T cell surface molecules regulating noncognate B lymphocyte activation. Role of CD2 and LFA-1

A central event in humoral responses is the Ag-mediated interaction of Th cells and B cells. This interaction leads to the activation of both cell types and results in cytokine secretion by the T cells and proliferation and secretion of Ig by the B cells. The proliferative and differentiative responses of B cells are dependent on contact-mediated signals and cytokines provided by the activated Th cells. Although the role of cytokines in B cell activation and differentiation is understood, the nature of the signals delivered by the activated Th cells and the molecules involved in this process are not known. In this study we have examined Ag-mediated "cognate" T-B cell interactions as well as B cell activation induced by contact with preactivated and fixed Th lymphocytes. Our results indicate that both the T cell surface molecules lymphocyte function associated Ag-1 and CD2 are important in the activation of T cells by Ag presented by B lymphocytes. This indicates that B cells have similar characteristics as other APC. However, once the T cells are activated, contact-mediated stimulation of resting B lymphocytes (the noncognate phase) is dependent on CD2 but not lymphocyte function associated Ag-1. Two lines of evidence indicate this; first, it is inhibited by blocking of CD2 on the T cells and, second, such stimulation is not efficiently mediated by a CD2- Th cell line. Thus, CD2 plays an obligatory role at several discrete stages of T cell-mediated activation of resting B lymphocytes.

Humoral immune functions in IL-4 transgenic mice

We have analyzed mice expressing IL-4 as a transgene, and found that expression of this lymphokine has profound effects on B cell function. B cells from transgenic mice exhibit phenotypic changes, including an increase in size and elevated expression of class II MHC. IL-4 increases the quantity of IgE produced by transgenic-derived B cells in response to LPS stimulation. In vivo, IL-4 markedly affects the serum Ig isotype repertoire. Serum levels of IgG1 and IgE are elevated, and levels of IgG2a, IgG2b, and IgG3 are depressed in IL-4 transgenic mice. Ag-specific antibody responses to immunization with hapten-carrier conjugates are also affected by IL-4. Transgenic mice show increased anti-hapten IgE and IgG1 and reduced anti-hapten IgG2a, IgG2b, and IgG3, compared with wild-type mice. Ag-specific IgE is substantially induced by T cell-dependent Ag, but not T cell-independent Ag, suggesting that cognate T-B interactions in addition to IL-4 are required for generating IgE responses in vivo. In vivo treatment with the anti-IL-4 mAb 11B11 reverses many of the isotype alterations in the transgenic mice, indicating that these changes arise as a direct consequence of IL-4 secretion.

Humoral immune functions in IL-4 transgenic mice

We have analyzed mice expressing IL-4 as a transgene, and found that expression of this lymphokine has profound effects on B cell function. B cells from transgenic mice exhibit phenotypic changes, including an increase in size and elevated expression of class II MHC. IL-4 increases the quantity of IgE produced by transgenic-derived B cells in response to LPS stimulation. In vivo, IL-4 markedly affects the serum Ig isotype repertoire. Serum levels of IgG1 and IgE are elevated, and levels of IgG2a, IgG2b, and IgG3 are depressed in IL-4 transgenic mice. Ag-specific antibody responses to immunization with hapten-carrier conjugates are also affected by IL-4. Transgenic mice show increased anti-hapten IgE and IgG1 and reduced anti-hapten IgG2a, IgG2b, and IgG3, compared with wild-type mice. Ag-specific IgE is substantially induced by T cell-dependent Ag, but not T cell-independent Ag, suggesting that cognate T-B interactions in addition to IL-4 are required for generating IgE responses in vivo. In vivo treatment with the anti-IL-4 mAb 11B11 reverses many of the isotype alterations in the transgenic mice, indicating that these changes arise as a direct consequence of IL-4 secretion.

Histologic analysis of T lymphocyte activation in reactive lymph nodes

As an approach to defining the anatomic sites of T cell activation in situ, we have developed an immunocytochemical stain for IL-2, a T cell-derived cytokine synthesized shortly after Ag-induced activation. Analysis of lymph nodes from mice immunized with keyhole limpet hemocyanin emulsified in CFA demonstrates that the IL2+ cells appear in a perivascular location 4 days after antigenic challenge. After germinal centers develop, IL-2+ cells are situated in a parafollicular pattern. Serial sections stained for different types of APC, including B cells, interdigitating dendritic cells, and macrophages, demonstrate a close physical association between IL-2+ cells and macrophages. These findings may have important implications for defining how APC bearing processed Ag and Ag-specific T cells interact in the complex environment of lymphoid tissues.

Histologic analysis of T lymphocyte activation in reactive lymph nodes

As an approach to defining the anatomic sites of T cell activation in situ, we have developed an immunocytochemical stain for IL-2, a T cell-derived cytokine synthesized shortly after Ag-induced activation. Analysis of lymph nodes from mice immunized with keyhole limpet hemocyanin emulsified in CFA demonstrates that the IL2+ cells appear in a perivascular location 4 days after antigenic challenge. After germinal centers develop, IL-2+ cells are situated in a parafollicular pattern. Serial sections stained for different types of APC, including B cells, interdigitating dendritic cells, and macrophages, demonstrate a close physical association between IL-2+ cells and macrophages. These findings may have important implications for defining how APC bearing processed Ag and Ag-specific T cells interact in the complex environment of lymphoid tissues.

Anergy in immature B lymphocytes. Differential responses to receptor-mediated stimulation and T helper cells

We have compared the responses of purified neonatal and adult B lymphocytes to stimulation by anti-Ig antibodies, which are functional analogues of Ag, and by Th cells. Neonatal B cells are markedly deficient in proliferative responses to anti-Ig antibodies + IL-4 or to anti-Ig conjugated to dextran, both of which induce strong proliferation of adult B cells in the absence of T lymphocytes. Anti-Ig antibodies actually inhibit the functional responses of neonatal B cells, even to polyclonal stimuli such as LPS. However, Th cells induce both proliferation and Ig secretion by neonatal B cells in the presence of Ag that bind to B cell Ig and are subsequently presented by the B cells. Thus, in neonatal B lymphocytes, cross-linking of membrane Ig in the absence of Th cells has a net inhibitory effect, and this inhibition is overcome by T cell help. These results also suggest that unresponsiveness or tolerance to thymus-independent Ag is induced in the B cells themselves, but tolerance to thymus-dependent proteins resides primarily in the T cell compartment.

Anergy in immature B lymphocytes. Differential responses to receptor-mediated stimulation and T helper cells

We have compared the responses of purified neonatal and adult B lymphocytes to stimulation by anti-Ig antibodies, which are functional analogues of Ag, and by Th cells. Neonatal B cells are markedly deficient in proliferative responses to anti-Ig antibodies + IL-4 or to anti-Ig conjugated to dextran, both of which induce strong proliferation of adult B cells in the absence of T lymphocytes. Anti-Ig antibodies actually inhibit the functional responses of neonatal B cells, even to polyclonal stimuli such as LPS. However, Th cells induce both proliferation and Ig secretion by neonatal B cells in the presence of Ag that bind to B cell Ig and are subsequently presented by the B cells. Thus, in neonatal B lymphocytes, cross-linking of membrane Ig in the absence of Th cells has a net inhibitory effect, and this inhibition is overcome by T cell help. These results also suggest that unresponsiveness or tolerance to thymus-independent Ag is induced in the B cells themselves, but tolerance to thymus-dependent proteins resides primarily in the T cell compartment.

T-cell-mediated activation of B cells

Antibody responses to protein antigens are dependent on helper T lymphocytes, which provide necessary growth and differentiation, inducing stimuli to antigen-specific B cells. Recent investigations have focused on the mechanisms of T-B interactions, and the nature of the T-cell-derived signals that are needed for activation and expansion of B cells.

Cytoplasmic tail deletion converts membrane immunoglobulin to a phosphatidylinositol-linked form lacking signaling and efficient antigen internalization functions

Membrane-bound immunoglobulin (mIg) is the antigen receptor on B lymphocytes mediating early events in antigen presentation and signal transduction. Wild-type human mIgM constructs transfected into the murine B-cell lymphoma A20 are expressed as transmembrane proteins with antigen presentation and signaling functions comparable to the endogenous mIgG2A; the transfected wild-type mIgM is internalized rapidly after anti-Ig cross-linking. Transfected constructs lacking the normal three-amino acid cytoplasmic tail are expressed exclusively as phosphatidylinositol-linked proteins, lack both antigen presentation and signal transduction functions, and are internalized slowly following anti-Ig binding. The molecular mass of the cytoplasmic tail-deleted phosphatidylinositol-linked Ig molecule is consistent with cleavage of the transmembrane residues during processing. Cytoplasmic domains may therefore regulate the mode of expression of membrane proteins and thereby influence their functional capabilities.

Activation of functionally distinct subsets of CD4+ T lymphocytes

The studies summarized in this review have established that cloned lines of CD4+ T cells that produce distinct cytokines differ markedly in their responses to different forms of antigenic stimulation. Furthermore, we are beginning to develop experimental systems for better defining the signals that stimulate the differentiation of resting T cells into functionally distinct subsets. From these studies it is possible to construct the following hypothetical model for the differentiation of mature CD4+ T cells. Resting cells produce IL2 as the principal growth factor, IFN gamma, and little or no IL4. Antigenic stimulation in the presence of IL4 (which may be produced by non-T cells) leads to the preferential expansion of IL4-producing cells. These cells secrete their cytokines maximally when stimulated with antigens presented by B cells, which are also the principal targets of these cytokines. Continued expansion of IL4-producing T cells may require antigen exposures that also stimulate the production of IL1 by macrophages. In the absence of IL4 and IL1 (and in the presence of costimulators that are not yet defined) the T cells that are preferentially expanded belong to the IL2-producing subset. In addition, each subset may produce cytokines that stimulate the expansion of that subset and inhibit the other (Fiorentino et al., 1989). It is apparent that a number of assays and reagents need to be developed if these results are to be extended to physiologic immune responses. First, it is important to identify surface molecules that may serve as phenotypic markers for functionally distinct subsets of CD4+ cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of cytokine secretion in murine leishmaniasis: correlation with disease progression or resolution

Susceptibility or resistance to infection with Leishmania major correlates with the ability of mice to produce characteristic panels of lymphokines in response to the parasite. To investigate the role of antigen-presenting cells in this phenomenon, we developed a model system which used congenic (H-2d) susceptible and resistant mice. L. major-specific T cells were isolated from infected BALB/c and B10.D2 mice, and the cells were restimulated in vitro on syngenic or congenic antigen-presenting cells. BALB/c L. major-reactive T cells restimulated with either antigen-presenting cell produced high levels of interleukin-4 and low levels of gamma interferon. In contrast, T cells from B10.D2 mice produced gamma interferon. Radiation-induced chimeras reconstituted with BALB/c bone marrow also produced more interleukin-4 in response to L. major than did chimeras reconstituted with B10.D2 bone marrow. To test whether this pattern of cytokine secretion was unique to infection with L. major, we infected the mice with a second intracellular pathogen, Mycobacterium bovis BCG. Mycobacterium-specific T cells from both BALB/c and B10.D2 mice produced interleukin-2 and no interleukin-4. Finally, when BALB/c mice were vaccinated with avirulent L. major, the induced resistance correlated with reduced production of interleukin-4 but no increase in gamma interferon production. Instead, T cells from the vaccinated mice produced high levels of tumor necrosis factor. This suggests that tumor necrosis factor, in addition to gamma interferon, may be involved in resistance to L. major and that interleukin-4 may inhibit the leishmanicidal activity of tumor necrosis factor and/or gamma interferon.

Heterogeneity of helper/inducer T lymphocytes. III. Responses of IL-2- and IL-4-producing (Th1 and Th2) clones to antigens presented by different accessory cells

Murine CD4+ T cell clones have been classified into at least two subsets, Th1 and Th2, on the basis of their distinct lymphokine secretion profiles and functions. In the present study, we compared the functional responses of Th1 and Th2 clones to Ag presentation by splenic B cells and peritoneal macrophages. Th2 clones secreted IL-4 in response to Ag presented by resting B cells, but their optimal proliferation required the addition of IL-1 or a source of IL-1. The degree of IL-1 dependence varied among the four Th2 clones examined. In contrast, Th1 clones secreted IL-2 and proliferated in response to Ag presented by both B cells and macrophages, without any requirement for exogenous IL-1. Furthermore, the proliferation of Th2 clones in response to Ag presented by splenocytes or macrophages was inhibited by an IL-1R antagonist. These results indicate that IL-1 is an important costimulator for the expansion of the Th2 subset of CD4+ T cells. The different requirements for the proliferation of Th1 and Th2 cells may be responsible for the preferential expansion of one or the other subset under different conditions of immunization.

Mutations of immunoglobulin transmembrane and cytoplasmic domains: effects on intracellular signaling and antigen presentation

The membrane-bound form of immunoglobulin serves as an antigen-specific receptor for B cells mediating signal transduction and antigen presentation. We have developed an assay that reconstitutes both these physiologic responses with respect to the antigen phosphorylcholine. By introducing specific mutations in the human Ig mu chain gene, we have shown that certain transmembrane residues and the short cytoplasmic domain are crucial for these two activities. Moreover, elimination of a single transmembrane hydroxyl group severely inhibits antigen presentation without affecting signal transduction, suggesting that these two functions are mediated by different protein interactions.

IL-4 induces allergic-like inflammatory disease and alters T cell development in transgenic mice

We have assessed the biologic role of IL-4 by fusing its gene to an immunoglobulin promoter/enhancer and introducing it into transgenic mice. By attenuating the transgene promoter through the insertion of E. coli lac operator sequences, we have created a series of animals that constitutively express varying amounts of IL-4. Overexpression of IL-4 results in a marked increase in serum IgE levels and the appearance of an inflammatory ocular lesion (blepharitis) with characteristic histopathologic features seen in allergic reactions. In addition, expression of the IL-4 transgene in the thymus perturbs T cell maturation, reducing the population of immature CD4+CD8+ thymocytes and peripheral T cells while increasing the population of mature CD8+ thymocytes. These results demonstrate that deregulation of a single cytokine gene in vivo can induce a complex inflammatory reaction resembling that observed in human allergic disease.

Heterogeneity of helper/inducer T lymphocytes. IV. Stimulation of resting and activated B cells by Th1 and Th2 clones

To test the hypothesis that resting and previously activated B lymphocytes differ in their proliferative and differentiative responses to various Th cell-derived stimuli, we have examined the interactions of purified small (resting) and large (activated) murine B cells with rabbit Ig-specific Th1 and Th2 clones in the presence of the Ag analogue, rabbit anti-mouse Ig antibody. Small numbers of Th2 cells induce strong Ag-dependent proliferation of and Ig secretion by both resting and activated B lymphocytes. In contrast, Th1 clones stimulate lower responses of activated B cells and fail to stimulate small resting B cells. An interaction with Th1 clones does make small B cells responsive to the Th2-derived cytokine, IL-4, indicating that Th1 clones are capable of delivering some but not all the stimuli necessary for the induction of humoral immunity. Finally, in order to compare the responses of small and large B cells to cognate interactions and secreted cytokines, we used an autoreactive I-Ak-specific Th2 line. This line induces proliferation of and Ig secretion by I-Ak expressing but not H-2d resting and activated B cells as a result of cognate interactions. However, when the H-2d B cells are bystanders in the presence of cytokine secretion by this Th2 line, or are directly exposed to Th2-derived cytokines, both small and large B cells are induced to proliferate but only the large B cells secrete antibody. These results indicate that the magnitude and nature of antibody responses depend on three principal factors: the cytokines produced by Th cells, the state of activation of the responding B lymphocytes, and whether the B cells are recipients of cognate help or are bystanders at the site of T cell stimulation. Our findings also confirm the view that cognate T-B interactions are most efficient for initiating B cell responses and may allow B cells to subsequently respond to a variety of T cell-derived cytokines.

Heterogeneity of helper/inducer T lymphocytes. IV. Stimulation of resting and activated B cells by Th1 and Th2 clones

To test the hypothesis that resting and previously activated B lymphocytes differ in their proliferative and differentiative responses to various Th cell-derived stimuli, we have examined the interactions of purified small (resting) and large (activated) murine B cells with rabbit Ig-specific Th1 and Th2 clones in the presence of the Ag analogue, rabbit anti-mouse Ig antibody. Small numbers of Th2 cells induce strong Ag-dependent proliferation of and Ig secretion by both resting and activated B lymphocytes. In contrast, Th1 clones stimulate lower responses of activated B cells and fail to stimulate small resting B cells. An interaction with Th1 clones does make small B cells responsive to the Th2-derived cytokine, IL-4, indicating that Th1 clones are capable of delivering some but not all the stimuli necessary for the induction of humoral immunity. Finally, in order to compare the responses of small and large B cells to cognate interactions and secreted cytokines, we used an autoreactive I-Ak-specific Th2 line. This line induces proliferation of and Ig secretion by I-Ak expressing but not H-2d resting and activated B cells as a result of cognate interactions. However, when the H-2d B cells are bystanders in the presence of cytokine secretion by this Th2 line, or are directly exposed to Th2-derived cytokines, both small and large B cells are induced to proliferate but only the large B cells secrete antibody. These results indicate that the magnitude and nature of antibody responses depend on three principal factors: the cytokines produced by Th cells, the state of activation of the responding B lymphocytes, and whether the B cells are recipients of cognate help or are bystanders at the site of T cell stimulation. Our findings also confirm the view that cognate T-B interactions are most efficient for initiating B cell responses and may allow B cells to subsequently respond to a variety of T cell-derived cytokines.

Anti-CD3 antibody induces unresponsiveness to IL-2 in Th1 clones but not in Th2 clones

Culture of murine T cells with immobilized (platebound) anti-CD3 antibody results in autocrine growth factor secretion in both Th1 (IL-2 producing) and Th2 (IL-4 producing) cells. Using a panel of murine T cell clones, we demonstrate that the IL-2-induced proliferation of Th1 clones is dramatically inhibited by immobilized anti-CD3 antibody, whereas that of Th2 clones is not. This unresponsiveness of Th1 clones to IL-2 is not due to decreases in IL-2R expression. Supernatants from Th1 or Th2 cell cultures fail to alter the effects of anti-CD3 on the two types of clones, suggesting that unresponsiveness induced in Th1 clones or the lack thereof in Th2 clones is not mediated by a stable cytokine(s). Accessory cells enhance the proliferation of Th1 cells exposed to low concentrations of anti-CD3, but the unresponsiveness induced by high concentrations of anti-CD3 is not prevented by accessory cells. Finally, soluble anti-CD4 antibody prevents the induction of the unresponsive state even at high concentrations of anti-CD3. These experiments demonstrate that two subsets of cloned CD4+ T cells differ in their responses to anti-CD3, and that CD4 molecules may play a critical role in regulating the outcome of receptor-mediated stimulation.

Anti-CD3 antibody induces unresponsiveness to IL-2 in Th1 clones but not in Th2 clones

Culture of murine T cells with immobilized (platebound) anti-CD3 antibody results in autocrine growth factor secretion in both Th1 (IL-2 producing) and Th2 (IL-4 producing) cells. Using a panel of murine T cell clones, we demonstrate that the IL-2-induced proliferation of Th1 clones is dramatically inhibited by immobilized anti-CD3 antibody, whereas that of Th2 clones is not. This unresponsiveness of Th1 clones to IL-2 is not due to decreases in IL-2R expression. Supernatants from Th1 or Th2 cell cultures fail to alter the effects of anti-CD3 on the two types of clones, suggesting that unresponsiveness induced in Th1 clones or the lack thereof in Th2 clones is not mediated by a stable cytokine(s). Accessory cells enhance the proliferation of Th1 cells exposed to low concentrations of anti-CD3, but the unresponsiveness induced by high concentrations of anti-CD3 is not prevented by accessory cells. Finally, soluble anti-CD4 antibody prevents the induction of the unresponsive state even at high concentrations of anti-CD3. These experiments demonstrate that two subsets of cloned CD4+ T cells differ in their responses to anti-CD3, and that CD4 molecules may play a critical role in regulating the outcome of receptor-mediated stimulation.

Antigen presentation by B lymphocytes: mechanisms and functional significance

B lymphocytes are remarkably efficient at presenting protein antigens that bind to their immunoglobulin (Ig) receptors to class II MHC-restricted CD4+ helper T cells. Ig molecules function as high-affinity binding sites for capturing antigens at limiting concentrations. In addition, Ig receptor-mediated endocytosis leads to an intracellular pathway of protein traffic that favors recycling of antigens and optimizes the processing of these antigens. As a result, small amounts of endocytosed antigens are capable of associating with MHC molecules and being presented in a form that can be specifically recognized by T lymphocytes. The antigen presenting function of B cells is particularly important in secondary antibody responses, which require low concentrations of antigens and MHC-restricted T-B cell cooperation.

Immunodeficiency in graft-versus-host disease (GVHD). II. Effects of GVHD-induced suppressor cells on CD4+ T cell clones

Lethally irradiated mice transplanted with H-2-matched, minor histocompatibility-disparate bone marrow develop graft-vs-host disease (GVHD) that is associated with severe immunodeficiency. Splenocytes from such mice contain radioresistant cells that profoundly suppress normal lymphocyte function. We now show that GVHD-induced suppressor cells also inhibit the proliferation of CD4+ T cell clones specific for different Ag and class II MHC determinants. These suppressors have a dominant anti-proliferative effect, because they inhibit DNA synthesis in response to receptor-mediated stimulation and growth-promoting lymphokines, without abolishing lymphokine secretion or lymphokine receptor expression by the cloned T cells. The implications of these findings, and the usefulness of T cell clones for studying immune suppression, are discussed.

Immunodeficiency in graft-versus-host disease (GVHD). II. Effects of GVHD-induced suppressor cells on CD4+ T cell clones

Lethally irradiated mice transplanted with H-2-matched, minor histocompatibility-disparate bone marrow develop graft-vs-host disease (GVHD) that is associated with severe immunodeficiency. Splenocytes from such mice contain radioresistant cells that profoundly suppress normal lymphocyte function. We now show that GVHD-induced suppressor cells also inhibit the proliferation of CD4+ T cell clones specific for different Ag and class II MHC determinants. These suppressors have a dominant anti-proliferative effect, because they inhibit DNA synthesis in response to receptor-mediated stimulation and growth-promoting lymphokines, without abolishing lymphokine secretion or lymphokine receptor expression by the cloned T cells. The implications of these findings, and the usefulness of T cell clones for studying immune suppression, are discussed.

Role of interleukin 1 in the activation of T lymphocytes

The activation of T lymphocytes requires their stimulation via clonotypic antigen receptors as well as nonantigen-specific costimulators, the best defined of which is the cytokine interleukin 1 (IL-1). Recent studies have shown that murine CD4+ helper T lymphocytes consist of two nonoverlapping subsets that selectively utilize interleukin 2 (IL-2) or interleukin 4 as their autocrine growth factors and are called Th1 and Th2 cells, respectively. We now show that IL-1 functions as a costimulator for the proliferation of Th2 but not of Th1 clones and only Th2 cells express high-affinity receptors for IL-1. Secretion of autocrine growth-promoting lymphokines by Th1 and Th2 cells occurs after stimulation via the antigen receptor-CD3 complex and is neither dependent on nor affected by IL-1. These findings suggest that the activation of T lymphocytes can be divided into two stages, lymphokine secretion and proliferation, and only proliferation requires costimulators such as IL-1. Moreover, the prevailing view that IL-1 functions as a costimulator by inducing secretion of IL-2 or expression of IL-2 receptors may not be generally applicable, because IL-2-producing Th1 clones do not express receptors for IL-1 and are insensitive to this cytokine.

Specific inhibition of lymphokine biosynthesis and autocrine growth using antisense oligonucleotides in Th1 and Th2 helper T cell clones

T helper cells have recently been divided into two subsets. The Th1 subset secretes and responds to IL-2 in an autocrine manner. The Th2 subset upon mitogen or antigen stimulation releases IL-4. Here we describe a novel technology that allowed us to confirm this distinction. We have used synthetic oligonucleotides complementary to the 5' end of mouse IL-2 and IL-4 to specifically block the biosynthesis of IL-2 or IL-4 in two murine helper T cell clones from the Th1 or Th2 subset. We show that the antisense IL-2 oligonucleotide inhibited the proliferation of the Th1 clone and had no effect on the Th2 clone. In parallel experiments, the antisense IL-4 oligonucleotide blocked the proliferation of the Th2 clone and not the proliferation of the Th1 clone. The inhibition was significantly reversed in both cases by the addition of the relevant lymphokine (IL-2 in the case of the Th1 clone, IL-4 in the case of the Th2 clone). Northern analysis, using cDNA probes specific for the two lymphokines, showed a decrease in the steady-state level of the relevant lymphokine mRNA, suggesting the specific degradation of the mRNA by an RNase H-like enzymatic activity. This strategy, which allows the specific blockade of the biosynthesis of a lymphokine, could be useful for future studies on the role of each T helper subset in physiological immune responses.

Immortalization of murine connective tissue-type mast cells at multiple stages of their differentiation by coculture of splenocytes with fibroblasts that produce Kirsten sarcoma virus

Mature connective tissue mast cells (CTMC) have not been previously available as a cell line from any species. Here we describe 15 novel mast cell lines (KiSV-MC) that were derived by coculturing murine splenocytes with fibroblasts that produce a Ki-ras-containing murine sarcoma virus. Some of the KiSV-MC lines are similar to CTMC in that they synthesize predominantly heparin proteoglycans, and contain up to 35 micrograms of histamine and 2.2 units of carboxypeptidase A/10(6) cells in secretory granules which stain red with Safranin. Other cell lines display phenotypic characteristics intermediate to CTMC and mucosal-like mast cells in being predominantly Safranin-, having lower amounts of histamine and carboxypeptidase A, and in synthesizing chondroitin sulfate E proteoglycans in preference to heparin proteoglycans. When the individual KiSV-MC lines were compared, a linear relationship was found between the number of Safranin+ granules, the cellular contents of histamine and carboxypeptidase A, and the biosynthesis of heparin relative to chondroitin sulfate E proteoglycans. Upon sensitization with monoclonal IgE and exposure to hapten-specific antigen, the cells exocytose the contents of their secretory granules. Thus, these immortalized cells provide the first source of CTMC-like lines for chemical and functional analysis and illustrate that murine mast cells can express a continuum of phenotypes.

Functional heterogeneity among human inducer T cell clones

Analysis of mouse CD4+ inducer T cells at the clonal level has established that a dichotomy among CD4+ T cell clones exists with regard to types of lymphokines secreted. Mouse T cell clones designated Th1 have been shown to secrete IL-2 and IFN-gamma, whereas T cell clones designated Th2 have been shown to produce IL-4 but not IL-2 or IFN-gamma. To determine if such a dichotomy in the helper inducer T cell subset occurred in man, we examined a panel of human CD4+ helper/inducer T cell clones for patterns of lymphokine secretion and for functional activity. We identified human T cell clones which secrete IL-4 but not IL-2 or IFN-gamma, and which appeared to correspond to murine Th2 clones. In marked contrast to murine IL-2 secreting Th1 clones which do not produce IL-4 or IFN-gamma, we observed that some human T cell clones secrete IL-2, and IFN-gamma as well as IL-4. Southern blot analysis indicated that these multi-lymphokine-secreting clones represented the progeny of a single T cell. IL-4 secretion did not always correlated with enhanced ability to induce Ig synthesis. Although one T cell clone which secreted IL-2, IL-4, and IFN-gamma could efficiently induce Ig synthesis, another expressed potent cytolytic and growth inhibitory activity for B cells, and was ineffective or inhibitory in inducing Ig synthesis. These results indicate that although the equivalent of murine Th2 type cells appears to be present in man, the simple division of T cells into a Th1 and Th2 dichotomy may not hold true for human T cells.

Functional heterogeneity among human inducer T cell clones

Analysis of mouse CD4+ inducer T cells at the clonal level has established that a dichotomy among CD4+ T cell clones exists with regard to types of lymphokines secreted. Mouse T cell clones designated Th1 have been shown to secrete IL-2 and IFN-gamma, whereas T cell clones designated Th2 have been shown to produce IL-4 but not IL-2 or IFN-gamma. To determine if such a dichotomy in the helper inducer T cell subset occurred in man, we examined a panel of human CD4+ helper/inducer T cell clones for patterns of lymphokine secretion and for functional activity. We identified human T cell clones which secrete IL-4 but not IL-2 or IFN-gamma, and which appeared to correspond to murine Th2 clones. In marked contrast to murine IL-2 secreting Th1 clones which do not produce IL-4 or IFN-gamma, we observed that some human T cell clones secrete IL-2, and IFN-gamma as well as IL-4. Southern blot analysis indicated that these multi-lymphokine-secreting clones represented the progeny of a single T cell. IL-4 secretion did not always correlated with enhanced ability to induce Ig synthesis. Although one T cell clone which secreted IL-2, IL-4, and IFN-gamma could efficiently induce Ig synthesis, another expressed potent cytolytic and growth inhibitory activity for B cells, and was ineffective or inhibitory in inducing Ig synthesis. These results indicate that although the equivalent of murine Th2 type cells appears to be present in man, the simple division of T cells into a Th1 and Th2 dichotomy may not hold true for human T cells.

The role of antigen-presenting B cells in T cell priming in vivo. Studies of B cell-deficient mice

Mice rendered B cell deficient by treatment with rabbit anti-mouse IgM (anti-mu) antibodies from birth fail to respond when primed with soluble protein antigens in CFA, as measured by T cell proliferation when challenged with antigen in vitro. The role of B cells in T cell priming in vivo was examined by adoptively transferring hapten-specific B cells into anti-mu mice, followed by immunization with haptenated Ag in CFA. The T cell proliferative response to OVA of anti-mu BALB/c mice was partially restored by the administration of TNP or FITC-specific B cells and immunization with TNP-OVA or FITC-OVA, respectively. This reconstitution was Ag-specific, inasmuch as hapten-binding B cells restored the T cell responses to OVA in mice immunized with the same hapten coupled to OVA. The mechanism of B cell reconstitution of T cell priming in anti-mu mice was addressed using parental to F1 B cell transfers. The Ia restriction pattern of the activated T cells from these mice indicated that both direct presentation of Ag by transferred B cells and antibody-mediated enhancement of Ag presentation by non-B, host Ag-presenting cells occurred. Thus, Ag-specific B lymphocytes play a critical role in priming of T cells in vivo.

The role of antigen-presenting B cells in T cell priming in vivo. Studies of B cell-deficient mice

Mice rendered B cell deficient by treatment with rabbit anti-mouse IgM (anti-mu) antibodies from birth fail to respond when primed with soluble protein antigens in CFA, as measured by T cell proliferation when challenged with antigen in vitro. The role of B cells in T cell priming in vivo was examined by adoptively transferring hapten-specific B cells into anti-mu mice, followed by immunization with haptenated Ag in CFA. The T cell proliferative response to OVA of anti-mu BALB/c mice was partially restored by the administration of TNP or FITC-specific B cells and immunization with TNP-OVA or FITC-OVA, respectively. This reconstitution was Ag-specific, inasmuch as hapten-binding B cells restored the T cell responses to OVA in mice immunized with the same hapten coupled to OVA. The mechanism of B cell reconstitution of T cell priming in anti-mu mice was addressed using parental to F1 B cell transfers. The Ia restriction pattern of the activated T cells from these mice indicated that both direct presentation of Ag by transferred B cells and antibody-mediated enhancement of Ag presentation by non-B, host Ag-presenting cells occurred. Thus, Ag-specific B lymphocytes play a critical role in priming of T cells in vivo.

Frequencies of IL-2- and IL-4-secreting T cells in naive and antigen-stimulated lymphocyte populations

The relative frequencies of IL-2-and IL-4-secreting precursors in naive and Ag-primed populations were investigated by using limiting dilution analysis. Cells capable of IL-4 production in lymphoid populations freshly isolated from mice were rare in comparison with those producing IL-2 when the cells were stimulated by nominal Ag, by alloantigens, or by mitogens. One cycle of in vitro Ag restimulation and rest, however, enabled us to detect high proportions of IL-4-secreting cells among keyhole limpet hemocyanin-primed lymph node cells. With cultures set up at monoclonal cell doses, it was shown that IL-2 and IL-4 are secreted by separate precursor populations at this stage of their development. The IL-4-secreting cells were further shown to be dependent upon the presence of IL-2, either secreted by separate precursors or exogenously added, for the production of detectable amounts of IL-4. Analysis of the frequencies of helper cells producing both IL-2 and IL-4 at various stages of the in vivo immune response and the requirements for their growth and differentiation should give a better understanding of the relative contributions of each cell type.

Frequencies of IL-2- and IL-4-secreting T cells in naive and antigen-stimulated lymphocyte populations

The relative frequencies of IL-2-and IL-4-secreting precursors in naive and Ag-primed populations were investigated by using limiting dilution analysis. Cells capable of IL-4 production in lymphoid populations freshly isolated from mice were rare in comparison with those producing IL-2 when the cells were stimulated by nominal Ag, by alloantigens, or by mitogens. One cycle of in vitro Ag restimulation and rest, however, enabled us to detect high proportions of IL-4-secreting cells among keyhole limpet hemocyanin-primed lymph node cells. With cultures set up at monoclonal cell doses, it was shown that IL-2 and IL-4 are secreted by separate precursor populations at this stage of their development. The IL-4-secreting cells were further shown to be dependent upon the presence of IL-2, either secreted by separate precursors or exogenously added, for the production of detectable amounts of IL-4. Analysis of the frequencies of helper cells producing both IL-2 and IL-4 at various stages of the in vivo immune response and the requirements for their growth and differentiation should give a better understanding of the relative contributions of each cell type.

Immunodeficiency in graft-versus-host disease. I. Mechanism of immune suppression

Irradiated CBA/J mice transplanted with H-2 compatible, minor histocompatibility disparate B10.BR bone marrow develop graft-versus-host disease (GVHD) if mature T lymphocytes are added to the marrow inoculum. In the setting of mild GVHD (receiving 10(4) or 10(5) T cells), by phenotypic analysis, lymphoid reconstitution occurs normally within 4 to 6 wk but there is a profound deficiency in the ability of splenic lymphocytes to respond to polyclonal activators such as LPS and Con A. This unresponsiveness is attributable to active suppression mediated by cells that express Thy-1 and can be removed with leucine methyl ester treatment. Thus, splenocytes from mice with GVHD suppress responses of normal T and B lymphocytes. Moreover, depletion of these suppressor cells restores normal function to splenocytes from mice with GVHD, and B cells isolated from these mice respond normally to T-dependent and -independent stimulation. Finally, IFN-gamma plays an important role in this suppression, because a neutralizing anti-IFN-gamma mAb significantly removes suppression of normal cells and restores functional responses of lymphocytes from mice with GVHD. These results provide insights into the mechanisms of immunodeficiency associated with GVHD, and suggest novel strategies for possible therapies for this disorder.

Immunodeficiency in graft-versus-host disease. I. Mechanism of immune suppression

Irradiated CBA/J mice transplanted with H-2 compatible, minor histocompatibility disparate B10.BR bone marrow develop graft-versus-host disease (GVHD) if mature T lymphocytes are added to the marrow inoculum. In the setting of mild GVHD (receiving 10(4) or 10(5) T cells), by phenotypic analysis, lymphoid reconstitution occurs normally within 4 to 6 wk but there is a profound deficiency in the ability of splenic lymphocytes to respond to polyclonal activators such as LPS and Con A. This unresponsiveness is attributable to active suppression mediated by cells that express Thy-1 and can be removed with leucine methyl ester treatment. Thus, splenocytes from mice with GVHD suppress responses of normal T and B lymphocytes. Moreover, depletion of these suppressor cells restores normal function to splenocytes from mice with GVHD, and B cells isolated from these mice respond normally to T-dependent and -independent stimulation. Finally, IFN-gamma plays an important role in this suppression, because a neutralizing anti-IFN-gamma mAb significantly removes suppression of normal cells and restores functional responses of lymphocytes from mice with GVHD. These results provide insights into the mechanisms of immunodeficiency associated with GVHD, and suggest novel strategies for possible therapies for this disorder.

Heterogeneity of helper/inducer T lymphocytes. II. Effects of interleukin 4- and interleukin 2-producing T cell clones on resting B lymphocytes

To compare the helper function of murine T cell clones that secrete IL-2 and IFN-gamma (Th1 cells) or IL-4 and IL-5 (Th2), purified resting B cells were stimulated with F(ab')2 rabbit anti-mouse Ig (RAMG) and rabbit Ig-specific, class II MHC-restricted cloned T cells belonging to the two subsets. Both Th2 clones examined induced strong proliferative responses of B cells in the presence of RAMG, as well as the secretion of IgM and IgG1 antibodies. In contrast, the Th1 clones tested failed to stimulate B cell growth or antibody secretion. Th2-mediated B cell activation was dependent on IL-4 and IL-5, and was also inhibited by IFN-gamma or IFN-gamma produced by Th1 cells present in the same cultures. However, the failure of Th1 cells to help resting B cells could not be reversed with neutralizing anti-IFN-gamma antibody. In addition to this inhibitory effect, IFN-gamma was required for the secretion of IgG2a antibody, particularly when B cells were stimulated with polyclonal activators such as LPS. Finally, both sets of T cell clones secreted lymphokines when stimulated with purified B cells and RAMG. These experiments demonstrate that T cells that differ in lymphokine production also differ in their ability to help B cells as a result of cognate interactions at low concentrations of antigens. Moreover, IL-4, IL-5, and IFN-gamma serve different roles in the T cell-dependent proliferative and differentiative responses of resting B lymphocytes.

Heterogeneity of helper/inducer T lymphocytes. I. Lymphokine production and lymphokine responsiveness

Antigen-specific, Ia-restricted helper/inducer T lymphocytes consist of subsets that can be distinguished by lymphokine secretion. One, called Th1, secretes IL-2 and the other, termed Th2, produces BSF-1/IL-4 in response to stimulation by lectin or antigen receptor signals, and each uses the respective lymphokine as its autocrine growth factor. Cloned lines representing Th2 cells proliferate in response to both IL-2 and their autocrine lymphokine, BSF-1/IL-4, but this proliferation is dependent on the synergistic costimulator activity of the monokine, IL-1. In contrast, Th1 clones proliferate only in response to IL-2, are unresponsive to BSF-1/IL-4, and their growth is unaffected by IL-1. These response patterns are not attributable to variations in culture conditions but apparently reflect intrinsic properties of the two T cell subsets. Moreover, the unresponsiveness of Th1 cells to BSF-1/IL-4 may be related to lower levels of expression of surface receptors for this lymphokine. These results may explain the observed heterogeneity among bulk populations of T cells in terms of lymphokine responsiveness and requirement for accessory factors (costimulators). In addition, our findings suggest that IL-2, unlike BSF-1/IL-4, is a fully competent growth factor that is potentially involved in antigen-independent expansion of bystander T cells present at sites of immune stimulation.

Antigen presentation by hapten-specific B lymphocytes. V. Requirements for activation of antigen-presenting B cells

Splenic B cells specific for the haptens, 2,4,6-trinitrophenyl (TNP) or fluorescein isothiocyanate (FITC) were cultured with a range of concentrations of unmodified or TNP- or FITC-conjugated conalbumin and the conalbumin + I-Ak-specific, interleukin (IL) 1-dependent helper T cell clone, D10 . G4, in the presence and absence of IL-1. Lymphokine secretion, T cell proliferation, and antibody secretion by B cells all exhibited identical antigen dose responses. Thus, hapten-binding B cells presented low concentrations of haptenated conalbumin for activation of both the T and the antigen-presenting B cells. Whereas proliferation of D10 . G4 required the addition of IL-1, both lymphokine production and stimulation of B cells to antibody secretion occurred without exogenous IL-1. These results demonstrate that when B lymphocytes function as presenting cells for antigens that bind to their immunoglobulin receptors, activation of the responding T cells and the B cells themselves occur at similar concentrations of antigen. Moreover, for functional T-B interactions, antigen-presenting B and responding T lymphocytes constitute a complete system that requires no other accessory stimuli, whereas clonal expansion of T cells is dependent on accessory factors such as IL-1. Finally, since D10 . G4 secretes IL-4 but neither IL-2 nor interferon-gamma, our results demonstrate that differentiation of B cells as a consequence of direct ("cognate") interactions with helper T cells as well as of bystander B cells can occur in the absence of IL-1, IL-2, and interferon-gamma.

Antigen presentation by hapten-specific B lymphocytes. V. Requirements for activation of antigen-presenting B cells

Splenic B cells specific for the haptens, 2,4,6-trinitrophenyl (TNP) or fluorescein isothiocyanate (FITC) were cultured with a range of concentrations of unmodified or TNP- or FITC-conjugated conalbumin and the conalbumin + I-Ak-specific, interleukin (IL) 1-dependent helper T cell clone, D10 . G4, in the presence and absence of IL-1. Lymphokine secretion, T cell proliferation, and antibody secretion by B cells all exhibited identical antigen dose responses. Thus, hapten-binding B cells presented low concentrations of haptenated conalbumin for activation of both the T and the antigen-presenting B cells. Whereas proliferation of D10 . G4 required the addition of IL-1, both lymphokine production and stimulation of B cells to antibody secretion occurred without exogenous IL-1. These results demonstrate that when B lymphocytes function as presenting cells for antigens that bind to their immunoglobulin receptors, activation of the responding T cells and the B cells themselves occur at similar concentrations of antigen. Moreover, for functional T-B interactions, antigen-presenting B and responding T lymphocytes constitute a complete system that requires no other accessory stimuli, whereas clonal expansion of T cells is dependent on accessory factors such as IL-1. Finally, since D10 . G4 secretes IL-4 but neither IL-2 nor interferon-gamma, our results demonstrate that differentiation of B cells as a consequence of direct ("cognate") interactions with helper T cells as well as of bystander B cells can occur in the absence of IL-1, IL-2, and interferon-gamma.

Inhibition of B lymphocyte activation by interferon-gamma

Helper/inducer T cell clones specific for protein antigens and class II MHC determinants consist of two nonoverlapping subsets. One (called Th1) secretes IL 2 and IFN-gamma and the other (Th2) produces BSF1 upon stimulation with antigen or polyclonal activators. By using hapten-binding normal B cells and the B lymphoma line WEHI-279 as assays for B cell helper (maturation) factors, we have shown that Th2 clone supernatants (SN) induce differentiation to antibody secretion, whereas Th1 SN do not. The failure of Th1 SN to activate B cells is due to inhibitory effects of IFN-gamma, because it can be reversed by a neutralizing monoclonal antibody specific for IFN-gamma. Thus, in the presence of this antibody, even Th1 SN stimulate B cell maturation maximally. Conversely, recombinant IFN-gamma inhibits proliferation and differentiation of B cells induced by active Th2 SN. These results demonstrate that IFN-gamma is a potent inhibitor of B lymphocyte activation and can be distinguished from growth and maturation-inducing helper factors that are produced by both subsets of helper T cells.

Inhibition of B lymphocyte activation by interferon-gamma

Helper/inducer T cell clones specific for protein antigens and class II MHC determinants consist of two nonoverlapping subsets. One (called Th1) secretes IL 2 and IFN-gamma and the other (Th2) produces BSF1 upon stimulation with antigen or polyclonal activators. By using hapten-binding normal B cells and the B lymphoma line WEHI-279 as assays for B cell helper (maturation) factors, we have shown that Th2 clone supernatants (SN) induce differentiation to antibody secretion, whereas Th1 SN do not. The failure of Th1 SN to activate B cells is due to inhibitory effects of IFN-gamma, because it can be reversed by a neutralizing monoclonal antibody specific for IFN-gamma. Thus, in the presence of this antibody, even Th1 SN stimulate B cell maturation maximally. Conversely, recombinant IFN-gamma inhibits proliferation and differentiation of B cells induced by active Th2 SN. These results demonstrate that IFN-gamma is a potent inhibitor of B lymphocyte activation and can be distinguished from growth and maturation-inducing helper factors that are produced by both subsets of helper T cells.

Retrovirus infection alters growth factor responses of T lymphocytes

A murine helper/inducer T cell clone, D10.G4, has been infected with Kirsten-murine sarcoma virus (KiSV) pseudotyped with an amphotropic murine leukemia virus. The resultant Ki-ras-expressing lines (KiSV-D10) remain dependent on exogenous factors for continued growth but display distinctly different mitotic responses to certain cytokines as compared to the uninfected parent clone. Unlike the parent D10.G4 cells, these KiSV-D10 cells can be maintained in vitro indefinitely in the presence of recombinant interleukin 2 (IL 2), and they all display a maximal proliferative response to purified or recombinant interleukin 1 (IL 1). The IL 1-induced proliferation is shown not to be dependent or secretion of the T cell autocrine growth factors IL 2 or B cell stimulatory factor-1 (BSF-1). The KiSV-D10 lines show certain differences from one another and parent D10.G4 cells in their secretory and proliferative responses to T cell receptor- and BSF-1 mediated signals. These viral oncogene-expressing T cell lines, which remain responsive to and dependent on physiologic growth factors, should prove valuable for analyzing the mechanisms of action of single oncogenes and the intracellular events in T lymphocyte activation.

Retrovirus infection alters growth factor responses of T lymphocytes

A murine helper/inducer T cell clone, D10.G4, has been infected with Kirsten-murine sarcoma virus (KiSV) pseudotyped with an amphotropic murine leukemia virus. The resultant Ki-ras-expressing lines (KiSV-D10) remain dependent on exogenous factors for continued growth but display distinctly different mitotic responses to certain cytokines as compared to the uninfected parent clone. Unlike the parent D10.G4 cells, these KiSV-D10 cells can be maintained in vitro indefinitely in the presence of recombinant interleukin 2 (IL 2), and they all display a maximal proliferative response to purified or recombinant interleukin 1 (IL 1). The IL 1-induced proliferation is shown not to be dependent or secretion of the T cell autocrine growth factors IL 2 or B cell stimulatory factor-1 (BSF-1). The KiSV-D10 lines show certain differences from one another and parent D10.G4 cells in their secretory and proliferative responses to T cell receptor- and BSF-1 mediated signals. These viral oncogene-expressing T cell lines, which remain responsive to and dependent on physiologic growth factors, should prove valuable for analyzing the mechanisms of action of single oncogenes and the intracellular events in T lymphocyte activation.

Antigen presentation by hapten-specific B lymphocytes. IV. Comparative ability of B cells to present specific antigen and anti-immunoglobulin antibody

The ability of trinitrophenyl (TNP)-binding murine B lymphocytes to present native rabbit IgG (RGG), TNP-modified RGG, and rabbit anti-mouse Ig (RAMG) to an Ia-restricted, RGG-specific helper/inducer T cell clone was compared. By three independent assays (lymphokine secretion, T cell proliferation, and B cell differentiation), TNP-RGG was presented at 10(2)- to 10(3)-fold lower concentrations than RGG, and RAMG at 10(2)- to 10(3)-fold lower concentrations than TNP-RGG. The available data suggest that the efficiency of antigen presentation is dependent primarily on the avidity of binding of a ligand to B cell surface Ig and/or the extent of subsequent endocytosis (modulation). Despite the observed quantitative differences between anti-Ig (RAMG) and specific antigen (TNP-RGG), these results demonstrate that qualitatively both are essentially similar in their ability to mediate specific T-B interactions. Thus, anti-Ig antibodies are valid models for analyzing cognate interactions between antigen-specific B and helper T lymphocytes.

Antigen presentation by hapten-specific B lymphocytes. IV. Comparative ability of B cells to present specific antigen and anti-immunoglobulin antibody

The ability of trinitrophenyl (TNP)-binding murine B lymphocytes to present native rabbit IgG (RGG), TNP-modified RGG, and rabbit anti-mouse Ig (RAMG) to an Ia-restricted, RGG-specific helper/inducer T cell clone was compared. By three independent assays (lymphokine secretion, T cell proliferation, and B cell differentiation), TNP-RGG was presented at 10(2)- to 10(3)-fold lower concentrations than RGG, and RAMG at 10(2)- to 10(3)-fold lower concentrations than TNP-RGG. The available data suggest that the efficiency of antigen presentation is dependent primarily on the avidity of binding of a ligand to B cell surface Ig and/or the extent of subsequent endocytosis (modulation). Despite the observed quantitative differences between anti-Ig (RAMG) and specific antigen (TNP-RGG), these results demonstrate that qualitatively both are essentially similar in their ability to mediate specific T-B interactions. Thus, anti-Ig antibodies are valid models for analyzing cognate interactions between antigen-specific B and helper T lymphocytes.

B-cell stimulatory factor 1 and not interleukin 2 is the autocrine growth factor for some helper T lymphocytes

Clonal expansion of T lymphocytes of the helper/inducer class is generally thought to be mediated by an interleukin 2 (IL-2)-dependent autocrine mechanism. Thus, T cells stimulated by antigens or mitogenic lectins secrete IL-2 and, under appropriate conditions, express membrane receptors for IL-2, and the specific hormone-receptor interaction induces cellular proliferation. Recent studies indicate that B-cell stimulatory factor 1 (BSF-1) is secreted by T cells and is capable of stimulating T-cell proliferation. We now report that BSF-1 and not IL-2 is the sole autocrine growth factor for certain cloned lines of inducer T lymphocytes. On stimulation by the lectin concanavalin A, anti-receptor antibody, or specific antigen with antigen-presenting cells, such clones secrete a lymphokine that stimulates DNA synthesis by the "IL-2 indicator line," HT2, but is identified as BSF-1 by specific inhibition with monoclonal antibodies. The proliferative response of such BSF-1-secreting clones to receptor-mediated signals is dependent on BSF-1 and not IL-2. These results demonstrate a function of BSF-1 and confirm the existence of a previously unknown autocrine pathway of T-cell activation.