Canine peripheral blood CID4+CD8+ double-positive T cell subpopulations exhibit distinct T cell phenotypes and effector functions

Canine peripheral blood CD4(+)CD8 alpha(+) double-positive (dp) Tcells represent a small population of T lymphocytes that are not only derived from single-positive (sp) CD4(+) but also from CD8(+) Tcells. Dp Tcells can be subdivided in three different subpopulations according to their expression levels of CD4 and CD8 alpha, i.e. the CD4(dim)CD8 alpha(bright), CD4(bright)CD8 alpha(bright), and the CD4(bright)CD8 alpha(dim) subsets. Previously we could show that total canine peripheral CD4(+)CD8 alpha(+) dp Tcells have features of activated Tcells. Here we demonstrate that the individual dp Tcell subsets distinctly differ in their activation status and phenotype. Thus, among the dp Tcell subsets the CD4(dim)CD8 alpha(bright) subpopulation has the lowest and the CD4(bright)CD8 alpha(bright) subset the highest frequency of CD25(+) cells pointing to the CD4(bright)CD8 alpha(bright) subset with the highest activation status. Consistent with that, analysis of CD44 vs. CD62L expression demonstrates that the CD4(bright)CD8 alpha(bright) subset contains the highest frequencies of effector-memory Tcells (TEM). Upon in vitro stimulation, the CD4(bright)CD8 alpha(bright) and CD4(dim)CD8 alpha(bright) dp Tcell subsets show the highest frequencies of IFN-alpha producing cells. Expression of granzyme B was found exclusively in the CD4(dim)CD8 alpha(bright) dp Tcell subset indicating cytotoxic potential of this dp Tcell subset. Furthermore, T-bet expression was restricted to the CD4(dim)CD8 alpha(bright) subset, while Foxp3(+) regulatory Tcells were detected in the CD4(bright)CD8 alpha(dim) dp Tcell subset. In conclusion, canine dp Tcells are phenotypically and functionally heterogeneous consistent with the diverse characteristics of their single-positive CD4(+) and CD8(+) Tcell progenitors. The data provide the basis for future in vivo studies to elucidate the role of individual dp Tcell subsets in host defense and/or immune pathological diseases of dogs. Copyright (C) 2017 Elsevier B.V. All rights reserved.