Effect of blocking antibodies against HER-1 and HB-EGF on MSC proliferation. Starting from both 5 × 10³ (A) and 10⁴ (B) cells, the enhancement of MSC proliferation induced by 5 ng/mL (▦), 25 ng/mL (▪), or 50 ng/mL (□) HB-EGF (1) is completely abolished by the addition of blocking antibodies against HER-1 (2) and HB-EGF (3). No inhibitory effect is obtained by adding to HB-EGF the same concentration of indifferent goat immunoglobulins (4), the blocking antibody isotype. Proliferation rate is assessed by MTT method at 72 hours and expressed as additional proliferation (the percentage of increase of MSC proliferation following HB-EGF addition [P1] compared with the spontaneous MSC proliferation that normally occurs without HB-EGF [P2]). In other words, (P1–P2)/P2 × 100. P < .01 refers to the difference between the arms without blocking antibodies and the others. The figure shows means ± SD of 5 different experiments.