Defining the impact of colonisation with Shiga toxin positive E. coli O157 on adaptive immunity in cattle
Beckett, Amy Elizabeth
Shiga producing E. coli (STEC) O157 is a zoonotic pathogen. In humans STEC O157 causes bloody diarrhoea and potentially fatal renal failure. Cattle are the major reservoir, where bacteria are limited to the intestinal tract and do not cause clinical signs of disease. Previous studies indicate that shiga toxins produced by STEC O157, suppress STEC-specific cellular immune responses in vivo. This study aimed to initially examine the humoral immune response in cattle following natural challenge and the effects of a toxoid vaccination on this humoral STEC specific-immune response. We determined a statistically significant suppression in Tir specific IgA in STEC O157 positive cattle compared to O157 negative cattle but not in super shedding cattle. Following toxoid vaccination we determined a significant increase in flagellin specific IgG1 antibody levels in toxoid vaccinated animals despite lower numbers of positive faecal samples compared to placebo vaccinated controls. These results suggest that shiga toxins produced by STEC O157 are actively suppressing the STEC specific immune response in natural colonisation. To clarify this suppression further calves were orally challenged with STEC O157 (either a PT21/28 Stx2c+, PT32 Stx2c+ or PT21/28 Stx2a+Stx2c+ strain) and their STEC specific immune responses monitored. STEC specific systemic antibody responses were variable and weak in some cases. STEC specific local antibody responses were only significantly increased following challenge with the PT21/28 Stx2a+Stx2c+ challenge. Transcripts for genes associated with immune responses, and in particular B cell activation, at the terminal rectum were analysed by reverse transcriptase quantitative PCR. Suppression of IL2RA transcripts was observed in calves challenged with PT21/28 Stx2a+Stx2c+ compared to control calves but not with the other two STEC O157 strains tested. This study also aimed to determine the effects of cattle colonisation with STEC O157 on the immune response to a non-bacterial T-cell dependent antigen, ovalbumin (OVA). Cattle were orally challenged with either a PT21/28 Stx2c+, PT32 Stx2c+ or PT21/28 Stx2a+Stx2c+ strain or unchallenged. Calves were subcutaneously immunised with OVA five days post challenge, on two separate occasions with a two week interval. Lymphocytes from lymph nodes local to the immunisation site demonstrated significantly increased OVA-specific proliferation and OVA-specific activation of CD4+ and CD8+ cells in calves that were challenged with the PT21/28 Stx2c+ strain (but not with the other two challenge strains), compared to unchallenged controls. These results indicate that colonisation with STEC O157 can alter local adaptive immune responses to non-bacterial antigens in a strain dependent manner, unexpectedly enhancing the immune response rather than suppressing it. Circulating T cell responses were unaffected. In conclusion this study provides some further evidence of adaption of the host immune response by STEC O157, which is strain dependent, and variable. It seems unlikely from the data in this study that STEC O157 colonisation is having a major impact on the responses of cattle to other vaccines or infections in the field.