Combined Parenteral and Oral Immunization Results in an Enhanced Mucosal Immunoglobulin A Response to Shigella Flexneri

Achieving a vigorous secretory immunoglobulin A (IgA) response in intestinal secretions usually requires multiple doses of antigen given orally, while systemic immunity is more easily attained by parenteral immunization. This study examines the role of combined parenteral and oral immunizations to enhance the early mucosal immune response to an enteropathogen. We have used a chronically isolated intestinal-loop model in rabbits as a probe to monitor kinetically the initial (primary) local immune response to shigella lipopolysaccharide (LPS) following combinations of parenteral immunization intramuscularly (i.m.) and oral stimulation with shigellae. Predictably, effective stimulation of systemic immunity was elicited when heat-killed preparations of Shigella sp. strain X16 were given i.m., as shown by strong serum IgG and weak intestinal IgA activity to shigella LPS. A single oral dose of live Shigella sp. strain X16 given to unprimed rabbits elicited only a typical weak IgA response in intestinal secretions. However, when an i.m. dose of heat-killed shigellae was followed 1 day later by an oral dose of live Shigella sp. strain X16, a hyperstimulation of the early secretory IgA response was elicited, and the response reached levels found previously only after multiple oral administrations of live shigellae. This stimulation did not require the use of an adjuvant. At the same time, the animals receiving this combined oral and i.m. regimen had a lower IgG antishigella LPS activity in serum compared with their response after receiving parenteral antigen in adjuvant alone. These findings indicate that while a dichotomy exists between the systemic and mucosal immune responses, careful orchestration of the stimulatory events can promote a vigorous early local IgA response.