The inhibition was not due to pertussis toxin (PT) action, and monocyte infection by the B. pertussis AC+ PT− mutant secreting a recombinant PT toxoid (21, 22) still blocked the M-CSF-driven differentiation of monocytes into macrophages to the same extent as an infection by the B. pertussis WT bacteria. Here, CSF1 is linked to infection.