Failure of T lymphocytes from elderly humans to enter the cell cycle is associated with low Cdk6 activity and impaired phosphorylation of Rb protein

Cell cycle analyses of activated T lymphocytes from elderly humans generally show that the proportion of noncycling cells increases with age. T cells that are not definitively blocked in G0 usually strain to traverse the G1 phase and may still be arrested at the G1/S boundary. The molecular mechanisms underlying these cell cycle arrests are unknown. Because G0/G1 and G1/S transitions are regulated in part by cyclin-dependent kinase Cdk6, we investigated the possibility that a loss of activity of this kinase is implicated in the age-related dysfunction of the cell cycle in its initial phases. G0/G1 and G1/S blocks were first confirmed by [(3)H]uridine and [(3)H]thymidine incorporation studies in anti-CD3 activated T lymphocytes derived from elderly donors. In the same cell preparations, in vitro phosphorylation of recombinant truncated Rb protein by immunoprecipitated Cdk6 was significantly decreased. The reduced Cdk6 activity was not attributable to a low level of the protein since a 24-h activation resulted in a comparable expression of the kinase in T cells from young and old individuals. However, at least two other mechanisms might be incriminated in the loss of Cdk6 activity: (1) a poor induction of the associated cyclin D2 upon anti-CD3 stimulation and (2) a delayed downregulation of the Cdk inhibitor p27 following cell activation. The low Cdk6 activity observed in T lymphocytes from the elderly was associated with a defective phosphorylation of the endogenous Rb protein and an increased sequestration of the E(2)F-1 transcription factor, possibly resulting in early cell cycle arrest.

Age-associated decline in cdk1 activity delays cell cycle progression of human T lymphocytes

Despite the repeatedly observed impaired proliferative response of T lymphocytes from aged donors, the precise molecular basis underlying such a defect is still poorly understood. The aim of this study was to determine whether cyclin-dependent kinase 1 (cdk1), a serine-threonine kinase required for entry into mitosis, is implicated in this age-associated dysregulation of the cell cycle. T lymphocytes derived from young and elderly donors were blocked in S phase by hydroxyurea after a 48-h activation by anti-CD3 Abs. Under these experimental conditions, only the cells that were already located beyond the S phase were able to complete the cell cycle, decreasing their DNA content from 4n to 2n chromosomes. Using this procedure, a delay in the accomplishment of mitosis could be observed in cells from elderly individuals, as evidenced by propidium iodide staining. In this age group, only a minimal cdk1 activity could be immunoprecipitated from cells sorted in G2/M after nocodazole block. The decrease in cdk1 activity observed in T lymphocytes from aged donors could be accounted for by at least three mechanisms: 1) a failure of these cells to express a sufficient amount of cdk1, 2) a reduced level of the associated cyclin B1, and 3) an incomplete dephosphorylation of the kinase on tyrosine. This low cdk1 activity is likely to postpone the progression through the G2/M transition and participates in the dysfunction of the cell cycle during the process of aging.

Susceptibility to apoptosis of T lymphocytes from elderly humans is associated with increased in vivo expression of functional Fas receptors

We recently showed that mature T lymphocytes derived from elderly humans were more susceptible to activation-induced cell death than similar cells from young individuals. Because this excessive apoptosis is unrelated to either the age-associated decrease in IL-2 production, a differential Bcl-2 expression or to a modification of the antioxidant pathway, we examined the possibility that the Fas receptor (FasR) is directly implicated in the generation of the unwarranted death signal. We investigated the expression and the function of FasR on T lymphocyte populations from healthy young and elderly individuals. We found that the frequency of FasR+ T cells increases as a function of age. The FasR expressed at the surface of freshly isolated T lymphocytes from elderly donors appear to be fully functional since their ligation by a cytocidal IgM anti-Fas mAb leads to a significant increase in DNA fragmentation in this cell population. Conversely, exposure of T cells derived from aged individuals to an antagonistic anti-FasR mAb partially prevents the age-related increase in apoptotic cell death. The population of FasR+ T lymphocytes is essentially constituted of previously activated CD45RO+ cells and also includes recently activated lymphocytes bearing the CD25 and CD69 activation markers. The accumulation of chronically and recently in vivo activated T-cells with age probably contributes to the amplification of the process of Fas-mediated cell death in T lymphocytes isolated from senescent organisms.

[Molecular mechanisms of age-related lymphocyte dysfunction]

Aging is classically accompanied by a dysregulation of the immunologic machinery. As a consequence, the immune response developed in senescent organisms is usually inappropriate, often inefficient, sometimes aberrant, and potentially detrimental. The age-associated immune dysfunction may be implicated to some degree in the extreme susceptibility of the elderly to infection and neoplasia and may even participate in various aspects of senescence. The current understanding of the molecular mechanisms underlying immunosenescence is still fragmentary. The most extensively studied phenomenon is the progressive decline in the proliferative capacities of T lymphocytes with aging. The loss of proliferative potential in response to antigenic challenge is a characteristic feature of immune senescence. It is directly implicated in the emergence of the age-related immune deficiency. The purpose of this review is to show how the accumulation of various biochemical lesions with advancing age leads to the failure of a critical cell function, namely the activation-induced lymphocyte proliferation. The biochemical modifications responsible for the defect in transduction and execution of the proliferative signal are analyzed as a function of age. The multiple alterations observed on the various biochemical pathways may appear as a consequence of a unique deleterious mechanism more fundamentally related to the process of senescence such as the inability to cope with oxidative stress.

Age-related tyrosine-specific protein phosphorylation defect in human T lymphocytes activated through CD3, CD4, CD8 or the IL-2 receptor

Although transmembrane signaling defect has been recognized as one of the major functional alterations involved in immune senescence, its biochemical nature as well as its precise molecular localization are still unknown. The available data indicate that an early step in the signaling cascade may be affected during the aging process. Because protein tyrosine kinases (PTK) are ubiquitously implicated in the initiation of physiological signals, they appear as prime candidates for age-related changes. The present investigation examined the effect of age on the activity of PTK associated with CD3, CD4, CD8 or the IL-2 receptor (IL-2R) in human T lymphocytes. By comparison with cells derived from young individuals, anti-CD3-activated T lymphocytes from elderly donors were more susceptible to herbimycin A, a PTK inhibitor known to prevent signal transduction by the T cell antigen receptor. This increased sensitivity of cells from senescent organisms to PTK inhibitors is most likely related to a lesser PTK activity since a significant decrease in the tyrosine phosphorylation of particular endogenous substrates was observed as a consequence of either CD3, CD4, CD8 or IL-2R activation. However, no age-related difference in tyrosine phosphorylation could be demonstrated when T cells were activated by pervanadate, a pharmacological activator of PTK. These results suggest that the intrinsic activity of the enzymes is preserved and that the age-associated defect in PTK activation occurs as a consequence of an upstream biochemical alteration. The defect in PTK activation could be the primary cause for the dysfunction of various components of the signaling cascade observed during the course of aging.

Excessive apoptosis of mature T lymphocytes is a characteristic feature of human immune senescence

Recent evidence suggests that apoptotic deletion of activated mature lymphocytes is an essential physiological process implicated in both the regulation of the immune response and the control of the overall number of immunocompetent cells. Tightly interrelated signaling mechanisms convey either activation or death messages, achieving the necessary equilibrium between cell proliferation and cell deletion. During the course of aging, numerous alterations of these signaling pathways may shift the balance toward cell death. In the present investigation, the reduced DNA synthesis of anti-CD3 activated T lymphocytes isolated from elderly individuals is associated with an important and early cell deletion from the cultures. Visualization of DNA fragmentation in the remaining activated cells argues in favour of the apoptotic nature of the cell deletion. Quantification of histone-associated DNA fragments shows that the apoptotic process is greatly amplified in activated lymphocytes derived from senescent organisms. Further analysis reveals that IL-2 deprivation does not play a significant role in the age-related increase in apoptosis. Partial correction of this excessive apoptosis by products that bypass the early steps of the signaling cascade suggests that transmembrane signaling defects are involved in this process. Exploration of the antioxidant pathway reveals that the increased susceptibility of lymphocytes from senescent organisms to apoptosis is not explained by a decreased Bcl-2 expression and is not influenced by a modification of the intracellular concentration of glutathione (GSH).

[Immunity and aging]

Immune senescence is usually understood as a progressive and unavoidable decline of the immune function with increasing age. In the light of recent observations, this classical concept needs to be revised. More than just a simple waning of activity, immune senescence appears as a true dysfunction in the immune system. On the biological level, the age-related changes of the complex network of interactions between the various participants of the immune system result in the loss of some activities and in increase of other activities. On the clinical level, the disturbances of this interactive network lead to an inefficient and sometimes inappropriate or aberrant immune response. A better understanding of the molecular basis of immune senescence will hopefully allow the identification and characterization of the underlying mechanisms. Prevention and treatment of some aspects of age-associated immune dysfunction are already possible by hormone replacement therapy.

Age-related effects in T cell activation and proliferation

Age-associated thymic involution manifests its effects in a variety of ways that are related to a loss of T cell function. These include the appearance of a non-functional subset of T cells that increase in representation with age. Moreover there is a loss of T cell proliferative ability, a decline in the synthesis and release of interleukin-2 (IL-2), a decline in the ability of the T cell to express the IL-2 receptor, and a loss of control activity. This loss of control is demonstrated by the age-related appearance of autoantibodies and an increase in the elaboration of inflammatory cytokines such as TNF, IFN, IL-6, and TGF. A major part of the basis for the loss of T cell function is an inability of the T cell to respond to activation signals that are transmitted through the membrane binding of specific stimulatory signals. Transduction events, differentiation signals, and a loss of control mechanisms are all parts of a complicated picture of age-related immune deficiencies.

Expression of the murine homologue of the cell cycle control protein p34cdc2 in T lymphocytes

The mammalian homologue of the cdc2 gene of the fission yeast Schizosaccharomyces pombe encodes a p34cdc2 cyclin-dependent kinase that regulates the cell cycle of a wide variety of cell types. Resting murine T lymphocytes contained no detectable p34cdc2 protein, histone kinase activity, or specific mRNA for the cdc2 gene. Activation of the T cells by immobilized anti-CD3 resulted in the expression of specific mRNA late in the G1 phase of the cell cycle, and p34cdc2 protein was detectable at or near G1/S. At this point in the cell cycle, the protein was phosphorylated at tyrosine and displayed no H1 histone kinase activity. As the cells progressed through the cycle, the amount of specific mRNA and p34cdc2 increased, and H1 histone kinase activity was detectable when the cells were blocked at G2/M by nocodazole. The activation of T cells by phorbol dibutyrate induced the expression of IL-2R but failed to induce the synthesis of IL-2 or the expression of cdc2-specific mRNA. Under these conditions, the activated cells failed to enter the S phase of the cell cycle. Because the presence of IL-2 added exogenously during activation by phorbol dibutyrate resulted in the expression of cdc2-specific mRNA and progression through the cell cycle, either IL-2 or the interaction with IL-2R may be involved in the expression of cdc2 and regulation of the G1/S transition.

Expression of the murine homologue of the cell cycle control protein p34cdc2 in T lymphocytes

The mammalian homologue of the cdc2 gene of the fission yeast Schizosaccharomyces pombe encodes a p34cdc2 cyclin-dependent kinase that regulates the cell cycle of a wide variety of cell types. Resting murine T lymphocytes contained no detectable p34cdc2 protein, histone kinase activity, or specific mRNA for the cdc2 gene. Activation of the T cells by immobilized anti-CD3 resulted in the expression of specific mRNA late in the G1 phase of the cell cycle, and p34cdc2 protein was detectable at or near G1/S. At this point in the cell cycle, the protein was phosphorylated at tyrosine and displayed no H1 histone kinase activity. As the cells progressed through the cycle, the amount of specific mRNA and p34cdc2 increased, and H1 histone kinase activity was detectable when the cells were blocked at G2/M by nocodazole. The activation of T cells by phorbol dibutyrate induced the expression of IL-2R but failed to induce the synthesis of IL-2 or the expression of cdc2-specific mRNA. Under these conditions, the activated cells failed to enter the S phase of the cell cycle. Because the presence of IL-2 added exogenously during activation by phorbol dibutyrate resulted in the expression of cdc2-specific mRNA and progression through the cell cycle, either IL-2 or the interaction with IL-2R may be involved in the expression of cdc2 and regulation of the G1/S transition.

T cell activation in the absence of interleukin 2 (IL 2) results in the induction of high-affinity IL 2 receptor unable to transmit a proliferative signal

Although interleukin 2 (IL 2) clearly up-regulates the expression of the p55 chain of the IL 2 receptor (IL 2R) little is known about its role in the induction of the high-affinity IL 2R. Resting T lymphocytes were induced to express IL 2R under experimental conditions in which IL 2 production was not induced or was prevented. Under these conditions high- and low-affinity IL 2R were easily demonstrated by Scatchard analysis. Northern blot analysis confirmed the accumulation of p55 specific mRNA and the absence of the IL 2 transcript. High-affinity IL 2R induced in the complete absence of IL 2 were unable to transmit a proliferative response unless exposed to extremely high concentrations of IL 2. The addition of picomolar amounts of recombinant IL 2 or the initiation of endogenous IL 2 production during the induction period restored the functionality of high-affinity IL 2R. Also, T cells induced to generate IL 2 displayed functional high-affinity IL 2R even in the presence of monoclonal antibodies blocking extracellular IL 2 and IL 2R. These results indicate that the presence of IL 2 during the early phase of T cell activation is an absolute requirement for the induction of fully operational high-affinity IL 2R and that low amounts of intracellular IL 2 are sufficient to confer functional properties to these receptors. The data also suggest that an intracellular as well as an extracellular high-affinity structure, expressed as a consequence of cell activation, is responsible for conferring competence to the high-affinity IL 2R involved in IL 2-dependent proliferation.

Restricted expression of mitogen-induced high affinity IL-2 receptors in aging mice

Several lines of indirect evidence suggest that the number and/or affinity of IL-2R expressed by activated T lymphocytes declines with age and that this decline is implicated in the age-related proliferative impairment of Ag or mitogen-stimulated T cells. In an attempt to provide a direct demonstration of such a defect, various experimental approaches were used to analyze the expression of high and low affinity IL-2R as well as their functional properties in relation to age in purified populations of murine T lymphocytes. IL-2R were induced by Con A-activation which involves a transmembrane signaling mechanism or by exposure to phorbol dibutyrate (PDBu) which bypasses such a pathway. Consistent with the previously reported age-related defect in signal transduction, a major deficiency in the expression of high affinity IL-2R was observed in mitogen-activated cells derived from aged animals. As expected, PDBu-induction circumvented the transmembrane signaling defect and resulted in the restoration of a measurable amount of high affinity IL-2R expressed by cells from aged mice early after activation. The functional properties of the IL-2R expressed as a consequence of Con A or PDBu induction were investigated by assessing the proliferative response induced through the high affinity IL-2R as compared to that mediated by the beta-chain alone. Although Con A-induction resulted in a decreased expression of high affinity IL-2R by T lymphocytes derived from aged mice, the ability of these receptors as well as that of their beta-chain component to transmit a proliferative signal was identical in both age groups. In contrast, PDBu induced in both cell populations the expression of functionally aberrant IL-2R, unable to signal for proliferation unless excessively high concentrations of rIL-2 were available. The quantitative minimal estimate of the frequency of Con A-activated, IL-2-responsive cells showed a fourfold age-associated decrease, confirming the inability of a subpopulation of T lymphocytes from aged mice to express a sufficient density of high affinity IL-2R as a consequence of mitogenic activation.

Restricted expression of mitogen-induced high affinity IL-2 receptors in aging mice

Several lines of indirect evidence suggest that the number and/or affinity of IL-2R expressed by activated T lymphocytes declines with age and that this decline is implicated in the age-related proliferative impairment of Ag or mitogen-stimulated T cells. In an attempt to provide a direct demonstration of such a defect, various experimental approaches were used to analyze the expression of high and low affinity IL-2R as well as their functional properties in relation to age in purified populations of murine T lymphocytes. IL-2R were induced by Con A-activation which involves a transmembrane signaling mechanism or by exposure to phorbol dibutyrate (PDBu) which bypasses such a pathway. Consistent with the previously reported age-related defect in signal transduction, a major deficiency in the expression of high affinity IL-2R was observed in mitogen-activated cells derived from aged animals. As expected, PDBu-induction circumvented the transmembrane signaling defect and resulted in the restoration of a measurable amount of high affinity IL-2R expressed by cells from aged mice early after activation. The functional properties of the IL-2R expressed as a consequence of Con A or PDBu induction were investigated by assessing the proliferative response induced through the high affinity IL-2R as compared to that mediated by the beta-chain alone. Although Con A-induction resulted in a decreased expression of high affinity IL-2R by T lymphocytes derived from aged mice, the ability of these receptors as well as that of their beta-chain component to transmit a proliferative signal was identical in both age groups. In contrast, PDBu induced in both cell populations the expression of functionally aberrant IL-2R, unable to signal for proliferation unless excessively high concentrations of rIL-2 were available. The quantitative minimal estimate of the frequency of Con A-activated, IL-2-responsive cells showed a fourfold age-associated decrease, confirming the inability of a subpopulation of T lymphocytes from aged mice to express a sufficient density of high affinity IL-2R as a consequence of mitogenic activation.

Depressed natural killer cell function in thermally injured adults: successful in vivo and in vitro immunomodulation and the role of endotoxin

Natural killer (NK) cells mediate host defense against infections and are regulated by interleukin 2 (IL-2) and other factors. We studied NK cell function in burn patients using a 51Cr release assay with K562 target cells. We found that peripheral blood lymphocytes from burn patients had depressed NK activity (target cell lysis = 22.0 +/- 3.1% vs 39.8 +/- 3.2% in healthy volunteers, P less than 0.001) and also a lower response to IL-2 (28.9 +/- 3.8% vs 53.2 +/- 4.3%, P less than 0.001). Thirteen burn patients were randomly assigned to receive either standard therapy or 5 days of intravenous polymyxin B in addition to standard therapy. After 2 weeks, the patients not receiving polymyxin B had a significant decline in peripheral blood NK activity (P less than 0.01) and response to IL-2 (P less than 0.05), while no decline in NK cell activity was seen in patients who received polymyxin B. Sera from burn patients was found to suppress the NK activity of lymphocytes from healthy adults by 5-75%. After using affinity chromatography to remove endotoxin, the sera from burn patients no longer suppressed NK cell activity. Circulating endotoxin appears to be involved in the suppression of NK activity in burn patients.

Signal transduction mechanisms in the immune system. Potential implication in immunosenescence

The essence of the immune system is the recognition of environmental agents foreign to the host. This recognition is accomplished at the cellular level through specific membrane receptors, which when bound by appropriate ligands trigger a complex response that results in the activation of immunocompetent cells. The receptor-mediated processes that activate resting lymphocytes involve transmembrane signaling mechanisms, which have been characterized in other cell systems and are currently being investigated in B and T lymphocytes. Lymphocyte activation, proliferation, and differentiation require a sequence of cellular reactions initiated by both antigen-receptor and growth factor-receptor interactions. Signals induced by antigens or mitogens initiate responses, including gene activation for the production of hormone-like lymphocytic growth factors and their specific receptors. The physiologic mitogenesis of lymphocytes is then regulated by interactions of these specific growth factors with receptors that transmit another set of signals across the cell membrane to induce a proliferative response. As a result of this complexity, there are discrepancies with regard to the nature of the signals required for lymphocytes to acquire the ability to proliferate, as distinguished from signals inducing proliferation. Deficiencies in the transmission of any of the multiple signals essential in lymphocyte immunobiology would be expressed in vivo as an immunodeficiency of the host. Investigations of the signaling mechanisms regulating lymphocyte function may therefore provide an understanding of the age-associated reduction in the functional capacity of the immune system.

Age-related defect in signal transduction during lectin activation of murine T lymphocytes

Interleukin 2 (IL-2) production and recognition are clearly involved in the age-associated proliferative defect of mitogen-stimulated T lymphocytes. The external signal delivered by mitogens is transmitted across the membrane via the release of two messenger molecules, diacylglycerol and inositol 1,4,5-trisphosphate (IP3), involved in the activation of protein kinase C (PK-C) and the elevation of cytosolic free Ca2+. In that Ca2+ mobilization and PK-C activation appear to be crucial events in the production of IL-2 and the expression of IL-2 receptors, a defect in transmembrane signaling would result in decreased synthesis and response to IL-2. We therefore examined PK-C activity and translocation, generation of inositol 1,4,5-trisphosphate, and cytosolic Ca2+ levels as a function of age in murine G0 T lymphocytes before and after exposure to mitogenic doses of concanavalin A (Con A). The basal levels and distribution of PK-C before and after direct activation of the enzyme by 2 or 20 nM phorbol myristate acetate were comparable in both age groups indicating no inherent age-associated functional defect in the enzyme. However, the Con A-induced PK-C translocation was reduced by 50% in cells from 24-mo-old animals. The Con A stimulation of G0 T lymphocytes increased free cytoplasmic Ca2+ concentration ([Ca2+]i) and the production of inositol phosphates to the same level, irrespective of the age of the donor. However, basal levels of both of these second messengers were consistently higher in lymphocytes derived from old mice. As a result, the net increase in inositol phosphates and [Ca2+]i was reduced by approximately the same extent as that observed for the translocation of PK-C. These results clearly point to an age-associated defect in the generation of phosphoinositide-derived second messengers and indicate that an alteration in signal transduction plays a primary role in the age-related impairment of the mitogen-induced, IL-2-mediated proliferative response of T lymphocytes.

Age-related defect in signal transduction during lectin activation of murine T lymphocytes

Interleukin 2 (IL-2) production and recognition are clearly involved in the age-associated proliferative defect of mitogen-stimulated T lymphocytes. The external signal delivered by mitogens is transmitted across the membrane via the release of two messenger molecules, diacylglycerol and inositol 1,4,5-trisphosphate (IP3), involved in the activation of protein kinase C (PK-C) and the elevation of cytosolic free Ca2+. In that Ca2+ mobilization and PK-C activation appear to be crucial events in the production of IL-2 and the expression of IL-2 receptors, a defect in transmembrane signaling would result in decreased synthesis and response to IL-2. We therefore examined PK-C activity and translocation, generation of inositol 1,4,5-trisphosphate, and cytosolic Ca2+ levels as a function of age in murine G0 T lymphocytes before and after exposure to mitogenic doses of concanavalin A (Con A). The basal levels and distribution of PK-C before and after direct activation of the enzyme by 2 or 20 nM phorbol myristate acetate were comparable in both age groups indicating no inherent age-associated functional defect in the enzyme. However, the Con A-induced PK-C translocation was reduced by 50% in cells from 24-mo-old animals. The Con A stimulation of G0 T lymphocytes increased free cytoplasmic Ca2+ concentration ([Ca2+]i) and the production of inositol phosphates to the same level, irrespective of the age of the donor. However, basal levels of both of these second messengers were consistently higher in lymphocytes derived from old mice. As a result, the net increase in inositol phosphates and [Ca2+]i was reduced by approximately the same extent as that observed for the translocation of PK-C. These results clearly point to an age-associated defect in the generation of phosphoinositide-derived second messengers and indicate that an alteration in signal transduction plays a primary role in the age-related impairment of the mitogen-induced, IL-2-mediated proliferative response of T lymphocytes.

G0 B cells activated by anti-mu acquire the ability to proliferate in response to B cell-activating factors independently from entry into G1 phase

In the present studies we investigated the early steps in B cell activation and determined that activation could be separated from entry into cell cycle. Purified B cells from BALB/c nu/nu mice, activated under controlled conditions by affinity-purified rabbit IgG anti-mu or F(ab')2 fragments prepared from the same antibodies, were sorted according to size by flow cytometry. Approximately 80% of the B cells were small and were shown to be in Go state by quantitative RNA analysis and by [3H]uridine and [3H]thymidine incorporation studies. The sorted Go B cells, when again incubated in serum-free medium with the appropriate anti-mu preparations, remained in Go. However, Go B cells in the presence of anti-mu have undergone significant change(s), in as much as they were now able to proliferate in response to soluble mediators. The ability to develop a proliferative response to B cell-activating factor(s), acquired independently from entry into cell cycle, characterizes another important step in early B cell activation and also indicates that Go B cells characterized on the basis of cell size, density, and/or RNA content may be in an activated state.

G0 B cells activated by anti-mu acquire the ability to proliferate in response to B cell-activating factors independently from entry into G1 phase

In the present studies we investigated the early steps in B cell activation and determined that activation could be separated from entry into cell cycle. Purified B cells from BALB/c nu/nu mice, activated under controlled conditions by affinity-purified rabbit IgG anti-mu or F(ab')2 fragments prepared from the same antibodies, were sorted according to size by flow cytometry. Approximately 80% of the B cells were small and were shown to be in Go state by quantitative RNA analysis and by [3H]uridine and [3H]thymidine incorporation studies. The sorted Go B cells, when again incubated in serum-free medium with the appropriate anti-mu preparations, remained in Go. However, Go B cells in the presence of anti-mu have undergone significant change(s), in as much as they were now able to proliferate in response to soluble mediators. The ability to develop a proliferative response to B cell-activating factor(s), acquired independently from entry into cell cycle, characterizes another important step in early B cell activation and also indicates that Go B cells characterized on the basis of cell size, density, and/or RNA content may be in an activated state.

A "lymphokine-like" soluble product that induces proliferation and maturation of B cells appears in the serum-free supernatant of a T cell hybridoma as a consequence of mycoplasmal contamination

The serum-free supernatant of a cloned murine T cell hybridoma supports the proliferation and maturation to Ig secretion of purified B cells (mu+ cells) from BALB/c nu/nu mice, but has no effect on the proliferation of nylon wool-selected BALB/c nu/+ splenic T cells. Although the supernatant activates B cells without co-stimulation, it synergizes with anti-mu for the proliferative response. The induction of B cell proliferation and maturation to Ig secretion is directly related to contamination of the hybridoma by Mycoplasma hyorhinis. Hybridoma cells freed of mycoplasma by detergent treatment fail to produce active supernatant, and reinfection of the treated cells reconstitutes the activity. Furthermore, deliberate infection of a mycoplasma-free unrelated T cell hybridoma, as well as the monocytic cell line P388D1, results in the production of supernatants with B cell proliferating activity. Mycoplasma organisms isolated from the supernatant induce B cell proliferation without subsequent maturation to Ig secretion. Gel filtration chromatography of the supernatant from mycoplasma-contaminated hybridoma cells yields two peaks of activity. The first peak, found at the exclusion limit of the gel, results in B cell proliferation without maturation and may be attributed to mycoplasma organisms. The second peak (average m.w. 90,000) results in B cell proliferation as well as differentiation to Ig secretion. A "lymphokine-like" soluble product released by Mycoplasma hyorhinis is most likely responsible for this B cell activation, because fractionation of the supernatant from deliberately contaminated P388D1 cells gives essentially the same results, and gel filtration of mycoplasma-free supernatants does not generate any active fractions. The possibility should be considered that mycoplasma-derived soluble products may be among the many factors controlling in vitro B cell growth and maturation.

A "lymphokine-like" soluble product that induces proliferation and maturation of B cells appears in the serum-free supernatant of a T cell hybridoma as a consequence of mycoplasmal contamination

The serum-free supernatant of a cloned murine T cell hybridoma supports the proliferation and maturation to Ig secretion of purified B cells (mu+ cells) from BALB/c nu/nu mice, but has no effect on the proliferation of nylon wool-selected BALB/c nu/+ splenic T cells. Although the supernatant activates B cells without co-stimulation, it synergizes with anti-mu for the proliferative response. The induction of B cell proliferation and maturation to Ig secretion is directly related to contamination of the hybridoma by Mycoplasma hyorhinis. Hybridoma cells freed of mycoplasma by detergent treatment fail to produce active supernatant, and reinfection of the treated cells reconstitutes the activity. Furthermore, deliberate infection of a mycoplasma-free unrelated T cell hybridoma, as well as the monocytic cell line P388D1, results in the production of supernatants with B cell proliferating activity. Mycoplasma organisms isolated from the supernatant induce B cell proliferation without subsequent maturation to Ig secretion. Gel filtration chromatography of the supernatant from mycoplasma-contaminated hybridoma cells yields two peaks of activity. The first peak, found at the exclusion limit of the gel, results in B cell proliferation without maturation and may be attributed to mycoplasma organisms. The second peak (average m.w. 90,000) results in B cell proliferation as well as differentiation to Ig secretion. A "lymphokine-like" soluble product released by Mycoplasma hyorhinis is most likely responsible for this B cell activation, because fractionation of the supernatant from deliberately contaminated P388D1 cells gives essentially the same results, and gel filtration of mycoplasma-free supernatants does not generate any active fractions. The possibility should be considered that mycoplasma-derived soluble products may be among the many factors controlling in vitro B cell growth and maturation.