Figure 9.

α₄ integrin supports terminal proliferation. (A) α₄β₁-Mediated adhesion is required for the proliferation of differentiating fetal liver erythroid cells. TER119⁻ fetal liver erythroid progenitor cells were cultured on V, Vo, VRGD⁻, or VoRGD⁻ substrates with Epo-containing media for 1 d and then in Epo-free media for a second day. At days 1 and 2, the number of live cells was counted. Only α₄β₁-binding substrates V and VRGD support an expansion in cell number similar to that seen on intact fibronectin. α₅β₁-Binding substrate Vo supports a moderate level of expansion, and VoRGD, which does not engage either integrin, does not support any cell expansion. (B) Blocking α₄ integrin with an antibody blocks proliferation. TER119⁺ cells were isolated and incubated with function-blocking antibodies to either α₄ or α₅ integrin and were cultured on fibronectin-coated or control substrates for 1 d. The mean normalized increase in cell number from three experiments is shown. *, P ≤ 0.05; **, P ≤ 0.01. (C and D) Expansion of erythroid progenitors during the second day occurs only in conditions in which α₄β₁ is engaged. TER119⁻ cells were cultured on Vo (C) or VRGD⁻ (D) substrates in Epo-containing media for the first day and were dissociated and transferred to wells coated with each of the various integrin-binding substrates in Epo-free media for a second day. Error bars represent the SD.