Sequence heterogeneity in PfMSP-119 protein, a leading vaccine candidate antigen for vaccination against erythrocytic stages of P. falciparum, may compromise its use as a vaccine candidate antigen. Based on sequence variations among different laboratory and field isolates, two prototypic alleles of P. falciparum MSP-1 represented by PfMAD20 and PfK1/Wellcome have been described in isolates from Africa, Asia and Latin America [15, 16, 24, 25]. The C-terminal 19 kDa region of MSP-1 (PfMSP-119) is a highly conserved region showing amino-acid alterations at only five positions out of 102 residues. Based on these alterations, about ten different PfMSP-119 allelic forms have been predicted in different field isolates across the world .
Here, sequence diversity in MSP-119 region, among Indian P. falciparum field isolates in malaria endemic villages in same geographical location was investigated. Some of these isolates have been previously analysed for sequence variations in the receptor binding domain of Pf EBA-175 . Sequence data of the 16th block of MSP-1 revealed that all the isolates were of PfMAD20 type. The present study's results and the data obtained from previous studies clearly demonstrate that MAD20 allelic form predominates among Indian PfMSP-1 isolates from different geographical locations [18, 20, 30]. In the 17th block, seven allelic forms was observed. Of these PfMSP-119 alleles, two alleles (E-TNR-L & E-TSR-F) have not been reported earlier, however, Qari et al (1998) have earlier predicted E-TNR-L. These allelic forms seem to have generated due to intragenic recombination between the two prototypic alleles as suggested earlier because all the substituted amino acids belonged to either one or other allelic form.
To know whether these five amino acid substitutions in the PfMSP-119 region generate variant specific immune responses in P. falciparum infected patients, three PfMSP-119 variants (Q-KNG-L, E-KNG-L & E-TSR-F) were expressed in an E. coli expression system with an N-terminal His tag. Proteins were purified on a Ni2+-NTA column. All the three PfMSP-119 recombinant forms were recognized well by an invasion inhibitory monoclonal antibody (12.10) and anti-PfMSP-119 (Q-KNG-L) polyclonal antibody. An earlier study  showed that the yeast secreted recombinant MSP-119 variants are recognized by polyclonal antibody raised against baculoproduced MSP1 (E-KNG) variant and by mAB 5B1 that is known to inhibit invasion. Recognition of all the PfMSP-119 variants by mAB 5B1 and 12.10 suggest that some of these protective epitopes are not variant specific. A preliminary investigation of protective type IgG1/IgG3 antibody response to Q-KNG-L sequence variant was carried out and significant IgG1/IgG3 responses were observed in the infected sera (data not shown). The frequency of IgG1 responses in different sera was quite similar while a significant difference was seen within IgG3 responses as previously shown .
To determine the presence of variant specific antibodies during the natural infection, sera from BK3-62, CM3-56 and PP3-18 patients were immuno-depleted using each of the three recombinant antigens and the depleted sera were analysed for reactivity with three variant antigens. Results demonstrated that sera from P. falciparum infected patients do contain cross-reactive as well as PfMSP-119 variant specific antibodies. At present, it is difficult to say up to what extent these variant specific antibodies contribute to protective immune responses. Another aspect that requires probing is the contribution of historic infections with parasite of different sequence types. However, data from a number of challenge studies have also provided evidences for the variant specific immune responses. Immunization of mice with recombinant PyMSP-119 protected mice against homologous but not heterologous sporozoite or blood stage challenges [32, 33]. In some cases where some level of protection to heterologous malarial challenge was seen, it was significantly lower than the homologous challenge [6, 34].