CSF1 and infection: We show that the action of subcytotoxic amounts of CyaA toxin and a low multiplicity of infection (MOI) by CyaA-secreting B. pertussis bacteria block macrophage colony-stimulating factor (M-CSF)-induced maturation of primary human monocytes into macrophage cells in vitro. Moreover, we show for the first time that action of a bacterial toxin can trigger apparent dedifferentiation of terminally differentiated human alveolar macrophages.