HA stimulates the production of a colony promoting activity in LTBMC. (A) LTBMC was treated with HA (100 μg/mL). CM was harvested at day 28 and tested for colony-promoting activity. Freshly isolated bone marrow cells were added to methylcellulose cultures and the colony formation by myelopoietic progenitors was determined in the presence of suboptimal doses of IL-3 (5% CM of Wehi-3B cells) and the addition of 20% of CM from either control LTBMC (lanes 1 and 3) or of CM from LTBMC treated with HA (lanes 2 and 4). After 7 days of culture, the numbers of colonies were counted. In lanes 3 and lane 4, hematopoietic progenitors were determined similarly to lanes 1 and 2, but antibodies directed against IL-6 (100 μg/mL) were added in addition. SE was calculated from 3 independent assays. (B) CM of control LTBMC or LTBMC cultured for 4 weeks in the presence of HA (100 μg/mL) or in the presence of HA’ase (0.1 U/mL) was assayed for IL-6 by ELISA.