Rapid Escape from Preserved Cross-Reactive Neutralizing Humoral Immunity without Loss of Viral Fitness in HIV-1-Infected Progressors and Long-Term Nonprogressors

A substantial proportion of human immunodeficiency virus type 1 (HIV-1)-infected individuals has cross-reactive neutralizing activity in serum, with a similar prevalence in progressors and long-term nonprogressors (LTNP). We studied whether disease progression in the face of cross-reactive neutralizing serum activity is due to fading neutralizing humoral immunity over time or to viral escape. In three LTNP and three progressors, high-titer cross-reactive HIV-1-specific neutralizing activity in serum against a multiclade pseudovirus panel was preserved during the entire clinical course of infection, even after AIDS diagnosis in progressors. However, while early HIV-1 variants from all six individuals could be neutralized by autologous serum, the autologous neutralizing activity declined during chronic infection. This could be attributed to viral escape and the apparent inability of the host to elicit neutralizing antibodies to the newly emerging viral escape variants. Escape from autologous neutralizing activity was not associated with a reduction in the viral replication rate in vitro. Escape from autologous serum with cross-reactive neutralizing activity coincided with an increase in the length of the variable loops and in the number of potential N-linked glycosylation sites in the viral envelope. Positive selection pressure was observed in the variable regions in envelope, suggesting that, at least in these individuals, these regions are targeted by humoral immunity with cross-reactive potential. Our results may imply that the ability of HIV-1 to rapidly escape cross-reactive autologous neutralizing antibody responses without the loss of viral fitness is the underlying explanation for the absent effect of potent cross-reactive neutralizing humoral immunity on the clinical course of infection.The need for an effective vaccine to prevent the global spread of human immunodeficiency virus type 1 (HIV-1) is well recognized. The ability to elicit broadly neutralizing antibodies (BrNAbs) is believed to be crucial to developing a successful vaccine, ideally to acquire protective immunity or, alternatively, to achieve a nonprogressive infection with viral loads sufficiently low to limit HIV-1 transmission (1, 39).During natural infection, antibodies that are able to neutralize autologous virus variants are elicited in the majority of HIV-1-infected individuals. Early in infection, these neutralizing antibodies (NAbs) are mainly type specific, due to the fact that they are primarily directed against the variable domains in the viral envelope, and allow for the rapid escape of HIV-1 from antibody neutralization (8, 9, 14, 15, 20, 28, 41). Escape from type-specific neutralizing humoral immunity has been associated with enormous sequence variation, particularly in variable loops 1 and 2 (V1V2) of the envelope protein where large insertions and deletions are observed, as well as with changes in the number of potential N-linked glycosylation sites (PNGS) in the envelope protein (8, 15, 19, 22, 25, 27-31, 41). The rapid escape of HIV-1 from autologous type-specific NAbs seems to be the underlying explanation for the absent correlation between autologous humoral immunity and HIV-1 disease course. Furthermore, we recently observed that the changes in envelope that are associated with escape from autologous neutralizing humoral immunity do not coincide with a loss of viral fitness (7), providing an additional explanation for the lack of protection from disease progression by the autologous type-specific NAb response.In the last couple of years, the focus of research has shifted toward neutralizing humoral immunity with cross-reactive activity, defined as the ability to neutralize a range of heterologous HIV-1 variants from different subtypes. It has become apparent that about one-third of HIV-1-infected individuals develop cross-reactive neutralizing activity in serum. However, the prevalence of cross-reactive neutralizing activity in serum was similar for HIV-infected individuals with a progressive disease course and long-term nonprogressors (LTNP) (11, 12, 34, 37).We studied the underlying explanation for this observation in three LTNP and three progressors who all had high-titer cross-reactive neutralizing activity in serum within 2 to 4 years after seroconversion (SC). In all individuals, we observed that the potent and cross-reactive neutralizing immunity was preserved during the entire course of infection. However, the presence of cross-reactive neutralizing activity in serum did not prevent rapid viral escape from humoral immunity, which coincided with changes in envelope similar to those described for escape from type-specific autologous humoral immunity. Although broadly neutralizing antibodies are assumed to target the more conserved epitopes that may lie in crucial parts of the viral envelope, escape from cross-reactive neutralizing activity did not coincide with a loss in viral fitness. Our findings underscore that vaccine-elicited cross-reactive neutralizing immunity should protect against HIV-1 acquisition, since protection from disease progression, even by humoral immunity with strong cross-reactivity, may be an unachievable goal.