Canine CD4+ CD8+ double-positive T cells can develop from CD4+ and CD8+ T cells

For a long time the expression of the CD4 and CD8 receptor on peripheral blood T cells was thought to be mutually exclusive. However, in canine peripheral blood, similar to other species as swine or human for example, mature CD4(+)CD8(+) double-positive (dp) T cells exist which simultaneously express both surface receptors and have features of activated T cells. Canine CD4(+)CD8(+)dp T cells are heterogeneous and can be divided into three subpopulations by their intensity of CD4 and CD8 alpha expression: CD4(bright) CD8 alpha(bright), CD4(dim)CD8 alpha(bright) and CD4(dim)CD alpha(dim). The number of CD4(+)CD8 alpha(+)dp T cells increases after in vitro stimulation of canine peripheral blood mononuclear cells (PBMC) raising the question of their progenitor(s). Thus, the aim of our study was to characterize the progenitor(s) of canine CD4(+)CD8 alpha(+)dp T cells. By cell tracing experiments we identified both CD4(+) single-positive (sp) and also CD8 alpha(+)sp T cells as progenitors of canine CD4(+)CD8 alpha(+)dp T cells after in vitro stimulation. CD4(+)sp T cells almost exclusively upregulate a CD8 alpha alpha homodimer, whereas CD8 alpha(+)sp T cells can become CD4(+)CD8 alpha beta(+) or CD4(+)CD8 alpha alpha(+). Even in the absence of other cells, highly purified CD4(+)sp T cells can become double-positive upon in vitro stimulation, whereas highly purified CD8 alpha(+)sp T cells fail to do so. However, CD8 alpha(+)sp T cells can additionally express CD4 when stimulated in the presence of CD4(-)CD8 alpha(-) double-negative (dn) cells or more efficiently when stimulated in the presence of CD4(+)sp T cells. Soluble factors secreted by CD4(+)sp T cells are sufficient for the upregulation of CD4 on CD8 alpha(+)sp T cells, but direct cell-cell contact between CD4(+)sp and CD8 alpha(+)sp T cells is more efficient. mRNA analysis shows that additional CD4 expression on CD8 alpha(+)sp T cells results from de nova synthesis. Thus, uptake of soluble CD4 or trogocytosis is less likely as mechanism for generation of canine double-positive T cells. CD4(+)CD8 alpha(+)dp T cells are highly activated independent of their origin except when generated in coculture of CD8 alpha(+)sp T cells with CD4(-)CD8 alpha(-)dn cells. Overall, in dog, CD4(+)sp T cells are the more potent progenitors of CD4(+)CD8 alpha(+)dp T cells compared to CD8 alpha(+)sp T cells. (C) 2014 Elsevier B.V. All rights reserved.