The characterization of parasite and parasite-induced host gene products in Plasmodium yoelii infected livers

Abstract

Amongst the various stages of the malaria parasite's life cycle, the liver stage has become an attractive target for vaccine design. The most convincing evidence for choosing the infected hepatocyte as the prime target for vaccine development is that the immunity induced by irradiated sporozoite vaccines is primarily directed against the infected hepatocytes. To date, only a handful of liver stage proteins have been identified. With the goal of understanding what goes on during the liver stage infection, the identification and characterization of both parasite and parasite-induced host responses were carried out. A modified differential display (DD) technique was used to identify P. yoelii liver stage gene products. Primers with 40% G+C produced the most number of DD bands exclusively expressed in the infected liver sample (d-DD bands). Selected d-DD bands were verified to be of parasite origin by PCR and sequence alignment. Five clones, designated P2T1L5, P2T1L6, P2T6L11, P2T7L12 and P2T7L13, were demonstrated to be P. yoelii liver stage gene products. P2T1L5 was also expressed during the sporozoite stage of the life cycle and P2T1L6 and P2T6L11 were present during blood stage cycle. Changes in parasite-induced host gene expression were characterized by a semi-quantitative RT-PCR assay. Ring3, semaphorin subclass 4 member G (sema 4g), glutamylcysteine synthetase (GCS) and p45 nuclear factor erythroid 2 (p45 NF E2), tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) expressions were quantitated. A systemic upregulation in Ring3 expression level similarly found in Toxoplasma infected livers, was suggestive of a generic immune response by the host to invading parasites. The remaining gene transcripts' upregulation were either tissue- or parasite-specific. These data initiated the investigation of the liver stage of Plasmodium life cycle. The five novel P. yoelii liver stage gene products identified may provide more information into the biology of liver stage parasite and to understand the parasite gene products that required by the parasites during the liver stage of its life cycle. Similarly, the upregulation of the host gene expression during Plasmodium liver stage infection suggests that host-parasite interactions are present and these findings may help understand how parasites evade host immune responses.