Results
All samples were rich in the molecules of interest; BM had significantly higher sACE2 and sNRP-1 but lower sCD26 than AF. Different isoforms of sACE2, sCD26 and sCD147 existed in AF compared to BM and might provide information about the origins of soluble forms of these mediators6 and dictate their biological function7. For example, the shorter isoform of ACE2 lacks SARS-CoV-2 spike high-affinity binding sites 7 so AF sACE2 might be less efficient in acting as a viral decoy. The lower molecular weight (MW) form of CD26 is typically associated with its membrane-bound form (which can be shed) compared to the higher MW of the naturally occurring soluble form 6 suggesting that BM sCD26 originates by shedding from mammary epithelial cells whereas AF contain the naturally occurring soluble form. As these two isoforms have shared and distinct properties there are likely different functional effects of sCD26 in AF and BM that might include the ability to bind SARS-CoV-2. CD147 evidently has various degrees of glycosylation, with BM exhibiting numerous isoforms in comparison to the one isoform observed in AF. The level of glycosylation is correlated to CD147 function, with low glycosylation (LG) forms being unable to self-aggregate and stimulate matrix metalloproteinase (MMP) induction which the high glycosylation (HG) forms are capable 8. SARS-CoV-2 severity has been associated with an increase in MMP activity9, 10. The presence of the LG forms in the BM and AF suggests a tighter regulation of MMP activity, potentially preventing severe disease.