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.