LFA-1-Mediated Adhesion to ICAM-2 Is Critical for Stromal Cell-Dependent Early T-Lymphoid Differentiation from Human Hematopoietic Precursors

The regulatory mechanism of human early T-lymphoid differentiation remains less defined. We previously reported that human telomerized bone marrow stromal cells support the generation of CD7⁺CD56^- early T- as well as CD10⁺CD19⁺ early B-lymphoid precursors from human hematopoietic precursors. Here we examined whether and how early T lymphopoiesis is regulated by interaction with stromal cells. Low or no levels of LFA-1 were expressed on CD34⁺CD38^-CD45RA^- immature hematopoietic precursors. However, high levels of LFA-1 were detected on CD34⁺CD38^-CD45RA⁺CD10⁺CD7^+/-CD19^- immature lymphoid precursors, while those of LFA-1 were diminished on CD34⁺CD38⁺CD45RA⁺CD10⁺CD19⁺ more mature pro B cells. On the other hand, ICAM-1 and ICAM-2 were expressed in a portion of the telomerized stromal cells. ICAM-3 was not detected. Various levels of ICAM-1, ICAM-2, or ICAM-3 were also expressed on hematopoietic and lymphoid precursors.

To examine the role of LFA-1-mediated adhesion to stromal cells or adjacent hematopoietic precccursors in early lymphoid differentiation, we examined the effect of anti-LFA-1 blocking antibody (Ab) on the differentiation of CD34⁺CD45RA^-CD7^-CD10^-CD38^lo/- hematopoietic precursors in the cultures on stromal cells or with conditioned medium (CM) obtained from cultures of stromal cells. In the cultures on stromal cells, anti-LFA-1 Ab strongly inhibited the generation of CD7⁺CD10^-CD45RA⁺ and CD7^-CD10⁺CD45RA⁺ lymphoid precursors from hematopoietic precursors after 21 days of culture. Significant number of CD14⁺monocytic cells was generated with or without anti-LFA-1 Ab. In the cultures with CM, anti-LFA-1 Ab showed marginable effect on lymphoid differentiation. To elucidate the effect of anti-LFA-1 Ab on more mature lymphoid precursors, CD7⁺CD10^-CD45RA⁺ or CD7^-CD10⁺CD45RA⁺ cells were isolated after culture of hematopoietic cells for 14 days and cultured on stromal cells in the presence or absence of anti-LFA-1 Ab. Anti-LFA-1 Ab remarkably inhibited the generation of CD7⁺ and CD10⁺CD19⁺ lymphoid cells from the CD7⁺CD10^-CD45RA⁺ early T-lymphoid precursors. Notably, few or no CD45RA⁺CD14^- lymphoid cells were detected. Anti-LFA-1 Ab did not affect B-lineage differentiation from CD10⁺CD19^-CD45RA⁺ early B-lymphoid precursors to CD10⁺CD19⁺CD45RA⁺proB cells. We next examined which ICAM ligand is responsible for the observed effects by anti-LFA-1 Ab. Anti-ICAM-2 Ab inhibited the generation of CD7⁺CD45RA⁺ and CD10⁺CD45RA⁺ lymphoid precursors from CD34⁺CD45RA^-CD7^-CD10^-CD38^lo/- hematopoietic precursors on stromal cells, as observed with anti-LFA-1 Ab. No effect was observed with anti-ICAM-1 Ab. Anti-ICAM-2 Ab further suppressed the generation of CD7⁺ and CD10⁺ lymphoid cells from the cultured CD7⁺CD10^-CD45RA⁺ early T-lymphoid precursors, but did not depress B-lymphoid differentiation from the cultured CD7^-CD10⁺CD45RA⁺ early B-lymphoid precursors.

Taken together, these data indicate that LFA-1-mediated adhesion to ICAM-2 is essential for stromal cell-dependent early lymphoid differentiation of hematopoietic and CD7⁺ early T-lymphoid precursors.