Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome in the Infant

Global Scope of the Problem

The HIV pandemic is a major public health problem, with cases reported to the WHO from virtually every country. At the end of December 2011, an estimated 34 million people (31.4-35.9 million) were living with HIV (an increase from 29.4 million in 2001), including 15.7 million women and 3.3 million children younger than 15 years. Nearly 97% of people living with HIV live in LMICs, and the adult HIV prevalence worldwide is estimated at 0.8%. Sub-Saharan Africa bears the brunt of the epidemic, accounting for 69% of HIV infections; approximately 23.5 million HIV-infected people live in sub-Saharan Africa, including more than 3 million children younger than 15 years, representing more than 90% of all children with HIV. South and Southeast Asia account for 4 million HIV-infected people, followed by 3 million people living with HIV in the Americas, including the Caribbean.

The complex and diverse epidemiology of HIV worldwide has evolved significantly after the peak of HIV incidence around 1997. Recent data from UNAIDS indicate an encouraging trend, with rates of new HIV infections continuing to decline among general populations. Overall, a greater than 20% decline in new HIV infections was noted in 2011 (when compared with 2001 incidence rates), with the most significant declines in the Caribbean and sub-Saharan Africa. In 2011, an estimated 2.5 million people became newly infected with HIV, including 330,000 children younger than 15 years. Although HIV testing centers have increased in many countries, approximately 50% of people living with HIV do not know their HIV status. In 2011, an estimated 1.7 million people died because of AIDS-related illness (24% fewer than in 2005), including 230,000 children younger than 15 years (20% less than in 2005).

Primary HIV infection among women of childbearing age fuels the perinatal HIV epidemic. Women constitute nearly one half of all HIV/AIDS infections worldwide, and more than one half of these (58%) living in sub-Saharan Africa. Young women and adolescent females are at highest risk of HIV acquisition because of a myriad of complex biologic, behavioral, and structural factors. In LMIC, antenatal HIV prevalence rates can vary considerably, ranging from less than 5% to greater than 35% in some high–HIV-burden countries. Although HIV prevalence among young pregnant women has declined in Kenya, Malawi, and Zimbabwe, most countries in Southern Africa have antenatal prevalence rates between 20% to 30%; rates can reach 35% to 40% in some antenatal clinics in South Africa, Swaziland, Lesotho, and Botswana. In 2009, an estimated 1.4 million HIV-infected women became pregnant, 90% of whom lived in 22 countries in sub-Saharan Africa and India. Approximately 900 HIV-infected infants are born each day.

In LMIC, children constitute 14% of new HIV infections worldwide and nearly a fifth of annual HIV deaths. Without access to combination ART, greater than 50% of HIV-infected children in sub-Saharan Africa die by their second birthday. In the absence of rapid identification and early access to ART, 26% of postnatal and 52% of perinatal HIV infections result in child mortality 1 year after acquiring HIV. Based on UNAIDS estimates, 17.3 million children have been orphaned by the AIDS epidemic.

United States

Mother-to-Child HIV Transmission: Rates, Timing, Risk Factors, and Mechanisms

Mother-to-child transmission of HIV can occur in utero, during labor and delivery, or postnatally through breastfeeding. In the United States, the transmission rate without PMTCT interventions is estimated to be 25% to 30%; in European studies, it was found to be lower, at 15% to 20%. A higher transmission rate of 25% to 45% has been observed among breastfeeding populations in sub-Saharan Africa. These variations in transmission rates likely reflect differences in infant feeding patterns, maternal and obstetric risk factors, viral factors, and methodologic differences among studies. Data suggest that most children are infected during the intrapartum period. Studies based on cord blood or newborn HIV polymerase chain reaction (PCR) testing indicate that 50% to 60% of the perinatal HIV transmission occurs around the time of labor and delivery.

An infant is considered to have been infected in utero if the HIV-1 genome can be detected by PCR or cultured from blood within 48 hours of birth. In contrast, a child is considered to have intrapartum infection if diagnostic assays, such as culture, PCR, and serum p24 antigen, are negative in blood samples obtained during the first week of life but became positive during the period from day 7 to 90 of life and the infant has not been breastfed. In the breastfed infant, 20% to 25% of HIV transmission occurs during pregnancy, 35% to 50% during labor and delivery, and another 25% to 35% during lactation.

Intrauterine Transmission

In utero transmission may occur through HIV infection in the placenta or fetal exposure to cell-free or cell-associated HIV in the amniotic fluid. In early studies using PCR and in situ hybridization technology, the virus was detected in some aborted fetuses of 8 to 20 weeks of gestational age and in amniotic fluid. Maternal decidual leukocytes, placental villous macrophages (Hofbauer cells), and endothelial cells stain positive for gp41 antigen and HIV nucleic acids. The placenta can be infected through CD4 ⁺ trophoblasts or through the occasional occurrence of chorioamnionitis. However, subsequent studies in animals and human fetuses reported almost no transmission during the first and second trimester of pregnancy. There is no clear predictive value for the identification of HIV in the placenta and the infection of the fetus or newborn, and important technical limitations exist in studies conducted on fetal or placenta tissues, particularly because of the difficulty in excluding contamination with maternal blood. Based on viral detection during the first 48 hours of birth, intrauterine transmission occurs in about 20% to 25% of infections. Statistical modeling data also suggest that most in utero HIV transmission occurs during the last few weeks before delivery when the vascular integrity of the placenta is disrupted.

Intrapartum Infection

Intrapartum transmission may occur because of direct exposure of the fetus or infant with infected maternal secretions during birth or maternal-to-fetal microtransfusions during uterine contractions. Intrapartum transmission is supported by studies failing to detect HIV in the first month of life in infants born to HIV-infected women but with subsequent detection of virus after 1 to 3 months of life. In a study by the French Collaborative Study Group, timing of transmission was estimated with a mathematical model. Data for 95 infected infants (infants seropositive at 18 months and infants who died of HIV disease before this age and who were exclusively bottle-fed) were used in the model, which indicated that one third of the infants were infected in utero less than 2 months before delivery (95th percentile). In the remaining 65% of cases (95% confidence interval [CI], 22% to 92%), the date of infection was estimated as the day of birth. The estimated median period between birth and the positivity of viral markers (HIV PCR or HIV culture) was 10 days (95% CI, 6% to 14%), and the 95th percentile was estimated at 56 days.

Additional evidence to support mucosal exposure to maternal virus during delivery as a likely route of transmission includes findings of increased HIV infection rates in first-born twins, increased risk associated with prolonged rupture of membranes, and the protective effect, although incomplete, of cesarean delivery before onset of labor. Some reports have documented an increased risk of MTCT associated with placental malaria infection, but not others.

Although numerous maternal, obstetric, infant, host genetic, and viral factors may modify perinatal HIV transmission risk, maternal plasma HIV RNA level is the strongest predictor of intrauterine and intrapartum transmission ( Box 22-1 ). Transmission can occur rarely, however, among pregnant women with low or undetectable serum levels of HIV around the time of labor and delivery. Other maternal risk factors associated with higher rates of perinatal HIV infection include women with advanced clinical disease, acute HIV infection during pregnancy, and low CD4 ⁺ counts. HIV viral load in cervicovaginal secretions is an independent risk factor for perinatal HIV transmission. Maternal genital ulcer diseases, especially herpes simplex virus (HSV) and syphilis and other coinfections (such as hepatitis C virus, hepatitis B virus, malaria, tuberculosis), may increase the risk. Behavioral risk factors including maternal substance abuse, cigarette smoking during pregnancy, and noncompliance to ART may also increase the risk of transmission.

Obstetric risk factors associated with increased risk of transmission include vaginal delivery, prolonged rupture of membranes, chorioamnionitis, and invasive obstetric procedures. Premature infants born to HIV-infected women have a higher rate of perinatal HIV infection than full-term infants. Data from a large international meta-analysis of 15 prospective cohort studies and a randomized controlled trial from Europe have shown that cesarean section performed before labor and rupture of membranes reduces perinatal transmission of HIV-1 by 50% to 87%, independent of the use of ART or zidovudine (ZDV) prophylaxis.

Viral and host genetic factors may also influence MTCT. Besides maternal viral load, other factors affecting transmission include viral subtype, circulating recombinant forms, resistance viral strains and replication kinetics, and fitness. Genetic and phylogenetic studies indicate that infant quasi-species are highly homogeneous and generally represent minor maternal variants, confirming that vertical transmission of clade B HIV occurs across a selective bottleneck. In one study, infant clones did not differ from the maternal clones in env length or glycosylation, and all infant variants used the CCR5 co-receptor but were not macrophage tropic. Preferential in utero transmission of HIV subtype C, compared with subtype A or D, has been reported.

Host genetic factors including maternal-infant human leukocyte antigen (HLA) concordance and maternal HLA class I homozygosity have been associated with increased risks of MTCT, whereas genetic variants of chemokine and chemokine receptors have yielded conflicting results. Genetic variants of Toll-like receptors (TLRs) and defensins (e.g., TLR9 and β-defensin 1), crucial determinants of innate immune response, may also influence host-virus interactions and MTCT.

Postnatal Infection

In LMIC, where prolonged breastfeeding (>6 months) is the cultural norm and safe replacement feeding is not affordable, feasible, sustainable, or safe, postnatal transmission of HIV through breast milk remains a significant challenge. In the absence of interventions, postnatal transmission of HIV through breastfeeding can account for one third to one half of all HIV infections globally and carries an estimated transmission risk of about 15% when breastfeeding is prolonged and continued into the second year of life.

Studies show that the highest risk of breast-milk HIV transmission occurs during the first 4 to 6 weeks of life, varying from 0.7% to 1% per week, with a lower but continued risk thereafter. In studies from sub-Saharan Africa, the risk of late postnatal HIV transmission after 4 to 6 weeks of age was 8.9 infections per 100 child-years of breastfeeding and relatively constant at approximately 0.7% per month of breastfeeding. Acute HIV infection in the mother during breastfeeding confers a higher risk of MTCT because of high levels of viral replication in maternal plasma and breast milk. In one report, the cumulative risk of breast-milk HIV transmission was 14% for chronic maternal infection compared with 25% to 30% for maternal infections acquired during late pregnancy or lactation.

Risk factors for breast-milk HIV transmission include women seroconverting during lactation, high HIV DNA or RNA level in maternal plasma and breast milk, low maternal CD4 ⁺ cell count, maternal symptomatic disease or AIDS, prolonged duration of breastfeeding, mixed infant feeding (breast milk plus formula milk) in the initial few months of life, oral lesions in the infant (e.g., candidiasis), abrupt weaning, and maternal breast problems (e.g., bleeding or cracked nipples, subclinical and clinical mastitis, and breast abscesses).

HIV has been isolated from cell-associated (DNA) and cell-free fractions of human breast milk from HIV-infected women and has been detected by culture or PCR in varying frequencies (39%-89%) in many studies. Breast-milk viral load levels are very high soon after birth. However, viral shedding may be intermittent, and variation in breast-milk viral load between left and right breasts during the first 3 months of lactation has been reported.

Mechanisms of Transmission

The mechanisms of MTCT are complex, requiring the virus to breach a mucosal barrier (placenta, the oral mucosa, or gastrointestinal [GI] tract). Mucosal transmission may be enhanced via the interaction of HIV with several C-type lectins, including dendritic cell–specific intercellular adhesion molecule-3–grabbing nonintegrin (DC-SIGN), expressed on dendritic cells located beneath the mucosal epithelium. DC-SIGN serves as an HIV-1 receptor, mediates virus adhesion to DCs, and recognizes pathogen-associated molecular patterns (PAMPs). DC-SIGN heightens HIV capture at mucosal entry sites, allowing its transport to CD4 T lymphocytes through transinfection. DC-SIGN is expressed on placental macrophages (Hofbauer cells), and genetic polymorphisms of this molecule have been associated with risk of MTCT.

In general, most infants escape infection despite an immature immune system and continued repetitive exposure to the virus (e.g., breastfeeding). Of the various routes of MTCT, transplacental HIV transmission is the least efficient. Some experts have hypothesized that fetal T-regulatory (Treg) cells may curb intrauterine transmission, given the ability of these cells to suppress inflammatory responses. Treg cells have been shown to suppress immune responses when exposed to maternal antigens in utero. Exposure to certain antigens (e.g., placental malarial infection) in the intrauterine period has been associated with increased Treg cells at birth. In addition, robust HIV-specific T-cell responses have been demonstrated in HIV-exposed, uninfected infants after depletion of Treg cells in vitro. Further, placental inflammation is associated with in utero. The gut of newborn infants has an abundance of memory CD4 ⁺ CCR5 ⁺ T cells, which are targets for HIV infection and facilitate MTCT. Likewise, fetal tissues and cells are more susceptible to HIV infection than adult cells. Thus other amniotic and/or placental factors may play a role in suppression of fetal gut inflammatory responses in utero.

Most intrapartum MTCT likely occurs via mucosal exposure. This is supported by studies demonstrating the protective effect of cesarean section, presence of HIV in infant gastric aspirates, and presence of CD4 ⁺ CCR5 ⁺ T cells in the neonatal gut. In addition, inflammatory cytokines and chemokines found in placentas of transmitting mothers are absent in placentas of mothers who transmit during intrapartum period. MTCT generally occurs in the presence of HIV-specific antibody transferred across the placenta during the third trimester of pregnancy. The role of maternal neutralizing and/or nonneutralizing antibodies in influencing MTCT risk is unclear, although in simian immunodeficiency models, broadly neutralizing antibodies can prevent infection and modify immune responses in infected neonatal macaques.

The mechanisms of breast-milk HIV transmission are poorly understood. A complex interplay of factors, including the nature and size of HIV reservoirs in breast milk, host susceptibility, various immune and nonimmune breast-milk factors, may be responsible for postnatal transmission during breastfeeding. Only a minority of breastfed infants born to HIV-infected mothers acquire postnatal infection by unknown mechanisms, despite prolonged and repeated daily exposure to HIV in breast milk. The reported probability of breast-milk transmission of HIV is extremely low, at 0.00064/L ingested and 0.00028/day of breastfeeding. Prolonged breastfeeding over a period of several months to years can result in postnatal transmission rates of greater than 15%.

The primary HIV reservoirs in human milk are RNA (as cell-free virus), DNA (as cell-associated virus integrated in latent T cells, and intracellular RNA (as cell-associated virus in activated T cells). HIV can gain entry into breast milk via passive transfer from the vascular compartment or through local replication in mammary gland tissues and breast milk. The mammary epithelium serves as an effective barrier to HIV entry. Compared with plasma, the concentration of cell-free HIV RNA in breast milk is approximately 2 logarithms lower. Cell-free, as well as cell-associated, viruses are associated with breast-milk HIV transmission. However, recent studies suggest that cell-associated virus (either latently infected or activated virus-producing T cells) in breast milk may be a stronger predictor for transmission of HIV to the infant than cell-free virus. cART administered during pregnancy or postpartum can suppress cell-free HIV RNA but not cell-associated HIV DNA load in breast milk. Another recent report found that cell-associated virus level (per mL) is more important for early postpartum HIV transmission (at 6 weeks) than cell-free virus.

Inflammatory breast conditions (e.g., mastitis or abscess) can result in increased mammary epithelial cell (MEC) permeability (rise in breast-milk sodium levels), viral shedding, and transmission. Subclinical mastitis (defined by elevated breast-milk sodium levels in asymptomatic women) can occur in mothers who do not breastfeed exclusively or during weaning. In such instances, infrequent breast emptying can lead to ductal inflammation and increased MEC permeability. Subclinical mastitis has been associated with increased breast-milk HIV loads and a higher risk of postnatal HIV transmission through breast milk. However, a study from Zimbabwe found that laboratory indicators of mastitis (e.g., sodium levels and cell counts) were not predictive of increased breast-milk HIV RNA levels.

The origin and evolution of HIV in breast milk is an area of ongoing research. HIV variants found in breast milk are genetically indistinguishable from those present in plasma. Studies indicate that viral evolution is limited in breast milk, reflecting restricted replication. A recent report demonstrated that breast milk may be initially seeded by an early HIV variant differing from the viral variants present in maternal plasma. Thereafter exchange of distinct viral sequences between plasma and breast milk results in multiple independent lineages of HIV without compartmentalization.

Although the virus enters through the infant’s mouth during breastfeeding, the precise portal of entry of HIV at various mucosal targets (e.g., oral mucosa, tonsils, GI tract) is unclear. HIV in breast milk may enter the submucosal layer of the infant gut because of breaks in the intestinal epithelial cell layer, which may occur from inflammation caused by nonexclusive breastfeeding or other coexisting pathogens. Low gastric acidity in neonates may facilitate entry of cell-free or cell-associated virus into intestinal epithelial cells. Mixed infant feeding may result in mucosal inflammation and immune activation in the infant GI tract, possibly facilitating HIV transfer across the gut lumen.

Although mucosal breaches facilitate viral entry, transmission of HIV may also occur through intact fetal oral and intestinal mucosal membranes. Transmission of HIV across the intestinal mucosal barrier is believed to occur after the virus attaches to immature dendritic cells of the infant gut, which then transport antigen to Peyer patches of the intestinal mucosa. Once the mucosal barrier is breached, HIV infects CD4 ⁺ T cells and disseminates to the draining lymph nodes and the lymphoid system.

Breast milk contains myriad factors with immunologic, antimicrobial, and antiinflammatory properties that may reduce the risk of transmission of HIV during breastfeeding among infants, despite repeated daily exposure to the virus. Some innate factors (e.g., mucin, polyanionic proteins, lysozyme, lactoferrin, bile salt–stimulated lipase, and secretory leucocyte protease inhibitor) in human milk exhibit strong HIV inhibitory activity in vitro. For example, bile salt–stimulated lipase can bind DC-SIGN and differentially inhibit viral capture and infection of CD4 T lymphocytes. Increased concentration of long-chain n-6 polyunsaturated fatty acids, human milk oligosaccharides, α-defensins, and erythropoietin in breast milk has been correlated with decreased risk of postnatal transmission of HIV during breastfeeding.

Breast milk also contains soluble/humoral and cellular factors that mediate innate and adaptive immune effects, including cytokines (interferon-γ [IFN-γ]), chemokines (regulated on activation, normal T-cell expressed and secreted [RANTES]), HIV-specific antibodies, CD8 ⁺ HIV-specific cytotoxic T lymphocytes, CD4 ⁺ HIV-specific helper T lymphocytes, natural killer (NK) cells, and macrophages. The source of HIV in breast milk is likely infected T cells or macrophages derived from gut-associated lymphoid tissue (GALT) via the enteromammary axis. CCR5-expressing memory CD4 T cells represent the majority of lymphocytes in breast milk. Compared with peripheral blood CD4 T ⁺ cells, breast-milk T lymphocytes exhibit primarily an effector memory phenotype (CD45RO ⁺ ), express more activation markers, display stronger expression of HIV co-receptors, and demonstrate mucosal homing receptors, identical to B cells located in GALT. The presence of breast-milk cellular immune responses against HIV has also been studied, but their significance in decreasing transmission is unclear. In recent studies, HIV–Gag-specific IFN-γ responses were detected among HIV-exposed breastfeeding infants and have been associated with early protection from MTCT.

Activated latently infected CD4 T ⁺ cells favor viral replication and release from the persistent stable HIV reservoirs in breast milk. Recent studies indicate that latently infected CD4 cells and spontaneously activated CD4 T ⁺ cells are not affected by maternal ART and may play an important role in residual breast-milk HIV transmission. Breast-milk cellular reservoirs and possible cell-to-cell HIV transfer from breast milk to the infant’s intestinal mucosae remain a concern regarding the feasibility of elimination of breastfeeding transmission of HIV. The biologic relevance of HIV-specific T cells in breast milk needs future research. Although breast milk contains maternal HIV-specific antibodies (secretory immunoglobulin A [IgA], secretory IgM, and IgG), their role in protection against postnatal HIV infection is uncertain. Likewise, the role of functionally effective neutralizing antibodies in preventing breast-milk HIV transmission is unknown and warrants future studies.

Viral Replication in Early Infant Infection

Human immunodeficiency virus strains transmitted from mother to child are typically minor maternal variants, predominantly macrophage-tropic and nonsyncytium-inducing R5 strains, that use CCR5 co-receptor to infect CD4 ⁺ T cells, although use of CXCR4 (X4) has been reported. Compared with adult cells, HIV replicates more readily in neonatal T lymphocytes and monocytes or macrophages, and this increased susceptibility is influenced by differential mRNA expression of several host genes associated with the virus life cycle. In vitro studies suggest that integration of HIV into highly conserved host genes in neonatal (cord blood) cells may contribute to increased gene expression and viral replication compared with adult cells. Analysis of many HIV structural, regulatory ( tat and rev ), and accessory ( vif, vpr, vpu, nef ) genes, has revealed conservation of functional domains of these genes during vertical transmission. In addition, the vif and vpr sequences of transmitting mothers were more heterogeneous and more functional than sequences of nontransmitting mothers.

Other HIV genes may also play a crucial role in virus transmission and pathogenesis. Functional domains in the HIV-1 functional LTR are conserved during vertical transmission, suggesting that a functional LTR sequence is crucial in viral gene expression, transmission, and pathogenesis. Other properties exhibited by vertically transmitted viruses include enhanced replication kinetics and fitness compared with nontransmitted viruses.

A recent study showed that fetal and neonatal intestine have a large subset of memory CD4 ⁺ CCR5 ⁺ T cells with predominantly a Th1 and Th17 phenotype, suggesting that the neonatal gut may be a primary target for HIV infection and replication, as observed in adults. A study in neonatal rhesus macaques demonstrated a rapid and profound loss of intestinal CD4 ⁺ T cells after acute SIV infection. Compared with adult macaques, intestinal CD4 ⁺ T-cell turnover and proliferation are markedly increased in infant macaques and may result in a more sustained pool of selective target cells for HIV replication. Increased viral replication may lead to a more rapid exhaustion of a finite precursor CD4 ⁺ T-cell pool, resulting in faster disease progression in infants.

Perinatal HIV infection is characterized by plasma RNA levels that rapidly reach very high levels, often exceeding 10 ⁵ to 10 ⁷ copies/mL during the first year of life, decreasing only slowly with age to a “setpoint” by approximately 5 years of age. In a study of 106 HIV-infected infants, the median plasma HIV RNA value at 1 month of age was 318,000 copies/mL, and it was common to see viral levels that exceeded 10 ⁶ copies/mL. In the absence of ART, HIV replication is very efficient, and plasma HIV RNA levels exceeding 1 million copies/mL are not unusual, with levels declining gradually over the first 24 to 36 months of life. A continued decrease in plasma HIV-1 RNA levels (mean, −0.2 to −0.3 logarithm decline/year) has been noted in vertically infected children through 5 to 6 years of age. Compared with adults, peak and setpoint viral load are usually greater than 1 logarithm greater in perinatal HIV infection. Viral loads are higher among infants who acquire HIV infection during the peripartum period, compared with those infected postnatally through breast milk. In contrast, the viral setpoint in many adults is attained within weeks of acute infection because of HIV-specific T-cell responses, remains relatively steady for several years during clinical latency, and is an important predictor for subsequent disease progression.

As in adults, higher infant viral loads correlate with a more rapid disease progression and high risk of mortality in the absence of ART, whereas lower levels of plasma HIV RNA with combination ART are associated with clinical benefit. These data indicate that high viral load is a critical determinant of pediatric disease progression and provide a strong argument for early and aggressive intervention with ART. In one study, significant age-related differences in HIV disease progression were noted in vertically infected children younger than 5 years. Younger children were at higher risk for disease progression or mortality compared with older children, irrespective of plasma viral load or CD4 lymphocyte count. In children older than 5 years, the risk of disease progression and mortality was similar to that noted in young adults; viral load and CD4 lymphocyte level were important predictors of HIV progression. In contrast, the clinical course of HIV infection in adolescents and adults is relatively slow, with progression to AIDS or death within 10 years among untreated patients.

The very high plasma HIV RNA levels in infants with HIV infection may be related to many factors, including the increased susceptibility of neonatal target cells to HIV-1 infection, a large and renewable CD4 T-cell pool size, presence of an active thymus, and delayed or ineffective HIV-specific immune responses.

Human Immunodeficiency Virus–Specific Immune Control

Vertically infected infants face many challenges in mounting a specific immune response to HIV. Perinatal HIV transmission occurs before the immune system is fully developed in an infant, allowing for more efficient viral replication and less efficient immunologic containment of the virus. The immunologic milieu in the neonate is tolerogenic, characterized by enhanced regulatory T-cell activity and increased interleukin 10 (IL-10) production. The adaptive immune response in early months of life is characterized by the preferential induction of Th17- and Th2-type immunity, which facilitates clearance of extracellular pathogens rather than intracellular pathogens (e.g., viruses). In addition, there is decreased production of type I IFNs and Th1 decrease in cytokines, such as IL-12 and IFN-γ .

In adults with acute HIV infection, HLA class I–restricted CD8 ⁺ (cytotoxic) T lymphocytes (CTL) play a crucial role in generation of HIV-specific immune responses to reduce viral load. CTL lyse HIV-infected cells through the recognition of virus-derived peptides (or “epitopes”) presented on the surface of the infected cells by HLA class I molecules. HLA-restricted CTL responses drive the evolution of HIV through selection of viral sequence polymorphisms (“immune escape mutations”).

Although HIV-specific CTL activity can be shown at a very early age, even in the fetus, the response is weak and less broad to optimally reduce viral load in infants. In addition, vertically transmitted maternal virus may be preadapted to the infant’s HLA alleles, affecting the infant’s ability to mount an early CTL response restricted by shared HLA alleles. HIV-specific CTL responses become more frequent and broad in infected infants with age. In one study, the Gag-specific CD8 T-cell response at 3 months of age correlated with clinical outcome and survival at 12 months. Another study found that robust functional CD8 T-cell responses are not detected until 3 years of age in vertically acquired children, primarily among those with adequate CD4 T-cell compartment. Polyfunctional HIV-specific CD8 T-cell responses are associated with slower disease progression in children as in adult HIV infection. Immune activation and the upregulation of programmed cell-death protein 1 (PD1), an inhibitory molecule produced by CD8 T cells, have been noted during the course of HIV infection in children. The narrow CTL responses in infants can result in viral escape mutants within the first year of life. In addition, persistent viral replication may lead to the selection of compensatory mutations that increase viral fitness. Certain HLA alleles (HLA-B ^∗ 27, HLA-B ^∗ 57, Cw-2, or DQB1-2) are associated with slower disease progression, indicating that CTL responses can have an important role in suppression of viral replication in pediatric HIV infection.

Compared with adults, HIV-specific CD4 T-cell responses are not detected initially in most vertically infected infants; subsequent responses remain narrow and low magnitude but increase with age. Studies in infant SIV-macaque model suggest that increased regulatory T-cell activity in early life may contribute to the suppression of viral specific CD4 T-cell responses. HIV-specific CD4 T-cell responses may also be important for T-cell–mediated viral control and enhance the capacity of HIV-specific CD8 T cells to suppress viral replication. In one study, the frequency of Gag-specific CD4 T cells in infected infants at 3 and 6 months of age correlated inversely with viral load. In another study, the magnitude of Gag-specific CD4 T-cell responses was associated with control of viral replication in ARV-naïve children.

The passive transfer of maternal, nonneutralizing antibodies could inhibit development of HIV-specific immune responses. Detection of antibodies mediating HIV-specific antibody-dependent, cell-mediated cytotoxicity (ADCC) is delayed in most vertically infected infants until after 1 year of age. Defective ADCC activity against HIV-infected target cells in neonates may also result in poor control of HIV. After the decline of passively transferred maternal antibody-dependent, cell-mediated cytotoxicity antibodies, the production of HIV-envelope cytotoxic antibodies is delayed in vertically infected infants. Although neutralizing antibodies can be generated during early infection, the precise role of neutralizing antibodies in limiting mother-to-child transmission of HIV is unclear and warrants further investigation.

The viral load in perinatally acquired HIV infection eventually decreases over time, although the precise mechanisms remain unknown. In the absence of ART, less than 10% of vertically infected children are well by 8 years of age, referred to as HIV long-term nonprogressors (LTNPs). There are fewer reports of LTNPs or “slow progressors” in children. In one study, lower levels of immune activation were noted among LTNP in the first weeks of perinatally acquired HIV infection. In adult HIV infection, immune activation results in CD4 ⁺ T-cell loss and is an important prognosis marker for disease progression than viral load. In contrast, CD8 T-cell activity may not be significant in the neonate given the tolerogenic regulatory T-cell environment and low-magnitude Th-cell responses. In pediatric slow progressors, the decline of HIV viremia eventually occurs because of increase in the breadth and magnitude of HIV-specific adaptive immune responses over time.

Immune Consequences in Human Immunodeficiency Virus Infection

Untreated HIV infection results in profound deficiencies in cell-mediated and humoral immunity caused by quantitative and qualitative defects. Progressive dysfunction of the immune system with selective depletion of CD4 ⁺ T cells occurs because of direct infection and immune activation. Increased immune activation can occur in HIV-infected children in association with decreased naïve T-cell numbers, increased apoptosis, and accelerated T-cell differentiation. In infants, thymic dysfunction associated with HIV infection may also have a significant impact on the developing immune system, leading to progressive depletion of thymic CD4 T-lymphocyte cells, dramatic decrease in cortical CD4/CD8 double-positive cells, and an increased percentage of CD8 cells.

Compared with adults, CD4 ⁺ T-cell depletion may be less striking in children because of their relative lymphocytosis. Flow cytometric analysis of lymphocyte subpopulations in healthy children revealed age-related changes in many of the different subgroups. Comparison of lymphocyte subsets in HIV-infected versus noninfected children younger than 2 years showed no difference for absolute CD8 ⁺ counts but clearly decreased levels of CD4 ⁺ cells. HIV-specific CD4 T-cell responses are crucial for B-cell antibody formation in addition to generation of HIV-specific CD8 T-cell responses to control viremia.

Abnormalities of the humoral immune system can precede or accompany abnormalities of the cellular immune system in HIV infection and consist of polyclonal B lymphocyte activation, resulting in hypergammaglobulinemia (especially IgG and IgA), abnormal primary or secondary antibody responses (T-lymphocyte–dependent and –independent antigens), decreased lymphocyte proliferation in response to an antigen; in addition, impaired function of other immune cells (monocytes/macrophages, NK lymphocytes, dendritic cells, and neutrophils) has been reported. Panhypogammaglobulinemia is noted in less than 10% of patients and is associated with poor prognosis.

Several studies indicate that HIV-infected children have reduced antibody responses to certain childhood vaccines (e.g., diphtheria, acellular pertussis vaccine). Reduced antibody responses after immunization and vaccine failures in HIV-infected infants may result from a poor primary immune response, failure to generate memory responses, or loss of memory cells. Most vertically infected infants who receive cART before 3 months of age develop antibody and lymphoproliferative responses to routine infant vaccines, although persistent HIV-specific immune responses are not detected. More recent studies have reported a sustained increase in peripheral blood CD5 cell counts and robust immune reconstitution among HIV-infected children after prolonged cART, permitting discontinuation of prophylactic therapy for opportunistic infections (OIs).

Human Immunodeficiency Virus Reservoirs

In individuals receiving suppressive ART, HIV can establish a state of latent infection in resting memory CD4 ⁺ T cells during development of immunologic memory responses. The size and distribution of the viral reservoir may be affected by chronic immune activation, inflammation, and immune dysfunction even in patients receiving potent ART. Although early initiation of ART in HIV-infected individuals can significantly reduce the size of the replication-competent resting CD4 ⁺ T-cell latent HIV reservoir, a stable pool of long-lived latently infected CD4 ⁺ T cells persists, representing a formidable challenge to HIV cure. The CD4 ⁺ T-cell latent HIV reservoir is established in vertically infected infants despite initiation of ART by 8 weeks of age. However, the size of the replication-competent resting CD4 ⁺ T cell in infants is associated with time to first undetectable viral load, suggesting that starting ART very early in infants may prevent establishment of a long-lived latent HIV reservoir. The recent finding of detectable viral load (relapse) after an earlier report of a “functional” cure in a perinatally infected infant who received cART within hours of birth is disappointing and warrants further studies.

Early Infant Diagnosis

Many advances have been made in the area of laboratory diagnosis of HIV infection. Routine HIV antibody testing (e.g., enzyme immunoassays, Western blot) cannot be used in infants for the diagnosis of HIV infection because of transplacental passage of maternal IgG antibodies to the virus that are present in infants up to 18 months of age. The diagnosis of HIV infection in infants warrants the use of PCR-based DNA or RNA assays (referred to as HIV nucleic acid amplification tests [NAATs]) that are highly sensitive and specific and now widely available in developed countries.

The HIV DNA PCR assay detects cell-associated proviral DNA and is the preferred test for early infant HIV diagnosis in the United States; approximately 30% to 40% of HIV-infected neonates will test positive by HIV DNA PCR assay by 48 hours of life, whereas 93% of infected infants will have a positive HIV DNA PCR assay result by 2 weeks of age. By 1 month of age, the sensitivity and specificity of HIV DNA PCR assays for detection of HIV subtype B are 90% to 100% and 95% to 100%, respectively. However, the HIV DNA PCR assay is less sensitive for identifying non–B subtype virus and has been associated with false-negative tests in patients with non–B subtype HIV infection.

HIV RNA PCR assay detects plasma viral RNA and can also be used for early infant diagnosis. The newer HIV RNA assay is as sensitive or more sensitive and as specific for detection of HIV subtype B compared with HIV DNA PCR assay. False-positive HIV RNA results can occur in HIV-exposed neonates, warranting the need to repeat any positive test for confirmation. In addition, a false-negative result can occur in neonates receiving ARV prophylaxis. In contrast, HIV DNA PCR assay can detect proviral DNA in peripheral blood mononuclear cells and will be positive in HIV-infected patients on ART with undetectable viral loads. The choice of NAATs also depends on the HIV subtype because of the genetic variability of HIV globally. HIV RNA PCR may be more sensitive than HIV DNA PCR test for detection of non–B subtype virus, such as subtype C, which is highly prevalent in sub-Saharan Africa. Therefore it is prudent to use HIV RNA PCR for diagnosis of infants born to women known or suspected to have non–B subtype HIV infection. In many studies, the sensitivity of HIV-1 RNA PCR is not affected by the presence of maternal or infant ZDV or nevirapine (NVP) prophylaxis.

HIV DNA or RNA PCR testing is recommended by the U.S. Public Health Service (PHS) at 14 to 21 days of age, and if test results are negative, repeat testing at 1 to 2 months of age and again at 4 to 6 months of age. Virologic testing is recommended at birth by some experts to diagnose in utero infection if mothers did not receive ART or prophylaxis during pregnancy or in other high-risk scenarios, but cord blood specimen should not be used because of possible contamination with maternal blood. It is assumed that children who have a positive HIV PCR result within the first 48 hours after birth were infected in utero, whereas those who are infected during the intrapartum period might become positive 2 to 6 weeks after birth. An infant is diagnosed with HIV infection if two separate blood samples test positive for HIV DNA or RNA PCR. If infection is confirmed, the infant should be promptly referred to a pediatric HIV specialist for consideration of cART and care to prevent rapid disease progression noted in some vertically infected infants.

HIV infection can be presumptively excluded in nonbreastfeeding HIV-exposed children younger than 18 months if (1) two negative HIV DNA or RNA PCR tests result from separate specimens, both of which were obtained at or after 2 weeks of age and one of which was obtained at or after 4 weeks of age; or (2) one negative HIV RNA or DNA PCR test results from a specimen obtained at or after 8 weeks of age; or (3) one negative HIV antibody test is obtained at or after 6 months of age; and no other laboratory (e.g., no subsequent positive PCR test results, if performed) or clinical (e.g., no AIDS-defining illness) evidence of HIV infection is obtained.

HIV infection can be definitively excluded in nonbreastfeeding HIV-exposed children younger than 18 months if (1) two negative HIV DNA or RNA PCR tests result from separate specimens, both of which were obtained at or after 1 month of age and one of which was obtained at or after 4 months of age; (2) two negative HIV antibody tests from separate specimens, both of which were obtained at or after 6 months of age; and no other laboratory (e.g., no subsequent positive PCR test results, if performed) or clinical (e.g., no AIDS-defining illness) evidence of HIV infection is obtained.

In children with two negative HIV DNA PCR test results, many physicians confirm the absence of HIV infection by documenting a negative HIV antibody test result at 12 to 18 months of age (“seroreversion”). A nonbreastfed infant is considered HIV negative if two antibody test samples drawn at least 1 month apart, and both obtained after 6 months of age, are negative. If HIV antibody testing is performed at 12 months of age in an HIV-exposed infant not known to be infected, and if the infant is still antibody positive, repeat testing at 18 months of age is recommended. Detection of HIV antibody in a child 18 months of age or older is diagnostic of HIV infection. Documentation of seroreversion may be more important when non–subtype B HIV is possible or present. In breastfed infants, definite exclusion of HIV infection is based on negative diagnostic test obtained at greater than 6 weeks after cessation of breastfeeding. Table 22-1 provides a diagram outlining the initial evaluation and clinical care of the HIV-exposed infant, as recommended by the American Academy of Pediatrics (AAP).

HIV isolation by culture is not recommended for routine diagnosis because culture is less sensitive, more expensive, needs a specialized laboratory, and results are not available for up to 28 days. Use of HIV-1 p24 antigen detection is not recommended for diagnosis of infant HIV-1 because of its poor sensitivity compared with HIV DNA PCR or culture.

Nonspecific laboratory parameters may also suggest HIV infection. Hypergammaglobulinemia, a nonspecific but early finding of HIV infection, is noted in up to 90% of perinatally infected infants by 6 months of age. CD4 counts must be interpreted within the bounds of the age-dependent normal range, and changes in counts may result in a decrease in the normal CD4 to CD8 T-lymphocyte count ratio of greater than or equal to 1.0 in vertically infected infants by 1 to 2 months of age, compared with HIV-exposed, uninfected infants.

Infectious Complications

Infections in HIV-infected infants not receiving combination ART can be serious or life threatening. The difficulty in treating these infectious episodes, their chronicity, and their tendency to recur distinguish them from the normal infections of early infancy. It is helpful to document each episode and to evaluate the course and frequency of their recurrences. In the pre-HAART era, the frequency of OIs varied by age, immune status, prior history of OI, and pathogen. With early infant HIV diagnosis and linkage to care and an ART program, the frequency of historically reported AIDS-defining illness and OIs has dramatically decreased among children living with HIV in the United States and other resource-rich countries. During the pre-ART era, the most common OIs (event rate, > 1.0/100 child-years) reported among U.S. children with vertically acquired infection included serious bacterial infections (e.g., pneumonia, bacteremia), PCP, disseminated MAC, herpes zoster, esophageal and tracheobronchial candidiasis, and CMV disease. Other rare OIs (event rate, <1.0/100 child-years) included CMV disease, cryptosporidiosis, tuberculosis (TB), systemic fungal infections, and toxoplasmosis. In the post-ART era, the rate of OI decreased from 12.5 to 0.8 cases per 100,000 person-years pre-ART and post-ART, respectively. Despite these impressive declines, HIV-infected children continue to experience the same types of OIs, even in the era of combination ART, highlighting the need for early recognition, treatment, and prevention.