| Tuberculosis (TB) is a kind of chronic communicable diseases. It has been accepted that the protective immunity of human against TB is mainly attributable to the cell-mediated immunity especially the T lymphocytes. But there are increasing opinions existed which argue that the B cell and humoral immunity are key part of immune system against TB. B cell could work as the antigen presenting cell, secrets antibodies and cytokines which play a regulatory role in immune response. B cell epitope is external motif of antigen, which could be recognized by B cell, and then stimulate B cells to produce antibodies or cytokines regulating immune response.Bacille Calmette-Guerin (BCG), is an attenuated derivative of Mycobacterium bovis. And so far BCG has been the only permitted vaccine against TB, even though there are debates focusing on the protection of BCG against TB. The mechanisms of BCG against TB remain poorly understood. Additionally there have many daughter strains of BCG with varied protective efficacy which result from the widespread use of BCG in the global scope. After this period, mother BCG strain produced a number of different clones, which present different protective antigen, different physical and chemical properties, different immune characteristics and even different protective efficacy.To in-depth understand the host humoral immune system, and provide useful data for the development of TB vaccines, we conduct study in different BCGs and 180 clinical isolates of Mycobacterium tuberculosis based on B cell epitopes. And then all raw data were analyzed using a variety of biological analysis software to illustrate the B cell epitopes distribution in BCG strains and MTB strains, clarify the varing protection in BCG strains and potential impact to different MTB strains causing disease.The results showed that 399 currently known B cell epitopes were encoded by 81 genes. Both in BCGs and in clinical isolates of Mycobacterium tuberculosis, most of B cell epitopes were highly conservative.Among 13 BCG genomes,321 epitopes didn’t show any changes. All B cell epitopes could be classified into 5 groups. The Group 1 contained 321 epitopes which were highly conservative in 13 BCGs. The Group 2 contained 15 epitopes which showed same SNPs in 13 BCGs. The Group 3 contained 26 epitopes which were lost in 13 BCGs. The Group 4 contained 13 epitopes which were lost in part of BCGs. The Group 5 contained 23 epitopes which were specific to BCGs respective. And BCG-Tokyo 172 and BCG-China, containing 357 conservative epitopes, possessed the most number of conservative epitopes. It could induce stronger anti-TB humoral immune response. The original BCG gradually lost part of B cell epitopes when it was spread to the world facing different environmental pressure, and then evolved into different BCG strains with varing protection. Taking into account this change, we could improve the TB vaccine by increasing more B cell epitopes in candidate vaccines.Among 180 clinical isolates of Mycobacterium tuberculosis,293 B cell epitopes didn’t have any change in coding sequence,104 B cell epitopes contained DNA change in coding sequence, and 78 epitope changed finally. Among 150 clinical isolates which having full length of B cell epitope coding genes, there was no any epitope lost. And 1 isolate had 12 changed epitopes, it was the isolate harboring most number of changed epitopes. There were 10 isolates having 397 B cell epitopes without any change. About 99.33%(149/150) clinical isolates had change epitopes less than 10. The epitopes distributed in clinical isolates were highly conservative.Based on the analysis of dN/dS value of B cell epitope coding genes, we found that genes coding B cell epitopes were conservative in all clinical isolates. Among 80 coding genes, only 27.5% genes show values of dN/dS>1, which meant genes were under pressure of positive selection. And the remaing genes were under purifying selection, mutuations were concentrated in non epitope region. Meanwhile we found some strains had DNA fragment insertion or deletion in 4 genes, which may have influence on the function of the proteins.The hyper conservative B cell epitopes and coding genes would make Mycobacterium tuberculosis being more sensitive to host and widely spread among the crowd. Both genes and B cell epitopes coding/non-coding region, in most cases, the dN/dS values are less than 1 which tend to be under purifying selection reminding that new tuberculosis vaccines should explore more variable regions of the MTBC. |
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