The modulation of ovine T-lymphocyte subsets and lymphocyte blastogenesis by leucine and leucine metabolites
Experiments were conducted to determine the effects of leucine (Leu) and its first catabolic product, [alpha]-ketoisocaproate (KIC), on lymphocyte blastogenesis and peripheral blood T-cell subsets percentages in sheep. In replicate experiments 1 and 2, when KIC was orally administered to sheep at 0.1% of the diet, KIC fed animals had significantly increased phytohemmagglutinin-P (PHA) and pokeweed mitogen (PWM) stimulated blastogenesis compared to controls (P < 0.05) and ConA stimulated blastogenesis compared to Leu fed animals (P < 0.05). Leu fed animals tended to have decreased lymphocyte blastogenesis compared to controls. In the same study, KIC increased the percentage of circulating T4 cells (P < 0.05) while Leu significantly decreased the percentage of circulating T19 cells (P < 0.05). In Experiment 3, in which ACTH treated sheep were fed Leu or KIC, KIC increased blastogenesis over controls and Leu fed animals also increased the percentage of T4 cells, while Leu decreased the percentage of T19 cells (P < 0.05). ACTH injection decreased lymphocyte blastogenic responses regardless of the dietary treatment and increased serum cortisol and insulin concentrations. In Experiment 4, Leu, KIC and other metabolites were added to PHA stimulated cultured ovine lymphocytes and lymphocyte proliferation was measured. Leu significantly suppressed blastogenesis but KIC had no effect. Thus, previous studies indicating that Leu suppressed blastogenesis in vivo may be attributed to a direct affect of the compound although the previously determined stimulatory effects of KIC in vivo are likely indirect. Further studies indicate a hydroxy acid metabolite of Leu ([beta]-hydroxy-[beta]-methylbutyrate; HMB) may be responsible for the stimulatory actions in vivo in that HMB increased blastogenesis 100% when added to mitogen stimulated lymphocyte cultures;In conclusion, feeding Leu decreases lymphocyte blastogenesis and the percentage of circulating T19 cells which may have directly decreased blastogenesis. Conversely, feeding KIC enhanced lymphocyte blastogenesis and increased T4 cells which may be the indirect result of the production of the metabolite, HMB. Feeding KIC also tended to enhance ACTH-suppressed lymphocyte blastogenesis which could overcome stress-induced immune suppression.