''Suppressor T cells'' were historically defined within the CD8 1 T-cell compartment and recent studies have highlighted several naturally occurring CD8 1 Foxp3 À Treg populations. However, the relevance of CD8 1 Foxp3 1 T cells, which represent a minor population in both thymi and secondary lymphoid organs of nonmanipulated mice, remains unclear. We here demonstrate that de novo Foxp3 induction in peripheral CD8 1 Foxp3 À T cells is counter-regulated by DC-mediated co-stimulation via CD80/CD86. CD8 1 Foxp3 1 T cells fail to develop in TCR-transgenic mice with Rag1 À/À background, similar to classical CD4 1 Foxp3 1 Tregs. Notably, both naturally occurring and induced CD8 1 Foxp3 1 T cells express bona fide Treg markers including CD25, GITR, CTLA4 and CD103, and show defective IFN-c production upon restimulation when compared with their CD8 1 Foxp3 À counterparts. However, utilizing DEREG transgenic mice for the isolation of Foxp3 1 cells by eGFP reporter expression, we demonstrate that induced CD8 1 Foxp3 1 T cells similar to activated CD8 1 Foxp3 À T cells only mildly suppress T-cell proliferation and IFN-c production. We therefore categorize CD8 1 Foxp3 1 T cells as a tightly controlled population sharing certain developmental and phenotypic properties with classical CD4 1 Foxp3 1 Tregs, but lacking potent suppressive activity.Keywords: CD8 . Foxp3 . TGF-b . Treg
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IntroductionFoxp3 is a master regulator of CD4 1 Treg function [1][2][3] and its mutation or the depletion of Foxp3 1 cells led to onset of multiorgan autoimmunity [4][5][6]. The vast majority of Foxp3 1 T cells are confined to TCR-ab 1 CD4 1 T cells, and little is known about CD8 1 T cells expressing Foxp3. Certain surface phenotypes such as CD28 À [7], CD122 1 [8], CD8aa 1 [9, 10], latencyassociated peptide (LAP) 1 [11] and restriction to the nonclassical MHCI molecule Qa-1 [12] have been linked with immunosuppressive functions of CD8 1 T cells. However, Foxp3 expression was either absent in these populations [8,9,[13][14][15]
716with the defining surface phenotype [11] or was not investigated specifically on a protein level [16]. Additionally, the isolation of viable CD8 1 Foxp3 1 populations was hampered by the nuclear localization of Foxp3 in conjunction with the occurrence of these cells at low numbers in nonmanipulated mice [2,17], rendering the identity and relevance of mouse CD8 1 Foxp3 1 T cells unclear.Classical CD4 1 Foxp3 1 Tregs develop either intrathymically (natural Tregs, nTregs) or in the periphery via conversion from Foxp3 À T cells (induced Tregs). Specialized dendritic cells (DC) can initiate the latter process by providing the key factors TGF-b and all-trans-retinoic acid (RA) [18,19]. Although natural and in vitro induced CD4 1 Foxp3 1 Tregs share key phenotypic and functional characteristics, they differ in the stability of Foxp3 expression, and different degrees of demethylation of an evolutionarily conserved region within the foxp3 locus (TSDR; Treg-specific demethylated region) have b...