University Archives

Yersinia pestis, the pathogen responsible for plague, evades the complement Membrane Attack Complex (MAC), through utilization of its adhesion protein Ail. Ail recruits the host complement regulatory protein Factor H (FH) present in serum, to the pathogen surface. FH is then able to recruit the protein Factor I (FI) which, along with FH, is able to cleave complement protein C3b into inactive iC3b, preventing formation of the MAC. Ail recruits FH by binding to short consensus repeats (SCRs) 5-7 (there are 20 SCRs within FH) and upon recruitment to the bacterial surface, FH is degraded by the Y. pestis protease Pla, but FH still associates with the bacterial cell surface after degradation. We sought to define which terminus of FH is cleaved off of the protein once it becomes associated with the bacterial surface and if the remaining FH fragment can recruit FI. Using monoclonal antibodies directed towards either the N-terminus or C-terminus of FH, we showed the C-terminus of FH is cleaved off by Pla. In vitro, in the absence of Y. pestis, this cleaved form of FH can bring FI and C3b together to generate iC3b. However, when pre-cleaved FH bound to an Ail+ strain lacking Pla (Dpla, to prevent further cleavage), FH could no longer facilitate iC3b generation. These data suggest loss of the C-terminal SCR19-20 C3b-binding site in FH renders Ail-bound FH unable to facilitate FI-mediated cleavage of C3b to iC3b. Thus, the ability of Ail to prevent MAC formation may be due to alternative modes of MAC inhibition, such as preventing C5b-C9 recruitment through Ail binding to vitronectin, or potentially by Ail directly inhibiting C9 polymerization in the membrane of Y. pestis. Future experiments will explore these possibilities.