Graft facilitating cells are derived from hematopoietic stem cells and functionally require CD3, but are distinct from T lymphocytes

Objective We previously demonstrated that CD8+/TCR− bone marrow cells facilitate engraftment of HSC in allogeneic recipients without causing graft-vs-host disease. Whether facilitating cells (FC) develop from T cells or represent a distinct lineage has not been determined. Methods In the present studies, we characterized the lineage derivation of FC, defined the role for the CD3 complex in allogeneic facilitation, and demonstrated syngeneic facilitation by FC but not T cells. Results We demonstrate for the first time that FC development and function is independent of T cells and cannot be replaced by them. Purified GFP+ HSC transplanted in syngeneic recipients produce GFP+ FC, which facilitate in secondary transplants, confirming that FC are derived from HSC. In addition, FC, but not T cells, potently facilitate the engraftment of suboptimal numbers of HSC in syngeneic recipients. Notably, FC contain the transcripts for CD3ε and CD3δ, but not TCRα or TCRβ, excluding the possibility of T-cell contamination. Genetic mutations that generate a functional deficiency in CD3 signaling significantly impair FC function in allogeneic facilitation (p = 0.006). Conclusion Taken together, these data clearly distinguish FC from T cells. Moreover, they indicate that FC require the CD3ε gene to facilitate allogeneic HSC engraftment. The unique function(s) of FC make them an attractive focus for new cell-based therapeutic approaches to enhance HSC engraftment while reducing toxicity, especially when limiting numbers of HSC are available.