SAR1 is a new IgG2a murine monoclonal antibody derived by immunization with a plant virus expressing the sequence GERDRDR from the C-terminal tail of the gp41 transmembrane glycoprotein of human immunodeficiency virus type 1 (HIV-1). SAR1 binds to peptides and proteins carrying the GERDRDR sequence, to some but not all preparations of purified virus, and to cells infected with all viruses tested. In a standard neutralization assay, SAR1 failed to neutralize, or neutralized poorly, a number of T cell line-adapted viruses. However, it was more effective at postattachment neutralization. This was measured by two assays, the inhibition of the syncytium production by input virus, and the inhibition of the production of infectious progeny virus. In general SAR1 was more effective at neutralizing progeny virus than inoculum virus. Fifty percent inhibition of progeny virus production by different HIV-1 strains was obtained with 2-26 microg/ml of SAR1. The SAR1 neutralizing epitope was mapped specifically to the gp41 C-terminal tail. SAR1 is an unusual, if not unique, antibody whose activity supports the view that part of the gp41 C-terminal tail is exposed on the outside of the virion.

Evidence has been presented which shows that part of the C-terminal tail of the gp41 transmembrane protein of human immunodeficiency virus type 1 (HIV-1) contains a neutralization epitope and is thus exposed on the external surface of the virion. Here, SAR1, a monoclonal antibody, which was stimulated by immunization with a plant virus expressing 60 copies of the GERDRDR sequence from the exposed gp41 tail, and has an unusual pattern of neutralization activity, giving little or no neutralization of free virions, but effecting modest post-attachment neutralization (PAN) of virus bound to target cells was investigated. Here, the properties of PAN were investigated. It was found that PAN could be mediated at 4 or 20 6C, but that at 20 6C maximum PAN required virus-cell complexes to be incubated for 3 h before addition of antibody. Further PAN appeared stable at 20 6C and could be mediated for at least 5 h at this temperature. In contrast, when virus-cell complexes formed at 20 6C but then shifted to 37 6C for various times before addition of SAR1, PAN was maximal after just 10 min, and was lost after 30 min incubation. Thus, PAN at 37 6C is transient and temperature-dependent. Since this scenario recalled the temperature requirements of virus-cell fusion, fusion of HIV-1-infected and non-infected cells was investigated, and it was found that SAR1 inhibited this process by up to 75 %, in a dose-dependent manner. However, antibodies to adjacent epitopes did not inhibit fusion. These data confirm the external location of the SAR1 epitope, implicate the gp41 C-terminal tail in the HIV-1 fusion process for the first time, and suggest that SAR1 mediates PAN by inhibiting virus-mediated fusion. INTRODUCTIONThe mature envelope protein of the virion of human immunodeficiency virus type 1 (HIV-1) is a trimer of heterodimers formed of the gp120 outer subunit and the gp41 transmembrane (tm) subunit. The gp120 serves to attach virus to the CD4 primary receptor and to the CXCR4 or CCR5 co-receptors on the surface of target cells, while the gp41 enables the viral and cell lipid bilayers to fuse together, resulting in the viral genome and associated proteins entering the cytoplasm and infecting the cell. Both gp120 and gp41 carry antibody neutralization epitopes (Levy, 1998).The gp41 subunit of HIV-1 and simian immunodeficiency viruses comprises the ectodomain, the tm domain, and a long C-terminal tail of approximately 150 aa residues (Gallaher et al., 1992). There is structural information on the ectodomain (Caffrey et al., 1998;Chan et al., 1997; Malashkevitch et al., 1998;Tan et al., 1997;Weissenhorn et al., 1997), but not on the C-terminal tail. The limits of the tm domain are not known exactly (West et al., 2001). Conventionally the tail is regarded as being located entirely inside the virion or the infected cell, but it has been proposed that about 40 residues of the tail region are looped out to the external surface of the virion (Cleveland et al., 2003;McLain et al., 2001). The main evidence for this relates to the neutra...

Herpes simplex virus (HSV) is common throughout the world and is a target for vaccine development. Transcutaneous immunisation is a novel technique that uses the application of vaccine antigens in solution on the skin in the presence of cholera toxin (CT) as an adjuvant. This study investigated the potential of transcutaneous immunisation in C3H mice, using CT co-administered with whole inactivated HSV-1 (CT+HSVi) or HSV-1 antigens extracted from infected Vero cells (CT+HSVag) or a control protein (CT+BSA). The application of any of the three vaccines on to bare mouse skin resulted in the migration of Langerhans cells from the epidermis and in the production of serum antibodies to CT. Both HSV preparations generated serum and mucosal (faecal) antibodies to HSV, with the CT+HSVi vaccine being a more potent stimulator of humoral immunity. The CT+HSVag vaccine, however, was the more potent stimulator of cell-mediated immunity, giving rise to a strong delayed type hypersensitivity response and lymphocyte proliferation in vitro. When the mice were challenged by epidermal inoculation of HSV, the CT+HSVag vaccine induced a higher level of protection than the CT+HSVi vaccine, a result which may indicate that the efficacy of HSV vaccines depends on stimulation of cell-mediated rather than humoral responses. The success of topical vaccination suggests that the transcutaneous route may offer a promising potential for novel vaccine delivery which merits further investigation.