Activation of B cells by autoreactive T cells: cloned autoreactive T cells activate B cells by two distinct pathways

Although the existence of autoreactive T cells has been widely reported, the functional capacities of these populations have been less well defined. Studies were therefore carried out to characterize the relationship of autoreactive T cells to antigen-specific major histocompatibility complex (MHC)-restricted T cells in their ability to act as helper cells for the induction of immunoglobulin synthesis by B cells. A number of autoreactive T cell lines and clones were isolated from antigen-primed spleen and lymph node cell populations. Autoreactive T cells were found to proliferate in response to direct recognition of syngeneic I-A or I-E subregion-encoded antigens in the absence of any apparent foreign antigen. It was shown that cloned autoreactive T cells were capable of activating B cell responses through two distinct pathways. After appropriate stimulation by syngeneic cells, autoreactive T cells polyclonally activated primed or unprimed B cells to synthesize IgM antibodies. These activated T cells functioned in these responses through an MHC-unrestricted pathway in which polyclonal responses were induced in both syngeneic and allogeneic B cells. These cloned autoreactive T cells were also able to activate IgG responses by primed B cells through a different activation pathway. In contrast to the polyclonal activation of IgM responses, the induction of IgG antibodies by the same cloned T cells required primed B cells and stimulation with the priming antigen. The activation of B cells to produce IgG was strongly MHC restricted and required the direct recognition by the autoreactive T cells of self MHC determinants expressed on the B cell surface, with no bystander activation of allogeneic B cells. These results indicate that cloned autoreactive T cells resemble antigen-specific MHC-restricted T cells in their ability to function as T helper cells through distinct MHC-restricted and MHC-unrestricted pathways.     

Analysis of two distinct B cell activation pathways mediated by a monoclonal T helper cell. II. T helper cell secretion of interleukin 4 selectively inhibits antigen-specific B cell activation by cognate, but not noncognate, interactions with T cells

A single monoclonal T helper (Th) clone can activate B cells in two distinct pathways; a cognate pathway requiring a major histocompatibility complex (MHC)-restricted T-B cell interaction, and a noncognate pathway not requiring an MHC-restricted T-B cell interaction. The present study was undertaken to investigate whether Th cells mediating a given immune response provide further regulatory function to B cells other than helper function. It was demonstrated that conditions of high antigen concentration which activate a noncognate B cell activation pathway simultaneously inhibit IgG responses. The inhibition is shown to be mediated by the T cell factor interleukin 4, produced by activated cloned Th cells. The inhibitory effect of this factor is directed to B cells and is MHC-unrestricted, antigen-nonspecific, and IgG class-specific. In addition to being susceptible to the effects of augmenting cells and suppressor cells, cloned Th cell populations can therefore themselves function as regulatory cells to inhibit IgG responses when stimulated with high dose of specific antigen. These results indicate that Th cells function to regulate B cells both positively and negatively, depending upon the activation conditions.     

Role of the major histocompatibility complex in T cell activation of B cell subpopulations: antigen-specific and H-2-restricted monoclonal TH cells activate Lyb-5+ B cells through an antigen-nonspecific and H-2-unrestricted effector pathway

Monoclonal T helper (TH) cell populations were employed to study the mechanism of activation of the Lyb-5+ B cell subpopulation in T cell-dependent antibody responses in vitro. It was demonstrated that monoclonal T cell populations were sufficient to help rigorously T-depleted unprimed (B + accessory) cells for direct plaque-forming cell responses to trinitrophenyl- (TNP) conjugated keyhole limpet hemocyanin (KLH). The activation of several lines of cloned (H-2b X H-2k)F1 TH cells was antigen (KLH) specific and H-2 restricted. Individual clones were restricted to H -2b, H-2k, or unique (H-2b X H-2k)F1 encoded determinants. Under the experimental conditions employed, responses mediated by cloned TH cells were found to result in the activation of the Lyb-5+ B cell subpopulation. The activation of Lyb-5+ B cells by cloned TH cells did not require covalent linkage of carrier and hapten, and responses could be stimulated in the presence of free KLH plus TNP conjugated to an irrelevant carrier. The H-2 restriction of TH cell function was shown to reflect a requirement for T cell recognition of determinants expressed by accessory cells, whereas no requirement existed for restricted T cell recognition of B cells. These findings suggest that the help provided by monoclonal TH cells, once activated, was both antigen nonspecific and H-2 unrestricted. Consistent with this interpretation, it was found that the supernatant of antigen-stimulated TH cells provided antigen-nonspecific help to T-depleted spleen cells. Thus, these results demonstrate that the activation of Lyb-5+ B cells by antigen-specific and H-2-restricted monoclonal TH cell populations is itself antigen nonspecific and H-2 unrestricted.     

Regulation of T15 idiotype dominance. III. Phosphocholine-specific B-cell activation in vivo requires major histocompatibility complex-restricted T-cell help

We have examined the requirement for major histocompatibility complex (MHC)-restricted T-cell help in the secondary in vivo antibody response to phosphocholine (PC). The memory response to PC has been demonstrated previously to be comprised of T15-dominant IgM and IgG3 plaque-forming cells (PFC) derived primarily from the Lyb-5+ B-cell subset, and IgG1 and IgG2 PFC, few of which bear the T15 idiotype and are predominantly derived from the Lyb-5- B-cell subset. Using carrier-primed (A X B)F1 T cells which have matured in a parentA chimeric environment so that "self" recognition is of the MHC determinants of parentA but not parentB, we have found that parentA PC-primed B cells, but not parentB PC-primed B cells, are activated. Even in the presence of an ongoing parentA anti-PC response, parentB PC-primed B cells were not activated, indicating that the restriction was between the helper T cell and the B cell, not between T-helper and accessory cells. MHC-restricted T-cell help was required by B cells producing T15+ and T15- IgM, IgG3, IgG1, and IgG2 responses.     

Analysis of two distinct B cell activation pathways mediated by a monoclonal T helper cell. I. MHC-restricted activation of B cells by an IL 2-dependent pathway

The present study was carried out to determine whether the MHC-restricted and MHC-unrestricted B cell activation pathways mediated by a single cloned Th cell are separable, and whether these two pathways are mediated by distinct mechanisms. It was demonstrated that the two B cell activating functions of a single cloned Th cell could be separated by their sensitivity to irradiation. It was shown that MHC-restricted B cell activation is mediated by a radiosensitive Th cell function, whereas MHC-unrestricted B cell activation is mediated by a radioresistant function of the same Th cell. In addition, it was shown that recombinant IL 2 can restore or replace the radiosensitive component of MHC-restricted cognate helper function.     

Streptococcal serotype carbohydrate represents a novel class of type 2 antigen which is T-independent

Our studies reported here, fully characterize two unique type 2 antigens trinitrophenol (TNP)-M1 serotype carbohydrates (TNP-M1 g and TNP-M1 c) derived from streptococci, which fail to induce antibody responses in xid or neonatal mouse splenic cultures. These antigens generate brisk responses in normal spleen and Peyer's patch cell cultures of xid mice, all of which suggest that responses are elicited in the Lyb-3+, 5+ B subpopulation. The antibody responses to TNP-M1 g (and TNP-M1 c) are not dependent upon T cells. Furthermore, TNP-M1 carbohydrates induce anti-TNP plaque-forming (PFC) responses in cultures of small, resting splenic B cell populations without an added T cell requirement. Thus two categories of type 2 antigens are distinguished, one which requires T cells or derived factors, e.g., TNP-Ficoll, and a second TNP-carbohydrate antigen TNP-M1 that does not. Studies of the mitogenic and polyclonal B cell activation properties of M1 carbohydrates indicated that B cell proliferation is induced in both xid (Lyb-3-, 5-) and normal (Lyb-3-, 5- and Lyb-3+, 5+) splenic B cell subpopulations, but that differentiation to IgM synthesis fails to occur in the Lyb-3-, 5- B cell subpopulation. Thus M1 carbohydrates are unique probes that allow the selective induction of proliferation and differentiation of mature B cells that are presumably Lyb-3+, 5+. Because the M1 serotype carbohydrates induce polyclonal IgM synthesis and antigen-specific responses in only the mature B cell population in the absence of T cells, whereas TNP-Ficoll and other type 2 antigens require T cells or their derived factors, the Lyb-3+, 5+ B cell subpopulation may consist of a T cell-dependent and a T cell-independent compartment for responses to different carbohydrate type 2 antigens.     

B cell response to fresh and effector T helper cells. Role of cognate T-B interaction and the cytokines IL-2, IL-4, and IL-6

The helper activity of resting T cells and in vitro generated effector T cells and the relative roles of cognate interaction, diffusible cytokines, and non-cognate T-B contact in B cell antibody responses were evaluated in a model in which normal murine CD4+ T cells (Th), activated with alloantigen-bearing APC, were used to support the growth and differentiation of unstimulated allogeneic B cells. Both "fresh" T cells, consisting of memory and naive cells, stimulated for 24 h, and "effector" T cells, derived from naive cells after 4 days of in vitro stimulation, induced the secretion of IgM, IgG3, IgG1, IgG2a, and IgA. Effector T cells were significantly better helpers of the response of small dense B cells, inducing Ig at lower numbers and inducing at optimal numbers 2- to 3-fold more Ig production than fresh T cells. The predominant isotype secreted was IgM. Supernatants derived from fresh T cell cultures contained moderate levels of IL-2, whereas those from effector cultures contained significant levels of IL-6 and IFN-gamma in addition to IL-2. The involvement of soluble factors in the B cell response was demonstrated by the ability of antibodies to the cytokines IL-2, IL-4, and IL-6 to each block Ig secretion. Antibodies to IL-5 and IFN-gamma had no effect on the T cell-induced response. Kinetic studies suggested that IL-4 acted during the initial stages of the response, whereas the inability of anti-IL-6 to block B cell proliferation suggested that IL-6 was involved in part in promoting differentiation of the B cells. The relative contributions of cognate (MHC-restricted) and bystander (MHC-unrestricted) T-B cell contact vs cytokine (non-contact)-mediated responses were assessed in a transwell culture system. The majority of the IgM, IgG3, IgG1, and IgG2a response induced by both fresh and effector T cells was dependent on cognate interaction with small, high density B cells. In contrast, a small proportion of these isotypes and most of the IgA secreted resulted from the action of IL-6 on large, presumably preactivated, B cells. The IgA response did not require cell contact or vary when fresh and effector cells were the helpers. The contribution of bystander contact in the overall antibody response to both T cell populations was minimal.(ABSTRACT TRUNCATED AT 400 WORDS)     

Separation of events mediating B cell proliferation and Ig production by using T cell membranes and lymphokines

The initiation by Th cells of B cell proliferation and differentiation to produce Ig involves both cell contact- and lymphokine-mediated signals. Plasma membrane-enriched fractions from stimulated, but not unstimulated, Th cells induced Ag nonspecific and MHC unrestricted proliferation of 60 to 70% of small dense B cells. Induction of stimulatory membrane activity was inhibited by cycloheximide, and the activity was eliminated by both protease and heat treatment of membranes. Membrane-stimulated B cells did not differentiate to secrete Ig; however, addition of a lymphokine-containing supernatant from activated Th cells or the combination of IL-4 and IL-5 resulted in substantial Ig production, predominantly of the IgM, IgG1, IgA, and IgE isotypes. The quantity and isotype distribution of the antibodies secreted were similar to those produced after B cell activation by the intact Th cells and Ag. Therefore, membranes from activated Th cells in combination with lymphokines normally secreted by such cells can replace intact Th cells and provide a defined system to identify molecular events important for B cell activation.     

The cell biology of T-dependent B cell activation

The requirement that CD4+ helper T cells recognize antigen in association with class II Major Histocompatibility Complex (MHC) encoded molecules constrains T cells to activation through intercellular interaction. The cell biology of the interactions between CD4+ T cells and antigen-presenting cells includes multipoint intermolecular interactions that probably involve aggregation of both polymorphic and monomorphic T cell surface molecules. Such aggregations have been shown in vitro to markedly enhance and, in some cases, induce T cell activation. The production of T-derived lymphokines that have been implicated in B cell activation is dependent on the T cell receptor for antigen and its associated CD3 signalling complex. T-dependent help for B cell activation is therefore similarly MHC-restricted and involves T-B intercellular interaction. Recent reports that describe antigen-independent B cell activation through coculture with T cells activated by anti-T-cell receptor or anti-CD3 antibodies suggest that cellular interaction with T cells, independent of antigen presentation or lymphokine secretion, induces or triggers B cells to become responsive to T-derived lymphokines, and that this may be an integral component of the physiological, antigen- and MHC-restricted T-dependent B cell activation that leads to antibody production.     

Contribution of Lyb 5+ and Lyb 5- B cells to the primary and secondary phosphocholine-specific antibody response

Due to a mutation on their X-chromosome, CBA/N mice lack the Lyb-5+ subset of B cells. The loss of this B cell subset results in a profound alteration in the immune response of these mice to the hapten phosphocholine (PC). Thus, when these mice are immunized with high doses of PC-KLH (200 micrograms) in CFA, they: 1) fail to produce IgM anti-PC antibodies; 2) produce little or no anti-PC antibody bearing the normally predominant T15-idiotype; and 3) produce IgG anti-PC antibodies only late in the primary response. In order to more fully delineate this defect in responsiveness to PC, the splenic focus assay was used to analyze Lyb-5- B cell precursors from both normal and immune defective mice. Lyb-5- cells were obtained from normal (CBA/N x DBA/2)F1 (CD) female spleens by treatment with anti-Lyb-5 serum and complement. These normal Lyb-5- cells and Lyb-5- cells from immune defective CD male mice were stimulated in vitro with either PC-Hy or TNP-Hy in the presence of Hy-primed T helper cells. The results demonstrate that primary Lyb-5- PC-specific B cells fail to respond in the splenic focus assay, while secondary Lyb-5- PC-specific precursors respond normally, and that both primary and secondary Lyb-5- TNP-specific precursors respond in the splenic focus assay. These data suggest that Lyb-5- PC-specific precursors must differentiate into memory cells before they can be activated to secrete antibody, and they also indicate that the Lyb-5- B cell subset may be composed of two subsets with different activation requirements.     

T cell activation of antigen-specific antibody responses by large B cells is MHC restricted

The activation of small, resting B cells for antibody synthesis by helper T cells has been proposed to require an MHC-restricted interaction between the T and B cells. Large, activated B lymphocytes were, in contrast, thought to be activated by an unrestricted pathway. We re-examined this issue and found that both large and small size fractionated murine B lymphocytes required an MHC-restricted interaction with helper T cells to be activated for specific antibody synthesis. Polyspecific antibody synthesis in the same cultures was not dependent upon an MHC-restricted T-B interaction for any size category of B cell. These results are interpreted as reflecting the ability of antigen-specific B cells to focus and present antigen to T cells, in contrast to B cells of random specificity, which have no effective focusing mechanism for a given experimental antigen. We found that the polyspecific response required much higher antigen concentrations than the antigen-specific response, a result consistent with the antigen-focusing hypothesis.     

IL-2 and a contact-mediated signal provided by TCR alpha beta + or TCR gamma delta + CD4+ T cells induce polyclonal Ig production by committed human B cells. Enhancement by IL-5, specific inhibition of IgA synthesis by IL-4.

To study the role of T cells in T-B cell interactions resulting in isotype production, autologous purified human splenic B and T cells were cocultured in the presence of IL-2 and Con A. Under these conditions high amounts of IgM, IgG, and IgA were secreted. B cell help was provided by autologous CD4+ T cells whereas autologous CD8+ T cells were ineffective. Moreover, CD8+ T cells suppressed Ig production when added to B cells cocultured with CD4+ T cells. Autologous CD4+ T cells could be replaced by allogeneic activated TCR gamma delta,CD4+ or TCR alpha beta,CD4+ T cell clones with nonrelevant specificities, indicating that the TCR is not involved in these T-B cell interactions. In contrast, resting CD4+ T cell clones, activated CD8+, or TCR gamma delta,CD4-,CD8- T cell clones failed to induce IL-2-dependent Ig synthesis. CD4+ T-B cell interaction required cell-cell contact. Separation of the CD4+ T and B cells by semiporous membranes or replacement of the CD4+ T cells by their culture supernatants did not result in Ig synthesis. However, intact activated TCR alpha beta or TCR gamma delta,CD4+ T cell clones could be replaced by plasma membrane preparations of these cells. Ig synthesis was blocked by mAb against class II MHC and CD4. These data indicate that in addition to CD4 and class II MHC Ag a membrane-associated determinant expressed on both TCR alpha beta or TCR gamma delta,CD4+ T cells after activation is required for productive T-B cell interactions resulting in Ig synthesis. Ig production was also blocked by mAb against IL-2 and the IL-2R molecules Tac and p75 but not by anti-IL-4 or anti-IL-5 mAb. The CD4+ T cell clones and IL-2 stimulated surface IgM-IgG+ and IgM-IgA+, but not IgM+IgG- or IgM+IgA- B cells to secrete IgG and IgA, respectively, indicating that they induced a selective expansion of IgG- and IgA-committed B cells rather than isotype switching in Ig noncommitted B cells. Induction of Ig production by CD4+ T cell clones and IL-2 was modulated by other cytokines. IL-5 and transforming growth factor-beta enhanced, or blocked, respectively, the production of all isotypes in a dose-dependent fashion. Interestingly, IL-4 specifically blocked IgA production in this culture system, indicating that IL-4 inhibits only antibody production by IgA-committed B cells.     

T cell regulation of B cell activation: antigen-specific and antigen-nonspecific suppressor pathways are mediated by distinct T cell subpopulations

The present studies were carried out to characterize the cellular events involved in the induction and function of carrier-specific Ts cells, which selectively regulate the generation of IgG responses by Lyb-5- B cells. It was demonstrated that this regulation is in fact mediated by two distinct suppressor pathways. In one pathway, carrier-primed Lyt-1 + 2 - Ts cells are specifically activated by in vitro reexposure to the priming antigen. After this specific activation, these Lyt-1 + 2 - Ts cells are able to suppress IgG responses in an antigen-nonspecific manner. This suppression requires the participation of unprimed Lyt-1 - 2 + T cells, and is effective in both the early and the late phases of antibody responses. A second suppressor pathway requires the antigen-specific activation of primed Lyt-1 - 2 + Ts cells. Suppression of antibody responses by activated Lyt-1 - 2 + Ts cells is highly carrier specific, in contrast to the nonspecific effector function of Lyt-1 + 2 - Ts cells, appears to act without requirement for additional T cell populations; and is effective only early in the course of the antibody response. Thus, it appears that two Ts cell populations may function through distinct mechanisms to regulate the generation of IgG Lyb-5- B cell responses.     

Functional heterogeneity of the Lyb-5- B cell subpopulation: mutant xid B cells and normal Lyb-5- B cells differ in their responsiveness to phenol-extracted lipopolysaccharide

In the present study, responses stimulated by phenol-extracted lipopolysaccharide (LPS(phenol)) and butanol-extracted LPS (LPS(butanol)) were used to assess the possibility that xid B cells might not be identical to the Lyb-5- B cells present in normal mice. It was found that xid B cells responded well only to LPS(butanol) whereas normal B cells responded well to both LPS(butanol) and LPS(phenol). Thus, LPS(butanol) appeared to be a TI-1 antigen and LPS(phenol) appeared to be a TI-2 antigen. In contrast to classical TI-2 responses, however, responses stimulated by LPS(phenol) did not exhibit a stringent requirement for accessory cells. Furthermore, if LPS(phenol) were a classical TI-2 antigen, it should only activate Lyb-5+ B cells. To determine if the responsiveness of normal B cells to LPS(phenol) were due, at least in part, to the stimulation of normal Lyb-5- B cells, the responsiveness of normal neonatal B cells and normal adult B cells that had been pretreated with anti-Lyb-5.1 + C was assessed. It was found that both normal neonatal B cells and normal adult Lyb-5- B cells did respond well to LPS(phenol). Thus, even though LPS(phenol) does not stimulate xid B cells, these data demonstrate that LPS(phenol) is different from other TI-2 antigens. More importantly, these data also demonstrate that xid B cells and normal Lyb-5- B cells are not identical. It is hypothesized that the normal Lyb-5- B cell subpopulation is heterogeneous, consisting of an Lyb-5(1)- and an Lyb-5(2)-B cell subset with the xid mutation blocking the differentiation of Lyb-5(1)-B cells into Lyb-5(2)-B cells.     

Antigen-induced T suppressor cells regulate the autoreactive T helper-B cell interaction

The T suppressor (Ts) cell population that functions to regulate antigen-specific MHC-restricted T helper (Th)-B cell interactions also regulates the activation of B cells by cloned autoreactive Th cells. Activated Ts cells were generated by in vivo priming and restimulation in vitro with high concentrations of the specific priming antigen. Once generated, this Ts population inhibits the Th-dependent activation of primed B cells by both antigen-specific and autoreactive T cells in an antigen-nonspecific manner. This suppression requires the participation of both Lyt-1+2- and Lyt-1-2+ T cells. It was also demonstrated that accessory cells were required for the induction of Ts cells. Moreover, the generation of suppression was MHC-restricted and required the recognition by T cells of Ia antigens on accessory cells. These studies demonstrate that the same or a very similar Ts cell population can function to inhibit the activation of B cells by antigen-specific as well as autoreactive T cells.     

Cloned allospecific human helper T cell lines induce an MHC-restricted proliferative response by resting B cells

To analyze helper T (Th) cell-induced B cell proliferation in man, we have cloned allospecific Th cells, grown them as long-term IL 2-dependent T cell lines (TCL), and analyzed their phenotypic and functional properties. The two TCL described in this report, A-7 and A-57, are both composed exclusively of T3+, T4+, T8- T cells blasts. In proliferative assays, with a panel of x-irradiated allogeneic stimulator cells, A-7 was found to proliferate in response to DR3-bearing cells, whereas A-57 responds to DR2-positive stimulators. Both TCL are capable of providing MHC-restricted polyclonal help for allogeneic B cells, as measured in the reverse hemolytic plaque assay. Of greater interest, x-irradiated A-7 and A-57 cells are capable of inducing a proliferative response by allogeneic B cells that is absolutely MHC restricted at the inductive (Th-APC) level. Thus, x-irradiated A-7 cells only trigger proliferation by DR3+ B cells, whereas A-57 cells selectively activate DR2+ B cells. In contrast, after antigen-specific activation, x-irradiated A-7 and A-57 cells can recruit a significant proliferative response by allogeneic B cells bearing "irrelevant" DR antigens. The possibility that Th-induced B cell proliferation may be restricted at the effector (Th-B cell) level was addressed by fractionating B cell populations into "activated" and "resting" subsets by discontinuous Percoll density gradient centrifugation and further purification by employing a monoclonal antibody directed against an antigen expressed on activated B cells (4F2). These studies demonstrate that activated B cells are readily and nonspecifically recruited to proliferate by activated Th cells, whereas optimal proliferative responses by resting B cells require MHC restricted Th-B cell interaction.     

Cognate interactions between helper T cells and B cells. III. Contact-dependent, lymphokine-independent induction of B cell cycle entry by activated helper T cells

An Ag-specific, IL-2-dependent Th clone induced the growth of B cells in a class II-restricted, Ag-specific, IL-2-dependent manner. The formation of stable Th-3.1-B cell conjugates was restricted by Ag and class II MHC. After activation of Th-3.1 by insolubilized anti-T3 (Th-3.1T3), Th-3.1T3 induced the growth of B cells in a class II unrestricted, Ag nonspecific manner. The formation of stable conjugates between Th-3.1T3 and B cells was also class II unrestricted and Ag nonspecific. Although the interaction of Th-3.1T3 and B cells was class II unrestricted, the interaction was inhibited by the combination of anti-IA and anti-IE mAb. This suggested that monomorphic domains of class II MHC molecules were involved in Th-3.1T3-B cell interaction. Fixed Th-3.1T3 but not fixed resting Th-3.1 induced B cell cycle entry, as measured by an increase in B cell RNA synthesis. Trypsin-treatment of Th-3.1T3 before fixation reduced their ability to activate B cells, indicating that cell surface proteins on Th-3.1T3 were required for enhanced B cell RNA synthesis. Anti-IL-4, anti-IL-2R, or anti-IFN-gamma did not affect the ability of Th-3.1T3 to induce heightened B cell RNA synthesis. Progression into S phase by B cells activated with fixed Th-3.1T3 was supported by the addition of soluble factors. When stimulated with fixed Th-3.1T3, EL4 supernatant (SN) enhanced B cell DNA synthesis. Depletion of IL-4, but not IL-2, from EL4 SN ablated its supportive capabilities. IL-4 alone was completely ineffective in supporting entry into S phase. Therefore, IL-4 and another activity(ies) in EL4 SN were necessary for B cell cycle progression into S phase. Taken together, these data suggest that after Th activation, Th cell surface proteins are expressed that mediate the binding of Th to B cells via recognition of nonpolymorphic domains of class II MHC molecules. Contact of Th-3.1T3 with B cells, not lymphokines, results in the entry of B cells into the cell cycle and heightened B cell lymphokine responsiveness. The addition of exogenous lymphokines supports the progression of Th-3.1T3-activated B cells into S phase.     

Idiotope antigens (Ab2 alpha and Ab2 beta) can induce in vitro B cell proliferation and antibody production

We previously showed that immunization of various strains of mice with three types of antigen--PC-Hy (nominal antigen), F6-Hy (Ab2 alpha-Hy, and 4C11-Hy (Ab2 beta-Hy)--induces a differential PC-specific, T15-Id+ antibody response. In this report, the in vitro phosphorylcholine (PC)-specific B cell responses induced by these three antigens were studied. A hemocyanin-specific long-term T helper cell line was used to provide help for primary and secondary in vitro T cell-dependent B cell responses. At low doses (0.005 to 0.5 micrograms/ml) of antigen, a significant increase in the proliferation of PC-OVA-primed BALB/c B cells was observed with Ab2-Hy or PC-Hy conjugate, but not unconjugate, antigens. Similar low doses of antigen could stimulate naive B cells to secrete IgM and stimulate PC-OVA- or 4C11-Hy-primed B cells to secret IgM and IgG1 anti-PC antibodies. The percentage of T15-Id of the PC-specific antibodies produced in the in vitro T-B culture was found to be less dominant than that produced by in vivo immunization, suggesting that certain regulatory mechanisms occur in the in vivo environment that may help to maintain the T15-Id dominance. Taken together, our in vivo and in vitro results indicate that idiotope antigens can function like nominal antigens to induce antigen-specific B cell responses. The mechanisms of thymic-dependent B cell activation induced by idiotope and nominal antigen are similar in that the T-B interaction is MHC-restricted and requires cognate recognition.     

Antigen-specific, MHC-restricted B cell activation by cell-free Th2 cell products. Synergy between antigen-specific helper factors and IL-4

Ag-specific and MHC-restricted Th clones of different Ag specificities and MHC haplotypes were tested for their ability to produce soluble factors capable of providing the signals required for B cell activation and IgG antibody production. Each of five Th clones tested generated significant helper activity in supernatants derived from coculture of the T cell clone with specific Ag and syngeneic APC. The same helper activity was detected in supernatants of clones stimulated with immobilized anti-CD3 antibody in the absence of Ag or APC. The secreted helper activity resembled the activity of the intact Th cells in that it was Ag-specific, carrier-hapten-linked and MHC-restricted. These T cell products functioned to activate only those B cells expressing MHC products which corresponded to the specificity of each Th clone. Thus, the specificity of the cell-free T cell product mimicked precisely that expressed by the intact Th cell and presumably mediated by the cell surface TcR. In addition to the apparent presence of specific helper factor in Th clone supernatants, a role for nonspecific lymphokines was also identified in these preparations. Although recombinant or purified IL-4 alone was not sufficient to stimulate hapten-primed B cells to secrete hapten-specific IgG antibodies, mAb specific for IL-4 blocked the induction of antibody secretion by Th cell supernatant. These results indicate that stimulation of B cells to produce hapten-specific IgG antibody requires at least two distinct signals: an Ag-specific T cell signal which is restricted by MHC products expressed on the B cells, and a nonspecific signal mediated at least in part by the lymphokine IL-4.     

Regulation of T15 idiotype dominance. I. Mice expressing the xid immune defect provide normal help to T15+ B cell precursors

The immune response to phosphocholine (PC) in many strains of mice is dominated by the T15 idiotype family of anti-PC antibodies. By introducing the CBA/N X-linked immune defect (xid gene) into these mice, one profoundly alters their ability to make a T15-predominant, IgM anti-PC response. This loss of T15 dominance in mice expressing the xid gene is not due to the presence of suppressor T cells or the lack of T15 idiotype-specific helper cells in these mice. Thus, one can reconstitute a T15 idiotype-dominant response in immune defective mice with B cells from normal mice, and in adoptive transfer assays the primed T helper cells from immune-defective mice provide qualitatively the same help to normal B cells as the T helper cells from normal mice. T15 idiotype dominance appears to be controlled by the expression and activation of Lyb-5+ PC-specific B cells. Thus, the majority of T15+ B cell precursors are restricted to this B cell subset, whereas the Lyb-5- B cell subset contains predominantly T15-, anti-PC B cell precursors, which produce mainly IgG antibodies after activation by PC-containing antigens.