Lipid requirements of human T lymphocytes stimulated with mitogen in serum-free medium. Membrane "fluidity" changes are an artefact of lipid (AL721) uptake by monocytes

The effect of aging on IgD receptor expression by T cells and its functional implications

Exposure to oligomeric or aggregated (a), but not to monomeric (m), IgD causes a rapid (within 1 h) upregulation of IgD-R expression on CD4+ T cells from young, but not from aged, mice and on both CD4+ and CD8+ T cells from all young and from approximately 65% of aged humans. In normal young (but not in IgD-/-) mice, this increase in IgD-R expression is associated with a marked increase in primary and secondary antibody responses, transferable to both aged and young mice with T cells from aIgD pretreated donors. In both species, immunization causes a rise in the IgD-R+ expression in vivo in the young. In mice, mIgD abolishes both the induction of IgD-R expression and augmentation of immune responses, suggesting that interaction between IgD-R+ T and IgD+ B cells is needed. In aged humans, the ability of peripheral blood lymphocytes to exhibit IgD-R expression in response to aIgD in vitro or to influenza vaccine in vivo is strongly correlated to the individual's ability to produce antibody. In T cells from aged mice, but not from aged IgD-non-responder humans, IgD-R are able to come to the cell surface if an additional signal has been supplied, such as by (ionomycin/thapsigargin + aIgD). Agents which induce IgD-R and augmentation of antibody production in aged and young mice include phosphatidylcholine and dehydroepiandrosterone sulfate. The immunoaugmenting effect of pretreatment with these agents appears indeed due to IgD-R+ T cells, because it is abolished by mIgD.

Correlation of lymphocyte lipid composition membrane microviscosity and mitogen response in the aged

Healthy aged and young blood donors were investigated for the role of membrane lipid composition in the age-related increase in membrane microviscosity and decline of mitogen responsiveness. Membrane microviscosity was shown to correlate positively with membrane cholesterol/phospholipid molar ratios, which were significantly elevated in the elderly. A positive correlation also was confirmed between lymphocyte membrane microviscosity, which was measured using the probe 1,6-diphenyl 1,3,5-hexatriene, and phytohemagglutinin responsiveness of cells from the same donor. Using stepwise regression statistical analysis, the variables age, cholesterol, cholesterol/total phospholipid and phosphatidyl ethanolamine/phosphatidyl choline molar ratios were all shown to have a significant positive influence on membrane microviscosity, whereas total phospholipids had a negative effect. No statistically significant difference was seen in content of any single saturated or unsaturated fatty acid between young and old donors. After pooling, however, the proportion of all unsaturated fatty acids was significantly higher in cells from the elderly as a consequence of an increase of n-6 and n-3 polyunsaturated fatty acids. Changes in lipid composition and physical properties of lymphocyte plasma membranes may, therefore, be responsible (at least partially) for the diminution of immune reactivity in old age.

Lipoprotein interactions with T cells: an update

The role of plasma lipoproteins in atherogenesis is well recognized but the physiological relevance of their immunoregulatory properties is still questioned. Here Karine Traill and colleagues outline the recent advances that have been made towards unravelling the mechanisms of immunoregulation by lipoproteins in vitro and consider whether any of these mechanisms are operative in vivo. In particular they address the possible detrimental effects of high serum lipoprotein levels on immune function and the question of whether hyperlipidemia (or hypercholesterolemia) should be considered a risk factor for diminished immunity, for example in old age.

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.

Approach to define "normal aging" in man. Immune function, serum lipids, lipoproteins and neopterin levels

In 53 healthy women with mean age 25.1 years (age range 21-34 years) and in 51 healthy women with mean age 82.1 years (age range 75-91 years), a panel of immunological and biochemical tests was performed. These tests, comprising lymphocyte mitogen responsiveness, phenotyping of lymphocytes, uptake of low density lipoprotein by T cells, serum levels of neopterin, lipids and lipoproteins, as well as routine blood chemistry, were investigated for a possible effect of age and of the classification according to the SENIEUR Protocol of admission criteria by the European Economic Community's Concerted Action Program on Aging (EURAGE). A highly significant effect of age on serum levels of neopterin, lipids and lipoproteins was found. No clear effects, however, of SENIEUR status on these variables was detected. As expected, age had a significant impact on mitogen responsiveness of T cells. Proportional numbers of helper/inducer and cytotoxic/suppressor T cells (as well as antigen density on these cells) were not influenced by age. SENIEUR classification did not affect these immunologic variables. Thus, most of the tested variables that are not included in the SENIEUR admission criteria appear to present information not yet covered by the SENIEUR variables. Various ways for a possible revision or extension of the SENIEUR Protocol are discussed.