Normalization Of Erythrocyte Morphology In Scarb1/- Mice By Bacterial Serum Opacity Factor Is Dependent On HDL-free Cholesterol

Compared to wild-type mice, HDL-receptor-deficient (Scarb1 -/- ) mice have higher plasma levels of free cholesterol (FC)-rich HDL and exhibit multiple pathologies—female infertility and deranged platelet and erythrocyte morphology and function. These pathologies are associated with a low mol% FC in ovaries, platelets, and erythrocytes, effects that are reversed by the HDL-lowering drug probucol. Some strains of Streptococcus pyogenes secrete a protein, serum opacity factor (SOF), which catalyzes the clouding i.e., opacification, of plasma. SOF specifically targets and quantitatively converts HDL into three products, that include a cholesteryl ester-rich microemulsion containing all the cholesterol content of >100,000 HDL particles as well as APOE and its heterodimer with APOA2 as its sole apolipoproteins. Delivery of SOF with an adeno-associated virus (AAV SOF ) constitutively lowers plasma HDL-FC and reverses female infertility in Scarb1 -/- mice. Thus, we tested the hypothesis—AAV SOF delivery to Scarb1 -/- mice will normalize erythrocyte morphology in an HDL-FC dependent way. The erythrocyte morphology and FC content expressed as mol% FC of three groups of mice—WT, untreated Scarb1 -/- mice (control) and Scarb1 -/- mice receiving AAV SOF —were compared and correlated with their respective HDL-mol% FC. Among Scarb1 -/- mice, AAV SOF treatment normalized reticulocyte number, erythrocyte morphology and erythrocyte mol% FC. Plasma- and HDL-mol% FC positively correlated (p < 0.0001) across all three groups of mice. HDL-mol% FC also positively correlated with erythrocyte mol% FC (p < 0.0001). Finally, the erythrocyte mol% FC positively correlated with both the number of reticulocytes (p = 0.006) and abnormal erythrocytes (p < 0.0001). Although less profound, AAV SOF treatment also reduced the FC contents of some extravascular tissues. HDL-FC spontaneously transfers from plasma lipoproteins to cell membranes in multiple tissues sites on a time scale of minutes to a few hours. FC-enrichment of erythrocytes, which are in contact with plasma HDL, was more profound than FC enrichment in tissues. AAV SOF treatment lowers both plasma HDL-FC and erythrocyte-FC and normalizes erythrocyte morphology and lipid composition in an HDL-FC dependent way.