The importance of myeloid-derived suppressor cells in the regulation of autoimmune effector cells by a chronic contact eczema

Induction of a chronic eczema is a most efficient therapy for alopecia areata (AA). We had noted a reduction in regulatory T cells during AA induction and wondered whether regulatory T cells may become recruited or expanded during repeated skin sensitization or whether additional regulatory cells account for hair regrowth. AA could not be cured by the transfer of CD4⁺CD25 ^high lymph node cells from mice repeatedly treated with a contact sensitizer. This obviously is a consequence of a dominance of freshly activated cells as compared with regulatory CD4⁺CD25⁺ T cells. Instead, a population of Gr-1⁺CD11b⁺ cells was significantly increased in skin and spleen of AA mice repeatedly treated with a contact sensitizer. Gr-1⁺CD11b⁺ spleen cells mostly expressed CD31. Expression of several proinflammatory cytokines as well as of the IFN-γ receptor and the TNF receptor I were increased. Particularly in the skin, Gr-1⁺ cells expressed several chemokines and CCR8 at high levels. Gr-1⁺CD11b⁺ cells most potently suppressed AA effector cell proliferation in vitro and promoted partial hair regrowth in vivo. When cocultured with CD4⁺ or CD8⁺ cells from AA mice, the Gr-1⁺CD11b⁺ cells secreted high levels of NO. However, possibly due to high level Bcl-2 protein expression in AA T cells, apoptosis induction remained unaltered. Instead, ζ-chain expression was strongly down-regulated, which was accompanied by a decrease in ZAP70 and ERK1/2 phosphorylation. Thus, a chronic eczema supports the expansion and activation of myeloid suppressor cells that, via ζ-chain down-regulation, contribute to autoreactive T cell silencing in vitro and in vivo.