Rickettsial Stimulation of Endothelial Platelet-Activating
6
INFECrION AN D M a y 1993, p. 2024-2029 0019-9567/93/052024-06$02.00/0 Copyright X ) 1993, American Society f o r Microbiology Rickettsial Stimulation of Endothelial Platelet-Activating Factor Synthesis T . STUART WALKER* AN D GERALD E . MELLOTT Muncie Center f o r Medical Education, Indiana University School o f Medicine a nd Ball State University, a n d Department o f Biology, Ball State University, Muncie, Indiana 47306 Received 2 1 December 1992/Accepted 1 8 February 1993 Endothelial cells (EC) synthesize platelet-activating factor (PAF) when activated b y agents such as A TP o r thrombin, a n d PA F production occurs as a consequence o f endothelial phospholipase A activity. Because interactions between prowazekii a n d a variety o f host cells result in t h e expression o f phospholipase A activity, we assessed t h e relative abilities of uninfected a n d rickettsia-infected EC t o synthesize PAF. Endothelial cells were infected with rickettsiae a n d examined a t intervals f o r rickettsial multiplication, EC viability, a n d PA F synthesis. By 2 4 h postinfection, 80 % o f t h e EC were infected with a n average o f 10.6 rickettsiae pe r cell; b y 7 2 h , t h e rickettsiae were t o o numerous t o count a n d t h e numbers o f viable EC began t o decrease. Both rickettsia-infected a n d sham-treated E C synthesized P A F when stimulated with either thrombin o r ATP, b ut rickettsia-infected EC synthesized about three times a s much P A F i n response t o cell activation a s di d their counterparts. Additionally, unlike their uninfected counterparts, rickettsia- infected EC synthesized significant amounts o f P AF i n t h e absence o f cell activation; rickettsia-infected EC synthesized a s much PA F in t he absence of activation as did uninfected E C i n response t o ATP. I n each case, essentially a l l o f t h e newly synthesized PA F remained with t h e cell pellet. Finally, EC incubated with high numbers o f rickettsiae (1,000 rickettsiae pe r E C ) f or 3 0 m i n synthesized more P A F when activated with ATP than d i d their sham-treated activated counterparts. I t h as long been recognized (34, 54 , 5 5 ) that rickettsioses a r e vascular diseases a n d that rickettsial invasion of t h e endothelium i s associated with profound changes i n vascular function. I n moribund patients, these changes culminate with shock due t o peripheral vascular collapse (34). It i s that this final condition occurs consequence of changes i n vascular tone a n d a profound generalized in- crease i n vascular permeability (34). Vascular changes a re also seen when mice a r e intravenously given large doses o f live typhus rickettsiae (19, 20, 22, 42). Although t h e mouse model differs from natural human infection i n that mice re killed before rickettsial multiplication occurs, t h e similarity o f their clinical pictures suggests that human infection a n d t h e mouse typhus toxic reaction share some mechanisms. Because Rickettsia prowazekii killed mice rapidly a nd b e - cause t h e injection of killed rickettsiae into mice di d not precipitate toxic manifestations such a s increased vascular permeability a nd shock (42), early investigators looked f o r a rickettsial toxin; none was found. I n t h e search f o r soluble mediators o f rickettsial disease, investigators have now turned their attention from t h e rick- ettsia t o the parasitized cell. The recent discovery that endothelial cells (EC) produce molecules that regulate vas- cular tone, thrombosis, a n d permeability raised t h e possibil- i t y that vascular changes that occur typhus might result, at least i n part, from rickettsial effects on t h e ability o f E C t o synthesize or express one or more o f these effector molecules. Among t h e effectors synthesized o r expressed b y E C ar e endothelium-derived relaxing factor ( 1 , 6 ) , t h e endo- thelins (13), tissue factor (25, 29), thrombomodulin (25), plasminogen activators (10), von Willebrand factor (11, 28), heparan sulfates (10), platelet-activating factor (PAF) (44), prostacyclin (21, 43), prostaglandin E 2 (PGE2) (21), interleu- * Corresponding author. k i n 1 (15), a nd angiotensin-converting enzyme (6). O ur laboratory recently reported (35) that EC heavily infected with R . prowazekii secreted as much a s 20 times a s much prostacyclin a s d id their uninfected counterparts. Raoult e t a l . (24) also reported that EC infected with Rickettsia cononi secreted increased amounts o f a n inhibitor o f plasminogen activator. Most recently, Sporn t a l . (28) reported that EC infected with spotted fever group rickettsiae released large von Willebrand factor monomers. Rickettsia-infected phago- cytes have also been reported t o secrete altered amounts of effectors; o u r laboratory reported that macrophages (36) a n d polymorphonuclear leukocytes (37) infected with typhus rickettsiae secreted more PGE2 a n d leukotriene B4 than d i d cells incubated with killed or no rickettsiae, a n d Manor a nd Sarov reported (16) that monocytes infected with spotted fever group rickettsiae secreted increased amounts o f tumor necrosis factor alpha a n d PGE2. PAF i 1-O-alkyl-2-acetyl glycero-3-phosphocholine a n d i s synthesized b y EC activated b y each of several cell-activat- i n g molecules, including thrombin, ATP, a n d calcium iono- phore A23187 (17, 23). P A F was s o named because of i t s procoagulant activity (14), b u t i t also promotes adherence o f leukocytes t o t h e endothelium (41), enhances leukocyte activities (14), a n d i s a potent hypotensive agent (30). The synthesis o f P AF by activated EC involves activation of a phospholipase A (45) a n d may involve protein kinase C (9). Because adherence o f R . prowazekii t o a variety o f host cells a n d intracellular rickettsial multiplication a r e accompanied b y phospholipase A activity (32, 47-51) a n d rickettsial mul- tiplication h a s been associated with increased secretion o f other phospholipase A-dependent endothelial products (35), we asked whether EC infected with rickettsiae synthesize PAF. We report that rickettsia-infected EC synthesized more P AF i n both t h e presence a n d absence o f cell-activat- i n g agents than d i d their uninfected counterparts. 2024 Vol. 1 , No. 5 byo n Au g u st24, 2009ia i. a sm . o rg Do w n lo a de dfro m
