SAFETY AND IMMUNOGENICITY OF THE LIVE ORAL AUXOTROPHIC SHIGELLA-FLEXNERI SFL124 IN ADULT VIETNAMESE VOLUNTEERS

The live, auxotrophic dependent Shigella flexneri Y vaccine strain SFL124 with a deleted aroD gene was tested in 30 healthy adult male Vietnamese volunteers. A single dose of 2 x 10(9) live bacteria was given orally to 15 volunteers, whereas 15 received three doses every other day. None of the volunteers reacted with fever or diarrhoea and SFL124 was excreted by all for a mean of 2.8 (single dose) and 2.6 (three doses) days. A total of 27 of 30 (90%) and 26 of 30 (87%) responded with significantly (0.001 < p < 0.01) increased antibody-secreting cell (ASC) numbers against Shigella flexneri Y lipopolysaccharide (LPS) and invasion plasmid-coded antigens (Ipa). A faecal IgA antibody response to LPS and Ipa was seen in 20 of the 30 (67%) volunteers against both antigens. Serum antibody responses were seen in 23 of 30 (77%) against the LPS and in 17 of the 30 against Ipa. The three-dose schedule elicited only somewhat stronger immune responses than the single-dose schedule. A booster dose of 2 x 10(9) live bacteria was given to half of the volunteers in each group after 6 months, the other half received the same dose after 12 months. Following the booster at 6 or 12 months (i) the excretion of SFL124 was significantly shorter (p < 0.05) than after primary vaccination; (ii) the anti-S. flexneri LPS and anti-Ipa faecal sIgA titres were significantly higher (p < 0.05 to p < 0.01) than after primary vaccination; (iii) the anti-LPS and anti-Ipa ASC responses were significantly lower (p < 0.05) and of shorter duration than after primary vaccination, and (iv) the serum anti-LPS and anti-Ipa responses were significantly elevated (p < 0.05) and similar to those seen after primary vaccination. The results indicate that SFL124 is a safe, live vaccine strain with a negligible reactogenicity in adults living in a Shigella endemic area. SFL124 induces specific immune responses against LPS and Ipa with a mucosal memory lasting for at least 1 year.