Lipopolysaccharides can convert heterologous red cells into thymus-independent antigens

Toll-like receptors on the fork roads between innate and adaptive immunity

There is a permanent interaction amid the innate and adaptive immune systems that leads to a defensive immune response against pathogens and contributes substantially to self-nonself discrimination. Toll-like receptors (TLRs) are essential molecules of the innate immune system that stimulate numerous inflammatory pathways and harmonize systemic defense against a wide array of pathogens. In addition to identifying unique molecular patterns associated with various sections of pathogens, TLRs may also recognize a number of self proteins and endogenous nucleic acids. Several reports have indicated that inappropriate stimulation of the TLR pathway via endogenous or exogenous ligands in animal models or humans may lead to the induction and/or prolongation of autoimmune response and tissue injury.

Control of B-cell responses by Toll-like receptors

Toll-like receptors (TLRs) detect microbial infection and have an essential role in the induction of immune responses. TLRs can directly induce innate host defence responses, but the mechanisms of TLR-mediated control of adaptive immunity are not fully understood. Although TLR-induced dendritic cell maturation is required for activation of T-helper (T(H)) cells, the role of TLRs in B-cell activation and antibody production in vivo is not yet known. Here we show that activation and differentiation of T(H) cells is not sufficient for the induction of T-dependent B-cell responses. We find that, in addition to CD4+ T-cell help, generation of T-dependent antigen-specific antibody responses requires activation of TLRs in B cells.

Chromatin-IgG complexes activate B cells by dual engagement of IgM and Toll-like receptors

Autoreactive B cells are present in the lymphoid tissues of healthy individuals, but typically remain quiescent. When this homeostasis is perturbed, the formation of self-reactive antibodies can have serious pathological consequences. B cells expressing an antigen receptor specific for self-immunoglobulin-gamma (IgG) make a class of autoantibodies known as rheumatoid factor (RF). Here we show that effective activation of RF+ B cells is mediated by IgG2a-chromatin immune complexes and requires the synergistic engagement of the antigen receptor and a member of the MyD88-dependent Toll-like receptor (TLR) family. Inhibitor studies implicate TLR9. These data establish a critical link between the innate and adaptive immune systems in the development of systemic autoimmune disease and explain the preponderance of autoantibodies reactive with nucleic acid-protein particles. The unique features of this dual-engagement pathway should facilitate the development of therapies that specifically target autoreactive B cells.

Effects of bacterial lipopolysaccharide on the production of colony-stimulating activity in C3H/HeJ mouse long-term bone marrow cultures

Bacterial lipopolysaccharide (LPS)-induced colony-stimulating activity (CSA) in murine long-term bone marrow culture system was investigated. Bone marrow culture cells of LPS-nonresponsive C3H/HeJ mice responded to LPS in terms of CSA production as efficiently as bone marrow culture cells of LPS-responsive C3H/slc mice. On the other hand, both peritoneal macrophages and bone marrow macrophages from C3H/HeJ mice did not produce CSA in vitro after treatment with LPS. Percoll density gradient separation of adherent layer cells in bone marrow cultures showed that two cell populations were present. One population was nonspecific esterase positive, productive of high CSA to LPS stimulation and light density cells, the other population was nonspecific esterase negative, productive of low CSA to LPS stimulation and high density cells, and CSA production stimulated by LPS in C3H/HeJ mice bone marrow culture cells was mainly attributed to the latter population of cells. These results suggest that CSA production stimulated by LPS in C3H/HeJ mice is regulated by different cell populations, respectively in vivo and in vitro.

Lipopolysaccharide and serum synergistically stimulate ornithine decarboxylase in Chinese hamster ovary cells

Lipopolysaccharide (LPS), the active component of bacterial endotoxin, caused no significant increase in ornithine decarboxylase (ODC) activity in serum-starved, Chinese hamster ovary fibroblasts. However, concurrent addition of LPS with 10% fetal bovine serum caused a synergistic 30 to 40-fold increase in enzyme activity as compared to the 10 to 20-fold increase seen after addition of serum alone. This synergism was not due to an alteration in the time course of enzyme induction after serum addition. The LPS-induced synergy of ODC induction by serum was inhibited by the concurrent addition of the specific LPS-antagonist, Polymyxin B.

Triggering of affinity-enriched B cells. Analysis of B cell stimulation by antigen-specific helper factor or lipopolysaccharide. I. Dissection into proliferative and differentiative signals

Proliferative and differentiative signals controlling the in vitro IgM response by unprimed, affinity-enriched B cells were studied using conditions under which as few as 2,000 B cells stimulated by antigen-specific, Ia-positive, allogeneically restricted, T cell-derived helper factor (Hf) or the polyclonal activator lipopolysaccharide (LPS) yielded on the average 400 antibody-forming cells (AFC) by direct plaque assay. Antigen alone induces neither B cell proliferation nor differentiation into AFC. Proliferation but not differentiation into AFC is induced when affinity-enriched B cells are cultured in the presence of Ag and Hf or LPS but in the absence of nonantigen-specific, radioresistant, accessory (A) cells. For the induction of a complete Hf- or LPS-mediated AFC response, cultures must be reconstituted with A cells or the secretory product(s) of these cells. The antigen-specific response depends strictly on the presence of the Hf specific for the relevant antigen, regardless of the cell cycle state of cooperating B cells. The differentiative signal from A cells is due, at least in part, to the presence of a Thy-1.2-bearing population of cells. In the case of the LPS-mediated, but not the Hf-mediated response. A cells can be substituted by using supernatant derived from an interleukin 2-secreting T lymphoma cell line (EL4). In the presence of histocompatible Hf and B cells, histoincompatible A cells can still cooperate in the immune response. However, the degree of allogeneic restriction between incompatible Hf and B cells is markedly increased if both B cells and A cells are incompatible with Hf.

Activation of human peripheral blood lymphocytes: effect of concanavalin A and lipopolysaccharide on in vitro synthesis of DNA and immunoglobulins

We studied the interaction of lipopolysaccharide (LPS) and concanavalin A (Con A) with regard to IgM and IgG production in in vitro cultures of human peripheral blood lymphocytes (PBL). In our system LPS alone over a wide range of concentrations did not stimulate detectable IgM or IgG production, while Con A at optimal (6 microgram/ml) and suboptimal (0.6 microgram/ml) mitogenic concentrations induced synthesis of small amounts of Ig. A marked enhancing effect was present when both Con A and LPS were added to the cultures. The different doses of LPS has similar effects on both classes of Ig, and typical dose-response curves were obtained. To evaluate the cellular basis of this synergism, the effect on cell proliferation was studied under identical experimental conditions in normal subjects and patients with X-linked agammaglobulinaemia (X-LA). Parallel cultures were set up after monocyte depletion by adherence on Petri dishes. On day 3, increasing doses of LPS were associated with progressive decreases in 3H-thymidine (3H-TdR) incorporation. Similar results were obtained with normal lymphocytes and those from X-LA patients. Monocyte depletion did not substantially alter the lymphocyte response pattern. The preferential induction of helper activities, either directly by helper stimulation or indirectly by suppressor inhibition, is suggested as a possible mechanism of the interaction observed.

In vitro studies on the immunological memory for antibody response to bovine serum albumin

Immunological memory for T and B cells was studied in an in vitro culture system with spleen cells from mice primed with bovine serum albumin (BSA). Spleen cells taken from mice immunized at various times previously with a single intravenous injection of alum-precipitated (AP) BSA and bacterial endotoxin (ET) were cultured in Marbrook's system with dinitrophenylated (DNP) BSA as the in vitro antigen. In the cultures of spleen cells obtained from mice primed more than 14 days previously an IgG-predominant anti-BSA response was generated. However, no anti-BSA response was observed in the culture of spleen cells taken from mice primed 7 days previously (day 7 spleen cells). The failure of day 7 spleen cells to generate an antibody response in vitro was shown to be attributable to both the lack of B memory cells and the effect of "suppressive" macrophages induced by ET. On the other hand, anti-BSA memory in the spleen of mice primed with AP-BSA plus ET and 2 months later challenged with AP-BSA matured within 7 days and declined rather quickly by 30 days after the challenge. The difference in the time course of the generation of memory between the spleen cells from primary and from secondary immunized mice might be attributable to the difference in the maturation of memory B cells, since the time course of the development of memory T cells after the secondard immunization was similar to that observed after primary immunization.

Dietary immunostimulation: interaction with BCG and LPS

The action of BCG and LPS mR595 used in conjunction with a formula-defined diet is dependent on the administration timing and resembles that of interacting adjuvants affecting different elements of the immune system.

Properties of antisera against lymphocytes of nude mice

The activity of rabbit antisera against nu/nu BALB/c lymphocytes was estimated in vivo and in vitro. It was found that anti-lymphocyte serum (ALS) against nu/nu lymph node cells suppressed the alloantigen reaction and the spontaneous rosette-forming cell (sRFC) or plaque-forming cell (PFC) formation for T-dependent (sheep red blood cells) and T-independent (lipopolysaccharide) antigens. ALS against nu/nu spleen cells affected only the sRFC and PFC for T-independent antigen. The former serum exhibited a high cytotoxicity for the suspensions enriched or depleted in B cells, while the latter was more cytotoxic for the suspension enriched in B cells. This may indicate that ALS anti-nu/nu spleen cells is specific for B lymphocytes, and ALS anti nu/nu lymph node cells is directed not only to B cells but also to a subpopulation of T lymphocytes. It may suggest the existence of a subpopulation of T lymphocytes in nu/nu lymph node cells.

T cell requirements for the expression of the lipopolysaccharide adjuvant effect in vivo: evidence for a T cell-dependent and a T cell-independent mode of action

The in vivo adjuvant effect of lipopolysaccharide (LPS) in mice was investigated with the soluble synthetic polypeptide antigen (T, G)-A--L, the antibody response to which is determined by the Ir-1A gene. With this specific antigen it can be demonstrated that the LPS adjuvant effect has the following modes of action: a) a T cell-dependent enhancement of primary and secondary IgM antibody response; b) a T cell-dependent enhancement of IgG secondary andibody response; and c) a T cell-dependent induction of switchover from IgM to IgG andibody in some strains of Ir-1A low responders. Although T cells are necessary for some aspects of the adjuvant effect, these data do not distinguish between a mechanism involving a direct interaction between LPS and T cells or a direct interaction of LPS and B cells with a general requirement for T cells for expression of IgG antibody.

Reversal of immunosuppression induced by murine leukemia viruses

BALB/c mice infected with Rowson-Parr virus, a lymphatic leukemia virus isolated from the Friend complex, undergo a rapid depression of antibody response. Spleen cells from these mice in culture show a similar deficit in the response to stimulation with sheep red cells and inhibit the reactivity of normal splenocytes. In an attempt to reverse this immunosuppression, near normal responses were obtained in vitro from infected splenocytes by increasing antigen dose, by adding E. coli lipopolysaccharide, or, more effectively, by cocultivating with small numbers of unfractionated or T cell-depleted peritoneal exudate cells (PC), whereas other manipulations proved ineffective. PC did not prevent the inhibition of normal splenocytes by infected spleen cells, but exhibited substantial restorative activity in vivo. In similar experiments, the immunosuppression exerted by the entire Friend complex could be reversed by PC in vitro but not in vivo. These results indicate that a functional deficit of macrophages may be partially responsible for the immunological impairment induced by leukemia viruses and suggest rational approaches to evaluate the relevance of this impairment to oncogenesis.

Analysis of the role of different cell types in the genetic regulation of antibody production to the thymus-independent synthetic polypeptide poly (DTyr, DGlu)-poly (DPro)--poly (DLys)

The immune response potential of mice to the thymus-independent synthetic polypeptide poly (DTyr, DGlu)-poly(DPro)--poly(DLys)[D(T,G)-Pro--L] is genetically regulated. The defect in the ability of low responder mice to mount an immune response to this antigen appears to be expressed in their B cell population since the presence of thymocytes, or addition of "educated T cells" or supernatant of T cells after stimulation with the antigen neither enhanced, nor suppressed the level of antibodies produced in both low and high responder mice. Low responsiveness could not be enhanced either by stimulation of macrophages or by injection of poly(A) - poly(U) in contrast to the significant effect of these agents on low responses to the thymus-dependent poly(LTyr, LGlu)-poly(LPro)--poly(LLys) [L(T,G)-Pro--L]. These results suggest that macrophages do not participate in the limiting step, or are not involved at all, in antibody production towards the thymus-independent polypeptide. The antibodies produced in response to D(T,G)-Pro--L were found to be mainly of the 7 S class. T cells are not required for the production of mercaptoethanol resistant antibodies to this immunogen since they were found in intact mice as well as in T cell depleted animals.

Cell mediated immunity and the inflammatory system

In this review we have considered the evidence for the existence of lymphokines and have focused on the specific mechanisms by which lymphokines modify the behavior of the various inflammatory cells. These mechanisms are based for the most part on in vitro observations. We have therefore discussed in detail data that document an in vivo role for various lymphokines. This evidence is based on experiments falling into two broad categories: the identification of lymphokines in tissue extracts and serum, and the demonstration of various biologic activities of exogenous lymphokines administered locally or systemically. The details of these kinds of experiments have been presented throughout the preceding discussion. The demonstration of lymphokines in vivo adds a new dimension to our ability to study human disease. The detection of migration inhibition factor in serum, for example, is technically simpler than studies of lymphocyte reactivity in patients with altered cellular immunity. Migration inhibition factor has already been found in the serum in several clinical settings, as described. The capacity of exogenous lymphokines for suppressing manifestations of delayed hypersensitivity may provide an explanation for the altered immunoreactivity often observed in some of those diseases. A final comment is in order regarding the significance of lymphokine production in the overall biologic scheme of things. In previous sections we noted that although activation of lymphocytes for lymphokine production by specific antigen is a property of T cells, B cells may be so activated nonspecifically by certain mitogens. Moreover, it has been shown that migration inhibition factor, or substances with similar biologic and physicochemical properties, may be found in certain replicating cultures of nonlymphoid cells. Also, we have recently shown that migration inhibitory activity, as well as certain other lymphokine-like activities such as macrophage and lymphocyte chemotaxis, may appear following the in vitro or in vivo infection of nonlymphoid as well as lymphoid cells by certain viruses. All these results suggest that lymphokine production, rather than "merely" representing an effector mechanism for cell mediated immunity and for certain kinds of helper functions in antibody synthesis, represents a general biologic phenomenon that may play a role in various aspects of host defense. Thus, such mediator substances should be more properly called "cytokines." Lymphokines represent a restricted set of cytokines made by one class of cells (lymphocytes) activated in certain unique ways. In this view the lmyphocyte has acquired some specialized means for triggering such production, not available to other cells.

Strain differences in the immunogenicity of aggregated human IgG and the adjuvant action of lipopolysaccharide on the low-responder strain of mice

Strain differences in the antibody response to human IgG (HGG) were observed when aggregated HGG was injected intravenously. Lipopolysaccharide (LPS) administered subsequently markedly enhanced the antibody response to HGG in low responder C57BL/6 mice as compared with that in high responder DDD, C3H/He or (C57BL/6 X DDD)F1 mice. Aggregate-free preparation of HGG at a dose of 0.5 mg induced immunological tolerance in all strains of mice tested. LPS injected subsequently converted tolerogenic, aggregate-free HGG into immunogen in DDD mice but not in C57BL/6 mice. To determine the correlation between adjuvanticity and mitogenicity of LPS, spleen cells from normal mice were cultured in the presence of LPS and 3H-thymidine uptake was measured. Spleen cells of DDD mice incorporated three times as much 3H-thymidine as those of C57BL/6 mice. There seems no strong correlation between both activities of LPS. The data obtained are discussed in terms of strain differences in the macrophage function for processing the antigen.

The role of adherent cells in the immune response. Fibroblasts and products released by fibroblasts and peritoneal cells can substitute for adherent cells

The primary immune response to sheep erythrocytes in adherent cell-depleted cultures was restored by adding a critical number of peritoneal cells. Complete substitution was achieved also with supernatants from allogeneic and syngeneic peritoneal cells. Both living fibroblasts and supernatants from fibroblast cultures were found to be highly efficient substitutes for adherent cells in both syngeneic and allogeneic systems. Supernatants from non-antigen-reated peritoneal cells and fibroblasts caused increased DNA synthesis and induction of polyclonal antibody synthesis in normal spleen cells. Thus, adherent cells need not function in the immune response by presenting antigen to the B cells via 'IgT' or by releasing signal-2 activity, which acts on lymphocytes that have already received signal 1.

The effect of lipopolysaccharide on the primary immune response to the hapten NNP

We have studied the effects of lipopolysaccharide (LPS) on the primary in vivo immune response to the hapten (4-hydroxy-3,5-dinitrophenyl)acetyl (NNP), with special reference to the avidity and affinity of the early appearing 19S and 7S antibodies. Comparisons were made of the immune response to NNP in groups of mice given either antigen alone, LPS alone, or antigen plus LPS. The avidity of antibodies induced by LPS plus antigen were similar to that found after injection of antigen alone, in spite of the fact that the antibodies were more numerous. However, when comparing the avidity of antibodies produced in animals given only LPS with those given LPS plus antigen, the latter group was often found to have fewer low-avidity 19S-antibody-producing cells. The affinity of 7S antibodies was also similar in the two groups given antigen or antigen plus LPS. Kinetic studies of the effect of LPS on the primary immune response to NNP showed that synergy was observed only before or after the peak response in groups given antigen alone. It is concluded that LPS under synergy conditions acts preferentially on specific antigen-sensitive cells, which are distinct from those that are activated to polyclonal antibody synthesis by LPS alone. Possible mechanisms for the adjuvant effect of LPS are discussed.

The effect of the thymus-independent antigens, collagen and synthetic collagen-like polypeptide, on the requirement of cell cooperation in the immune response to thymus-dependent antigens

The effect of thymus-independent antigens on the need for cell cooperation in the immune response to thymus-dependent antigens was investigated. Irradiated recipient mice transplanted with either bone marrow cells or a mixture of bone marrow and thymus cells, were immunized with the thymus-independent antigen (Pro-Gly-Pro)n covalently conjugated to the thymus-dependent ovalbumin, or with a mixture of (Pro-Gly-Pro)n and ovalbumin. In both cases an effective response towards ovalbumin was observed in the absence of thymus cells as was found for the thymus-independent (Pro-Gly-Pro)n. The same effect on ovalbumin was demonstrated when a mixture of the thymus-independent collagen and ovalbumin was used for immunization. On the other hand, when irradiated reconstituted mice were immunized with a mixture of ovalbumin and the thymus-dependent gelatin, which is the denatured product of collagen, cell-to-cell cooperation was required for an immune response to both immunogens. The effect of (Pro-Gly-Pro)n and collagen on the response to the thymus-dependent ovalbumin in vivo was observed in in vitro experiments using sheep red blood cells (SRBC) as the immunogen as well. In the presence of reduced and carboxymethylated (RCM) Ascaris collagen and (Pro-Gly-Pro)n, nude spleen cells could produce significant numbers of plaque-forming cells towards SRBC. Thus, (Pro-Gly-Pro)n and collagen can deliver the signal required to stimulate B cells to produce antibody towards thymus-dependent antigens in the absence of T cells. In contrast to the results with (Pro-Gly-Pro)n and collagen, the thymus-independent synthetic polypeptide poly(DTyr, DGlu-)-poly(DPro)--poly(DLys) did not affect the requirement for cell cooperation of the thymus-dependent immunogens, ovalbumin and SRCB. It thus appears that the ability to substitute for T cells for antibody production towards thymus-dependent immunogens is not a general characteristic of thymus-independent antigens.

Histocompatibility antigens acting as helper determinants for tumor-associated antigens of murine lymphosarcoma

The mechanism of effectiveness of allogeneic versus syngeneic immunization in mounting a humoral specific immune response against tumor-associated antigens of murine urethan-induced thymic lymphosarcomas was studied. C57BL mice immunized with untreated or blocked C3Hf lymphoma cells yielded sera with complement-dependent cytotoxicity for C57BL lymphosarcomas, whereas they did not respond to repeated injections of blocked syngeneic lymphoma cells, indicating a helper activity of alien alloantigens in the reflex of tumor antigens. Thymectomized, lethally-irradiated and T, but not B-reconstituted mice produced active sera, suggesting a T-dependent response. The anti-tumor activity of the serum ran parallel with the anti-histocompatibility one. The tumor-associated and the alien histocompatibility determinants had to be present on the same cell membrane. In addition, active sera were obtained when immunizing syngeneic lymphoma cell inocula were followed by allogeneic normal immunocompetent cells. It is suggested that either a helper mechanism with B and T cells cooperating in recognizing haptenic tumor-associated and carrier histocompatibility antigens or a graft-versus-host reaction determining an abnormal induction of the immune system is needed in order to detect the weak tumor-associated antigens of urethan-induced lymphomas.

Mechanism of b-lymphocyte activation: failure to obtain evidence of a direct role of the Ig receptors in the triggering process

Experiments were designed to test two hypotheses of B-cell activation by antigen: the cross-linking concept, postulating that a suitable degree of antigen-induced cross-linking of the Ig receptors is sufficient for immunocyte triggering, and the two-signal hypothesis, suggesting that a first signal delivered by antigen interacting with the Ig receptors followed by a second signal given by, for example, a polyclonal B-cell activator is necessary for activation. The results did not support either of these hypotheses. Thus, the hapten FITC coupled to human serum albumin and human gammaglobulin in different conjugation ratios failed to activate B cells, whether the hapten-protein conjugates were soluble or precipitated, whether the experiments were carried out in the presence or absence of different concentrations of sera from different species, and irrespective of the day of assay. Furthermore, the same FITC-protein conjugates or FITC itself coupled to Sepharose particles failed to induce a specific anti-FITC response, even though a range of 10-9-fold concentrations of FITC were used. In contrast, FITC coupled to lipopolysaccharide (LPS) regularly induced a primary anti-FITC response in all the above systems, whether FITC-LPS was soluble or coupled to Sepharose particles. The conjugation ratio of FITC to LPS was within the range of epitope densities used with FITC-protein conjugates. Analogous studies were performed with the above compounds and, in addition, NNP-cap and fowl gammaglobulin, added alone or together with LPS to lymphocyte cultures. In no case did the antigen plus LPS give a better specific anti-FITC response than LPS alone, irrespective of the culture conditions, the epitope densities, the physical form of the conjugates, and whether they were bound to Sepharose particles or not, although this would be expected in terms of the two-signal concept. The results are compatible with the one nonspecific signal hypothesis, ascribing a passive role to the Ig receptors and an active triggering function to thymus-independent antigens. Therefore, the ability to trigger B cells directly will depend on the nature of the carrier, triggering being achieved if the carrier is a polyclonal B-cell activator; the epitope density and the degree of cross-linking of Ig receptors are unimportant for delivering the triggering signal, although they can facilitate the binding of the conjugate to the specific B cells.

Functional characteristics of Peyer's patch lymphoid cells. II. Lipopolysaccharide is thymus dependent

This study shows that LPS is not mitogenic in cultures containing B cells, or B cells and accessory adherent cells or ME, unless T cells are present. This observation rules out models of induction of antibody synthesis in which it is assumed that the delivery of a mitogenic signal by the interaction of LPS with the membrane of the B cell is in itself sufficient for B-cell induction (19). Further, it makes unlikely the proposed extrapolation of such a model to other so-called thymus-independent antigens, e.g., PVP, levan, dextran, and SIII (19). The mitogenic action of LPS appears to be due to its ability to complete an inductive stimulus to B cells (13). We interpret the observed thymus dependence of the B-cell response to LPS in light of a model in which two signals are obligatory for B-cell induction (14). The first signal in the inductive pathway is delivered to the antigen-sensitive cell via a conformational change in the receptor upon interaction with antigen. The second signal is delivered via the thymus-derived cooperating system. Since LPS can induce immune responses to both immunogenic and nonimmunogenic ligands (9-13) we envision that one signal is delivered to the B cell via specific binding of the ligand to the B-cell antigen receptor, while a second signal is delivered as a result of T-cell cooperation via membrane-bound LPS. This has been termed abnormal induction (20). In this example LPS is the foreign membrane-bound determinant in question although histocompatibility antigens (21, 22), viral determinants, or surface bound lectins could act similarly. In light of the above model, one observation should be pointed out. LPS inhibits the induction of a SRBC response in normal Peyer's patch cells to which adherent cells or ME is added. This inhibition appears to be a T-cell-mediated effect because it is abolished by partial depletion of the T-cell population by antitheta treatment. Since the induction of IgM producing PFC is being measured, the T-cell-dependent LPS inhibition could act either (a) by induction of T-cell "suppression" (23, 24) of the normal cooperating system required for a SRBC response, or (b) by the induction of such high levels of cooperating function (13) as to be inhibitory to a SRBC IgM response. Our observations contrast sharply with prior reports which describe LPS as a thymus-independent antigen (2-4) and a B-cell mitogen (5-8) capable of stimulating immune responses in the absence of T-cell cooperation (2-12). This demonstration of the thymus dependence of LPS stimulation has been possible because Peyer's patches from congenitally athymic (nude) mice are functionally a highly purified B-cell population devoid of T cells and accessory adherent cells. In this respect, earlier studies relied on nude spleen cultures and spleen cultures from thymectomized, lethally irradiated, and bone marrow-reconstituted mice (3, 4, 6-13). These spleen cultures which contain B cells and accessory adherent cells are recognized to be deficient but not devoid of the thymus-derived contribution to the inductive stimulus (12, 13). It could be argued that the presence of T cells and adherent cells is in fact required for the antigen-specific effect and not for the LPS effect. However, this is unlikely since our experiments show that LPS is not directly mitogenic for B cells and does not stimulate background anti-SRBC PFC. It seems unlikely that Peyer's patch antigen-sensitive cells differ from antigen-sensitive cells in the spleen in their mechanism of induction. We have shown that Peyer's patch B cells can be specifically induced by antigen, and Peyer's patch T cells mediate cooperating and killer functions. Alternately, the possibility that Peyer's patch B cells were not stimulated by LPS as a result of prior cryptic exposure to LPS (13) in the intestinal tract was excluded since cultures containing B cells, T cells, and adherent cells or ME were stimulated to DNA synthesis by LPS. The reason that certain antigens appear to be thymus independent may be that their repeating polymeric nature permits inductive interactions at very low levels of thymus-derived cooperation (see reference 20 for quantitative considerations). It has been stated that the inductive properties of all thymus-independent antigens are directly related to their ability to act as B-cell mitogens (19). The observation that LPS is thymus dependent for its B-cell mitogenic activity makes us question the thymus independence of any antigen.

Mechanism of thymus-independent immunocyte triggering. Mitogenic activation of B cells results in specific immune responses

The present experiments were performed in order to analyze the mechanism by which thymus-independent antigens (nonspecific B-cell mitogens) can induce specific immune responses to antigenic determinants present on the same molecule. The hapten NNP was coupled to the B-cell mitogen, lipopolysaccharide (LPS). The conjugate retained full mitogenic activity and bound specifically to NNP-reactive cells. NNP-LPS activated polyclonal as well as specific anti-NNP antibody synthesis, but the optimal concentrations for induction of specific anti-NNP cells were several orders of magnitude lower than the concentrations required for polyclonal activation. These low concentrations failed to activate nonspecific cells, but they induced specific thymus-independent responses of high-avidity NNP-specific cells with the typical kinetics of antigenic responses in vitro. Furthermore, hapten-specific cells were paralyzed by NNP-LPS concentrations that were optimal for induction of polyclonal activation. Specific activation and paralysis could be abolished by free hapten indicating that selective binding of NNP-LPS to hapten-specific cells was responsible for the specificity of the response. However, the triggering signal lacked specificity, since high-avidity specific anti-NNP cells could still be activated by stimulating concentrations of NNP-LPS in the presence of free hapten, even though the Ig receptor combining sites were presumably occupied by NNP. The findings show that B cells with specific Ig receptors for the antigenic determinants on mitogen molecules preferentially bind these molecules and become activated at concentrations still unsufficient to trigger other B cells that lack specific receptors. It is suggested that activation for primary IgM responses in B cells is the result of "one nonspecific signal." This nonspecific signal is provided by the mitogenic properties of some antigens (highly thymus independent or, alternatively, by nonspecific T-cell factors (for highly T cell-dependent antigens), or both, and the surface structures responsible for triggering are not the Ig receptors. The specific Ig receptors only act as passive focusing devices for nonspecific stimuli, entitling the cell to be selectively activated, even though both the signal and the receptors for the triggering are nonspecific.