Cohort characteristics and HLA allele frequencies
The study included 433 infants who participated in a perinatal HIV-1 transmission study between July 1999 and December 2002 (). Maternal CD4 T cell counts and maternal HIV-1 viral load results were available at 32 weeks of pregnancy for 433 and 332 mother–infant pairs, respectively. The mean maternal CD4 count was 474 cells/μl (range: 6–1628 cells/μl) and the mean maternal HIV-1 viral load was 4.6 log10 copies/ml (standard deviation 0.9 log10 copies/ml). At delivery, mean maternal viral load was lower than at 32 weeks of gestation (4.2 vs 4.6 log10 copies/ml) due to administration of perinatal zidovudine. Delivery HIV-1 viral load data, only available for a subset of 322 mother–infant pairs, were highly correlated with viral loads at 32 weeks of pregnancy (correlation coefficient = 0.68), indicating that the 32-week maternal viral load is a good marker for overall HIV-1 RNA levels in this cohort.
Among the 433 infants in the cohort, 322 (74%) were breast-fed at birth and 298 (73%) breast-fed after 1 month of age with a median duration of breast-feeding of 9 months (range: 1–12 months). Seventy-six (18%) infants were HIV-1 infected during 12 months of follow-up: 29 (38%) of the 76 were HIV-1 infected at birth, 36 (47%) between birth and the first month of life, and 11 (14%) between 1 and 12 months of age. The cumulative probability of HIV-1 infection was 7% at birth, 15% at 1 month, and 18% at 12 months. Infant mortality was 11% during follow-up with 49 deaths occurring among the 433 infants enrolled in the study, 26 (53%) of whom were HIV-1 infected and 23 (47%) of whom were HIV-1 uninfected at the last time of testing. Excluding infants who died, 257 (67%) infants completed 12 months of follow-up and 31 (7%) infants were lost to follow-up. Infants who were HIV-1 infected were not more likely to be lost to follow-up than infants who were HIV-1 uninfected (8% of 76 infected infants versus 7% of 357 uninfected infants; p = 0.8).
The prevalence of each of the 11 class I alleles evaluated in this cohort is depicted graphically in . The most frequently expressed alleles were A*02 (34%), A*30 (27%), Cw*06 (30%), and Cw*07 (33%). An additional 51 alleles were typed in the cohort with 26 (51%) among these expressed by fewer than 5% of the 433 infants.
Infant HLAs associated with mother-to-child HIV-1 transmission
Early HIV-1 acquisition was defined as occurring at or before month 1 and late acquisition defined as occurring after month 1. While the 65 early infections may have occurred in utero, intrapartum, or through exposure to colostrum or breast milk, the 11 late infections resulted exclusively from exposure to HIV-1 in breast milk across oropharyngeal and gastrointestinal mucosa.
The B*18 haplotype was protective against early HIV-1 acquisition in this cohort. After adjusting for maternal CD4 T cell count and HIV-1 viral load at 32 weeks of pregnancy, infants expressing HLA B*18 were 74% less likely to become infected than infants not expressing this allele (RR = 0.26; 95% CI = 0.04–0.82) (). Two (5%) infants were HIV-1 infected at age 1 month among the 43 infants with B* 18 compared to 63 (16%) of 390 infants not expressing this haplotype. The overall test for the B locus was significant when HLA B*18 and HLA B*53 were included in the analysis (p = 0.02), but lacked power when all 28 B alleles were incorporated into the model (p = 0.72). These analyses were repeated adjusting for maternal HIV-1 viral load at the time of delivery, rather than at 32 weeks of pregnancy. In this analysis, the association between B*18 and protection persisted, the strength of the association was unchanged (RR = 0.28; 95% CI = 0.05–0.89), and the overall p value for the B locus remained significant (p = 0.03).
Risk of Infant HIV-1 Infection by 1 Month of Age
While the overall test for the A locus was not statistically significant (p = 0.27), an association between early infection risk and HLA A*29 expression was observed in the adjusted analysis including both maternal CD4 T cell count and HIV-1 viral load at 32 weeks gestation (RR = 2.0; 95% CI = 1.0–3.8) (). Infants with A*29 had a 2-fold increased risk of infection before month 1, with 13 (25%) of the 52 infants expressing HLA A*29 becoming infected by month 1 compared to 52 (14%) of the 381 infants without this allele.
A trend toward protection was observed for HLA A*02 in an unadjusted analysis that did not control for maternal CD4 count and HIV-1 viral load. Infants expressing HLA A*02 were approximately 50% less likely to become HIV-1 infected before month 1 (RR = 0.55; 95% CI 0.3–0.9), an observation that is consistent with previous reports that also did not adjust for maternal viral load.7
We did not observe significant protection against HIV-1 acquisition for A*02 in the analysis that adjusted for the effect of maternal immunosuppression and viral load on vertical transmission risk (). Future studies focusing on HLAs A*02 and A*29 may clarify their contribution to mother-to-child HIV-1 transmission and disease progression.
In our analysis of late transmission events, risk of HIV-1 acquisition via breast milk after month 1 was not associated with the A, B, or C locus (). This may be because our evaluation of late HIV-1 infection was limited by the small number of infants (n = 11) who became HIV-1 infected between 1 and 12 months of age. Thus, our analysis may have lacked power to detect a significant difference in late transmission risk. However, we did observe that no infants expressing the B*18 allele acquired HIV-1 after month 1, suggesting that HLA B*18 may in fact protect against late breast milk infection (). We also observed that both Cw*07 and Cw*08 were associated with late HIV-1 acquisition (RR = 4.0; 95% CI = 1.0–18.0 and RR = 7.2; 95% CI 1.2–37.3, respectively). High relative risk estimates and wide confidence intervals for these two alleles reflect the fact that few infants were infected after month 1 and may not represent the true effect of these alleles on HIV-1 acquisition risk ().
Risk of Infant HIV-1 Infection After Month 1 for Breast-fed Infants