Homologues Of Macrophage Migration Inhibitory Factor Identified From Malaria Parasites

Malaria is one of the most severe infectious diseases in the world and responsible for 2 millions of deaths every year.The pathogenesis of malaria is complicated,multifactor and poorly understood.And up to now,mechanisms for malaria parasite escaping from host immune attack have not been understood clearly.No malaria-derived molecule has been found in regulating the host immune response directly.In this research work,we identified two malaria-derived MIF homologues(PfMIF and PyMIF) from Plasmodium falciparum and Plasmodium yoelii,did preliminary functional analysis and investigated their levels in both human and rodent animal.First,we have identified the sequence and structure character of PfMIF and PyMIF molecules.The Pfmif gene includes one intron(280bp) and two exons(79bp and 272bp), of which coding a 351 nucleotides Pfmif gene and encodes a 116 amino acid polypeptide. By comparing its amino acid sequences within other 9 organisms,we found that the PfMIF protein sequence is 27%to 30%identical to the MIF family sequences as 17 residues are highly conserved especially the proline at position 2,which is critical for isomerase function of vertebrate MIF.In the evolutional tree analysis,Pfmif has relative far relationship to the mammalian and nematode MIF.The three-dimensional structures of PfMIF and PyMIF predicted by Geno3D soft were same to HuMIF to have two antiparallel a-helixes and sixβ-sheets,forming aβαβββαββstructure(1).We also identified the polymorphism of Pfmif genome in Plasmodium falciparum from 9 strains and there is no difference in exon and only a microsatellite in the middle of the single intron.The gene sequence of Pymif identified from P.yoelii is 484bp,encoding a 351bp open reading frame.The amino acid sequence of it is highly conserved,76%identical and 91% positive to that of Pfmif.Besides these,we found 7 homologues of mif genes from other 7 Plasmodium species and there is no much different intraspecies.Specially,all these sequences have 4 cysteines and forming a typical structure of CC chemokine family:CC (-C-C).However,all of them have no C-X-X-C motif,which is the sequence feature of MIF family.Then we have made the functional comparison of PfMIF with HuMIF in enzymatic and chemotactic activities.When we identified the tautomerase of PfMIF with three substrates,_L-Dopachrome methyl ester,Phenylpyruvate and p-Hydroxyphenylpyruvate,we found that the PfMIF was active in tautomerizing all of the substrates,but the specific activities of them were less than 5%of the HuMIF.The mutant PfMIF didn't have any measurable catalytic activity on the three substrates.For chemotaxis assay,we found that PfMIF had such a significant chemotactic activity to human monocytes as the efficient concentration was as low as 0.1ng/ml,which was greatly comparable to HuMIF. Furthermore,the chemotactic index of PfMIF was obviously not dose-dependent.For analysis of the role of malaria derived MIF in infection,we have made some primary functional research of them in vivo and in vitro.PfMIF molecule can be detected in the supernatant of cultured PRBC in vitro without stimulated and the level of it increases from 0 to 48h after synchronization.When we transfected the pEGFP vector with Pfmif encoding region into Cos7 cells,the rPfmif molecules can also be detected in the supernatant and the secretion level increased from 24h to 72h after transfection.By PfMIF-specific sandwich ELISA,3 serum samples of Plasmodium falciparum infected-patients were confirmed to have PfMIF molecule.Besides the study of PfMIF,we have detected the transcription and expression of PyMIF in vivo.After we infected mice with P.yoelii,the mRNA of Pymif was detectable from day 4^th,when the parasitemia was about 3%,and the up-regulation of Pymif mRNA in peripheral blood of mice remained till death.Meanwhile,the PyMIF protein was detected from day 4,enhanced each day till day 7^th when the parasitemia was about 30%but fall down on the day of death(day 8).Since the first nematode-derived MIF homologue was reported in 1998,more and more parasite-derived molecules were paid attention.It provides a new conception of study the relationship between host and parasite.For malaria parasite which is in symbiosis with host during a long period of evolution,this definitely exists a mechanism that benefits for them and also did not lead the host died out.Here we first reported the primary functions of the homologues of MIF in malaria parasite in vitro and in vivo.Our results provide a new sight in escaping host immune attack and also provide a new clue in the research of malaria pathogenesis.