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

The mucosal Immoral immune response elicited following Shigella flexneri in fection in patients living in Antananarivo districts (Madagascar Island) wa s evaluated by measuring the gut-derived, circulating immunoglobulin A (IgA ) antibody-secreting cells (ASC) specific for the major bacterial antigen l ipopolysaccharide (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, comp ared to the number in healthy controls, early after the onset of disease. T he number of anti-homologous LPS IgA ASC varied among individuals and peake d 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 hete rologous S. flexneri serotypes was weak. These data indicate that S. flexne ri 2a and 1a are the predominant strains responsible for shigellosis in thi s area of endemicity and that the anti-LPS antibody response following natu ral infection is mainly directed against serotype-specific determinants.