Helper cell function of primed T cells. II. T-T cell Synergism between Ig+ and Ig− subpopulations of primed thymocytes: a mechanism for amplification of helper cell function

Thymocytes collected from spleens of lethally irradiated and thymocyte-reconstituted mice, 6 h after “education” with SRBC, exert a markedly augmented helper cell function, especially for the 7 S response. Pretreatment of primed thymocytes with a rabbit anti-mouse Ig serum and complement abolished the augmented helper cell function, although it had no effect on unprimed cells. Treatment of 6 h primed T cells with an anti-Θ serum and complement also abolished the augmented helper cell function. However, in striking contrast with unprimed T cells where such treatment entirely abolished the helper cell function, the anti-Θ treated 6 h T cells are still able to provide a significant helper cell function of the same magnitude as untreated nonprimed cells. This result suggests that a T cell subpopulation 6 h after priming becomes resistant to lysis by anti-Θ serum and supports a substantial helper cell function. Since the sum of plaque-forming cells (PFC) given by anti-Ig and anti-Θ-treated 6 h primed cells was far smaller than the PFC given by untreated cells, a T-T synergism is implicated as a mechanism for the augmented helper cell function of 6 h primed thymocytes. The amplification was also eliminated following treatment with a mouse anti SRBC serum indicating that some primed cells carry antigen on their surface. The data from various recombinations of primed cells treated with one of these three antisera, as well as primed cells treated sequentially with various combinations of two of the three antisera, suggested that the Ig⁺ and antigen-carrying cells belong to the same subpopulation which is resistant to lysis by anti-Θ serum. These cells are distinct from the rest which remain Ig^? and still sensitive to lysis by anti-Θ serum (Θ⁺). The Ig and antigen most likely represent a cytophilic complex present on the surface of a subpopulation of T cells. Thymocytes collected 5 days after education show little or no synergistic effect. The T-T cell synergism described here may represent a basic regulatory function of T cells.     

Antigen presentation for T cell interleukin-2 secretion is a late acquisition of neonatal B cells.

The ability of B lymphocytes to process and present antigen to helper T cells is essential to initiate T cell-B cell interactions in humoral immune responses. Here we describe the developmental acquisition of the antigen-presenting function of B cells as measured by the ability of B cells to stimulate a T cell hybrid to interleukin (IL)-2 secretion. Neonatal splenic B cells are not adult-like in their ability to process and present the model protein antigen pigeon cytochrome (Pc), which enters the B cell through fluid-phase pinocytosis, until 21 to 28 days of life. The ability of neonatal B cells to process and present antigen which enters the cell bound to surface Ig is not adult-like until 28 days of age. When neonatal B cells acquire antigen-presenting cell (APC) function, surface IgM facilitates antigen processing. The delayed acquisition of APC function cannot be accounted for solely by a deficiency in major histocompatibility complex MHC class II, ICAM-1, or LFA-1 as neonatal B cells express adult levels of these molecules by 7-14 days after birth. Moreover, the ability of neonatal B cells to present a peptide fragment of Pc which does not require processing is adult like by day 14. Furthermore, neonatal B cells are capable of binding, internalizing and degrading radiolabeled antigen, suggesting a more subtle level of regulation. In contrast to neonatal B cells, immature B cells in the adult bone marrow and adult B cells undergoing antigen-driven differentiation to memory B cells, as defined by the loss of the J11D marker, are competent to process and present antigen resulting in T cell IL-2 secretion. Thus, developing B cell subpopulations in the adult and in the neonate can be distinguished. Only neonatal B cells are deficient in their ability to stimulate T cells to IL-2 production.     

Engagement of the B lymphocyte antigen receptor induces presentation of intrinsic immunoglobulin peptides on major histocompatibility complex class II molecules

By means of the clonotypic variable region, the immunoglobulin (Ig) is a tumorspecific antigen on B cell neoplasms. We report that engagement of the B cell antigen receptor (BcR) promotes presentation of peptides derived from the B cell's intrinsic Ig to major histocompatibility complex (MHC) class II-restricted T cells. Thus, anti-Ig endowed normal, ex vivo B lymphocytes from H-2^d, Ig constant heavy chain allotype b (IgC_H^b) mice with the capacity to stimulate an I-A^d-restricted T cell clone which recognizes the γ2a^b 435–451 allopeptide. The corresponding self γ2a^a peptide is cryptic and 6000-fold less antigenic than the γ2a^b allopeptide. Even so, the syngeneic B cell lymphoma A20 which expresses surface (s) IgG2a^a, was also recognized by the T cells after BcR ligation. Thus, anti-Ig triggered the disclosure of a cryptic tumor antigen determinant. We propose that autoantigens, by engaging the BcR of self-reactive B cells, induce presentation of intrinsic Ig peptides to which the T helper cell (Th) repertoire is not tolerant. In this way, B cells with anti-self potential may be activated without Th recognition of nominal autoantigen.     

H-2-restricted T-B cell interactions involved in polyspecific B cell responses mediated by soluble antigen

Maturation of polyspecific B cells to Ig secretion was induced in response to the interaction of helper T cells with soluble antigen. The in vitro propagated, keyhole limpet hemocyanin (KLH)-primed C57BL16 T cells used in these experiments proliferated in response to antigen presented on I-A-compatible antigen-presenting cells and were depleted of alloreactive cells. These T cells induced a strong polyspecific plaque-forming cell response from normal C57BL/6 B cells over a wide range of KLH concentrations (100 μg/ml to 0.01 μg/ml). Culture conditions were established whereby H-2-restricted T cell-macrophage interactions were nonlimiting, allowing a direct analysis of H-2-restricted T-B cell interactions involved in polyspecific B cell induction. The requirements for B cell induction/amplification were dependent on the size of the responding B cell population. Small B cells were activated only at high concentrations of KLH and required a direct H-2-restricted interaction with KLH-primed helper T cells. In contrast, large B cells were induced/amplified over the full range of KLH concentrations tested in an H-2-unrestricted manner, limited only by the H-2-restricted, antigen-dependent activation of T helper cells. The requirement for high antigen concentration in the activation of small B cells is proposed to reflect a requirement for antigen binding to the B cell surface via nonspecific interactions. This binding was not in itself stimulatory, but expression of antigen in conjunction with B cell surface Ia antigens provided a focus for a direct interaction with Ia-restricted, antigen-specific T helper cells (or factors). In contrast, the H-2-unrestricted induction/ amplification of large B cells was mediated solely as a consequence of the helper T cell activation which occurred at the lowest concentrations of KLH tested. T helper cell activation is proposed to lead to the production of non-antigen-specific, non-Ia-restricted factors supporting the continued growth of blasted B cells. Thus, while an H-2-restricted T cell-macrophage interaction is an obligate requirement for polyspecific B cell responses, the requirement for a direct H-2-restricted T-B cell interaction varies with the pre-existing state of activation of the responding B cell population.     

The Immunoglobulin Receptors on B Cells Bind Antigen, Focus Activation Signals to Them and Initiate Antigen-Presentation

We do not agree with the analysis of Langman and Cohn on the function of Ig receptors. We have reviewed the available literature regarding anti-Ig activation of B cells and found it contradictory and unconvincing. We have presented experimental evidence on the inability of Ig receptors on B cells to mediate activation or tolerogenic signals. We suggest that the Ig receptors serve to focus antigen to specific B cells so the B cells can be activated by TI antigens or helper T cells. The Ig molecules also bind foreign antigen and thereby initiate internalization and antigen processing. The processed peptides are exported to the membrane, where they associate with MHC class II antigens, thus transforming B cells into efficient antigen-presenting cells.     

Functional properties of subsets of T lymphocytes defined by specific antigens.

Heteroantisera raised to the acute lymphocytic T (ALL) cell line HSB2 and to Sézary cells react with distinct subpopulations of T lymphocytes. Each antiserum reacts with a different T cell antigen and defines a distinct subpopulation that represents approximately 50% of peripheral blood T lymphocytes. The anti-HSB2-positive subpopulation contained suppressor cells for pokeweed mitogen-dependent immunoglobin (Ig) synthesis whereas the anti-Sézary cell serum-positive population included helper cells for Ig synthesis and mixed lymphocyte responder cells.     

Evidence for the ontogenic precedence of suppressor T cell functions in the human neonate

The study was undertaken to elucidate some functional characteristics of T cell subsets in human cord blood. A comparison of the cellular interactions involved in the in vitro regulation of pokeweed (PWM) vs. Epstein-Barr virus (EBV)-driven B cell differentiation was done in vitro in short-term cultures of lymphocytes from newborns or adults. T cell subsets were isolated using the monoclonal antibodies OKT4⁺ and OKT8⁺. OKT4⁺ but not OKT8⁺ T lymphocytes from adults as well as neonates suppressed EBV-induced immunoglobulin (Ig) secretion of B lymphocytes from adults. This inhibition was mediated through gamma-type interferon (IFN-γ). B cells from newborns were not inhibitable by OKT4⁺ lymphocytes as a result of their insensitivity to IFN-γ. Helper activity for PWM-induced Ig secretion was exclusively contained within the OKT4⁺ population from adult T cell donors. This function was normally not detectable in any of the neonatal T cell subsets. OKT8⁺ cells from both adults and neonates suppressed PWM-induced Ig secretion, but required the collaboration with cells within the OKT4⁺ population. The suppressor activity in the PWM system was not IFN-γ-mediated. Thus, suppressor functions for EBV-vs. PWM-induced Ig synthesis were mediated through different pathways. There was no evidence of a unique suppressor system in the newborn. In the neonate, suppressor T cell activities are developed before T helper functions, a circumstance for which there is good evolutionary reason.     

Differential effects of glucocorticosteroids on the functions of subpopulations of helper T lymphocytes

The in vitro effects of dexamethasone on the activities of Th₁ and Th₂ helper cell subpopulations were examined in secondary (IgG) responses to hapten-carrier conjugates under conditions where the selective or dominant expression of their individual activities could be observed. In the induction of IgG anti-hapten responses, carrier-primed Th₁ cells cooperate with hapten-primed B cells by linked recognition of antigen, while carrier-primed Th₂ cells cooperate with hapten-specific B cells by unlinked recognition of antigen. The function of carrier-primed Th₂ cells was resistant to inactivation by dexamethasone. In contrast, the function of carrier-primed Th₂ cells was abolished in the presence of pharmacologic concentrations of the steroid. The differential effects of dexamethosone on the activities of the two subpopulations of helper T cells could not be attributed to selective inhibition of a subpopulation of B cells which cooperates with Th₂ cells.     

The effect of age on the cognate function of CD4+ T cells

Summary: With increasing age, the ability to produce protective antibodies in response to immunization declines, resulting in reduced efficacy of vaccination. We have examined how reductions in CD4⁺ T‐cell function contribute to reduced humoral responses, using a model that allows us to compare identical numbers of antigen‐specific naive T cells from young and aged T‐cell receptor transgenic mice. Naive cells from aged mice exhibit reduced responses, both in vitro and in vivo. In vitro, responses of aged T cells can be enhanced by addition of interleukin (IL)‐2. In vivo, using an adoptive transfer model with young hosts, naive cells from aged mice exhibit significant reductions in cognate helper function, leading to reduced B‐cell expansion and differentiation. These age‐related defects could be overcome by prior in vitro T helper 2 effector generation with aged T cells. This improvement in cognate function of the aged effectors may be related to the enhancement of CD154 expression, which occurs on aged T cells in the presence of exogenous IL‐2. We also found no difference in B‐cell expansion and differentiation when young cells were transferred to young or aged hosts. Our results indicate that age‐related reductions in humoral responses are mainly due to defects in the cognate helper function of naive CD4⁺ T cells from aged individuals.     

Profile of multiple lymphocyte functional defects in acquired hypogammaglobulinemia,derived from in vitro cell recombination analysis

Immunoregulatory defects in patients can be assigned to B cells, T helper cells, or T suppressor cells by means of a modification of the in vitro PWM-stimulated Ig biosynthesis assay. When lymphocyte subpopulations from patient and normal are recombined, multiple internal comparisons within a single experiment reveal the functional activity of each subpopulation. By this technique we studied nine adult patients with acquired panhypogammaglobulinemia and one with isolated IgG deficiency. Low total Ig production by the B cell fraction was demonstrated in seven of the nine panhypogammaglobulinemic patients. In four of these seven, IgG and IgA secretion was markedly reduced compared with IgM. In the two patients with normal total Ig production, elevated IgM compensated for lower IgA and IgG. The one patient with no surface Ig⁺ cells had a B cell defect, but no T cell defect. Reduced T help and excessive T suppression characterized two of the four patients with the most severe B cell defects. Three patients were anergic by delayed hypersensitivity skin testing and failed to sensitize to DNCB, but the patient with the most severe T cell defects in vitro was not among them. One of these three patients showed a mild T cell help defect and suppressor excess and the other two had pure B cell defects. Thus, anergy and T regulator function were not correlated. In three instances where hypersuppression was more evident by adding patient unfractionated cells than by adding patient T cells, suppression by the T-depleted fraction could be demonstrated. No cases of radiation-resistant T suppression were revealed among the seven patients tested. Subnormal total protein synthesis, noted in six of seven patients with low Ig production, was invariably less marked than the Ig defect and often affected both T and B cells. One additional patient with pure IgG deficiency of 2 yr duration was essentially normal in her in vitro lymphocyte function. The general applicability of this experimental design for analysis of positive or negative immunoregulatory abnormalities is emphasized.     

Evaluation of accessory cell heterogeneity. II. Failure of dendritic cells to activate antigen-specific T helper cells to soluble antigens

The activation of antigen-specific T cells requires Ia⁺, antigen-presenting accessory cells (AC). Dendritic cells (DC) and macrophages (M?) isolated from spleen and peritoneal exudate were tested as AC for the activation of T helper cells and the induction of T cell proliferation. The cell separations to obtain DC and splenic M? were performed by discontinuous bovine serum albumin gradients, adherence on petri dishes and rosetting with opsonized sheep erythrocytes. DC as well as the M? were able to induce antigen-specific T cell proliferation, but only the M? and not the DC activated antigen-specific T helper cells which help B cells for antibody production to soluble antigens. Keyhole limpet hemocyanin-specific T cells repeatedly stimulated with DC and antigen also did not express helper activity. The failure of DC to induce T helper cells was not due to the activation of a suppressor pathway. Thus, dendritic cells, although very efficient as AC in the induction of various T cell functions, are not able to activate T helper cells required for carrier-specific T-B cooperation and therefore cannot be the sole accessory cells. Based on these results and on previous data using Ia⁺ tumor cell line as AC, we confirm the existence of functional AC heterogeneity.     

Enhanced activation of human T cell clones specific for virus-like particles expressing the HIV V3 loop in the presence of HIV V3 loop-specific polyclonal antibodies

Recombinant virus-like particles (VLP), formed by the yeast Ty p1 protein, carrying the HIV gp120 V3 loop on their surface (V3-VLP) have been tested in vitro for immunogenicity and antigenicity by using VLP p1-specific human CD4⁺ T cell lines and clones. VLP-specific human T cell lines and clones were generated from normal individuals, indicating that VLP-specific precursor cells present in the peripheral lymphocyte pool can be induced to expand clonally upon antigen challenge in vitro, in the absence of previous immunization. It was also shown that V3-specific polyclonal antibodies enhance V3-VLP-induced activation of VLP-specific T cell clones. Antibody-dependent potentiation has been shown previously in other antigen systems, and it depends on enhanced uptake of complexed antigen by Fc receptor-positive antigen-presenting cells. Since in this case antigen is internalized by presenting cells as a complex, it can be inferred that a similar event of antibody-mediated antigen uptake can take place with V3-specific B cells, resulting in presentation by the B cells of T helper epitopes derived from processing of the VLP p1 moiety. This suggests that T helper cells specific for the carrier VLP p1 protein can be activated to provide help to V3-specific B cells in the presence of the appropriate antigen construct.     

Phenotypic and functional characterization of CD11c+ dendritic cell population in mouse Peyer's patches

The antigen-presenting cell system in the gastrointestinal tract, one of three main sites (skin and lung being the others) of primary antigen contact, is poorly understood. Our study focused on dendritic cells (DC) as possible candidates for antigen uptake, processing and presentation in mucosal inductive sites, such as Peyer's patches (PP). To investigate the morphology, immunophenotype and stimulatory activity of intestinal DC, a procedure was developed to obtain a cell population by using collagenase digestion of PP, density centrifugation and cell sorting on the basis of CD11c expression. The resultant low-density cell fraction consisted of a nonadherent cell population expressing different intensities of CD11c that could at least be characterized by typical DC morphology (e.g. abundant cytoplasma with veil-like cytoplasmatic dendrites, irregularly shaped nuclei, multivesicular and multilamellar bodies), constitutive levels of surface MHC class II, the presence of macrophage-specific markers, such as F4/80, Mac-I and Fc receptors, respectively, on subpopulations of CD11c⁺ sorted cells and expression of adhesion and co-stimulatory receptors like ICAM-1 and CD44. The capability of this low-density CD11c⁺ fraction to stimulate T cell responses was demonstrated in primary allogeneic mixed-lymphocyte reactions (MLR). Herein, we show that the freshly isolated CD11c⁺ cells showed weak accessory function, but develop this capacity following short-term culture in vitro in the presence of granulocyte/macrophage colony-stimulating factor. Although the nature and functional capacity of the isolated CD11c⁺ needs further clarification, these preliminary results describing phenotype and accessory function provide some evidence that these cells isolated from the PP may be immature forms of DC and play a crucial role as antigen-presenting cells with important implications for understanding the complex network regulating intestinal antigen uptake, processing and presentation.     

Antigen-specific activation of B cells in vitro after recruitment of T cell help with superantigen

Background: Human B cells can proliferate in vitro after stimulation with anti-Ig and via the CD40 molecule. Superantigens like SEA which bind to MHC class II antigens on, e.g. B cells can polyclonally activate T cells via interaction with their TcR. The activated T cell subsequently activates the B cells to proliferation and Ig-production. Objectives: To investigate whether superantigen could be used to direct polyclonal T cell help to human B cells stimulated by antigen in a restricted manner resulting in production of antigen-specific antibodies in vitro. Study design: Purified B cells were preincubated with the antigen in manners allowing crosslinking of surface-Ig. The antigen exposed B cells were then cultured together with autologous CD4⁺ helper T cells and in the presence of various concentrations of SEA. Antibody production was measured by ELISA after 7–12 days of culture. Results: Antigen-specific activation of B cells could be obtained after stimulating the B cells with antigen or anti-surface-Ig antibodies in the presence of T helper cells and SEA. The degree of B cell activation (proliferation as well as antibody production) depended on the dose of antigen as well as on the dose of SEA used. Increased crosslinking of surface-Ig on antigen-specific B cells enhanced Ig production. Specific antibody production to a secondary recall antigen (tetanus toxoid) and to primary antigens (DNP and GM2) were obtained. The specific B cell response was dependent on contact between T and B cells. Conclusion: the results obtained demonstrate that the superantigen SEA can recruit T cell help to human B cells specifically stimulated by antigens, resulting in production of antigen reactive antibodies in vitro.     

Antigen-binding human T suppressor cells and their association with the HLA-DR locus

The ability of human lymphocytes to bind antigen was studied by direct binding of ¹²⁵I-labeled streptococcal protein antigen, followed by autoradiography. T-enriched lymphocytes depleted of adherent cells and B cells showed specific binding of ¹²⁵I-labeled streptococcal antigen (SA) at 4°C and in the presence of sodium azide. Further depletions of the T-enriched population by the monoclonal T4 or T8 antiserum and complement revealed that the antigen-binding T cell is T4^?, T8⁺. This was confirmed by positive selection of T8 cells, by rosetting with ox red blood cells and by the binding of SA by in vitro induced suppressor but not helper cells. Antigen specificity of binding to the suppressor cells was established by complete inhibition with the SA but no inhibition with keyhole limpet hemocyanin. A characteristic dose-response of binding 1 or 10 ng SA to HLA-DRw6 lymphocytes and 1000 ng SA to DR4,1,2,3 or 5 lymphocytes was found. A comparison of the dose-responses of antigen-binding T⁺ suppressor cells with those of helper and suppressor functions showed that the dose of SA which binds to suppressor cells is similar to the dose required to induce helper but not suppressor function. A plausible interpretation of these observations is that the T8⁺ antigen-binding suppressor cells might function as “contrasuppressor cells” which compete successfully for the membrane receptors of helper cells, thereby preventing suppression by the major subset of suppressor cells.     

Normal human B cells express endogenous sheep erythrocyte (E) receptor after appropriate in vitro culture

When normal human B cells from blood, tonsil or spleen are cultured with 20–30% autologous or allogeneic T cells and mitogen, the majority of cells rosette with sheep erythrocytes (E) after 3–5 days in culture. These E⁺ cells are of transformed appearance and several points indicate that many are derived from B cells. Some simultaneously possess surface immunoglobulin (sIg) or a B cell antigen [detected with BA1 monoclonal antibody (mAb)] and E receptor. Many do not stain with anti-T cell mAb (UCHT1, OKT4, OKT8), while cultured T cells continue to express these antigens. Furthermore, many E⁺ cells appear when the original T cells have been irradiated. All the cultured E⁺ cells stain with OKT11, an mAb against the E receptor, and their positivity cannot therefore be attributed to mitogen-induced nonspecific stickiness. The E positivity of the B cells was shown to be endogenous since passive acquisition of E receptor shed by T cells was excluded in a number of ways; no phenotypic changes were observed when supernatants from cultures containing many E⁺ sIg⁺ non-T cells were added to B cells, or when the B and T cells were separated by a Millipore membrane; and E receptor was re-expressed after stripping with trypsin or pronase. The intimate presence of T cells is essential for the expression of E receptors by B cells, and this helper capacity was shown to reside within the OKT4-defined helper T cell subpopulation. The significance of the expression of E receptor by B cells is discussed in relation to the recent in vitro and in vivo demonstration of E⁺ sIg⁺ cells in certain leukemias and in relation to the specificity of E rosetting as a marker of T cells.     

Absence of an antigen-specific helper T cell required for the expression of the T 15 idiotype in mice treated with anti-mu antibody

T-dependent anti-phosphorylcholine (PC) plaque-forming cell (PFC) responses were studied in BALB/c mice. Helper T cells derived from normal, carrier-primed donors induced anti-PC PFC responses dominated by the T15 idiotype. Helper T cells derived from carrier-primed mice that had been treated from birth with anti-μ antibody, so that they were lacking B cells and circulating antibody bearing the T15 idiotype, also provided helper cell function for the anti-PC antibody response. In contrast to the response induced by helper T cells from normal mice, T helper cells from μ-suppressed mice induced an anti-PC antibody response which was mainly non- T15 in character. The failure to induce antibody formation by T15⁺ B cells was not due to suppressor cells but rather to the lack of an Lyt-1⁺ helper T cell population which is necessary for predominant T15 production. This latter cell population was shown to be present in carrier-primed normal but missing or diminished in carrier- primed anti-μ-treated BALB/c mice. It required carrier priming for the expression of its helper function, but its function did not require the antigen (carrier) to be physically linked to the hapten. From this, we conclude that dominant production of the T15 idiotype involves the synergistic activity of two antigen-specific helper T cells. The helper cell population which is required for predominant T15 production is greatly diminished in mice treated with anti-μ antibody from birth. Hence, the production of circulating T15⁺ antibody induced by environmental antigens or the appearance of T 15-bearing B cells would appear to be required for the induction of helper T cells which enhance T15⁺ anti-PC antibody synthesis.     

Major histocompatibility complex-restricted cellular interactions determining B cell activation

Requirements for antigen- and major histocompatibility complex (MHC)-restricted cellular interactions for an in vitro secondary anti-2,4-dinitrophenyl (DNP) response, as well as the accompanying polyspecific immune response, are assessed. Induction of the polyspecific B cell response requires only the MHC-restricted, antigen-specific activation of helper T cells. Once the latter is accomplished, the polyspecific Ig response is not limited by B cell MHC. In contrast, MHC-restricted T helper cell activation is not sufficient for the activation of long-term primed, DNP-specific B cells. Their activation also depends on antigen presentation in conditions of linked recognition, i.e., hapten covalently linked to carrier, as well as recognition of B cell MHC by helper T cells. Moreover, the results demonstrate that the MHC-restricted T-macrophage and T-B cell interactions are independent and rule out the involvement of MHC-restricted 3 cell-MHC, interactions. The differential induction requirements for DNP-specific and polyspecific B cell responses are considered to reflect different states of B cell activation. When DNP-primed B cells are derived from animals recently challenged with antigen, their induction requirements are identical to those for polyspecific B cells. The requirements for linked recognition and syngenicity with helper T cells are lost and their amplification is mediated by antigen-specific, MHC-restricted helper T cell activation (T cell-mac-rophage) alone. Activation requirements limiting the induction of a bystander erythrocyte response mediated by protein-specific T cells (keyhole limpet hemocyanin) are considered within this context. At low keyhole limpet hemocyanin concentrations, induction requirements for sheep red blood cell-specific bystander B cells parallel those for polyspecific B cells and recently boosted DNP-specific B cells. However, at high antigen concentrations, bystander B cells syngeneic to T helper cells are preferentially induced. This latter phenomenon is considered to reflect antigen presentation by B cells. We conclude that the differing requirements for MHC-restricted T-macrophage and T-B cell interactions in different B cell responses depend primarily on the pre-existing state of B cell activation at the time of challenge.     

Normal B cells fail to secrete interleukin-12

Interleukin-12 is a key regulatory cytokine produced by antigen-presenting cells (APC) which drives the development of interferon-γ (IFN-γ)-producing cells and promotes cell-mediated immunity. Following subcutaneous immunization with protein antigen in adjuvant, dendritic cells (DC) but not small nor large B cells in immune lymph nodes express antigenic complexes and secrete substantial amounts of bioactive IL-12 p75 upon antigen-specific interaction with T cells. We have analyzed secretion of IL-12 p40 and p75 by cell populations enriched in DC, macrophages or B cells in response to nonspecific stimulation or to interaction with antigen-specific CD4⁺ cells. These APC populations do not produce IL-12 constitutively but, upon stimulation with heat-fixed Staphylococcus aureus and IFN-γ, IL-12 p40 and p75 are secreted by DC and macrophages, whereas B cells fail to produce IL-12. B cells also fail to secrete IL-12 in response to stimulation with LPS and IFN-γ. Co-culture with CD4⁺ T hybridoma cells and antigen induces IL-12 secretion by DC. Up-regulation of IL-12 secretion by interaction with antigen-specific CD4⁺ T cells is abrogated by anti-class II monoclonal antibodies (mAb), by soluble CD40 molecules and by anti-CD40 ligand mAb, demonstrating a positive feedback between T cells and DC mediated by TCR-peptide/class II and by CD40-CD40 ligand interactions. Expression of class II and CD40 molecules is comparable in B cells and DC, and both APC types activate CD4⁺ T cells. Yet, even upon interaction with antigen-specific T cells, B cells fail to secrete IL-12. The capacity of B cells to present antigen but not to secrete IL-12 may explain their propensity to selectively drive T helper type 2 cell development.