Lung cell proliferation is accelerated by alveolar macrophage co-culture. A: Neoplastic LM2 cells were cultured alone (Δ) or with primary BAL macrophages from tumor-bearing mice (●) in serum-free media. B-D: (B) LM2, (C) JF32 and (D) E10 cell lines were co-cultured with BAL macrophages (MØ) from naïve (grey bars) or tumor-bearing (black bars) mice, or the MH-S macrophage cell line (green bars) for 72 hrs in serum-free media. E: E10 cells were co-cultured as in (D), but in media containing 10% serum. Epithelial cells cultured in the absence of macrophages were controls (blue bars). Proliferation was determined as described, and mean ± SD pooled from at least 3 independent experiments plotted as fold-change from control (normalized to 1). * P < 0.05, ** P < 0.01 and *** P < 0.001 versus control; # P < 0.01, ## P < 0.001 between (A) conditions at the indicated time points, or (C, E) between proliferation induced by primary macrophage source (naïve vs. tumor-bearing mice).