Chick embryos were orally immunised at day 16 of incubation by injection of heat-killed Campylobacter jejuni organisms into the amniotic fluid. The response to vaccination was observed at 5 d after hatching or, in some birds which received a postnatal oral booster vaccination, at 7 d after hatching, and the response was observed at 14 d of age. 2. The titres of antibody in serum, bile and intestinal scrapings, the distribution of immunoglobulin-containing cells in the spleen, duodenum and ileum and the expression on peripheral blood leukocytes (PBL) of the T cell surface markers CD3, CD4 and CD8 were determined. 3. Whereas low titres of anti-flagellin antibody were detected in serum, bile and intestinal scrapings of unimmunised birds, high titres were observed in immunised birds. 4. An increase in antibody of all isotypes was detectable in serum but the elevation in IgA antibody in intestinal scrapings and bile was particularly striking. This response was reflected in a dramatic increase in immunoglobulin-containing cells, detected by fluorescent histology, particularly those associated with IgA and IgM isotypes in the spleen and intestine of immunised birds. 5. Secondary oral boosting after hatching resulted in a depression in serum anti-flagellin antibody in immunised birds compared to pre-boosting titres (although still significantly higher than in non-immunised controls) but an increase in IgA antibody in intestinal scrapings and bile. The number of immunoglobulin-containing cells was also increased after boosting. 6. Neither immunisation regimen caused a significant change in the numbers of circulating CD3, CD4 or CD8 T cells. 7. These results indicate that in ovo oral immunisation with C. jejuni antigens stimulates the precocious development of immunity in chicks.
