Formation of multi-nucleated cells by human blood monocytes. Monocytes were isolated from human blood and infected with BCG (pMV261) (D, E, F) or BCG (pAS-MDP1) (G, H, I), respectively. Uninfected cells (A, B, C) served as negative control. Blood monocytes activated with LPS and IFN-γ are shown in K, L, M. The cells were stained with Diff-Quick after three (A, D, G, K), four (B, E, H, L) and 11 (C, F, I, M) days. Micrographs were taken with a magnification of 200 ×. Arrows mark multi-nucleated cells.