Journal of General Virology

Fig. 3. CD4-dependent and CD4-independent infection. For CD4-dependent infection, gp120 first interacts with CD4 at the cell surface. This interaction induces conformational changes in Env that allow a secondary interaction with a 7TM coreceptor to occur. Following coreceptor binding, further conformational changes result in fusion of viral and cellular membranes. CD4-independent viruses can bypass CD4 and interact directly with a coreceptor molecule to initiate fusion. The ability to infect cells in the absence of CD4 is associated with HIV-2 and SIV strains and correlates with exposed coreceptor binding sites, broadened cell tropism and enhanced sensitivity to neutralization.

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