily for HSV-1 infection. The O-desulfated heparins had little or no inhibitory effect on HSV-1 infection but inhibited HSV-2 infection. Using a series of intertypic recombinant mutant viruses, we found that susceptibility to O-desulfated heparins can be transferred to HSV-1 by the gene for glycoprotein C of HSV-2 (gC-2). This supports the notion that the envelope glycoproteins of HSV-1 and HSV-2 interact with different affinities for different structural features of heparin. To determine if the modified heparin compounds inhibited plaque formation by competing with cell surface HS for viral attachment, binding studies were also performed. As anticipated, most compounds inhibited binding and plaque formation in parallel. However, several compounds inhibited the binding of HSV-1 to cells during the initial attachment period at 4؇C; this inhibitory effect was reversed when the cells and inoculum were shifted to 37؇C. This temperature-dependent differential response to modified heparin compounds was evident primarily when glycoprotein C of HSV-1 (gC-1) was present in the virion envelope. Minimal temperaturedependent differences were seen for HSV-1 with gC-1 deleted and for HSV-2. These results suggest differences in the interactions of HSV-1 and HSV-2 with cell surface HS that may influence cell tropism.