Studies performed early in the HIV/AIDS epidemic indicated that HIV-specific T cells were infrequent and narrowly-targeted in infants(46
), but subsequent data suggest that these quantitative deficiencies may be largely attributable to the early stage of infection, as acute HIV infection is now known to induce a relatively narrow T cell response in adults as well as children(48
). Indeed our data support other recent pediatric studies demonstrating that HIV-specific CD8 T cells can be detected in most infants during the first months of life(10
), and further indicate that this response progressively broadens throughout childhood, with HIV-specific T cell frequencies among older children approximating those of adults(11
). Thus, there do not appear to be substantial quantitative
differences in the CD8 response to HIV during infancy. However, in the present study, we describe several age-related qualitative
differences in the HIV-specific T cell response that may contribute to the inability of infants to restrict viral replication. These differences include a propensity of infant CD8 T cells to target variable proteins such as Nef or Env rather than Gag, a lower degree of functionality among infant CD8 T cells, and a paucity of virus-specific CD4 T cell help during early infancy.
Our results in clade B -infected infants and young children support and extend those of another recent pediatric study which described age-related differences in the distribution of responses at the viral protein level among clade C-infected African infants (10
). This prior study found that Env-specific responses predominated during the first six months of life, while responses to Gag were very uncommon. We observed a similar paucity of Gag-specific T cell responses, with only 17% of infants in this age group exhibiting a Gag-specific response. However in our cohort, responses to Nef were slightly more common than to Env during early infancy, perhaps reflecting differences in class I HLA allele frequencies or HIV-1 subtypes of the populations studied. Interestingly, both Nef- and Env-specific responses have previously been associated with higher levels of HIV viremia in clade C-infected Africans(16
), and a similar association of Nef responses and poor viral control was observed in our multivariate model. Both Nef and Env are characterized by a relatively high degree of sequence variability, making them potentially elusive targets in a rapidly evolving virus, in contrast to Gag which is relatively conserved. Despite this sequence heterogeneity, preferential targeting of Nef was previously observed in a cohort of acutely infected adults, whose dominant responses shifted after prolonged antigen exposure to include Gag, Pol, and Env(50
). The cross-sectional nature of our study did not permit us to differentiate whether the observed age-related differences reflect a temporal shift, in which acute-phase responses to Nef or Env are lost or eclipsed by late-emerging Gag-specific responses, or instead reflect a survival bias favoring infants who mount T cell response to Gag. The prospective longitudinal studies that would be required to distinguish these two possibilities may not be ethically feasible in the HAART era, as infants are at high risk for rapid progression to AIDS without early initiation of HAART(51
The infrequent recognition of Gag by infant T cells may have important implications for viral control. In the present study, subjects who recognized at least one Gag epitope exhibited significantly lower HIV RNA levels than those who did not, and the frequency of Gag-specific T cells correlated inversely with viral load after adjustment for age. These findings are in agreement with other recent studies indicating that Gag-specific T cell responses are associated with better control of HIV viremia in adults (16
). The potential mechanism underlying this association remains uncertain. The gag
sequence is highly conserved relative to other HIV proteins, and the virus may be less able to escape from Gag-specific T cells due to functional constraints on the Gag protein, resulting in more durable antiviral T cell responses. Indeed, the inability of HIV to tolerate sequence changes within highly conserved Gag epitopes without great cost to viral replicative fitness has been postulated as an explanation for the association of HLA-B*27 and B*57 with spontaneous viral containment and long-term nonprogressive HIV disease(52
). An alternative explanation for the superior efficacy of Gag-specific CTL is that HIV-infected cells have been demonstrated to present Gag-derived CD8 T cell epitopes within 2 hours of infection, prior to viral integration, transcription, and Nef-mediated downregulation of HLA(56
). This phenomenon appears to be unique to Gag epitopes, owing to cross-presentation of the abundant Gag protein particles present in internalized virions, and could confer an ability to lyse infected target cells before the release of progeny virions, which would be highly advantageous for viral containment.
Our data add to mounting evidence that there is a generalized impairment of CD4 responses to HIV and other viruses such as CMV during early childhood(10
). Previous studies have shown that virus-specific proliferative responses to both CMV(43
) and HIV(42
) are largely absent during the first five years of life. Perinatally CMV-infected infants exhibit a deficiency of virus-specific IFNγ production by CD4, but not CD8, T cells when compared to adults with acute CMV infection (41
). The results of the present study indicate that there is a similar temporal pattern in the emergence of CD4-mediated IFNγ responses to HIV among infants. We observed a virtual absence of HIV-specific CD4 cells during the first year of life, despite high-frequency CD8 responses in many of these infants, followed by a linear increase in the frequency of Gag-specific CD4 cells during early childhood. These results are in agreement with prior studies, mostly limited to measurement of Gag-specific IFNγ production, which found HIV-specific CD4 T cells to be absent during the first year of life (10
), and detectable in older children but generally at lower frequencies than those reported in adults(44
). Although our study is more comprehensive in that it assessed production of multiple cytokines in response to all HIV proteins, responses were nonetheless absent or of extremely low magnitude in the majority of infants. A generalized deficit in T cell help during infancy could have important consequences for the ability of infants to control viral infections, as it might be expected to compromise CD8 effector function and/or memory potential, similar to the “unhelped CD8 T cell” phenotype described following vaccination of CD4-depleted mice(39
). This impairment of T cell help during early infancy may help to explain the lower frequency of polyfunctional CD8 T cells observed among younger infants in the present study. It remains to be determined whether this age-related deficiency in virus-specific CD4 responses is due to developmental differences in antigen presenting cells, costimulatory pathways, or CD4 T cells themselves, or is instead due to active suppression by CD4 T regulatory cells(61
), which are particularly abundant during infancy.
Mounting empiric evidence for the superior antiviral efficacy of Gag-specific T cells, combined with theoretical considerations such as the high degree of Gag sequence conservation and the kinetic advantage of early Gag epitope presentation, make HIV-Gag a particularly attractive vaccine target. The mechanism underlying the inefficient targeting of Gag by infants is unclear, and solving this puzzle will likely require a better understanding of the general determinants of immunodominance in the setting of viral infections – which may in turn lead to approaches to manipulate the hierarchy of epitopes targeted by vaccine-induced immune responses. Our data indicate that optimal immunization of neonates may require strategies to skew the immunodominance hierarchy away from that observed in natural pediatric infection, and toward induction of Gag-specific responses.