Association of HIV-1 Envelope-Specific Breast Milk IgA Responses with Reduced Risk of Postnatal Mother-to-Child Transmission of HIV-1

Abstract

Infants born to HIV-1-infected mothers in resource-limited areas where replacement feeding is unsafe and impractical are repeatedly exposed to HIV-1 throughout breastfeeding. Despite this, the majority of infants do not contract HIV-1 postnatally, even in the absence of maternal antiretroviral therapy. This suggests that immune factors in breast milk of HIV-1-infected mothers help to limit vertical transmission. We compared the HIV-1 envelope-specific breast milk and plasma antibody responses of clade C HIV-1-infected postnatally transmitting and nontransmitting mothers in the control arm of the Malawi-based Breastfeeding Antiretrovirals and Nutrition Study using multivariable logistic regression modeling. We found no association between milk or plasma neutralization activity, antibody-dependent cell-mediated cytotoxicity, or HIV-1 envelope-specific IgG responses and postnatal transmission risk. While the envelope-specific breast milk and plasma IgA responses also did not reach significance in predicting postnatal transmission risk in the primary model after correction for multiple comparisons, subsequent exploratory analysis using two distinct assay methodologies demonstrated that the magnitudes of breast milk total and secretory IgA responses against a consensus HIV-1 envelope gp140 (B.con env03) were associated with reduced postnatal transmission risk. These results suggest a protective role for mucosal HIV-1 envelope-specific IgA responses in the context of postnatal virus transmission. This finding supports further investigations into the mechanisms by which mucosal IgA reduces risk of HIV-1 transmission via breast milk and into immune interventions aimed at enhancing this response.

Importance: Infants born to HIV-1-infected mothers are repeatedly exposed to the virus in breast milk. Remarkably, the transmission rate is low, suggesting that immune factors in the breast milk of HIV-1-infected mothers help to limit transmission. We compared the antibody responses in plasma and breast milk of HIV-1-transmitting and -nontransmitting mothers to identify responses that correlated with reduced risk of postnatal HIV-1 transmission. We found that neither plasma nor breast milk IgG antibody responses were associated with risk of HIV-1 transmission. In contrast, the magnitudes of the breast milk IgA and secretory IgA responses against HIV-1 envelope proteins were associated with reduced risk of postnatal HIV-1 transmission. The results of this study support further investigations of the mechanisms by which mucosal IgA may reduce the risk of HIV-1 transmission via breastfeeding and the development of strategies to enhance milk envelope-specific IgA responses to reduce mother-to-child HIV transmission and promote an HIV-free generation.

FIG 1

Magnitude of breast milk neutralization in uninfected and postnatal transmitting and nontransmitting mothers. The breast milk neutralization 50% inhibitory dose (ID₅₀) was measured by TZM-bl pseudovirus assay against tier 1 C.MW965 (A), tier 1 B.MN.3 (B), and tier 2 C.1209BMH5 (C). Samples from nine uninfected controls were included to represent nonspecific background activity. Tier 1 C.MW965 neutralization responses among transmitting and nontransmitting women were significantly above background levels observed in uninfected control women (Wilcoxon test). Each dot represents milk from one donor, and medians are indicated with black horizontal lines.

FIG 2

Antibody-dependent cell-mediated cytotoxicity (ADCC) activity and ADCC antibody (Ab) titer against gp120-coated and HIV-1-infected target cells in breast milk from uninfected and postnatal transmitting and nontransmitting mothers. (A and B) Maximum observed ADCC activity measured as percent granzyme B (GzB)-positive target cells (A), and ADCC Ab titers of breast milk samples against 4403BMC5 gp120-coated target cells in the ADCC GranToxiLux (ADCC-GTL) assay (B). (C and D) Breast milk maximum percent specific ADCC killing activity (C), and ADCC Ab titers against 4403BMC5 HIV-1-infected target cells in the ADCC-Luc assay (D). Samples from nine uninfected controls were included to represent nonspecific background activity and were tested in two independent experiments, for a total n = 18 in the ADCC-Luc assay. Maximum ADCC activities and Ab titers among transmitting and nontransmitting women were above background levels observed in uninfected control subjects as indicated (Wilcoxon test). Each point represents an individual donor, except in panels C and D, where duplicate testing of the uninfected controls is included, and all 18 data points are shown. Medians are indicated with black horizontal lines.

FIG 3

Breast milk total IgA binding to select HIV-1 envelope (Env) antigens in uninfected and postnatal transmitting and nontransmitting mothers. The magnitude of the milk IgA binding responses measured by binding antibody multiplex assay (BAMA) against A1.con env03 gp140 (A) and B.con env03 gp140 (B) in transmitting women were similar to responses of 30 uninfected women, whereas the nontransmitting women had significantly higher IgA responses against these Env antigens than uninfected women, as indicated (Wilcoxon test). Binding magnitude is reported as mean fluorescent intensity (MFI). (C) Breast milk total IgA binding to B.con env03 gp140 measured by enzyme-linked immunosorbent assay (ELISA). The magnitude of binding, measured as the area under the curve among transmitting and nontransmitting women, was significantly above background levels observed in uninfected control subjects (Wilcoxon test) when measured by ELISA. Each dot represents milk from one donor, and medians are indicated with black horizontal lines.