Strain dependent cellular immune responses in cattle following Escherichia coli O157:H7 colonisation

Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 causes haemorrhagic diarrhoea and potentially fatal renal failure in humans. Ruminants are considered to be the primary reservoir for human infection. Vaccines that reduce shedding in cattle are only partially protective and their underlying protective mechanisms are unknown. Studies investigating the response of cattle to colonisation generally focus on humoral immunity, leaving the role of cellular immunity unclear. To inform future vaccine development, we studied the cellular immune response of cattle during EHEC O157:H7 colonisation. Calves were challenged with either a phage type (PT) 21/28 strain possessing the Shiga toxin (Stx) 2a and Stx2c genes or a PT32 strain possessing the Stx2c genes only. T-helper cell associated transcripts at the terminal rectum were analysed by RT-qPCR. Induction of IFNγ and T-bet was observed, with peak expression of both genes at 7 days in PT32 challenged calves, whilst up regulation was delayed, peaking at 21 days in PT21/28 challenged calves. Cells isolated from gastro-intestinal lymph nodes demonstrated antigen-specific proliferation and IFNγ release in response to Type III secreted proteins (T3SPs); however responsiveness was suppressed in cells isolated from PT32 challenged calves. Lymph node cells showed increased expression of the proliferation marker Ki67 in CD4(+) T-cells from PT21/28, NK cells from PT32 and CD8(+) and γδ T-cells from both PT21/28 and PT32 challenged calves following ex vivo re-stimulation with T3SPs. This study demonstrates that cattle mount cellular immune responses during colonisation with EHEC O157:H7, the temporality of which is strain dependent, with further evidence of strain-specific immunomodulation.