Human immunodeficiency virus type 1 is a major cause of morbidity and mortality worldwide and there
is urgent demand for a protective vaccine. A major goal of vaccine development is the elicitation of
antibodies capable of neutralizing diverse strains. In order to achieve this goal it is necessary to
understand the dynamic relationship between neutralizing antibodies (NAbs) and HIV-1, in vivo.
In humans, HIV-1 rapidly escapes from NAbs, confirming that humoral responses inhibit virus
replication. However, neutralizing responses are commonly detected in viraemic patients and the clinical
impact of NAbs on HIV-1 control is unclear. To investigate this further, viral load (VL) and NAb activity
were assessed longitudinally in patients enrolled into a clinical trial of short-course antiretroviral therapy
(ART), administered in early infection. The aims of this study were two-fold: i) to understand the
importance of VL in the control of NAb responses and ii) to assess whether NAbs contribute to durable
control of VL set-point.
A high-throughput pseudovirus neutralization assay was developed, using automated counting
procedures to quantify infected TZM-bl reporter cells. The assay was used to assess NAb responses with
autologous viruses derived from 22 patients. Seven patients with low VL set-points (<104 RNA copies/ml)
failed to develop neutralizing responses throughout the 48-144 week follow-up period. In contrast, the
remaining patients developed progressively-increasing neutralizing plasma titres (IC50) that correlated
with the extent and timing of VL rebound after cessation of ART. This suggests that the production of
NAbs depends on the duration and extent of viraemia in early infection. Viral load was poorly predictive
of neutralizing responses against heterologous isolates assayed in 38 patients, suggesting that other
factors are important in the production of antibodies with cross-neutralizing activity.
Depletion of specific immunological compartments can yield crucial information as to their functional
importance in vivo. We took advantage of a unique opportunity to investigate the role of NAbs and the
consequences of their depletion in an HIV-1 infected human. Three years after cessation of short-course
ART, the patient was treated for pre-existing low-grade lymphoplasmacytic lymphoma by antibodymediated
depletion of CD20+ B cells using rituximab. This treatment was followed by a 1.7 log10 rise in
HIV-1 VL which spontaneously reversed. Autologous NAb responses decreased as viraemia flared, and
recovered as VL was controlled. Antibodies were found to target the CD4 binding site (CD4bs), as shown
by competitive-binding assays. Sequence analysis revealed diversification through generation of new
variants as NAbs decreased, with subsequent selection of NAb-resistant mutants at sites consistent with
the binding data. These data suggest that B cell function contributed to long-term control of VL in this
individual and that NAbs may be more important in controlling HIV-1 infection than previously
suspected.