A growing consensus suggests that eliciting neutralizing antibodies (NAbs) will be necessary to achieve protection from HIV infection through immunization. However, induction of a broadly cross-neutralizing antibody response has thus far not been achieved by any candidate vaccines. Identification of HIV-infected individuals with broadly cross-neutralizing antibodies to HIV may provide important information regarding the titer and breadth of neutralizing antibodies and the frequency, phenotype, and specificities of HIV-specific B cells that produce these antibodies. Thus far, efforts to determine the mechanism(s) of broad cross-neutralization have been hampered by a number of obstacles. These include a lack of cohorts with broad responses, a lack of techniques to purify antigen-specific cells, and inefficient cloning techniques. To address these obstacles and better understand the mechanism of broad neutralizing antibody responses to HIV, we have begun efforts in three major areas. The first is to screen for and recruit patients that represent the best available example of a broad neutralizing antibody response to HIV. The second is to develop techniques to detect and enrich for HIV-specific B cells. The third is to develop techniques that will permit high-throughput cloning of HIV-specific B cells that will permit careful study of specificities that mediate broad cross-neutralization. We recently reported on the magnitude and breadth of the HIV-specific antibody response and frequency of HIV-specific B cells in our cohorts. We have screened 113 patients for sera able to neutralize diverse HIV strains. We determined the breadth and titer of these sera against a diverse array of HIV envelopes, many of which are difficult to neutralize, and identified several patient sera showing potent and broad HIV-1 neutralization. Sera able to neutralize at least 4 of 5 viral isolates were found in more than one-third of progressors and slow progressors, but much less frequently in long-term non-progressors (LTNPs). This low level of neutralizing antibodies in LTNPs is believed secondary to reduced amounts of HIV antigens in these patients. Most envelope (Env)-specific antibody-secreting B cells were CD27hiCD38hi plasmablasts, an activated population of antibody-secreting cells. The total plasmablast frequency was higher in HIV-infected patients than in uninfected donors: 0.0031% of B cells and 0.047% of plasmablasts secreted Env-specific IgG in an ELISpot assay. More recently, we developed a new method of labelling HIV-specific B cells with Env gp140 protein, which enhances our ability to produce human monoclonal antibodies from patients with broad cross-neutralization. We found 0.09% of B cells to be Env-specific by this method, a frequency far higher than indicated by ELISpot. gp140-labelled B cells were predominantly CD27+ and surface IgG+. These data describe the breadth and titer of serum NAbs and the frequency and phenotype of HIV-specific B-cells in a cohort of patients with broad cross-neutralizing antibody responses that are potential goals for a preventive HIV vaccine. In addition, we found that considerable breadth and potency can be found in approximately 20% of HIV-infected patients. These data suggest that broad neutralization of HIV is achievable in humans. However, a considerable fraction of HIV-infected patients (30-75%) have some decrease in antibody responses to pathogens other than HIV. Thus, these results suggest that the 20% figure likely underestimates the fraction of HIV-uninfected vaccinees that may achieve broad and potent neutralization.
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