Magnitude and breadth of the neutralizing antibody response in the RV144 and Vax003 HIV-1 vaccine efficacy trials

Abstract

Background: A recombinant canarypox vector expressing human immunodeficiency virus type 1 (HIV-1) Gag, Pro, and membrane-linked gp120 (vCP1521), combined with a bivalent gp120 protein boost (AIDSVAX B/E), provided modest protection against HIV-1 infection in a community-based population in Thailand (RV144 trial). No protection was observed in Thai injection drug users who received AIDSVAX B/E alone (Vax003 trial). We compared the neutralizing antibody response in these 2 trials.

Methods: Neutralization was assessed with tier 1 and tier 2 strains of virus in TZM-bl and A3R5 cells.

Results: Neutralization of several tier 1 viruses was detected in both RV144 and Vax003. Peak titers were higher in Vax003 and waned rapidly in both trials. The response in RV144 was targeted in part to V3 of gp120.vCP1521 priming plus 2 boosts with gp120 protein was superior to 2 gp120 protein inoculations alone, confirming a priming effect for vCP1521. Sporadic weak neutralization of tier 2 viruses was detected only in Vax003 and A3R5 cells.

Conclusion: The results suggest either that weak neutralizing antibody responses can be partially protective against HIV-1 in low-risk heterosexual populations or that the modest efficacy seen in RV144 was mediated by other immune responses, either alone or in combination with neutralizing antibodies.

Figure 1.

Neutralizing antibody (NAb) responses as measured in TZM-bl cells. NAbs were assessed against a panel of 6 tier 1 reference strains of Env-pseudotyped viruses in the TZM-bl assay. Samples for RV144 included plasma from 112 vaccine recipients and 20 placebo recipients obtained at 2 weeks after the fourth inoculation (visit 8). Samples for Vax003 included serum from 90 vaccine recipients and 30 placebo recipients obtained at 2 weeks after the second (visit 5) and fourth (visit 9) inoculation. All results for placebo recipients in Vax003 were negative (not shown). Top, Positive response rates (frequency of positive results at ≥1:20 plasma dilution) against each of 6 tier 1 reference viruses (listed on the x-axis). Middle, Box plots of NAb titers against each virus. For the box plots, 25% of values lie below the box, 25% lie above the box, and 50% lie below the horizontal line (the median) inside the box. Vertical lines above the box extend to a distance 50% greater than the height of the box; points beyond this are unusually high values (outliers). Bottom, Magnitude-breadth (M-B) curves of 50% inhibitory dose (ID₅₀) NAb titers against all 6 viruses (x-axis, neutralization titers; y-axis, fraction of viruses neutralized). Dashed lines represent subject-specific responses. Solid lines represent group averages.

Figure 2.

Longevity of the neutralizing antibody (NAb) response in RV144 and Vax003. A and B, Titers of NAbs against MN.3 and TH023.6 at 2 weeks and at 6, 12, 18, 24, 30, and 36 months after final boosting in RV144. C, Positive response rates (frequency of positive results at ≥1:20 plasma dilution) against MN.3 and TH023.6 at each time point shown in A and B. D and E, Box plots of ID₅₀ NAb titers against 6 tier 1 viruses at 2 weeks and 6 months after the fourth inoculation. F, Positive response rates against the 6 tier 1 viruses at 2 weeks and 6 months after the fourth inoculation. Six-month longevity was the longest period available in the Vax003 trial design. Abbreviation: ID₅₀, 50% inhibitory dose.

Figure 3.

Kinetics of the neutralizing antibody (NAb) response in RV135. Serum NAbs for 45 vaccine recipients and 14 placebo recipients were assessed at baseline and at 2 weeks after the third and fourth inoculations (after the first and second protein inoculation) in the TZM-bl assay. All results for placebo recipients were negative (data not shown). Abbreviation: ID₅₀, 50% inhibitory dose.

Figure 4.

Heightened sensitivity of A3R5 cells for detecting neutralizing antibody (NAbs). Serum from 7 vaccine recipients and 2 placebo recipients in Vax003 (2 weeks after the fourth inoculation) were assayed for neutralizing activity against C1080.c03 Env.IMC.LucR in TZM-bl and A3R5 cells by using an identical stock of virus in both assays. Placebo recipients are labeled 03–066 and 03–055.

Figure 5.

Neutralizing antibody (NAb) responses as measured in A3R5 cells. NAbs were assessed against a panel of tier 1 and tier 2 reference strains of Env.IMC.LucR viruses in the A3R5 assay. Plasma (RV144) and serum (Vax003) samples are the same as those described in Figure 1. Top, Positive response rates (frequency of positive results at ≥1:20 plasma dilution) against each virus (listed on the x-axis). Because of limited sample volumes, 2 viruses (427299 and CM244) were not assayed at visit 5 for Vax003 (nt, not tested). Middle, Box plots of NAb titers against each virus (7 viruses for RV144 and 5 viruses for Vax003). For the box plots, 25% of values lie below the box, 25% lie above the box, and 50% lie below the horizontal line (the median) inside the box. Vertical lines above the box extend to a distance 50% greater than the height of the box; points beyond this are unusually high values (outliers). Bottom, Magnitude-breadth (M-B) curves of NAbs against the 5 viruses that were common to the assays performed for RV144 and Vax003 (C1080, C3347, R2184, CM235, and 644039). Neutralization titer is shown on the x-axis; the fraction of viruses neutralized is shown on the y-axis. Dashed lines represent subject-specific responses. Solid lines represent group averages. Abbreviation: ID₅₀, 50% inhibitory dose.