Interpretation
It is well known that breastfeeding protects babies against gastrointestinal and respiratory infections25–28. IgA represents around 90% of all immunoglobulins present in human milk and its concentration is higher in the colostrum, decreasing during the first year of lactation9. Due to its low degradation and absorption rate in the infant’s gastrointestinal system, IgA is the most important immunoglobulin in human milk, protecting the infant against infections at mucosa level. Therefore, anti-SARS-CoV-2 IgA in human breast milk could protect the infant against the infection at a local level, similarly to what happens with other viral infections28. Recently, it has been demonstrated that anti-SARS-CoV-2 antibodies in breast milk neutralize the virusin-vitro 16,30,31.
In our study, most of the colostrum samples tested positive for IgA, irrespective of the time of SARS-CoV-2 infection, and it was the only immunoglobulin present in mature milk. In contrast, IgG was present in less than 2% of the colostrum samples and in none of the mature milk samples. When evaluating longitudinal changes in the concentrations of immunoglobulins both, in the colostrum and the mature milk, a significant reduction in IgA levels was found, similarly to what happens with other viral infections9. Importantly, IgA was even present in the colostrum from mothers with a negative serological status at the time of delivery, contrary to what happened with IgG, which was more likely to be present when maternal levels of IgG in serum were higher. A possible explanation for this could be related to the fact that IgA is secreted from Peyer patches, while IgG is mostly filtered from maternal plasma32. Peyer patches belong to the Gastrointestinal Antigen Linfoide Tissue (GALT) system and they represent maternal immunological memory33,34. This system is responsible of secreting antibodies against common infections prevalent in maternal living area35. IgM is also secreted by this system, but at much lower concentrations.
According to the severity of the disease, we have found higher concentrations of colostrum immunoglobulins in women with severe symptoms (pneumonia) as compared to those with mild or no symptoms. A higher immunological response has also been previously demonstrated in non-pregnant population36.
In this study 26 samples from women with active disease at the time of delivery were tested by rRT-PCR-SARS-CoV-2 and all of them were negative. Evidence suggesting the presence of SARS-CoV-2 in the breast milk is conflicting2–8,13 and it is possible that crossed-contamination was responsible of positive results16. Goad et al investigated the presence of cell-specific expression of angiotensin-converting enzyme 2 (ACE2), proteases TMPRSS2, and cathepsins CTSB and CTSL in breast epithelium and they did not find co-expression of ACE2/TMPRSS2 or ACE2/CTSB/L, which is important for the entry of the virus into the cell. Therefore, they concluded that there was no risk of vertical transmission of SARS-CoV-2 in neonates through breastfeeding37.