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Journal of Virology, December 2007, p. 12775-12784, Vol. 81, No. 23
0022-538X/07/$08.00+0     doi:10.1128/JVI.00624-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Human Immunodeficiency Virus-Specific CD8⁺ T-Cell Activity Is Detectable from Birth in the Majority of In Utero-Infected Infants

Christina F. Thobakgale,^1* Dhanwanthie Ramduth,¹ Sharon Reddy,¹ Nompumelelo Mkhwanazi,¹ Chantal de Pierres,¹ Eshia Moodley,¹ Wendy Mphatswe,¹ Natasha Blanckenberg,¹ Ayanda Cengimbo,¹ Andrew Prendergast,² Gareth Tudor-Williams,³ Krista Dong,⁴ Prakash Jeena,¹ Gupreet Kindra,¹ Raziya Bobat,¹ Hoosen Coovadia,¹ Photini Kiepiela,¹ Bruce D. Walker,^1,4,5 and Philip J. R. Goulder^1,2,4

HIV Pathogenesis Programme, Doris Duke Medical Research Institute, University of KwaZulu-Natal, Durban, South Africa,¹ Department of Paediatrics, Nuffield Department of Medicine, Peter Medawar Building for Pathogen Research, South Parks Rd., Oxford OX1 3SY, United Kingdom,² Department of Paediatrics, Imperial College of London, London, United Kingdom,³ Partners AIDS Research Center, Massachusetts General Hospital, 13th St., Bldg. 149, Charlestown, Boston, Massachusetts 02129,⁴ Howard Hughes Medical Institute, Chevy Chase, Maryland⁵

Received 23 March 2007/ Accepted 28 August 2007

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Human immunodeficiency virus (HIV)-infected infants in sub-Saharan Africa typically progress to AIDS or death by 2 years of life in the absence of antiretroviral therapy. This rapid progression to HIV disease has been related to immaturity of the adaptive immune response in infants. We screened 740 infants born to HIV-infected mothers and tracked development and specificity of HIV-specific CD8⁺ T-cell responses in 63 HIV-infected infants identified using gamma interferon enzyme-linked immunospot assays and intracellular cytokine staining. Forty-four in utero-infected and 19 intrapartum-infected infants were compared to 45 chronically infected children >2 years of age. Seventy percent (14 of 20) in utero-infected infants tested within the first week of life demonstrated HIV-specific CD8⁺ T-cell responses. Gag, Pol, and Nef were the principally targeted regions in chronic pediatric infection. However, Env dominated the overall response in one-third (12/36) of the acutely infected infants, compared to only 2/45 (4%) of chronically infected children (P = 0.00083). Gag-specific CD4⁺ T-cell responses were minimal to undetectable in the first 6 months of pediatric infection. These data indicate that failure to control HIV replication in in utero-infected infants is not due to an inability to induce responses but instead suggest secondary failure of adaptive immunity in containing this infection. Moreover, the detection of virus-specific CD8⁺ T-cell responses in the first days of life in most in utero-infected infants is encouraging for HIV vaccine interventions in infants.

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An association between human immunodeficiency virus (HIV)-specific CD8⁺ and CD4⁺ T-cell responses and control of viral replication has been well documented in acute adult infection (2, 4, 13-15), but it is less clearly established what role CD8⁺ T cells play in control of HIV in acute pediatric infection, which occurs both in utero (IU) and intrapartum (IP). In adults, the appearance of HIV-specific CD8⁺ T-cell immune responses in acute infection is temporally associated with a rapid decline in viremia from an average of 10 million copies/ml at peak to a median set point of 30,000 copies/ml (26, 33). In pediatric infection, there is typically no such rapid decline in viremia following acute infection, with viral loads remaining in an excess of 100,000 copies/ml over the first year of life, and decreasing only slowly over the next 2 to 3 years of life in survivors (31, 36).

A potential explanation for the absence of a dramatic decline in viremia in early pediatric infection is either low-frequency HIV-specific CD8⁺ T-cell activity and/or ineffective CD8⁺ T-cell activity in infancy. Previous studies of limited numbers of HIV-infected infants have demonstrated that HIV-specific CD8⁺ T-cell responses can be detected at low frequency in some infants (5, 24, 35). Moreover, when detected in infancy, the CD8⁺ T-cell responses generated had no immediate benefit in clinical outcome (5, 23, 32), and no studies have compared IU, IP, and chronic pediatric infection. Recent reports that CD8⁺ T-cell responses in infants can exert selection pressure in vivo in known CD8⁺ T-cell epitopes within the first few months of life (10, 20, 29) suggest that in some instances, at least, these responses may be functional. In addition, slow progression to disease has been well described in children who express HLA-B*27 or HLA-B*57 (10, 11), suggesting that CD8⁺ T-cell responses can be important in pediatric infection as in adult infection.

The aim of these studies was to examine a large cohort of infants born to HIV-infected mothers and to determine both the age at which HIV-specific CD8⁺ T-cell responses are induced in infected infants and the specificity of these responses using a panel of overlapping peptides spanning all the HIV proteins. These studies were undertaken in Durban, South Africa, a country in which its estimated that there are >100 newly infected infants born each day (http://www.avert.org/worldstats.htm).

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Study subjects. Sixty-three HIV type 1 (HIV-1)-infected infants born to HIV-positive mothers were enrolled from St. Mary's and Prince Mshiyeni Hospitals in Durban, South Africa, from 2003 to 2005 (Table 1). HIV-1-seropositive mothers were recruited during the last trimester of pregnancy, and a single dose of nevirapine was given to the mothers during labor and to the infants within 48 h of birth, according to the HIVNET-012 protocol (16, 17). A total of 719 mothers were enrolled and screened, and 740 infants were born to the mothers; 623 infants were uninfected, 41 were untraceable, and only 75 were infected. Sixty-three of the 75 infants met the clinical criteria for enrollment into the study. Exclusion criteria comprised prematurity, intrauterine growth restriction, and congenital anomaly (28).

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TABLE 1. Characteristics of the cohort of acutely infected infants at the time of first ELISPOT assay

Forty-five HIV-1-positive, antiretroviral therapy-naïve children with chronic infection between ages of 2 and 12 years were recruited from the Pediatric HIV outpatient clinic at King Edward VIII Hospital and McCord Hospital, both in Durban, South Africa. The median absolute CD4 count in this cohort of children with chronic HIV infection was 559, with a median CD4 percentage of 17 and a median viral load of 110,000 copies/ml (Table 2). The mothers gave written informed consent for participation of their children in both studies. These studies were approved by all the participating Institutional Review Boards.

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TABLE 2. Characteristics of the cohort of chronically infected children at the time of the first ELISPOT assay at baseline visit

Diagnosis of HIV-1 infection in infants. Infants were diagnosed as HIV infected following detection of plasma HIV RNA by RNA PCR (Roche Amplicor assay). Blood was collected on day 1 and day 28 of life. A positive plasma viral load (>400 RNA copies/ml) on day 1 or day 28 was followed by a confirmation test before enrollment of the infant into the study. Infants with detectable virus on day 1 were defined as IU infected (n = 44), and infants with undetectable virus on day 1 but with detectable virus on day 28 were defined as IP infected (n = 19). Since the majority of infants were breast fed, it is possible that this "IP" group includes some early breast milk mother-to-child transmission.

Viral load and CD4 measurement. Plasma viral loads were measured using either the Roche Amplicor Monitor assay detection limit of 400 HIV-1 RNA copies/ml plasma) or the Roche Ultrasensitive assay detection limit of 50 RNA copies/ml plasma), according to the manufacturer's instructions. CD4 counts were determined from fresh whole blood using Tru-Count technology and analyzed on a four-color flow cytometer (Becton Dickinson) according to the manufacturer's instructions.

Isolation of PBMCs. Blood was collected in EDTA tubes and processed within 6 h of collection. Peripheral blood mononuclear cells (PBMCs) were isolated from whole blood using Ficoll-Histopaque (Sigma, St. Louis, MO) density gradient centrifugation and were used fresh in enzyme-linked immunospot (ELISPOT) assays.

Synthetic HIV-1 peptides. A panel of 410 overlapping peptides (18-mers with a 10-amino-acid overlap) spanning the entire HIV-1 clade C consensus sequence were synthesized on an automated peptide synthesizer (MBS 396; Advanced ChemTech) and used in a matrix system in screening assays.

ELISPOT assays. Screening for T-cell responses was done ex vivo using the gamma interferon (IFN-) ELISPOT assay as previously described (18). The individually recognized peptides within the pools were determined by the use of a second ELISPOT assay.

Freshly isolated PBMCs were plated in 96-well polyvinylidene difluoride-backed plates (MAIP S45; Millipore) that had been previously coated with 100 µl of anti-human IFN- monoclonal antibody 1-D1k (0.5 µg/ml; Mabtech) overnight at 4°C. Peptides were added at a final concentration of 2 µg/ml to a 96-well plate with 100 µl of R10 medium at 50,000 or 100,000 cells/well. Negative controls with cells and medium only were run in quadruplicate along with two positive controls containing phytohemagglutinin. The plate with contents was incubated overnight at 37°C with 5% CO₂ and then processed. Following overnight incubation, the plate was washed with cold phosphate-buffered saline (PBS), and 0.5 µg/ml of IFN- monoclonal antibody biotinylated secondary antibody (7-B6-1; Mabtech) was added and left for 90 min in the dark at room temperature. The plate was then washed with cold PBS, and 0.5 µg/ml of streptavidin-alkaline phosphatase conjugate antibody (Mabtech) was added and left for 45 min in the dark at room temperature. IFN--producing cells were noted by direct visualization of the plate following development with alkaline phosphatase color reagents (Bio-Rad).

The IFN--secreting cells were quantified by counting the number of spots per well using an automated ELISPOT plate reader (AID ELISPOT reader system; Autoimmun Diagnostika GmbH, Strasburg, Germany). Results were expressed as number of spot-forming cells (SFC) per million PBMCs after subtractions of values for background wells. A response was defined as positive, using previously adopted criteria (1, 18), if it was 100 SFC/million PBMCs and 3 standard deviations above the mean for four background wells containing PBMCs but no peptide. The mean background levels in all assays were always less than 120 SFC/10⁶ cells, with a range of 0 to 120 SFC/10⁶ cells.

Quantitation of CD8⁺ T-cell responses towards each HIV protein was undertaken from the ELISPOT assays as follows. Following subtraction of the background in each well, the numbers of SFC for each well containing peptides within a particular protein were summed; only positive wells with responses of 100 SFC were used to calculate responses to each protein. Wells with responses of <100 SFC were treated as negative and were assigned a value of 90 SFC for statistical analyses. To calculate the relative contribution of each protein to the total CD8⁺ HIV-specific response for each study subject, the total response to all nine HIV proteins was summed, and the contribution of each individual protein was derived by dividing the protein-specific response by the total response. Any protein-specific response that was <100 SFC/million PBMCs represented 0% contribution to the total response.

Flow cytometric intracellular cytokine staining. Freshly isolated PBMCs (0.5 x 10⁶) were incubated at 37°C with 5% CO₂ for 90 min with peptide pools at a final concentration of 2 µg/ml per peptide following stimulation with anti-CD28 and anti-CD49 antibodies (Becton Dickinson). Brefeldin (Sigma) was added, and cells were incubated for a further 4.5 h at 37°C with 5% CO₂. Cells were then stained with anti-human allophycocyanin-conjugated CD8 and anti-human phycoerythroerythrin-conjugated CD4 antibodies (Becton Dickinson), washed, fixed, and permeabilized as previously described (30) before addition of anti-IFN--fluorescein isothiocyanate (FITC), anti-interleukin-2 (IL-2)-FITC, or anti-tumor necrosis factor alpha (TNF-)-FITC. Following 20 min of incubation, the cells were washed, resuspended in 200 µl of PBS, and acquired on a FACSCalibur (Becton Dickinson). Duplicate negative controls with PBMCs alone together with a positive control containing PBMCs stimulated with phytohemagglutinin were included in the assays. For the infant cohort, a minimum of 150,000 events were collected per subject, and a minimum of 100,000 events were collected for the cohort of chronically infected children.

The total CD4⁺ and CD8⁺ T-cell responses were obtained after subtracting the mean of two negative controls. Reference ranges were obtained from intracellular cytokine staining for Gag IFN- in a total of 23 HIV-uninfected infants between the ages of 1 week and 17 months. A response was considered positive if it was above 0.07% for CD8 responses and above 0.02% for CD4 responses. Gag-specific CD8⁺ responses ranged from 0.00 to 0.07% and CD4⁺ responses from 0.00 to 0.02% in these 23 uninfected controls.

HLA typing. DNA for HLA typing was extracted using a Puregene DNA isolation kit for blood (Gentra Systems, Minneapolis, MN) according to the manufacturer's instructions. HLA class I typing was done by DNA PCR using sequence-specific primers as previously described (18).

Statistical analysis. Fisher's exact test was used to compare proportions of IU- and IP-infected infants with early detectable responses and also to compare the numbers of responders among acutely and chronically infected children. The Mann-Whitney test was used to compare differences in the magnitude and contribution of each protein to the overall total response of the nine HIV proteins targeted by both acutely and chronically infected children. The Mann-Whitney test was also used to evaluate CD4⁺ and CD8⁺ T-cell responses measured by intracellular cytokine staining in acute and chronic children.

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HIV-specific CD8⁺ T-cell responses are detectable from day 1 of life in infected infants. To determine whether IU HIV infection induces an adaptive immune response above the level of any response in exposed, uninfected infants, we enrolled 10 randomly selected HIV-infected mothers and tested their newborn infants' HIV-specific CD8⁺ T-cell responses on day 1 of life, blind to diagnosis. Three of 10 had detectable CD8⁺ T-cell responses on day 1 of life, and all subsequently proved to be HIV infected by plasma HIV RNA determination (Fig. 1A). The remaining seven subjects who all had undetectable CD8⁺ T-cell responses (<100 SFC/million PBMCs) proved to be HIV uninfected.

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FIG. 1. Early detection of CD8+ T-cell responses in HIV-infected infants. (A) CD8+ T-cell responses on day 1 of life in 10 infants born to HIV-positive mothers. (B) Detection of HIV-specific CTL responses in acutely infected infants. Infants were grouped by weeks depending on the earliest time that the first assay could be done. For both plots, infants with CD8+ T-cell responses above the horizontal dotted line (100 SFC/million PBMCs) were defined as having positive responses and those below as having a negative response. The vertical dotted line divides IU- and IP-infected infants. Responses of <100 SFC were assigned 90 SFC and treated as negative.

In order to further define adaptive immune responses in acute infant infection, we studied an expanded cohort of infants who were HIV infected on day 1 of life (defined as IU infected) and other infants who were HIV negative on day 1 but became HIV positive when retested on day 28 (defined as IP-infected infants). A total of 63 infected infants were identified from 719 mothers enrolled in the study; 70% of these infants (44/63) were IU infected, and 30% (19/63) were IP infected (Table 1). The initial IFN- ELISPOT assays were performed as soon after diagnosis as feasible, before antiretroviral therapy was initiated, at a median of 9 days of age (range, 1 to 92 days) for IU-infected infants and at a median of 55 days of age (range, 31 to 105 days) for IP-infected infants. CD8⁺ T-cell responses were detectable in 39/63 infants (29 of 44 IU infected and 10 of 19 IP infected). In three of these infants, all of whom had detectable responses, peptides within Env and Vif were not included in the assay because of a paucity of PBMCs.

HIV-specific CD8⁺ T-cell responses were detected in 14 of 20 IU-infected infants tested within 1 week of birth, and the majority of IU-infected infants tested after 4 weeks of life had detectable responses (Fig. 1B and data not shown). Overall, a greater proportion (29/44; 65%) of IU-infected infants had detectable cellular immune responses compared to IP-infected infants (10/19; 52%) (not significant), with a median follow-up of the IP-infected infants of 52 days (range, 31 to 105 days). By comparison, all of 45 chronically infected children had detectable CD8⁺ T-cell responses when initially tested at year 2 of life or greater (median, 580 SFC/million PBMCs).

Frequent targeting of Env in early pediatric infection and of Nef in chronic infection. To identify the proteins principally targeted in early and chronic pediatric infection, the magnitudes of the responses to each of the nine HIV proteins were documented from the first time point at which assays were performed for the 36 acutely infected infants (Fig. 2A) and 45 chronically infected children (median age of 6 years) (Table 2; Fig. 2B) who had detectable responses upon comprehensive screening in ELISPOT assays. There was a greater number of responders to Gag, Pol, Vif, Nef, and Vpr (P = 0.0011, 0.0001, 0.0051, 0.0008, and 0.0051, respectively, by Fisher's exact test) among chronically infected children than among acutely infected infants. Further analysis confirmed a larger magnitude of responses to Gag, Pol, and Nef (P = 0.0012, 0.0002, and 0.0410, respectively, by Mann-Whitney test) in chronically infected than in acutely infected children. There were no significant differences between acute and chronically infected children in responses to Vpu, Tat, Env, and Rev.

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FIG. 2. Hierarchy of responses to HIV proteins targeted by acutely (A) and chronically (B) infected children. Analysis was made from the 36 acutely infected infants (n = 39; 3 were excluded due to insufficient cells to carry out full matrix screen) and 45 chronically infected children, all of whom made responses to at least one of the nine HIV proteins on the first ELISPOT assay. The first ELISPOT assay was undertaken in infants at a mean of 31 days of age (range, 1 day to 105 days; interquartile range, 7 to 46 days). The number of infants showing positive responses to each protein is shown in parentheses. Negative responses were assigned 90 SFC/million PBMCs (dotted line); values of 100 SFC/million PBMCs were defined as positive responses and those of <100 SFC/million PBMCs negative (see Materials and Methods).

We next compared the proportional contribution of each protein to the overall total response per child in both acute and chronic pediatric infection (Fig. 3A). The overall contribution of the Env and Rev responses was greater in acute infection than in chronically infected children (P < 0.0001 and P = 0.0074), and in chronically infected children the contribution of the Nef-specific response to the total response was greater than in acute infection (P = 0.0027 by Mann-Whitney test) (Fig. 3B). Of particular significance is the predominant targeting of Env in acute infection: in one-third (12/36) of infants Env was the dominant target, compared to 4% (2/45) of the chronically infected children (P = 0.00083 by Fisher's exact test) (Fig. 3A).

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FIG. 3. (A) Relative contribution of protein response to overall response in acute and chronic infection. A reanalysis of the same data shown in Fig. 2 is used to show the contribution of each protein to the total HIV-specific response in the acutely infected infants (top panel) and chronically infected children (bottom panel) who had significant responses on the first ELISPOT assay. (B) Comparison of the contributions of responses to different proteins in acutely (A) and chronically (C) HIV-infected children with detectable responses.

To determine whether the initial responses observed in children are maintained over time we tracked the responses longitudinally in four infected infants (Fig. 4). These were the only 4 of 20 followed longitudinally from birth who did not meet the clinical and immunological WHO criteria to receive highly active antiretroviral therapy (HAART) within the first 12 months of age. Two of the infants, 349-AC and 447-AC, illustrate the initially high contribution to the total HIV-specific response made by Env-specific CD8⁺ T cells in early pediatric infection. 447-AC required HAART after 14 months of life. 349-AC initially had dominant Env responses, which decreased over the first 12 months and were later replaced by Gag- and Pol-specific responses; development of the latter coincided with a significant decrease in viral load (186,000 RNA copies/ml at 14 months versus 2,280 RNA copies/ml at 24 months) and an increase in CD4 counts from 30% at 14 months to 41% at 24 months without HAART (Fig. 4A and B).

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FIG. 4. Longitudinal measurement of CD8+ T-cell responses, CD4, and viral load in therapy-naïve subjects A-349 (A), A-447 (B), A-133 (C), and A-517 (D). The CD4 counts and viral load measurements correspond to the time points at which CD8+ T-cell responses were determined.

133-AC, who achieved successful control of viremia without the need for antiretroviral therapy (to 494 copies/ml and a CD4 percentage of 29 at 31 months), showed a strongly dominant CD8⁺ Gag-specific response that persisted. Similarly, 517-AC initially had dominant Rev-specific responses that were soon replaced by dominant Gag-specific responses (Fig. 4C and D). Although viremia has remained high in this child (>1 x 10⁶ copies/ml), the CD4 percentage has also remained at high levels of >30%. However, these four anecdotal cases are insufficient to allow an analysis of specificity of the CD8⁺ response in infants and subsequent immunological and virological control of HIV infection.

Weak CD4⁺ Gag-specific T-cell activity in early pediatric infection. Since CD8⁺ T cells require functional CD4⁺ T-cell help to sustain their effector activity (7), we examined Gag-specific CD8⁺ and CD4⁺ T-cell activity in infants with early infection. We focused on Gag-specific responses since Gag is the dominant target for HIV-specific T-helper activity (30) and limitations on cell numbers precluded analysis of CD4⁺ T-cell responses to non-Gag proteins. In addition, Gag-specific CD4⁺ T-cell responses have been detected in acute HIV infection of adults (39). Gag-specific responses were measured at 2, 4, and 6 months of age only, as 50 and 75% of the infants required HAART by 6 and 12 months of age, respectively. The infants with acute infection had detectable CD8⁺ T-cell responses at all three time points (median Gag-specific CD8⁺ T-cell responses were 0.095, 0.155, and 0.14% at 2, 4, and 6 months, respectively). These responses were lower than but did not differ significantly from those measured in chronically infected children (median, 0.31%) (Fig. 5A). In contrast, infants with acute infection had significantly lower CD4⁺ T-cell responses at all time points than the chronically infected children, with median Gag-specific CD4⁺ T-cell responses being 0.01%, 0.01%, and 0.02% CD4 at 2, 4, and 6 months, respectively, compared to 0.06% for chronically infected children (P < 0.0001, P < 0.0001, and P < 0.0008, respectively) (Fig. 5B).

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FIG. 5. (A and B) Gag-specific CD8+ and CD4+ T-cell responses in acutely and chronically infected children by intracellular cytokine staining. (C and D) Gag-specific CD8+ and CD4+ IFN-, IL-2, and TNF- at 6 months of life in acutely infected infants. All infants studied were antiretroviral therapy naïve at time of analysis. Dashed lines indicate the upper limit of responses detected from the same assays undertaken with 23 HIV-uninfected control infants (see Materials and Methods). Horizontal bars indicate median responses in HIV-infected study subjects.

We also compared Gag-specific CD8⁺ (Fig. 5C) and CD4⁺ (Fig. 5D) IFN-, IL-2, and TNF- production in the same of group of infants at 6 months of life. IL-2 and TNF- responses were significantly less frequently detected than IFN-. In contrast, adults can generate substantial HIV-specific CD4⁺ IFN- and IL-2 responses during acute HIV infection (39), while chronically infected children on therapy show an increase in the frequency of IL-2-secreting CD4⁺ T cells (6).

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
These studies describe the early HIV-specific T-cell responses in pediatric infection. CD8⁺ T-cell responses are detectable from the first days of life in the majority (70%) of IU-infected infants. Responses were broadly directed, although Env-specific CD8⁺ T-cell activity in particular contributed significantly more to the total HIV-specific response in acutely infected infants than to that in chronically infected children (P < 0.0001). Gag-specific CD4⁺ T-cell responses were weak or undetectable in the first 6 months of life, contrasting with chronic pediatric or acute adult infection.

Previous studies using noncomprehensive screening assays and in very limited numbers of subjects have demonstrated weak HIV-specific CD8⁺ T-cell responses in infancy (24, 25). HIV-specific cytotoxic T lymphocytes (CTL) have been detected in cord blood at birth previously (24), and human cytomegalovirus-specific CTL have been detected in cord blood IU (27), indicating that immune responses to HIV can be detected very early in life. The studies described here demonstrate not only that these early responses can arise but that they are detectable in a high proportion (70%) of infants at the earliest time point that they were tested.

Studies evaluating the specificity of the initial CD8⁺ T-cell response in infants infected with HIV have not been systematically undertaken, and the finding that Env-specific CD8⁺ T-cell activity contributes substantially to this initial response is of interest in relation to recent studies with adults indicating that Env-specific CD8⁺ T-cell responses are associated with higher viral loads and Gag-specific CD8⁺ T-cell responses with lower viral loads (4, 19, 32). More studies will be needed to determine if this is related to the lack of a rapid reduction in viral load observed in early pediatric infection compared to early adult infection, where Env makes a smaller contribution to the initial CD8⁺ T-cell responses (22).

It was not possible here to relate the specificity of the early CD8⁺ T-cell response and progression in this cohort, as two-thirds of the study subjects were enrolled in a study of early HAART in pediatric HIV infection. Of the 20 infants who did not receive early HAART, only 2 achieved viral loads of <100,000 in the first year. The finding of early CD8⁺ T-cell responses combined with persistently high viral loads in the first year of life suggests that these CD8⁺ T cells are ineffective, and additional functional studies will be needed to determine how this apparent dysfunction compares to that seen in adults (4, 8). Comprehensive phenotypic analysis of CD8⁺ T cells in adults has demonstrated the association of polyfunctional CD8⁺ T cells with control of viremia (3). These comprehensive analyses were not undertaken with these study infants, but the lack of IL-2 and TNF- responses suggests that the majority of the CD8⁺ T-cell responses detectable via the IFN- ELISPOT assay may be monofunctional.

Limitations imposed by cell numbers allowed only Gag-specific CD4⁺ T-cell responses to be assessed. Gag was chosen since it is consistently the dominant target for HIV-specific CD4⁺ T-cell responses (30). The weak or undetectable Gag-specific CD4⁺ T-cell activity in acutely infected infants seen in this study also contrasts with acute adult infection, where adults present with high levels of CD4⁺ T-cell responses during acute infection (39). In addition, Gag-specific CD4⁺ T-cell activity has been reported in chronic infection in children >5 years old who have spontaneously controlled viremia without antiretroviral therapy (9). The marked absence of HIV-specific CD4⁺ T-cell activity even to 6 months of age suggests a fundamental reason why CD8⁺ T cells in infected infants are ineffective (12, 21). These findings of a lack of HIV-specific CD4⁺ T-cell activity are consistent with other studies (24), in one case showing minimal CD4⁺ T-cell responses in HIV-infected children until 3 to 5 years of age (34). Furthermore, where detectable, HIV-specific CD4⁺ T-cell responses have been reported to be type 2 as opposed to type 1 (37, 38) and therefore less likely to support the induction and maintenance of HIV-specific CTL activity. Of concern, and again in contrast to what is observed in acute adult infection (33), early treatment with antiretroviral therapy in infected infants did not result in increased HIV-specific CD4⁺ T-cell responses (D. Ramduth et al., unpublished data). However, the extent to which these findings result from or cause the persistent high levels of viremia observed in early pediatric HIV infection is not known.

The encouraging aspect of these data is that IU-infected infants mount CD8⁺ T-cell responses from the first day of life, while those infected IP have detectable responses a month after infection. Although 85% of infected infants (in this cohort) met current WHO criteria to initiate HAART within 12 months of infection, it is also clear that a small minority (2/20) of infected infants showed viral loads of <10,000 and a CD4 percentage of >30 by 24 months of age. Thus, spontaneous control of HIV is possible in pediatric HIV infection. Identification of greater numbers of "relative controller" children will facilitate further definition of what constitutes an effective immune response in early pediatric HIV infection. Moreover, the fact that the neonatal immune system can generate adaptive immune responses to HIV provides important information for the development of vaccines to prevent peripartum infection.

 
This study was funded by the Doris Duke Charitable Foundation (grant 20011031 to P.J.R.G), The Wellcome Trust (a grant to P.J.R.G), and Bristol Myers Squibb Secure the Future (grant RES116/01 to P.J.R.G.).

We thank the mothers and children for participation in this study, the clinic team at St. Mary's Hospital antenatal clinic and Prince Mshiyeni Hospital, and the HIV Pathogenesis Programme (HPP) team: T. Cele, T. Skhakhane, K. Mngwenya, T. Mchunu, D. Sindane, T. Phahla, M. Mbambo, M. Vanderstok, Z. Mncube, T. Moodley, N. Khan, K. Nair, K. Bishop, K. Ngumbela, and C. Day. We also thank G. Robbins and H. Ribaudo at Partners AIDS Research Center at Harvard for the statistical analysis input.

 
* Corresponding author. Mailing address: HIV Pathogenesis Programme, Doris Duke Medical Research Institute, University of KwaZulu-Natal, Durban, South Africa. Phone: 27 31 260 4608. Fax: 27 31 260 4036. E-mail: Thobakgalec{at}ukzn.ac.za

Published ahead of print on 19 September 2007.

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Journal of Virology, December 2007, p. 12775-12784, Vol. 81, No. 23
0022-538X/07/$08.00+0     doi:10.1128/JVI.00624-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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