Predominance of serotype-specific mucosal antibody response in Shigella flexneri-infected humans living in an area of endemicity.

The mucosal humoral immune response elicited following Shigella flexneri infection in patients living in Antananarivo districts (Madagascar Island) was evaluated by measuring the gut-derived, circulating immunoglobulin A (IgA) antibody-secreting cells (ASC) specific for the major bacterial antigen lipopolysaccharide (LPS). Fifty, 34, 11, and 5% of the S. flexneri-positive patients were infected with serotypes 2a, 1a, 4a, and 3a, respectively. The total number of IgA ASC in infected patients increased significantly, compared to the number in healthy controls, early after the onset of disease. The number of anti-homologous LPS IgA ASC varied among individuals and peaked between days 5 and 10 after the onset of the disease. In the S. flexneri 1a- and 2a-infected patients, the level of IgA ASC cross-reactivity to heterologous S. flexneri serotypes was weak. These data indicate that S. flexneri 2a and 1a are the predominant strains responsible for shigellosis in this area of endemicity and that the anti-LPS antibody response following natural infection is mainly directed against serotype-specific determinants.