The University of Queensland, School of Biological Sciences and Centre for Marine Science, Brisbane, QLD 4072, Australia.
Vet Microbiol. 2013 Feb 22;162(1):151-9. doi: 10.1016/j.vetmic.2012.08.018. Epub 2012 Sep 1.
Streptococcus iniae causes invasive infections in fresh and saltwater fish and occasional zoonoses. Vaccination against S. iniae is complicated by serotypic variation determined by capsular polysaccharide. A potential target for serologically cross-protective vaccines is the M-like protein SiMA, an essential virulence factor in S. iniae that is highly conserved amongst virulent strains. The present study determined how SiMA is regulated and investigated potential as a cross-protective vaccine for fish. Electrophoretic mobility shift suggested that SiMA is regulated by the multigene regulator Mgx via a binding site in the -35 region of the simA promoter. Moreover, expression of simA and mgx was highly correlated, with the highest level of simA and mgx expression during exponential growth under iron limitation (20-fold increase in relative expression compared to growth in Todd-Hewitt broth). Based on these results, a vaccination and challenge experiment was conducted in barramundi (Lates calcarifer) to determine whether SiMA is protective against S. iniae infection and cross-protective against a different capsular serotype. The challenge resulted in 60% mortality in control fish. Formalin-killed bacterins prepared from the challenge strain resulted in 100% protection, whereas bacterins prepared from a serotypically heterologous strain resulted in significantly reduced protection, even when culture conditions were manipulated to optimise SiMA expression. Moreover, recombinant SiMA protein was not protective against the challenge strain in spite of eliciting specific antibody response in vaccinated fish. Specific antibody did not increase oxidative activity or phagocytosis by barramundi macrophages. Indeed incubating S. iniae with antisera significantly reduced phagocytosis. Lack of specific-antibody mediated opsonisation in spite of 100% protection against challenge with the homologous vaccine suggests that other immune parameters result in protection of challenged fish.
无乳链球菌可引起淡水和海水鱼类的侵袭性感染，偶尔也会引发人类动物传染病。无乳链球菌的疫苗接种很复杂，这是由荚膜多糖决定的血清型变异引起的。一种具有血清学交叉保护作用的疫苗的潜在靶标是 M 样蛋白 SiMA，它是无乳链球菌的一种必需毒力因子，在毒力株中高度保守。本研究确定了 SiMA 的调控方式，并研究了其作为鱼类交叉保护疫苗的潜力。电泳迁移率变动表明，SiMA 通过 simA 启动子-35 区的结合位点，由多基因调控因子 Mgx 调控。此外，simA 和 mgx 的表达高度相关，在铁限制下指数生长期的表达水平最高（与在 Todd-Hewitt 肉汤中生长相比，相对表达水平增加了 20 倍）。基于这些结果，在军曹鱼（Lates calcarifer）中进行了疫苗接种和攻毒实验，以确定 SiMA 是否对无乳链球菌感染具有保护作用，以及是否对不同荚膜血清型具有交叉保护作用。对照组鱼的死亡率为 60%。由攻毒株制备的福尔马林灭活菌苗可提供 100%的保护，而由血清型不同的菌株制备的菌苗则提供显著降低的保护，即使在优化 SiMA 表达的培养条件下也是如此。此外，尽管重组 SiMA 蛋白在接种鱼中引发了特异性抗体反应，但它并不能抵抗攻毒株。特异性抗体不能增加军曹鱼巨噬细胞的氧化活性或吞噬作用。事实上，用抗血清孵育无乳链球菌会显著降低吞噬作用。尽管用同源疫苗接种 100%保护免受攻毒，但未观察到特异性抗体介导的调理作用，这表明其他免疫参数导致受攻毒鱼得到保护。
