4The tetraspanins are a broadly expressed superfamily of transmembrane glycoproteins with over 30 members found in humans and with homologues conserved through distantly related species, including insects, sponges, and fungi. Members of this family appear to form large integrated signaling complexes or tetraspanin-enriched microdomains (TEMs) by their association with a variety of transmembrane and intracellular signaling/cytoskeletal proteins (49). These interactions link tetraspanins to an array of physiological functions and, in consequence, to numerous endogenous pathologies, including cancer development and inherited disorders (Table 1).Tetraspanins are also known to have roles in the pathology of infectious diseases such as diphtheria, malaria, and numerous viral infections (Table 1). The literature currently indicates that specific tetraspanin family members are selectively associated with specific viruses and affect multiple stages of infectivity, from initial cellular attachment to syncytium formation and viral particle release. Thus, the relationship of tetraspanins with viruses appears to be particularly complex.Here, we will consider this data in the context of recent developments in tetraspanin biology, particularly in our understanding of the architecture and function of TEMs. With the benefit of recent insights into tetraspanin function in cell fusion events and intracellular trafficking, we discuss common features of tetraspanin/viral associations which indicate a fundamental role for TEMs in a number of viral infections. We will also consider the existing therapeutic strategies for human immunodeficiency virus (HIV), hepatitis C virus (HCV), and human T-cell lymphotropic virus type 1 (HTLV-1), focusing on the potential therapeutic value of targeting TEMs, using peptide reagents based on tetraspanin extracellular regions.

Human immunodeficiency virus type 1 (HIV-1) infection of human macrophages can be inhibited by antibodies which bind to the tetraspanin protein CD63, but not by antibodies that bind to other members of the tetraspanin family. This inhibitory response was limited to CCR5 (R5)-tropic virus and was only observed using macrophages, but not T cells. Here, we show that recombinant soluble forms of the large extracellular domain (EC2) of human tetraspanins CD9, CD63, CD81, and CD151 produced as fusion proteins with glutathione S-transferase (GST) can all potently and completely inhibit R5 HIV-1 infection of macrophages with 50% inhibitory concentration values of 0.11 to 1.2 nM. Infection of peripheral blood mononuclear cells could also be partly inhibited, although higher concentrations of EC2 proteins were required. Inhibition was largely coreceptor independent, as macrophage infections by virions pseudotyped with CXCR4 (X4)-tropic HIV-1 or vesicular stomatitis virus (VSV)-G glycoproteins were also inhibited, but was time dependent, since addition prior to or during, but not after, virus inoculation resulted in potent inhibition. Incubation with tetraspanins did not decrease CD4 or HIV-1 coreceptor expression but did block virion uptake. Colocalization of fluorescently labeled tetraspanin EC2 proteins and HIV-1 virions within, and with CD4 and CXCR4 at the cell surfaces of, macrophages could be detected, and internalized tetraspanin EC2 proteins were directed to vesicular compartments that contained internalized dextran and transferrin. Collectively, the data suggest that the mechanism of inhibition of HIV-1 infection by tetraspanins is at the step of virus entry, perhaps via interference with binding and/or the formation of CD4-coreceptor complexes within microdomains that are required for membrane fusion events.

Summary Members of the tetraspanin superfamily of proteins are implicated in a variety of complex cell processes including cell fusion. However, the contribution of individual tetraspanins to these processes has proved difficult to define. Here we report the use of recombinant extracellular regions of tetraspanins to investigate the role of specific members of this family in the fusion of monocytes to form multinucleated giant cells (MGC). In contrast to their positive requirement in sperm–egg fusion, previous studies using antibodies and knockout mice have indicated a negative regulatory role for tetraspanins CD9 and CD81 in this process. In an in vitro model of fusion using human monocytes, we have confirmed observations that antibodies to CD9 and CD81 enhance MGC formation; however, in contrast to previous investigations, we found that all members of a panel of antibodies to CD63 inhibited fusion. Moreover, recombinant proteins corresponding to the large extracellular domains (EC2s) of CD63 and CD9 inhibited MGC formation, whereas the EC2s of CD81 and CD151 had no effect. The potent inhibition of fusion and binding of labelled CD63 EC2 to monocytes under fusogenic conditions suggest a direct interaction with a membrane component required for fusion. Our findings indicate that the tetraspanins CD9, CD63 and CD81 are all involved in MGC formation, but play distinct roles.