In this cohort of breast-feeding women and their infants, mother-child HLA concordance was associated with increased risks of overall, in utero, and breast milk HIV-1 transmission. Additionally, maternal homozygosity was associated with the risk of transmission overall and via breast-feeding. The effects of HLA concordance on transmission appeared to derive predominantly from the HLA-B locus, but this is difficult to confirm with our data.
Increased susceptibility to vertical HIV-1 transmission due to mother-child HLA concordance has several possible biological mechanisms. Infants whose HLA is the same as their mothers may be less able to recognize HIV-1 that has evolved to evade maternal immune responses via HLA-mediated selection. Concordance might also decrease the likelihood of infant alloimmune responses against maternally derived lymphocytes. Among mother-infant pairs with more concordance, HLA molecules on the surface of HIV-1—infected or -uninfected maternal cells will be recognized as “self” by cytotoxic T lymphocytes or NK cells and will be less likely to be destroyed. Infant alloimmune responses may also target maternal HLA molecules incorporated into the HIV-1 envelope when virus buds from host cells [14
]. Thus, increased concordance could restrict T cell destruction of cell-free HIV-1, in addition to reducing destruction of HIV-1—infected maternal cells.
Our observation that mother-child HLA concordance was associated with increased HIV-1 transmission risk is consistent with previous findings and adds to them by demonstrating that concordance is also associated with a significantly greater risk of breast milk HIV-1 transmission. A study of 125 mother-child pairs in Kenya without antiretroviral prophylaxis found that each additional concordant class I allele was associated with a 2.6-fold increased risk of perinatal infection [6
]. A United States— based study in which mothers and infants were given zidovudine found a 4-fold increased transmission risk before 6 weeks of age among pairs who were completely concordant at 1 or more class I loci compared with pairs who shared only 3 alleles [7
]. Neither of these studies found an association between HLA concordance and HIV-1 transmission via breast-feeding.
One strength of the present study is that the sampling schedule permitted more precise estimation of the timing of infection. Thus, infections could be classified as having been in utero, peripartum, or breast milk. The only other study of whether HLA concordance influences breast milk transmission classified infections occurring before 6 months of age as early and infections occurring after 6 months as being transmitted through breast-feeding. In our sample, nearly 50% of infections occurring through breast-feeding happened before the age of 6 months. These would have been classified as early infections by the criteria of MacDonald et al. and may have increased the likelihood of observing an association between concordance and early transmission rather than breast-feeding [6
]. Another difference that may have increased our power to detect an association is that both of the previous studies used serologic HLA typing, whereas we used molecular-based typing, which is more specific. Serologically defined HLA alleles are broad, whereas many alleles that can be identified only by molecular techniques have distinct epitope-binding motifs.
In the present study, mothers with HLA homozygosity had higher plasma HIV-1 RNA loads, and any maternal homozygosity was associated with an increased risk of HIV-1 transmission overall and via breast-feeding. We are not aware of any studies that have specifically focused on HLA homozygosity and HIV-1 transmission. Previous studies have examined HLA homozygosity as it relates to HIV-1 disease progression [4
]. Carrington et al. [4
] found that homozygosity at any HLA class I locus was associated with more rapid disease progression. One mechanism through which maternal homozygosity might increase vertical transmission risk is accelerated HIV-1 progression and higher viral loads in homozygous mothers. However, we observed a strong association between maternal HLA homozygosity and HIV-1 transmission overall and through breast-feeding, even after adjusting for maternal HIV-1 load, which suggests that the effect of maternal HLA homozygosity on transmission risk is not due only to its effect on maternal disease progression. Because this association is independent of viral load antenatally, another mechanism for the influence of maternal HIV-1 homozygosity on vertical transmission risk must be considered. It is possible that HIV-1—infected cells of homozygous mothers elicit a weaker alloimmune response in their infants, which in turn may lead to increased survival of maternal HIV-1—infected cells and increased risk of transmission.
In addition to associations between concordance at the HLA-A, -B, and -C loci together, there were associations with overall and in utero transmission for the HLA-B locus individually and with overall transmission after adjustment for A and C locus concordance. These results suggest that associations observed when concordance was classified according to the class I loci considered together may have been driven by the B locus. The importance of HLA-B alleles in HIV-1 transmission and disease progression has been demonstrated in past studies. First, a study of HIV-1—discordant couples in Zambia found that increased concordance at the HLA-B locus was associated with a 2-fold increased transmission risk [15
]. In another study, Kiepiela et al. demonstrated a dominant influence of HLA-B alleles on CD8+
T cell responses against HIV-1 [16
Our observation that HLA concordance and maternal homozygosity were associated with in utero and breast milk transmission but not with peripartum transmission might be explained by the roles played by cell-associated and cell-free virus in transmitting HIV-1 from mother to child. It is not well established whether cell-free or cell-associated virus is more influential in vertical HIV-1 transmission. However, in vitro data suggest that in utero transmission is caused more by cell-associated virus than by cell-free virus [17
]. Additionally, cell-associated virus may be more readily transmitted through breast-feeding than cell-free virus, whereas intrapartum transmission may be caused by both types of virus [20
]. Thus, HLA concordance and homozygosity might have stronger associations with increased risks of in utero and breast milk transmission to the extent that alloimmune responses act primarily on cell-associated virus. The role played by HLA concordance in increasing vertical transmission of cell-associated virus via breast-feeding is further supported by findings that HLA concordance increased the risk of vertical transmission of human T cell lymphotrophic virus type 1, which is transmitted only via cell-to-cell interaction [22
]. This adds to a growing body of evidence indicating that cell-associated HIV-1 is critically important for in utero and breast milk transmission [17
There were several limitations to our study. First, because maternal and infant HLA typing was conducted in different laboratories, there is a possibility of measurement error when concordance scores are computed. Measurement error of this kind would most likely decrease the indicated number of shared alleles, biasing our results toward the null. Hence, the true effect of concordance on transmission could be even greater than the effects observed here. Second, because a limited number of transmission events occurred through breast-feeding, these analyses could have been underpowered. Third, our ability to determine the timing of infections, although very good compared with earlier studies, was nevertheless less than perfect. Late in utero infections and early infections via breast-feeding may have been misclassified as intrapartum infections because of the inability to differentiate the specific route of transmission in these cases.
In conclusion, mother-child HLA concordance and maternal HLA homozygosity increased the risk of HIV-1 infection in infants for overall, in utero, and breast-feeding transmission. When HLA-B concordance was considered individually, there is evidence that it increased the risk of in utero and overall transmission. These data are consistent with evidence in the literature indicating that HLA-mediated immune responses are important drivers of HIV-1 selection and HLA diversity, and they support the hypothesis that the HLA-B locus is particularly important. Our results also suggest a role for alloimmune responses in vertical transmission of HIV-1, which is intriguing because similar studies have indicated that concordance and alloimmune responses may also alter the risk of heterosexual HIV-1 transmission [14
]. Further study of the role played by HLA class I and II and cellular alloimmune responses in both vertical and heterosexual HIV-1 transmission is warranted.