Changes in T cell subpopulations of sheep due to age and dietary protein intake; association with protective immunity to Trichostrongylus colubriformis

Purified lymphocytes from peripheral blood and cannulated intestinal lymph were examined by flow cytometry, to study the effect of age, level of dietary protein, and prior exposure to helminths on their composition. This was then compared with animals resistant to Trichostrongylus colubriformis infection. When young lambs consumed a low protein diet, they developed significantly lower protective immunity to the parasite than age-matched animals offered a high protein diet, or older animals irrespective of diet. In the peripheral blood there was an increased percentage of CD4+, CD5+ and CD8+ cells with increasing age (from 8 to 51 weeks). The percent of CD4+ cells in lambs that repeatedly ingested infective larvae was significantly increased in both peripheral blood and mesenteric lymph, compared with non-immunized animals. There was a significant interaction of dietary protein and immunization, as the percentage of T19+ cells from mesenteric lymph in young lambs that consumed a high protein diet increased when they were sensitized and challenged with larvae, while this was not apparent in those that consumed a low protein diet. The increase of peripheral and local CD4+ cells may contribute to development of protective immunity to the parasite. In addition, local T19+ cell populations in young animals were influenced by the status of protein in the diet, which may in turn influence immunity.