Lymphoid tissue inducer cells: architects of CD4 immune responses in mice and men.

In this review, we summarize the current understanding of the multiple functions of the mouse lymphoid tissue inducer (LTi) cells in: (i) the development of organized lymphoid tissue, (ii) the generation and maintenance of CD4-dependent immunity in adult lymphoid tissues; and (iii) the regulation of central tolerance in thymus. By contrast with mouse LTi cells, which have been well described, the human equivalent is only just beginning to be characterized. Human LTi-like cells expressing interleukin (IL)-22 have been identified recently and found to differentiate into natural killer (NK) cells. The relationship of LTi cells to NK cells is discussed in the light of several studies reporting a close relationship in the mouse between LTi cells and transcription factor retinoid-related orphan receptor gammat-dependent IL-22 producing NK cells in the gut. We also outline our data suggesting that these cells are present in adult human lymphoid tissues.