Understanding the mechanisms of an effective neutralizing antibody response to HIV is one of the highest priorities in the field of HIV-specific immunity. In this regard, the inability of the humoral response of most vaccinees to cross-neutralize multiple strains of HIV is believed to be a major obstacle to the design of effective vaccines. In 2006 we observed that sera from subpopulation of our chronically-infected cohorts had considerable neutralization breadth extending across clades. Much of the work done prior to that time had not focused on patients selected for broadly cross-neutralizing antibodies to HIV-1. In addition, a relatively small number of monoclonal neutralizing antibodies existed at the time. For these reasons we began to recruit a cohort of individuals screened for broadly cross-neutralizing antibodies to HIV. We, in collaboration with John Mascola at the Vaccine Research Center (VRC), used these sera to systematically dissect the means by which these patients cross-neutralize. We thus far have identified 30 such patients and are continuing to accrue additional subjects. A number of fundamental questions had not been addressed with regard to the HIV-specific humoral immune response of these patients. For example, it was not known if these patients had genetic or clinical characteristics, or HIV-specific cellular immune response characteristics in common. It was also not known whether neutralization was mediated by a few B cell clones directed to conserved epitopes or by an extremely polyclonal response to many epitopes. Given that our patients are infected with clade B viruses and should not have experienced infection by viruses belonging to multiple clades, we hypothesized that cross-neutralization is mediated through conserved epitopes on HIV envelope. In addition, although considerable work had been done on patient sera, very little had been done on HIV-specific B cells. The phenotype and immunoglobulin class of HIV-specific B cells in comparison to responses to other viruses remained poorly defined. Further, it remained unclear whether patients with broad cross-neutralizing activity are unique with regard to these parameters. One primary objective of our work on the humoral response to HIV is to understand the basis of a broadly cross-neutralizing antibody response in our patients. It was not known whether there are common features of the humoral response of such patients with regard to specificity. It was also not known whether neutralization was mediated by a few B cell clones directed to conserved epitopes or by an extremely polyclonal response to many epitopes. To understand the specificity and diversity of epitopes targeted by the B-cell response in patients with broad sera, we have initiated a collaborative effort to isolate monoclonal antibodies. This work has resulted in several publications using the targeted strategy of isolating monoclonal antibodies from sorted gp140-labelled B-cells. Collaborators at Rockefeller University have observed that gp140-labelled B-cells contain numerous clones with some neutralizing activity that may have an additive effect to produce breadth. However, individual monoclonal antibodies with extraordinary potency and breadth have been isolated in our laboratory or at the VRC. One of these monoclonals, named VRC01, is the most broadly neutralizing antibody isolated to date. Antibody-dependent cellular cytotoxicity (ADCC) is thought to potentially play a role in vaccine-induced protection from HIV-1. The characteristics of such antibodies remain incompletely understood. Furthermore, correlates between ADCC and HIV-1 immune status are not clearly defined. In 2012, we screened the sera of 20 HIV-1+ patients for ADCC. Normal human peripheral blood mononuclear cells were used to derive HIV-infected CD4+ T cell targets and autologous, freshly isolated, natural killer (NK) cells in a novel assay that measures GrB and HIV-1-infected CD4+ T cell elimination (ICE) by flow cytometry. We observed that complex sera mediated greater levels of ADCC than anti-HIV-1 envelope glycoprotein (Env)-specific mAbs, and serum-mediated ADCC correlated with the amount of IgG and IgG1 bound to HIV-1-infected CD4+ T cells. No correlation was detected between ADCC and viral load, CD4+ T cell count or neutralization of HIV-1SF162 or other primary viral isolates. Sera pooled from clade B HIV-1+ individuals exhibited breadth in killing targets infected with HIV-1 from clades A/E, B, and C. Taken together, these data suggest that the total amount of IgG bound to an HIV-1-infected cell is an important determinant of ADCC, and that polyvalent antigen-specific Abs are required for a robust ADCC response. In addition, Abs elicited by a vaccine formulated with immunogens from a single clade may generate a protective ADCC response in vivo against a variety of HIV-1 species. Increased understanding of the parameters that dictate ADCC against HIV-1-infected cells will inform efforts to stimulate ADCC activity and improve its potency in vaccinees. Through this work we continue to provide a better understanding of the specificities and functions of the HIV-specific humoral immune response that is most likely to provide protection from infection. Over the coming years we anticipate that this work, in the context of work from other groups, will greatly enhance our knowledge of what features of this response should be induced in vaccination strategies.
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