Differences in complement activation of serum-resistant and serum-sensitive Klebsiella pneumoniae isolates

The gram-negative bacteria Klebsiella pneumoniae are genetically heterogeneous and a common cause of sepsis and bacteremia in humans. The complement system is the first line of defence against bacteria when they invade the body. We previously investigated K. pneumoniae isolates from sepsis patients. We found that complement factor (C) 3 is deposited on all isolates independent of serum sensitivity, but the membrane attack complex (MAC) was only formed on the serum-sensitive isolates. To investigate the mechanism for serum resistance, we incubated one serum-sensitive and one serum-resistant isolate in human serum and identified bound complement factors by mass spectrometry. The serum-sensitive isolate had all expected complement factors bound, including C4, while the serum-resistant isolate had only C3 bound. The serum resistance was caused by a fast cleavage of C3b to iC3b. Thereby, the C5 convertase, and thus MAC, cannot be formed. To confirm the role of C4 in serum sensitivity, C4 was inhibited by the nanobody hC4Nb8, resulting in the survival of the serum-sensitive isolate. This suggests that C4 is indispensable for MAC formation through the classical and lectin pathways. In contrast, when activated selectively, the alternative pathway primarily leads to the generation of iC3b, thereby enabling serum resistance by bypassing MAC assembly.