Identification of albumin as the serum factor essential for the growth of activated human lymphocytes

Serum albumin improves recovery from spinal cord injury

A neuroprotective factor is shown to be present in mammalian serum. This factor is identified by Western blotting to be serum albumin. The serum factor and albumin both protected cultured spinal cord neurons against the toxicity of glutamate. The inability of K252a, a blocker of the high affinity tyrosine kinase receptor for members of the nerve growth factor family, to block the neuroprotective effect of the serum factor established that the serum factor is not a member of the nerve growth factor family. Post-injury injection of albumin intravenously or into the site of injury immediately after injury both improved significantly locomotor function according to Basso-Beattie-Bresnahan assessment and spontaneous locomotor activity recorded with a photobeam activity system. Albumin has multiple mechanisms whereby it may be neuroprotective, and it is a potentially useful agent for treating neurotraumas.

The Anti-Apoptotic Activity of Albumin for Endothelium Is Mediated by a Partially Cryptic Protein Domain and Reduced by Inhibitors of G-Coupled Protein and PI-3 Kinase, but Is Independent of Radical Scavenging or Bound Lipid

Increased vascular disease occurs with low albumin (human serum albumin, HSA), possibly reflecting specific inhibition of endothelial apoptosis reported for tissue culture. Despite the reported specificity for endothelial protection by HSA, the high but physiological concentrations needed appear more consistent with non-specific low-affinity interactions. We reconcile this contradiction by demonstrating protection is mediated by a partially cryptic HSA protein domain, which becomes more exposed and active following cyanogen bromide fragmentation (p < 0.001). Also, although others reported HSA radical scavenging and bound lipids as important for inhibiting apoptosis in non-endothelial cell types, we demonstrate the protective effect for endothelium is unaffected when HSA radical scavenging is blocked by alkylation, or following delipidation. Further probing the mechanism responsible, we found that the G-coupled protein inhibitors pertussis toxin and suramin reduced protection of endothelium by HSA (p < 0.005), while the tyrosine kinase inhibitor genistein had no effect. Consistent with a role for phosphoinositide 3 kinase (PI3K) was inhibition by both wortmannin and LY294002 (p < 0.05), as well as phosphorylation of Akt, while MAP kinase inhibitors had no effect. We conclude the active site in HSA inhibiting endothelial apoptosis is partially cryptic, and acts via a G-coupled protein PI3K-dependent mechanism.

Endocytosis of three serum proteins of a multigene family and of arachidonic acid in human lectin-stimulated T lymphocytes

Alpha-fetoprotein (AFP), serum albumin (SA), and vitamin D binding protein (DBP) are members of a multigene family of proteins showing high structural homology. AFP and SA exhibit a reciprocal relation during development and carry mostly fatty acids, while DBP carries vitamin D and its metabolites in the plasma. Covalent conjugates of these proteins with horseradish peroxidase (HRP) were used to follow by cytochemistry, at the electron microscope level, the protein uptake and intracellular pathways in peripheral blood human lymphocytes stimulated to blast formation by phytohemagglutinin (PHA). Transferrin (Tf), an iron-binding plasma protein, was used as a control. Combined with the results of competition and saturability experiments reported elsewhere, the ultrastructural observations are in favour of a specific endocytosis of the four proteins through cell surface receptors. Tf and AFP enter the cells via small vesicles and endosomes and move to multivesicular bodies (MVBs) and tubular vesicular elements located in the Golgi-centrosphere region to be finally recycled back into the medium. A noncovalent conjugate of AFP-HRP with 3H arachidonic acid [3H-(20:4)] is strongly internalized at 37 degrees C in PHA-stimulated lymphocytes; the autoradiographic labelling, localized in cellular membranes and mostly in lipid droplets, was only occasionally associated with organelles where the presence of AFP-HRP was cytochemically detected. SA, which competes with AFP for a common binding site on the surface of activated T cells, is endocytosed through small vesicles, endosomes, and MVBs before being released in a degraded form from the cells, in agreement with the localization of SA-HRP in lysosome-like organelles. DBP-HRP is poorly internalized through noncoated vesicles, endosomes, and MVBs and is finally routed to lysosomes. The physiological role of AFP and SA would be to mediate the transfer of fatty acids into cells, while that of DBP would be to facilitate the intracellular delivery of vitamin D.

Oxysterol-induced cell death in human leukemic T-cells correlates with oxysterol binding protein occupancy and is independent of glucocorticoid-induced apoptosis

In eukaryotic cells oxysterols inhibit cholesterol biosynthesis and cell growth. A potent oxysterol, 25-hydroxycholesterol, was used to investigate the biological effects of oxysterols on three clonal lines of either glucocorticoid-sensitive or -resistant CEM cells, human leukemic T-lymphocytes. In addition, the glucocorticoid sensitivity of an oxysterol-resistant CEM cell line was tested. Oxysterols blocked growth and caused the lysis of cells regardless of their glucocorticoid response. All cells studied herein possessed an oxysterol binding protein with high affinity for 25-hydroxycholesterol. For all clones grown in serum-free medium, the half-maximal cytolytic concentration of 25-hydroxycholesterol (20-40 nM) correlated with its affinity (Kd = approximately 31 nM) for this oxysterol binding protein. Both cholesterol and mevalonate reversed 25-hydroxycholesterol cytotoxicity; 3-6 microM cholesterol or 0.1 mM mevalonate decreased 60 nM 25-hydroxycholesterol cytotoxicity by 50%. This cholesterol or mevalonate reversal appeared possible even after several days of 60 nM oxysterol treatment. The protective effect of cholesterol could be overcome by increasing 25-hydroxycholesterol concentrations. Cholesterol and mevalonate did not prevent glucocorticoid-mediated lymphocytolysis. Furthermore, the oxysterol-resistant line was sensitive to dexamethasone lysis. These data support the hypothesis that oxysterols and glucocorticoids act independently to block the growth of human leukemic lymphoblasts.

Serum-free cell culture

Cell culture is one important tool when studying cellular functions and molecular biology. It is also a basic method in most virological investigations. Serum has been an obligatory component in most cell culture media. During the last decades serum-free, chemically defined media have been developed, that are supplemented with a number of substances with specific cellular activities. The main developments of defined media are presented. Examples are given of investigations with different cell types.

Stimulatory effect of serum albumin on the proliferation of serum-free SV40-transformed Balb/c 3T3 cells

Commercial serum albumins have been found to be able to stimulate the proliferation of Balb/c 3T3 cells transformed by SV40, but not that of the normal counterpart. The effect is most pronounced with crystalline samples of albumin depleted of both globulin and fatty acid components, and depends on conditions used for the attachment and on seeding density. Physical and chemical treatments aimed to remove tightly bound impurities do not abolish the activity of fatty acid free serum albumin, thus supporting the idea that albumin per se is mitogenic towards these cells.

Effect of fetal and adult bovine albumins on human lymphocyte proliferation in vitro

Fetal and adult bovine albumins were isolated and compared with the commercially obtained fraction V of Cohn, in terms of their ability to support cell proliferation, when added as a supplement to serum-free lymphocyte cultures activated with phytohemagglutinin. It was found that fetal albumin is the least efficient of the three proteins in permitting lymphoproliferation. A difference between the kinetics of proliferation of lymphocyte cultures supplemented with albumin and with serum was also observed.

In vitro activation of bone marrow-derived T-and non-T-cell subsets by alpha-fetoprotein

Alpha-Fetoprotein (AFP) is a major serum glycoprotein during embryonic and early postnatal life. A number of diverse biologic functions have been attributed to AFP, including osmotic and carrier function and immunosuppressive activity. In this study we demonstrate that AFP selectively stimulates in vitro proliferation of two distinct subsets of adult murine bone marrow cells. One population of AFP-reactive bone marrow cells expresses surface receptors for soybean agglutinin (SBA) lectin. SBA+ bone marrow cells are resistant to cytotoxic pretreatment with T-cell-specific antisera and are not retained on Ig-anti-Ig affinity columns. The absence of conventional T- and B-cell markers, coupled with the presence of SBA receptors, suggests that AFP-activated non-T bone marrow cells may belong to an immature set of B lymphocytes. A second population of AFP-responsive bone marrow cells expresses the Thy-1+ Lyt 1+2- phenotype characteristic of conventional mature adult T helper cells. The potential physiological relevance of the mitogenic effects of AFP on bone marrow cells with respect to immunoregulatory processes in the fetal/newborn environments is discussed.

A defined basal medium for study of in vitro T-cell responses

A basal serum-free medium has been devised which supports short-term murine T-cell proliferative and functional assays in the absence of added lipids or serum albumin. Immune responses obtained with this medium are totally dependent on cell-cell interactions known to generate the lymphokines and monokines essential for replication and differentiation. Background activity is minimal, allowing better control of the specificity of response than is possible with serum-containing media. This medium is also suitable for the identification of added agents which modulate immune responses, and may facilitate the purification of secreted factors.

Effects of different protein supplements on mitogen responses of human peripheral blood lymphocytes

We evaluated the effects of 6 supplements often used in human lymphocyte cultures, including fetal calf serum, autologous human serum, pooled human AB serum, hypogammaglobulinemic human serum, bovine serum albumin and human serum albumin. Lymphocyte proliferation of unstimulated and mitogen activated peripheral blood mononuclear cells was measured by [3H]thymidine incorporation. The response of cells stimulated with the T-cell mitogen phytohemagglutinin-P were significantly lower when cultured in bovine serum albumin supplemented media, but were otherwise not supplement dependent. In contrast, responses of cells stimulated with the B-cell mitogens Cowan I strain of S. aureus and antisera against the mu or delta chain of human immunoglobulin were significantly effected by supplement. Cultures containing fetal calf serum and bovine serum albumin had high background responses without a proportional rise in cellular proliferation when B-lymphocyte-specific mitogens were utilized. Autologous human serum and pooled human AB serum contained immunoglobulin which interacted with each of the B cell mitogens, thus limiting their usefulness as in vitro supplements. Cells grown in human serum albumin supplemented media had minimal background and high stimulated responses to B-cell mitogens. These results indicate that human serum albumin is an optimal supplement for in vitro human lymphocyte proliferative assays since it supports high stimulated cell responses with low background activity, is devoid of immunoglobulin and had minimal variability among lots.

Propagation of large quantities of poliovirus in human lymphoblastoid cells grown in a serum-free medium

All three types of poliovirus replicated to good titers in Epstein-Barr virus-transformed human lymphoblastoid cells isolated from patients with infectious mononucleosis. Cells were grown in a serum-free medium, which consisted of enriched Eagle's medium with addition of 0.1% albumin, 0.1% Intralipid and 1 mg/l of transferrin. Large quantities of virus could be produced in 2- to 20-l roller flasks or in spinner flasks. The propagated poliovirus gave an antibody response that complies with requirements for vaccine production.

Lymphokine production and lymphocyte transformation by human mononuclear cells in a serum-free medium

In order to study the effects of purified serum proteins, such as complement components, on human lymphoid cell functions, we have developed a serum-free culture medium that supports lymphocyte transformation and lymphokine production. The medium is comprised of RPMI 1640 containing commercially available purified human serum albumin (HSA). Optimal proliferative responses to mitogens and antigens were noted at HSA concentrations of 2.5-10.0 mg/ml, and at a cell density of 1 X 10(6) mononuclear cells/ml. In medium supplemented with 5 mg/ml HSA, it was found that lymphocyte transformation responses, release of interleukin 2 and leukocyte inhibition factor (LIF), and the one-way mixed lymphocyte reaction proceeded in a manner analogous to that in plasma-supplemented cultures. Stimulation of LIF release by C3b was seen in HSA-supplemented cultures but not in serum-free conditions or in cultures containing 12% autologous plasma. This system can thus be used to examine cell-mediated immune responses without the interference of plasma proteins other than HSA.

A comitogenic serum glycoprotein in T lymphocyte activation

The production of Interleukin-2 is induced in helper cells, probably of T-cell origin, after stimulation by Interleukin-1. PHA is known to induce production of Interleukin-1 and Interleukin-2 in human PBL. We observed that PHA-induced mitogenesis in PBL requires the presence of a 90-KD-serum glycoprotein, which we were able to distinguish from other known serum proteins of similar molecular weight. Its biological activity can be destroyed by removal of its sialic acid residues. Evidence presented in this paper indicates that the function of this protein is related to the induction of synthesis and/or release of Interleukin-2. The presence of PHA does not seem to be essential at the level of Interleukin-2 production; the glycoprotein is able to induce Interleukin-2 in cooperation with a soluble mediator, which is produced by adherent cells. This mediator causes T-cell mitogenesis in PBL, provided serum or the 90-KD glycoprotein is present in the culture. It is not able to support the growth of a CTL line. We suggest the name PHILIP (Plasmatic Human Interleukin-2-Inducing Protein) for the 90-KD molecule.

Transferrin support of stimulated lymphocytes

Transferrin was tested for its ability to replace serum in supporting mitogen and allogeneic cell stimulated human lymphocyte proliferation. Although transferrin, at concentrations greater than 5 microgram/ml, was incapable of completely replacing the serum used to support phytohemagglutinin, Concanavalin A, and pokeweed mitogen, stimulated human lymphocytes, in the absence of serum it significantly augmented the proliferative responses observed for mitogen, yet not allogeneically-stimulated cells. Augmentation is not due to a nonspecific protein effect and appears to be independent of the metal content of transferrin. The mechanism of growth support appears to involve an effect of transferrin following the G1 phase in the initial cell cycle.

Development of a serum-free defined culture medium for lymphoblast transformation tests of mouse spleen and thymus cells

By analysis of thymidine uptake during the first 4 h of incubation and by examining the responsiveness of unfractionated mouse spleen cells upon mitogenic or allogeneic stimulation, some serum factors of major importance for in vitro cultivation of lymphocytes have been examined. Albumin and l-alanine are essential for the maximal preservation of the in vivo-initiated lymphocyte activity during the first hours of incubation in vitro. Transferrin plays a major role in the lymphocyte proliferation, induced by mitogens in vitro, and, finally, zinc and selenium exert a clear enhancing effect on the response to an allogeneic stimulation. The AATSZ medium (RPMI 1640 enriched with l-alanine, albumin, transferrin, zinc chloride, and sodium selenite) enables a proliferation of the same magnitude as or higher than FCS medium. The kinetics are the same, and the cell viability is comparable, but standardization is much simpler with AATSZ. This is primarily because FCS binds some mitogens and contains inhibitors. Consequently, the standardization of such a culture system is dependent on variations from serum batch to serum batch. On the other hand, the current composition of the AATSZ medium promotes the sticking capacity of T lymphocytes and does not support growth of all lymphoid cell lines. Consequently, this defined medium, although not yet suitable as optimal medium for all lymphocyte functions, can advantageously be used for short-term studies of most murine lymphocyte functional and cooperation studies in vitro.

Induction of immunoglobulin-producing human peripheral blood lymphocytes in serum-free medium

Induction of immunoglobulin-secreting cells from human peripheral blood lymphocytes in a serum-free culture medium was studied. Albumin, transferrin, insulin and fibronectin can replace serum entirely for support of pokeweed mitogen (PWM)-stimulated B lymphocytes, measured by a reverse hemolytic plaque assay using protein A-coated red cells. In this serum-free system, growth and maturation to IgM and IgG secretion occur at the same or higher efficiency as in conventional serum-containing medium, with maximum numbers of plaque-forming cells on day 6 at optimal dose of PWM, 0.5-5 micrograms/ml. This system can be used to avoid the interference from undefined serum components.

Evaluation of murine thymocyte stimulation using a defined culture medium

The ability of RPMI 1640 enriched with FCS or AATSZ (L-alanine, BSA, human transferrin, zinc chloride, and sodium selenite) to support mitogen-induced activation (G0-G1 shift) and proliferation (G1-S shift) of thymocytes has been investigated. The two culture media were found to be equally supportive. In terms of viability, differences were detected in the number of recovered viable cells, but this could be related to alterations in adherent properties, rather than viability of the cells. For the examination of a PHA-induced proliferation, IL-1-containing suppernatants, deriving from normal or induced peritoneal macrophages, were prepared. The supportive capacity of these preparations showed no significant difference between AATSZ and FCS. Despite the excellent supportive capacity for the mitogen-stimulated thymocyte cultures, the AATSZ medium was not able to support all established cell lines tested. A T cell (MOLT 4F) and a macrophage cell line (SK 1) grew equally well in AATSZ- and FCS-enriched medium, but a B cell (U 266) and a null-cell line (Reh) did not proliferate at all. When cells from the latter two lines were cultured in AATSZ medium, they did not complete the RNA-synthesis required for DNA-synthesis, as judged by cytofluorography. From the experiments presented it is concluded that the AATSZ medium offers several advantages, such as easy standardization of culture conditions, and no essential disadvantages for studying mitogen-stimulated thymocytes in vitro. On the other hand, some lymphoid cell lines do require culture conditions that the AATSZ medium cannot provide.

Role of serum in inhibition of cultured lymphocytes by lysophosphatidylcholine

Serum was heated at various temperatures to inactivate components which might be involved in the regulation of lysophosphatidylcholine (lysoPC) levels in rabbit lymph-node cell cultures. Cells cultured in medium containing serum preheated for 20 min at 66 degrees C ("66 degrees C-serum") were inhibited much more by exogenous lysoPC (5 microM) than were cells cultured in medium containing control serum ("38 degrees C-serum"). This was observed over a 20 h culture period as a slow increase in inhibition of cell labelling with [3H] uridine, which reflected cytotoxic cell damage. Heating serum at 66 degrees C caused (i) conversion of monomeric albumin to highly polymeric forms which were deficient in lysoPC-binding activity, (ii) transfer of lysoPC from albumin to lipoproteins, predominantly high density lipoproteins, and (iii) inhibition of two lysoPC metabolizing activities (which were detected only at low levels in control serum). Addition of albumin to cultures containing 66 degrees C-serum decreased the toxicity of lysoPC to the same extent as did the addition of control serum with an equivalent albumin content. Thus, albumin was the major heat-labile factor protecting cells against lysoPC. However, cell inhibition by lysoPC was dependent on the sequence of heating serum and lysoPC addition. Inhibition was small when lysoPC was added before heating the serum. This could not be explained by a detectable difference in the binding of lysoPC to serum components. Furthermore, although radioactive labelling of cells with [14C] lysoPC was increased in 66 degrees C-serum, this did not correlate with cell inhibition. Increased labelling with [14C] lysoPC occurred several hours before significant cell inhibition was evident and was not affected by the sequence of heating and lysoPC addition. Since preincubation of lysoPC with 66 degrees C-serum increased the inhibition, it is suggested that the heated serum lysoPC generates another factor which is responsible for the cytotoxic effects observed.

Human lymphocyte transformation induced by mitogens and antigens in a serum-free tissue culture system

The use of serum to supplement lymphocyte tissue culture media introduces uncontrolled variables; different serum sources, lots and concentrations can produce variability in experimental results, serum can stimulate or inhibit lymphocytes, and components of serum can react with substances whose effects on lymphocytes are being studied. To avoid these problems, we studied the ability of human peripheral mononuclear cells to survive and to respond to stimulation in an entirely synthetic medium, RPMI-1640 supplemented with L-glutamine, gentamicin, HEPES buffer and magnesium. Optimal cell concentration in this serum-free RPMI-1640 was 2.5 x 10(6) cells/ml, whereas optimal cell concentration in serum containing RPMI-1640 was 1 x 10(6) cells/ml. In this serum-free RPMI-1640, 50% of the cellular input was recovered as viable cells after 7 days of culture, which was similar to results in serum containing RPMI-1640. Mononuclear cell transformation transformation was induced by phytohemagglutinin, concanavalin A, pokeweed mitogen, streptolysin O and candida. Optimal doses of stimulants and the kinetics of the responses were similar in serum-free and serum containing RPMI-1640. This system can be used to avoid the problems inherent in systems which supplement tissue culture media with serum.

Dialyzable serum components can support the growth of hybridoma cell lines in vitro

Conditions are defined for the in vitro growth of antibody producing hybrid myeloma cell lines in an immediate environment free of non-dialyzable serum components. Continuous access to low molecular weight serum growth factors was critical to the maintenance of cell viability and antibody production. This requirement was met by use of two-chambered Marbrook vessels, whose geometry allows a small chamber containing metabolically active cells to communicate through a dialysis membrane with a large adjacent reservoir filled with serum-supplemented medium. This tissue culture technique permits the rapid isolation of proteins secreted by cells in vitro, and should permit the further definition of minimal nutritional requirements for the growth of differentiated cells in vitro.

Effects of ascorbic acid on 3H-thymidine incorporation by isolated mouse thymocytes

The 15-min incorporation of 3H-thymidine (Tdr) and 3H-uridine (Ur) into nucleic acids of freshly isolated mouse thymocytes decreased steadily with time of preincubation at 37 degrees C. Sodium ascorbate at 5 mM prevented the decline of 3H-Tdr incorporation by preventing the decrease in its uptake. No such effect was noted on the incorporation of 3H-Ur, suggesting that ascorbate might be more specific for cells in or near the S phase of the cell cycle. The ene-diol group on the ascorbate molecule was required for this function, as ascorbyl-2-sulfate was ineffective and dehydroascorbate (DHA) reduced 3H-Tdr incorporation even further. Ascorbate was also inhibitory at a lower concentration (0.1 mM) or lower cell density. Thiols such as dithiothreitol or reduced glutathione seemed to act like 0.1 mM rather than 5 mM ascorbate. The inhibition by 0.1 mM ascorbate was presented by 0.1% bovine serum albumin (BSA), catalase or anaerobiosis. BSA had its own protective effects on the cells, since at 0.1% it increased the uptake of both 3H-Tdr and 3H-Ur. The combined effects of 5 mM ascorbate and 0.1% BSA on 3H-Tdr uptake were additive, but some synergism was noted at the lower BSA concentrations. These results suggest that with low concentrations of ascorbate (0.1 mM) oxidative reactions occur in vitro, resulting in the accumulation of the toxic hydroxyl radical (. OH). High concentrations apparently override this inhibition by a mechanism possibly involving an increase in critical cellular thiol groups.

Transferrin can replace serum for in vitro growth of mitogen-stimulated T lymphocytes

Activation of human T cells by mitogens was compared in cultures containing serum, human serum albumin or purified human transferrin as growth support. The mitogenic effect of the lectins leucoagglutinin, concanavalin A and Wistaria floribunda agglutinin was measured as incorporation of [3H]thymidine into the cellular DNA of the lymphocytes. Three different preparations of transferrin were all able to fully substitute serum or serum albumin as growth promotors, when present at concentrations of 10 microgram/ml or more. A small contamination of transferrin in the human serum albumin preparations used was shown to be responsible for their growth-supporting effect, while no need for the presence of albumin itself could be demonstrated.

The binding of gitoxin to human plasma proteins

The binding of gitoxin, digitoxin and digoxin to human plasma proteins was measured by ultracentrifugation and equilibrium dialysis. At concentrations in the range of therapeutic plasma levels, protein binding amounted, respectively, to 85, 92 and 20%, the last two values being consistent with data reported in the literature. The affinity of purified human albumin was not significantly different for the three cardiac glycosides tested. No other protein than albumin was found to bind gitoxin in human plasma.

Elucidation of an A and L system for amino acid transport in the human lymphoblast using a membrane filtration technique

Optimum conditions have been established for the measurement of amino acid transport by human lymphoblastoid cell lines using a membrane-filtration technique. The parameters we found to be important for the reproducibility of the method are: the types and combination of filters, the strength of the vacuum applied to the filters and the density of the cultures at the time of harvesting and during uptake and filtration. We found that bovine serum albumin added to phosphate buffered saline (PBS) glucose in which the cells are washed, resuspended and assayed is essential for the maintenance of viability, the prevention of clumping and the retention of the accumulated amino acid. Using this procedure we have characterized two transport systems for the neutral amino acids; an A and an L system, which are similar but not identical to the A and L systems characterized in rodent cell lines. These A and L systems have characteristically lower Km's and Vm's for alanine and phenylalanine, when compared to rodent cell lines. In addition, we find alpha-AIB to be a poor competitor of alanine and phenylalanine uptake.

Role of fatty acids in growth-promoting effect of serum albumin on hamster cells in vitro

Dialyzed serum albumin had considerable growth-promoting effect on cultivated hamster cells. This effect was virtually lost on removal of the fatty acids, and it was completely restored by recombination of the fatty acid-free albumin with the isolated and purified fatty acids. The role of albumin itself appeared to be largely that of a carrier of fatty acids, protecting the cells against toxic effects of fatty acids in free solution. This conclusion was based on two observations: Fatty acids in the absence of albumin were growth-inhibitory except in extremely dilute solutions, and beta-lactoglobulin, a protein possessing, like albumin, the ability to bind and release fatty acids, could replace albumin in the presence of fatty acids with similar growth-promoting effect. Examination of individual molecular types of fatty acids showed that all unsaturated acids tested were growth-promoting, whereas the saturated acids were growth-inhibiting, with the exception of stearic acid in low concentrations. Although the possibility of a mitotic triggering effect was not excluded, the fatty acids presumably stimulated growth by providing substrate for cellular metabolism, since there was a direct relationship between the degree of growth stimulation and the duration of exposure of cells to the fatty acids.

Effect of serum and serum components on the mitogen-induced proliferation of murine lymphocytes

By increasing the concentration of human serum, the dose-response profile of the Concanavalin A-induced proliferation of murine spleen lymphocytes becomes broader and the optimal cell proliferation takes place at higher concentrations of mitogen. It is shown that this behavior is due to the globulin fraction of serum which greatly decreases the binding of Concanavalin A to lymphocytes. Cohn fractions III and IV have a similar effect. On the other hand, cell proliferation takes place also in the absence of Pooled Human Serum at appropriate concentrations of Concanavalin A and within a very narrow range of cell concentrations. In the presence of the albumin fraction of serum, proliferation takes place over a wider range of mitogen and cell concentrations. As a result of these phenomena, the mitogenic effect of Concanavalin A varies greatly in function of the concentration of serum. Analogous results were obtained with fetal calf serum.