HIV-1 entry. For HIV-1 to enter its target T-cell or macrophage, its envelope glycoprotein gp120 binds the cellular receptor CD4. This induces a conformational change in gp120 that allows it to associate with a coreceptor such as the chemokine receptor CCR5 or CXCR4. The coreceptor is actually more important than CD4; it is necessary, and in some cases sufficient, for HIV-1 to enter its target cells. Association with the coreceptor induces a dramatic conformational change in the envelope glycoprotein gp41 that allows its amino-terminus to associate with the target-cell membrane, facilitating mixing of the viral and cellular lipids and ultimately entry of the viral capsid into the cell. CCR5 is the primary coreceptor used during transmission and replication during the asymptomatic period of infection. We have studied CCR5 extensively, mapping a domain in the amino-terminus of the receptor that is critical for HIV-1 entry. This domain is rich in tyrosine and acidic residues, and we have shown that the tyrosines in this region are modified by the addition of sulfate. These sulfates are quite important for HIV-1 replication. For example, peptides based on the amino-terminus of CCR5 can inhibit HIV-1 entry only if they are sulfated, and sulfated peptides can complement the inability of a CCR5 lacking an amino-terminus to support HIV-1 entry.

The anti-HIV-1 immune response. We have shown that antibodies can mimic CCR5 in a number of ways, including by sulfating the tyrosines in their antigen-binding regions. These antibodies may be important because the virus has greater difficulty escaping from an antibody that so closely mimics its obligate receptor. We are currently investigating these antibodies, their role in controlling infection in the population of long-term non-progressing patients, and how to better elicit them. We are using recent advances in mass spectroscopy (using the Center's own state-of the-art LTQ-FT spectrometer) to provide a higher resolution view of the antibody and T-cell receptor response to HIV-1 in several ways. For example, we are quantifying in infected individuals and macaques the elimination and disproportionate use of specific V and J chains, the efficiency over the time of infection of somatic hypermutation, and the frequency of antibody tyrosine sulfation.

Other viral receptors. In 2003, we identified the receptor, ACE2, necessary (and apparently sufficient) for infection of cells by the SARS virus. We have also defined a small region of the SARS-CoV S protein that functions as its receptor-binding domain. This work provides two immediate therapeutic approaches to SARS (a much easier problem than HIV-1). More importantly this has led us to further develop the mass spectrometric approach that we used to identify the SARS-CoV receptor in an effort to identify additional receptors of enveloped viruses.