Impact of feeding diets without mineral P supplement on the immune system of two laying hen strains

Minimizing the addition of mineral phosphorus (P) to diets of laying hens is of great interest in poultry farming and might be possible due to the ability of poultry to utilize phytate. However, certain changes in nutrient composition can impair the immune system and affect animal health and welfare. The objective of this study was to investigate the impact of a complete renunciation of mineral P supplementation on the immune system of 2 commercial laying hen strains, before and after the onset of egg laying. At the age of 15 and 20 wk, Lohmann Brown-Classic (LB) and Lohmann LSL-Classic (LSL) hens were placed in metabolic units and fed a corn-soybean-based diet either without supplemented P (P-) or with 1 g/kg supplemented mineral P (P+). After 4 wk, numbers of various leukocyte types in blood, spleen and cecal tonsils, functionality of lymphocytes and gene expression of pro- and anti-inflammatory cytokines were measured. The P- diet increased proportions of natural killer cells and CD4⁺CD25^high T cells in the cecal tonsils. Moreover, expression levels of IL-2 and IL-10 at the age of 24 wk were higher with the P- diet compared to the P+ diet. However, the P- diet led to lower immune cell counts in the blood, such as total leukocytes, total T cells and CD4⁺ T cells. No effects of mineral P renunciation were seen in blood antibody concentrations and splenocyte proliferation. Likewise, immune cell counts in spleen and cecal tonsils were not affected. Our findings suggest that the renunciation of mineral P supplementation modifies immune regulation within the gut-associated immune system and fosters an anti-inflammatory condition. In addition, higher immune cell subpopulations related to innate and humoral immunity as well as higher IFN-γ and SOD2 expression in general were found in LB hens, whereas a more pronounced cellular immunity and higher expressions of IL-1β, IL-10, TNF-α and iNOS were found in LSL hens, reconfirming and extending our knowledge of strain-specific differences.