The impact of HIV exposure and maternal Mycobacterium tuberculosis infection on infant immune responses to bacille Calmette-Guérin vaccination

Maternal HIV Status and Risk of Infant Mycobacterium tuberculosis Infection as Measured by Tuberculin Skin Test

BACKGROUND

The effect of maternal HIV on infant Mycobacterium tuberculosis (Mtb) infection risk is not well-characterized.

METHODS

Pregnant women with/without HIV and their infants were enrolled in a longitudinal cohort in Kenya. Mothers had interferon gamma-release assays (QFT-Plus) and tuberculin skin tests (TST) at enrollment in pregnancy; children underwent TST at 12 and 24 months of age. We estimated the incidence and correlates of infant TST-positivity using Cox proportional hazards regression.

RESULTS

Among 322 infants, 170 (53%) were HIV-exposed and 152 (47%) were HIV-unexposed. Median enrollment age was 6.6 weeks [interquartile range (IQR): 6.1-10.0]; most received Bacillus Calmette-Guerin (320, 99%). Thirty-nine (12%) mothers were TST-positive; 102 (32%) were QFT-Plus-positive. Among HIV-exposed infants, 154 (95%) received antiretrovirals for HIV prevention and 141 (83%) of their mothers ever received isoniazid preventive therapy (IPT). Cumulative 24-month infant Mtb infection incidence was 3.6/100 person-years (PY) [95% confidence interval (CI): 2.4-5.5/100 PY]; 5.4/100 PY in HIV-exposed infants (10%, 17/170) versus 1.7/100 PY in HIV-unexposed infants (3.3%, 5/152) [hazard ratio (HR): 3.1 (95% CI: 1.2-8.5)]. More TST conversions occurred in the first versus second year of life [5.8 vs. 2.0/100 PY; HR: 2.9 (95% CI: 1.0-10.1)]. Infant TST-positivity was associated with maternal TST-positivity [HR: 2.9 (95% CI: 1.1-7.4)], but not QFT-Plus-positivity. Among HIV-exposed children, Mtb infection incidence was similar regardless of maternal IPT.

CONCLUSIONS

Mtb infection incidence (by TST) by 24 months of age was ~3-fold higher among HIV-exposed children, despite high maternal IPT uptake. Overall, more TST conversions occurred in the first 12 months compared to 12-24 months of age, similar in both HIV-exposed and HIV-unexposed children.

Bifidobacterium infantis associates with T cell immunity in human infants and is sufficient to enhance antigen-specific T cells in mice

Bacille Calmette-Guerin (BCG) vaccine can elicit good TH1 responses in neonates. We hypothesized that the pioneer gut microbiota affects vaccine T cell responses. Infants who are HIV exposed but uninfected (iHEU) display an altered immunity to vaccination. BCG-specific immune responses were analyzed at 7 weeks of age in iHEU, and responses were categorized as high or low. Bifidobacterium longum subsp. infantis was enriched in the stools of high responders, while Bacteroides thetaiotaomicron was enriched in low responders at time of BCG vaccination. Neonatal germ-free or SPF mice orally gavaged with live B. infantis exhibited significantly higher BCG-specific T cells compared with pups gavaged with B. thetaiotaomicron. B. infantis and B. thetaiotaomicron differentially affected stool metabolome and colonic transcriptome. Human colonic epithelial cells stimulated with B. infantis induced a unique gene expression profile versus B. thetaiotaomicron. We thus identified a causal role of B. infantis in early-life antigen-specific immunity.

The protective role of maternal genetic immunization on maternal-fetal health and welfare

Pregnancy is a critical period associated with alterations in physiologic, biologic, and immunologic processes, which can affect maternal-fetal health through development of several infectious diseases. At birth, neonates have an immature immune system that makes them more susceptible to severe viral infections and diseases. For this reason, different maternal nutritional and immunization interventions have been used to improve the immune and health status of the mother and her neonate through passive immunity. Here, we reviewed the protective role of maternal immunization with different types of vaccines, especially genetic vaccines, during pregnancy in maternal-fetal health, immune response, colostrum quality, immune response, and anti-oxidative status. For this purpose, we have used different scientific databases (PubMed and Google Scholar) and other official web pages. We customized the search period range from the year 2000 to 2023 using the key words "maternal immunization" OR "gestation period/pregnancy" OR "genetic vaccination" OR "maternal-fetal health" OR "micronutrients" OR "neonatal immunity" "oxidative stress" OR "colostrum quality". The evidence demonstrated that inactivated or killed vaccines produced significant immune protection in the mother and fetus. Furthermore, most recent studies have suggested that the use of genetic vaccines (mRNA and DNA) during pregnancy is efficient at triggering the immune response in mother and neonate without the risk of undesired pregnancy outcomes. However, factors such as maternal redox balance, nutritional status, and the timing of immunization play essential roles in regulating immune response inflammatory status, antioxidant capacity, and the welfare of both the pregnant mother and her newborn.

Human immunodeficiency virus and antiretroviral therapy-mediated immune cell metabolic dysregulation in children born to HIV-infected women: potential clinical implications

Commencing lifelong antiretroviral therapy (ART) immediately following HIV diagnosis (Option B+) has dramatically improved the health of HIV-infected women and their children, with the majority being of HIV-exposed children born uninfected (HEU). This success has led to an increasing population of HIV-infected women receiving ART during pregnancy and children exposed to ART in utero. Nonetheless, a small proportion of children are still infected with HIV (HEI) each year. HEI children suffer from reduced immunocompetence and host-defence, due to CD4+ T lymphocyte depletion, but also dysregulation of other immune cells including CD8+ T lymphocytes, natural killer (NK) cells, macrophages including B lymphocytes. Furthermore, although HEU children are uninfected, altered immune responses are observed and associated with increased vulnerability to infections. The mechanisms underlying immune dysregulation in HEU children remain poorly described. Building on early studies, emerging data suggests that HIV/ART exposure early in life affects cell metabolic function of HEU children. Prenatal HIV/ART exposure has been associated with dysregulation of mitochondria, including impaired DNA polymerase activity. Furthermore, dysregulation of oxidative phosphorylation (OXPHOS) causes a decreased generation of adenosine triphosphate (ATP) and increased production of reactive oxygen species (ROS), resulting in oxidative stress. These altered metabolic processes can affect immune cell viability and immune responses. Recent studies have indicated that immune-metabolic dysregulation may contribute to HIV-associated pathogenesis and clinical observations associated with HIV and ART exposure in HEU/HEI children. Given the critical role metabolic processes in immune cell functioning, immune-metabolic dysregulation in HEU and HEI children may have implications in effective host-defence responses against pathogens, as well as efficacy of standard ART regimens and future novel HIV cure approaches in HEI children. At the same time, targeting metabolic pathways of immune cells may provide safer and novel approaches for HIV cure strategies. Here, we review the current literature investigating immune-metabolic dysregulation in paediatric HIV pathogenesis.

What's Old and New in Tuberculosis Vaccines for Children

Tuberculosis (TB) is a leading cause of global child mortality. Until the turn of the 21st century, Mycobacterium bovis bacille Calmette-Guerin (BCG) was the only vaccine to prevent TB. The pediatric TB vaccine pipeline has advanced in the past decade to include the evaluation of novel whole cell vaccines to replace infant BCG and investigation of subunit and whole cell vaccines to boost TB immunity during adolescence. We describe the history of BCG, current TB vaccine candidates in clinical trials, and the challenges and opportunities for future TB vaccine research in children. Children are a critical target population for TB vaccines, and expansion of the pediatric TB vaccine pipeline is urgently needed to end the TB pandemic.

Defective Monocyte Enzymatic Function and an Inhibitory Immune Phenotype in Human Immunodeficiency Virus-Exposed Uninfected African Infants in the Era of Antiretroviral Therapy

BACKGROUND

Human immunodeficiency virus-exposed uninfected (HEU) infants are a rapidly expanding population in sub-Saharan Africa and are highly susceptible to encapsulated bacterial disease in the first year of life. The mechanism of this increased risk is still poorly understood. We investigated whether human immunodeficiency virus (HIV)-exposure dysregulates HEU immunity, vaccine-antibody production, and human herpes virus amplify this effect.

METHODS

Thirty-four HIV-infected and 44 HIV-uninfected pregnant women were recruited into the birth cohort and observed up to 6 weeks of age; and then a subsequent 43 HIV-infected and 61 HIV-uninfected mother-infant pairs were recruited into a longitudinal infant cohort at either: 5-7 to 14-15; or 14-15 to 18-23 weeks of age. We compared monocyte function, innate and adaptive immune cell phenotype, and vaccine-induced antibody responses between HEU and HIV-unexposed uninfected (HU) infants.

RESULTS

We demonstrate (1) altered monocyte phagosomal function and B-cell subset homeostasis and (2) lower vaccine-induced anti-Haemophilus influenzae type b (Hib) and anti-tetanus toxoid immunoglobulin G titers in HEU compared with HU infants. Human herpes virus infection was similar between HEU and HU infants.

CONCLUSIONS

In the era of antiretroviral therapy-mediated viral suppression, HIV exposure may dysregulate monocyte and B-cell function, during the vulnerable period of immune maturation. This may contribute to the high rates of invasive bacterial disease and pneumonia in HEU infants.

Factors influencing maternal microchimerism throughout infancy and its impact on infant T cell immunity

Determinants of the acquisition and maintenance of maternal microchimerism (MMc) during infancy and the impact of MMc on infant immune responses are unknown. We examined factors which influence MMc detection and level across infancy and the effect of MMc on T cell responses to BCG vaccination in a cohort of HIV exposed, uninfected and HIV unexposed infants in South Africa. MMc was measured in whole blood from 58 infants using a panel of quantitative PCR assays at day one and 7, 15, and 36 weeks of life. Infants received BCG at birth, and selected whole blood samples from infancy were stimulated in vitro with BCG and assessed for polyfunctional CD4+ T cell responses. MMc was present in most infants across infancy with levels ranging from 0-1,193/100,000 genomic equivalents and was positively impacted by absence of maternal HIV, maternal-infant HLA compatibility, infant female sex, and exclusive breastfeeding. Initiation of maternal antiretroviral therapy prior to pregnancy partially restored MMc levels in HIV exposed, uninfected infants. Birth MMc was associated with an improved polyfunctional CD4+ T cell response to BCG. These data emphasize that both maternal and infant factors influence MMc, which may subsequently impact infant T cell responses.

Immune Response of Neonates Born to Mothers Infected With SARS-CoV-2

IMPORTANCE

Although several studies have provided information on short-term clinical outcomes in children with perinatal exposure to SARS-CoV-2, data on the immune response in the first months of life among newborns exposed to the virus in utero are lacking.

OBJECTIVE

To characterize systemic and mucosal antibody production during the first 2 months of life among infants who were born to mothers infected with SARS-CoV-2.

DESIGN, SETTING, AND PARTICIPANTS

This prospective cohort study enrolled 28 pregnant women who tested positive for SARS-CoV-2 infection and who gave birth at Policlinico Umberto I in Rome, Italy, from November 2020 to May 2021, and their newborns. Maternal and neonatal systemic immune responses were investigated by detecting spike-specific antibodies in serum, and the mucosal immune response was assessed by measuring specific antibodies in maternal breastmilk and infant saliva 48 hours after delivery and 2 months later.

EXPOSURES

Maternal infection with SARS-CoV-2 in late pregnancy.

MAIN OUTCOMES AND MEASURES

The systemic immune response was evaluated by the detection of SARS-CoV-2 IgG and IgA antibodies and receptor binding domain-specific IgM antibodies in maternal and neonatal serum. The mucosal immune response was assessed by measuring spike-specific antibodies in breastmilk and in infant saliva, and the presence of antigen-antibody spike IgA immune complexes was investigated in breastmilk samples. All antibodies were detected using an enzyme-linked immunosorbent assay.

RESULTS

In total, 28 mother-infant dyads (mean [SD] maternal age, 31.8 [6.4] years; mean [SD] gestational age, 38.1 [2.3] weeks; 18 [60%] male infants) were enrolled at delivery, and 21 dyads completed the study at 2 months' follow-up. Because maternal infection was recent in all cases, transplacental transfer of virus spike-specific IgG antibodies occurred in only 1 infant. One case of potential vertical transmission and 1 case of horizontal infection were observed. Virus spike protein-specific salivary IgA antibodies were significantly increased (P = .01) in infants fed breastmilk (0.99 arbitrary units [AU]; IQR, 0.39-1.68 AU) vs infants fed an exclusive formula diet (0.16 AU; IQR, 0.02-0.83 AU). Maternal milk contained IgA spike immune complexes at 48 hours (0.53 AU; IQR, 0.25-0.39 AU) and at 2 months (0.09 AU; IQR, 0.03-0.17 AU) and may have functioned as specific stimuli for the infant mucosal immune response.

CONCLUSIONS AND RELEVANCE

In this cohort study, SARS-CoV-2 spike-specific IgA antibodies were detected in infant saliva, which may partly explain why newborns are resistant to SARS-CoV-2 infection. Mothers infected in the peripartum period appear to not only passively protect the newborn via breastmilk secretory IgA but also actively stimulate and train the neonatal immune system via breastmilk immune complexes.

Non-IFNγ Whole Blood Cytokine Responses to Mycobacterium tuberculosis Antigens in HIV-exposed Infants

BACKGROUND

HIV-exposed uninfected (HEU) infants have increased risk of tuberculosis (TB). Testing for Mycobacterium tuberculosis (Mtb) infection is limited by reduced Quantiferon (QFT) sensitivity in infants and tuberculin skin test (TST) cross-reactivity with Bacillus Calmette-Guérin vaccine. Our objective is to assess if non-IFNγ cytokine responses to Mtb-specific antigens have improved sensitivity in detecting Mtb infection in HEU infants compared with QFT.

METHODS

HEU infants were enrolled in a randomized clinical trial of isoniazid preventive therapy (IPT) to prevent Mtb infection in Kenya (N = 300) and assessed at 12 months postrandomization (14 months of age) by TST and QFT-Plus. Non-IFNγ cytokine secretion (IL2, TNF, IP10, N = 229) in QFT-Plus supernatants was measured using Luminex assay. Logistic regression was used to assess the effect of IPT on Mtb infection outcomes in HEU infants.

RESULTS

Three of 251 (1.2%) infants were QFT-Plus positive. Non-IFNγ Mtb antigen-specific responses were detected in 12 additional infants (12/229, 5.2%), all TST negative. IPT was not associated with Mtb infection defined as any Mtb antigen-specific cytokine response (odds ratio = 0.7, P = 0.54). Mtb antigen-specific IL2/IP10 responses had fair correlation (τ = 0.25). Otherwise, non-IFNγ cytokine responses had minimal correlation with QFT-Plus and no correlation with TST size.

CONCLUSIONS

We detected non-IFNg Mtb antigen-specific T-cell responses in 14-month HEU infants. Non-IFNg cytokines may be more sensitive than IFNg in detecting infant Mtb infection. IPT during the first year of life was not associated with Mtb infection measured by IFNg, IL2, IP10 and TNF Mtb-specific responses.

Factors That Influence Infant Immunity and Vaccine Responses

The neonatal period and early infancy are times of increased vulnerability to infection. The immune system of infants undergoes rapid changes and a number of factors can influence the maturation and function of the early infant immune system, amongst these factors are maternal infections and immunity. Infants who are HIV-exposed, but uninfected show important immune alterations, which are likely to be associated with the increased morbidity and mortality observed in these infants. Maternally derived antibodies are crucial in early life to protect infants from infection during the time when their own immune system is becoming more experienced and fully mature. However, maternal antibodies can also interfere with the infant's own antibody responses to primary vaccination. Preterm infants are particularly vulnerable to infection, having not had the opportunity to benefit from the transplacental transfer of maternal antibodies in late pregnancy. In addition, further differences have been observed in the innate and adaptive immune system between preterm and term infants. Here, we focus on maternal influences on the infant immune system, using HIV and maternal vaccination as examples and finish by considering how prematurity impacts infant immune responses to vaccination.

The Protective Role of Maternal Immunization in Early Life

With birth, the newborn is transferred from a quasi-sterile environment to the outside world. At this time, the neonatal immune system is inexperienced and continuously subject to a process of development as it encounters different antigenic stimuli after birth. It is initially characterized by a bias toward T helper 2 phenotype, reduced T helper 1, and cytotoxic responses to microbial stimuli, low levels of memory, and effector T and B cells and a high production of suppressive T regulatory cells. The aim of this setting, during fetal life, is to maintain an anti-inflammatory state and immune-tolerance. Maternal antibodies are transferred during pregnancy through the placenta and, in the first weeks of life of the newborn, they represent a powerful tool for protection. Thus, optimization of vaccination in pregnancy represents an important strategy to reduce the burden of neonatal infections and sepsis. Beneficial effects of maternal immunization are universally recognized, although the optimal timing of vaccination in pregnancy remains to be defined. Interestingly, the dynamic exchange that takes place at the fetal-maternal interface allows the transfer not only of antibodies, but also of maternal antigen presenting cells, probably in order to stimulate the developing fetal immune system in a harmless way. There are still controversial effects related to maternal immunization including the so called "immunology blunting," i.e., a dampened antibody production following infant's vaccination in those infants who received placentally transferred maternal immunity. However, clinical relevance of this phenomenon is still not clear. This review will provide an overview of the evolution of the immune system in early life and discuss the benefits of maternal vaccination. Current maternal vaccination policies and their rationale will be summarized on the road to promising approaches to enhance immunity in the neonate.

Maternal Latent Mycobacterium tuberculosis Does Not Affect the Infant Immune Response Following BCG at Birth: An Observational Longitudinal Study in Uganda

Background: BCG has low efficacy in tropical countries. We hypothesized that maternal latent Mycobacterium tuberculosis (M.tb) infection (LTBI) results in fetal tolerance to mycobacterial antigens and impaired responses to BCG immunization.

Methods: We enrolled 132 LTBI-positive and 150 LTBI-negative mothers and their babies in Entebbe, Uganda. Infants were BCG-immunized at birth. Cord blood and samples at weeks 1, 4, 6, 10, 14, 24, and 52 were analyzed for cytokine/chemokine responses to M.tb antigens by Luminex 17-plex assay in 6-day whole blood cultures and antibody responses by ELISA. Of the 17 Luminex analytes, seven (IL-2, IL-5, IL-10, IL-13, IL-17A, TNF, and IFN-γ) were included in the main analysis as they were considered most likely to represent T cell responses. Immune sensitization was defined as a detectable cord blood cytokine response to PPD for any of the seven cytokines. Patterns of cytokine and antibody responses were compared between infants of mothers with and without LTBI using linear mixed models adjusting for confounders.

Results: Most infants (73%) were sensitized in utero to M.tb antigens, with no overall difference seen between infants born to mothers with or without LTBI. Patterns of post-BCG cytokine and antibody responses to mycobacterial antigens were similar between the two infant groups.

Conclusions: Our data do not support the hypothesis that maternal LTBI results in an impaired response to BCG immunization, in Ugandan infants. BCG vaccination at or shortly after birth is likely to be beneficial to all infants, irrespective of maternal LTBI status.

Human Immunodeficiency Virus Exposure but Not Early Cytomegalovirus Infection Is Associated With Increased Hospitalization and Decreased Memory T-Cell Responses to Tetanus Vaccine

BACKGROUND

Human immunodeficiency virus (HIV)-exposed, uninfected (HEU) infants experience high rates of infectious morbidity. We hypothesized that early cytomegalovirus (CMV) infection was associated with increased hospitalization rates and decreased vaccine responses in HEU compared with HIV-unexposed (HUU) infants.

METHODS

Among infants enrolled in the Tshipidi study in Botswana, we determined CMV infection status by 6 months of age and compared hospitalization rates and responses to tetanus and Bacille Calmette-Guérin vaccines among HEU and HUU vaccinees.

RESULTS

Fifteen of 226 (6.6%) HEU infants and 17 (19.3%) of 88 HUU infants were CMV-infected by 6 months. The HEU infants were approximately 3 times as likely to be hospitalized compared with HUU infants (P = .02). The HEU peripheral blood cells produced less interleukin (IL)-2 (P = .004), but similar amounts of interferon-γ, after stimulation with tetanus toxoid. Antitetanus immunoglobulin G titers were similar between groups. Cellular responses to purified protein derivative stimulation did not differ between groups. Maternal receipt of 3-drug antiretroviral therapy compared with zidovudine was associated with increased IL-2 expression after tetanus toxoid stimulation. The infants' CMV infection status was not associated with clinical or vaccine response outcomes.

CONCLUSIONS

We observed that increased rates of hospitalization and decreased memory T-cell responses to tetanus vaccine were associated with HIV exposure and incomplete treatment of maternal HIV infection, but not early CMV infection.

Bacille Calmette-Guérin Vaccine Strain Modulates the Ontogeny of Both Mycobacterial-Specific and Heterologous T Cell Immunity to Vaccination in Infants

Differences in Bacille Calmette-Guérin (BCG) immunogenicity and efficacy have been reported, but various strains of BCG are administered worldwide. Since BCG immunization may also provide protection against off-target antigens, we sought to identify the impact of different BCG strains on the ontogeny of vaccine-specific and heterologous vaccine immunogenicity in the first 9 months of life, utilizing two African birth cohorts. A total of 270 infants were studied: 84 from Jos, Nigeria (vaccinated with BCG-Bulgaria) and 187 from Cape Town, South Africa (154 vaccinated with BCG-Denmark and 33 with BCG-Russia). Infant whole blood was taken at birth, 7, 15, and 36 weeks and short-term stimulated (12 h) in vitro with BCG, Tetanus and Pertussis antigens. Using multiparameter flow cytometry, CD4+ T cell memory subset polyfunctionality was measured by analyzing permutations of TNF-α, IL-2, and IFN-γ expression at each time point. Data was analyzed using FlowJo, SPICE, R, and COMPASS. We found that infants vaccinated with BCG-Denmark mounted significantly higher frequencies of BCG-stimulated CD4+ T cell responses, peaking at week 7 after immunization, and possessed durable polyfunctional CD4+ T cells that were in a more early differentiated memory stage when compared with either BCG-Bulgaria and BCG-Russia strains. The latter responses had lower polyfunctional scores and tended to accumulate in a CD4+ T cell naïve-like state (CD45RA+CD27+). Notably, BCG-Denmark immunization resulted in higher magnitudes and polyfunctional cytokine responses to heterologous vaccine antigens (Tetanus and Pertussis). Collectively, our data show that BCG strain was the strongest determinant of both BCG-stimulated and heterologous vaccine stimulated T cell magnitude and polyfunctionality. These findings have implications for vaccine policy makers, manufacturers and programs worldwide and also suggest that BCG-Denmark, the first vaccine received in many African infants, has both specific and off-target effects in the first few months of life, which may provide an immune priming benefit to other EPI vaccines.

Tuberculosis susceptibility and protection in children

Children represent both a clinically important population susceptible to tuberculosis and a key group in whom to study intrinsic and vaccine-induced mechanisms of protection. After exposure to Mycobacterium tuberculosis, children aged under 5 years are at high risk of progressing first to tuberculosis infection, then to tuberculosis disease and possibly disseminated forms of tuberculosis, with accompanying high risks of morbidity and mortality. Children aged 5-10 years are somewhat protected, until risk increases again in adolescence. Furthermore, neonatal BCG programmes show the clearest proven benefit of vaccination against tuberculosis. Case-control comparisons from key cohorts, which recruited more than 15 000 children and adolescents in total, have identified that the ratio of monocytes to lymphocytes, activated CD4 T cell count, and a blood RNA signature could be correlates of risk for developing tuberculosis. Further studies of protected and susceptible populations are necessary to guide development of novel tuberculosis vaccines that could facilitate the achievement of WHO's goal to eliminate deaths from tuberculosis in childhood.

Beyond Passive Immunity: Is There Priming of the Fetal Immune System Following Vaccination in Pregnancy and What Are the Potential Clinical Implications?

Infection is responsible for over half a million neonatal deaths worldwide every year, and vaccination in pregnancy is becoming increasingly recognized as an important strategy for the protection of young infants. Increasing evidence suggests that exposure to maternal infection in utero may "prime" the developing immune system, even in the absence of infant infection. It is also possible that in utero priming may occur following maternal vaccination, with antigen-specific cellular immune responses detectable in utero and at birth. However, this remains a topic of some controversy. This review focuses on the evidence for in utero priming and the clinical implications for vaccination in pregnancy, considering whether in utero priming following vaccination could provide protection independent of antibody-mediated passive immunity, the possible effects of vaccination on subsequent infant vaccinations, their potential "non-specific" effects, and how the design and timing of vaccination might affect prenatal priming. Looking forward, we describe other possible options for quantifying antigen-specific cellular responses, including MHC tetramers, novel proliferation and cytokine-based assays, and animal models. Together, these may help us address future research questions and establish more robust evidence of fetal immune system priming.

The Effect of Human Immunodeficiency Virus and Cytomegalovirus Infection on Infant Responses to Vaccines: A Review

The success of prevention of mother to child transmission programs over the last two decades has led to an increasing number of infants who are exposed to human immunodeficiency virus (HIV), but who are not themselves infected (HIV-exposed, uninfected infants). Although the morbidity and mortality among HIV-exposed, uninfected infants is considerably lower than that among HIV-infected infants, they may remain at increased risk of infections in the first 2 years of life compared with their HIV-unexposed peers, especially in the absence of breastfeeding. There is some evidence of immunological differences in HIV-exposed, uninfected infants, which could play a role in susceptibility to infection. Cytomegalovirus (CMV) may contribute to the increased immune activation observed in HIV-exposed, uninfected infants. Infants born to HIV-infected women are at increased risk of congenital CMV infection, as well as early acquisition of postnatal CMV infection. In infants with HIV infection, CMV co-infection in early life is associated with higher morbidity and mortality. This review considers how HIV infection, HIV exposure, and CMV infection affect infant responses to vaccination, and explores possible immunological and other explanations for these findings. HIV-infected infants have lower vaccine-induced antibody concentrations following tetanus, diphtheria, pertussis, hepatitis B, and pneumococcal vaccination, although the clinical relevance of this difference is not known. Despite lower concentrations of maternal-specific antibody at birth, HIV-exposed, uninfected infants respond to vaccination at least as well as their HIV-unexposed uninfected peers. CMV infection leads to an increase in activation and differentiation of the whole T-cell population, but there is limited data on the effects of CMV infection on infant vaccine responses. In light of growing evidence of poor clinical outcomes associated with CMV infection in HIV-exposed, uninfected infants, further studies are particularly important in this group. A clearer understanding of the mechanisms by which maternal viral infections influence the developing infant immune system is critical to the success of maternal and infant vaccination strategies.

Safety and Immunogenicity of Newborn MVA85A Vaccination and Selective, Delayed Bacille Calmette-Guerin for Infants of Human Immunodeficiency Virus-Infected Mothers: A Phase 2 Randomized, Controlled Trial

Background

Vaccination of human immunodeficiency virus (HIV)-infected infants with bacille Calmette-Guérin (BCG) is contraindicated. HIV-exposed newborns need a new tuberculosis vaccination strategy that protects against tuberculosis early in life and avoids the potential risk of BCG disease until after HIV infection has been excluded.

Methods

This double-blind, randomized, controlled trial compared newborn MVA85A prime vaccination (1 × 108 PFU) vs Candin® control, followed by selective, deferred BCG vaccination at age 8 weeks for HIV-uninfected infants and 12 months follow-up for safety and immunogenicity.

Results

A total of 248 HIV-exposed infants were enrolled. More frequent mild-moderate reactogenicity events were seen after newborn MVA85A vaccination. However, no significant difference was observed in the rate of severe or serious adverse events, HIV acquisition (n = 1 per arm), or incident tuberculosis disease (n = 5 MVA85A; n = 3 control) compared to the control arm. MVA85A vaccination induced modest but significantly higher Ag85A-specific interferon gamma (IFNγ)+ CD4+ T cells compared to control at weeks 4 and 8 (P < .0001). BCG did not further boost this response in MVA85A vaccinees. The BCG-induced Ag85A-specific IFNγ+ CD4+ T-cell response at weeks 16 and 52 was of similar magnitude in the control arm compared to the MVA85A arm at all time points. Proliferative capacity, functional profiles, and memory phenotype of BCG-specific CD4 responses were similar across study arms.

Conclusions

MVA85A prime vaccination of HIV-exposed newborns was safe and induced an early modest antigen-specific immune response that did not interfere with, or enhance, immunogenicity of subsequent BCG vaccination. New protein-subunit and viral-vectored tuberculosis vaccine candidates should be tested in HIV-exposed newborns.

Clinical Trials Registration

NCT01650389.

Vaccine responses in newborns

Immunisation of the newborn represents a key global strategy in overcoming morbidity and mortality due to infection in early life. Potential limitations, however, include poor immunogenicity, safety concerns and the development of tolerogenicity or hypo-responsiveness to either the same antigen and/or concomitant antigens administered at birth or in the subsequent months. Furthermore, the neonatal immunological milieu is polarised towards Th2-type immunity with dampening of Th1-type responses and impaired humoral immunity, resulting in qualitatively and quantitatively poorer antibody responses compared to older infants. Innate immunity also shows functional deficiency in antigen-presenting cells: the expression and signalling of Toll-like receptors undergo maturational changes associated with distinct functional responses. Nevertheless, the effectiveness of BCG, hepatitis B and oral polio vaccines, the only immunisations currently in use in the neonatal period, is proof of concept that vaccines can be successfully administered to the newborn via different routes of delivery to induce a range of protective mechanisms for three different diseases. In this review paper, we discuss the rationale for and challenges to neonatal immunisation, summarising progress made in the field, including lessons learnt from newborn vaccines in the pipeline. Furthermore, we explore important maternal, infant and environmental co-factors that may impede the success of current and future neonatal immunisation strategies. A variety of approaches have been proposed to overcome the inherent regulatory constraints of the newborn innate and adaptive immune system, including alternative routes of delivery, novel vaccine configurations, improved innate receptor agonists and optimised antigen-adjuvant combinations. Crucially, a dual strategy may be employed whereby immunisation at birth is used to prime the immune system in order to improve immunogenicity to subsequent homologous or heterologous boosters in later infancy. Similarly, potent non-specific immunomodulatory effects may be elicited when challenged with unrelated antigens, with the potential to reduce the overall risk of infection and allergic disease in early life.

What Have We Learnt about BCG Vaccination in the Last 20 Years?

A number of new tuberculosis (TB) vaccines have been or are entering clinical trials, which include genetically modified mycobacteria, mycobacterial antigens delivered by viral vectors, or mycobacterial antigens in adjuvant. Some of these vaccines aim to replace the existing BCG vaccine but others will be given as a boosting vaccine following BCG vaccination given soon after birth. It is clear that the existing BCG vaccines provide incomplete and variable protection against pulmonary TB. This review will discuss what we have learnt over the last 20 years about how the BCG vaccine induces specific and non-specific immunity, what factors influence the immune responses induced by BCG, and progress toward identifying correlates of immunity against TB from BCG vaccination studies. There is still a lot to learn about the BCG vaccine and the insights gained can help the development of more protective vaccines.

Maternal BCG scar is associated with increased infant proinflammatory immune responses

Introduction

Prenatal exposures such as infections and immunisation may influence infant responses. We had an opportunity to undertake an analysis of innate responses in infants within the context of a study investigating the effects of maternal mycobacterial exposures and infection on BCG vaccine-induced responses in Ugandan infants.

Material and methods

Maternal and cord blood samples from 29 mother-infant pairs were stimulated with innate stimuli for 24 h and cytokines and chemokines in supernatants were measured using the Luminex® assay. The associations between maternal latent Mycobacterium tuberculosis infection (LTBI), maternal BCG scar (adjusted for each other’s effect) and infant responses were examined using linear regression. Principal Component Analysis (PCA) was used to assess patterns of cytokine and chemokine responses. Gene expression profiles for pathways associated with maternal LTBI and with maternal BCG scar were examined using samples collected at one (n = 42) and six (n = 51) weeks after BCG immunisation using microarray.

Results

Maternal LTBI was positively associated with infant IP-10 responses with an adjusted geometric mean ratio (aGMR) [95% confidence interval (CI)] of 5.10 [1.21, 21.48]. Maternal BCG scar showed strong and consistent associations with IFN-γ (aGMR 2.69 [1.15, 6.17]), IL-12p70 (1.95 [1.10, 3.55]), IL-10 (1.82 [1.07, 3.09]), VEGF (3.55 [1.07, 11.48]) and IP-10 (6.76 [1.17, 38.02]). Further assessment of the associations using PCA showed no differences for maternal LTBI, but maternal BCG scar was associated with higher scores for principal component (PC) 1 (median level of scores: 1.44 in scar-positive versus −0.94 in scar-negative, p = 0.020) in the infants. PC1 represented a controlled proinflammatory response. Interferon and inflammation response pathways were up-regulated in infants of mothers with LTBI at six weeks, and in infants of mothers with a BCG scar at one and six weeks after BCG immunisation.

Conclusions

Maternal BCG scar had a stronger association with infant responses than maternal LTBI, with an increased proinflammatory immune profile.

Impact of In Utero Exposure to Malaria on Fetal T Cell Immunity

Pregnancy-associated malaria, including placental malaria, causes significant morbidity and mortality worldwide. Recently, it has been suggested that in utero exposure of the fetus to malaria antigens may negatively impact the developing immune system and result in tolerance to malaria. Here, we review our current knowledge of fetal immunity to malaria, focusing on the dynamic interactions between maternal malaria infection, placental development, and the fetal immune system. A better understanding of the long-term impact of in utero malaria exposure on the development of natural immunity to malaria, immune responses to other childhood pathogens, and vaccine immunogenicity is urgently needed. This may guide the implementation of novel chemoprevention strategies during pregnancy and facilitate the push toward malaria vaccines.

HIV-Exposed Uninfected Infants in Zimbabwe: Insights into Health Outcomes in the Pre-Antiretroviral Therapy Era

The ZVITAMBO trial recruited 14,110 mother-infant pairs to a randomized controlled trial of vitamin A between 1997 and 2000, before the availability of antiretroviral therapy for HIV prophylaxis or treatment in Zimbabwe. The HIV status of mothers and infants was well characterized through 1-2 years of follow-up, leading to the largest cohort to date of HIV-exposed uninfected (HEU) infants (n = 3135), with a suitable comparison group of HIV-unexposed infants (n = 9510). Here, we draw on 10 years of published findings from the ZVITAMBO trial. HEU infants had increased morbidity compared to HIV-unexposed infants, with 50% more hospitalizations in the neonatal period and 30% more sick clinic visits during infancy, particularly for skin infections, lower respiratory tract infections, and oral thrush. HEU children had 3.9-fold and 2.0-fold higher mortality than HIV-unexposed children during the first and second years of life, respectively, most commonly due to acute respiratory infections, diarrhea/dysentery, malnutrition, sepsis, and meningitis. Infant morbidity and mortality were strongly related to maternal HIV disease severity, and increased morbidity remained until maternal CD4 counts were >800 cells/μL. HEU infants were more likely to be premature and small-for-gestational age than HIV-unexposed infants, and had more postnatal growth failure. Here, we propose a conceptual framework to explain the increased risk of infectious morbidity, mortality, and growth failure among HEU infants, hypothesizing that immune activation and inflammation are key drivers of both infection susceptibility and growth failure. Future studies should further dissect the causes of infection susceptibility and growth failure and determine the impact of ART and cotrimoxazole on outcomes of this vulnerable group of infants in the current era.

Tuberculosis Infection in Early Childhood and the Association with HIV-exposure in HIV-uninfected Children in Rural Uganda

BACKGROUND

In high tuberculosis (TB) burden countries, a significant proportion of the latent TB reservoir is established by the age 5 years. There are critical knowledge gaps in our understanding of the age-specific prevalence of TB infection and the influence of HIV exposure on TB infection in the first 5 years of life among HIV-uninfected children in sub-Saharan Africa.

METHODS

We measured TB infection with the Quantiferon Gold-in-Tube (QFT) and tuberculin skin tests (TST) in 447 children ≤60 months and their 284 HIV-infected and IV-uninfected mothers in rural Uganda.

RESULTS

The overall prevalence of TB infection in children ≤60 months by TST was 24% (95% confidence intervals [CI]: 19.9-27.9). The prevalence of TST positivity was highest among children in their first year of life (36%; 95% CI: 26.0-45.9) and declined with age to 19% at 36-60 months of age, χ test for trend P = 0.014. In contrast, 4% (95% CI: 1.9-5.87%) of children had a positive QFT, and there was no trend detected with age, P = 0.576. QFT positivity was detected as early as 5 months. HIV-exposed uninfected children had significantly higher odds of TB infection by QFT (odds ratio [OR]: 21.2; P = 0.008; 95% CI: 2.2-204.7) or positive TST or QFT (OR, 2.4; P = 0.020; 95% CI: 1.2-5.1) compared with HIV-unexposed uninfected children, adjusting for age, BCG vaccination and a positive maternal TST or QFT.

CONCLUSIONS

An appreciable prevalence of TB infection was detected in early childhood. HIV-exposed uninfected children have a higher risk for TB infection compared with children born to HIV-uninfected mothers.

Delayed BCG immunization does not alter antibody responses to EPI vaccines in HIV-exposed and -unexposed South African infants

BACKGROUND

Bacille Calmette-Guérin (BCG) is routinely given at birth in tuberculosis-endemic settings due to its protective effect against disseminated tuberculosis in infants. BCG is however contraindicated in HIV-infected infants. We investigated whether delaying BCG vaccination to 14 weeks of age affected vaccine-induced antibody responses to Haemophilus influenzae type b (Hib)-conjugate, pertussis, tetanus and Hepatitis B (HBV) vaccines, in HIV-exposed uninfected (HEU) and -unexposed uninfected (HUU) infants.

METHODS

Infants were randomized to receive BCG at birth or at 14 weeks of age. Blood was taken at 14, 24, and 52 weeks of age and analyzed for Hib, pertussis, tetanus and HBV specific antibodies.

RESULTS

BCG was given either at birth (106 infants, 51 HEU) or at 14 weeks of age (74 infants, 50 HEU). The timing of BCG vaccination did not influence the antibody response to any antigen studied. However, in a non-randomized comparison, HEU infants had higher Hib antibody concentrations at weeks 14 and 24 (p=0.001 and <0.001, respectively) and pertussis at week 24 (p=0.003). Conversely, HEU infants had lower antibody concentrations to HBV at 14 and 52 weeks (p=0.032 and p=0.031) with no differences in tetanus titres.

CONCLUSIONS

HIV exposure, but not the timing of BCG vaccination, was associated with antibody concentrations to Hib, pertussis, HBV and tetanus primary immunization.

CLINICAL TRIAL REGISTRATION

DOH-27-1106-1520.

HIV-exposed, uninfected infants: new global challenges in the era of paediatric HIV elimination

The number of infants infected with HIV is declining with the rise in interventions for the elimination of paediatric HIV infection, but the number of uninfected infants exposed to HIV through their HIV-infected mothers is increasing. Interest in the health outcomes of HIV-exposed, uninfected infants has grown in the past decade, with several studies suggesting that these infants have increased mortality rates, increased infectious morbidity, and impaired growth compared with HIV-unexposed infants. However, heterogeneous results might reflect the inherent challenges in studies of HIV-exposed, uninfected infants, which need large populations with appropriate, contemporaneous comparison groups and repeated HIV testing throughout the period of breastfeeding. We review the effects of HIV exposure on mortality, morbidity, and growth, discuss the immunological abnormalities identified so far, and provide an overview of interventions that could be effective in this susceptible population. As the number of infants infected with HIV declines, the health needs of HIV-exposed, uninfected infants should be prioritised further, to ensure that post-2015 Sustainable Development Goals are achieved.

The impact of maternal infection with Mycobacterium tuberculosis on the infant response to bacille Calmette-Guérin immunization

Bacille Calmette–Guérin (BCG) immunization provides variable protection against tuberculosis. Prenatal antigen exposure may have lifelong effects on responses to related antigens and pathogens. We therefore hypothesized that maternal latent Mycobacterium tuberculosis infection (LTBI) influences infant responses to BCG immunization at birth. We measured antibody (n = 53) and cellular (n = 31) responses to M. tuberculosis purified protein derivative (PPD) in infants of mothers with and without LTBI, in cord blood and at one and six weeks after BCG. The concentrations of PPD-specific antibodies declined between birth (median [interquartile range (IQR)]) 5600 ng ml⁻¹ [3300–11 050] in cord blood) and six weeks (0.00 ng ml⁻¹ [0–288]). Frequencies of PPD-specific IFN-γ-expressing CD4⁺T cells increased at one week and declined between one and six weeks (p = 0.031). Frequencies of IL-2- and TNF-α-expressing PPD-specific CD4⁺T cells increased between one and six weeks (p = 0.019, p = 0.009, respectively). At one week, the frequency of PPD-specific CD4⁺T cells expressing any of the three cytokines, combined, was lower among infants of mothers with LTBI, in crude analyses (p = 0.002) and after adjusting for confounders (mean difference, 95% CI −0.041% (−0.082, −0.001)). In conclusion, maternal LTBI was associated with lower infant anti-mycobacterial T-cell responses immediately following BCG immunization. These findings are being explored further in a larger study.

Factors influencing innate immunity and vaccine responses in infancy

Despite significant progress in reducing the burden of mortality in children under the age of five, reducing mortality in newborns remains a major challenge. Infection plays a significant role in infant deaths and interventions such as early vaccination or antenatal immunization could make a significant contribution to prevention of such deaths. In the last few years, we have gained new insights into immune ontogeny and are now beginning to understand the impact of vaccines, nutrition and environmental factors on 'training' of the immune response in early life. This review article sets out to explain why vaccine responses can be heterogeneous between populations and individuals by providing examples chosen to illustrate the impact of host, pathogen and environmental factors on shaping the immune 'interactome' in young children.

Placental transfer of anti-group B Streptococcus immunoglobulin G antibody subclasses from HIV-infected and uninfected women to their uninfected infants

OBJECTIVES

Placental antibody transfer is impaired in the context of HIV infection, which may render HIV-exposed, uninfected infants vulnerable to group B Streptococcus (GBS) disease. The GBS antibody response predominately consists of immunoglobulin G2 (IgG2) antibody. Thus we determined whether concentration and placental transfer of anti-GBS antibody subclasses was altered in HIV-infected compared with HIV-uninfected mothers.

DESIGN

A retrospective analysis of anti-GBS antibody subclasses in 38 HIV-infected and 33 HIV-uninfected mothers and their uninfected infants.

METHODS

Sera were analysed using a novel flow cytometric assay that quantified binding of IgG1, IgG2, IgG3 and IgG4 to serotype (ST)Ia, STIII and STV GBS bacteria.

RESULTS

IgG2 binding to GBS STIa and V was lower in HIV-infected women compared with HIV-uninfected women. Moreover, IgG2 binding to GBS STIa was also lower in HIV-exposed, uninfected infants compared with unexposed infants. However, there were no statistically significant differences in the transplacental transfer ratio of IgG2 for any GBS serotype. The transplacental transfer of total IgG was reduced for GBS STIII and V and IgG1 subclass for STIII; placental transfer of all other subclasses was comparable in HIV-affected and HIV-unaffected pregnancies.

CONCLUSION

Anti-GBS IgG2 placental transfer is not affected by HIV infection. This is important for functional antibody against the capsular polysaccharide of GBS and provides confidence that maternal GBS vaccination may result in functional activity in HIV-infected and uninfected women.

Altered Memory T-Cell Responses to Bacillus Calmette-Guerin and Tetanus Toxoid Vaccination and Altered Cytokine Responses to Polyclonal Stimulation in HIV-Exposed Uninfected Kenyan Infants

Implementation of successful prevention of mother-to-child transmission of HIV strategies has resulted in an increased population of HIV-exposed uninfected (HEU) infants. HEU infants have higher rates of morbidity and mortality than HIV-unexposed (HU) infants. Numerous factors may contribute to poor health in HEU infants including immunological alterations. The present study assessed T-cell phenotype and function in HEU infants with a focus on memory Th1 responses to vaccination. We compared cross-sectionally selected parameters at 3 and 12 months of age in HIV-exposed (n = 42) and HU (n = 28) Kenyan infants. We measured ex vivo activated and bulk memory CD4 and CD8 T-cells and regulatory T-cells by flow cytometry. In addition, we measured the magnitude, quality and memory phenotype of antigen-specific T-cell responses to Bacillus Calmette-Guerin and Tetanus Toxoid vaccine antigens, and the magnitude and quality of the T cell response following polyclonal stimulation with staphylococcal enterotoxin B. Finally, the influence of maternal disease markers on the immunological parameters measured was assessed in HEU infants. Few perturbations were detected in ex vivo T-cell subsets, though amongst HEU infants maternal HIV viral load positively correlated with CD8 T cell immune activation at 12 months. Conversely, we observed age-dependent differences in the magnitude and polyfunctionality of IL-2 and TNF-α responses to vaccine antigens particularly in Th1 cells. These changes mirrored those seen following polyclonal stimulation, where at 3 months, cytokine responses were higher in HEU infants compared to HU infants, and at 12 months, HEU infant cytokine responses were consistently lower than those seen in HU infants. Finally, reduced effector memory Th1 responses to vaccine antigens were observed in HEU infants at 3 and 12 months and higher central memory Th1 responses to M. tuberculosis antigens were observed at 3 months only. Long-term monitoring of vaccine efficacy and T-cell immunity in this vulnerable population is warranted.

Anti-group B Streptococcus antibody in infants born to mothers with human immunodeficiency virus (HIV) infection

BACKGROUND

HIV-exposed uninfected infants have increased infection risk and mortality compared to HIV-unexposed infants. HIV-exposed infants may be at increased risk of invasive GBS disease due to reduced maternal antibody against GBS.

METHODS

We quantified antibodies that bind to the surface of whole Group B Streptococcus (GBS) of serotypes Ia, Ib, II, III and V using novel flow cytometry assays in South African HIV-infected and non-infected mothers and their uninfected infants. Antibody-mediated complement C3b/iC3b deposition onto GBS of these serotypes was also quantified by a novel flow cytometry assay.

RESULTS

Geometric mean concentration (GMC) of both surface-binding anti-GBS antibody and antibody-mediated complement deposition onto GBS were reduced in HIV-infected women (n=46) compared to HIV-uninfected women (n=58) for ST1a (surface-binding: 19.3 vs 29.3; p=0.003; complement deposition: 2.9 vs 5.3 SU/mL; p=0.003), STIb (24.9 vs 47.6; p=0.003; 2.6 vs 4.9 SU/mL; p=0.003), STII (19.8 vs 50.0; p=0.001; 3.1 vs 6.2 SU/mL; p=0.001), STIII (27.8 vs 60.1; p=0.001; 2.8 vs 5.3 SU/mL; p=0.001) and STV (121.9 vs 185.6 SU/mL; p<0.001) and in their infants for STIa (complement deposition 9.4 vs 27.0 SU/mL; p=0.02), STIb (13.4 vs 24.5 SU/mL; p=0.02), STII (14.6 vs 42.7 SU/mL; p=0.03), STIII (26.6 vs 62.7 SU/mL; p=0.03) and STV (90.4 vs 165.8 SU/mL; p=0.04). Median transplacental transfer of antibody from HIV-infected women to their infants was reduced compared to HIV-uninfected women for GBS serotypes II (0.42 [IQR 0.22-0.59] vs 1.0 SU/mL [0.42-1.66]; p<0.001), III (0.54 [0.31-1.03] vs 0.95 SU/mL [0.42-3.05], p=0.05) and V (0.51 [0.28-0.79] vs 0.75 SU/mL [0.26-2.9], p=0.04). The differences between infants remained significant at 16 weeks of age.

CONCLUSIONS

Maternal HIV infection was associated with lower anti-GBS surface binding antibody concentration and antibody-mediated C3b/iC3b deposition onto GBS bacteria of serotypes Ia, Ib, II, III and V. This may render these infants more susceptible to early and late onset GBS disease.