Immune responses to Shigella lipopolysaccharide and invasion plasmid antigens in Shigella infected Vietnamese patients

Immune responses were studied in 19 adult male Vietnamese patients with bacillary dysentery. Shigella flexneri was isolated in faeces from 17 patients (S. flexneri 2a (n = 11), S. flexneri 4a (n = 4), S. flexneri 3a (n = 1), and S. flexneri 3b (n = 1)) and S. dysenteriae 2 was isolated from 2 patients. Thirty healthy, adult males, without diarrhoea 3 months prior to the study, served as controls. The use of enzyme immune assay (EIA) showed that S. flexneri infected patients developed intestinal sIgA responses against S. flexneri Y lipopolysaccharide (LPS) and Shigella invasion plasmid antigens (Ipa), with peak values seen 2 weeks after the onset of diarrhoea. The sIgA titres were significantly higher (0.001 < P < 0.05 for anti-Shigella Ipa sIgA and 0.01 < P < 0.05 for anti-S. flexneri Y LPS sIgA) in the patients than in the healthy controls. Enzyme-linked immunospot (ELISPOT) analyses of peripheral blood mononuclear cells (PBMC) showed that S. flexneri infected patients developed significant antibody secreting cell (ASC) responses against S. flexneri Y LPS and Shigella Ipa (0.001 < P < 0.05), which peaked 1 week after the onset of diarrhoea. IgA-ASC predominated, followed by IgG-ASC, whereas IgM-ASC were least common. The number of Shigella Ipa-specific ASC was higher than the number of S. flexneri Y LPS-specific ASC. With use of EIA it was shown that S. flexneri infected patients had significantly (0.001 < P < 0.05) higher serum IgA and IgG titres than the healthy controls. All 17 patients had anti-S. flexneri Y LPS titres, and 11 patients had anti-Shigella Ipa titres exceeding the cut-off titres (mean ± 2S.D.) seen in the controls. The highest IgA titres were seen 1 week, and the highest IgG titres 2 weeks, after the onset of diarrhoea.