Proliferation Of Murine Spleen Cells Research Articles

Maca against Echinococcosis?-A Reverse Approach from Patient to In Vitro Testing.

Drug-based treatment of alveolar echinococcosis (AE) with benzimidazoles is in most cases non-curative, thus has to be taken lifelong. Here, we report on a 56-year-old male AE patient who received standard benzimidazole treatment and biliary plastic stents, and additionally self-medicated himself with the Peruvian plant extract Maca (Lepidium meyenii). After 42 months, viable parasite tissue had disappeared. Based on this striking observation, the anti-echinococcal activity of Maca was investigated in vitro and in mice experimentally infected with Echinococcus multilocularis metacestodes. Albendazole (ABZ)-treated mice and mice treated with an ABZ+Maca combination exhibited a significantly reduced parasite burden compared to untreated or Maca-treated mice. As shown by a newly established UHPLC-MS/MS-based measurement of ABZ-metabolites, the presence of Maca during the treatment did not alter ABZ plasma levels. In vitro assays corroborated these findings, as exposure to Maca had no notable effect on E. multilocularis metacestodes, and in cultures of germinal layer cells, possibly unspecific, cytotoxic effects of Maca were observed. However, in the combined treatments, Maca inhibited the activity of ABZ in vitro. While Maca had no direct anti-parasitic activity, it induced in vitro proliferation of murine spleen cells, suggesting that immunomodulatory properties could have contributed to the curative effect seen in the patient.

Purification and characterisation of a xylose-specific mitogenic lectin from Fusarium sambucinum

A xylose-specific intracellular lectin, showing hemagglutination only with rabbit erythrocytes was purified from mycelium of Fusarium sambucinum which was designated as FSL. An array of anion exchange chromatography on Q-Sepharose and gel-exclusion chromatography on Sephadex G-100 resulted in 84.21% yield and 53.99-fold purification of lectin with specific activity of 169.53 titre/mg. Molecular weight of FSL determined by SDS-PAGE was 70.7 kDa, which was further confirmed by gel-exclusion chromatography. Native-PAGE analysis of FSL showed its monomeric nature. FSL was observed to be a glycoprotein containing 2.9% carbohydrate. Hapten inhibition profile of FSL displayed its strong affinity towards D-xylose (MIC 1.562 mM), L-fucose (MIC 6.25 mM), D-mannose (MIC 3.125 mM), fetuin (MIC 15.62 μg/mL), asialofetuin (MIC 125 μg/mL) and BSM (MIC 3.125 μg/mL). Affinity of FSL towards xylose is rare. FSL was found stable over a pH range 6.0–7.5 and upto 40 °C temperature. Hemagglutination activity of FSL remained unaffected by divalent ions. Lectin concentration of 5 μg/mL was found sufficient to stimulate proliferation of murine spleen cells and its concentration 75 μg/mL exhibited highest mitogenic potential. FSL exhibited maximum mitogenic stimulatory index of 14.35. The purification, characterisation and mitogenicity of F. sambucinum lectin has been reported first time.

Enhanced immunogenicity of pneumococcal surface adhesin A (PsaA) in mice via fusion to recombinant human B lymphocyte stimulator (BLyS)

BackgroundB lymphocyte stimulator (BLyS) is a member of the tumor necrosis factor superfamily of ligands that mediates its action through three known receptors. BLyS has been shown to enhance the production of antibodies against heterologous antigens when present at elevated concentrations, supporting an immunostimulatory role for BLyS in vivo.MethodsWe constructed a fusion protein consisting of human BLyS and Pneumococcal Surface Adhesin A (PsaA) and used this molecule to immunize mice. The immunostimulatory attributes mediated by BLyS in vivo were evaluated by characterizing immune responses directed against PsaA.ResultsThe PsaA-BLyS fusion protein was able to act as a co-stimulant for murine spleen cell proliferation induced with F(ab')2 fragments of anti-IgM in vitro in a fashion similar to recombinant BLyS, and immunization of mice with the PsaA-BLyS fusion protein resulted in dramatically elevated serum antibodies specific for PsaA. Mice immunized with PsaA admixed with recombinant BLyS exhibited only modest elevations in PsaA-specific responses following two immunizations, while mice immunized twice with PsaA alone exhibited undetectable PsaA-specific serum antibody responses. Sera obtained from PsaA-BLyS immunized mice exhibited high titers of IgG1, IgG2a, IgG2b, and IgG3, but no IgA, while mice immunized with PsaA admixed with BLyS exhibited only elevated titers of IgG1 following two immunizations. Splenocytes from PsaA-BLyS immunized mice exhibited elevated levels of secretion of IL-2, IL-4 and IL-5, and a very modest but consistent elevation of IFN-γ following in vitro stimulation with PsaA. In contrast, mice immunized with either PsaA admixed with BLyS or PsaA alone exhibited modestly elevated to absent PsaA-specific recall responses for the same cytokines. Mice deficient for one of the three receptors for BLyS designated Transmembrane activator, calcium modulator, and cyclophilin ligand [CAML] interactor (TACI) exhibited attenuated PsaA-specific serum antibody responses following immunization with PsaA-BLyS relative to wild-type littermates. TACI-deficient mice also exhibited decreased responsiveness to a standard pneumococcal conjugate vaccine.ConclusionThis study identifies covalent attachment of BLyS as a highly effective adjuvant strategy that may yield improved vaccines. In addition, this is the first report demonstrating an unexpected role for TACI in the elicitation of antibodies by the PsaA-BLyS fusion protein.ReviewersThis article was reviewed by Jonathan Yewdell, Rachel Gerstein, and Michael Cancro (nominated by Andy Caton).

The Effects of Dehydroepiandrosterone (DHEA) onIn VitroSpleen Cell Proliferation and Cytokine Production

Dehydroepiandrosterone (DHEA), a weak androgenic steroid, has been associated with enhancing immune responses and upregulating resistance against viral, parasitic, and bacterial infections. The objective of this study was to assess the effects of DHEA on murine spleen cell viability, proliferation, and cytokine production following in vitro stimulation with the mitogens concanavalin A (ConA) and lipopolysaccharide (LPS). Results showed that exposure to 6 microM DHEA significantly decreased the viability and proliferation of murine spleen cells stimulated with LPS, whereas no effect was seen on murine spleen cells stimulated with ConA. DHEA did influence the production of both ConA-induced and LPS-induced cytokines. DHEA also significantly reduced the mitogen-induced production of the proinflammatory cytokine interleukin-1 (IL-1) as well as the Th1 cytokines IL-2 and interferon-gamma (IFN-gamma). Increasing concentrations of DHEA significantly increased the production of the Th2 cytokine IL-10 but had no effect on the production of the Th2 cytokine IL-4, the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha), or IL-6. These results suggest that DHEA may be an important factor for increasing Th2 cytokine production and decreasing Th1 and proinflammatory cytokine production. This study provides a more comprehensive understanding of the effects of DHEA on the rates of cell proliferation, cell viability, and cytokine production.

Mycobacterial di-O-acyl-trehalose inhibits mitogen- and antigen-induced proliferation of murine T cells in vitro.

2,3-Di-O-acyl-trehalose (DAT) is a glycolipid located on the outer layer of the Mycobacterium tuberculosis cell envelope. Due to its noncovalent linkage to the mycobacterial peptidoglycan, DAT could easily interact with host cells located in the focus of infection. The aim of the present work was to study the effects of DAT on the proliferation of murine spleen cells. DAT was purified from reference strains of M. tuberculosis, or M. fortuitum as a surrogate source of the compound, by various chromatography and solvent extraction procedures and then chemically identified. Incubation of mouse spleen cells with DAT inhibited in a dose-dependent manner concanavalin A-stimulated proliferation of the cells. Experiments, including the propidium iodide exclusion test, showed that these effects were not due to death of the cells. Tracking of cell division by labeling with 5,6-carboxyfluorescein diacetate succinimidyl ester revealed that DAT reduces the rounds of cell division. Immunofluorescence with an anti-CD3 monoclonal antibody indicated that T lymphocytes were the population affected in our model. Our experiments also suggest that the extent of the suppressive activity is strongly dependent on the structural composition of the acyl moieties in DATs. Finally, the inhibitory effect was also observed on antigen-induced proliferation of mouse spleen cells specific for Toxoplasma gondii. All of these data suggest that DAT could have a role in the T-cell hyporesponsiveness observed in chronic tuberculosis.

Synthesis and Biological Evaluation of Nonylprodigiosin and Macrocyclic Prodigiosin Analogues

Nonylprodigiosin (4) and various of its analogues have been prepared by Suzuki cross-coupling reactions of a well accessible pyrrolyl triflate with (hetero)aryl boronic acid derivatives bearing alkenyl side chains. The resulting alkenes or dienes were subjected to metathesis dimerization or ring-closing metathesis (RCM) reactions, respectively, by using a ruthenium indenylidene complex as the catalyst. The biological activity of the products thus obtained was tested in two different assays monitoring i) the proliferation of murine spleen cells induced by lipopolysaccharides (LPS) and concanavalin A (Con A), and ii) the vacuolar acidification of baby hamster kidney (BHK) cells. Compounds 4 and 21 suppressed Con A-induced T-cell proliferation much more potently than LPS-induced B-cell proliferation. Furthermore, compounds 4 and 26 markedly inhibited vacuolar acidification, although other compounds exhibited no or only marginal effects. Thus, the immunosuppressive activity of prodigiosins toward T-cell proliferation seems to be mediated through cellular targets distinct from vacuolar acidification, and the prodigiosin analogues might be powerful tools to dissect these biological responses. The X-ray crystal structure of the macrocyclic product 25 has been determined, showing that the replacement of one pyrrole ring of the parent compound 4 by a phenyl group does not alter the overall electronic features of the remaining heterocyclic ring system of these alkaloids.

Synthesis and biological evaluation of nonylprodigiosin and macrocyclic prodigiosin analogues.

Nonylprodigiosin (4) and various of its analogues have been prepared by Suzuki cross-coupling reactions of a well accessible pyrrolyl triflate with (hetero)aryl boronic acid derivatives bearing alkenyl side chains. The resulting alkenes or dienes were subjected to metathesis dimerization or ring-closing metathesis (RCM) reactions, respectively, by using a ruthenium indenylidene complex as the catalyst. The biological activity of the products thus obtained was tested in two different assays monitoring i) the proliferation of murine spleen cells induced by lipopolysaccharides (LPS) and concanavalin A (Con A), and ii) the vacuolar acidification of baby hamster kidney (BHK) cells. Compounds 4 and 21 suppressed Con A-induced T-cell proliferation much more potently than LPS-induced B-cell proliferation. Furthermore, compounds 4 and 26 markedly inhibited vacuolar acidification, although other compounds exhibited no or only marginal effects. Thus, the immunosuppressive activity of prodigiosins toward T-cell proliferation seems to be mediated through cellular targets distinct from vacuolar acidification, and the prodigiosin analogues might be powerful tools to dissect these biological responses. The X-ray crystal structure of the macrocyclic product 25 has been determined, showing that the replacement of one pyrrole ring of the parent compound 4 by a phenyl group does not alter the overall electronic features of the remaining heterocyclic ring system of these alkaloids.

Defensins act as potent adjuvants that promote cellular and humoral immune responses in mice to a lymphoma idiotype and carrier antigens.

Defensins released by neutrophils are able to kill a broad spectrum of microbes. They also induce leukocyte migration in vitro and elicit inflammatory leukocyte responses at s.c. injection sites in mice. In vitro experiments showed that human defensins enhanced concanavalin A-stimulated murine spleen cell proliferation and IFN-gamma production. This led us to examine the effects of human defensins on specific immune responses in vivo. BALB/c mice were immunized with 50 microg of keyhole limpet hemocyanin (KLH) adsorbed to aluminum hydroxide and administered with defensins in aqueous solution. Intraperitoneal administration of defensins significantly increased the production of KLH-specific IgG1, IgG2a and IgG2b antibodies 14 days after immunization. In vitro splenic KLH-specific proliferative responses were higher in mice treated with KLH and defensins than in those treated with KLH alone. Increased IFN-gamma and, to a lesser extent, IL-4 production were also detected in the supernatants of ex vivoKLH-activated spleen cells from mice treated with defensins. Finally, defensins significantly enhanced the antibody response to a syngeneic tumor antigen, lymphoma Ig idiotype and also augmented resistance to tumor challenge. These results indicate that defensins act as potent immune adjuvants by inducing the production of lymphokines, which promote T cell-dependent cellular immunity and antigen-specific Ig production. Thus, defensins appear to function as neutrophil-derived signals that promote adaptive immune responses.

Syntheses of biotinylated alpha-galactosylceramides and their effects on the immune system and CD1 molecules.

A representative alpha-galactosylceramide (alpha-GalCer), KRN7000, has strong immunostimulatory and antitumor activity. Recent studies demonstrated that KRN7000-pulsed antigen-presenting cells (APC) can activate natural killer T (NKT) cells, a novel T-cell lineage, and CD1d molecules on APC play an important role in the activation of NKT cells. However, it remains unclear whether alpha-GalCers actually bind to CD1d molecules. To address this question, we synthesized three kinds of biotinylated alpha-GalCer and a biotinylated beta-GalCer and found that the biotinylated alpha-GalCers significantly stimulate the proliferation of murine spleen cells, but the biotinylated beta-GalCer does not and that all biotinylated compounds bind to CD1d molecules.

Effects of 3α- and 3β-galactosylated α-g galactosylceramides on the immune system

We compared effects of α-galactosylceramide (α-GalCer) and its 3α- or 3β-galactosylated derivatives on the proliferation of murine spleen cells. The 3α-galactosylated α-GalCer showed stongger proliferative response than the parental α-GalCer, but the 3β-galactosylated α-GalCer possessed weaker activity than the α-GalCer. In addition, α-Gal-3-Cer did not show immunostimulatory activity.

Immunostimulatory activities of mono- or diglycosylated α-galactosylceramides

We examined the effects of 2″- or 3″-monoglycosylated α-galactosylceramides (α-GalCers) and 2″,3″-diglycosylated α-GalCers on allogeneic MLR and the proliferation of murine spleen cells. It was found that their ceramide portions greatly affect their immunostimulatory activities, and that the 3″-hydroxyl group plays a more important role in the immunostimulatory effects of α-GalCer derivatives than the 2″-hydroxyl group.

2-amino-3-methylimidazo[4,5-f]quinoline-mediated immunosuppression involves inhibition of protein kinase C in murine splenocytes.

The effects of the food-borne mutagenic and carcinogenic heterocyclic amine, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), on the proliferation of murine spleen cells was examined. A significant and dose-related suppression of the proliferative response to concanavalin A (Con A) was observed by the treatment of IQ. IQ also decreased markedly the level of proliferative response induced by PMA and ionomycin and the IL-2 secretion by T cells. Treatment of PMA resulted in the recovery of IQ-induced suppression of IL-2 production, but the addition of ionomycin (Io) had no effect. IQ decreased PKC activity in both the membrane fraction and the cytosol fraction and inhibited the phosphorylation of MARCKS protein. These results suggest that the suppression of spleen cell proliferation induced by IQ might be associated with the inhibition of PKC activity and the subsequent IL-2 secretion of T cells.

Purified protein from Salmonella typhimurium inhibits the interleukin-2 response of murine splenic T-lymphocytes activated with anti-CD3 antibody.

Previously, we demonstrated that the immunosuppression induced by a purified preparation of Salmonella typhimurium-derived inhibitor of T-cell proliferation (STI) can be observed in terms of suppression of the proliferation of murine spleen cells stimulated with a mitogenic lectin. In the present study, I observed that STI inhibited the interleukin-2 (IL-2) response of purified murine splenic T lymphocytes stimulated with anti-CD3 antibody. The flow cytometric analysis of IL-2 receptor (IL-2R) expression on T cells showed that STI specifically suppressed the expression of IL-2R beta and IL-2R gamma. Furthermore, when the IL-2-dependent T-cell line CTLL-2 was incubated with STI, the growth of CTLL-2 cells was significantly inhibited. These results suggest that the target cells for STI are T cells themselves, and that the suppression of T-cell proliferation induced by STI might involve a defect in the IL-2 receptor (IL-2R) function of T cells.

A purified protein from Salmonella typhimurium inhibits proliferation of murine splenic anti-CD3 antibody-activated T-lymphocytes

In a previous study, we observed that the purified substance Salmonella typhimurium-derived inhibitor of T-cell proliferation (STI) had an immunosuppressive effect, demonstrated as the suppression of mitogenic lectin-induced proliferation of murine spleen cells. In the present study, we confirmed the immunosuppressive effect of STI, which suppressed the proliferation of murine splenic T-lymphocytes activated with the anti-CD3 antibody (Ab) and phorbol 12-myristate-13 acetate (PMA) and this phenomenon was accompanied by augmentation of interferon-γ (IFN-γ) secretion and inhibition of interleukin-2 (IL-2) secretion. Furthermore, the augmentation of IFN-γ secretion caused IL-2 receptor α chain (IL-2R α) over expression on T-cells. However, the addition of an anti-IFN-γ Ab and recombinant IL-2 (rIL-2) did not reverse the suppressed T-cell proliferation, although the level of IL-2R α expression on T-cells recovered to around normal. Furthermore, Western blotting using an anti-phosphotyrosine Ab showed that IL-2R-mediated tyrosine phosphorylation of protein substrates in T-cells was inhibited by incubation with STI for 48 h and this inhibition was not reversed by adding the anti-IFN-γ Ab and rIL-2. These results suggest that STI-induced suppression of T-cell proliferation involves a defect in IL-2R function and/or IL-2 signaling pathway in T-cells.

A purified protein from Salmonella typhimurium inhibits proliferation of murine splenic anti-CD3 antibody-activated T-lymphocytes.

In previous study, we observed that the purified substance Salmonella typhimurium-derived inhibitor of T-cell proliferation (STI) had an immunosuppressive effect, demonstrated as the suppression of mitogenic lectin-induced proliferation of murine spleen cells. In the present study, we confirmed the immunosuppressive effect of STI, which suppressed the proliferation of murine splenic T-lymphocytes activated with the anti-CD3 antibody (Ab) and phorbol 12-myristate-13 acetate (PMA) and this phenomenon was accompanied by augmentation of interferon-gamma (IFN-gamma) secretion and inhibition of interleukin-2 (IL-2) secretion. Furthermore, the augmentation of IFN-gamma secretion caused IL-2 receptor alpha chain (IL-2R alpha) over expression on T-cells. However, the addition of an anti-IFN-gamma Ab and recombinant IL-2 (rIL-2) did not reverse the suppressed T-cell proliferation, although the level of IL-2R alpha expression on T-cells recovered to around normal. Furthermore, Western blotting using an anti-phosphotyrosine Ab showed that IL-2R-mediated tyrosine phosphorylation of protein substrates in T-cells was inhibited by incubation with STI for 48 h and this inhibition was not reversed by adding the anti-IFN-gamma Ab and rIL-2. These results suggest that STI-induced suppression of T-cell proliferation involves a defect in IL-2R function and/or IL-2 signaling pathway in T-cells.

Tepoxalin, a novel immunomodulatory compound, synergizes with CsA in suppression of graft-versus-host reaction and allogeneic skin graft rejection.

Tepoxalin, a dual 5-lipoxygenase and cyclooxygenase inhibitor with nonsteroidal antiinflammatory effects, has recently been shown to suppress NF kappa B transactivation and inhibit T cell proliferation via a mechanism very different from cyclosporine (CsA). In this report, we demonstrate that this novel immunosuppressive effect of tepoxalin is manifested in in vivo transplantation models. Tepoxalin suppressed murine spleen cell proliferation in a mixed lymphocyte reaction (MLR) with an IC50 of 1.3 microM. Coadministration of tepoxalin and CsA in MLR cultures showed an additive inhibitory effect. Oral administration of tepoxalin at 12 mg/kg/day to mice suppressed local graft-versus-host (GVH) responses by about 40% (n = 10). Combination of tepoxalin and CsA at suboptimal doses synergized their immunosuppressive effects on GVH responses (n = 20). In skin transplantation, the median survival time of allogeneic BALB/cByJ (H-2d) mouse skin grafted onto C3H/HeJ (H-2 kappa) mice was 10.5 days (n = 8), and was prolonged to 15.0 days (n = 9) for recipient mice administered tepoxalin at 50 mg/kg/day. Coadministration of suboptimal doses of tepoxalin (12.5 mg/kg/day) and CsA (50 mg/kg/day) prolonged skin graft rejections dramatically (55% of the grafts survived for more than 40 days, n = 9). Taken together, these results demonstrate that tepoxalin is a potent immunomodulatory compound that, when combined with CsA, provides synergistic immunosuppressive activity. The fact that tepoxalin and CsA act on different transcription factors, NF kappa B and NFAT respectively, might explain the synergistic suppressive effects when both compounds were used. Tepoxalin could be an important addition to the cohort of immunosuppressive therapies currently used in solid organ and bone marrow transplantations.

Immunosuppreession induced bySalmonellainfection is correlated with augmentation of interleukin-2 receptor α chain expression in murine splenic lymphocytes

In a previous study, we observed that the suppression of T-cell proliferation induced by Salmonella cell-free extract was associated with augmentation of IL-2 receptor (IL-2R) alpha chain expression. In this study, we also observed this kind of augmentation of IL-2R alpha in Salmonella-infected mice. Phytohaemagglutinin (PHA)-stimulated proliferation of murine spleen cells was significantly suppressed when the mice were infected with Salmonella typhimurium. However, expression of the alpha chain but not the beta chain of IL-2R in lymphocytes was augmented by the infection. Analysis of the IL-2R-positive cell-population showed that the augmentation of IL-2R alpha was not specific to certain cell subpopulations. Furthermore, the inhibition of PHA-stimulated murine spleen cell proliferation and the augmentation of IL-2R alpha expression induced by the infection in lymphocytes was completely reversed by treatment with anti-interferon-gamma monoclonal antibody (anti-IFN-gamma Ab). These results suggest that the suppression of T-cell proliferation induced by Salmonella infection was associated with augmentation of IL-2R alpha expression in an IFN-gamma production-dependent manner in the same way as the suppression of T-cell proliferation induced by Salmonella cell-free extract.

Immunosuppreession induced by Salmonella infection is correlated with augmentation of interleukin-2 receptor α chain expression in murine splenic lymphocytes

In a previous study, we observed that the suppression of T-cell proliferation induced by Salmonella cell-free extract was associated with augmentation of IL-2 receptor (IL-2R) α chain expression. In this study, we also observed this kind of augmentation of IL-2Rα in Salmonella-infected mice. Phytohaemagglutinin (PHA)-stimulated proliferation of murine spleen cells was significantly suppressed when the mice were infected with Salmonella typhimurium. However, expression of the α chain but not the β chain of IL-2R in lymphocytes was augmented by the infection. Analysis of the IL-2R-positive cell-populylation showed that the augmentation of IL-2Rα was not specific to certain cell subpopulations. Furthermore, the inhibition of PHA-stimulated murine spleen cell proliferation and the augmentation of IL-2Rα expression induced by the infection in lymphocytes was completely reversed by treatment with anti-interferon-γ monoclonal antibody (anti-IFN-γ Ab). These results suggest that the suppression of T-cell proliferation induced by Salmonella infection was associated with augmentation of IL-2Rα expression in an IFN-γ production-dependent manner in the same way as the suppression of T-cell proliferation induced by Salmonella cell-free extract.

Inhibition of T-cell proliferation induced by a cell-free Salmonella typhimurium extract does not involve a nitric oxide-mediated mechanism.

In a previous study, we observed that a cell-free Salmonella typhimurium extract induced suppression of mitogen-induced T-cell proliferation and that this suppression involved nonresponsiveness of T-cells to interleukin-2 (IL-2) and augmentation of IL-2 receptor (IL-2R) expression. In this study, we found that inhibition of phytohemagglutinin (PHA)-stimulated murine spleen cell proliferation induced by a cell-free S. typhimurium extract was reversed by treatment with an anti-interferon-gamma monoclonal antibody (anti-IFN-gamma Ab), but not by interleukin-4 or NG-monomethyl-L-arginine, which is known to inhibit nitric oxide (NO)-secretion from spleen cells in culture. However, IL-2R expression was augmented by treatment with the extract, although this was independent of an NO-mediated mechanism. Only anti-IFN-gamma Ab treatment reduced the augmented IL-2R expression to a normal level. These results suggest that the suppression of T-cell proliferation induced by the Salmonella cell-free extract is associated with augmentation of IL-2R expression in an NO production-independent manner.

A cell-free extract ofSalmonella typhimuriuminhibits mitogen-induced proliferation of murine splenic T lymphocytes

In this study, a cell-free extract of Salmonella inhibited T cell mitogen-induced proliferation of spleen cells from non-immunized mice. The proliferation of murine spleen cells stimulated with a T cell mitogen, such as phytohaemagglutinin (PHA) or concanavalin A (ConA) was suppressed significantly when the cells were treated with a sonicate of S. typhimurium, but not of E. coli. The agent(s) responsible for the suppressive effect existed mainly in the soluble fraction of S. typhimurium, whereas the membrane fraction possessed minimal activity. The T cell proliferation suppression paralleled the level of interleukin-2 (IL-2) secretion. Addition of phorbol 12-myristate-13 acetate (PMA) to the cultures restored IL-2 secretion to normal levels, although proliferation remained suppressed and was not reversed by treatment with recombinant IL-2. These results suggest that the suppression of T cell proliferation induced by a soluble Salmonella fraction is associated with inhibition of IL-2 secretion and the response of T cells to IL-2 and the former effect is dependent upon the inhibition of the stimulatory activity of protein kinase C on IL-2 secretion. This type of suppression may explain a mechanism of immunosuppression induced by murine typhoid fever.