Roles of the Amino Terminal Region and Repeat Region of the <em>Plasmodium berghei</em> Circumsporozoite Protein in Parasite Infectivity

<div><p>The circumsporozoite protein (CSP) plays a key role in malaria sporozoite infection of both mosquito salivary glands and the vertebrate host. The conserved Regions I and II have been well studied but little is known about the immunogenic central repeat region and the N-terminal region of the protein. Rodent malaria <em>Plasmodium berghei</em> parasites, in which the endogenous CS gene has been replaced with the avian <em>Plasmodium gallinaceum</em> CS (<em>PgCS</em>) sequence, develop normally in the <em>A. stephensi</em> mosquito midgut but the sporozoites are not infectious. We therefore generated <em>P. berghei</em> transgenic parasites carrying the <em>PgCS</em> gene, in which the repeat region was replaced with the homologous region of <em>P. berghei</em> CS (<em>PbCS</em>). A further line, in which both the N-terminal region and repeat region were replaced with the homologous regions of <em>PbCS</em>, was also generated. Introduction of the <em>PbCS</em> repeat region alone, into the <em>PgCS</em> gene, did not rescue sporozoite species-specific infectivity. However, the introduction of both the <em>PbCS</em> repeat region and the N-terminal region into the <em>PgCS</em> gene completely rescued infectivity, in both the mosquito vector and the mammalian host. Immunofluorescence experiments and western blot analysis revealed correct localization and proteolytic processing of CSP in the chimeric parasites. The results demonstrate, <em>in vivo,</em> that the repeat region of <em>P. berghei</em> CSP, alone, is unable to mediate sporozoite infectivity in either the mosquito or the mammalian host, but suggest an important role for the N-terminal region in sporozoite host cell invasion.</p>