Despite the recent demonstration of partial HIV-1 vaccine efficacy in humans6
, the immune responses required to protect against acquisition of infection have remained unclear. Preclinical studies of HIV-1 vaccine candidates have begun to elucidate immunologic correlates of protection against neutralization-sensitive viruses1–3
, but no study has to date reported vaccine protection against acquisition of heterologous, neutralization-resistant virus challenges1,7,8
. Mucosal SIVmac251 infection of rhesus monkeys represents a stringent preclinical model of a highly pathogenic, neutralization-resistant virus swarm1,9,10
, and repetitive mucosal challenges more closely mimic sexual HIV-1 transmission in humans than do single high-dose challenges10
. We therefore performed two studies to evaluate the protective efficacy of optimized adenovirus/poxvirus and adenovirus/adenovirus vector-based vaccines against repetitive, heterologous, intrarectal SIVmac251 challenges in rhesus monkeys.
In the first study, 40 Indian-origin rhesus monkeys (Macaca mulatta
) that did not express the class I alleles Mamu-A*01
, and Mamu-B*17
associated with spontaneous virologic control11–13
were immunized by the intramuscular route with the following vaccine regimens expressing SIVsmE543 Gag-Pol and Env immunogens (N=8/group): (i) DNA prime, MVA boost; (ii) MVA prime, MVA boost; (iii) Ad26 prime, MVA boost; (iv) MVA prime, Ad26 boost; and (v) sham controls. Groups were balanced for susceptible and resistant TRIM5α alleles1,14
. Monkeys were primed once at week 0 with 2×1010
vp Ad26 vectors or 108
pfu MVA vectors, or three times at weeks 0, 4, and 8 with 5 mg DNA vaccines. Animals were then boosted once at week 24 with 2×1010
vp Ad26 vectors or 108
pfu MVA vectors.
The vaccine regimens elicited different profiles of cellular and humoral immune responses, as measured by IFN-γ ELISPOT assays (, Supplementary Fig. 1
), multiparameter intracellular cytokine staining (ICS) assays8,15–17
(, Supplementary Fig. 2
), cellular immune breadth (Supplementary Fig. 3
), SIVmac251 Env-specific binding antibody ELISAs (), tier 1 neutralizing antibody (NAb) assays against tissue culture laboratory adapted (TCLA) tier 1 SIVsmE660 (CP3C-P-A8) and SIVmac251 (TCLA) pseudoviruses (), and antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cell-mediated virus inhibition (ADCVI) assays (Supplementary Fig. 4
). Tier 2 NAb responses against neutralization-resistant SIVsmE660 (CR54-PK-2A5) and SIVmac251 (SIVmac251.30) pseudoviruses, however, were below the 50% neutralization cutoff for positivity, although positive trends were observed in all vaccinated groups (Supplementary Fig. 4
Immunogenicity and protective efficacy of the adenovirus/poxvirus vaccines
To evaluate the protective efficacy of these vaccine regimens, all monkeys were challenged repetitively beginning at week 52 (six months following the boost immunization) with six intrarectal inoculations of the heterologous virus SIVmac251 utilizing a 1:1000 dilution (930 TCID50
) of our challenge stock9
. After the first challenge, 75% of sham control monkeys became infected, as compared with only 12–25% of the animals that received the heterologous vector regimens DNA/MVA, Ad26/MVA, and MVA/Ad26 (). The percent uninfected animals declined proportionately with each challenge, and the majority of vaccinees and all controls were infected by the end of the challenge protocol. Monkeys that received the Ad26/MVA and MVA/Ad26 vaccines required 3 challenges to infect 50% of animals in each group, whereas only 1 challenge was required to infect 50% of animals in the control group (P=0.004 and P=0.006, respectively, Wald tests, proportional hazard model). The heterologous vector regimens also exhibited decreased hazard ratios of 0.17 (CI 0.05–0.57) to 0.20 (CI 0.06–0.63) as compared with the controls, corresponding to an 80–83% reduction in the per-exposure probability of infection (; vaccine efficacy VE = 1 – hazard ratio), utilizing the statistical approach of Self et al.4
and Gilbert et al.5
. These data demonstrate vaccine protection against acquisition of infection following repetitive, heterologous, IR SIVmac251 challenges.
Control monkeys exhibited peak viral loads on day 14 following infection and then relatively stable mean setpoint viral loads of 5.85 log SIV RNA copies/ml (Supplementary Fig. 5
). The Ad26/MVA and the MVA/Ad26 vaccines resulted, respectively, in at least 2.32 and 1.08 log reductions of mean setpoint viral loads compared with sham controls for over 250 days (P=0.0037 for each vaccine versus sham, Wilcoxon rank-sum tests) (, Supplementary Fig. 5
). Moreover, half the animals in the Ad26/MVA group either demonstrated rapid and durable virologic control to undetectable levels (; N=3) or remained uninfected (; N=1). The Ad26/MVA and MVA/Ad26 vaccines also afforded a survival advantage as compared with the controls (P=0.025, log-rank test) (Supplementary Fig. 6
We next evaluated the immunologic correlates of protection against acquisition of infection, defined as the number of challenges required to establish infection, and virologic control, defined as setpoint viral loads. Our pre-specified primary immunologic correlates analysis (Supplementary Table 1
) demonstrated that protection against acquisition of infection was best correlated with Env binding ELISA antibody responses (; P<0.0001, Spearman rank-correlation test) and tier 1 NAb titers (; P=0.0034) immediately prior to challenge. Protection against acquisition of infection also correlated with V2-specific antibodies that presumably represent a subset of total Env binding antibodies (; P<0.0001). Virologic control was correlated with Gag ELISPOT breadth (; P=0.0002) and magnitude (; P=0.0058) immediately prior to challenge, consistent with our previous observations18
Correlates of protection against acquisition of infection and virologic control with the adenovirus/poxvirus vaccines
In our exploratory immunologic correlates analysis, we evaluated 35 humoral and cellular immune parameters at both peak and memory timepoints prior to challenge as possible immunologic correlates of acquisition and virologic control following challenge. No additional immune parameters were significantly correlated with protection against acquisition of infection in this analysis after multiple comparison adjustments (Supplementary Table 2
). Gag-, Pol-, and Env-specific effector memory CD8+ T lymphocyte responses exhibited trends towards protection against acquisition but did not achieve statistical significance according to our pre-specified criteria. In contrast, multiple humoral and cellular immune responses were significantly correlated with virologic control (Supplementary Table 3
), including Env ELISA, NAb, and ADCC responses as well as Gag ELISPOT magnitude and breadth, Pol ELISPOT magnitude, and Env CD4+ effector memory responses. These data suggest a model in which protection against acquisition of infection is correlated with vaccine-elicited Env-specific antibody responses, whereas virologic control may be correlated with both T lymphocyte and antibody responses. These distinct immunologic correlates likely reflect fundamentally different biologic requirements for blocking establishment of infection at the mucosal site of inoculation compared with controlling viral replication after infection has become disseminated19
. However, the actual mechanisms of protection remain to be determined.
We next evaluated directly the hypothesis that Env was critical for blocking acquisition of infection in this system. In the second study, 40 rhesus monkeys that did not express the class I alleles Mamu-A*01
, and Mamu-B*17
were immunized by the intramuscular route with Ad35 prime20
, Ad26 boost21
vaccine regimens expressing (i) SIVsmE543 Gag-Pol (N=16); (ii) SIVsmE543 Gag-Pol and Env (N=16); and (iii) sham controls (N=8). Groups were balanced for susceptible and resistant TRIM5α alleles1,14
. Monkeys were primed once at week 0 with 2×1010
vp Ad35 vectors and boosted once at week 24 with 2×1010
vp Ad26 vectors. Cellular immune responses were assessed by IFN-γ ELISPOT assays (, Supplementary Fig. 7
) and multiparameter ICS assays in both the periphery (, Supplementary Fig. 8
) and in colorectal mucosa (Supplementary Fig. 9
). Env-specific humoral immune responses were assessed by ELISAs in both the periphery () and in colorectal mucosa (Supplementary Fig. 10
), tier 1 NAb assays (), and ADCC assays (Supplementary Fig. 11
). Only marginal tier 2 NAb responses were observed (Supplementary Fig. 11
Immunogenicity and protective efficacy of the adenovirus/adenovirus vaccines
We assessed protective efficacy of these vaccine regimens against repetitive, heterologous, intrarectal SIVmac251 challenges as described in the first study. After the first challenge, 50% of sham control monkeys became infected, as compared with only 12% of the animals that received the Gag-Pol-Env vaccine (). The monkeys that received the Gag-Pol-Env vaccine required 4 challenges to infect 50% of animals in each group, whereas only 1 challenge was required to infect 50% of animals in the control group (; P=0.002, Wald test, proportional hazard model). Moreover, the Gag-Pol-Env vaccine resulted in a decreased hazard ratio of 0.20 (CI 0.07–0.55), corresponding to an 80% reduction in the per-exposure probability of infection. In contrast, the Gag-Pol vaccine afforded only a marginal protective effect, demonstrating the critical role of Env in blocking acquisition of infection in this model. The Gag-Pol and Gag-Pol-Env vaccines resulted in, respectively, at least 1.59 log and 2.18 log reductions of setpoint viral loads as compared with controls (, Supplementary Fig. 12
; P=0.0006 and 0.0002, respectively, Wilcoxon rank-sum tests). Immunologic correlates of protection against acquisition of infection were consistent with the first study, and both peripheral () and rectal () Env-specific IgG correlated with reduced acquisition risk.
Correlates of protection against acquisition of infection with the adenovirus/adenovirus vaccines
Taken together, these data demonstrate that optimized adenovirus/poxvirus and adenovirus/adenovirus vector-based vaccines afforded significant protection against acquisition of infection following highly pathogenic, heterologous, neutralization-resistant SIVmac251 challenges in rhesus monkeys (, , Supplementary Fig. 13
). Although several studies have previously shown partial protection against acquisition of neutralization-sensitive virus challenges1–3
, no HIV-1 vaccine candidate has to date blocked acquisition of heterologous, difficult-to-neutralize virus challenges, including Ad57
, and CMV8
vaccines. In particular, a recent study demonstrated that a DNA/Ad5 vaccine afforded partial protection against acquisition of SIVsmE660, which is a neutralization-sensitive tier 1A virus in TZM-bl neutralization assays, but the same vaccine afforded no efficacy against neutralization-resistant SIVmac2511
, highlighting important differences in the stringencies between these two SIV challenge models as well as potentially important phenotypic differences between Ad serotypes17
. However, we note that the acquisition effect in the present study was relative rather than absolute and that the majority of vaccinees became infected by the end of the challenge protocol.
Our studies also demonstrate that inclusion of Env in the vaccine was required for the acquisition effect (), despite an 18% difference in the amino acid sequences between the vaccine strain and challenge virus. Moreover, our immunologic correlates analyses (, ; Supplementary Tables 1–3
) suggest that Env-specific antibodies may be critical for blocking acquisition of infection, whereas multiple cellular and humoral immune responses may correlate with virologic control, although the actual mechanisms of protection remain to be determined. In addition, the RV144 immunologic correlates analyses raised the hypothesis that vaccine-elicited V1/V2-specific antibodies may reduce HIV-1 acquisition risk in humans22
. Our data (, ) are consistent with this hypothesis, although it remains unclear whether V2-specific antibodies actually protect or simply represent a marker for other Env-specific antibodies or other protective factors.
Considerable efforts are currently underway to identify and to reverse engineer potent, broadly reactive monoclonal antibodies23,24
. Although the induction of such NAb responses by a vaccine would be highly desirable, no Env immunogens have to date been developed that can elicit these responses. Our findings suggest that a substantial degree of protection can be achieved against stringent virus challenges even in the absence of high titers of tier 2 NAbs, perhaps reflecting the importance of antibody effector functions that may not be fully measured by traditional virus neutralization assays. Of note, the partial protection in the present study was observed with vectored Env and without a purified Env protein subunit boost. The degree to which an Env protein boost may further improve the protective efficacy afforded by these vaccine regimens remains to be determined.
In summary, our data demonstrate the first proof-of-concept that vaccination can protect against acquisition of stringent, heterologous, neutralization-resistant SIVmac251 challenges in rhesus monkeys. These findings, together with the observations of a critical requirement for Env and the distinct immunologic correlates of protection against acquisition of infection and virologic control, pave novel paths forward for HIV-1 vaccine development.