Reply: In vitro culture conditions, antiphospholipid antibodies and trophoblast function.

Sir, WethankDrsClark andLaskin for their interest in our recentlypublished systematic review of the effects of antiphospholipid antibodies on cultured placental cells in vitro.We agreewithmany of the authors’ comments and their suggestions for future investigations. However, in suggesting that trophoblast plugs prevent antiphospholipid antibodies accessing the placenta in early gestation, we believe that Drs Clark and Laskin have misinterpreted the reports of trophoblast plugs in the uterine spiral arteries. Firstly, it has been shown that potentially as few as 20% of spiral arteries are fully plugged during the first trimester of human pregnancy (Meekins et al., 1997), which if true, means maternal blood-borne antibodies would readily access early gestation placentae. Secondly, trophoblast plugs in the spiral arteries are widely agreed to be only loosely cohesive (Boyd and Hamilton, 1970; Ramsey and Donner, 1980). This loose cohesionmeans that while the plugs prevent the passage ofmaternal red blood cells into the intervillous space, maternal plasma may still pass through the plugs to access the placenta. Supporting this, hysteroscopic examination reveals that placental villi are bathed in a clear fluid prior to 12weeks of gestation (Jaffe et al., 1997) and histologic specimens of implantation sites from as early as 50 days gestation show clear tracks of fluid leading from the spiral arteries to the intervillous space (Burton et al., 1999). The flowofmaternal plasma to and from the placenta is corroborated by the finding of substantial numbers of placenta-derived extracellular vesicles (syncytial nuclear aggregates, microand nanovesicles) in thematernal peripheral blood fromas early as 6weeks of gestation (Covone et al., 1984; Knight et al., 1998; Askelund and Chamley, 2011; Salomon et al., 2014). Thus, unlike maternal red blood cells, soluble factors including antiphospholipid antibodies have access to the first trimester placenta despite the presence of trophoblast plugs in the spiral arteries. Therefore, these autoantibodiesmayhavedirect deleterious effects on the placenta from the beginning of placental development.