Robust and Specific Secretory IgA Against SARS-CoV-2 Detected in Human Milk

Abstract

The SARS-CoV-2 immune response in human milk has not yet been examined, although protecting infants and young children from COVID-19 is critical for limiting community transmission and preventing serious illness and death. Here, milk samples from eight COVID-19-recovered and seven COVID-19-suspected donors were tested for antibody (Ab) binding to the SARS-CoV-2 Spike protein. All samples exhibited significant specific IgA reactivity to the full Spike, whereas 80% exhibited significant IgA and secretory (s)Ab binding to the Receptor-Binding Domain (RBD). Additionally, 67% samples exhibited IgG and/or IgM binding to RBD. IgA and sAb titers were highly correlated, indicating most IgA to be sIgA. Overall, these data indicate that a robust sIgA-dominant SARS-CoV-2 Ab response in human milk after infection should be expected in a significant majority of individuals. Further research is highly warranted to determine Ab functionality and the potential for exploiting extracted milk sIgA for therapeutic use.

Keywords: Immunology; Pediatrics; Virology.

Graphical abstract

Figure 1

All Human Milk Samples Collected from COVID-19-Recovered Donors Exhibit IgA Reactivity Against the Full SARS-CoV-2 Spike Trimer (A) Full titration against Spike. NEG (i.e., negative)/segmented lines: pre-pandemic controls. COV/solid lines: milk from COVID-19-recovered donors. (B) Endpoint dilution titers. Experiments were performed in duplicate and repeated twice. Mean with SEM is shown. Dotted lines indicate positive cutoff value (mean OD or endpoint titer of negative control milk samples +2 ×SD). Segmented line indicates 10× positive cutoff value.

Figure 2

Eighty Percent of Human Milk Samples Collected from COVID-19-Recovered Donors Exhibit IgA and Secretory Antibody Reactivity Against the Receptor-Binding Domain (RBD) of the SARS-CoV-2 Spike (A) Full titration against RBD, measuring IgA binding. (B) IgA endpoint dilution titers. (C) Full titration against RBD, measuring secretory antibody binding. (D) Secretory antibody endpoint dilution titers. NEG (i.e., negative)/segmented lines: pre-pandemic controls. COV/solid lines: milk from COVID-19-recovered donors. Experiments were performed in duplicate and repeated twice. Mean with SEM is shown. Dotted lines indicate positive cutoff value (mean OD or endpoint titer of negative control milk samples +2 × SD).

Figure 3

The RBD-Specific IgA Response in Milk is Dominant and Not Necessarily Concurrent with a Measurable IgG or IgM Response (A and B) Full titrations against RBD, measuring IgG (A) and IgM (B) binding are shown. NEG (i.e., negative)/segmented lines: pre-pandemic controls. COV/solid lines: milk from COVID-19-recovered donors. Experiments were performed in duplicate and repeated twice. Mean with SEM is shown. Dotted lines indicate positive cutoff value (mean OD or endpoint titer of negative control milk samples +2 × SD).

Figure 4

Milk from COVID-19-Recovered Donors Exhibits Significantly Greater IgA, Secretory Antibody, and IgG Binding Against RBD Compared with Controls (A–D) Grouped OD values for undiluted milk are shown for IgA (A), secretory antibody (B), IgG (C), and IgM (D). Experiments were performed in duplicate and repeated twice. Mean with SEM is shown. (E) Correlated IgA- versus secretory Ab-binding activity. (F) Correlated IgG- versus IgM binding activity. For correlation tests, OD values for undiluted milk were used in 2-tailed Spearman correlation tests. SC: secretory component.