Antigen-specific suppressor factors produced by T cell hybridomas for delayed-type hypersensitivity.

This study describes the generation of T hybridoma lines which secret factors specifically suppressing delayed-type hypersensitivity (DTH) to sheep red blood cells (SRBC). AKR strain-derived T lymphoma BW 5147 cells were fused with spleen cells from mice primed with SRBC and containing antigen-specific T suppressor cells for DTH. Supernants from the derived hybridomas were tested for suppression of either expression of induction of DTH to SRBC. Six lines produced specific suppressor activity for the expression of DTH; 4 lines produced suppressor activity for the induction of DTH of which only one line was antigen-specific. These lines were passaged in normal AKR mice, and the serum obtained had activity up to 10(-4) dilution. The factor was effective across the H-2 barrier.     

Suppressor T cells for delayed-type hypersensitivity to Japanese encephalitis virus.

The delayed-type hypersensitivity (DTH) to Japanese encephalitis virus (JEV) and the suppressor cells controlling it and the antibody-forming cells in inbred Swiss mice have been studied. JEV induces DTH, with a peak response at day 7 following infection which persists at low levels at least up to 119 days. Suppressor activity appeared on day 18. It was transferable by immune spleen cells. Treatment of spleen cells with anti-Thy-1.2 antisera and complement abrogated the suppressor activity. The homogenate of the spleen was equally effective in mediating suppression of DTH and the humoral response as measured by direct antibody plaque-forming cell (IgM-PFC) assay. The suppressor activity was antigen-specific both on DTH and T helper for antibody response as the immune responses against SRBC or Coxsackie B4 virus were not suppressed. The suppressor cells were sensitive to cyclophosphamide treatment when the drug was given 48 hr before their appearance. It is, therefore, concluded that in JEV infection of mice, antigen-specific suppressor T cells are generated, both for DTH and IgM antibody, which are cyclophosphamide-sensitive and mediate suppression through soluble product(s).     

Regulation of delayed-type hypersensitivity. I. T suppressor cells for delayed-type hypersensitivity to sheep erythrocytes in mice.

Mice injected with 1 X 10(8) sheep red blood cells (SRBC) into the footpad showed high levels of delayed-type hypersensitivity (DTH) to SRBC 4-8 days after the injection. In contrast, mice injected intravenously with 1 X 10(9) SRBC were unresponsive to DTH induction through 1 X 10(8) SRBC injected into the footpad. This suppression of DTH was maintained for at least 6 weeks and was transferable spleen, lymph node and thymus cells to normal syngeneic recipients. Bone marrow cells, on the other hand, did not contain the suppressor cells. The suppression of DTH was antigen-specific in that DTH to chicken red blood cells and contact sensitivity to 2,4-dinitrofluorobenzene was not affected. The suppressor cells were theta-positive and Ig-negative. They appeared in the spleen in optimum number 3-4 days after induction. The suppressor cells affected both the induction and manifestation of DTH. The presence of suppressor and effector cells for DTH inducible by different routes of antigenic presentation reflects the dynamic balance in the regulation of DTH.     

Rapid loss of feedback suppressor T-cell activity after priming in vivo.

T cells from antigen-primed mice have a diminished capacity to mediate feedback suppression when compared to T cells from unprimed mice. This was demonstrated using an in vitro model of B-cell induced feedback suppression in which spleen cells from mice primed with sheep erythrocytes (SRBC) activate feedback suppressor T-cell precursors to mediate suppression of the primed spleen cell response. The addition of splenic T cells from unprimed mice to cultures of spleen cells from SRBC-primed mice resulted in suppression of the secondary IgM and IgG anti-SRBC response. In contrast, no suppression was detected when T cells from mice primed with SRBC were added to the primed spleen cell cultures. The loss of suppression by T cells from primed mice was antigen-specific and was detectable by 24 hr after priming, coinciding with the appearance after priming of T-cell enhancing activity. The reduced suppressive activity could be due to changes in the active T-cell subset itself or to the appearance of cells or factors within the T-cell population that block or mask detection of feedback suppression. In either case, the present finding suggests that priming of a host not only activates feedback suppression induction mechanisms, but also rapidly affects the ability of the T-cell population to develop effective feedback suppression.     

Regulation of delayed-type hypersensitivity. II. Specific suppressor factor for delayed-type hypersensitivity to sheep erythrocytes in mice.

An antigen-specific suppressor factor for delayed-type hypersensitivity (DTH) to sheep red blood cells (SRBC) in mice is described. Lymph node cells and spleen cells from mice injected intravenously with 1 x 10(9) SRBC 4 days previously were incubated in vitro for 48 h in culture medium. Supernatant obtained from the culture inhibited the induction of DTH to SRBC in normal mice. It also suppressed the expression of DTH in presensitized mice. The suppression is specific as the suppressor factor had no effect on the DTH to noncross-reacting antigen, chicken red blood cells. Treatment of the spleen cells with anti-theta serum and complement prevented the production of the suppressor factor, whereas treatment with anti-Ig serum and complement had no effect. Suppressor factor produced by H-2k mice suppressed the DTH in H-2b mice. The factor thus seems to act across the H-2 barrier. The suppressor factor was not removed by adsorption with goat anti-mouse immunoglobulin immunoadsorbent, but could be adsorbed by SRBC. It was stable at 56 degrees C for 1 h, but was partially inactivated by freezing and thawing. The factor has a molecular weight of less than 35 000 daltons.     

Modification of Immune Response by Cinnarizine

Abstract

Effects of cinnarizine on immune response in mice were investigated. Mice were orally administered with cinnarizine and were immunized with sheep red blood cells (SRBC) intravenously. Numbers of plaque forming cells (PFC) to SRBC in spleen of these mice were assayed and delayed-type hypersensitivity (DTH) response to SRBC was measured. 1) PFC response in immunization with 5 × 10⁶ cells/mouse of SRBC was enhanced by administration of 25 mg/kg of cinnarizine, while the response in immunization with 5 × 10⁸ cells/mouse was suppressed by 25 to 200 mg/kg of cinnarizine. 2) From study on timing of administration, suppression of PFC response by 6.25 to 200 mg/kg of cinnarizine was observed at 24 hr. after the immunization. 3) 12.5 to 200 mg/kg of cinnarizine suppressed polyclonal B cell activation induced by lipopolysaccharide (LPS). 4) Colchicine induced suppressor T cell inactivation was prevented by administration of 50 mg/kg of cinnarizine and it was suggested that cinnarizine may induce suppressor T cells from the study of adoptive cell transfer system. 5) 50 mg/kg of cinnarizine showed the suppression of DTH response in expression phase, but not in induction phase. It was concluded that immune responses in mice were modified by cinnarizine.     

Modification of Immune Response by Cinnarizine

Effects of cinnarizine on immune response in mice were investigated. Mice were orally administered with cinnarizine and were immunized with sheep red blood cells (SRBC) intravenously. Numbers of plaque forming cells (PFC) to SRBC in spleen of these mice were assayed and delayed-type hypersensitivity (DTH) response to SRBC was measured. 1) PFC response in immunization with 5 × 10⁶ cells/mouse of SRBC was enhanced by administration of 25 mg/kg of cinnarizine, while the response in immunization with 5 × 10⁸ cells/mouse was suppressed by 25 to 200 mg/kg of cinnarizine. 2) From study on timing of administration, suppression of PFC response by 6.25 to 200 mg/kg of cinnarizine was observed at 24 hr. after the immunization. 3) 12.5 to 200 mg/kg of cinnarizine suppressed polyclonal B cell activation induced by lipopolysaccharide (LPS). 4) Colchicine induced suppressor T cell inactivation was prevented by administration of 50 mg/kg of cinnarizine and it was suggested that cinnarizine may induce suppressor T cells from the study of adoptive cell transfer system. 5) 50 mg/kg of cinnarizine showed the suppression of DTH response in expression phase, but not in induction phase. It was concluded that immune responses in mice were modified by cinnarizine.     

Characterization of a third-order suppressor T cell (Ts3) induced by cryptococcal antigen(s).

Previous studies from our laboratory have shown that a high dose of cryptococcal culture filtrate antigen (CneF) administered intravenously induces a complex suppressor cell cascade which down-regulates the cell-mediated immune response to Cryptococcus neoformans antigens. The primary objective of this investigation was to determine whether a suppressor cell induced by immunization is required for efferent suppression of the cryptococcal delayed-type hypersensitivity (DTH) response. Our approach to this problem was to immunize CBA/J mice with CneF emulsified in complete Freund adjuvant and then 6 days later to collect spleen cells from the immunized mice and adoptively transfer these cells along with C. neoformans-specific second-order suppressor T cells (Ts2) to naive syngeneic recipients at the time of footpad challenge of the recipients with CneF. To establish which populations of cells in the spleens of immunized mice play a suppressive role, mass cytolysis with specific antibodies and complement was performed before the spleen cells were transferred to naive animals. Since the phenotype of the cells responsible for the transfer of the cryptococcal DTH response had not been completely determined, we first demonstrated that the cells responsible for DTH were L3T4+ Lyt-2- cells. Subsequently, we established that a Thy-1+ L3T4- Lyt-2+ I-J+ cell population induced by immunization was required along with C. neoformans-specific Ts2 cells for efferent suppression of the cryptococcal DTH response. In addition, we demonstrated that the suppressor cells in the immune cell population were derived from cyclophosphamide-sensitive precursors. These data indicate that a third suppressor cell population is required for efferent suppression of the cryptococcal DTH response. As in the azobenzenearsonate and 4-hydroxy-3-nitrophenyl acetyl hapten suppressor models, the Ts2 cells in the circuit mediate their effects through this third suppressor component. Since the mode of induction and the phenotype of the third C. neoformans-specific suppressor cells are similar to those reported for Ts3 cells in other antigen-specific suppression models, we referred to this third suppressor cell in the C. neoformans-specific suppressor cell cascade as a Ts3 cell.     

Regulation of delayed-type hypersensitivity IV. Antigen-specific suppressor cells for delayed-type hypersensitivity induced by lipopolysaccharide and sheep erythrocytes in mice.

Mice injected subcutaneously with 1 x 10(8) sheep red blood cells (SRBC) developed high levels of delayed-type hypersensitivity (DTH) to SRBC 4-8 days after injection. Such DTH was suppressed when 100 microgram lipopolysaccharide (LPS) was injected intravenously 1-2 days before or at the time of SRBC injection. This suppression of DTH was transferable by spleen, lymph node, thymus and bone marrow cells to sensitized or normal syngeneic recipients, but could not be transferred by serum. Suppressor cells were not induced by LPS alone or SRBC alone, and they were antigen-specific since DTH to chicken red blood cells was not affected. The suppressor cells appeared in the spleen in optimum number 3-4 days after induction. They were theta-negative and Ig-positive as judged by antiserum plus complement treatment and by Ig rosette separation. Attempts to obtain soluble suppressor factor from the suppressor cells by sonication or in vitro incubation were unsuccessful. Mitomycin C treatment of the suppressor cells completely abolished the suppressor activity. Thus, LPS, in conjunction with antigen, appears to induce a population of specific suppressor B cells which are capable of regulating T cell function.     

Nonspecific suppression of primary antibody responses and presence of plastic-adherent suppressor cells in Toxoplasma gondii-infected mice.

The effect of Toxoplasma infection on primary antibody responses to both T-dependent and T-independent antigens was examined in mice. Drastic suppression of primary responses to sheep erythrocytes (SRBC) occurred when mice were immunized 7 days after infection. The suppression was observed in both 2-mercaptoethanol-sensitive and -resistant hemagglutinin responses. Anti-dinitrophenol (DNP) immunoglobulin E and G1 responses to DNP-conjugated keyhole limpet hemocyanin were also suppressed by infection. It was suggested that the suppressive effect is nonspecific for the antigens and immunoglobulin classes produced. Anti-DNP responses to DNP-Ficoll, a T-independent antigen, were suppressed by infection, but the suppressive effect was weaker than that on the responses to SRBC. This suggests that both T and B cells are suppressed by infection. In vitro responses of infected mouse spleen cells to SRBC and DNP-Ficoll confirmed the results of in vivo experiments. In addition, the presence of plastic-adherent suppressor cells was demonstrated in the spleen cells of infected mice, which suppressed the ability of normal mouse spleen cells to mount an SRBC-specific plaque-forming cell response. These plastic-adherent suppressor cells appeared to be a major cause of nonspecific suppression of primary antibody responses in Toxoplasma-infected mice.     

Regulation of delayed-type hypersensitivity V. Suppressor cell memory in antigen-specific suppression of delayed-type hypersensitivity to sheep erythroeytes

Mice primed i. v. with 10⁹ sheep red blood cells (SRBC) produce antigen-specific T suppressor (Ts) cells which inhibit both the induction and the expression of delayed-type hypersensitivity (DTH). These Ts cells are detectable in the spleen and lymph nodes 3–5 days after priming but are largely absent by 6 days. The transient detect-ability of the Ts cells contrasts sharply with the profound antigen-specific suppression which persists in primed donor mice for at least a year. Evidence is presented that this long-term impairment of DTH is maintained, at least in part, by memory Ts cells which are Thy-1⁺, cyclophosphamide-resistant and antigen-specific. Although they appear to be co-induced with the short-lived primary Ts cells and localize initially in the lymphoid organs, they are present in the long-lived circulating pool of T cells and can be adoptively transferred by celomic parabiosis. Memory Ts cells are readily reactivated by lower doses of SRBC which would induce T effector cells rather than Ts cells in naive animals. Reactivated memory Ts cells seem to generate a population of antigen-specific secondary Ts cells which again localizes in the lymphoid organs and can adoptively suppress the induction and expression of DTH to SRBC.     

In vitro immunosuppressive effect of serum from orally immunized mice.

Mice were orally immunized by 4 daily intragastric (IG) intubations of 4 X 10(9) SHEEP RED BLOOD CELLS (SRBC). The serum of such mice (OS), taken two weeks after the first intubation, was compared to normal mouse serum for its suppressive effect on in vitro primary responses of normal spleen cells to SRBC. OS regularly depressed the in vitro response to less than 10% of the normal response. The suppressive effect was dose-dependent, antigen-specific and removable by absorption with SRBC. When cultures were stimulated with SRBC pre-exposed to OS, the response was also depressed. The suppression was less when the addition of OS was delayed. OS from BALB/c mice efficiently depressed in vitro responses of spleen cells from other strains. Spleen cells, taken 2 weeks after IG immunization, responded normally to in vitro challenge with SRBC. The suppressive factor had the molecular size of an IgG and could be removed by anti-Ig and anti-IgG1 but not by anti-alpha or anti-mu immunosorbents. Purified IgG1 was more suppressive than IgG2a and IgG2b despite a similar anti-SRBC antibody content.     

Modulation of the immune responses against SRBC after oestriol treatment in mice.

Delayed-type hypersensitivity (DTH), primary direct and indirect PFC, memory antibody response and suppressor cell induction against sheep red blood cells (SRBC) have been examined in oestriol (E3) pretreated mice. The results showed that DTH and primary direct and indirect PFC responses were suppressed by E3 treatment. These suppressive effects could, however, be overcome when oestrogenized mice were given supra-optimal doses of SRBC for each response. On the other hand, the memory antibody response was markedly enhanced when E3 was given prior to the primary antigen stimulation. Induction of the suppressor cells for the antibody response seemed not to be affected by E3 treatment, but the characterization of the suppressor cells revealed that those obtained from E3 treated mice were surface immunoglobulin positive (sIg+) cells whereas those from control mice were not. These results were discussed in terms of the altered antigen distribution due to the activated phagocytic activity of the reticuloendothelial system (RES) after E3 treatment.     

Major histocompatibility gene complex (MHC)-coded determinants on antigen-specific suppressor factor for delayed-type hypersensitivity and surface phenotypes of cells producing the factor

Antigen-specific suppressor factor for delayed-type hypersensitivity (DTH) to sheep red blood cells (SRBC) was obtained by incubating in vitro spleen cells from CBA mice (H-2k) injected intravenously 3 days previously with 1 x 10(9) SRBC. The suppressor factor was characterized for major histocompatibility gene complex (MHC)-coded antigenic determinants by passing the factor through immunosorbents coupled with appropriate alloantisera. The suppressor factor was absorbed by anti-H-2k, anti-Iak and anti-I-Jk immunosorbents but was not retained by anti-Ias, anti-I-Js, anti-I-Ak, anti-I-E/Ck or anti-H-2Kk immunosorbents. In addition, the factor bound to an immunosorbent coupled with rabbit antibodies against carbohydrate-defined Ia antigens. Furthermore, the suppressive activity that was absorbed was quantitatively recovered in the acid eluates from the immunosorbents. Treatment of the spleen cells with anti-Lyt-1.1 antiserum and complement completely abrogated their ability to elaborate the suppressor factor in vitro. In contrast, treatment with anti-Lyt-2.1 or anti-Iak antiserum and complement had no effect. Thus, it appears that the suppressor factor for DTH to SRBC bears I-J subregion-coded determinants, and its production is dependent on cells which have the Lyt-1+,2- and Ia- phenotype.     

Influences of anti-mouse interleukin-6 receptor antibody on immune responses in mice

In the present study, we examined the effect of anti-IL-6 receptor antibody (MR16-1) on humoral and cellular immune responses in mice. MR16-1 did not affect antigen-specific antibody production in either the primary or secondary response in mice immunized with dinitro-phenyl (DNP)-keyhole limpet haemocyanin (KLH) in saline. DNP-KLH immunization with complete Freund's adjuvant (CFA) markedly augmented anti-DNP antibody production and induced interleukin 6 (IL-6) production in serum. In this case, MR16-1 significantly suppressed antibody production and further increased serum IL-6 levels. Regarding the cellular response, we studied the effect on the delayed-type hypersensitivity (DTH) response. DTH response was induced in mice by the immunization with Mycobacterium butyricum with incomplete Freund's adjuvant and following antigen challenge into the footpad 14 days after immunization. When MR16-1 was injected immediately after immunization, the DTH response was significantly suppressed and enlargement of the spleen was also suppressed. This suppressive effect was observed, when MR16-1 was administered on day 0, but not on days 5 and 10. Again, serum IL-6 levels were much higher in MR16-1-treated mice compared with controls. Furthermore, spleen cells from control mice released IL-2 and INFgamma by the stimulation of antigen in vitro. In contrast, spleen cells from MR16-1-treated mice produced these cytokines at a marginal level. In contrast, MR16-1 did not suppress the DTH response, when it was injected immediately after antigen challenge. Our results suggest that IL-6 does not always involve antibody production, although IL-6 augments antibody production, and that IL-6 is essential for the induction of Th1 cells.     

Antibody-mediated allotype suppression in adult mice: the role of antigen, effector isotype and regulatory T cells

It has been reported (Contemp. Top. Immunobiol. 1974. 3:41) that allotype-specific T suppressor cells can be induced after monoclonal anti-allotype treatment of neonatal (BALB/c X SJL)F1 (Igha/b) mice. Here we show that (BALB/c X CB20)F1 adult-derived spleen cells (SC) are, by contrast, potently suppressed by monoclonal allotype-specific reagents, (when transferred into irradiated BALB/c recipients) in the absence of primary T suppressor cell induction. Such suppression is only induced in activated B cells [exposed to lipopolysaccharide or sheep red blood cells (SRBC)], and is probably dependent on the isotype of the anti-allotype sera administered. For example, two independently produced IgG1 monoclonal reagents raised against the Igh-1b allotype were poorly suppressive or nonsuppressive, whereas an IgG3 and an IgG2a monoclonal antibody induced a 90% suppression of the target allotype in transferred adult SC. It was found that suppression was not due to a depletion of antigen-specific T cell help since: (a) the addition of SRBC-educated T cells did not break suppression and (b) suppressed SC were as good a source of T cell help as normal SC, in the response of virgin or memory B cell (Thy-1-depleted) responses to SRBC in vivo. Suppression was maintained in suppressed cells which had been rechallenged with SRBC after transfer into a second irradiated recipient, but was not induced in normal SC when these were admixed with an equal number from this suppressed SC population. These findings point to a possible mechanism for the regulation of B cell expression, through the formation of an antibody-Ig receptor complex at the surface of the B lymphocyte. After complexing the target cell is either deleted or inactivated. The response to SRBC was reduced or ablated for at least 70 days after treatment with a single dose of anti-allotype serum.     

Impairment of antibody response and of suppressor cell ontogenesis by early injection of hydrocortisone.

The appearance of antibody responsiveness against sheep red blood cells (SRBC) was studied in young mice treated with hydrocortisone hemisuccinate (HHC) either during gestation or around delivery (last 5 days of gestation or day of birth). The plaque forming cell (PFC) response was normal at 2 weeks and depressed at 4 weeks in animals treated during the last 10 days of pregnancy. Conversely, when HHC was only given around delivery, PFC response was depressed at 2 weeks and normal at 4 weeks. The presence of suppressor cells in unimmunized 1--2-week-old spleen cells was evaluated by co-culturing them with adult spleen cells from mice immunized against SRBC. Suppressive activity was found at high level at 8 days, and had already declined at 15 days in non-treated mice. Spleen cells from mice treated with HHC around delivery showed reduced suppressive activity at 8 days but this activity was very high on the 15th day, greater than in normal mice. Such highly suppressive activity correlated with a high level of Lyt-123 T cells. These data on suppressor cells indicate that the paradoxical effect of HHC in mice treated during gestation or around birth is probably explained by the differential effect of HHC on the kinetics of suppressor cells in ontogeny.     

MHC- and Igh-unrestricted interaction between L3T4+, Lyt-2- Ts inducer cells and L3T4-, Lyt-2+ Ts effector cells is required for T cell-dependent suppression of DTH to histocompatibility antigens

This paper describes the characteristics of T suppressor inducer (Ts ind) cells which can interact with T suppressor effector (Ts eff) cells and thereby can account for suppression of delayed-type hypersensitivity (DTH) to alloantigens. Adoptive transfer of spleen cells from mice intravenously (i.v.) injected with allogeneic spleen cells one day earlier induced an antigen-specific state of suppression in the recipients. This became apparent when DTH was induced by subcutaneous (s.c.) immunization of the recipients three days after transfer. The induction of suppression after adoptive transfer of spleen cells required Thy-1+, L3T4+, Lyt-1+2- cells. These cells that by themselves did not exert a suppressive effect induced a state of suppression in recipient mice by activation of recipient-type Ts eff cells. Therefore, the former cell type was classified as Ts ind cell. When athymic nude mice were used as recipients, Lyt-2+ precursors of Ts eff cells had to be transferred together with the Ts ind cells to induce a state of suppression in these mice. The Ts ind cells could activate Ts eff cells in MHC- and Igh-incompatible recipients. The results are discussed in relation to previously described immunoregulatory T cell pathways.     

Specific IgM Enhances and IgG Inhibits the Induction of Immunological Memory in Mice

The effect of priming mice with IgM anti-SRBC (sheep erythrocytes) together with SRBC or IgG anti-SRBC together with SRBC on the development and expression of memory cells was studied. Mice primed with specific IgM and SRBC showed a much more efficient secondary plaque-forming cell and serum antibody response after challenge with SRBC in an adoptive transfer system than did controls primed with SRBC only. The expression of this enhanced memory of IgM-primed spleen cells was counteracted by the high levels of internal IgG anti-SRBC (also the result of priming with IgM) when the mice, instead of being tested in adoptive transfers, were challenged directly. The antigen-specific feedback suppression of the primary antibody response by specific IgG antibodies was also seen to inhibit partially the development of memory cells. The suppressive effect on priming could be demonstrated both in adoptive transfer systems and after direct boost of the same mice that received the primary immunization. Both the IgM enhancement and the IgG suppression of memory cell development were antigen-specific, since no effect on the antibody response to a non-cross-reacting antigen, horse erythrocytes, was seen. The effect of these up- or down-regulations of immunological memory could be demonstrated after secondary injections as long as 90-280 days after priming.     

Composite activities on B cells of products released by T cells activated by concanavalin A.

Supernatants from murine spleen cells cultured for 48 h in the presence of 1.0 μg of concanavalin A (Con A) induced polyclonal antibody synthesis in cultures of spleen cells from both normal and athymic mice in the absence of exogeneous antigen. Moreover, the Con A-induced supernatant rescued B cells which had been rendered unresponsive by a tolerogen [haptenated poly(D Glu,D Lys)]. The capacity of the supernatant to induce cell proliferation was studied under both high and low-density culture conditions. In contrast to antibody secretion, proliferation was only detectable in low-density cultures. The Con A-induced supernatant also contained suppressive components, since the primary anti-sheep red blood cell (SRBC) response was markedly suppressed when the antigen added to cultures consisted of SRBC which had previously been used for absorbing the supernatants. Absorbed supernatants displayed enhanced helper activity indicating that only the suppressor component was removable by antigen. The suppressive component was eluted from erythrocytes with ammonium thiocyanate and was itself strongly suppressive when added to cultures with fresh erythrocytes. Furthermore, the suppressive component proved to be highly antigen-specific, as the SRBC-absorbed factor did not affect the response to horse RBC. The results indicate that supernatants from Con A-activated spleen cells contain both helper and suppressor factors, the latter having easily demonstrable antigen specificity. They further suggest that a nonantigen-derived signal is sufficient to trigger both proliferation and antibody synthesis by B cells.