| Modulation of gut microbiota, by administration of antibiotics or probiotics has the potential to alter the immune response. A series of studies were conducted in order to examine this in the chicken. To establish if subtherapeutic and prophylactic doses of virginiamycin have an effect on systemic and mucosal immune responses, chickens were immunized with experimental antigens systemically and orally. Systemic antibody responses were greater in birds fed diets containing virginiamycin compared to birds fed diets with no antibiotics. To determine if members of the commensal intestinal bacteria, Lactobacillus acidophilus, Lactobacillus reuteri, and Lactobacillus salivarius, could differentially induce expression of cytokine genes in chicken lymphoid tissue cells, mononuclear cells isolated from cecal tonsils and spleens were co-cultured with one of the three live bacteria and host gene expression was analyzed. Different pro- and anti-inflammatory cytokine profiles were induced, with L. acidophilus inducing primarily cytokines associated with a T helper 1 response, while L. salivarius induced a more anti-inflammatory response. To further examine the molecular mechanisms of these phenomena, the expression of immune system genes in chicken cecal tonsil and spleen cells in response to structural constituents of L. acidophilus was investigated. Using a low-density chicken immune system microarray, it was shown that cecal tonsil cells responded more rapidly than spleen cells to bacterial stimuli, with the most potent stimulus for cecal tonsil cells being bacterial DNA. Further examination revealed the involvement of STAT2 and STAT4 signaling pathways in cellular responses to bacterial DNA. To determine if commensal bacteria could alter the systemic immune response, the final study examined effects of the above three bacteria individually or in combination on the induction of the antibody- and cell-mediated immune responses in vivo. Chickens received bacteria weekly via oral gavage starting on day of hatch, and subsequently were immunized with two experimental antigens and two commercial vaccines. L. acidophilus and L. salivarius increased the antibody response and decreased the cell-mediated immune response. Further examination demonstrated that L. salivarius induces a type 2 response. In conclusion, antibiotic growth promoters and commensal bacteria, in the form of probiotics, can affect the systemic immune response of chickens. |
