Cytokine mRNA expressed in tuberculin skin test biopsies from BCG‐vaccinated and Mycobacterium bovis inoculated cattle

Summary To obtain a better understanding of the delayed‐type hypersensitivity reaction to Mycobacterium bovis, we measured the expression of cytokine mRNA from tuberculin skin test biopsies of cattle. Non‐vaccinated and BCG‐vaccinated cattle were inoculated intratracheally with a low dose of virulent M. bovis or sham‐inoculated and 20 weeks later were skin tested with tuberculin. At necropsy 1–2 weeks later, tuberculous lesions were found in six of the nine non‐vaccinated and three of the nine BCG‐vaccinated animals. All of the lesioned and the majority of the non‐lesioned M. bovis inoculated cattle showed a distinct skin swelling response to tuberculin, irrespective of vaccination. However, cattle with tuberculous lesions displayed larger skin swelling responses than non‐lesioned cattle. Tuberculin‐induced expression of IFN‐γ, IL2, IL4, IL10 and TNF‐α mRNA occurred in the skin biopsies of all of the lesioned, M. bovis inoculated animals except for an absence of tuberculin‐induced TNF‐α mRNA expression in two animals. A lower proportion of the non‐lesioned M. bovis inoculated cattle displayed tuberculin‐induced expression of the five cytokine mRNA. There was no evidence of a unique pattern of cytokine expression which could be used to distinguish between diseased and protected animals. By 28 weeks after vaccination, the three BCG‐vaccinated, sham‐inoculated cattle displayed minimal skin swelling response to tuberculin, but tuberculin‐induced expression of IFN‐γ, IL2, IL4, TL10 and TNF‐α mRNA was observed in skin biopsies of all of these animals. The pattern of cytokine mRNA expressed in the tuberculin skin test reaction site of cattle appeared to be typical of a mixed T helper O‐like cell phenotype response and there was no clear‐cut relationship between the size of the tuberculin skin swelling response and cytokine mRNA expression.