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.