Effects of the color additive caramel color III and 2-acetyl-4(5)-tetrahydroxybutylimidazole (THI) on the immune system of rats

Administration of ammonia caramel color (AC) to rats may decrease blood lymphocyte counts, specifically in rats fed a diet low in vitamin B 6 . This effect is associated with 2-acetyl-4(5)-(1,2,3,4-tetrahydroxybutyl)imidazole (THI). To characterize and compare the effects of AC and THI and to study the influence of dietary pyridoxine, two studies in rats were conducted. Weanling rats fed a diet containing 2–3 ppm pyridoxine and exposed to 4% AC or 5.72 ppm THI in drinking water for 4 weeks showed reduced cell numbers in spleen and popliteal lymph nodes, as well as in the blood. Flow cytometric analyses demonstrated a comparable reduction in B and T lymphocytes. In blood, spleen, and popliteal lymph nodes, CD4 + lymphocytes were more reduced than CD8 + cells. The number of bone marrow cells was not affected. Although thymus weight and cell number were not affected either, a decreased cortex over medulla area ratio and an increase in medullary cell density largely due to an increase in CD4 + thymocytes was observed. Decreased numbers of ED2 + macrophages were observed in the thymic cortex and in the spleen red pulp. All effects observed were either less pronounced or absent in a study with rats fed a diet containing 11–12 ppm pyridoxine. The effects of AC and THI on the immune system were similar. Whereas AC exposure was associated with changes in vitamin B 6 status, THI did not induce obvious effects on vitamin B 6 parameters. It is proposed that the effects of AC and THI on the immune system are not caused by a disturbance of vitamin B 6 metabolism, but may in fact result from a disturbance of a specific PLP-dependent process.