Optimizing the Protection of Cattle against Escherichia coli O157:H7 Colonization through Immunization with Different Combinations of H7 Flagellin, Tir, Intimin-531 or EspA

Enterohemorrhagic Escherichia coli (EHEC) are important human pathogens, causing hemorrhagic colitis and hemolytic uraemic syndrome in humans. E. coli O157:H7 is the most common serotype associated with EHEC infections worldwide, although other non-O157 serotypes cause life-threatening infections. Cattle are a main reservoir of EHEC and intervention strategies aimed at limiting EHEC excretion from cattle are predicted to lower the risk of human infection. We have previously shown that immunization of calves with recombinant versions of the type III secretion system (T3SS)-associated proteins EspA, intimin and Tir from EHEC O157:H7 significantly reduced shedding of EHEC O157 from experimentally-colonized calves, and that protection could be augmented by the addition of H7 flagellin to the vaccine formulation. The main aim of the present study was to optimize our current EHEC O157 subunit vaccine formulations by identifying the key combinations of these antigens required for protection. A secondary aim was to determine if vaccine-induced antibody responses exhibited cross-reactive potential with antigens from other EHEC serotypes. Immunization with EspA, intimin and Tir resulted in a reduction in mean EHEC O157 shedding following challenge, but not the mean proportion of calves colonized. Removal of Tir resulted in more prolonged shedding compared with all other groups, whereas replacement of Tir with H7 flagellin resulted in the highest levels of protection, both in terms of reducing both mean EHEC O157 shedding and the proportion of colonized calves. Immunization of calves with recombinant EHEC O157 EspA, intimin and Tir resulted in the generation of antibodies capable of cross-reacting with antigens from non-O157 EHEC serotypes, suggesting that immunization with these antigens may provide a degree of cross-protection against other EHEC serotypes. Further studies are now required to test the efficacy of these vaccines in the field, and to formally test the cross-protective potential of the vaccines against other non-O157 EHEC.

肠出血性大肠杆菌（Enterohemorrhagic Escherichia coli, EHEC）是一类重要的人类致病菌，可引发人类出血性结肠炎与溶血性尿毒综合征。大肠杆菌O157:H7是全球范围内与EHEC感染关联最密切的常见血清型，不过其他非O157血清型也可引发危及生命的感染。牛是EHEC的主要储存宿主，旨在限制牛体内EHEC排泄的干预策略，预计可降低人类感染风险。
我们此前已证实，使用源自EHEC O157:H7的Ⅲ型分泌系统（type III secretion system, T3SS）相关蛋白EspA、紧密素（intimin）与Tir免疫犊牛，可显著减少实验定植犊牛的EHEC O157排菌量；且在疫苗配方中添加H7鞭毛蛋白，能够增强免疫保护效果。本研究的主要目标为优化现有EHEC O157亚单位疫苗配方，通过筛选确定保护所需的关键抗原组合；次要目标则是探究疫苗诱导的抗体应答是否具备与其他EHEC血清型抗原发生交叉反应的潜能。
免疫EspA、紧密素与Tir可降低攻毒后EHEC O157的平均排菌量，但未改变定植犊牛的平均比例。去除Tir会导致排菌时长较其余所有组别更长；而用H7鞭毛蛋白替换Tir，则可实现最优保护效果，无论是在降低平均EHEC O157排菌量还是定植犊牛比例层面均是如此。
使用重组EHEC O157 EspA、紧密素与Tir免疫犊牛，可诱导产生能够与非O157 EHEC血清型抗原发生交叉反应的抗体，提示针对这些抗原的免疫可为其他EHEC血清型提供一定程度的交叉保护。
当前仍需开展进一步研究，以在田间场景下验证这些疫苗的效力，并正式评估该疫苗针对其他非O157 EHEC的交叉保护潜能。