Effect of exogenous lipids and lipoproteins on the primary immune response in vitro

Nucleic acid-binding properties of low-density lipoproteins: LDL as a natural gene vector

The role of apo B-100 as a transcription factor is indicated by the presence of regions in its primary structure that are similar to the DNA-binding domains of the transcription factors ISGF3gamma, STATs, IRFs, and SREBPs as well as by the presence of 11 RNA-binding KH domains. The Apo B-100 sequence also contains numerous bipartite nuclear localization sequences (NLS). A modified gel shift assay was used to show binding of highly purified preparations of human LDLs to fragmented genomic DNA, plasmid DNA, synthetic oligonucleotides (ISRE, 5'-GGGAAACCGAAACTG and E/C, E-box motif and CCAAT, adipocyte-specific genes promoter site), and total RNA from human liver. LDL was observed to bind preferentially to plasmid DNA containing the hCMV IE2 promoter region. In experiments using human liver total RNA, RNA for five different genes was recovered from LDL and VLDL bands. Gene transfection experiments using human skin fibroblast cells were used to study the gene transfer capacity of LDL. Cells transfected with a pEGFP-N1 plasmid DNA and LDL expressed functional GFP, as indicated by fluorescence, at approximately 3 hrs after transfection. Our results strongly support an alternative role for apo B-100, in toto or perhaps as functional fragments, in the control of gene expression and as gene transfer vector.

Immunosuppression by pulmonary surfactant: mechanisms of action

Pulmonary surfactant has been shown by this group to suppress peripheral blood lymphocyte responses to mitogens and alloantigens in a dose dependent manner, though the mechanism of action of the suppressive effect is not clearly understood. To try to clarify this, attempts were made to reverse the effects of preincubation with surfactant, obtained by bronchoalveolar lavage from pigs, on lymphocytes and accessory monocytes obtained from the blood of normal volunteers, by washing and incubating the cells in medium alone for various periods up to 24 hours. Immunosuppression, measured as the reduction in thymidine incorporation in response to the mitogen phytohaemagglutinin, could not be reversed by these methods. The addition of indomethacin (up to 100 micrograms/ml for 72 hours) also had no effect, indicating that the immunosuppression was not related to synthesis of prostaglandins. Incubation with surfactant for as little as two hours before addition of mitogen suppressed in vitro lymphoproliferative responses by half, but surfactant added two hours after mitogen had no observed effect. Preincubation of purified lymphocytes in surfactant, before they were cultured with accessory monocytes and mitogen, caused significant suppression of response, but preincubation of purified monocytes had no suppressive effect. There was no change in the intensity of HLA-DR expression on monocytes. These results support the hypothesis that surfactant exerts its effects on the resting uncommitted lymphocyte rather than on antigen presenting monocytes.

Development of a serum-free medium for in vitro immune responses by using beta-cyclodextrin. Demonstration of the requirements for polyamines

A serum-free medium has been developed which is able to support primary antibody responses by cultured murine lymphocytes. This medium is based on RPMI 1640 that is supplemented with beta-cyclodextrin, insulin, transferrin, albumin, low density lipoprotein, putrescine and L-alanine as substitutes for fetal calf serum. Omission of anyone of these components resulted in a marked decrease of antibody responses. By employing the serum-free culture conditions, it was clearly demonstrated that polyamines such as putrescine, spermidine and spermine had positive effects on the development of antibody-forming cells. This serum-free medium supported the antibody response to sheep erythrocytes, trinitrophenyl (TNP)-Ficoll or TNP-lipopolysaccharide as efficiently as 10% fetal calf serum-containing medium. In addition, murine lymphocytes proliferated in response to an antigen or a mitogen equally well in these two types of medium.

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

A correlation has been sought between the effects of lipids on membrane fluidity and mitogen responsiveness of human peripheral blood lymphocytes (PBL). Cholesterol and the reputedly potent membrane fluidizing agent AL721 (1) were used for these studies. However, the large AL721 induced increase in membrane "fluidity", assessed by steady state polarization of the probe 1,6-diphenyl-1,3,5-hexatriene (DPH), was found to be an artefact arising from lipid uptake by monocytes. Mitogen responses were enhanced by AL721 but unaffected by cholesterol. It is concluded that AL721 does not exert its effect through enhanced triggering of cells by altered membrane fluidity but rather that lymphocytes require an exogenous source of phospholipids/triglycerides for optimal growth in vitro, although they can synthesize sufficient cholesterol to meet their own needs.

Lymphocyte membrane lipid composition and mitogen responsiveness in chickens: role of membrane "fluidity"

After establishing optimal conditions for measuring the membrane lipid packing density ("fluidity") of chicken peripheral blood lymphocytes, the fluidity was modulated in vitro by incubation in cholesterol or phospholipid ("active lipid", AL)-enriched serum-free tissue culture medium. The effect of these lipids on mitogen responsiveness was then investigated, the aim being to determine whether the observed enhancement/suppression was membrane mediated, i.e. explainable by fluidity changes. Chicken peripheral blood lymphocytes exhibited no requirement for exogenous cholesterol; low concentrations did not affect the mitogen response while the higher concentrations, which induced a measurable decrease in membrane fluidity, were usually mildly suppressive. Pre-incubation did not increase this suppressive effect and we believe it not to be membrane mediated. AL, at low concentrations which induced no changes in membrane fluidity, prolonged the phytohemagglutinin response, enhancement being evident only after the peak; we interpret this as a nutrient effect. At the higher concentrations, which induced large increases in fluidity, a transient enhancement was followed by suppression; suppression was delayed in onset when AL was added 4 h after phytohemagglutinin stimulation. It is therefore an early event which may be mediated through changes in membrane fluidity.

Modulation by lipoproteins of amphotericin B-induced immunostimulation

Previous reports indicate that amphotericin B (AmB) and amphotericin methyl ester (AME) are potent adjuvants and polyclonal B-cell activators, and that most mouse strains can be classified as high or low responders to AmB and AME. In the present study, an inbred strain with very high plasma cholesterol concentration (HC strain) proved to be a low responder. Responses of HC mice to other immune stimuli were normal, suggesting that HC lymphoid cells expressed selectively weak responses to AmB and AME. Plasma levels of low-density lipoproteins (LDL), very-low-density lipoproteins (VLDL), and high-density lipoprotein subfraction (HDL2) were very much higher in HC mice than in AKR mice, an AmB-high responder strain. Low responses in vitro to AME were observed with lymphoid cells from HC mice and from AKR----HC bone marrow chimeras, i.e., AmB-high responder strain lymphocytes from a low responder host. However, enhanced AME responses were induced by a 2- or 48-hr preincubation of splenocytes from either HC or AKR mice in medium containing lipoprotein-depleted fetal calf serum. Taken together these studies indicate that plasma lipoproteins can inhibit lymphocyte responses to AME; this seems to account for the low AmB and AME responses of the HC strain. The mechanism of this lipoprotein-induced inhibition remains obscure, but it cannot be accounted for by competitive binding of AmB by lipoproteins.

The relationship between adjuvant and mitogenic effects of amphotericin methyl ester

The antifungal polyene amphotericin B (AmB) and its methyl ester derivative (AME) both show potent murine immunostimulant as well as B-cell activating effects. Under certain experimental conditions, AME is a much more potent polyclonal B-cell activator (PBA) than AmB. Notable features of the murine B-cell stimulation induced by AME include: (i) High concentrations of AME (50-100 microgram/ml) are required and even at this level exhibit little or no spleen cell toxicity. (ii) Several lines of evidence suggest that the B-cell activating properties of AME are not involved in the cellular mechanism of adjuvant activity in vivo. (iii) There is a strong correlation between the magnitude of the in vitro PBA effects and the in vivo adjuvant effects of AME in a survey of different mouse strains. This evidence suggests that there is genetic control of the murine lymphoid cell-stimulatory effects of AME and that a small number of genes determines the responsive phenotype.

Enhancement of formylmethionyl-leucyl-phenylalanine-induced directional locomotion of human neutrophils by serum low-density lipoproteins

Human low-density lipoproteins (LDL) significantly increased neutrophil locomotion in a gradient of the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP), while LDL alone had no effect on either directional or random locomotion. The enhancing effect on directional locomotion was constantly observed at FMLP concentrations of 10(-8) M and 5 X 10(-8) M after addition of 50 microgram LDL/ml.

Modulation of concanavalin A-induced lymphocyte stimulation by human low-density lipoproteins

Spleen lymphocytes and T cells were stimulated by concanavalin A in the presence of various concentrations of human low-density lipoproteins (LDL). The proliferative responses were measured by [14C]thymidine incorporation. Low LDL doses (5-50 microgram/ml) significantly enhanced the stimulation of splenic lymphocytes and T cells. Further, LDL had the capacity to partially relieve the suppression produced by supraoptimal doses of concanavalin A.

Identification of phospholipids and neutral lipids in human gingival fluid

Gingival fluid was collected from children with a gingival index score of 2. Lipids were extracted, purified, and separated by thin-layer nanochromatography and identified. Lysophosphatidylcholines, sphingomyelins, phosphatidyl-cholines-ethanolamines-serines-inositols, polyglycerophosphatides, diphosphatidylglycerols, non-esterified fatty acids, mono- and di-glycerides, and cholesterols were observed. Serum origin of these lipids was possible, but enrichment by salivary bacterial or tissue lipids cannot be eliminated.