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Double expressor switch intermediates
Clearance of Malaria Sporozoites Inhis article entitled ‘The role of
Kupffer cells in the clearance of malaria sporozoites from the circulation”, Vreden has comprehensively reviewed our current knowledge of the route taken by malaria sporozoites in invading hepatocytes. It now seems clear that Kupffer cells are not essential for successful penetration and maturation of sporozoites in hepatocytes, either in vitro or in viva. Nevertheless, I agree with Vreden that, in spite of this, Kupffer cells still might well play a role in mediating in vivo sporozolte invasion of hepatocytes. However, I disagree with him In his exclusion of the possibility of sporozoite invasion through liver sinusoidal endothellal cells during sporozoite passage from blood to hepatocyte. We fint suggested the direct passage of sporozoltes through endothellal cells when we found that Plosmodrum berghel sporozoites injected Intravenously into Sprague-Dawley rats had invaded hepatocytes within two minute+. Our more recent studies have shown a similar rapid invasion by P. berghei sporozoltes rn Norway-Brown rats and P. yoelli sporozoites in BALB/c mace. Vreden argues against direct Invasion through endothelial cells, based on the relatively small diameter of the fenestrae that perforate them (about
Reply Vanderberg has a good point by clalmlng that the possibility of sporozoite passage through intact endothelial cells (EC) cannot be excluded with the argument that the size of the fenestrae IS too small. Indeed. active penetration must still be consldered as a possible route of invasion into the liver parenchyma. However, there are some arguments that do not favor thts mechanism. Besides a remark within brackets In a publication of Vanderberg er a/. I, there are, to my knowledge, no publications that demonstrate sporozolte penetration of EC. In contrast, Mels et 01. have produced several electron microscopical studies which not only demonstrate lntemallzation of sporozoltes into Kupffer cells (KC), but
Double Expressor Switch Intermediates Recently, It was noted that double expressor switch intermedlates (DE) expressing a mlxed variant surface glycoprotein (VSG) coat exist during antlgenic variation in African trypanosomesl. This discovery valldates our predlcitor+. whtch until recently has been met with scepticlsm4. I would like to draw the attention of parasitologists to the significance of this finding. Using a mathematical model, we showed that real-life parasitaemia can be retrieved theoretlcally only if one assumes 24
0. I pm) and the considerably larger diameter of sporozoites (about I km) that would have to pass through these cells. Dismissing the possibility of trans-endothelial cell passage, Vreden concludes that ‘The only other way to reach the hepatocytes is passage through Kupffer cells’. However, this argument presumes that sporozoites are immobile organisms that can only enter passively into the space of Dlsse. It is well established that passive filtration of small particles, such as liposomes and chylomicrons, through the endothelial fenestrae IS size restricted3,4. However, sporozoites are quite motile and have the ability to penetrate rapidly and actively through the plasma membranes of macrophages and other cellss. No one would deny the ability of sporozoites to first penetrate the intact basement membrane and plasma membrane of mosquito salivary gland cells and ultimately penetrate the intact plasma membrane of hepatocytes, yet none of these invaded cells even possesses fenestrae. Therefore, how can It be argued that sporozoites are not capable of passing through liver sinusoidal endothelial cells because they are too big to fit through the fenestrae? One of the most Interesting new areas of study of sporozoite biology cited by Vreden relates to specific binding of the clrcumsporozoite protein to heparan sulfate proteoglycans associated with the surface
also their escape on the parenchymal side of these cells*,3. The vldeomlcroscopical study of Vanderberg et al. also shows that sporozoites can escape macrophagesq. Liver schlzonts are located preferentially In hepatocytes that are situated in those areas where the afferent blood vessels empty Into the sinusoids2. It is probably not a coincidence that KC are concentrated Itthese zones of the liver acinls. Recently, we reported that when rats are pretreated with gadoliniumchloride (which does not ellmlnate, but only Inactivates KC), the number of developing liver schizonts drops slgnlticantly’? This finding supports the theory that KC play an important part in the homing of sporozoites in the liver, but it still does not exclude a role for the EC.
the existence of DE in the maloritv of blood-stage switches; the dlffeyen; DE combinations, which result from stochastic switches, are expected to vary In susceptibility to the Immune response against the predecessor VSG. Our results suggest that the dynamics of parasitaemla depend prlmat-ly on these two factors. It appears that our former result has now been validated and it seems to me crucial also to check the second one. Validation of these results may have significant implications for future control of the diseases caused by African ttypanosomes.
membrane of hepatocfles6,‘. It remains to be determined whether this hepatocyte surface receptor is expressed across the endothelial cell fenestrae for recognition by sporozoites in the blood (as proposed by these investigators) or whether the receptor is encountered only by sporozoltes that have already penetrated into the space of Disse. In either case, direct invasion by sporozoite across the endothelial cell barner remains a plausible possibility. However, the rapidity with which sporozoites have been shown to move and invade cells may make it difficult to catch one In the act. References I Vreden, S.G.S. (I 994) Porasitoiogy Today IO, 304-308 2 Shin, S.J.,Vanderberg J.P. and Tetzakts, J.A. (I 982) ,I Protorool 29,448-454 3 De Zanger, R. and Wlsse. E. ( 1982) in Sinusoidoi i~ver Cells (Knook, D.L. and Wisse, E., eds), pp 69-76, Elsevler Biomedical Press 4 Scherphof, G. (I 983) 5101.Ceil 47, 47-58 5 Vanderberg, J,P., Chew, S. and Stewart, M.J. (I 990)]. Protozooi. 37. 528-536 6 Cerami. C. et al. ( 1992) Ceil 70, IO2 I- I033 7 Frevett, U. et al. (I 993) j, Exp. Med. 177, I 287p I298 Jerome P. Vanderberg Department of MedIcal and Molecular Parasitology New York Univenlty School of Medlclne
New York NY IO0 16. USA
References I Vanderberg. J.P. and Stewart M.J. (I 990) Buii. WHO 6% (Suppl.), 74-79 2 Meis, J.F.G.M. et ai (I 985) Parasitology 86, 232-242 3 MeIs, J.F.G.M. et ai (I 985) Z. Porasitenkd.7 I,
473-483 4 Vandenberg, J,P,,Chew, S. and Stewart, M.]. (I 990)). Protozooi 37, 528-536 5 Seljelld, R. (I 980) in Mononucieor Phogocytes: functional Aspects (Van Furth, R., ed.). pp 157-l 99, MartInus Nijhoff 6 Vreden, S.G.S. ( 1994) in lnflammotory MedIaton ,n the Development ofLiver Schrzonts, pp 93 105, Nijmegen Univenlty Press Stephen G.S. Vreden Department of lnfectlous Diseases Unlverslty Hospital Leaden Rijnsburgetweg IO. PO Box 9600 2300 RC Leaden. The Netherlands
References I Boti, P. and Rudenko, G. (I 994) Soence 264, I fI72- I 873 2 Agur, Z., Abtn, D. and Van der Ploeg, L.H.T.
( 1989) Proc.NotI Acod Ser.USA 86, 9626-9630 3 Agur, Z. ( I99 I ) PorosrtologyToday 8, 128-129 4 Barry J.D. and Turner, C.M.R. ( 1992) ParasitologyToday 8. I29 Zvia Agur Department
of Cell Research and Immunolom
Tel AVIV Univenlty Ramat Aviv 49978 Tel Aviv, Israel Poroatoiogy Today, vol.
I i, no. i, / 995
Kupffer cells in the clearance of malaria sporozoites from the circulation”, Vreden has comprehensively reviewed our current knowledge of the route taken by malaria sporozoites in invading hepatocytes. It now seems clear that Kupffer cells are not essential for successful penetration and maturation of sporozoites in hepatocytes, either in vitro or in viva. Nevertheless, I agree with Vreden that, in spite of this, Kupffer cells still might well play a role in mediating in vivo sporozolte invasion of hepatocytes. However, I disagree with him In his exclusion of the possibility of sporozoite invasion through liver sinusoidal endothellal cells during sporozoite passage from blood to hepatocyte. We fint suggested the direct passage of sporozoltes through endothellal cells when we found that Plosmodrum berghel sporozoites injected Intravenously into Sprague-Dawley rats had invaded hepatocytes within two minute+. Our more recent studies have shown a similar rapid invasion by P. berghei sporozoltes rn Norway-Brown rats and P. yoelli sporozoites in BALB/c mace. Vreden argues against direct Invasion through endothelial cells, based on the relatively small diameter of the fenestrae that perforate them (about
Reply Vanderberg has a good point by clalmlng that the possibility of sporozoite passage through intact endothelial cells (EC) cannot be excluded with the argument that the size of the fenestrae IS too small. Indeed. active penetration must still be consldered as a possible route of invasion into the liver parenchyma. However, there are some arguments that do not favor thts mechanism. Besides a remark within brackets In a publication of Vanderberg er a/. I, there are, to my knowledge, no publications that demonstrate sporozolte penetration of EC. In contrast, Mels et 01. have produced several electron microscopical studies which not only demonstrate lntemallzation of sporozoltes into Kupffer cells (KC), but
Double Expressor Switch Intermediates Recently, It was noted that double expressor switch intermedlates (DE) expressing a mlxed variant surface glycoprotein (VSG) coat exist during antlgenic variation in African trypanosomesl. This discovery valldates our predlcitor+. whtch until recently has been met with scepticlsm4. I would like to draw the attention of parasitologists to the significance of this finding. Using a mathematical model, we showed that real-life parasitaemia can be retrieved theoretlcally only if one assumes 24
0. I pm) and the considerably larger diameter of sporozoites (about I km) that would have to pass through these cells. Dismissing the possibility of trans-endothelial cell passage, Vreden concludes that ‘The only other way to reach the hepatocytes is passage through Kupffer cells’. However, this argument presumes that sporozoites are immobile organisms that can only enter passively into the space of Dlsse. It is well established that passive filtration of small particles, such as liposomes and chylomicrons, through the endothelial fenestrae IS size restricted3,4. However, sporozoites are quite motile and have the ability to penetrate rapidly and actively through the plasma membranes of macrophages and other cellss. No one would deny the ability of sporozoites to first penetrate the intact basement membrane and plasma membrane of mosquito salivary gland cells and ultimately penetrate the intact plasma membrane of hepatocytes, yet none of these invaded cells even possesses fenestrae. Therefore, how can It be argued that sporozoites are not capable of passing through liver sinusoidal endothelial cells because they are too big to fit through the fenestrae? One of the most Interesting new areas of study of sporozoite biology cited by Vreden relates to specific binding of the clrcumsporozoite protein to heparan sulfate proteoglycans associated with the surface
also their escape on the parenchymal side of these cells*,3. The vldeomlcroscopical study of Vanderberg et al. also shows that sporozoites can escape macrophagesq. Liver schlzonts are located preferentially In hepatocytes that are situated in those areas where the afferent blood vessels empty Into the sinusoids2. It is probably not a coincidence that KC are concentrated Itthese zones of the liver acinls. Recently, we reported that when rats are pretreated with gadoliniumchloride (which does not ellmlnate, but only Inactivates KC), the number of developing liver schizonts drops slgnlticantly’? This finding supports the theory that KC play an important part in the homing of sporozoites in the liver, but it still does not exclude a role for the EC.
the existence of DE in the maloritv of blood-stage switches; the dlffeyen; DE combinations, which result from stochastic switches, are expected to vary In susceptibility to the Immune response against the predecessor VSG. Our results suggest that the dynamics of parasitaemla depend prlmat-ly on these two factors. It appears that our former result has now been validated and it seems to me crucial also to check the second one. Validation of these results may have significant implications for future control of the diseases caused by African ttypanosomes.
membrane of hepatocfles6,‘. It remains to be determined whether this hepatocyte surface receptor is expressed across the endothelial cell fenestrae for recognition by sporozoites in the blood (as proposed by these investigators) or whether the receptor is encountered only by sporozoltes that have already penetrated into the space of Disse. In either case, direct invasion by sporozoite across the endothelial cell barner remains a plausible possibility. However, the rapidity with which sporozoites have been shown to move and invade cells may make it difficult to catch one In the act. References I Vreden, S.G.S. (I 994) Porasitoiogy Today IO, 304-308 2 Shin, S.J.,Vanderberg J.P. and Tetzakts, J.A. (I 982) ,I Protorool 29,448-454 3 De Zanger, R. and Wlsse. E. ( 1982) in Sinusoidoi i~ver Cells (Knook, D.L. and Wisse, E., eds), pp 69-76, Elsevler Biomedical Press 4 Scherphof, G. (I 983) 5101.Ceil 47, 47-58 5 Vanderberg, J,P., Chew, S. and Stewart, M.J. (I 990)]. Protozooi. 37. 528-536 6 Cerami. C. et al. ( 1992) Ceil 70, IO2 I- I033 7 Frevett, U. et al. (I 993) j, Exp. Med. 177, I 287p I298 Jerome P. Vanderberg Department of MedIcal and Molecular Parasitology New York Univenlty School of Medlclne
New York NY IO0 16. USA
References I Vanderberg. J.P. and Stewart M.J. (I 990) Buii. WHO 6% (Suppl.), 74-79 2 Meis, J.F.G.M. et ai (I 985) Parasitology 86, 232-242 3 MeIs, J.F.G.M. et ai (I 985) Z. Porasitenkd.7 I,
473-483 4 Vandenberg, J,P,,Chew, S. and Stewart, M.]. (I 990)). Protozooi 37, 528-536 5 Seljelld, R. (I 980) in Mononucieor Phogocytes: functional Aspects (Van Furth, R., ed.). pp 157-l 99, MartInus Nijhoff 6 Vreden, S.G.S. ( 1994) in lnflammotory MedIaton ,n the Development ofLiver Schrzonts, pp 93 105, Nijmegen Univenlty Press Stephen G.S. Vreden Department of lnfectlous Diseases Unlverslty Hospital Leaden Rijnsburgetweg IO. PO Box 9600 2300 RC Leaden. The Netherlands
References I Boti, P. and Rudenko, G. (I 994) Soence 264, I fI72- I 873 2 Agur, Z., Abtn, D. and Van der Ploeg, L.H.T.
( 1989) Proc.NotI Acod Ser.USA 86, 9626-9630 3 Agur, Z. ( I99 I ) PorosrtologyToday 8, 128-129 4 Barry J.D. and Turner, C.M.R. ( 1992) ParasitologyToday 8. I29 Zvia Agur Department
of Cell Research and Immunolom
Tel AVIV Univenlty Ramat Aviv 49978 Tel Aviv, Israel Poroatoiogy Today, vol.
I i, no. i, / 995