The Immunochemistry of Shigella flexneri O‐Antigens

Abstract

Structural analysis of a representative group of Shigella flexneri lipopolysaccharides has shown that the 0-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, Yl which consists of N-acetylglucosaminyl-( 1 +2)-rhamnosyl- ( 14) ) -rhamnose and a-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 0-antigens, X and Y variants are the consequence of enzyme defects that interrupt the biosynthesis of the complete smooth lipopolysaccharides. Yl variants probably result from defects in the specific UDP-glucose transferases of serotypes l a and 2a while Y, variants probably result from defects in the analogous UDPglucose 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 Yz 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.The Shigella flexneri are a serologically heterogeneous group of dysentery bacilli whose 0-antigens are lipid A-polysaccharide-lipid B-polypeptide complexes comparable in their gross structural and biological properties [I -41 to the analogous substances first isolated from Sh. shigae [5] and now known to be present in many other gram-negative bacilli of the family Enterobacteriaceae [6]. I n addition to their unequivocal importance in medicine, these 0-antigens are of special relevance to the biology of gram-negative bacilli in that comparative structural analysis with other genera will almost certainly reveal similarities and differences of the kind that will help t o elucidate the taxonomy, biosynthesis, gene...

Structure and biology of Shigella flexneri O antigens

“…JEexneri x E. coli hybrid LPSs from all serotypes (Manson et al, 1970) are identical with that of classical variant Y strains. However, the LPS from the original variant Y of Hiss and Russell (1903) which many observers thought closer to W (serotype 2a), has proved to be structurally and serologically identical with serotype-2a LPS (Simmons, 1971~). Fourth, the view that variant X is a degraded form is consistent with the observation that it is a de-acetylated serotype 3a.…”

“…Analysis of S. JEexneri 0-spec@ and group serodeterminants. Evidence for the molecular groupings that determine the 0-specific and group factors has come from combined chemical and serological studies in which structurally-defined oligosaccharides obtained by acid hydrolysis of the S.JEexneri 0-specific polysaccharides (Simmons, 1969) were used to inhibit the homologous complement fixation systems (Simmons, 1971b). The factor-I system was inhibited by a-~-Glc-( 1 +4)-GlcNAc ; the factor-I1 system by ~t-~-Glc-( 1 -+4)-Rha; the factor-IV system by a-~-Glc-( 1 +6)-GlcNAc and the factor-V system by a-~-Glc-( 1 + 3)-Rha.…”

“…The common basal sugars 2-keto-3-deoxy-octonate, L-glycero-D-manno-heptose phosphate, D-glucose, D-galactose and Nacetyl-D-glucosamine are incorporated into the growing basal structure in that order and defects in the genes controlling this biosynthetic pathway result in an incomplete basal structure of rough (R) specificity (Johnston et al, 1967). The 0-specific side-chains (each consisting of six to eight repeating pentasaccharide units) may be further subdivided into two regions-a primary unbranched chain of L-rhamnose and N-acetyl-D-glucosamine identical to the variant Y antigen, and secondary side-chains of a-D-glucosyl and 0-acetyl residues which are substituted on the primary chain in a position that is unique for each serotype (Simmons, 1969). The specificity of the linkage of the glucose secondary side-chain is governed by specific UDP-glucose transferases that are phage-dependent and map near the lac locus (Manson et al, 1970).…”

Stereochemical Aspects of Antigenic Specificity in Polysaccharide Determinants

“…Although some of these limitations also apply to the Xhigella flexneri (dysentery bacilli) 0-antigens that form the subject of this communication, this family of structurally related polymers probably offers the best model a t present available for studying the effects of polysaccharide stereochemistry on antigenic specificity for the following reasons. Firstly, the structures of their 0-specific side chains have been elucidated [5] and the antigenic determinants of their common group and type specific factors completely characterised by complement fixation inhibition studies [6,7]. Secondly, structural studies infer that the biosynthesis of this group of antigens is completed from a precursor variant Y-type polysaccharide simply by incorporating a-glucosyl secondary side chains in a position that is unique to each serotype [5].…”

“…Firstly, the structures of their 0-specific side chains have been elucidated [5] and the antigenic determinants of their common group and type specific factors completely characterised by complement fixation inhibition studies [6,7]. Secondly, structural studies infer that the biosynthesis of this group of antigens is completed from a precursor variant Y-type polysaccharide simply by incorporating a-glucosyl secondary side chains in a position that is unique to each serotype [5]. This implies that the configurations ofthe cryptic (hidden) Y structures in these polymers are probably very similar (if not identical) and that the steric differences between the completely synthesised 0-antigens are determined solely by the M e r e n t linkages of their a-glucosyl side chains [5].…”