Arginine (ARG) is an immunologic modulator due in part to its role as a substrate for leukocytes. Therefore, the objective of these studies was to evaluate the impact of dietary ARG on ARG utilization in peripheral blood mononuclear cells (PBMC) and thymocytes in broiler chicks as they provide a model that is not confounded by de novo ARG synthesis. Several experiments were performed to measure: 1) ARG transporters as markers of ARG utilization by leukocytes; 2) thymocyte proliferation; 3) PBMC phenotype, and 4) ARG’s impact on the acute phase response (APR). In experiment 1, chicks were fed diets containing 1.20% or 1.35% dietary ARG. On d7, PBMC CAT-1 mRNA abundance was 2-fold higher in chicks fed 1.35% ARG than in those fed 1.20% ARG (P<0.05). Between d3 and d14, 1.20% ARG increased thymocyte y+LAT-2 mRNA abundance 2.5-fold (P<0.05) while 1.35% ARG did not. In experiments 2 and 3, chicks were fed diets containing 1.1%, 1.3%, and 1.5% dietary ARG. On d10, 1.5% ARG reduced thymocyte proliferation compared to 1.1% and 1.3% ARG (P<0.05). The percentage of peripheral blood B cells tended (P=0.06) to have an age by ARG interaction. On d14, but not on d10 or 17, the percentage of monocytes from chicks fed 1.5% was higher than from those fed 1.1% and 1.3% ARG (P<0.05). In experiment 3, chicks from four replicate pens per treatment were not injected (control) or injected subcutaneously with Salmonella typhimurium lipopolysaccharide (LPS; 1 mg/kg BW) to initiate the APR. At 2 h post LPS injection, there tended (P=0.08) to be a LPS by ARG interaction for TGF-β4 mRNA abundance. Therefore, these studies indicate that the optimal level of dietary ARG supplementation for utilization by ARG transporters in leukocytes is higher than what is sufficient for growth and is both age and leukocyte dependent. Additionally, these studies indicate that dietary ARG levels above the NRC requirement may be beneficial for the abundance of PBMC sub-populations. These studies also indicate that the APR may be manipulated by dietary ARG.

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