The Antigenicity of Shigella Sonnei
Authors: Branham Sara E. , Carlin Sadie A. 
Published: January 2023

Discussion and Summary
Some cultures of S. sonnei are remarkably antigenic. Among the strains of this species studied by the authors this great antigenicity has been limited to those of phase I, and all phase I cultures that were examined have shown this property. The active immunity induced by those organisms extended to some degree against S. paradysenteriae I (V) and II (W) also, but was not noted with the few strains of III (Z) included. This extension of immunity to cover these so-called “Flexner” strains has not been demonstrable in passive mouse protection tests. Neither active nor passive immunity induced by S. sonnei has been found to extend to any marked degree to other Shigella or to other phases of S. sonnei.
Cross immunity has been shown, in many instances, to arise as a result of the presence of common antigens, as shared, for example, between Salmonella typhi and S. schottmülleri (19), Pneumococcus Types I and III (20), Pneumococcus Types I, II and III (21), etc. Dubos (22) has reviewed many of the findings in this field of experimentation.
Less clearly explained, on the basis of antigenic structure, is the nonspecific protection described by Zahl and Hutner (23). These authors prepared endotoxins from strains of Shigella, Salmonella, and the unrelated Rhodospirillum. Mice, immunized with these, were protected best against the homologous species, but there was some protection against other species also. These authors postulated a single thermostable toxin in all gram-negative bacteria. That this is true to some extent most workers with gram-negative groups of bacteria will agree. But such an “endotoxin” does not produce an immunity comparable with that described by Longfellow and Luippold, Kolchin and Gross, Dubos, and the present authors.
In the absence of further and more detailed experiments, it is not felt that undue significance can be attached to the cross protection reported in this study between S. sonnei and other species of Shigella. However, these findings deserve further investigation.
During recent years many attempts have been made to produce an immunizing agent against bacillary dysentery, using strains of S. paradysenteriae (Flexner) and S. dysenteriae (Shiga).
It would seem possible that a useful and practical agent has been overlooked and that phase I of S. sonnei may have such a role. Infections due to S. sonnei are quite common, and, in themselves, constitute a public health problem. Infections due to “Flexner” and “Sonne” strains often occur together in the same outbreak. In the experiments described in this paper S. sonnei (I) has protected mice well against itself, and there have been indications that it often protects against the I (V) and II (W) types of S. paradysenteriae (Flexner) as well as, or better than, some of these “Flexner” strains do against themselves. The extent to which this cross protection actually occurs can be determined only by more thorough study. The data presented here concerning such cross protection are not dramatic, and might not be considered immunologically significant, but they indicate a trend.
It is possible that there are differences in regard to cross protection among individual strains as well as among types. In fact, the apparent differences among types may be due to the strains used.
The usefulness of S. sonnei, phase I, as an immunizing agent would depend, to some extent, upon the role of phase II in causing bacillary dysentery, since there appears to be little cross protection between the phases. The phase II strains which have been available in this study have had neither antigenicity nor virulence. Whether these properties are constantly lacking cannot be decided on the basis of so few strains. A phase study of Sonne strains immediately upon isolation is in progress and will be reported in a subsequent communication.