Studies were conducted to test the hypothesis altering intestinal immune function in broiler chickens affects health and susceptibility to diseases. Treatments tested included vitamin A deficiency (negative modulation), Lactobacillus-based probiotic stimulation (positive modulation), and their combination. Vitamin A deficiency impaired gut-associated lymphoid tissues (GALT) with an effect best characterized as a reduction in intraepithelial lymphocyte (IEL) subpopulations, mainly CD4+ T cells. This change in IEL lowered resistance to Eimeria acervulina infection, resulting in greater oocyst shedding. Systemic immunity (e.g., interferon-γ (IFN-γ) secretion and ability of splenic T lymphocytes to respond to in vitro mitogen) was also impaired. Lactobacillus-based probiotic increased certain IEL subpopulations within the GALT, namely the main effector immune cells CD4+ and CD8+ T cells. The probiotic also stimulated early cellular immune responses against E. acervulina (e.g., IFN-γ and interleukin-2 (IL-2)). The probiotic also provided protection to different inoculation rates of E. acervulina pathogen as manifested by lower oocyst shedding. In experiments with combined treatments, the probiotic had immunostimulatory effects even in birds compromised by a hypovitaminosis A as evidenced by lower oocyst shedding and higher IFN-γ and IL-2 levels in probiotic birds compared to controls or vitamin A-deficient birds. Humoral response to E. acervulina was unaffected by probiotic. Collectively, the project showed clear evidence of the immunostimulatory actions of a Lactobacillus-based probiotic, which enhanced immunity against an enteric disease and an immunosuppressive agent in broiler chickens. The research also expanded basic understanding of the function of gut mucosal immunity in broiler chickens and the underlying immunological changes associated with probiotic treatment. Further, given the economic impact of infectious enteric diseases in modern poultry production, probiotic supplementation offers a simple way to maintain normal beneficial microflora and enhance the birds' resistance to enteric infections, and, thereby, providing an additional tool for reducing morbidity and mortality caused by such infections.
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