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Ated using the bl2seq program for the 500 most abundant peptides. The protein Mast Cell Carboxypeptidase A Precursor (CPA3), which had the highest final score, produced matches to the 52 10781694 peptides with the sum of scores equal 917.3. Of these peptides more than half (27) were aligned against a single site with the cumulative score equal 575.4 (Figure 5). This score was comparable to the scores corresponding to the major epitopes recognized by mouse sera on the PAP molecule. To test if the serum antibodies of the melanoma patient will recognize the whole CPA3 protein we analyzed the 293T cells lysate (purchased fromthe origene.com) overexpressing the recombinant CPA3 protein using Western blotting. As shown in Figure 5, serum from the melanoma patient but not from the healthy donor reacted with a protein band corresponding to the size of the CPA3 protein in the 293T cells lysate overexpressing the recombinant CPA3 protein. No reactivity was detected in the control 293T cells lysate.DiscussionWe have demonstrated that the lists of peptide sequences generated by the NGS of DNA from RPPDL selected for binding to serum antibodies can be used for identifying protein antigens recognized by serum antibodies, when the humoral immune response is at least partially directed to linear epitopes. The simple BLAST-based algorithm of the SAS strategy generated a list of candidate proteins that contained multiple sequence matches to different peptides from the list. The PAP protein used for immunization of mice was one of the top fifteen candidates for the three out of four immunized mice. It is noteworthy that the proteins that were ranked higher than the PAP protein shared the same motif with the PAP, suggesting that antibodies against the PAP are likely to cross-react with the identified candidate proteins. Moreover, for the two out of three sera that showed a response to linear epitopes of the PAP protein, the score for the PAP epitope was the highest among the similar epitopes in other proteins thus making the PAP protein the top candidate for the target of immune response. In addition to identifying the candidate proteins recognized by antibodies, the SAS also exposed quantitatively the “fineSerum Antibody Repertoire ProfilingFigure 3. Motifs identified by MEME software for the top candidate antigens selected for the PAP1 and PAP3 antisera. Motifs for the antigens are related to the motifs identified for the corresponding top 500 the most abundant peptides. doi:10.1371/journal.pone.0067181.gstructure” of humoral immune response against the antigen. SAS not only can identify the epitopes recognized by antibodies on the protein sequence but also can show the 86168-78-7 chemical information strength of the response against these epitopes, since the number of peptides containing the epitope sequence 307538-42-7 should be proportional to the titer of antibodies.Table 2. Epitopes predicted by the SVMTriP inside the PAP protein.Rank 1 2Location 334?53 136?55 360?Epitope ETQHEPYPLMLPGCSPSCPL LLWQPIPVHTVPLSEDQLLY VGPVIPQDWSTECMTTNSHQScore 2.505 1.152 1.The table shows the hypothetical epitopes predicted by the SVMTriP software on the PAP protein. In bold are the sequences that produce matches to the peptides recognized by serum antibodies of the Pap3 serum. doi:10.1371/journal.pone.0067181.tThe antibody responses against the linear epitopes of the human PAP protein appear to be limited in mice only to few epitopes. Each of the PAP1, PAP2 and PAP3 antisera recognized only a single major linear epitop.Ated using the bl2seq program for the 500 most abundant peptides. The protein Mast Cell Carboxypeptidase A Precursor (CPA3), which had the highest final score, produced matches to the 52 10781694 peptides with the sum of scores equal 917.3. Of these peptides more than half (27) were aligned against a single site with the cumulative score equal 575.4 (Figure 5). This score was comparable to the scores corresponding to the major epitopes recognized by mouse sera on the PAP molecule. To test if the serum antibodies of the melanoma patient will recognize the whole CPA3 protein we analyzed the 293T cells lysate (purchased fromthe origene.com) overexpressing the recombinant CPA3 protein using Western blotting. As shown in Figure 5, serum from the melanoma patient but not from the healthy donor reacted with a protein band corresponding to the size of the CPA3 protein in the 293T cells lysate overexpressing the recombinant CPA3 protein. No reactivity was detected in the control 293T cells lysate.DiscussionWe have demonstrated that the lists of peptide sequences generated by the NGS of DNA from RPPDL selected for binding to serum antibodies can be used for identifying protein antigens recognized by serum antibodies, when the humoral immune response is at least partially directed to linear epitopes. The simple BLAST-based algorithm of the SAS strategy generated a list of candidate proteins that contained multiple sequence matches to different peptides from the list. The PAP protein used for immunization of mice was one of the top fifteen candidates for the three out of four immunized mice. It is noteworthy that the proteins that were ranked higher than the PAP protein shared the same motif with the PAP, suggesting that antibodies against the PAP are likely to cross-react with the identified candidate proteins. Moreover, for the two out of three sera that showed a response to linear epitopes of the PAP protein, the score for the PAP epitope was the highest among the similar epitopes in other proteins thus making the PAP protein the top candidate for the target of immune response. In addition to identifying the candidate proteins recognized by antibodies, the SAS also exposed quantitatively the “fineSerum Antibody Repertoire ProfilingFigure 3. Motifs identified by MEME software for the top candidate antigens selected for the PAP1 and PAP3 antisera. Motifs for the antigens are related to the motifs identified for the corresponding top 500 the most abundant peptides. doi:10.1371/journal.pone.0067181.gstructure” of humoral immune response against the antigen. SAS not only can identify the epitopes recognized by antibodies on the protein sequence but also can show the strength of the response against these epitopes, since the number of peptides containing the epitope sequence should be proportional to the titer of antibodies.Table 2. Epitopes predicted by the SVMTriP inside the PAP protein.Rank 1 2Location 334?53 136?55 360?Epitope ETQHEPYPLMLPGCSPSCPL LLWQPIPVHTVPLSEDQLLY VGPVIPQDWSTECMTTNSHQScore 2.505 1.152 1.The table shows the hypothetical epitopes predicted by the SVMTriP software on the PAP protein. In bold are the sequences that produce matches to the peptides recognized by serum antibodies of the Pap3 serum. doi:10.1371/journal.pone.0067181.tThe antibody responses against the linear epitopes of the human PAP protein appear to be limited in mice only to few epitopes. Each of the PAP1, PAP2 and PAP3 antisera recognized only a single major linear epitop.

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