Infrequent detection of HIV-1-specific, but not cytomegalovirus-specific, CD8(+) T cell responses in young HIV-1-infected infants

Role of Early Life Cytotoxic T Lymphocyte and Natural Killer Cell Immunity in Paediatric HIV Cure/Remission in the Anti-Retroviral Therapy Era

Only three well-characterised cases of functional cure have been described in paediatric HIV infection over the past decade. This underlines the fact that early initiation of combination antiretroviral therapy (cART), whilst minimising the size of the viral reservoir, is insufficient to achieve cure, unless other factors contribute. In this review, we consider these additional factors that may facilitate functional cure in paediatric infection. Among the early life immune activity, these include HIV-specific cytotoxic T-lymphocyte (CTL) and natural killer (NK) cell responses. The former have less potent antiviral efficacy in paediatric compared with adult infection, and indeed, in early life, NK responses have greater impact in suppressing viral replication than CTL. This fact may contribute to a greater potential for functional cure to be achieved in paediatric versus adult infection, since post-treatment control in adults is associated less with highly potent CTL activity, and more with effective antiviral NK cell responses. Nonetheless, antiviral CTL responses can play an increasingly effective role through childhood, especially in individuals expressing then 'protective' HLA-I molecules HLA-B*27/57/58:01/8101. The role of the innate system on preventing infection, in shaping the particular viruses transmitted, and influencing outcome is discussed. The susceptibility of female fetuses to in utero mother-to-child transmission, especially in the setting of recent maternal infection, is a curiosity that also provides clues to mechanisms by which cure may be achieved, since initial findings are that viral rebound is less frequent among males who interrupt cART. The potential of broadly neutralising antibody therapy to facilitate cure in children who have received early cART is discussed. Finally, we draw attention to the impact of the changing face of the paediatric HIV epidemic on cure potential. The effect of cART is not limited to preventing AIDS and reducing the risk of transmission. cART also affects which mothers transmit. No longer are mothers who transmit those who carry genes associated with poor immune control of HIV. In the cART era, a high proportion (>70% in our South African study) of transmitting mothers are those who seroconvert in pregnancy or who for social reasons are diagnosed late in pregnancy. As a result, now, genes associated with poor immune control of HIV are not enriched in mothers who transmit HIV to their child. These changes will likely influence the effectiveness of HLA-associated immune responses and therefore cure potential among children.

Preservation of lymphocyte functional fitness in perinatally-infected and treated HIV+ pediatric patients displaying sub-optimal viral control

Background

Host–pathogen dynamics associated with HIV infection are quite distinct in children versus adults. We interrogated the functional fitness of the lymphocyte responses in two cohorts of perinatally infected HIV+ pediatric subjects with early anti-retroviral therapy (ART) initiation but divergent patterns of virologic control. We hypothesized that sub-optimal viral control would compromise immune functional fitness.

Methods

The immune responses in the two HIV+ cohorts (n = 6 in each cohort) were benchmarked against the responses measured in age-range matched, uninfected healthy control subjects (n = 11) by utilizing tests for normality, and comparison [the Kruskal–Wallis test, and the two-tailed Mann–Whitney U test (where appropriate)]. Lymphocyte responses were examined by intra-cellular cytokine secretion, degranulation assays as well as phosflow. A subset of these data were further queried by an automated clustering algorithm. Finally, we evaluated the humoral immune responses to four childhood vaccines in all three cohorts.

Results

We demonstrate that contrary to expectations pediatric HIV+ patients with sub-optimal viral control display no significant deficits in immune functional fitness. In fact, the patients that display better virologic control lack functional Gag-specific T cell responses and compared to healthy controls they display signaling deficits and an enrichment of mitogen-stimulated CD3 negative and positive lymphocyte clusters with suppressed cytokine production.

Conclusions

These results highlight the immune resilience in HIV+ children on ART with sub-optimal viral control. With respect to HIV+ children on ART with better viral control, our data suggest that this cohort might potentially benefit from targeted interventions that might mitigate cell-mediated immune functional quiescence.

An HLA-I signature favouring KIR-educated Natural Killer cells mediates immune control of HIV in children and contrasts with the HLA-B-restricted CD8+ T-cell-mediated immune control in adults

Natural Killer (NK) cells contribute to HIV control in adults, but HLA-B-mediated T-cell activity has a more substantial impact on disease outcome. However, the HLA-B molecules influencing immune control in adults have less impact on paediatric infection. To investigate the contribution NK cells make to immune control, we studied >300 children living with HIV followed over two decades in South Africa. In children, HLA-B alleles associated with adult protection or disease-susceptibility did not have significant effects, whereas Bw4 (p = 0.003) and low HLA-A expression (p = 0.002) alleles were strongly associated with immunological and viral control. In a comparator adult cohort, Bw4 and HLA-A expression contributions to HIV disease outcome were dwarfed by those of protective and disease-susceptible HLA-B molecules. We next investigated the immunophenotype and effector functions of NK cells in a subset of these children using flow cytometry. Slow progression and better plasma viraemic control were also associated with high frequencies of less terminally differentiated NKG2A+NKp46+CD56^dim NK cells strongly responsive to cytokine stimulation and linked with the immunogenetic signature identified. Future studies are indicated to determine whether this signature associated with immune control in early life directly facilitates functional cure in children.

In adults, immune control of HIV is strongly influenced by antiviral CD8+ T-cell responses restricted by ‘protective’ HLA class I molecules, such as HLA-B*57, and by ‘disease-susceptible’ HLA class I molecules such as HLA-B*58:02. By contrast, Natural Killer (NK) cells responses make a smaller, albeit significant, contribution. In this study, we evaluate in children living with HIV the contribution of NK cell responses to immune control of HIV, in an age group where HIV-specific CD8+ T-cell responses have less impact on disease outcome. A cohort of >300 therapy-naïve children living with HIV shows that a genetic signature favouring a KIR-education on NK cells is associated with slow progression and better viraemic control. Consistent with this, we observed control of HIV viraemia and lower total HIV DNA levels among children was associated with a less differentiated NKG2A+NKp46+ CD56^dim NK cell population that functionally was highly responsive to cytokine stimulation. Thus, the study identifies a signature that can impact future therapeutic strategies to achieve remission in children.

Impact of Early Versus Late Antiretroviral Treatment Initiation on Naive T Lymphocytes in HIV-1-Infected Children and Adolescents - The-ANRS-EP59-CLEAC Study

Background

The early initiation of antiretroviral therapy (ART) in HIV-1-infected infants reduces mortality and prevents early CD4 T-cell loss. However, the impact of early ART on the immune system has not been thoroughly investigated in children over five years of age or adolescents. Here, we describe the levels of naive CD4 and CD8 T lymphocytes (CD4/CD8TN), reflecting the quality of immune reconstitution, as a function of the timing of ART initiation (early (<6 months) versus late (≥24 months of age)).

Methods

The ANRS-EP59-CLEAC study enrolled 27 children (5-12 years of age) and nine adolescents (13-17 years of age) in the early-treatment group, and 19 children (L-Ch) and 21 adolescents (L-Ado) in the late-treatment group. T lymphocytes were analyzed by flow cytometry and plasma markers were analyzed by ELISA. Linear regression analysis was performed with univariate and multivariate models.

Results

At the time of evaluation, all patients were on ART and had a good immunovirological status: 83% had HIV RNA loads below 50 copies/mL and the median CD4 T-cell count was 856 cells/µL (interquartile range: 685-1236 cells/µL). In children, early ART was associated with higher CD8TN percentages (medians: 48.7% vs. 31.0%, P = 0.001), and a marginally higher CD4TN (61.2% vs. 53.1%, P = 0.33). In adolescents, early ART was associated with low CD4TN percentages and less differentiated memory CD8 T cells. CD4TN and CD8TN levels were inversely related to cellular activation and gut permeability.

Conclusion

In children and adolescents, the benefits of early ART for CD8TN were clear after long-term ART. The impact of early ART on CD4TN appears to be modest, because pediatric patients treated late respond to HIV-driven CD4 T-lymphocyte loss by the de novo production of TN cells in the thymus. Our data also suggest that current immune activation and/or gut permeability has a negative impact on TN levels.

Clinical Trial Registration

ClinicalTrials.gov, identifier NCT02674867.

Therapeutic vaccination of SIV-infected, ART-treated infant rhesus macaques using Ad48/MVA in combination with TLR-7 stimulation

Globally, 1.8 million children are living with HIV-1. While antiretroviral therapy (ART) has improved disease outcomes, it does not eliminate the latent HIV-1 reservoir. Interventions to delay or prevent viral rebound in the absence of ART would be highly beneficial for HIV-1-infected children who now must remain on daily ART throughout their lifespan. Here, we evaluated therapeutic Ad48-SIV prime, MVA-SIV boost immunization in combination with the TLR-7 agonist GS-986 in rhesus macaque (RM) infants orally infected with SIV_mac251 at 4 weeks of age and treated with a triple ART regimen beginning 4 weeks after infection. We hypothesized immunization would enhance SIV-specific T cell responses during ART-mediated suppression of viremia. Compared to controls, vaccinated infants had greater magnitude SIV-specific T cell responses (mean of 3475 vs 69 IFN-γ spot forming cells (SFC) per 10⁶ PBMCs, respectively, P = 0.01) with enhanced breadth of epitope recognition and increased CD8⁺ and CD4⁺ T cell polyfunctionality (P = 0.004 and P = 0.005, respectively). Additionally, SIV-specific gp120 antibodies against challenge and vaccine virus strains were significantly elevated following MVA boost (P = 0.02 and P < 0.001, respectively). GS-986 led to expected immune stimulation demonstrated by activation of monocytes and T cells 24 hours post-dose. Despite the vaccine-induced immune responses, levels of SIV DNA in peripheral and lymph node CD4⁺ T cells were not significantly different from controls and a similar time to viral rebound and viral load set point were observed following ART interruption in both groups. We demonstrate infant RMs mount a robust immunological response to this immunization, but vaccination alone was not sufficient to impact viral reservoir size or modulate rebound dynamics following ART release. Our findings hold promise for therapeutic vaccination as a part of a combination cure approach in children and highlight the importance of a pediatric model to evaluate HIV-1 cure interventions in this unique setting of immune development.

While antiretroviral therapy (ART) has improved disease outcome and reduced HIV-1 transmission, it is not a cure, as interruption of ART results in rapid viral rebound due to the persistent latent reservoir. Interventions to induce HIV-1 remission in the absence of ART would be highly beneficial to children living with HIV-1, sparing them from the associated adherence requirements, side effects, and cost of ART. Here, we used our previously established pediatric model of oral SIV infection and ART suppression of viremia in infant rhesus macaques (RMs) to evaluate the safety and efficacy of an Ad48-SIV prime, MVA-SIV boost therapeutic vaccine approach plus TLR-7 stimulation. Our study demonstrates this vaccination strategy is immunogenic in infants; however, unlike previously reported results in adult RMs using a similar approach, vaccination did not result in a difference in the level of CD4⁺ T cell-associated SIV DNA or viral rebound dynamics after ART interruption when compared to control infant RMs. These results highlight the importance of pre-clinical studies using pediatric models and indicate potential HIV-1 cure strategies may differentially impact adults and children.

Impact of Early Antiretroviral Therapy Initiation on HIV-Specific CD4 and CD8 T Cell Function in Perinatally Infected Children

Early initiation of antiretroviral therapy (ART) in vertically HIV-infected children limits the size of the virus reservoir, but whether the time of treatment initiation (TI) can durably impact host immune responses associated with HIV infection is still unknown. This study was conducted in PBMC of 20 HIV-infected virally suppressed children on ART (mean age 9.4 y), classified as early treated (ET; age at ART initiation ≤0.5 y, n = 14) or late treated (LT; age at ART initiation 1-10 y, n = 6). Frequencies and functions of Ag-specific CD4 (CD40L⁺) and CD8 (CD69⁺) T cells were evaluated by intracellular IL-2, IFN-γ, and TNF-α production with IL-21 in CD4 or CD107a, granzyme B and perforin in CD8 T cells following stimulation with HIV gp140 protein (ENV) or GAG peptides by multiparameter flow cytometry. ET showed a higher proportion of cytokine-producing ENV- and GAG-specific CD4 and CD8 T cells compared with LT. In particular, ET were enriched in polyfunctional T cells. RNA sequencing analysis showed upregulation of immune activation pathways in LT compared with ET. Our results suggest that timing of TI in HIV-infected children has a long-term and measurable impact on the quality of the HIV-specific T cell immune responses and transcriptional profiles of PBMC, reinforcing the importance of early TI.

Deep sequence analysis of HIV adaptation following vertical transmission reveals the impact of immune pressure on the evolution of HIV

Human immunodeficiency virus (HIV) can adapt to an individual’s T cell immune response via genomic mutations that affect antigen recognition and impact disease outcome. These viral adaptations are specific to the host’s human leucocyte antigen (HLA) alleles, as these molecules determine which peptides are presented to T cells. As HLA molecules are highly polymorphic at the population level, horizontal transmission events are most commonly between HLA-mismatched donor/recipient pairs, representing new immune selection environments for the transmitted virus. In this study, we utilised a deep sequencing approach to determine the HIV quasispecies in 26 mother-to-child transmission pairs where the potential for founder viruses to be pre-adapted is high due to the pairs being haplo-identical at HLA loci. This scenario allowed the assessment of specific HIV adaptations following transmission in either a non-selective immune environment, due to recipient HLA mismatched to original selecting HLA, or a selective immune environment, mediated by matched donor/recipient HLA. We show that the pattern of reversion or fixation of HIV adaptations following transmission provides insight into the replicative cost, and likely compensatory networks, associated with specific adaptations in vivo. Furthermore, although transmitted viruses were commonly heavily pre-adapted to the child’s HLA genotype, we found evidence of de novo post-transmission adaptation, representing new epitopes targeted by the child’s T cell response. High-resolution analysis of HIV adaptation is relevant when considering vaccine and cure strategies for individuals exposed to adapted viruses via transmission or reactivated from reservoirs.

Highly mutable pathogens utilise genetic variations within T cell epitopes as a mechanism of immune escape (viral adaptation). The diversity of the human leucocyte antigen (HLA) molecules that present viral targets to T cells in human populations partially protects against rapid population-level accumulation of human immunodeficiency virus (HIV) adaptations through horizontal transmissions. In contrast, vertical transmissions occur between haplo-identical mother/child pairs, and potentially include adaptive changes through father-mother-child transmission, representing a pathway to complete pre-adaptation to HLA alleles in child hosts over only two transmission events. We utilised next-generation sequencing to examine HIV evolution in the unique setting of vertical HIV transmission. We predict the in vivo replicative cost and immune benefit of specific HIV adaptations that could be used to inform vaccine design and cure strategies to combat viral immune adaptation.

Human Immunodeficiency Virus Type 1 DNA Decay Dynamics With Early, Long-term Virologic Control of Perinatal Infection

Background.

Early antiretroviral therapy (ART) limits proviral reservoirs, a goal for human immunodeficiency virus type 1 (HIV-1) remission strategies. Whether this is an immediate or long-term effect of virologic suppression (VS) in perinatal infection is unknown.

Methods.

We quantified HIV-1 DNA longitudinally for up to 14 years in peripheral blood mononuclear cells (PBMCs) among 61 perinatally HIV-1-infected youths in the Pediatric HIV/AIDS Cohort Study who achieved VS at different ages. Participants in group 1 (n = 13) were <1 year of age and in group 2 (n = 48) from 1 through 5 years of age at VS. Piecewise linear mixed-effects regression models assessed the effect of age at VS on HIV-1 DNA trajectories during VS.

Results.

In the first 2 years following VS, HIV-1 DNA levels decreased by -0.25 (95% confidence interval [CI], -.36 to -.13) log10 copies/million PBMCs per year and was faster with early VS by age 1 year compared with after age 1 (-0.50 and -0.15 log10 copies/million PBMCs per year, respectively). Between years 2 and 14 from VS, HIV-1 DNA decayed by -0.05 (95% CI, -.06 to -.03) log10 copies/million PBMCs per year and was no longer significantly different between groups. The estimated mean half-life of HIV-1 DNA from VS was 15.9 years and was shorter for group 1 compared to group 2 at 5.9 years and 18.8 years, respectively (P = .09). Adjusting for CD4 cell counts had no effect on decay estimates.

Conclusions.

Early effective, long-term ART initiated from infancy leads to decay of HIV-1-infected cells to exceedingly low concentrations desired for HIV-1 remission strategies.

Role of HIV-specific CD8+ T cells in pediatric HIV cure strategies after widespread early viral escape

Recent studies have suggested greater HIV cure potential among infected children than adults. A major obstacle to HIV eradication in adults is that the viral reservoir is largely comprised of HIV-specific cytotoxic T lymphocyte (CTL) escape variants. We here evaluate the potential for CTL in HIV-infected slow-progressor children to play an effective role in "shock-and-kill" cure strategies. Two distinct subgroups of children were identified on the basis of viral load. Unexpectedly, in both groups, as in adults, HIV-specific CTL drove the selection of escape variants across a range of epitopes within the first weeks of infection. However, in HIV-infected children, but not adults, de novo autologous variant-specific CTL responses were generated, enabling the pediatric immune system to "corner" the virus. Thus, even when escape variants are selected in early infection, the capacity in children to generate variant-specific anti-HIV CTL responses maintains the potential for CTL to contribute to effective shock-and-kill cure strategies in pediatric HIV infection.

Very early combination antiretroviral therapy in infants: prospects for cure

PURPOSE OF REVIEW

A single case of sustained HIV control in the absence of antiretroviral therapy or HIV-specific immune responses ensued following 18 months of combination antiretroviral therapy initiated at 30 h of age in a perinatally HIV-infected child (the Mississippi child). This case provides proof-of-concept that delay in HIV viremic rebound may ensue following very early treatment (VET) in perinatal infection, likely through marked reduction of latent replication-competent HIV reservoirs.

RECENT FINDINGS

The latent HIV reservoir remains the critical barrier to remission. Several studies indicate that the earlier effective combination antiretroviral therapy is initiated, the smaller the size of the HIV reservoir. The unique ability of perinatally infected neonates to initiate VET at the time of birth maximizes the potential benefits of limiting latent reservoir size and permitting reservoir decay, likely lengthening the duration of remission and limiting the capacity for re-establishment of viremia.

SUMMARY

This article covers the rationale and feasibility of VET to achieve sustained virologic remission in perinatal infection. Recent studies highlighting the effects of VET on biomarkers of HIV persistence in perinatal HIV infection are reviewed as well as implications and challenges for cure research in pediatric populations.

In-utero infection with HIV-1 associated with suppressed lymphoproliferative responses at birth

In-utero exposure to HIV-1 may affect the immune system of the developing child and may induce HIV-1-specific immune responses, even in the absence of HIV-1 infection. We evaluated lymphoproliferative capacity at birth among 40 HIV-1-uninfected infants born to HIV-1-infected mothers and 10 infants who had acquired HIV-1 in utero. Cord blood mononuclear cells were assayed using [(3) H]-thymidine incorporation for proliferation in response to HIV-1 p55-gag and the control stimuli phytohaemagglutinin (PHA), Staphylococcus enterotoxin B (SEB) and allogeneic cells. In response to HIV-1 p55-gag, eight (20%) HIV-1-exposed, uninfected (EU) infants had a stimulation index (SI) ≥ 2 and three (30%) in-utero HIV-1 infected infants had SI ≥2. The frequency and magnitude of responses to HIV-1 p55-gag were low overall, and did not differ statistically between groups. However, proliferative responses to control stimuli were significantly higher in EU infants than in infants infected in utero, with a median SI in response to PHA of 123 [interquartile range (IQR) 77-231] versus 18 (IQR 4-86) between EU and infected infants, respectively (P < 0·001). Among infected infants, gestational maturity was associated with the strength of HIV-1 p55-gag response (P < 0·001); neither maternal nor infant HIV-1 viral load was associated. In summary, EU and HIV-1-infected infants mounted HIV-1-specific lymphoproliferative responses at similar rates (20-30%), and although global immune function was preserved among EU infants, neonatal immune responses were significantly compromised by HIV-1 infection. Such early lymphoproliferative compromise may, in part, explain rapid progression to AIDS and death among HIV-1-infected infants.

Early initiation of combination antiretroviral therapy in HIV-1-infected newborns can achieve sustained virologic suppression with low frequency of CD4+ T cells carrying HIV in peripheral blood

BACKGROUND

A human immunodeficiency virus type 1 (HIV-1)-infected infant started on combination antiretroviral therapy (cART) at 30 hours of life was recently reported to have no detectable plasma viremia after discontinuing cART. The current study investigated the impact of early cART initiation on measures of HIV-1 reservoir size in HIV-1-infected children with sustained virologic suppression.

METHODS

Children born to HIV-1-infected mothers and started on cART within 72 hours of birth at 3 Canadian centers were assessed. HIV serology, HIV-1-specific cell-mediated immune responses, plasma viremia, cell-associated HIV-1 DNA and RNA, presence of replication-competent HIV-1, and HLA genotype were determined for HIV-1-infected children with sustained virologic suppression.

RESULTS

Of 136 cART-treated children, 12 were vertically infected (8.8%). In the 4 who achieved sustained virologic suppression, HIV serology, HIV-1-specific cell-mediated immune responses (Gag, Nef), and ultrasensitive viral load were negative. HIV-1 DNA was not detected in enriched CD4(+) T cells of the 4 children (<2.6 copies/10(6) CD4(+) T cells), whereas HIV-1 RNA was detected (19.5-130 copies/1.5 µg RNA). No virion-associated HIV-1 RNA was detected following mitogenic stimulation of peripheral blood CD4(+) T cells (5.4-8.0 million CD4(+) T cells) in these 4 children, but replication competent virus was detected by quantitative co-culture involving a higher number of cells in 1 of 2 children tested (0.1 infectious units/10(6) CD4(+) T cells).

CONCLUSIONS

In perinatally HIV-1-infected newborns, initiation of cART within 72 hours of birth may significantly reduce the size of the HIV-1 reservoirs. Cessation of cART may be necessary to determine whether functional HIV cure can be achieved in such children.

Immunity to HIV in Early Life

The developing immune system is adapted to the exposure to a plethora of pathogenic and non-pathogenic antigens encountered in utero and after birth, requiring a fine balance between protective immunity and immune tolerance. In early stages of life, this tolerogenic state of the innate and adaptive immune system and the lack of immunological memory render the host more susceptible to infectious pathogens like HIV. HIV pathogenesis is different in children, compared to adults, with more rapid disease progression and a substantial lack of control of viremia compared to adults. Plasma viral load remains high during infancy and only declines gradually over several years in line with immune maturation, even in rare cases where children maintain normal CD4 T-lymphocyte counts for several years without antiretroviral therapy (ART). These pediatric slow progressors also typically show low levels of immune activation despite persistently high viremia, resembling the phenotype of natural hosts of SIV infection. The lack of immunological memory places the fetus and the newborn at higher risk of infections; however, it may also provide an opportunity for unique interventions. Frequencies of central memory CD4+ T-lymphocytes, one of the main cellular reservoirs of HIV, are very low in the newborn child, so immediate ART could prevent the establishment of persistent viral reservoirs and result in "functional cure." However, as recently demonstrated in the case report of the "Mississippi child" who experienced viral rebound after more than 2 years off ART, additional immunomodulatory strategies might be required for sustained viral suppression after ART cessation. In this review, we discuss the interactions between HIV and the developing immune system in children and the potential implications for therapeutic and prophylactic interventions.

Reduced markers of HIV persistence and restricted HIV-specific immune responses after early antiretroviral therapy in children

OBJECTIVE

Understanding the extent to which early antiretroviral therapy (ART) can limit the establishment and persistence of the HIV reservoir is an important step to designing interventions aimed at achieving HIV cure. We measured the markers of HIV persistence and HIV-specific immunity in early treated children.

DESIGN

This is a cross-sectional study that enrolled 15 children older than 2 years of age who initiated ART before 6 months of age and had sustained viral suppression. Total and integrated HIV DNA, and 2-LTR circles in CD4 T cells, HIV antibody response by fourth generation HIV enzyme immunoassay, and CD4 and CD8 T-cell responses to gag/env peptides by intracellular cytokine staining of CD4 and CD8 T cells were measured.

RESULTS

The median current age was 6.3 years and age at ART initiation was 17 weeks. The median duration of viral suppression was 6 years, and all had HIV RNA less than 50 copies/ml. The median CD4 T cells was 44%. The median total HIV DNA was 132 copies/10 CD4 T cells (range 11-1804) and integrated HIV DNA was 17 copies/10 CD4 T cells (range 0-516), and no one had detectable 2-LTR circles. Nine of the 15 children (60%) had undetectable or extremely low integrated HIV DNA (<20 copies/10 CD4 T cells). All except one (93%) had undetectable HIV-specific CD4/CD8 cell responses and seven (47%) had nonreactive enzyme immunoassay.

CONCLUSION

Early ART resulted in very low levels of markers of HIV persistence and undetectable HIV-specific immune responses in the majority of HIV-infected children who started ART before 6 months of age.

The impact of differential antiviral immunity in children and adults

The course of immune maturation has evolved to favour survival at each stage of development in early life. Fetal and neonatal immune adaptations facilitate intrauterine survival and provide early postnatal protection against extracellular pathogens, but they leave infants susceptible to intracellular pathogens such as viruses that are acquired perinatally. This Review focuses on three such pathogens--HIV, hepatitis B virus and cytomegalovirus--and relates the differential impact of these infections in infants and adults to the antiviral immunity that is generated at different ages. A better understanding of age-specific antiviral immunity may inform the development of integrated prevention, treatment and vaccine strategies to minimize the global disease burden resulting from these infections.

Immunology of infants through adolescents: responses to emulate for HIV vaccines

PURPOSE OF REVIEW

The major target groups for an HIV vaccine include breastfeeding infants and adolescents. Differential immune maturity in these age groups may significantly impact vaccine efficacy, and should be taken into account when developing vaccines. Here we review these differences, with an emphasis on the immune response to vaccines for HIV and other pathogens. Recommendations for potential adaptation of current HIV vaccines are also made.

RECENT FINDINGS

An effective neonatal vaccine needs to be immunogenic in the presence of maternal antibody, and must induce cytotoxic T-lymphocyte responses, neutralizing antibody responses, both systemic and mucosal. There is renewed hope in the possibility of stimulating neutralizing antibodies with HIV vaccination. DNA vaccines are promising for neonates, but will need appropriate boosting. Certain adjuvants and vector delivery systems are more suitable for neonates. Adolescents may have stronger immune responses to HIV vaccines than adults, and will also require induction of mucosal neutralizing humoral and cellular immunity.

SUMMARY

Some current HIV vaccine strategies may need adaptation for neonates and suitable product development should be accelerated. Vaccines could induce better responses in adolescents and therefore should not be discarded prematurely. Development of vaccines that have potential for these age groups is an urgent global priority.

Suboptimal immune reconstitution in vertically HIV infected children: a view on how HIV replication and timing of HAART initiation can impact on T and B-cell compartment

Today, HIV-infected children who have access to treatment face a chronic rather than a progressive and fatal disease. As a result, new challenges are emerging in the field. Recent lines of evidence outline several factors that can differently affect the ability of the immune system to fully reconstitute and to mount specific immune responses in children receiving HAART. In this paper, we review the underlying mechanisms of immune reconstitution after HAART initiation among vertically HIV-infected children analyzing the possible causes of suboptimal responses.

Restricted genetic diversity of HIV-1 subtype C envelope glycoprotein from perinatally infected Zambian infants

Background

Mother-to-child transmission of HIV-1 remains a significant problem in the resource-constrained settings where anti-retroviral therapy is still not widely available. Understanding the earliest events during HIV-1 transmission and characterizing the newly transmitted or founder virus is central to intervention efforts. In this study, we analyzed the viral env quasispecies of six mother-infant transmission pairs (MIPs) and characterized the genetic features of envelope glycoprotein that could influence HIV-1 subtype C perinatal transmission.

Methodology and Findings

The V1-V5 region of env was amplified from 6 MIPs baseline samples and 334 DNA sequences in total were analyzed. A comparison of the viral population derived from the mother and infant revealed a severe genetic bottleneck occurring during perinatal transmission, which was characterized by low sequence diversity in the infant. Phylogenetic analysis indicates that most likely in all our infant subjects a single founder virus was responsible for establishing infection. Furthermore, the newly transmitted viruses from the infant had significantly fewer potential N-linked glycosylation sites in Env V1-V5 region and showed a propensity to encode shorter variable loops compared to the nontransmitted viruses. In addition, a similar intensity of selection was seen between mothers and infants with a higher rate of synonymous (dS) compared to nonsynonymous (dN) substitutions evident (dN/dS<1).

Conclusions

Our results indicate that a strong genetic bottleneck occurs during perinatal transmission of HIV-1 subtype C. This is evident through population diversity and phylogenetic patterns where a single viral variant appears to be responsible for infection in the infants. As a result the newly transmitted viruses are less diverse and harbored significantly less glycosylated envelope. This suggests that viruses with the restricted glycosylation in envelope glycoprotein appeared to be preferentially transmitted during HIV-1 subtype C perinatal transmission. In addition, our findings also indicated that purifying selection appears to predominate in shaping the early intrahost evolution of HIV-1 subtype C envelope sequences.

Intrahost and interhost variability of the HIV type 1 nef gene in Brazilian children

Many aspects of HIV-1 pathogenesis are affected by Nef protein activity, and efforts have been made to study variation in the nef gene and how that variation relates to disease outcome. We studied the genetic diversity of the nef gene in distinct clones obtained from the same patient (intrahost) and in sequences obtained from different hosts (interhost). The set of sequences analyzed was obtained from HIV-1-infected Brazilian children and contained 112 clones from 25 children (intrahost samples), as well as 55 sequences from epidemiologically unlinked children (interhost samples). We found extensive site polymorphisms and amino acid length variations, mainly in the amino terminal region of the nef gene, between the myristoylation motif (MGxxxS) and the MHC-1 downregulation motif (Rxx). Analysis of the sequences deposited in the Los Alamos HIV sequences database ( www.hiv.lanl.gov ) indicated that the most frequent motif at the MHC-1 downregulation site in the subtype B strain is R(86%)A(64%)E(82%) (n = 1040) and R(78%)T(74%)E(56%) in the subtype C strain (n = 549). Conversely, the Brazilian subtype B isolates presented the motif R(81%)T(62%)E(67%) at this site (n = 64). A detailed analysis of selective pressures identified a concentration of codons under strong positive selection in the amino terminal region of the nef gene. We also determined that different sites are under positive selection in the subtype B and subtype C viruses. The amino acid composition in the MHC-1 downregulation motif of the nef gene in our sequences may indicate a distinct adaptive pattern of HIV-1 subtype B to the Brazilian host population.

Identification of human immunodeficiency virus-1 specific CD8+ and CD4+ T cell responses in perinatally-infected infants and their mothers

BACKGROUND

There are few data describing the specificity, breadth and magnitude of T cell responses to HIV-1 in infancy.

METHODS

HIV-specific CD8+ and CD4+ T cell responses to peptide pools representing Gag, Env, Pol, Nef and the regulatory regions (Reg) were simultaneously measured in 18 perinatally-infected infants and 14 of their chronically-infected mothers, using a whole blood interleukin-2 and interferon-gamma flow cytometric intracellular cytokine staining assay.

RESULTS

HIV-specific CD8+ T cell responses were detected in all the infants aged 6 weeks and older (range 0.1-6.62%) and their mothers (range 0.1-4.89%). HIV-specific CD4+ T cell responses were detected in 33% of the infants (range 0.11-0.54%) and 73% of the mothers (range 0.16-0.84). CD8+ T cell responses in the mothers were almost equally spread between the variable (Nef, Reg and Env) and conserved proteins (Gag and Pol). Conversely, CD8+ T cell responses to the more variable proteins dominated in the perinatally-infected infants comprising 74% of the total response. Interestingly, mothers and infants shared responses to at least one peptide pool, whereas only one mother-infant pair shared a peptide pool targeted by CD4+ T cells. Two in-utero-infected infants tested at birth had CD8+ T cell responses, and one of them had an Env-specific CD4 T cell response.

CONCLUSION

Our observations that HIV-specific CD8+ and CD4+ T cell responses can be detected in perinatally-infected infants from 6 weeks of age and that CD8+ T cell responses predominantly target the variable proteins have important implications for HIV vaccine design.

GENERALIZED PARTIALLY LINEAR MIXED-EFFECTS MODELS INCORPORATING MISMEASURED COVARIATES

In this article we consider a semiparametric generalized mixed-effects model, and propose combining local linear regression, and penalized quasilikelihood and local quasilikelihood techniques to estimate both population and individual parameters and nonparametric curves. The proposed estimators take into account the local correlation structure of the longitudinal data. We establish normality for the estimators of the parameter and asymptotic expansion for the estimators of the nonparametric part. For practical implementation, we propose an appropriate algorithm. We also consider the measurement error problem in covariates in our model, and suggest a strategy for adjusting the effects of measurement errors. We apply the proposed models and methods to study the relation between virologic and immunologic responses in AIDS clinical trials, in which virologic response is classified into binary variables. A dataset from an AIDS clinical study is analyzed.

Empirical Likelihood based Inference for Additive Partial Linear Measurement Error Models

This paper considers statistical inference for additive partial linear models when the linear covariate is measured with error. To improve the accuracy of the normal approximation based confidence intervals, we develop an empirical likelihood based statistic, which is shown to be asymptotically chi-square distributed. We emphasize the finite-sample performance of the proposed method by conducting simulation experiments. The method is used to analyze the relationship between semen quality and phthalate exposure from an environment study.

Deficiency of HIV-Gag-specific T cells in early childhood correlates with poor viral containment

Perinatal HIV infection is characterized by a sustained high-level viremia and a high risk of rapid progression to AIDS, indicating a failure of immunologic containment of the virus. We hypothesized that age-related differences in the specificity or function of HIV-specific T cells may influence HIV RNA levels and clinical outcome following perinatal infection. In this study, we defined the HIV epitopes targeted by 76 pediatric subjects (47 HIV infected and 29 HIV exposed, but uninfected), and assessed the ability of HIV-specific CD8 and CD4 T cells to degranulate and produce IFN-gamma, TNF-alpha, and IL-2. No responses were detected among HIV-uninfected infants, whereas responses among infected subjects increased in magnitude and breadth with age. Gag-specific responses were uncommon during early infancy, and their frequency was significantly lower among children younger than 24 mo old (p = 0.014). Importantly, Gag responders exhibited significantly lower HIV RNA levels than nonresponders (log viral load 5.8 vs 5.0; p = 0.005). Both the total and Gag-specific T cell frequency correlated inversely with viral load after correction for age, whereas no relationship with targeting of other viral proteins was observed. Functional assessment of HIV-specific T cells by multiparameter flow cytometry revealed that polyfunctional CD8 cells were less prevalent in children before 24 mo of age, and that HIV-specific CD4 cell responses were of universally low frequency among antiretroviral-naive children and absent in young infants. These cross-sectional data suggest that qualitative differences in the CD8 response, combined with a deficiency of HIV-specific CD4 cells, may contribute to the inability of young infants to limit replication of HIV.

Delayed early antiretroviral treatment is associated with an HIV-specific long-term cellular response in HIV-1 vertically infected infants

Antiviral T-cell immune responses appear to be crucial to control HIV replication. Infants treated before the third month of life with highly active antiretroviral treatment (HAART) did not develop a persistent HIV-specific immune response. We evaluated how delayed initiation of HAART after 3 months of age influences the development of HIV-1-specific T-cell responses during long-term follow-up in 9 HIV-1 vertically infected infants. These data suggest that a longer antigenic stimulation, due to a larger window for therapeutic intervention with HAART, is associated with the establishment of a persistent specific HIV immune response resulting in a long-term viral control of vertically infected infants.

Human immunodeficiency virus-specific CD8+ T-cell activity is detectable from birth in the majority of in utero-infected infants

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.

Evaluation of change in CD4+ cell counts in AIDS clinical trials

To evaluate the antiretroviral activity of antiretroviral agents and to compare the effects of two different antiretroviral agents, we propose a non-parametric mixed-effects model to investigate change of CD4+ counts. The proposed model and methods are applied to analyse the data from PACTG345 study. Population and individual patterns of change of CD4+ counts and a reference band are obtained. Our results indicate that treatment with high-dose ritonavir is significantly superior compared with low-dose ritonavir.

Selective pressures of human immunodeficiency virus type 1 (HIV-1) during pediatric infection

Pediatric HIV-1 infection presents remarkable features that are distinct from those observed in adult infection. In vertically HIV-1-infected children, the viral load declines more slowly, and the cytotoxic T-lymphocyte response emerges late, only after the sixth month of life. This response generally tends to be narrow and less intense than that seen in adults. While the nuances of immune response at the cellular level during pediatric HIV-1 infection have been addressed, there is a lack of studies focusing on the consequences of this delayed and narrowed immune response at the population level. To better explore these features, we evaluated the selection regimen in gag, pol and env gene fragments of HIV-1 during pediatric infection. We estimated the number of nonsynonymous substitutions (d(N)) and synonymous substitutions (d(S)) codon-by-codon, using the maximum likelihood method and a modified counting method. Notably, both methods indicated a similar intensity of selection (measure by mean d(N)/d(S) ratio) between children and adults. Additionally, sites under positive selection were equally distributed along HIV genes and the location of these sites was analogous between children and adults. Therefore, the selective regimen in HIV during pediatric infection is equally broad and intense likewise the observed in adults. Unexpectedly, our phylogenetic-based analysis enabled us to identify two regions in the env gene of HIV with distinct adaptive functions. The first region, located in the vicinity of V3 loop, contains sites that might increase viral fitness within-host during antibody attack and virus-cell interaction. The second region, restricted to amino acids 334-368 of Gp160, contains sites that might increase viral fitness during interhost transmission at the population level.

Pediatric HIV-1-specific cytotoxic T-lymphocyte responses suggesting ongoing viral replication despite combination antiretroviral therapy

Human immunodeficiency virus-1 (HIV-1)-specific cytotoxic T-lymphocyte (CTL) responses are common in infected adults and usually exhibit rapid decay after combination antiretroviral therapy (ART). CTLs develop later in the first year of life, and the fate of HIV-1-specific responses in perinatally infected children after ART is less well described. HIV-1-specific CTL responses were measured in 17 perinatally infected children and adolescents (ages 3-20 y) receiving combination ART. Seven had prolonged viral suppression (<400 copies/mL) for 2.5-5.3 y and 10 had persistent viremia (median, 77,550 copies/mL). HIV-1-specific CTL responses were tested by interferon (IFN)-gamma enzyme-linked immunospot (ELIS-pot) assays using 53 overlapping peptide pools spanning the entire HIV-1 proteome. HIV-1-specific CTL responses were detected in 14 of 17 individuals. Responses to one to four viral proteins were found in eight of 10 individuals with persistent viremia and six of seven with prolonged viral suppression. The magnitude and breadth of CTL responses were similar between groups. HIV-1-specific CTL responses were present in the majority of perinatally infected subjects, irrespective of viremia at evaluation. Because ART-treated infected adults usually have rapid decay of responses, these data suggest viral replication below the limits of detection is more persistent in combination ART-treated perinatally infected pediatric subjects. The long-term clinical implications of these findings remain to be determined.

Poor recognition of HIV-1 Nef protein by CD8 T cells from HIV-1-infected children: impact of age

Recognition of various HIV proteins by CD8 T cells from HIV-infected children was determined by two functional assays. First, using an Elispot assay, we show that 80% of patients recognized Gag, 77% recognized Pol, 61% recognized Env, 44% recognized Nef and 29% recognized Vif. Frequencies of Gag-, Pol-, and Env-specific IFN-gamma producing CD8 T cells were higher than frequencies of Nef and Vif-specific CD8 T cells. The poor recognition of Nef by ex vivo CD8 T cells was confirmed by CTL assays performed in HAART naïve children: 25% of children had positive response against Nef versus 44, 63 and 62% for Env, Gag, and Pol, respectively. Memory Gag-specific CTL were positively correlated with age, whereas Nef-specific CTL were negatively correlated with age. The poor Nef-specific CD8 T cell response in HIV-infected children contrasts with dominance of Nef-specific responses in infected adults.

Vaccines to prevent transmission of HIV-1 via breastmilk: scientific and logistical priorities

Mother-to-child transmission (MTCT) of HIV-1 is the major mode of paediatric infection. The rapidly increasing incidence of MTCT worldwide has resulted in an urgent need for preventive strategies. Antiretroviral regimens can prevent intrapartum HIV transmission; however, these regimens do not prevent HIV transmission through breastfeeding. Furthermore, children who escape MTCT are again at risk of infection when they become sexually active as adolescents. An infant vaccine regimen, begun at birth, would hence be a more attractive strategy and might also provide the basis for lifetime protection. Unique features of MTCT and paediatric HIV disease could be helpful in understanding correlates of immune protection and could facilitate rapid assessment of vaccine efficacy. Thus, there is compelling rationale to develop safe, effective HIV vaccines for use in infants and children. Here, we discuss the scientific and logistical challenges for the development of paediatric HIV vaccines; available vaccines and completed or planned paediatric vaccine trials are also discussed.

Structured treatment interruptions with tenofovir monotherapy for simian immunodeficiency virus-infected newborn macaques

We demonstrated previously that prolonged tenofovir treatment of infant macaques, starting early during infection with virulent simian immunodeficiency virus (SIVmac251), can lead to persistently low or undetectable viremia even after the emergence of mutants with reduced in vitro susceptibility to tenofovir as a result of a K65R mutation in reverse transcriptase; this control of viremia was demonstrated to be mediated by the generation of effective antiviral immune responses. To determine whether structured treatment interruptions (STI) can induce similar immunologic control of viremia, eight newborn macaques were infected with highly virulent SIVmac251 and started on a tenofovir STI regimen 5 days later. Treatment was withdrawn permanently at 33 weeks of age. All animals receiving STI fared much better than 22 untreated SIVmac251-infected infant macaques. However, there was a high variability among animals in the viral RNA set point after complete drug withdrawal, and none of the animals was able to achieve long-term immunologic suppression of viremia to persistently low levels. Early immunologic and viral markers in blood (including the detection of the K65R mutation) were not predictive of the viral RNA set point after drug withdrawal. These results, which reflect the complex interactions between drug resistance mutations, viral virulence, and drug- and immune-mediated inhibition of virus replication, highlight the difficulties associated with trying to develop STI regimens with predictable efficacy for clinical practice.

Failure to eradicate HIV despite fully successful HAART initiated in the first days of life

Highly active antiretroviral therapy (HAART) started shortly after birth resulted in reversion of human immunodeficiency virus (HIV) plasma viremia, proviral DNA in PBMC, viral culture, and serum HIV antibodies to negative. Discontinuation of HAART 2 years after apparent HIV eradication, however, was followed by virus replication, CD4 decline, and destruction of HIV-specific lymphocytes, epitomizing the impossibility of HIV eradication.

CD8 T cell effector maturation in HIV-1-infected children

HIV-1 infection generates maturational responses in overall CD4 and CD8 T cell populations in adults, with elevated expression of lytic effector molecules perforin and granzyme B, and reduced expression of CCR7 and CD45RA. Here, we have found that these marked effects were significantly less pronounced in children, both in terms of the skewed CCR7/CD45RA expression profile as well as the increased perforin expression. Similar to adults, HIV-specific CD8 cells in children were largely CD27+ CD45RA- and lacked perforin. However, one pediatric subject with late-stage infection displayed robust expansion of Gag 77-85-specific CD8 T cells which were perforin+ and lytic, but lacked expression of CD27 and IFNgamma. Our data indicate that the T cell effector maturation induced by HIV-1 infection is markedly weaker in children as compared to adults. The data also suggest, however, that the perforin-deficient state of HIV-specific CD8 T cells in children may be reversible.

HIV-1-specific CD8+ T cell responses and viral evolution in women and infants

CD8+ T lymphocyte responses play an important role in controlling HIV-1 replication but escape from CD8+ T cell surveillance may limit the effectiveness of these responses. Mother-to-child transmission of CD8+ T cell escape variants may particularly affect CD8+ T cell recognition of infant HIV-1 epitopes. In this study, amino acid sequence variation in HIV-1 gag and nef was examined in five untreated mother-infant pairs to evaluate the potential role of CD8+ T cell responses in the evolution of the viral quasispecies. Several CD8+ T cell escape variants were detected in maternal plasma. Evaluation of infant plasma viruses at 1-3 mo documented heterogeneity of gag and nef gene sequences and mother-to-child transmission of CD8+ T cell escape variants. Infant HLA haplotype and viral fitness appeared to determine the stability of the escape mutants in the infant over time. Changes in CD8+ T cell epitope sequences were detected in infants' sequential plasma specimens, suggesting that infants are capable of generating virus-specific CD8+ T cell responses that exert selective pressures in vivo. Altogether, these studies document that HIV-1-specific CD8+ T cell responses contribute to the evolution of the viral quasispecies in HIV-1-infected women and their infants and may have important implications for vaccine design.

Correlates of delayed disease progression in HIV-1-infected Kenyan children

Without treatment most HIV-1-infected children in Africa die before their third birthday (>89%) and long-term nonprogressors are rare. The mechanisms underlying nonprogression in HIV-1-infected children are not well understood. In the present study, we examined potential correlates of delayed HIV disease progression in 51 HIV-1-infected African children. Children were assigned to progression subgroups based on clinical characterization. HIV-1-specific immune responses were studied using a combination of ELISPOT assays, tetramer staining, and FACS analysis to characterize the magnitude, specificity, and functional phenotype of HIV-1-specific CD8(+) and CD4(+) T cells. Host genetic factors were examined by genotyping with sequence-specific primers. HIV-1 nef gene sequences from infecting isolates from the children were examined for potential attenuating deletions. Thymic output was measured by T cell rearrangement excision circle assays. HIV-1-specific CD8(+) T cell responses were detected in all progression groups. The most striking attribute of long-term survivor nonprogressors was the detection of HIV-1-specific CD4(+) Th responses in this group at a magnitude substantially greater than previously observed in adult long-term nonprogressors. Although long-term survivor nonprogressors had a significantly higher percentage of CD45RA(+)CD4(+) T cells, nonprogression was not associated with higher thymic output. No protective genotypes for known coreceptor polymorphisms or large sequence deletions in the nef gene associated with delayed disease progression were identified. In the absence of host genotypes and attenuating mutations in HIV-1 nef, long-term surviving children generated strong CD4(+) T cell responses to HIV-1. As HIV-1-specific helper cells support anti-HIV-1 effector responses in active disease, their presence may be important in delaying disease progression.

Longitudinal assessment of human immunodeficiency virus type 1 (HIV-1)-specific gamma interferon responses during the first year of life in HIV-1-infected infants

Human immunodeficiency virus type 1 (HIV-1) infection results in different patterns of viral replication in pediatric compared to adult populations. The role of early HIV-1-specific responses in viral control has not been well defined, because most studies of HIV-1-infected infants have been retrospective or cross-sectional. We evaluated the association between HIV-1-specific gamma interferon (IFN-gamma) release from the cells of infants of 1 to 3 months of age and peak viral loads and mortality in the first year of life among 61 Kenyan HIV-1-infected infants. At 1 month, responses were detected in 7/12 (58%) and 6/21 (29%) of infants infected in utero and peripartum, respectively (P = 0.09), and in approximately 50% of infants thereafter. Peaks of HIV-specific spot-forming units (SFU) increased significantly with age in all infants, from 251/10(6) peripheral blood mononuclear cells (PBMC) at 1 month of age to 501/10(6) PBMC at 12 months of age (P = 0.03), although when limited to infants who survived to 1 year, the increase in peak HIV-specific SFU was no longer significant (P = 0.18). Over the first year of life, infants with IFN-gamma responses at 1 month had peak plasma viral loads, rates of decline of viral load, and mortality risk similar to those of infants who lacked responses at 1 month. The strength and breadth of IFN-gamma responses at 1 month were not significantly associated with viral containment or mortality. These results suggest that, in contrast to HIV-1-infected adults, in whom strong cytotoxic T lymphocyte responses in primary infection are associated with reductions in viremia, HIV-1-infected neonates generate HIV-1-specific CD8+-T-cell responses early in life that are not clearly associated with improved clinical outcomes.

T cell-mediated immune responses in human newborns: ready to learn?

Infections with intracellular pathogens are often more severe or more prolonged in young infants suggesting that T cell-mediated immune responses are different in early life. Whereas neonatal immune responses have been quite extensively studied in murine models, studies of T cell-mediated immunity in human newborns and infants are scarce. Qualitative and quantitative differences when compared with adult immune responses have been observed but on the other hand mature responses to certain vaccines and infectious pathogens were demonstrated during the postnatal period and even during foetal life. Herein, we review the evidence suggesting that under appropriate conditions of stimulation, protective T cell-mediated immune responses could be induced by vaccines in early life.

A vaccinia-based elispot assay for detection of CD8+ T cells from HIV-1 infected children

HIV-specific CD8+ T lymphocytes participate in the control of viral replication in infected patients. These responses are of low intensity in young infants and are decreased by antiretroviral therapy. In the present study, we report on a recombinant Vaccinia virus (rVV)-based Elispot assay for the detection of HIV-specific CD8+ T cells immediately after isolation of peripheral blood mononuclear cells (PBMC). The rVV-based assay was highly sensitive; 48 out of 50 children had a positive response against the rVV encoding HIV Env-Gag-Pol antigen. Interferon-gamma was produced by CD8+ T cells, and CD14+/15+ cells were the main cell subset presenting antigens expressed by rVV. We observed that the cell input per well had a critical influence on the sensitivity of the assay. Results from the ex vivo Elispot assay correlated poorly with those of the 51Cr release assay performed after expansion of PBMC in vitro; thus, both assays gave information on different subsets and/or functions of the HIV-specific T cell response.

Not All Tetramer Binding CD8+ T Cells Can Produce Cytokines and Chemokines Involved in the Effector Functions of Virus-Specific CD8+ T Lymphocytes in HIV-1 Infected Children

In the pediatric human immunodeficiency virus type-1 (HIV-1) infection, the presence of cytotoxic T lymphocytes (CTL) is associated with a slow progression to AIDS. The secretion of cytokines by CTLs may be critical in the control of viral infection. We used the combination of cell surface and intracellular staining to study the functionality of tetramer binding CD8+ T cells recognizing two HIV-1 immunodominant epitopes, in peripheral blood mononuclear cells from HIV-1-infected children. A fraction of tetramer positive CD8+ T cells produce cytokines (IFN-gamma, TNF-alpha) or chemokines (CCL4, CCL5) after ex vivo stimulation with the cognate peptide. There was a negative correlation between the plasma viral load and the percentage of CD8+ Tetramer Gag+ T cells secreting IFN-gamma. This is the first report in the context of pediatric HIV-1 infection showing that only a fraction of HIV-1-specific CD8+ T cells have the capacity to produce cytokines and chemokines implicated in their antiviral functions.

Maternal versus paternal inheritance of HLA class I alleles among HIV-infected children: consequences for clinical disease progression

OBJECTIVE

When children acquire HIV infection from their mothers (with whom they share at least 50% of their HLA alleles), they acquire virus with a history of encounter with maternal HLA-mediated immune responses. We investigated whether maternal HLA selection pressures on the virus would adversely influence clinical outcomes of HIV-infected children.

METHODS

We tested whether time to AIDS diagnosis or death, among a cohort of 59 HIV-infected children in New York City followed from birth for up to 12 years, was associated with maternally- or paternally-inherited child HLA class I alleles, and with HLA similarity between mother and child.

RESULTS

HIV-infected children with an HLA allele usually associated with slow disease experienced a slower progression to AIDS or death only if the allele was paternally inherited. If the allele was present in the mother, no association was observed. Children who were homozygous or who shared both alleles with their mothers at more than one HLA class I locus were more likely to progress to AIDS or death than other children (relative hazard, 3.46; 95% confidence interval, 1.24-9.71).

CONCLUSION

Genetic similarity between mother and child may compromise the child's capacity to control HIV replication when the virus is acquired from the mother. HLA-mediated selective pressures on the virus in a transmitting mother-infant pair may undermine future HLA-mediated viral control in the child.

Human cytomegalovirus proteins pp65 and immediate early protein 1 are common targets for CD8+ T cell responses in children with congenital or postnatal human cytomegalovirus infection

Recombinant modified vaccinia Ankara- and peptide-based IFN-gamma ELISPOT assays were used to detect and measure human CMV (HCMV)-specific CD8(+) T cell responses to the pp65 (UL83) and immediate early protein 1 (IE1; UL123) gene products in 16 HCMV-infected infants and children. Age at study ranged from birth to 2 years. HCMV-specific CD8(+) T cells were detected in 14 (88%) of 16 children at frequencies ranging from 60 to >2000 spots/million PBMC. Responses were detected as early as 1 day of age in infants with documented congenital infection. Nine children responded to both pp65 and IE1, whereas responses to pp65 or IE1 alone were detected in three and two children, respectively. Regardless of the specificity of initial responses, IE1-specific responses predominated by 1 year of age. Changes in HCMV epitopes targeted by the CD8(+) T cell responses were observed over time; epitopes commonly recognized by HLA-A2(+) adults with latent HCMV infection did not fully account for responses detected in early childhood. Finally, the detection of HCMV-specific CD8(+) T cell responses was temporally associated with a decrease in peripheral blood HCMV load. Taken altogether, these data demonstrate that the fetus and young infant can generate virus-specific CD8(+) T cell responses. Changes observed in the protein and epitope-specificity of HCMV-specific CD8(+) T cells over time are consistent with those observed after other primary viral infections. The temporal association between the detection of HCMV-specific CD8(+) T cell responses and the reduction in blood HCMV load supports the importance of CD8(+) T cells in controlling primary HCMV viremia.

Reconstitution of virus-specific CD4 proliferative responses in pediatric HIV-1 infection

Gag-specific CD4 proliferative responses correlate inversely with HIV-1 RNA levels in infected adults, and robust responses are characteristic of long-term nonprogressive infection. However, strong responses are seldom detected in adult subjects with progressive infection and are not generally reconstituted on highly active antiretroviral therapy (HAART). To date, the role of HIV-1-specific Th responses in children has not been thoroughly examined. We characterized Gag-specific CD4 responses among 35 perinatally infected subjects, including 2 children who spontaneously control viremia without antiretroviral therapy, 21 children with viral loads (VL) of <400 on HAART, and 12 viremic children. Gag-specific Th activity was assessed by lymphoproliferative assay, and responses were mapped using overlapping Gag peptides in an IFN-gamma ELISPOT. Robust proliferative responses were detected in the children exhibiting spontaneous control of viremia, and mapping of targeted Gag regions in one such subject identified multiple epitopes. Among children >or=5 years old, 14 of 17 subjects with VL of <400 on HAART demonstrated a significant p24 proliferative response (median p24 stimulation index, 20), in contrast with only 1 of 9 viremic children (median p24 stimulation index, 2.0; p = 0.0008). However, no subject younger than 5 years of age possessed a significant response, even when viremia was fully suppressed. When compared with adults with VL of <400 on HAART, Th responses among children with VL of <400 were both more frequent (p = 0.009) and of greater magnitude (p = 0.002). These data suggest that children may have a greater intrinsic capacity to reconstitute HIV-1-specific immunity than adults, and may be excellent candidates for immune-based therapies.

[Update on antiretroviral treatment in HIV infection]

In recent years, major advances have been made in the care of HIV-infected children, particularly in antiretroviral treatment, which have dramatically improved survival and quality of life. The goal of highly active antiretroviral therapy (HAART), which includes at least three potent drugs, is the maximal and most durable suppression of viral replication possible, which is often not achieved despite clear immunologic and clinical improvement. There are still major barriers to achieving this goal, mainly the difficulty of permanent adherence to complex regimens and treatment-related toxicities. Adverse events are frequent, including a high prevalence of metabolic complications with unknown consequences in the future. These drawbacks of antiretroviral treatment are leading to a more conservative initial approach, as well as to research into simpler and less toxic therapeutic options. New strategies should continue to be developed to overcome the still important limitations of current antiretroviral treatment.

Mature CD8(+) T lymphocyte response to viral infection during fetal life

Immunization of newborns against viral infections may be hampered by ineffective CD8(+) T cell responses. To characterize the function of CD8(+) T lymphocytes in early life, we studied newborns with congenital human cytomegalovirus (HCMV) infection. We demonstrate that HCMV infection in utero leads to the expansion and the differentiation of mature HCMV-specific CD8(+) T cells, which have similar characteristics to those detected in adults. High frequencies of HCMV-specific CD8(+) T cells were detected by ex vivo tetramer staining as early as after 28 weeks of gestation. During the acute phase of infection, these cells had an early differentiation phenotype (CD28(-)CD27(+)CD45RO(+), perforin(low)), and they acquired a late differentiation phenotype (CD28(-)CD27(-)CD45RA(+), perforin(high)) during the course of the infection. The differentiated cells showed potent perforin-dependent cytolytic activity and produced antiviral cytokines. The finding of a mature and functional CD8(+) T cell response to HCMV suggests that the machinery required to prime such responses is in place during fetal life and could be used to immunize newborns against viral pathogens.

Comprehensive screening reveals strong and broadly directed human immunodeficiency virus type 1-specific CD8 responses in perinatally infected children

Advances in antiviral therapy have dramatically shifted the demographics of pediatric human immunodeficiency virus type 1 (HIV-1) infection in the developed world, and a growing proportion of perinatally HIV-1-infected children are now entering their second or even third decade of life. Although cellular immune responses to HIV are known to be weak in early infancy, the magnitude, breadth, and specificity of responses later in childhood have not been characterized in detail. We performed a comprehensive characterization of HIV-1-specific CD8 responses in 18 perinatally infected children (age range, 6 to 17 years), most of whom were on antiviral therapy, using both previously defined HIV-1 epitopes and overlapping peptides spanning all HIV-1 proteins. Multispecific responses were detected in all subjects and accounted for a median of 0.25 to 0.3% of all peripheral blood mononuclear cells that was similar to the magnitude seen in HIV-infected adults. CD8 responses were broadly directed at an average of 11 epitopes (range, 2 to 27 epitopes) and targeted nearly all HIV-1 proteins, with the highest proportion in Gag. Responses were readily detected even in those children with suppressed viremia on highly active antiretroviral therapy, although the breadth (P = 0.037) and the magnitude (P = 0.021) were significantly lower in these subjects. Each child recognized only a small minority of the HIV-1 optimal epitopes defined for his or her class I HLA alleles. Together, these data indicate that perinatally infected children who survive infancy mount a robust HIV-1-specific CD8 response that is much stronger than previously thought and is comparable in magnitude and breadth to that of adults. Moreover, this response has the potential to be broadened to target more epitopes, making these children attractive candidates for immunotherapeutic interventions.