Immune responses to Shigella dysenteriae 1 and Shigella flexneri lipopolysaccharide and polysaccharide antigens in Bangladeshi patients with shigellosis

Abstract

The immune responses of adult Bangladeshi patients to diarrhoea caused by Shigella dysenteriae type 1 ( n = 20) and Shigella flexneri ( m = 12) were analysed and compared with that of 20 healthy individuals. Antigen-specific antibody secreting cells (ASC) were estimated by ELISPOT, the peripheral mononuclear cell response was determined by a lymphocyte proliferation assay, and serum and mucosal antibody responses were measured by ELISA. Purified lipopolysaccharides (LPS) and O-polysaccharides (PS) from S. dysenteriae type 1 or S. flexneri Y strains, respectively, were used as antigens. A significant increase ( P < 0.001) in the number of LPS-specific IgA-ASC was observed with a peak 6–8 days after the onset of disease followed by a rapid decline within 16 days. The mucosal IgA responses were similar. Serum IgA titres were highest 9–11 days after the onset of diarrhoea and significantly higher ( P < 0.001) than in the control group. The serum IgG levels in S. dysenteriae 1 group were almost twice ( P = 0.008) the level measured in the control group. The mean IgG titres in the S. flexneri group, however, were only slightly higher ( P > 0.05) than that seen in the control group during the acute phase (3–5 days after onset). A strong specific cellular immune response ( P < 0.001) to the homologous S. dysenteriae 1 and S. flexneri O PS antigens was observed in both groups 6–8 days after the onset of symptoms. The immune responses observed suggest that the patients had previously been exposed to shigella. The study demonstrates a good correlation between humoral and cellular immune responses which may play concomitant protective roles in the host defence against shigellosis. Informed consent was obtained from study patients and controls in accordance with the guidelines of the ethical review committee of International Centre for Diarrhoeal Diseases Research, Bangladesh.