Karine N. Traill

Lipids and lymphocyte function

Lipids have a variety of important biological functions, serving as storage and transport forms of metabolic fuel, as structural membrane components, and as cell-surface components conferring, for example, tissue and species specificity. Not surprisingly, because the immune system functions through a complex series of stimulatory and regulatory interactions, there has been speculation about the role of lipids as lymphocyte membrane components. Here Kay Traill and George Wick discuss recent research on the possible influence of lipids on receptor binding, signal transmission and the stimulation of lymphocytes to effector function.

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 Communitys 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.

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.

Simple method for comparing large numbers of flow cytometry histograms exemplified by analysis of the CD4 (T4) antigen and LDL receptor on human peripheral blood lymphocytes.

We have developed a simple method for comparing the relative fluorescence intensity (FI) of flow cytometry histograms. It entails assessment of the FI (equivalent to the fluorescence-activated cell sorter (FACS) channel) of the 50th or 75th percentiles of either positively stained cells or the total cell population. We illustrate the method with dilution curves of 1) monoclonal antibodies against the T4 surface antigen of human peripheral blood lymphocytes and 2) fluorescent low density lipoprotein (LDL) binding to the human peripheral blood lymphocytes LDL receptor. We demonstrate the versatility of the method by characterizing the binding properties of fluorescent LDL to their receptors. Binding was shown to be specific and of high affinity, and to reach a steady state plateau at about 2 hr; the affinity of fluorescent LDL for the receptor was found to be two to three times higher than that of the unlabeled LDL.

Chicken thrombocytes. Isolation, serological and functional characterisation using the fluorescence activated cell sorter

Different procedures for enrichment of peripheral blood leukocytes (PBL) from whole chicken blood have been compared in terms of the percentage yield of lymphocytes and thrombocytes. The yield of thrombocytes was low in the buffy coat cells of heparinised blood, but high in PBL enriched from heparinised blood over a Ficoll-Paque density gradient and in PBL prepared from citrated blood by either technique. The fluorescence activated cell sorter (FACS) has been used to separate thrombocytes from lymphocytes in PBL by sorting of the negative cells after staining with specific anti-T and anti-B cell sera (or anti-immunoglobulin). The sorted cells were more than 99% viable, more than 98% thrombocytes by antigenic and morphological criteria and did not respond in vitro to T and B cell mitogens. Absorption studies on a turkey anti-bursa cell serum (ABST) which is cross-reactive for B cells and thrombocytes (8) indicated that the cross-reactivity is attributable to two (or more) antibody specificities in the serum rather than to one antibody directed at shared determinants on the two cell types.

Lack of correlation between serum cholesterol levels, lymphocyte plasma membrane fluidity and mitogen responsiveness in young and aged chickens.

Chickens were studied in an attempt to demonstrate correlations between serum lipid levels and peripheral blood lymphocyte (PBL) plasma membrane fluidity and mitogen responsiveness: (a) in the laying hen; (b) during aging; and (c) following dietary manipulation of serum cholesterol of young and aged chickens. The membrane fluidity of PBL from laying hens was significantly greater than that of immature birds. However, no direct correlation was found between serum lipid levels, nor the serum free cholesterol/phospholipid (FC/Pl) mole composition and PBL membrane fluidity in any of the age-groups tested. Likewise, no correlation was found either between serum FC/Pl mole ratio or membrane fluidity and mitogen responsiveness of PBL from birds up to 5 years of age nor was there any evidence for a decline in mitogen responsiveness up to this age. Supplementation of diets with 1% cholesterol induced hypercholesterolemia, mainly in the very low density lipoprotein (VLDL) fraction, but membrane fluidity and mitogen responsiveness remained unaffected.

High density lipoprotein uptake by freshly isolated human peripheral blood T lymphocytes

A high density lipoprotein (HDL) receptor/binding site has been identified on peripheral blood lymphocytes, and some of its properties were compared with those of HDL receptors on other cell types. Binding studies were performed using fluorescent (dioctadecylindocarbocyanine)-labelled HDL (DiI-HDL) and analyzed by fluorescence microscopy and quantitative flow cytometry. Uptake of low levels of DiI-HDL during a 2 h incubation at 37 degrees C was a property of all lymphocytes, i.e. not of one particular subset only. Visual inspection of these cells in the fluorescence microscope revealed both membrane and cytoplasmic fluorescence, indicating that DiI-HDL become internalized during the 2 h incubation; internalization appeared to be a receptor-mediated process. In competitive binding studies, apo E-free HDL competed effectively for DiI-HDL binding, whereas LDL competed very weakly. Two features of DiI-HDL uptake are demonstrated which are unique to lymphocytes: 1) it was enhanced 3-6-fold by inclusion of EDTA in the incubation medium or by incubating in Ca2+/Mg2+ free medium, and 2) it was saturable at 37 degrees C.