IncreasedexpressionofFasbyhematopoieticprogenitorsinaplasticanemia(AA)suggeststhatFas/Fasligand(FasL)systemplaysakeyroleintheformationofseverepancytopenia.Tofurtherconfirmtheabovehypothesis,Tcellsfrom8patientswithAAweresystematicallystudiedfortheirFasL'sdistributionpattern,releasingmannerandproapoptoticactivity,comparedwithnormalrestingTcellsandartificiallyactivatedTcellblasts.TheresultsdemonstratedthatAATcellsabnormallyexpressedlowlevelsofmembrane-boundFasLandcontainedhighlevelsofintracellularFasLwhichcouldbetriggeredtoreleasebyhigh-dosephytohemagglutinin(PHA)pulse-stimulation.ThesupernatantsfromthePHA-stimulatedAATcellshadapparentcytotoxicityagainstFasL-sensitiveJurkatcells,whichcouldbesignificantlyinhibitedbymonoclonalantibodyagainstFasLinadose-dependentmanner,ornearlycompletelyabrogatedbyultracentrifugation.TheabovephenomenaalsoappearedonartificiallyactivatedTcellblasts,butthiswasnotthecaseonnormalrestingTcells.TheseresultsindicatethatAATcellisatypeof'preactivated'Tlymphocyte,characterizedbyoverexpressionofFasL,especiallyintracellularFasLwhichcanbestimulatedtoreleaseinbioavtiveexosomesboundform.Takentogether,ourdataprovidefurtheranddirectevidenceforthehypothesisthatTcellsmightmediatethedestructionofhematopieticprogenitorinAAthroughFas/FasLsystem.
