Recent developments in production and purification of malaria antigens: Some aspects of serum requirements for continuous cultivation of Plasmodium falciparum*

Mt1e;irn of the World Heal)t Organiza.o't, 5? (Suppl. 1): 27-31 (1979) Some aspects of serum requiremenits for continuous cultivation of Plasmodium falciparum* JAMES B. JENSEN' As possible substitutes or supplementsfor the human serum requirenmentsfor continu- ous cultures of Plasmodium falciparum several different animal sera have been tried, in- cluding: fresh fetal bovine, shfeep, horse, adult bovine, newborn calf, swine, commercially supplied fresh-frozen or lyophilized human sera, fatty acid-free bovine or humnan serum albumin, red cell extract, Physiogel, and a serum substitute composed of Bacto-peprone, Yeastolate, Lactalbumnin hydrolysate, polyvinylpyrrolidone, and bovine insulin. None were comparable to the huiman serum, nor did they exhibit anyserum-sparing effect. Plasma con- tamiing citrate-phosphate-dextrose (CPD) was nearly as good asserum, but dialysedplasma orserum would not support cultures ofP. falciparurm. Freshz human serumn variedfrom lot to lot in irs culture supporting properties especially if the concentration of serunm in the medium was reduced below 10%. At 10% concentrations, however, most sera supported the cultures very well. The first continuous cultures ofPlasmodiumfalci- parum were started by diluting infected erythrocytes of A otus trivirgatus with human type AB red cells in RPMI 1640 culture medium supplemented with human type AB Rh + serum at the rate of 150 ml/litre (1). Once the cultures were established they were maintained in human typeA Rh + cells in RPMl 1640 containing 1O/o of human type A Rh+ serum. The initial use ofAB Rh + cells and serum was to avoid in- compatibility wich the Aotus blood, and the sub- sequent switch to typeA Rh+ cclls and serum was due to the greater availability of this latter blood type. In addition, A Rh + serum has the advantage of allow- ing one to use either type A or 0 erythrocytes which, in the USA and Europe, comprise nearly 80% of all types. The need for human serum as a supplement in these cultures has many disadvantages, in particular, poss- ible contamination with hepatitis viruses and other pathogens, high cost, and, in malarious regions, the presence of possibly inhibitory imminological fac- tors. Thus complete or partial replacement of human serum in P.falciparunm cultures is obviously a desir- able goal. Commercially supplied fetal bovine sera have not been acceptable (2), but perhaps some other animal sera would be. This report examines some aspects of * Financial suppot l ror this invesugation was supplied by USAID Contract No. 1373. Assistant Professor of Parasitology, Laboratory of Parasit- olog, The Rocketeller University, Ncw York, NY 1002J, USk. Present addres- Department of Microbiology and Public Healtl Michigan State University, East Lansing, MI 48824, USA. the serum requirements for continuous cultures of P.falciparum with emphasis on replacement or sup- plementation of human serum. MATERIALS AND METHODS All sera and serum supplements or replacements were tested in cultures of P.falciparum against human type A Rh+ serum using the Petri dish- candle jar technique (2). Generally, at subculture, Petri dishes were set up from a common pool of in- fected blood, strain FCR-3/FMG (3) and, divided into groups of 4 dishes each. The control group was supplied with RPMI 1640 medium containing 100 ml/litre (107o) of fresh human type A Rh + serum (New York Blood Center). The experimental groups contained the same medium with the addition of the different sera or supplements to be tested. Cultures were grown for 2 cycles (96 h) at which time thin films were made from all dishes. Because all the parasites in a givenI experimnent originated from a common pool the parasite counts at the end of the experiment were directly comparable. Comrnerciaily available sera Fetal bovine sera, from individual fetuses of differ- ent breeds,' was quick-frozen and transported on dry ice directly to me. Fresh-frozen pooled human serum, horse, sheep, bovine, and newborn calf sera,b and a Kindly provided by Dr Rex Spendlove, Sterilc Systems, Inc., Logan, UT, USA b All froin M;iles Laboratories, Inc., Kankakee, IL 60901, USA. 3865 -27- 28 J B. JENSEN fresh swine serumc were also tested. Commercially supplied fresh lyophilized human sera were tested against our standard serum, lyophilized in our own laboratory. Serum supplements or replacements As supplements or replacements for human serum the following were tested: Physiogel, a gelatin based plasma-expander,d bovine insulin,' fatty acid-free bovine or human serum albumin,' D-a-tocopherol (vitamin E)" and a serum substitute containing Bacto- peptone,fLactalbumin hydrolysate,e Yeastolate,J and polyvinylpyrrolidone (PVP),' as reported by Las- fargues et al. (4). Red cell extract was prepared by making a 20% suspension of washed human erythro- cytes in RPMI 1640 and freeze-thawing the suspen- sion twice. The red cell stroma were removed by pelleting at 35 000 g for 2 h at 4IC. The supematant extract was used at a dilution of 100 ml/litre in RPMI 1640 with or without serum. The clotting elements were removed from fresh CPD-plasma by adding, one drop at a time, 1 ml of a 10% solution of CaCI2 per 100 ml of plasma. After 30-40 min of stirring at moderate speeds with a mag- netic stirrer, the clot would be formed around the magnetic bar. To remove the citrated preservatives, 12 ml ofplasma was dialysed through a 2-cm diameter membrane having a pore size equivalent to MW 6000-8000, against 2 litres of RPMI 1640 medium (over 48 h). In one experiment, 30 ml of serum was dialysed against 500 ml of distilled water containing 25 mmol/ litre HEPES. The HEPES buffered dialy- sate was then lyophilized and reconstituted in RPMI 1640 medium without HEPES. To one portion of this reconstituted dialysate was added 10 ml of dialysed serum (the non-dialysable fraction). These fractions, the reconstituted dialysate, the non-dialysable frac- tion, and a combination of both were tested for their ability to support cultures of P.falciparum. RESULTS AND DISCUSSION Substitution or supplementation ofhuman serum The results obtained using commercially supplied animal and human sera are found in Table 1. In general, none of the sera were comparable to freshly obtained human serum for continuous cultivation of P.faciparum. It was previously reported (2) that fetal t From Irvine Scientific, Fountain Valley, CA, USA. d From ZentraUaboratorium Blutspendedienst, SRK, 3000 Berne 22, Switzerland. e From Sigma Chemical Co., St Louis, MO, USA J Difco Laboratories, Detroit, Ml, USA Table 1. Comparison of the growth of P. falciparum cultures supplemented wrth fresh human type A Rh + serum and commercially avaitable animal and human sera. All sera, except controls, were heat inactivated for 30 min at 56 DC Growth Serum in companson Comments with control 1%l control serum a 100 the mean parasitasmia at 96 h for controls in all experiments was 6 B%, representing an average increase of 60-70-fold bovine, fetal 35 b the first few cycles were encouraging, but growth declined with each subculture bovine, adut 7 2 bovine, newborn 1 0 horse 190 sheep 2 3 sunne 14 0 after 48 h, cultures showed some agglutination of RBCs, consideble haemolysis at 96 h human, frozen c 27 0 human, lyophilized 4 4 haemoysis began after 72 h tn vitro; IMiles) became extensive at 96 h human, lyophiizedd 17 6 hernolyss began after 48 h in vitro,(Difcol became extersve at 96 h human, lyophilized 100 stored at 23 IC for 6 weeks before in laboratory reconsituting by addition of distilled water B Fresh human serum obtained from freshly clotted blood supplied by the New York Blood Center -b The average of all samAples tested for the first 96 hin vitro c Commercially pooled "fresh" frozen human serum from Miles Laboratones, Inc d Reconsituted and added to culture medium at a rate equivalern to 200 ml of serumr per litre of nedium bovine sera (FBS) would not support cultures of P.falciparum. However, owing to the poor handling of mosi commercially supplied FBS and the resulting variation in quality (5) it was felt that FBS obtained fresh from individual fetuses might adequately replace human serum in the parasite cultures. Accordingly, sera were obtained from 6 bovine fetuses: 2 Holsteins, 2 Herefords, I Black Angus, and 1 Hereford-Holstein cross. Initially, these sera showed promise, some sup- porting parasite growth very well during the first few cycles. The trophozoites and schizonts grown in these sera, however, were generally smaller than those seen in human serum controls and with repeated sub- cultures the parasite numbers gradually declined. The longest cultures grown in FBS survived 32 days, repre- senting 8 subcultures, but they eventually died out. The only other animal sera that supported reasonable CONTINUOUS CULTIVATION OF P. FALCIPARUM29 parasite growth were horse and swine sera. In both cases there was some growth with many healthy look- ing parasites. The swine serum, however, caused ag- glutination of the erythrocytes after 48 h in vitro and extensive haemolysis was seen by 96 h. Perhaps fresh horse or fetal swine sera might prove valuable as a replacement for human serum. Human serum from freshly clotted blood was no longer satisfactory if it was stored at 4°C for more than 2-3 weeks (2). However, such sera can be stored for long periods at -20 DC without measurable loss of nutrient status. Commercially supplied 'fresh- frozen" pooled hurman serum was not comparable to that prepared and stored frozen in our laboratory. Growth in the commercially supplied serum was only 27% of the level seen in the controls. Two sources of "fresh-lyophilized" human serum were even less suit- able, giving parasite growth equal to 4.487o and 17.6870 of the controls, respectively. Also, these lyophilized sera produced extensive haemnolysis in the cultures. Conversely, when fresh serum was lyophilized in our laboratory and stored at 23 °C for 6 weeks before being reconstituted there was no change in its ability to support growth, and no haemolysis. These findings suggest that the methods employed by commercial suppliers of human and animal sera lead to a marked reduction in their suitability for supporting P.falci- parum cultures. Partial or complete replacement of human serum using fatty acid-free bovine or human serum albumin was not possible, in spite of the report by Siddiqui & Richmond-Crum (6). Likewise, a serum substitute, described by Lasfarques et al. (4). composed in part of Bacto-peptone, Yeastolate. Lactalbumin hydroly- sate, bovine insulin, and polyvinylpyrrolidone (PVP) did not support any parasite growth. Supplemen- tation of 2.587. or 108o human serum with 100 IU of D-a-tocopherol (vitamin E)/ml or with a 5% red cell extract was not beneficial. The red cell extract caused extensive clumping of the erythrocytes in the cultures, but was not otherwise deleterious. Aspects ofhumnan serum requirements The first continuous cultures ofP.falciparwn were grown in RPMI 1640 medium supplemented with 150 ml/litre of human type AB Rh + serum (1). This con- centration of serum was later reduced to 100 ml/Iitre. In some attempts at serum replacement or supplemen- tation, the concentration of human serum was reduced to 25 ml or 50 m/llitre when the supplements were added to see if these substances would make it possible to economize on serum. It soon became ap- parent that fresh human serum varied greatly from lot to lot with regard to its ability to promote cultivation of P.fakciparurn. Data pertaining to the ability of different lots of human sera to support cultures of P.falciparum at different concentrations are summarized in Table 2. Some sera, such as A-32, A-49, and A-50 were nearly as nutritious at 50 ml/litre concentrations as they were at 100 ml/litre, which was our standard concentration. Some sera, such as A-32 and A-58 promoted excellent parasite growth even at concentrations as low as 25 mi/litre. In other serum samples, namely A-17, A-23, A-43, anid A-47, concentrations of 50 ml/litre sup- Table 2. Assuming the concentration of serum in medium of 100 ml/litre to be "standard", this table compares the growtha of P. falciparum in different serum concentrations against the standard for 9 different samples of human type A Rh + serum Serumn concentrabon imlllitrel Serum 25 50 75 100 150 200 sarnple (standard) A-32 312 (681 460 (100) 444 (971 460 (1001 474 (1031 A-43 385 (591 639 (99, 648 t(100) 730 (113) A-47 500 (51) 749 (91) 825 (100) 710 (86) 638 177) A-17 186 (46) 391 (100) A-23 286 (40) 712 1100) A-49 517 (91) 566 tl00) A-5o 606 81) 1100 300 1531 A-58 472 (691 58 :1001 A-67 9 I1) 29 (5) 635 (1001 a The dataare repornedas numberof parasites per 10 OO0erylhrocyies (med,nfrom4Petrioishesi,. Iale gu-esm parenthesesgive ne growth asa percentageofthat in the standard 29 J. B JENSEN ported growth rates at approximately one-half the level seen with the standard 100 ml/litre concen- tration. One serum, A-67, was especially poor, giving essentially no growth at concentrations of 25 ml and 50 mJ/litre. Generally, when a serum sample sup- ported very good parasite growth at reduced concen- trations it usually did less well at concentrations greater than the 100 ml/litre standard, as seen with sera A-47 and A-48, which at 200 ml/litre concen- tration supported 77% and 53% of the growth seen with the standard. These findings suggest that human serum varies considerably with regard to its ability to support P.falciparum cultures at concentrations of less than 100 mI/litre, but that generally any such deficiencies are not apparent when serum is used at a concentration of 100 mI/litre. It might be of interest to know whether a given donor's serum varies with regard to its culture promoting properties over a 24-h cycle in a way that might be correlated with natural fluctuations in hormones and other factors. It is becoming increasingly common in many blood banks to separate the plasma immediately from the cellular elements ofwhole blood and to use these frac- tions separately. Accordingly, 1 attempted to deter- mine whether plasma containing citrate-phosphate- dextrose preservatives (CPD) could be used in place of serum and whether it was necessary to remove the CPD by dialysis. When 12 ml of CPD-plasma was dia- lysed (membrane pore size equivalent to MW 6000- 8000) against RPMI 1640 medium it would no longer support parasite growth (nor would similarly dialysed serum) whereas the non-dialysed CPD-plasma (clot- ting elements removed by addition of CaCI2) sup- ported growth nearly as well as our standard serum. Even after 4 successive subcultures (20 continuous days) the CPD-plasma gave a growth level equal to or better than 70% of that of standard serum. In ad- dition, erythrocytes that were stored (4°C) in 90% haematocrit were still good for cultivation of P.falci- parum for up to 21 days. Erythrocytes stored at 40-50%o haematocrit, however, were good for culture purposes for up to 30 days or more (2). These findings indicate that one could take a unit of freshly drawn blood, separate the plasma and cells (to 90%o haema- tocrit), precipitate the clotting elements by addition of CaCl2, and use both fractions for P.faiparum cultures. The discovery that dialysed serum would not sup- port parasite growth may prove to be a useful tool for elucidating the nutritional qualities of human serum in P.faliparum cultures. In one experiment, serum was dialysed against distilled water containing 25 mmol/litre HEPES and the dialysate was lyophilized and then reconstituted in RPM) 1640 without HEPES. This medium was divided into two aliquots, one of which received a poition of the dialysed serum (the non-dialysable fraction). All fractions were then tested to see which, if any, would support parasite growth. Although none of the fractions or combi- nations supported the cultures as well as the non-dia- lysed controls, significant differences were noted be- tween the various fractions and combinations. The dialysed serum did not support parasite growth, whereas tbe lyophilized and reconstituted dialysate produced 3 times more parasites over 96 h. The com- bination of reconstituted dialysate and the non-dia- lysable fraction of the dialysed serum produced a 9-fold increase in parasites during the same period. These findings suggest that low (less than 8000) mol- ecular weight components, not supplied by RPMI 1640 medium, are necessary for cultivation ofP.falci- parum. Such components might include trace ele- ments, polypeptides, or small protein molecules, such as some hormones. Such a material may not necess- arily be a direct requirement of the parasite itself, but may offer physiological support to the erythrocyte, which in turn, supports the parasite. ACKNOWLEDGEMENTS It is a pleasure to acknowledge Mi Hcriberto Borrero for technical assistance and Mrs Diane Greene for her help with the manuscript. RJSUM1 COMMENT SATISFAIRE LES BESOINS EN SERUM POUR LA CULTURE CONTINUE DE PLASMODIUM FALCIPARUM Dans l'espoir de trouver un substitut ou uncompltment au strum humain requis pour la culture continue de Plasmo- dium falciparum, les strums de differents animaux ont Wtt mis A 1'essai, i savoir )es serums de bceuf Mtal frais, de mouton, de cheval, de bIeut adulte, de veau nouveau-nt, de porc, les serums humains fournis par le commierce sous forme congelte ou lyophilisee, las6rumalbunune- humaine ou bovine-exempte d'acides gras, un extrait de globules 30 CONTINUOUS CULTIVATION OF P. rALCIPARUM, 31 rouges, Physiogel, et un ersatz de s6rum compose de Bacto- peptone, Yeastolate, hydrolysat de lactalbumine, PVP et insuline bovine. Aucun de ces produits n'a donn6 des resul- tats comparables A ceux obtenus avec lc s&rum humain, et aucun n'a permnis d'konomnser cc derruer. Un plasma con- tenant du citrate-phosphate-dextrose (CPD) s'est r&v6I6 pTeSqU'aUSSi satisfaisant que le serum, mais un plasma ou un strum dialyscs ne peuvenl servir A entretenir une culture de P.falciparum. Les propri&tts du serum humain frais pour l'entretien des culturcs varient d'un lot i I'autre, nolamment lorsque la concentration du serum dans lc milieu est inf6- ricure A 10%o. Mais, A une concentratLon de 10%ae, la plupart des serums ont prouv6 leur aptitude A l'entretien des cultures. REFERENCES 1. TRAGER, W. & JENSEN, J. B. SCienCe, 193: 673-675 (1976). 2. JENSEN, J. B. & TRArER, W. Journal of parasitology, 63: 883-886 (1977). 3. JENSEN, J. B. & TRAGER, W. A merican journal of tropi- cal medicine and hygiene, 27: 743-746 (1978). 4. LASFARQUES, E. Y. ET AL In vitro, 8: 494-500 (1973). 5. HONN, K. V. ET AL. Proceedings of thre Societyfor Ex- perimental Biology and Medicine, 149: 344-347 (1975). 6. SIDDIQUI, W. & RICHMOND-CRUM, S. M. Journal of parasrtology, 63: 583-584(1977).