We, therefore, used our CEACAM1-humanized mouse model [42–44] in combination with deficiencies in C3 (C3−/-), C5 (C5−/-), C3aR1 (C3ar1-/-) C5aR1 (C5ar1−/-) or C5aR2 (C5ar2−/-) to monitor Nme nasal colonization following intranasal infection with 105 CFU. Here, C3AR1 is linked to infection.