Human Herpesvirus-6 (HHV-6) is a lymphotropic $\beta$-herpesvirus. Epidemiological studies have shown it to be a major etiologic agent of roseola infantum. HHV-6 has been linked to several illnesses and is also considered an important cofactor in AIDS. Although HHV-6 infects a variety of cells in vivo and in vitro, complete viral replication occurs most efficiently in T lymphocytes. Following primary infection, HHV-6 persists in a latent state and can be reactivated under conditions of acquired or induced immunosuppression. The nature of HHV-6 immunosuppression remains unresolved. To address this, the cytokine repertoire expressed by T cells following HHV-6 infection and the T cell anergy induced by HHV-6 were studied. Results show that HHV-6 infected T cells secreted increased amounts of IL-1$\beta$, IL-6, IL-10, IFN-$\gamma$ and TNF-$\alpha$ when compared to mock-infected T cells. The enhanced cytokine release was mediated by binding of the virus to cells and did not require viral replication. HHV-6 exposed T cells showed a significant impairment in their proliferative response to mitogen as well as to activation signals delivered through surface molecules or other transmembrane pathways. Some virus transcription was required to suppress lymphocyte proliferation. HHV-6 infected T cells express a soluble suppressive factor(s) which inhibits the lymphoproliferative response of uninfected cells. The suppressive factor requires its native conformation for activity and its suppressive activity is reduced when neutralized with anti-HHV-6 serum. NK cytolysis in PBMCs exposed to the suppressor factor could be enhanced by the addition of IL-15. These results suggest a mechanism whereby the immune system may respond to acute immunosupvressive HHV-6 infection.

To identify the cellular receptor for Enterovirus 70 (EV70), mice were immunized with HeLa cells or HeLa cell membranes. Hybridomas were generated and screened for the production of antibody that protected HeLa cells against infection by EV70, and one monoclonal antibody (mAb), EVR1, was isolated. EVR1 recognized a trypsin-resistant HeLa cell surface epitope, and interfered with EV70 binding to HeLa cell monolayers. The specificity of EVR1 for EV70 was demonstrated by the inability of this mAb to protect HeLa cells against infection by poliovirus (PV) or coxsackievirus B3 (CVB3). Furthermore, EVR1 did not bind to monkey kidney (LLC-MK$\sb2)$ cells, nor did it protect these cells against EV70 infection. In Western blots and immunoprecipitations, EVR1 recognized a HeLa cell glycoprotein of approximately 75 kDa. The properties of this protein suggested that it may be decay-accelerating factor (DAF/CD55). DAF is a 70-75 kDa glycosyl-phosphatidylinositol (GPI)-anchored membrane protein that is a regulator of complement activation, and that has also been shown to act as a receptor for a number of human enteroviruses. Several experiments were performed to demonstrate that EVR1 recognized DAF. HeLa cells treated with phosphatidylinositol-specific phospholipase C (PI-PLC) had a greatly reduced ability to bind EVR1, indicating that the ligand of EVR1 is attached to cells via a GPI anchor. Furthermore, EVR1 bound to Chinese hamster ovary (CHO) cells constitutively expressing human DAF, and DAF-specific mAbs inhibited EV70 binding to HeLa cells and protected them against EV70 infection. Using a vaccinia virus based system, human DAF was transiently expressed in murine NIH/3T3 cells. Transient expression of human DAF conferred virus binding activity to these cells, and a stably transformed NIH/3T3 cell line expressing human DAF supported low levels of EV70 replication. To map the EV70 binding site of DAF, chimeric DAF/membrane cofactor protein (MCP) molecules were transiently expressed in NIH/3T3 cells and tested for their ability to bind EV70. Expression of chimeric receptors, in which individual extracellular short consensus repeat (SCR) domains of DAF were exchanged with those of MCP indicated that DAF SCR1 contained sequences essential for EV70 binding. Sequences within DAF SCR2 may also play a minor role in EV70 binding, but SCRs 3 and 4 were not specifically required to retain binding activity of the molecule. Similarly, the juxtamembrane domain and the GPI anchor of DAF were not specifically required for EV70 binding. Chimeric receptor expression was also used to identify DAF SCR1 as the binding site for EVR1. These results indicate that DAF acts as a HeLa cell receptor for EV70. However, whether or not DAF is necessary or sufficient to mediate EV70 infection of cells is not yet clear. The widespread in vivo distribution of DAF and the restricted in vivo tropism of EV70, as well as the discovery that other human enteroviruses also use DAF as a receptor indicate that DAF expression is not the sole determinant of EV70 tropism. In addition, it remains to be determined if EV70 uses DAF homologs or entirely different surface molecules as receptors on cells of non-human origin. The identification of co-receptors or accessory factors that govern the specificity of EV70 infection awaits further investigation.

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