T3 Molecule Research Articles

Minimum binding energy and size of the doubly muonic T3 molecule

AbstractThe minimum energy and size of doubly muonic T3 molecule in which two of the electrons are replaced by the much heavier muons are calculated by the well‐known variational method. The calculations show that the system possesses two minimum positions, one at a typically muonic distance and the second at the atomic distance. It is shown that at the muonic distance the effective charge, zeff, is 2.88. We assumed a symmetrical planar vibrational model between two minima and an oscillation potential energy is approximated in this region. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 377–383, 2003

Impact of Gene Cloning, Disruption and Over-Expression of Iodothyronine Deiodinases on Thyroid Hormone Homeostasis

Thyroxine (T4) is the main product of thyroid secretion, a pro-hormone that must be activated by deiodination to T3 in order to initiate thyroid hormone action. This deiodination reaction occurs in the phenolic-ring (outer-ring deiodination, ORD) of the T4 molecule and is catalyzed by two selenocysteine-containing deiodinases, i.e. D1 and D2. As a counter point to the activation pathway, both T4 and T3 can be irreversibly inactivated by deiodination of the thyrosyl-ring (inner-ring deiodination, IRD), a reaction catalyzed by D3, the third member of the selenodeiodinase group. Due to its substantial physiological plasticity, D2 is considered the critical T3-producing deiodinase in humans. Recently, the observations made in the D1-deficient C3H mouse mice were expanded by the development of mice with generalized targeted disruption or cardiac-specific over-expression of the D2 gene. The results obtained indicate that the selenodeiodinases constitute a physiological system contributing with the thyroid hormone homeostasis during adaptation to changes in iodine supply, cold exposure, in patients with thyroid dysfunction and perhaps during starvation and illness.

Thermodynamics of the Collapsing Phase Transition in a Single Duplex DNA Molecule

We observed changes in the conformation of individual single giant DNAs (T4DNA) using fluorescence microscopy at different temperatures, 3∼53 °C, in an aqueous solution containing 1.2 mM spermidine. We found that individual DNAs take either elongated coil or collapsed globule conformation and that the relative population of the globule state increases with an increase in temperature. Based on this temperature dependence, the change in entropy for the coil−globule transition was deduced to be ΔS = 32 ± 8 kB, where kB is the Boltzmann constant. In other words, at TC = 301 K the change in enthalpy is ΔH =32 ± 8kBTC(= 19 ± 5 kcal/mol chains). The thermodynamics upon the transition in a single T4 molecule from the elongated coil to the collapsed globule is discussed in terms of statistical physics by considering the different contributions to the change in entropy; i.e., elasticity, mixing with the solvent, the translation of counterions, hydration, and charge neutralization. We conclude that the small positive change in entropy for the transition is attributed to the competition between the large positive value originated from the translational entropy on the exchange of counterions and large negative values from the other contributions.

New noncompetitive immunoassays of small analytes

We developed a novel noncompetitive immunoassay format for monoepitopic analytes and describe here a model assay for triiodothyronine (T3), performed on Ciba Corning's ACS:180 analyzer. Acridinium ester (AE)-labeled bivalent anti-T3 was incubated with the sample, producing AE-anti-T3/T3 complexes and unreacted AE-anti-T3. Controlled-pore glass particles (CPG) with immobilized diiodothyronine (T2) were then added in excess, to bind AE-anti-T3 possessing two unoccupied binding sites but not AE-anti-T3 bound to one or two T3 molecules. Paramagnetic particles (PMP) with immobilized anti-AE were then added to the same cuvette to capture AE-anti-T3/T3 complexes; AE-anti-T3 bound to the surface of CPG, however, was not captured, because of steric hindrance. After the incubation, the PMP was magnetically separated to remove the liquid phase and the suspended CPG from the cuvette. The chemiluminescence associate with the PMP remaining in the cuvette was then measured. This noncompetitive T3 assay exhibited a 10-fold lower detection limit than the equivalent competitive T3 assay, i.e., 0.3 vs pg/test. Imprecision (CV) in the clinically significant range was 6% or less. The assay also displayed two- to sevenfold lower cross-reactivities and a wider dynamic range.

Structural aspects of oligothienyls from X-ray powder diffraction data

The structures of oligothienyls (T4–T6) has been evinced from x-ray powder diffraction data using the Rietveld full-profile analysis on particularly pure samples, we were able to overcome preferred orientation effects. Both T4 and T6 molecules display no internal symmetry, are planar within 10° and present a herringbone arrangement, the angle between mean planes of adjacent molecules ranges from 70°(T4) to 60°(T6). Intra and intermolecular energy minimization is in progress.

T Cell Receptor‐independent Cell‐mediated Cytotoxicity by Nude Mouse Lymphokine‐activated Killer Cells

Lymphokine‐activated killer (LAK) cells, which can lyse a variety of tumor cells, can be induced from both normal and athymic nude mouse spleen cells by culture with high doses of recombinant interleukin 2 (rIL‐2). LAK cells generated from nude mouse spleen cells (Nude‐LAK cells) express just Thy 1.2 antigen, but not CD4 and CDS antigens. Nude‐LAK cells express neither T3 molecule, T cell receptor (TCR) αβ nor TCR γδ on their cell surface. The lack of TCR expression on Nude‐LAK cells was confirmed by the results of northern blot analysis. LAK cells generated from normal mouse spleen cells (Nor‐LAK) express TCR α, β transcripts, while Nude‐LAK cells express only sterile TCR β transcript, but not TCR α transcript. TCR γδ transcripts were scarcely detected in both Nor‐LAK cells and Nude‐LAK cells. Thus, it is strongly suggested that Nude‐LAK cells can recognize and lyse tumor cells by TCR‐independent mechanisms. Monoclonal antibody against lymphocyte function‐associated antigen (LFA‐1) molecule can block the Cytotoxicity of Nude‐LAK cells, indicating an important role of such accessory molecules in Nude‐LAK cell‐mediated Cytotoxicity.

Novel and sensitive noncompetitive (two‐site) enzyme immunoassay for haptens with amino groups

A novel and sensitive noncompetitive (two-site) enzyme immunoassay for haptens with amino groups is described. L-Thyroxine (T4) was used as a model hapten. T4 was indirectly biotinylated with glutathione as spacer between T4 and biotin molecules and trapped onto anti-(T4-bovine serum albumin) IgG-coated polystyrene balls. After washing the polystyrene balls to eliminate unreacted biotin and other biotinylated substances, biotinylated T4 was eluted from the polystyrene balls with HCl and was reacted with anti-(T4-bovine serum albumin) Fab'-horseradish peroxidase conjugate. The complex formed was trapped onto streptavidin-coated polystyrene balls. Peroxidase activity bound to the polystyrene balls was assayed by fluorimetry. The detection limit of T4 was 78 fg (0.1 fmol)/tube, which was 50-fold lower than the limit by competitive enzyme immunoassay using the same antibody and T4-peroxidase conjugate. This noncompetitive enzyme immunoassay was applied to the measurement of total T4 in serum. Serum levels of total T4 in 10 healthy subjects aged 25-40 yr were 94 +/- 13 (SD) micrograms/L (range, 78-114). The principle of this noncompetitive enzyme immunoassay may allow it to measure other haptens with amino groups more sensitively than can competitive immunoassays.

The alternative pathway of T cell activation: Biology, pathophysiology, and perspectives for immunopharmacology

The application of monoclonal antibodies and recombinant mediators to studies of T cell activation has led to a new concept regarding the central mechanisms underlying specific immune responses in man. Stimulation of human T cells to express their functional programs with regard to immunoregulatory activities and effector functions can be mediated through several distinct mechanisms or pathways. We report on the recently discovered T3-Ti antigen receptor independent mode of human T cell activation, namely, the T11-mediated “alternative pathway”. Recent evidence supports the notion that this pathway plays an important role in the immune response in man and that failure to activate T cells through T11 is associated with immunodeficiency. The characterization of functional epitopes of the T11 molecule along with functional investigations on patients suffering from etiologically different cases of immunodeficiency provides important perspectives for future pharmacological interventions into the human immune system. It seems likely that immunologic disorders such as autoimmune disease and immunodeficiencies result from overamplification or blockades of the “alternative pathway of T cell activation” and that the T11 epitope represents a potential site for selective inhibition of the “alternative pathway of T cell activation,” e.g., by means of synthetic peptide analogues. Conversely, high affinity ligands to the T11 epitope might be suitable for immunostimulation immunodeficiencies that result from circulating blocking factors of the LFA- 3 T11 interaction.

Triggering of T-lymphocytes via Either T3-Ti or T11 Surface Structures Opens a Voltage-insensitive Plasma Membrane Calcium-permeable Channel: Requirement for Interleukin-2 Gene Function

Stimulation of human T-lymphocytes via either the surface T3-Ti antigen-major histocompatibility complex receptor complex or the T11 molecule results in clonal proliferation through a calcium-dependent mechanism. To investigate this signal transduction, plasma membrane calcium-permeable channels were characterized in T-lymphocytes by means of whole cell or single channel patch-clamp recordings. Stimulation of T-lymphocytes via either structure results in opening of an identical set of voltage-insensitive plasma membrane Ca2+-permeable channels through the action of a diffusible second messenger. Previous work with excised inside-out patches suggests that inositol 1,4,5-trisphosphate is the activating second messenger of the voltage-insensitive T-cell Ca2+-permeable channel. Since there is a significant increase in phosphoinositide turnover after stimulation via either the T3-Ti or T11 pathway, it is suggested that triggering of either structure opens a common set of channels through this mechanism. Furthermore, currents flowing through Ca2+-permeable channels are apparently autoregulated, as inward conductance is abolished by elevation of Ca2+ concentration in the bathing solution. In particular, the steady-state rise in interleukin-2 (T-cell growth factor) mRNA is dependent on the rise of [Ca2+]i resulting from ion movement across this channel.

Diminished Expression of the T Cell Receptor on the Expanded Lymphocyte Population in Mrl/Mp- 1pr/1pr Mice

MRL mice homozygous for the recessive lpr gene develop an accelerated autoimmune syndrome and massive lymphadenopathy. Because the function of the expanded lymph node population is unclear, we have studied the subunits of the T cell receptor for antigen (TcR). DNA and RNA were prepared from MRL/Mp-lpr/lpr (lpr) and congenic MRL/Mp(-)+/+ (+/+) mice by standard techniques and studied by Southern blot, northern blot, and dot blot analysis using the cDNAs TT11, specific for the TcR alpha chain; 86T5, specific for the TcR beta chain; and T3 delta; specific for the subunit of the T3 molecule. Surface protein was immunoprecipitated with antisera 8177, which recognizes TcR framework determinants, and resolved by diagonal SDS-PAGE. FACS analysis was performed with a monoclonal antibody to murine T3, and with the KJ16-133 and F23.1 monoclonal antibodies, which recognize determinants encoded by the V beta 8 subfamily of beta chain variable region genes. When compared with +/+ controls, surface TcR density as detected by immunofluorescence using all three antibodies was significantly diminished on lpr spleen and lymph node cells, as well as on lpr lymph node cells which had been depleted of L3T4+ and Ly2+ cells by negative selection. There appeared, however, to be selective expression of the genes encoding the epitopes binding F23.1. Southern blot analysis of DNA showed polyclonal rearrangements of the TcR beta chain genes. There were increased alpha, beta, and T3 delta RNA transcripts in the double negative lymph node cells. The paradoxical decrease in TcR surface expression in the setting of large quantities of full length transcript is yet to be explained.

Infection of B lymphocytes by the human immunodeficiency virus and their susceptibility to cytotoxic cells.

The T4 molecule (CD4) is an important component of the human immunodeficiency virus (HIV) receptor. As yet, no other component has been demonstrated. We report here that two cell lines, a B lymphoblastoid cell line (Gupta) and a glial cell line (HEB) derived from human embryonal brain tissue, are productively infectable with two distinct isolates of HIV as judged by electron microscopy and immunological and virological studies. These two cell lines do not display detectable surface CD4 glycoprotein. However, using S1 nuclease analysis, we have found that both cell lines do express low levels of CD4 mRNA. Neither of them produced syncytia formation upon HIV infection, a recognized feature of HIV-infected cells strongly expressing the CD4 glycoprotein. It is conceivable that the CD4 mRNA is translated, resulting in meager surface expression of CD4 molecules undetectable by conventional techniques. Therefore, infection with HIV may be one of the most sensitive methods of demonstrating low levels of CD4 expression by human cells. Furthermore, HIV-infected Gupta cells have here been shown to be more susceptible to the lytic activity of natural killer (NK) cells than their uninfected counterparts. These phenomena may be important for pathogenesis of HIV-associated disorders.

Interdependence of CD3-Ti and CD2 activation pathways in human T lymphocytes.

Human T lymphocytes can be activated through either the antigen/MHC receptor complex T3-Ti (CD3-Ti) or the T11 (CD2) molecule to proliferate via an IL-2 dependent mechanism. To investigate the relationship of these pathways to one another, we generated and characterized Jurkat mutants which selectively express either surface CD3-Ti or CD2. Here we show that CD3-Ti- mutants fail to be stimulated by either pathway to increase phosphoinositide turnover, mobilize calcium or induce the IL-2 gene. The activation capacity of these mutants via CD2 as well as CD3-Ti can be restored following reconstitution of surface CD3-Ti expression upon appropriate DNA transfer (e.g. Ti beta subunit cDNA into Ti beta- Jurkat variants). Collectively, these results demonstrate that CD3-Ti and CD2 pathways are interdependent and that phosphoinositide turnover is linked to the CD3-Ti complex.

Adhesion domain of human T11 (CD2) is encoded by a single exon.

The 50-kDa T11 (CD2) T-lymphocyte surface glycoprotein facilitates physical adhesion between T-lineage cells and their cognate cellular counterparts (cytotoxic T-lymphocytes-target cells, helper T lymphocytes-antigen-presenting cells, or thymocytes-thymic epithelium) as well as signaling through the antigen-specific T3-Ti receptor complex. To examine the relationship between the structure and function of the T11 molecule, we have utilized a baculoviral expression system to produce milligram quantities of the hydrophilic extracellular T11 segment. Enzyme cleavage, microsequencing, and HPLC analyses of the expressed protein in conjunction with genomic cloning information show that the domain involved in cellular adhesion is encoded by a single 321-base-pair exon.

Differentiation in vitro of T3+ large granular lymphocytes with characteristic cytotoxic activity from an isolated hematopoietic progenitor colony.

Blast colonies were developed by rIL-3 from the spleen cells of mice pretreated with 5-fluorouracil (5-FU) in the methylcellulose cultures. When such IL-3-induced blast colonies were individually lifted up and recultured in the presence of rIL-3 and recombinant erythropoietin (rEpo), a variety of hematopoietic colonies developed from every single colony, including neutrophils, macrophages, eosinophils, megakaryocytes, mast cells, and erythroblasts. The results indicated that IL-3-induced blast colonies consisted of multipotential hematopoietic progenitor cells. By culturing individual IL-3-induced blast colonies in the presence of rIL-2 and irradiated peritoneal macrophages, on the other hand, the proliferation of homogeneous lymphoid cells was observed in 5 of 24 wells, each of which received a single blast colony. Morphologically, they were typical large granular lymphocytes (LGL), and thus it was indicated that LGL could be differentiated directly from the hematopoietic progenitor cells utterly in vitro by rIL-2 and accessory macrophages. From one of these culture wells, a continuous LGL line, IL3B1, was successfully obtained. The proliferation of IL3B1 was dependent on IL-2 in the presence of accessory macrophages, but they no longer responded to IL-3, nor to another T cell growth factor, IL-4. Flow cytometric analysis indicated that the phenotype of IL3B1 was Thy-1+,T3+,L3T4-,Lyt-2-,T200+ Asialo GM1+, whereas that of original IL-3-induced blast cells was Thy-1+,T3-,L3T4-,Lyt-2-,B220-. The results suggested that the expression of T3 molecules was induced in the process of LGL differentiation from the hematopoietic progenitor cells in vitro. Conforming to this, it was revealed that both gamma and beta chain genes of the TCR were rearranged in IL3B1. Northern blot analysis indicated that IL3B1 had abundant mRNA for gamma chain, while mRNA for beta chain was rather faint. Functionally, IL3B1 exhibited typical NK-patterned cytotoxic activity against a panel of tumor cell targets. In addition, they showed significant cytotoxic activity against normal bone marrow cells, as well as various factor-dependent myelogenous progenitor cell lines, all of which were resistant to endogenous NK activity of the fresh spleen cells. These results indicated that at least a set of T3+ LGL with rearranged TCR genes could be directly differentiated from isolated hematopoietic progenitor cells in vitro. Results also suggested that such a prethymically differentiated subset of T-lineage lymphocytes, namely T3+ double-negative LGL, had particular cytotoxic activity in addition to conventional NK activity, which might well contribute to feedback regulation of hematopoiesis.

T4 endocytosis and phosphorylation induced by phorbol esters but not by mitogen or HIV infection.

The T4 (CD4) molecule has been shown to facilitate the interactions of T cells with HLA class II determinants, to function as a signal transducing molecule, and to serve as a receptor for HIV. Recent studies demonstrated that both phorbol esters and antigen stimulation induced the rapid and transient modulation and phosphorylation of T4 on an IL-2-dependent line of cloned peripheral blood T4+ cells. In the current study, we define the kinetics of T4 phosphorylation and internalization induced by phorbol esters and determine the extent to which this metabolic pathway is required for T cell proliferation, activation, and HIV infection. On both peripheral blood T4+ cells and the T cell line Sup-T1, the modulation and internalization of surface T4 induced by phorbol 12, 13-dibutyrate (PDB) was preceded by rapid and transient phosphorylation. On both cell types, by 48 h, T4 was reexpressed on the cell surface in a nonphosphorylated form and was shown to be resistant to phosphorylation and internalization when these cells were reexposed to PDB. In contrast, T4 on the surface of PBL was neither phosphorylated nor down-modulated when PBL were stimulated by PHA, indicating that these effects were not simply the result of T cell activation or proliferation. In additional studies, we demonstrate that this pathway for T4 phosphorylation and internalization is not required for HIV infection by showing that 1) the binding of the HIV gp 120 envelope to T4 does not induce phosphorylation of T4, 2) Sup-T1 cells that are rendered resistant to phorbol ester-induced T4 internalization and phosphorylation by prolonged culture in PDB remain highly susceptible to HIV infection, and 3) clones of HIV-producing cells expressing high levels of surface T4 that is complexed with viral envelope remain susceptible to PDB-induced modulation of T4. This observation suggests that, at least on lymphoid cells, HIV penetration does not occur exclusively by R-mediated endocytosis.

Tritiated D‐ala1‐peptide T binding: A pharmacologic basis for the design of drugs which inhibit HIV receptor binding

AbstractThe HIV virus initiates its infectious cycle through a high‐affinity binding interaction between the envelope protein gp 120 and its receptor, the T4 (or CD4) molecule. An octapeptide sequence, termed peptide T, present in the second variable region of gp 120 from the ARV isolate, has been implicated as the attachement site. The core peptide required for activity has been further refined to a pentapeptide, and homologous pentapeptides are similarly positioned in 21 other sequenced HIV isolates. Utilizing a novel direct binding assay of 3H‐D‐ala1‐peptide T, we now report that synthetic peptides derived from these other isolates are potent competitors of peptide T binding to T4‐positive lymphocytes and brain membranes. Direct peptide T binding is also competable by purified virion gp 120, indicating that these ligands are interactive at the same receptor. Peptide T has sequence relatedness to the peptide vasoactive intestinal peptide (VIP), and VIP and its relevant homologous pentapeptide, VIP[7–11], are also potent inhibitors of peptide T binding. To determine the essential structural features responsible for receptor activity we have studied a series of synthetic peptides substituted with single D‐amino acid residues. These data reveal that the tyrosine of position 7 in peptide T, present in all natural viral isolates, is obligate for receptor activity. Structure/function analysis for a large number of analogs is presented. Significantly, this binding assay is highly correlated with peptide bioactivity in several independent systems, indicating that this methodology can be used for rapid screening of novel, potential anti‐AIDS therapeutics whose target is inhibition of virus gpl20 binding.

Structure and expression of the human and mouse T4 genes.

The T4 molecule may serve as a T-cell receptor recognizing molecules on the surface of specific target cells and also serves as the receptor for the human immunodeficiency virus. To define the mechanisms of interaction of T4 with the surface of antigen-presenting cells as well as with human immunodeficiency virus, we have further analyzed the sequence, structure, and expression of the human and mouse T4 genes. T4 consists of an extracellular segment comprised of a leader sequence followed by four tandem variable-joining (VJ)-like domains, a transmembrane domain, and a cytoplasmic segment. The structural domains of the T4 protein deduced from amino acid sequence are precisely reflected in the intron-exon organization of the gene. Analysis of the expression of the T4 gene indicates that T4 RNA is expressed not only in T lymphocytes, but in B cells, macrophages, and granulocytes. T4 is also expressed in a developmentally regulated manner in specific regions of the brain. It is, therefore, possible that T4 plays a more general role in mediating cell recognition events that are not restricted to the cellular immune response.

The T11 (CD2) molecule is functionally linked to the T3/Ti T cell receptor in the majority of T cells.

Most mature human T lymphocytes express both the multichain T3 (CD3)/Ti T cell receptor for antigen (TCR), and the biochemically distinct 55-kDa T11 (CD2) glycoprotein. Stimulating the T11 molecule causes profound T cell proliferation and functional activation in vitro, but the relationship of T11-mediated activation to antigenic stimulation of T lymphocytes in vivo remains unknown. We now present evidence that T11 function is directly linked to TCR components in T3/Ti+ T11+ human T cells. First, we found that stimulating peripheral blood T cells with the mitogenic combination of anti-T11(2) cells with the mitogenic combination of anti-T11(2) plus anti-T11(3) monoclonal antibodies caused the phosphorylation of TCR T3 chains. The predominance of T3-gamma-phosphorylation that occurred in anti-T11(2) plus anti-T11(3)-treated T cells is similar to the pattern previously observed in antigen-stimulated T cell clones. Second, T11 function depended upon concurrent cell-surface expression of the TCR. Thus, when peripheral blood T cells were deprived of cell surface T3/Ti by anti-T3 modulation, anti-T11(2) plus anti-T11(3)-induced mitogenesis and transmembrane signal generation in the form of calcium mobilization were inhibited. The mechanism of TCR-T11 interdependence was investigated in a series of TCR-deficient variants of a T cell lymphoblastoid cell line. T3/Ti negative variants expressed cell surface T11, but anti-T11(2) plus anti-T11(3) failed to cause detectable calcium mobilization. The TCR-deficient variants also failed to express T11(3) activation epitopes after incubation with anti-T11(2) antibodies, suggesting that T11(3) expression is an essential and TCR-dependent intermediate in the T11 activation mechanism in these cells. Taken together, our results suggest that T11 function depends upon cell-surface expression of TCR in many T3/Ti+ T11+ T lymphocytes, and T11-mediated activation is intimately interconnected with TCR activation mechanisms. A model in which stimulating signals delivered via T11 may be a part of antigenic activation of T lymphocytes is presented.

Auto-tumor immunity in patients with solid tumors: participation of CD3 complex and MHC class I antigens in the lytic interaction

In a number of experiments performed with blood lymphocytes of patients, the high density subset lysed autologous tumor cells separated from the surgical specimens. The lysis was abrogated by pretreatment of the effector cells with antibodies (OKT3) directed against the T3 molecule associated with the T cell receptor and by pretreatment of the target cells with antibodies (W6/32) directed against the monomorphic part of the MHC class I antigens. This subset lyses only autologous tumor cells. The selectivity and the characteristics shared with antigen specific cytotoxic T lymphocytes (CTL) suggest that the auto-tumor lysis by the effectors reflects an immune response against the tumor cells. The low density lymphocytes, separated from the blood, can lyse both auto- and allogeneic tumor cell. In the autologous system, incubation of the effectors with the mAb OKT3 had no inhibitory effect and incubation of the targets with the anti W6/32 mAb inhibited their lysis only in some experiments. The nature of the reactivity of the LD lymphocytes remains to be defined. Whether it is similar to the indiscriminative natural killing or whether part of these lymphocytes are antigen(s) specific and exhibit a high avidity interaction with the target remains to be seen. It is possible that both types of target recognition occur since the LD lymphocyte population is heterogeneous.

Clonal analysis of B cell growth and differentiation activities induced from T lymphocytes upon triggering of T3-Ti and T11 pathways.

The cellular origin of B cell growth factors (BCGF) and differentiation factors (BCDF) was investigated in the present study. For this purpose, T4+ and T8+ T cell clones were obtained from human peripheral blood, activated via stimulation of either the antigen/MHC receptor (T3-Ti molecular complex) or the antigen-independent alternative pathway (T11 molecule), and subsequently examined for their capacity to induce B cell proliferation and immunoglobulin production. The results showed that 1) BCGF is produced by both T4+ and T8+ T cells at the population level as well as at the clonal level; 2) BCDF activity, in contrast, is largely but not exclusively restricted to the T4+ subset; and 3) both the T3-Ti and T11 pathways activate individual clonal T cell populations to promote B cell growth and differentiation.