Membrane lipid modification and immune function

Lipids, membrane receptors, and enzymes: effects of dietary fatty acids

Dietary polyunsaturated fatty acids can significantly affect many biochemical and physiologic functions that are related to inflammatory, immune, and protective reactions. The different types of fatty acids can impact on energy metabolism, determine the lipid composition of membranes, and influence eicosanoid synthesis, all of which are relevant to prevention of and recovery from illness. In this paper, the effects of dietary polyunsaturated fatty acids on membrane composition, membrane-associated enzyme and receptor functions, signal transduction, second messenger, and eicosanoid generation are summarized. The differential effects of the polyunsaturated fatty acids of the n-6 and n-3 families are reviewed in the context of optimizing levels in diets.

Absence of unsaturated fatty acid synthesis in murine T lymphocytes

Stearic acid is toxic for T lymphocytes in vitro but has little effect on B lymphocytes. To investigate the molecular basis for this difference, purified murine T and B lymphocytes were compared for their abilities to incorporate and metabolize stearic acid. Unstimulated T and B cells incorporated identical amounts of stearic acid into six different phospholipids and four neutral lipids. After mitogen stimulation, fatty acid uptake was increased in both lymphocyte types, but cell-specific differences were seen in the distribution of stearic acid among the various cellular lipids. Doses of stearic acid that selectively inhibited T-cell proliferation resulted in a 5-fold greater accumulation of distearoylphosphatidylcholine in T cells than in B cells. Whereas T cells did not desaturate the exogenously derived stearic acid, up to 25% of the saturated fatty acid was converted to oleic acid in B cells. These findings suggested a deficiency of stearoyl-CoA desaturase (acyl-CoA, hydrogen-donor:oxygen oxidoreductase, EC 1.14.99.5) activity in T cells, which was confirmed by subsequent studies. Cell-free extracts from B cells displayed nearly 20-fold more stearoyl-CoA desaturase activity than T-cell extracts, and the level of stearoyl-CoA desaturase mRNA was 30-fold higher in B cells. Collectively, our data indicate that murine T cells are deficient in unsaturated fatty acid synthesis. The deficiency of stearoyl-CoA desaturase in T cells may represent the basis for the differing sensitivities of T and B lymphocytes to inhibition by saturated fatty acids.

Suppression of adjuvant arthritis in rats by cholesterol

Dietary cholesterol suppressed adjuvant arthritis, a chronic inflammatory disease, in rats, but did not significantly affect carrageenin edema, an acute inflammation. When rats were fed a high-cholesterol diet beginning 10 days before injection of adjuvant, the development of the adjuvant-induced arthritis was greatly suppressed. Cholesterol feeding prevented hypertrophy of the adrenal gland in arthritic rats, but had little influence on the serum corticosterone level. A significant positive correlation was observed between the adrenal weight and the severity of the arthritis. These findings suggest that the effect of cholesterol feeding is not due to increased adrenal sterol synthesis. Dietary cholesterol also prevented hypertrophy of the spleen, but had no effect on atrophy of the thymus in adjuvant-treated rats. Cholesterol-fed rats showed a significant decrease in the serum lipid peroxide level and a significant increase in the serum copper level. Adjuvant treatment not only enhanced hypercholesterolemia produced by cholesterol feeding, but also the level of free cholesterol in serum. These results suggest that dietary cholesterol may exert some effect on the immune response through changes in spleen and liver functions.