TNF-alpha is critical to facilitate hemopoietic stem cell engraftment and function.

The use of tolerogenic cells as an approach to induce tolerance to solid organ allografts is being aggressively pursued. A major limitation to the clinical application of cell-based therapies has been the ability to obtain sufficient numbers and also preserve their tolerogenic state. We previously reported that small numbers of bone marrow-derived CD8+/TCR- graft facilitating cells (FC) significantly enhance hemopoietic stem cell (HSC) engraftment in allogeneic and syngeneic recipients. Although the majority of FC resemble precursor plasmacytoid dendritic cells (p-preDC), p-preDC do not replace FC in facilitating function. In the present studies, we investigated the mechanism of FC function. We show for the first time that FC significantly enhance HSC clonogenicity, increase the proportion of multipotent progenitors, and prevent apoptosis of HSC. These effects require direct cell:cell contact between FC and HSC. Separation of FC from HSC by transwell membranes completely abrogates the FC effect on HSC. p-preDC FC do not replace FC total in these effects on HSC function. FC produce TNF-alpha, and FC from TNF-alpha-deficient mice exhibit impaired facilitation in vivo and loss of the in vitro effects on HSC. Neutralizing TNF-alpha in FC similarly blocks the FC effect. The antiapoptotic effect of FC is associated with up-regulation of Bcl-3 transcripts in HSC and blocking of TNF-alpha is associated with abrogation of up-regulation of Bcl-3 transcripts. These data demonstrate a critical role for TNF-alpha in mediating FC function. FC may have a significant impact upon the safe use of chimerism to establish tolerance to transplanted organs and tissue.