Macrophage migration inhibitory factor (MIF) was the first cytokine to be identified almost 40 years ago. Homologues of MIF have been isolated recently from invertebrates, making it an interesting molecule from an evolutionary as well as functional perspective. The present study represents the first report of MIF homologues in apicomplexan parasites, belonging to the genus Eimeria. A single full-length clone was isolated from Eimeria acervulina that shared between 35 and 38% amino acid identity with MIFs of vertebrates. A MIF cDNA from Eimeria tenella shared 64% amino acid identity with E. acervulina MIF. The mRNA expression was highest in merozoites, whereas developing oocysts and sporozoites expressed low to undetectable levels. Protein expression patterns were nearly identical to that observed by reverse transcriptase polymerase chain reaction (RT-PCR), suggesting strong developmental regulation. Immunofluorescence staining and co-localisation studies of E. acervulina merozoites indicated that MIF is distributed throughout the cytosol, and appears to be concentrated in the apical end of the parasite. The presence of MIF was detected in excretory/secretory (ES) products collected from E. acervulina merozoites, and isoelectric focusing indicated that three MIF isoforms are present in this stage. Phylogenetic analysis revealed that apicomplexan MIF sequences form a sister relationship to MIF-like molecules from Arabidopsis thaliana. © 2006.
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