The immunochemistry of Shigella flexneri O-antigens. The structure and biosynthesis of the O-specific side chains of some representative serotypes.

Abstract

Structural analysis of a representative group of Shigella flexneri lipopolysaccharides has shown that the O‐specific side chains of these polymers consist of a primary unbranched chain of N‐acetylglucosamine and rhamnose substituted with secondary side chains of glucose. Sh. flexneri Y variants which lack the glucose secondary side chains and are thus structurally identical with the primary side chains of the different serotypes, belong to one of two types namely, Y1 which consists of N‐acetylglucosaminyl‐(1→2)‐rhamnosyl‐(1→4)‐rhamnose repeating units bound together by a (1→6)‐linkage or Y2 which consists of N‐acetylglucosaminyl‐(1→3)‐rhamnosyl‐(1→4)‐rhamnose repeating units bound together by a (1→4)‐linkage. The different smooth Sh. flexneri serotypes are synthesised from these variant Y1 or Y2 precursor structures by the action of specific UDP‐glucose transferases which incorporate the glucose secondary side chains in a position that characterises each serotype. The structural sequences that seem to play an immunodominant role in antigen factors I, II, III, IV, V and 7,8 are α‐glucosyl‐(1→4)‐N‐acetyl‐glucosamine, α‐glucosyl‐(1→4)‐rhamnose, O‐acetyl‐α‐glucosyl‐(1→2)‐rhamnose, α‐glucosyl‐(1→6)‐N‐acetylglucosamine, α‐glucosyl‐(1→3)‐rhamnose and α‐glucosyl‐(1→2)‐rhamnose respectively. The group antigen 3,4 in which rhamnose is the immunodominant sugar, seems to be related to the internal rhamnosyl‐(1→4)‐rhamnose sequence. The serotype 6 antigen (VI) used in this study is so different structurally from its analogues that this organism is no longer regarded as a true Sh. flexneri species. From the structural relationship of the different O‐antigens, X and Y variants are the consequence of enzyme defects that interrupt the biosynthesis of the complete smooth lipopolysaccharides. Y1 variants probably result from defects in the specific UDP‐glucose transferases of serotypes 1a and 2a while Y2 variants probably result from defects in the analogous UDP‐glucose transferases of serotypes 4a, 3a, 5 and variant X. Variant X is an intermediate stage in the biosynthesis of serotypes 3a and 5 from the precursor Y2 structure. Further evidence for the role of UDP‐glucose transferases in conferring type specificity on the cryptic Y structures comes from two sources. Firstly, the type specific or T‐loci of Petrovskaya that map near the lac locus are those in which glucose is the immunodominant sugar and loss of this locus by genetic recombination results in a glucose‐deficient Y‐type hybrid. Secondly, the structures proposed show that all phage conversions of Sh. flexneri reported to date, can be explained in terms of single enzyme changes involving specific UDP‐glucose transferases. From the limited number of antigens studied, it can be predicted that the expression of type specific antigens will depend on the presence of group factors, as the former are always end group determinants which cannot be incorporated into the growing molecule before biosynthesis of the more central regions which determine group antigenicity.