Associated Project

The objective of the study Hofmann et al. 2021 was to investigate the effect of standard or reduced levels of dietary Ca and P at the laying peak on the number of various immune cell types in blood, spleen and cecal tonsils, in vitro mitogen-induced lymphocyte proliferation, and cytokine production of splenocytes, as well as the concentrations of immunoglobulin (Ig)M, IgY, and IgA in blood and IgA concentration in bile in two laying hen strains. The results reveal substantial effects of the dietary Ca/P content as well as the genetic background of laying hens on the number and function of immune cells in lymphatic tissues and blood. Immune parameters were generally higher when fed the reduced P diet compared to standard diets, suggesting a positive effect on immune function. Irrespective of the strain, diets low in P concentration stimulated the number of circulating B cells and IgA concentration in bile. LB hens apparently profit more from diets low in P concentration, since only this hen strain showed increased number of peripheral monocytes and γδ T cells in cecal tonsils, as well as higher in vitro IFN-γ production of spleen-derived lymphocytes when fed reduced P diets, whereas these leukocyte types were unaffected in LSL hens. In conclusion, several enhancing effects of the low P diets on the number of innate and adaptive immune cells as well as antibody concentration were seen in both strains, however with more pronounced effects in LB hens.

The objective of the second study (Schmucker et al., submitted) was to characterize the immune system of LB and LSL hens throughout their productive life span. Immune cell numbers in blood as well as in spleen and CT were investigated at periods representing different maturation and reproductive stages of the hens (week of life 9/10, 15/16, 23/24, 29/30, and 59/60). Measurements at spleen level included also functional assessments.

Prominent changes occurred at the transition to egg laying activity (week 15/16 - 23/24) in blood and spleen. Cytotoxic T cells and γδ T cells, thus the cellular arm of the immune system, decreased in number, while phagocytotic immune cells increased, which resulted also in an increased ratio of heterophils to lymphocytes. Regarding immune function, pokeweed mitogen-induced IFN-γ production of splenocytes was higher at the onset as well as at peak of laying activity (week 24 and 30), whereas mitogenic stimulated proliferation did not change with the respective periods of life. The temporal dynamics of immune parameters in blood and spleen throughout their productive life span had a similar pattern for both strains, with only few interactive effects of strain and week of life for a few immune cell types.

Summarizing, the results of study 2 show that whereas further development of the immune system in young and growing hens most probably contributes to an enhancement of immunocompetence, this effect is reversed by the onset of laying activity. The alterations in immune cell distribution thereby point to a shift towards humoral and innate immune responses and are maintained during ongoing egg-laying activity, which might challenge immunocompetence of laying hens.