The most reported immunological abnormality of HEU infants pertains to the frequency of immune cell subsets. Cluster of differentiation (CD) 4 T-cells have been relatively well studied in HEU infants, owing to both the vulnerability of CD4 T-cells to HIV infection and their important role as regulators of the immune system and acquired immunity. Lower CD4 T-cell counts (28, 31–37) and to a lesser extent lower CD8 T-cell counts (32, 33) have been reported in multiple studies contrasting HEU infants to HU peers. Maternal HIV viral load has been proposed as a correlate for subsequent HEU T-cells counts. At 2 and 6 months of age, HEU infants born to mothers with viral load >1000 copies/ml had lower CD4 T-cell counts compared to HEU infants born to mothers with viral load <50 copies/ml at the time of delivery (35). Decreased counts of circulating CD4 T-cells may limit antigenic coverage and subsequent response, eventually cumulating in increased severity of infections.

However, differences between HEU vs. HU T-cell counts may be more nuanced. It has been proposed that the difference in the quantity of circulating T-cells detected in HEU infants is in part due to difference in frequencies of subsets of CD4 T-cells. HEU newborns had lower CD4 to CD8 T-cell ratio, lower CD4 naïve, and CD8 naïve T-cell percentages, increased percentage of activated (CD8+ CD38^bright) CD8 T-cells and memory (CD4+ CD45RO+) T-cells, augmented double-negative (CD3+ CD4− CD8−) and immature (CD4− CD8− CD5− CD44+) T-cells when compared to HU newborns (34). The elevated number of immature (CD4− CD8− CD5− CD44+) T-cells suggests that the physiologic thymic maturation pathway may be impaired in HEU children (34). In another study, activated (CD4+ HLA− DR+ CD38+) and memory (CD4+ CD45RA− RO+) T-cells were higher among HEU infants compared to infants born to HIV-uninfected women (38). The increase in the percentage of activated and memory T-cells and the diminished numbers of naïve T-cells suggests that this subset of lymphocytes had experienced antigenic exposure that could have resulted from either in utero exposure of the fetus to antigens or to non-antigen specific cell differentiation.

The altered numbers and subtypes of T-cells in HEU infants may or may not result in impaired T-cell immune function (21).

Several studies have shown that HEU infants express altered CMI function and display impaired T-cell maturation. The CMI response of HEU infants has been studied primarily by measuring cytokine production and cell proliferation to both HIV-specific and non-HIV-specific antigens with reports of hyper- or hypo-activity depending on the stimulant (Table 1). It is proposed that increased CMI response in HEU infants may result from in utero exposure to HIV antigens and/or other antigens related to other infections in the HIV-infected mothers. By contrast, attenuation of the CMI response might be related to limited CD4 T-cells repertoire and progenitor cell dysfunction that might lead to T-cells with compromised function.

In summary, the most consistent findings are that HEU infants have lower CD4 T-cell, lower CD8 T-cell counts, increased activated and memory T-cells as compared to infants born to HIV-uninfected women and there is compelling evidence that HEU infants have HIV-specific cellular memory response. HIV and associated antigenic exposure can take place during gestation. Data support that HIV can cross the placenta during gestation as HIV has been detected in fetal tissues (60). HIV exposure appears to be able to activate HIV-specific memory responses (45–48, 61). The HIV-specific T-cell immune response, at least in part, is suppressed by Treg cells (61). Depletion of Treg cells increased HIV-specific T-cell immune responses among HEU infants (61).

It should be noted that there is controversy regarding the T-cell response to non-HIV-specific stimulation. Mixed hyper- and/or hypo-responsiveness to T-cell activation and the different pro-inflammatory cytokines profile may stem from different experimental conditions, variable population characteristics, maternal HIV viral load, and ARV exposure.

If the HIV-infected in utero milieu leads to priming of the HEU immune system, it follows that this takes place in a complex manner. This may be a simplified explanation for the complex reality that leads to diversity in HEU hyper- vs. hypo-responsiveness that may be deemed immune dysregulation. It further stands to reason that there will be population variation in such immune dysregulation, which may explain both discordant results and how many HEU infants go on to lead relatively healthy lives compared to some of their peers. Ultimately, it is suspected that for some stimuli the afflicted HEU infant will be hyper-responsive, while being normal or hypo-responsive to other stimuli.

Data on the immunogenicity and seroprotection provided to HEU infants following primary immunization with hepatitis B vaccine (HBV) suggest that the majority of HEU infants produce adequate humoral immune response.

In South Africa, prevaccination (at 2 and 6 weeks) hepatitis B surface antigen (HBsAg) IgG levels, presumed maternally derived, were significantly higher among HEU than HU infants, with one-quarter of HEU infants having sero-protective levels (HBsAg IgG > 10 mIU/ml) (63). Following primary vaccination according to the EPI, HBsAg IgG levels at 3 months (after two HBV doses given at 6 and 10 weeks of age) were similar between HEU and HU infants and continued to be equivalent through the first 2 years of life (63). A subsequent study of South African HEU infants reported that >99% of HEU infants developed sero-protective antibody levels (HBsAg IgG >10 mIU/ml) 1 month after completing the primary HBV series (64). This small South African series would suggest that HEU infants do indeed produce adequate acquired immunity in response to HBV vaccination.

The prevalence of non-responders to primary series of HBV was also described. Among a cohort of Brazilian HEU infants vaccinated with HBV (at birth, 1 and 6 months), 6.7% of HEU infants were non-responders to HBV (HBsAg IgG <10 mIU/ml) 1 month after the third dose of HBV (65). This study supports earlier findings which reported that about 8% of HEU infants did not respond to HBV (66) and more recent data from India which found that of HEU infants vaccinated with three doses of recombinant HBV (at 6, 10, and 14 weeks), 11.5% were non-responders 3 months after the third dose of the vaccine (67). The reported prevalence of non-responders to HBV among HEU infants, 6.7–11.5% (65–67) is comparable to the 5–10% of non-responders reported among healthy population (73).

In sum, though timing of vaccination and population characteristics are variable, it does appear that the majority of HEU infants are capable of mounting a sufficient immune response for HBV vaccination to confer adequate seroprotection. What remains to be determined is how resilient these responses are and whether they wane over time. There may also be a sub-population within HEU infants that are at higher risk and do not mount adequate immune responses to HBV vaccination.

Studies that evaluated the immunogenicity and seroprotection following tetanus toxoid (TT) vaccination reports that HEU infants develop robust immune response. One month following the completion of the vaccination series with diphtheria-tetanus toxoid-cellular pertussis-Haemophilus influenzae type b (DTP-Hib) (given at 2, 4, and 6 months) in Brazil, all HEU infants showed sero-protective immune response to TT (anti-TT IgG >0.1 IU/ml) (65). And following the completion of 3 DTP vaccines doses (at 6, 10, and 14 weeks) in South Africa, anti-TT IgG levels were similar between HU and HEU infants, and the majority of HEU developed sero-protective levels to TT (63, 64, 68). Lowest prevaccination anti-TT IgG levels were associated with greatest vaccine-induced immune responses at 16 weeks (68).

Data also support that HEU infants mount a quantitative immune response to diphtheria toxoid (DT) vaccine that is comparable to HU infants. Specifically, Brazilian HEU infants vaccinated with DTP-Hib (at 2, 4, and 6 months) has been shown to have similar anti-DT IgG levels compared to infants not exposed to HIV, 1 month after the 3rd dose of the vaccine (65). Moreover, the percentage of non-responders to DT vaccine (anti-DT IgG <0.1 IU/ml) was not significantly different between the groups, with 94.7% of HEU subjects and 98.2% of HU infants, having protective levels against DT after completion of primary series of three DTP doses (65). These findings were supported by a study in South Africa that found antibody levels against DT to be similar between HEU and HU, 1 month following the completion of primary vaccination series with diphtheria-toxoid, tetanus-toxoid, whole cell pertussis-HibCV (DTwP-HibC) according to the EPI (at 6, 10, and 14 weeks) (64). In addition, a high proportion (>99%) of HEU children developed sero-protective levels to DT vaccine (anti-DT IgG ≥ to 0.01 and 0.1 IU/ml) (64).

Though limited to only two studies on the subject, it appears as though HEU infants are not significantly different from peers with respect to response to DT vaccine.

Evaluation of the immune response to 7-valent pneumococcal conjugate vaccine (PCV) showed higher pneumococcal-specific antibody levels following the second (68) and third 7-valent PCV-doses (69) in HEU infants as compared to HU controls. Specifically, Jones et al. found that among a cohort of South African HEU infants vaccinated with two doses of 7-valent PCV (at 6 and 14 weeks), HEU infants had higher levels of pneumococcal-specific antibody levels than HU at 16 weeks of age (68). Moreover, infants with the lowest prevaccination pneumococcal-specific antibody levels had the most robust immune responses to the pneumococcal vaccine at 16 weeks (68). However, the clinical significance of these robust results is complicated by lack of well-established correlate of protection for collective antibody response to pneumococcal vaccine (68).

The largest study to date to report the immune response following primary PCV immunization in HEU infants described a cohort of HEU infants in South Africa (69). Prevaccination (age <6 weeks), HEU infants had lower titers and lower proportion of children with serotype-specific anti-capsular antibody ≥0.35 μg/ml to all serotypes (4, 6B, 9V, 14, 18c, 19F, and 23 F) compared to HU infants (69). At 10 weeks, antibody levels following the first 7-valent PCV-dose (given at age 6 weeks) were lower for four serotypes (6B, 14, 19F, and 23F) and the proportion of children who had serotype-specific anti-capsular antibody ≥0.35 μg/ml were lower for most serotypes (6B, 9V, 14, 19F, and 23F) in HEU as compared to HU infants (69). Following the second 7-valent PCV-dose (at 10 weeks), the antibody levels measured at 14 weeks of age were similar in HEU and HU infants for the majority of serotypes (6B, 9V, 14, 18c, 19F, and 23 F) (69). Three to six weeks following the third 7-valent PCV-dose (given at 14 weeks), the serotype-specific anti-capsular antibody levels were higher among HEU infants as compared to HU infants for all serotypes (69). When integrated, these data suggest that a two-dose primary series schedule in HEU children may afford similar protection against vaccine serotype-specific invasive pneumococcal disease as compared to HU children. However, it is noteworthy that the functionality of anti-pneumococcal antibodies as a correlate of protection is not well founded, and when assessed for opsonophagocytic activity there was an absence of consistency in the pattern between HEU and HU infants (69).

Current data on the immunogenicity of Haemophilus influenzae type b (Hib) vaccine administered to HEU infants is reassuring as HEU infants vaccinated according to the EPI mounted robust and immune responses that were similar to HU infants following vaccination with Hib vaccine.

Within 1 month following the completion of primary immunization (6, 10, and 14 weeks), South African HEU infants had similar anti-Hib antibody levels as compared with HU at the age of 16 weeks (64, 68). Gaensbauer et al. reported findings from a cohort of Ugandan HEU infants vaccinated with Hib polysaccharide conjugate vaccine at 6, 10, and 14 weeks of age. By 12 weeks of age (following two Hib vaccine doses), 91% of HEU had protective levels (anti-Hib IgG >1.0 μg/ml), as did all infants by 24 weeks after three doses. Interestingly, maternally derived anti-Hib IgG levels at birth did not correlate with anti-Hib IgG levels at 12 or 24 weeks. At 48 weeks, the vast majority (98.3%) of HEU still retained protective levels (70).

The duration of HEU infant immune response to primary Hib vaccination has been derived from longitudinal studies. Data from a South African cohort showed that although prevaccination anti-Hib IgG levels (at 2 and 6 weeks) were lower among HEU compared to HU infants, antibody levels were similar between both groups at 3 months (after two doses given at 6 and 10 weeks) and until 2 years of age (63).

Thus, current data suggest that HEU infants mount immune response to Hib vaccine similar to HU infants that lasted until 2 years of age.

Several studies have demonstrated that HEU infants mount higher immune response to primary vaccination with whole cell pertussis vaccine (given according to the EPI) compared to HU infants (63, 64, 68). Jones et al. reported data from a South African cohort and found that despite having lower anti-pertussis titers at birth, HEU infants had higher anti-pertussis antibody levels compared to HU 2 weeks following the completion of primary vaccination with thee doses of DTP-Hib vaccine (at 6, 10, and 14 weeks) (68). Infants with the lowest prevaccination anti-pertussis levels exhibited the greatest vaccine responses at 16 weeks (68). Reikie et al. has found that at 3 months, anti-pertussis IgG levels were similar between South African cohort of HEU and HU infants after two doses of DTP vaccine (at 6, 10 weeks), and higher for HEU as compared to HU at the age of 6 and 12 months (63). Moreover, Simani et al. reported that 1 month following the primary DTP-HibC vaccine series (at 6, 10, and 14 weeks) the anti-pertussis-toxoid titers were higher in HEU than in HU infants (64). It should be noted that the aforementioned studies used whole cell pertussis vaccine as compared to acellular pertussis vaccine that is currently employed by most developed countries. Moreover, the read out measure is critical as measuring collective antibody response to multiple antigens (63, 68) might be less accurate than antigen-specific responses. The lack of well-established sero-protective antibody levels is another challenge for extrapolating these finding to clinical settings. Overall, though more data are required on acellular pertussis vaccination among HEU infants, it does appear as though HEU infants mount satisfactory responses to whole cell pertussis vaccination.

Measles vaccine generated both comparable (63) and lower immune response (71) among HEU infants as compared to HU infants.

The proportion of South African HEU infants with sero-protective titers against measles was similar compared with HU infants, 28 weeks following the first measles vaccine dose (administered at nearly 40 weeks of age) and 2 weeks after the booster dose (administered at nearly 68 weeks of age) (71). However, waning immunity in HEU was noticed 9.5 months post booster, when HU had higher anti-measles titers and were more likely to have seroprotective titers (94.3%) compared with HEU (79.6%) infants, bringing the durability of HEU immune response into question (71). In contrast to the previous findings, the average levels of anti-measles virus antibodies were nearly indistinguishable between South African HEU and HU infants prevaccination (at 2 and 6 weeks), at 12 months (post 1^st vaccine dose given at age of 9 months) and at 2 years (post 2^nd vaccine dose given at 18 months) (63).

The long-term vaccine-induced immune memory against measles, rubella, and mumps was recently explored (72). A large US cohort of HEU children was evaluated at the age of 7–15 years, among which 87% of HEU infants had received two doses of measles, mumps, and rubella vaccine. The prevalence of measles seroprotection, rubella seroprotection, and mumps seropositivity were 99, 98, and 97%, respectively (72). Thus, although limited to one study, it appears that HEU infants retain long-term immunity to the measles, mumps, and rubella vaccine.