In vivo efficacy of human immunodeficiency virus neutralizing antibodies: estimates for protective titers

Abstract

The definition of plasma neutralizing antibody titers capable of controlling human immunodeficiency virus (HIV) infection in vivo is considered a critical step in vaccine development. Here we provide estimates for effective neutralization titers by assessing samples from a recent passive immunization trial with the neutralizing monoclonal antibodies (MAbs) 2G12, 2F5, and 4E10 using an analytic strategy that dissects the contributions of these MAbs to the total neutralization activity in patient plasma. Assessment of neutralization activities for six responding patients with partial or complete control of viremia during the MAb treatment and for the eight nonresponding patients revealed a significant difference between these groups: Among responders, MAb-mediated activity exceeded the autologous neutralization response by 1 to 2 log units (median difference, 43.3-fold), while in the nonresponder group, the autologous activity prevailed (median difference, 0.63-fold). In order to reach a 50% proportion of the responders in our study cohort, MAb neutralizing titers higher than 1:200 were required based on this analysis. The disease stage appears to have a significant impact on the quantities needed, since titers above 1:1,000 were needed to reach the same effect in chronic infection. Although our analysis is based on very small sample numbers and thus cannot be conclusive, our data provide a first estimate on how in vitro-measured neutralizing antibody activity can relate to in vivo efficacy in controlling HIV infection and may therefore provide valuable information for vaccine development. Interestingly, lower neutralizing antibody levels showed an effect in acute compared to chronic infection, suggesting that in early disease stages, therapeutic vaccination may show promise. Equally, this raises hopes that a preventive vaccine could become effective at comparatively lower neutralizing antibody titers.

Figures

FIG. 1.

Dissection of MAb contributions to neutralization activity. Total, MAb-mediated, and patients' (autologous) antibody (Ab)-mediated neutralization activities (70% NT) against the heterologous virus JR-FL (A and B) and the autologous viruses (C and D) were evaluated for all 14 patients. Data are means from two to four independent experiments. (A and C) Representative profiles of neutralization activities of plasma from patient NAB04 against JR-FL (A) and the autologous virus of patient NAB04 (C), analyzed throughout the trial. The MAb infusion period and off-treatment phase (MAb washout) are indicated. Gray squares, viral load (VL; expressed as copies of HIV RNA/ml of patient plasma); black circles with solid lines, total neutralization activity in plasma (combined autologous-Ab and MAb activities); dashed lines, autologous-Ab activity. The calculated MAb NTs are shown in separate panels as dark red, orange, and green areas for 2G12, 2F5, and 4E10, respectively. The sum of the activities of all three MAbs is shown as a light blue area in the top panel. (B and D) Summary of total neutralization activity (black) and MAb-mediated neutralization activity (red) for each of the 14 individuals during the trial against JR-FL (B) and autologous viruses (D).

FIG. 2.

Dissection of MAb contribution to neutralization activity. (A and C) Comparison of mean MAb-mediated NT70s between weeks 2 and 12 for nonresponding and responding patients against the heterologous virus JR-FL (A) and the autologous viruses (C). Dark red, orange, and green bars represent 2G12-, 2F5-, and 4E10-mediated activities, respectively. Cumulative MAb NTs (sum of all three MAb NTs) (blue bars) for each patient were used to compare the effects of MAb neutralization in responders and nonresponders. Median (interquartile range [IQR]) MAb NTs and P values by the Mann-Whitney U test are provided. (B and D) Simple linear regression analysis of interdependency between total and MAb-mediated neutralization activities against the heterologous virus JR-FL (B) and autologous viruses (D) using robust methods of variance estimation to adjust for intrapatient clustering of measurements. For regression model output and measures of goodness of fit, see insets. Note that confidence intervals (CI) not encompassing zero indicate statistically significant slopes. Black and red symbols represent nonresponders and responders, respectively. Data are means from two to four independent experiments.

FIG. 3.

Influence of MAb-mediated neutralization activity on treatment outcome. Shown are analyses of mean NTs between weeks 2 and 12. (A) Cumulative MAb NT70s (orange bars) and autologous neutralization activity (green bars) against autologous viruses in nonresponders and responders. (B) Comparison of total (black), autologous-antibody-mediated (green), and MAb-mediated (orange) NT70s against autologous viruses in nonresponding (open symbols) and responding (solid symbols) patients. Group comparisons were performed using the Mann-Whitney test. (C) Ratio of MAb-induced responses to autologous responses in responding (solid circles) versus nonresponding (open circles) patients. Group comparisons were performed using the Mann-Whitney test.

FIG. 4.

Prediction of MAb NTs required for protection. (A) Estimated MAb NTs against the autologous viruses required to achieve 50% protection among responding patients according to the method of Reed and Muench (15, 22). The gray shaded area represents the threshold of MAb NT activity required to achieve a 50% proportion of responders. (B) Analysis as described for panel A of a patient cohort subgrouped into acute and chronic individuals. Blue and green shaded areas represent the thresholds of MAb NT activity required to achieve a 50% proportion of responders among acute and chronic individuals, respectively.