Canine CD4+CD8+ double positive T cells in peripheral blood have features of activated T cells

In dogs a CD4+CD8+ double positive T cell subpopulation exists that has not been phenotypically defined yet. We demonstrate that canine CD4+CD8+ T cells are mature CD1a− and TCRαβ+ T cells. To analyse the activation potential of CD4+CD8+ T cells, PBMC from dogs vaccinated against canine distemper virus (CDV) were re-stimulated with CDV. Upon antigen-specific stimulation, the CD4+CD8+ T cell fraction increases and consists nearly exclusively of proliferated cells. Similarly, other features of activated effector/memory T cells such as up-regulation of CD25 and MHC-II as well as down-regulation of CD62L (L-selectin) were observed in CD4+CD8+ T cells after stimulation. Canine CD4+CD8+ T cells are less abundant, but more heterogeneous than porcine ones, comprising a small proportion expressing the β chain of CD8 in addition to the CD8α chain, like human CD4+CD8+ T cells. In summary, this analysis provides the basis for functional characterisation of the in vivo relevance of CD4+CD8+ T cells in T-cell mediated immunity.