Evasion of Yersinia enterocolitica serotype 03 from complement-mediated killing.

Using isogenic strains of Y. enterocolitica serotype 03 (isolate 75) differing in LPS side chains (S and R), plasmid content (p+ and p-) or selective failure of YOP1 expression (YOP1-) we observed comparable C9 consumption via the alternative pathway by all strains. Differences became apparent in the bactericidal assays in which the 75S p+ strain was resistant whereas the plasmid-negative S and R strains were killed. Increased but submaximal resistance was observed with the 75R p+ and 75S YOP1- strains indicating that LPS side chains and plasmid-encoded factors other than YOP1 also contribute to serum resistance. Deposition of C9 and terminal complement complex (TCC) formation were greatly reduced on the resistant 75S p+ strain compared to the sensitive 75S p- strain. The 75S YOP1- variant behaved like the 75S p- strain with respect to C9 deposition and TCC formation suggesting that YOP1 prevents terminal C activation. A comparison of TCC deposited on resistant and sensitive Y. enterocolitica revealed no differences with respect to their salt- and protease-resistance and their size. We conclude from our experiments that serum resistance of 75S p+ depends on several surface components YOP1 being the most important. The YOP1 protein clearly interferes with C9 deposition and TCC formation and thereby contributes to serum resistance of plasmid-positive, YOP1 expressing Y. enterocolitica.