Susceptibility ofDifferent Strains ofMicetoHepatic Infection withPlasmodium berghei

using an antibody toaP.fakciparum heatshock protein. Themean numberofEEparasites per 100 cm2forC57BL/6 and A/Jstrains was significantly higher thanthatforBALB/c(2,190260, 88 38,and6 2,respectively). The proportion ofinoculated sporozoites transforming intoliver schizonts was 8.2%inC57BL/6and<1% in C3HVHeJ, DBA/1, andSwiss CD-1/ICR mice. Nonspecific inflammatory infiltrates aroundEEparasites were less prevalent inliver sections fromC57BL/6 micethaninthose fromBALB/cmice, whichcontributed tothe decrease indeveloping EE stages inBALB/c mice.Thesedataindicate thattheC57BL/6-P. berghei systemis preferable forinvestigating thebiology andimmunology ofliver stageparasites. Theexoerythrocytic (EE)stageofthemalaria parasite inthe hostliver isthestagebetween thesporozoite, whichoriginates fromthebiteofan infected anopheline mosquito, andthe erythrocytic stage, whichresults fromtherelease ofmerozoites fromhepatocytes. TheEEformsofPlasmodium spp.were the last tobediscovered andremain theleast known(19, 23)due totheinherent difficulties ofexperimentally manipulating a vital organsuchastheliver andtothesmall size andsparse distribution ofthese parasites. Consequently, animal models of experimental malaria were pursued (6). Thamnomys surdaster, a treerat, isthenatural hostofPlasmodium berghei, butitis notamenable toroutine useinthelaboratory. Therefore, these ratshavebeenreplaced bymore manageable strains ofmice. Inbred strains ofmiceareusedtostudy thebasic biology of themalaria parasite, testvaccines, andinvestigate theimmune mechanisms theyelicit. Datagathered fromdifferent mouse strains concerning theirradiated-attenuated sporozoite vac- cine(4,15)were useful fordesigning vaccine trials inhumans (2,3,7-9,17). Previous studies haveshownthatBALB/cmice havea lowsusceptibility toP.berghei hepatic infection; only 0.01%ofinoculated sporozoites infect theliver (13). Thislow susceptibility was correlated withtheability ofBALB/cmiceto elicit acellular inflammatory responseagainst theEEstages of P.berghei. Thisinnate inflammatory responsebeganeliminat- ingdeveloping EE stages as early as 24h post-sporozoite inoculation (13, 20). Because theinnate responseinBALB/cmicecouldinterfere withthestudy ofthecellular protective mechanisms involved insporozoite-induced