Infants of HIV-1-positive mothers have passively acquired HIV-specific NAbs at the time of virus exposure, and thus represent a unique setting to examine the potential for such antibodies to impact the risk of HIV-1 acquisition. Here we found that infants who remained uninfected with HIV had NAb breadth and potency at birth similar to those who became HIV infected. Thus, despite the presence of these HIV-specific antibodies, there is no evidence that they provide any protection from infection in exposed infants. There was no correlation between the infant infection status at 6 weeks, 6 months, or 24 months of life and the breadth and potency of passively acquired HIV-specific NAbs in infants.
Virtually all infants (99%) had antibodies capable of neutralizing at least one of the viruses that were generated from a panel of eight envelope variants representing both vertically and sexually transmitted HIV-1s of diverse subtypes (4
). Importantly, this virus panel included variants that were sensitive to CD4 binding site antibodies (VRC01 and b12), antibodies that recognized a quaternary epitope on the surface unit (PG9), and antibodies directed to the membrane proximal domain of the transmembrane protein (4E10 and 2F5). Thus, the selected viruses provided a surrogate measure of whether antibodies directed to the epitopes recognized by broadly neutralizing antibodies, which are presumably accessible on variants that are neutralized by the MAbs, may play a role in protection. However, there was no association between the infants' ability to neutralize any of these viruses individually and subsequent acquisition, even when controlling for maternal viral load.
In order to determine whether unique characteristics of the maternal viruses could make them less sensitive to NAbs targeted to conserved epitopes, we examined whether viruses from women in this cohort were recognized by HIV-1-specific MAbs with broad specificities. The maternal viruses exhibited a range of neutralization sensitivities to 2F5, VRC01, and PG9 similar to what has been reported for a large panel of diverse HIV-1 variants (4
), suggesting that maternal viruses are not unusual in their susceptibilities to broad and potent MAbs. The neutralization sensitivities of the maternal variants were also similar to those of the viruses used to examine the breadth of the antibodies present in infant plasma. Thus, the virus panel was representative of the viruses that an infant encounters with respect to susceptibility to NAbs targeted to epitopes that are recognized by broad and potent HIV-specific MAbs.
The breadth and potency of the NAb responses acquired by the infants through maternal passive transfer were comparable to those observed in chronically infected women who were screened with a subset of the same viruses (36
). This suggests efficient transfer of NAb antibodies to infants and allowed us to examine whether protection was afforded by HIV-specific NAbs at levels similar to those found in natural HIV-1 infection. Our finding that passively acquired NAbs at these levels are not broadly protective in the setting of MTCT is consistent with studies of passive antibody protection in macaques, including a recent study showing protection using a low-dose challenge model designed to more closely mirror antibody levels in natural HIV-1 infection (18
). In this study and several other studies of passive transfer of HIV-1 antibodies, the macaques were exposed to a challenge virus that encodes a highly sensitive HIV-1 envelope variant (SHIVSF162
) that is highly sensitive to the MAb tested (Ig-Gb12; IC50
, 0.29 μg/ml [19
]). Indeed, based on in vitro
), more than half of all circulating viral variants studied to date would be predicted to require at least ~100-fold-more b12 to achieve a similar potency of neutralization. Thus, NAbs at this level would not be expected to protect against most of the diverse variants to which high-risk humans are exposed. Highly neutralization-sensitive variants of the type used in the macaque studies may be even more rare in the setting of MTCT, where viral evolution in the mother in response to continued antibody pressure may further increase the fraction of variants that are more resistant to maternal antibodies.
Indeed, one caveat to these experiments is the fact that there are likely to be viruses that were already selected in the mother to be less susceptible to her specific repertoire of NAbs. Because HIV-1 continually evolves to escape NAb pressure (39
), most individuals generally harbor a mixture of variants that range in neutralization sensitivity. Thus, even if the mother has a broad antibody response, the more resistant variants could be transmitted despite the presence of a strong NAb response. This model is supported by studies of autologous NAb responses in the setting of MTCT, showing that viruses that are transmitted are among the more neutralization-resistant viruses in the mother (10
). However, while the aggregate data indicate selection for more resistant variants in MTCT, there are cases where the viruses transmitted to infants are still relatively sensitive to maternal NAbs (53
). Collectively, these findings suggest that there may be a threshold of NAbs required for protection, perhaps one that may also be influenced by the presence of other cofactors that affect MTCT. In some cases, the levels of HIV-1-specific NAbs found may be adequate to provide some protection, but only against highly neutralization-sensitive variants. In this context, it is perhaps not surprising that a broad and potent NAb response would not predict protection because breadth and potency against a panel of viruses do not measure the neutralization sensitivity against the most neutralization-resistant virus in the population.
Despite the unique aspects of MTCT noted above, the finding that the breadth and potency of the NAb response are not correlates of protection from HIV-1 acquisition is not specific to this setting. Similar results were observed among a small number of cases of HIV-1 superinfection (3
). As with infants vertically infected with HIV-1, individuals who became superinfected did not exhibit any deficits in the breadth or potency of NAb responses compared to controls, and in some cases, they had NAbs specific for the superinfecting strain near the time of exposure (3
). Collectively, these studies strongly suggest that the levels of HIV-specific NAbs found in natural HIV-1 infection, while potentially capable of providing partial protection against the most sensitive HIV-1 envelope variant, are not adequate to provide protection against HIV-1 infection with diverse, circulating strains of HIV-1 that span a range of neutralization sensitivities.
The focus of this study was on antibodies present in the infant at the time of exposure to HIV, as this is most relevant for considering the potential of antibodies elicited through immunization to protect exposed individuals. Even when the analyses were restricted to infants infected at a time when the antibody levels were highest (within 6 weeks or 6 months of life), there was no correlation between infant infection and NAb breadth and potency. However, these studies did not address the potential roles of maternal antibodies in blood, genital secretions, or breast milk or infant antibodies in the gut in impacting infant infection. It is unknown whether the repertoire of maternal antibodies is precisely reflected in the antibodies transferred to her infant. Indeed, several studies suggest that antibodies in the mother, the index case, may provide some protection (2
) even if antibodies in the infant, the exposed individual, do not protect, as suggested by this study.
We cannot rule out that there could be viruses that would predict the ability of NAbs to protect that were not included in the virus panel used here. In a Thai study of maternal antibodies and MTCT, there was evidence that the neutralizing potential against one particular regional HIV-1 strain was correlated with infant infection (2
). It is difficult to compare these studies directly because the prior study examined antibodies in the maternal index case, while ours focused on antibodies in the exposed infant. But these studies collectively raise the possibility that breadth and potency as currently measured using virus panels may not be ideal for predicting protective benefit; rather, there may be particular HIV-1 variants that provide a better surrogate measure of protective antibodies that remain to be identified. These studies may also suggest that our current NAb assays are not detecting responses that are relevant in transmission. Such assays measure the potential of antibodies to neutralize cell-free virus, and they do not address whether antibodies capable of neutralizing cell-associated virus may play a role. Such antibodies may be particularly important in protection in the setting of MTCT, where there is a stronger association with the levels of cell-associated virus and risk of infant infection than with cell-free virus levels (41
Current vaccine efforts to prevent HIV-1 infections are focused on immunization of an uninfected population, and that is why the studies described here examined NAbs in the infant. One notable difference between passively transferred antibodies and those found in vaccinees is that the latter would be elicited in an immunocompetent host who had the potential to develop more potent NAb responses, particularly to a highly immunogenic vaccine. There is currently considerable interest in whether NAbs elicited by vaccination can protect exposed humans from HIV-1, but there is not yet data demonstrating that such protection is possible. In this first detailed study of the protective effect of neutralizing antibodies in infants, no benefit was observed, at least when NAb responses were measured against a virus panel. As noted above, this does not preclude a protective effect of NAbs in the index case. However, it is important to distinguish these two situations when drawing inferences for vaccine strategies, given that current efforts are not directed at immunizing an infected population to prevent viral spread. Indeed, MTCT offers a unique setting to compare the role of NAbs in protection in the index case versus in the exposed partner.
While our studies suggest that a broad and potent HIV-1 response at levels found in natural HIV-1 infection may not be adequate to provide protection from infection, they do not rule out the possibility that NAbs, if present at higher levels and/or of different specificities, could provide protection from infection. Indeed, other protective viral vaccines, such as those for hepatitis B virus (HBV) and human papillomavirus (HPV), induce NAb levels that are similar to, or even significantly higher than, those found in natural infection (6
). At present, the NAb responses induced by current HIV-1 vaccine immunogens are at a considerably lower level than those found in chronic HIV-1 infection, where there is continual antigenic stimulation by an evolving viral quasi-species (for examples, see references 14
). Thus, a major unmet challenge is to identify HIV-1 immunogens that can elicit broad NAb responses of higher potency than those elicited during natural infection.
There are promising new HIV-specific broad antibodies that have been isolated from infected individuals (50
), and there is evidence that some MAbs are more efficacious in passive-transfer studies than would be predicted based on in vitro
neutralization measures (19
). However, until there are SHIV/macaque models that better mirror the neutralization properties of circulating strains of HIV, it is hard to define the levels of antibodies needed for protection in these models. Thus, it is critical that we continue to evaluate immune correlates of protection in exposed human populations, such as infants of HIV-1-positive mothers, to define the potential for specific immune responses to protect against diverse circulating strains of HIV.