Short communication: Lymphocyte proliferative responses in cattle naturally infected with digital dermatitis consist of CD8+ and γδ-T cells but lack CD4+ T cells.

Digital dermatitis is an infectious disease of cattle and the leading cause of lameness. This disease is complicated by the reoccurrence of the lesions and the observation of lesions on more than one limb at different time points, indicating infection may not result in a protective immune response. The objective of this study was to characterize the peripheral blood cellular response in naturally infected and naïve cattle to bacterial antigens derived from pathogens associated with digital dermatitis lesions. Peripheral blood mononuclear cells were isolated from dairy cattle identified as having active or chronic lesions during routine hoof-trimming. Following bacterial antigen stimulation, cells were analyzed for proliferation and phenotype by flow cytometry, and culture supernatants were analyzed for IFN-γ secretion. Digital-dermatitis-infected animals had greater serum antibody titers to treponemal antigens, higher percentages of proliferating CD8+, γδ-T cells, and B cells, and increased IFN-γ secretion in vitro when compared with responses of naïve animals. No increase in proliferation of CD4+ T cells was detected in infected or naïve cattle. Although CD8+ and γδ-T cell responses may be antigen specific, the memory nature or long-lived response is yet unknown. The lack of responsiveness of CD4+ memory cells to treponemal antigens could explain the high rate of reoccurrence of digital dermatitis in infected animals.