It is generally agreed that a potent and broad NtAb response by effective vaccines is needed to confer protection against HIV-1 infections. Thus, to aid in the design of vaccines, we characterized variants that are vertically transmitted, in a setting where antibodies are present, and examined the role of NtAb in selecting these variants during transmission. We examined the sensitivity of a total of 96 envelope variants from 12 MTCT pairs, each from an independent PCR amplification, to neutralization by maternal plasma. We compared 32 variants from the infants to 64 variants from the mothers and found that near the time of transmission, variants from infants were more resistant to neutralization by maternal plasma than was the overall virus population from their mothers.
The maternal viral variants displayed a wide range of neutralization sensitivities, even within the quasispecies from a single subject. The neutralization-sensitive variants may reflect the fact that these viruses are expressed from cells that have not undergone rapid turnover, such as “quiescently” infected resting T cells and/or monocytes/macrophages. Alternatively, the contemporaneous presence of both neutralization escape and sensitive variants may suggest the emergence of escape variants before the clearance of sensitive strains. Indeed, this dynamic has previously been observed during chronic HIV-1 infection (35
). The presence of variants that were sensitive to neutralization was not surprising given that we selected cases in which there was evidence for neutralization of the maternal primary isolate by her autologous plasma.
The selection of NtAb-resistant variants during MTCT is in apparent contrast to the results seen with five cases of heterosexual transmission in discordant couples in which the viruses that were transmitted were among the more neutralization-sensitive variants in the index case (11
). One explanation for the difference is that sexual transmission occurs in the absence of preexisting NtAb in the exposed person; thus, the fitness of a variant for transmission may be determined by other properties of the virus. In contrast, in the setting of MTCT, infants passively acquire maternal antibodies that usually persist for approximately 10 months after birth (12
). In this regard, MTCT represents a model of transmission that occurs in the face of NtAb pressure. In this setting, it may be more difficult for vaccines that induce NtAb with suboptimal breadth or potency to block the transmitted strains, as the bar for neutralization may be high.
The transmitted variants were capable of eliciting NtAb response in the infants. In fact, in two cases, NtAbs responses were high (IC50s of 4,108 and 21,650). These infant NtAbs were unlikely to be derived from maternal antibodies because in the limited maternal follow-up plasmas tested, the NtAb titers of maternal plasmas were much lower than those of the infant plasmas from the same time points (data not shown). These results suggest that the transmitted envelopes are able to elicit NtAb response in the newly infected hosts.
Transmembrane-directed antibodies 2F5 and 4E10 displayed greater breadth of neutralization against the transmitted variants than did antibodies directed to the envelope surface unit (biz and 2G12), which is consistent with previous observations that 2F5 and 4E10 generally show greater breadth of neutralization than biz and 2G12 (3
). None of the viruses were neutralized by 2G12, and biz was also mostly ineffective, with a few exceptions. Most importantly, TriMab (mix of 4E10, 2F5, and 2G12), which is being tested for efficacy in blocking MTCT, failed to neutralize 3 of 10 variants and neutralized several others with limited potency. On the basis of these observations, we conclude that the MAbs tested in this study may have limited benefit in protecting against most vertically transmitted non-subtype B viruses. It is unclear whether the vertically transmitted variants are more resistant to these MAbs and other NtAbs than viruses transmitted by other routes or viruses present during chronic infection. The fact that infant viruses could be neutralized by the infants' antibodies suggests that vertically transmitted viruses are capable of being neutralized by antibodies of the proper specificity. The lack of potency of the MAbs tested here may in part reflect the fact that the viruses under study are subtype A, C, and D variants, whereas the MAbs were derived from subtype B-infected persons. Thus, future passive strategies may benefit by combining the most potent of these with additional MAbs from subjects infected with diverse strains.
We also found that compared to variants in the index case, vertically transmitted variants contained significantly fewer PNGS in envelope, specifically in regions near the stem of the V1V2 loop, at positions 339 and 363 in C3, 392 in V4, and 465 in V5. This was somewhat surprising given that neutralization resistance is typically associated with increased PNGS in viruses that evolve during chronic infection (8
). Yet in the case of the transmitted viruses examined here, an inverse correlation appears to exist, suggesting that there may be other advantageous properties for less-glycosylated viruses during transmission. As predicted on the basis of the predicted structure of CD4-bound HIV-1 gp120 (22
), glycans in V1V2 loop may play a role in occlusion of the CD4 binding site, and glycans at positions 276 in C2, 363 in C3, and 465 in V5 are located proximally to residues directly involved in CD4 binding (22
). Removal of these glycans could potentially expose the CD4 binding site, thereby increasing viral infectivity.
There was no difference in the lengths of envelope variable loops between mother and infant viruses. Yet greater sensitivity to neutralization by maternal plasma was found to correlate with shorter variable loops in the envelope sequence, suggesting that length variation may be one mechanism for altering neutralization sensitivity of these viruses. However, features of envelope other than neutralization sensitivity, such as glycosylation and affinity for CD4 or coreceptor, may also be important characteristics of viruses that are transmitted.
In summary, we identified two mechanisms in the selection of vertically transmitted variants: escape from maternal NtAb and reduced glycosylation in envelope. The latter feature, represented by fewer PNGS in envelope, was also characteristic of variants transmitted heterosexually (9
), indicating that this may be a common property of transmitted variants independent of transmission mode. However, escape from NtAb was observed in vertically transmitted variants but the opposite—namely, neutralization sensitivity—was observed in some cases of sexually transmitted variants (11
) although not in others (15
). It remains to be determined whether there are common characteristics of all transmitted variants, independent of viral subtype and mode of transmission, that may provide insights into the selective pressures that occur during HIV-1 transmission.
For vaccines that aim to block HIV-1 spread by all modes of transmission, the variability in sensitivity to NtAb among variants of different subtypes or selected by different modes of transmission may need to be considered. The panel of vertically transmitted viruses characterized here may represent a useful screen for defining the ability of vaccine immunogens to elicit NtAbs against strains transmitted in regions of high HIV-1 prevalence. These viruses may also constitute a useful panel for assessing the potency of antibody cocktails under consideration as tools for passive therapy to prevent MTCT of HIV-1.