The Impact of Colistin Resistance on the Activation of Innate Immunity by Lipopolysaccharide Modification

Colistin resistance is caused by different lipopolysaccharide (LPS) modifications, and we propose to evaluate the effect on the innate immune response of in vivo and in vitro colistin resistance acquisition. We used 2 pairs of isogenic strains: (1) Escherichia coli ATCC25922, susceptible to colistin and its isogenic transconjugant-carrying mcr-1 gene; and (2) OXA-48, CTX-M-15 K. pneumoniae susceptible to colistin (CS-Kp) isolated from a urinary infection and its colistin-resistant variant (CR-Kp) from the same patient after prolonged treatment with colistin. No mutation of described genes for colistin resistance (pmrA, pmrB, mgrB. phoP/Q and crrAB) were found in the CR-Kp genome; however, LPS modifications were characterized by negative-ion MALDI-TOF. The strains were co-cultured with human monocytes to determine their survival after phagocytosis and induction to apoptosis. Also, monocytes were stimulated with bacterial LPS to study cytokine and immunecheckpoint production. The addition of 4-amino-4-deoxy-l-arabinose (Ara4N) to lipid A of CR-Kp accounted for the colistin resistance. CR-Kp survived significantly longer inside human monocytes after being phagocytosed compared with the CS-Kp strain, whereas no significant differences were observed for the E. coli isogenic strains. In addition, LPS from CR-Kp induced both higher apoptosis in monocytes and higher levels of cytokine and immune checkpoint production than LPS from CS-Kp. This effect was strictly the opposite for E. coli. Our data reveal a variable impact of colistin resistance on the innate immune system, depending on the responsible mechanism. Adding Ara4N to LPS increases bacterial survival after phagocytosis and elicits a higher inflammatory response than its colistin-susceptible counterpart.