IL-33 and IgE stimulate mast cell production of IL-2 and regulatory T cell expansion in allergic dermatitis.

BACKGROUND: We have previously shown that mast cells (MCs) suppress chronic allergic dermatitis in mice. The underlying mechanism involves MC-derived IL-2, which supports regulatory T cell (Treg) response at the site of inflammation. However, it is not clear what are the factors that drive MCs to produce IL-2. OBJECTIVE: To understand the mechanisms that lead to IL-2 production from MCs in chronic allergic dermatitis. METHODS: Isolated murine bone marrow-derived MCs (BMMCs) were incubated with various stimulators, and IL-2 production was assessed by RT-PCR and ELISA. The response of signalling pathways was evaluated by MAPK inhibitors and Western blot analysis. The effect of MC-IL-2 on Tregs was studied by incubation of splenic T cells with conditioned media obtained from activated BMMCs. Dermatitis was elicited by repeated exposures of mouse ears to oxazolone. MCs in mouse and human skin samples were evaluated by immunostaining. RESULTS: BMMCs released IL-2 in response to IL-33, and IL-2 production was further enhanced by concomitant FcεRI activation. The effect of IL-33 was mediated by activation of the MAPK family members. IL-2 in conditioned media from IL-33 and IgE-stimulated BMMCs led to considerable expansion of Tregs in vitro. IL-33 levels were elevated in oxazolone-challenged ears along with increased numbers of IL-2-expressing MCs. Human skin with chronic inflammation also contained IL-2-expressing MCs that colocalized with IL-33 staining in the dermis. CONCLUSIONS: IL-33, in collaboration with IgE, is critical for MC-IL-2 production in allergic skin disease, thus leading to Treg stimulation and suppression of allergic dermatitis.

AllergoOncology - the impact of allergy in oncology: EAACI position paper.

Th2 immunity and allergic immune surveillance play critical roles in host responses to pathogens, parasites and allergens. Numerous studies have reported significant links between Th2 responses and cancer, including insights into the functions of IgE antibodies and associated effector cells in both antitumour immune surveillance and therapy. The interdisciplinary field of AllergoOncology was given Task Force status by the European Academy of Allergy and Clinical Immunology in 2014. Affiliated expert groups focus on the interface between allergic responses and cancer, applied to immune surveillance, immunomodulation and the functions of IgE-mediated immune responses against cancer, to derive novel insights into more effective treatments. Coincident with rapid expansion in clinical application of cancer immunotherapies, here we review the current state-of-the-art and future translational opportunities, as well as challenges in this relatively new field. Recent developments include improved understanding of Th2 antibodies, intratumoral innate allergy effector cells and mediators, IgE-mediated tumour antigen cross-presentation by dendritic cells, as well as immunotherapeutic strategies such as vaccines and recombinant antibodies, and finally, the management of allergy in daily clinical oncology. Shedding light on the crosstalk between allergic response and cancer is paving the way for new avenues of treatment.

Reevaluation of reserpine-induced suppression of contact sensitivity. Evidence that reserpine interferes with T lymphocyte function independently of an effect on mast cells.

It has been suggested that reserpine blocks expression of delayed hypersensitivity (DH) by depleting tissue mast cells of serotonin (5-HT), thereby preventing a T cell-dependent release of mast cell 5-HT necessary to localize and to amplify the DH response. However, reserpine blocks expression of DH in mast cell-deficient mice. We therefore decided to reevaluate the mechanism by which reserpine abrogates expression of cellular immunity, and investigated whether the drug might interfere with T cell activity in vitro or in vivo. At concentrations as low as 4 microM, reserpine profoundly suppressed baseline or antigen-augmented levels of [3H]thymidine incorporation by immune lymph node cells obtained from mice sensitized to the contactant oxazolone [I-LNC(Ox)]. This effect was observed both with I-LNC derived from normal mice and with I-LNC derived from congenitally mast cell-deficient W/Wv mice, cell preparations that lacked detectable mast cells, histamine, and 5-HT. Furthermore, treatment of I-LNC with reserpine (20 microM) for 1 h in vitro virtually abolished the ability of these cells to transfer CS to naive mice. This was not a cytolytic effect, as the viability of the I-LNC treated with reserpine was not affected, and washing of the reserpine-treated I-LNC before transfer fully restored their ability to orchestrate a CS response. The action of the drug was not mediated by an effect on mast cells, since the experiment could be performed using mast cell-deficient W/Wv mice as both donors and recipients of I-LNC. In addition, the effect was specific for the treated cells: mice that received reserpine-treated I-LNC(Ox) intravenously together with untreated I-LNC(DNFB) did not develop CS to Ox but responded normally to DNFB; and local intradermal injection of reserpine-treated I-LNC(Ox) which failed to transfer reactivity to Ox, did not interfere with the development of CS to DNFB at the same site. Finally, cotransfer experiments indicated that the effect of reserpine on the transfer of CS was not due to activation of suppressor cells. Our findings strongly suggest that whatever effects reserpine might have on immunologically nonspecific host cells, the drug's effects on sensitized T cells are sufficient to explain its ability to block cell-mediated immune responses in vivo.

Synaptotagmin II negatively regulates Ca2+-triggered exocytosis of lysosomes in mast cells.

Synaptotagmins (Syts) I and II are believed to act as Ca2+ sensors in the control of neurotransmission. Here we demonstrate that mast cells express Syt II in their lysosomal fraction. We further show that activation of mast cells by either aggregation of FcepsilonRI or by Ca2+ ionophores results in exocytosis of lysosomes, in addition to the well documented exocytosis of their secretory granules. Syt II directly regulates lysosomal exocytosis, whereby overexpression of Syt II inhibited Ca2+-triggered release of the lysosomal processed form of cathepsin D, whereas suppression of Syt II expression markedly potentiated this release. These findings provide evidence for a novel function of Syt II in negatively regulating Ca2+-triggered exocytosis of lysosomes, and suggest that Syt II-regulated secretion from lysosomes may play an important role in mast cell biology.

Interleukin-6 and soluble intercellular adhesion molecule-1 in acute brain ischaemia.

Inflammation plays a critical role in the pathogenesis of atherothrombosis. Our aim was to examine the association between plasma concentrations of inflammatory biomarkers and severity and outcome of acute brain ischaemia. Plasma samples were collected within 36 h of symptom onset in patients with acute brain ischaemia, and assessed by conventional ELISA kits for concentration of interleukin-6 (IL-6) and soluble intercellular adhesion molecule-1 (sICAM-1). Patients were assessed serially for stroke severity (National Institute of Health stroke scale) and outcome during follow-up (modified Rankin Scale, mRS; and Stroke Impact Scale-16, SIS). Patients (n = 113, 65% men, mean age 64 +/- 12 years) had a mean IL-6 concentrations of 5.1 +/- 5.0 pg/ml and sICAM-1 of 377 +/- 145 ng/ml. IL-6, but not sICAM-1, concentrations were strongly associated with stroke severity (P < 0.01 at all serial assessments). Ln-transformed IL-6 levels (per 1 SD) were associated with disability (mRS > or = 2, OR = 1.7; 95% CI 1.1-3.0) and poor physical function (SIS < or = 85, OR = 1.7; 95% CI 1.0-2.8). Further adjustment for baseline stroke severity, however, eliminated these associations. Our results suggest that high plasma concentrations of the inflammatory biomarker IL-6 but not sICAM-1 are associated with stroke severity and poorer functional outcome. IL-6 does not add, however, additional prognostic information for stroke outcome beyond that conveyed by the stroke severity.

Glucocorticoids decrease tissue mast cell number by reducing the production of the c-kit ligand, stem cell factor, by resident cells: in vitro and in vivo evidence in murine systems.

The local delivery of glucocorticoids to tissues significantly decreases mast cell number. This pharmacologic effect of glucocorticoids is believed to be one of the mechanisms by which glucocorticoids regulate allergic inflammation. To determine the mechanism by which glucocorticoids are able to exert this effect, we first applied the glucocorticoid fluocinonide to mouse dermis and observed that the decrease in mast cell number was associated with an increase in mast cell apoptosis. This did not appear to be due to a direct effect of the glucocorticoid on mast cells, as the addition of 0.01-1.0 microM of the glucocorticoid dexamethasone into stem cell factor (SCF)-dependent mast cell cultures did not enhance mast cell death. However, addition of dexamethasone to cultured fibroblasts did result in a downregulation of SCF mRNA and a significant decrease in SCF protein production. Similarly, immunohistochemistry performed on fluocinonide-treated mouse dermis revealed a decrease in immunoreactive SCF. Administration of SCF at sites of fluocinonide administration to the dermis abolished the mast cell-depleting effect of this glucocorticoid. Thus, glucocorticoids decrease tissue mast cell number by downregulating tissue SCF production required for the survival of local mast cells. This observation may be applicable to the design of improved strategies to treat mast cell-mediated disorders.

Suppression of experimental autoimmune diseases and prolongation of allograft survival by treatment of animals with low doses of heparins.

The ability of activated T lymphocytes to penetrate the extracellular matrix and migrate to target tissues was found to be related to expression of a heparanase enzyme (Naparstek, Y., I. R. Cohen, Z. Fuks, and I. Vlodavsky. 1984. Nature (Lond.). 310:241-243; Savion, N., Z. Fuks, and I. Vlodavsky. 1984. J. Cell. Physiol. 118:169-176; Fridman, R., O. Lider, Y. Naparstek, Z. Fuks, I. Vlodavsky, and I. R. Cohen. 1987. J. Cell. Physiol. 130:85-92; Lider, O., J. Mekori, I. Vlodavsky, E. Baharav, Y. Naparstek, and I. R. Cohen, manuscript submitted for publication). We found previously that heparin molecules inhibited expression of T lymphocyte heparanase activity in vitro and in vivo, and administration of a low dose of heparin in mice inhibited lymphocyte traffic and delayed-type hypersensitivity reactions (Lider, O., J. Mekori, I. Vlodavsky, E. Baharav, Y. Naparstek, and I. R. Cohen, manuscript submitted for publication). We now report that treatment with commercial or chemically modified heparins at relatively low doses once daily (5 micrograms for mice and 20 micrograms for rats) led to inhibition of allograft rejection and the experimental autoimmune diseases adjuvant arthritis and experimental autoimmune encephalomyelitis. Higher doses of the heparins were less effective. The ability of chemically modified heparins to inhibit these immune reactions was associated with their ability to inhibit expression of T lymphocyte heparanase. There was no relationship to anticoagulant activity. Thus heparins devoid of anticoagulant activity can be effective in regulating immune reactions when used at appropriate doses.

Inhibition of T cell adhesion to extracellular matrix glycoproteins by histamine: a role for mast cell degranulation products.

Mast cells, which are capable of releasing a multitude of preformed and newly generated biological mediators and cytokines, are involved in various inflammatory processes. We studied whether histamine, a mast cell degranulation product, influences the adhesive interactions of T cells with extracellular matrix (ECM) glycoproteins, an event that occurs at sites of inflammation and is mediated primarily by virtue of cell-surface receptors of the beta 1-integrin subfamily. A prerequisite of lymphocyte-ECM interactions is activation of the cells, which modulates the affinity of the otherwise inactive integrins. Isolated rat CD4+ T cells were preincubated with histamine and activated with phorbol myristate acetate (PMA), and their ability to adhere to immobilized ECM components (fibronectin and laminin) was determined. Preincubation with histamine resulted in a 40-50% decrease in the adhesion of the CD4+ cells to both fibronectin or laminin. The notion that inhibition of T cell adhesion to ECM proteins by histamine-induced increase of the cells' intracellular levels of cAMP, thus interfering with calcium influx-associated events that occur during T cell activation, is supported by the finding that T cell adhesion was also abrogated by pharmacological inducers of cAMP. When the T cells were preincubated with supernatants of immunologically activated mast cells and then activated with PMA, a 40-50% inhibition of their adhesion to fibronectin or laminin was also observed. The inhibitory moiety present in the mast cell degranulation supernatants was resistant to heat (80 degrees C). Histamine exerted its suppressive effect on adhesion of T cells via their H2 receptors, as pretreatment with H2 antagonists abrogated the inhibitory effect. Thus, both purified histamine and mast cell-secreted histamine appear to be capable of affecting T cell interactions with ECM.

Activated T lymphocytes induce degranulation and cytokine production by human mast cells following cell-to-cell contact.

Activated mast cells reside in close apposition to T cells in some inflammatory processes. In this study, we analyzed whether this close physical proximity affects human mast cell degranulation and cytokine release. Thus HMC-1 human mast cells or primary bone marrow-derived human mast cells were cocultured with activated and with resting T cells. Mast cells cocultured with activated T cells released histamine and beta-hexosaminidase and produced tumor necrosis factor alpha (TNF-alpha), an effect that peaked at 20 h. Kinetics of histamine release paralleled the formation of heterotypic aggregates. Separation of the two cell populations with a porous membrane prevented mediator release and TNF-alpha production. Addition of the PI3-kinase inhibitor, wortmannin, inhibited the heterotypic adhesion-associated degranulation but not TNF-alpha production. These data thus indicate a novel pathway through which human mast cells are activated to both release granule-associated mediators and to produce cytokines in association with heterotypic adhesion to activated human T cells.

Osteoporosis as the sole presentation of bone marrow mastocytosis.

Three (young) adults with severe generalized osteopenia and vertebral compression fractures were studied. Extensive clinical and laboratory investigations were not contributory. Undecalcified bone biopsies demonstrated multiple mast cell granulomas in the marrow in two patients and numerous mast cells diffusely distributed throughout the bone marrow in the third patient. Mast cells may serve as a pathogenic agent in osteoporosis. Therefore, we conclude that isolated skeletal mastocytosis without clinical evidence of mast cell mediator release should be sought in the evaluation of a patient with unexplained severe bone loss.

Increased cytosolic free calcium in lymphocytes of Alzheimer patients.

Free cytosolic calcium content [Ca2+]i was determined in peripheral blood mononuclear cells (PBMC) from healthy volunteers, Alzheimer's disease and multi-infarct dementia patients. Measurement of [Ca2+]i by the fluorescent dye quin-2, before and at several time intervals during incubation with phytohemagglutinin (PHA), showed a higher resting [Ca2+]i in PBMC of Alzheimer's disease patients as compared to controls and multi-infarct dementia patients. However, the addition of supra-optimal PHA doses (100 micrograms/ml) induced strikingly higher [Ca2+]i levels in Alzheimer's disease patients (1647 +/- 200 nM versus 398 +/- 27 nM in controls, and 346 +/- 40 nM in multi-infarct dementia patients). The increased [Ca2+]i concentration was also found after a specific stimulation with a monoclonal anti-CD3 antibody. The results may have important implications in understanding the pathophysiology of Alzheimer's disease and suggest that [Ca2+]i may prove diagnostically valuable.

Secretion of stem cell factor by alveolar fibroblasts in interstitial lung diseases.

Sarcoidosis (SA) and diffuse interstitial fibrosis (DIF) are characterized by alveolitis, mast cell hyperplasia and increased fibroblast proliferation. Stem cell factor (SCF) stimulates proliferation of hematopoietic progenitor cells involved in mast and stromal cell interaction. We assessed the role of SCF secreted by alveolar fibroblasts (AFb) in the development of fibrosis of DIF and SA in six patients with SA and six patients with DIF. Bronchoalveolar lavage (BAL) was performed by conventional methods. A total of 500 cells were differentially counted from Giemsa-stained cytopreps. AFb and supernatants were recovered from long-term cultures of BAL cells and from 24 h cultures of confluent AFb. Levels of SCF were measured by ELISA. Alpha actin content of AFb was characterized by immunohistochemistry. The expression of AFb mRNA for IL1-alpha and beta, TGF-beta, IFN-gamma, IL-2, IL-4, IL-5 and IL-6 was determined by RT-PCR. There was a lymphocytic predominance in the SA patients and an increase in neutrophils and eosinophils in DIF. SCF secreted by AFb from DIF was significantly higher than in SA. TNF + IL-1 significantly decreased the secretion of SCF by AFb. There was a positive correlation between SCF levels and the percentage eosinophils but not for metachromatic cells. Alpha-actin expression of AFb in DIF was significantly higher than in SA. Cytokine mRNA was extracted from AFb of two SA and two DIF patients. The profile showed that only in stimulated AFb isolated from the DIF patients can IL-5 transcripts be visualized. In conclusion, AFb can contribute to the onset of fibrosis by secreting SCF and IL-5 which, in turn, may recruit eosinophils.

Morphological evidence for chronic mast cell activation after prolonged exposure with supernatants from chronic graft-versus-host splenocytes.

Chronic graft-vs.-host disease (cGVHD) includes a syndrome of inflammatory and fibrotic changes in some respects resembling scleroderma. In the present study we have quantitated the number of peritoneal mast cells (MC) in mice with cGVHD induced across minor histocompatibility barriers. MC were evaluated by staining with toluidine blue. The number of MC decreased significantly (by 25%) at the onset of the cGVHD fibrosis (day 12). Around day 35, MC were virtually undetectable, and started to reappear on day 130. Upon clinical recovery (day 200) a dramatic increase in MC numbers was found (about 8-fold). In addition, we evaluated by electron microscopy the morphology of peritoneal MC, obtained from normal mice and rats, that had been co-cultured on 3T3 fibroblast monolayer in the presence of splenocyte supernatants from mice with cGVHD or from control mice. After 6-8 days of continuous incubation with the cGVHD splenocyte supernatant, MC appeared to be activated, since they displayed an array of heterogenous granules. Few of the granules were dense; many were swollen and pale. Rare granule extrusion was evident. This would indicate that MC underwent a slow activation process due to a factor(s) present in the cGVHD supernatant, different from the classical acute anaphylactic activation.

Prostaglandin E2 production by chronic graft-versus-host disease dermal fibroblasts.

Chronic graft versus host disease (cGVHD) across minor histocompatibility barriers is associated with the development of cutaneous fibrosis, disappearance of mast cells and immunosuppression. The idea, which has been the basis of our previous and present studies, is that fibroblasts are not only a target for modulation in cGVHD, but also have effector roles in this condition. In the present study we investigated the production of prostaglandin E2 (PGE2) and of collagen by cultured dermal fibroblasts obtained from cGVHD and control mice. Early in the development of the disease (Day 8) cGVHD fibroblasts generated constitutively more PGE2 (3-fold) than did control fibroblasts. Thereafter, PGE2 production declined to near normal levels by Day 20 post cGVHD induction. On the other hand, at this time point cGVHD fibroblasts displayed an enhanced synthesis of collagen as compared to the control fibroblasts and to earlier time points. Therefore, PGE2 synthesis appears to inversely correlate with collagen synthesis by cGVHD fibroblasts. We propose that fibroblasts may contribute to the development of immunosuppression, which characterizes the early phase of cGVHD.

1,25-Dihydroxyvitamin D3 enhances degranulation of mast cells.

The mast cell lines rat basophilic leukemia (RBL) and mouse C57 cells respond to IgE/antigen complexes by degranulation. Treatment of these cells with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), (10-100 nM) for 24-48 h enhanced IgE/antigen-induced exocytosis as monitored by release of hexosaminidase. A short term incubation with the hormone did not affect exocytosis, ruling out a rapid non genomic mechanism. The presence of vitamin D receptors, demonstrated by immunoblotting and the lack of effect of 24,25(OH)2D3 suggest a role for these receptors in the enhancing effect. 1,25(OH)2D3 also enhanced exocytosis induced by the calcium ionophore A23187 in the presence or absence of phorbol ester indicating modulation of events distal to signal transduction. 1,25(OH)2D3 enhanced exocytosis in the presence of cytochalasin D, indicating that the action of the hormone is not due to effects on microfilament structure. The results of this study suggest that 1,25(OH)2D3 may affect the allergic or pro-inflammatory potential of mast cells.

Aberrant crypt foci in human colons: distribution and histomorphologic characteristics.

Aberrant crypt foci (ACF) are one of the earliest putative preneoplastic, and in some cases, neoplastic lesions in human colons. These microscopic lesions, identified on methylene blue-stained mucosa with a low-power-magnification microscope, are thought to be closely related to the earliest steps in multistage colonic tumorigenesis. We investigated the distribution pattern and histomorphological features of ACF in 74 patients with sporadic colorectal cancer. The distribution pattern shows a slightly higher prevalence with older age. The prevalence of the ACF in sigmoid colon was significantly higher in patients with colorectal cancer as compared with patients with benign colonic diseases. Also, significantly more ACF were detected in distal parts of the large bowel (descending, sigmoid colon, and rectum) than in proximal parts. Of 42 microdissected lesions, 12 were dysplastic and 30 were hyperplastic foci. The average size of dysplastic lesions was significantly larger than hyperplastic foci. More apoptotic bodies were found in dysplastic lesions. These lesions also showed an upward expansion of proliferative compartment and higher proliferation indices expressed as proliferating cell nuclear antigen-labeling index. Lymphoid follicles were frequently observed in the base of both hyperplastic and dysplastic foci (40% and 66.6%, respectively). The coincidence of lymphoid follicles was 2.5 to 8 times higher than expected. These features may be related to further progression of selected ACF during colorectal tumorigenesis.

Lymphoid tissues and the immune system in mastocytosis.

The association of mast cells and lymphoid tissues may reflect either regional overproduction of growth factors for mast cells or a predisposition for mast cells at certain sites within the body, particularly the liver, lymph nodes, and spleen. The significant increase in mast cell number associated with mastocytosis is not sufficient to generate a change in either T-cell or B-cell functions, as evaluated by analyzing cytokine phenotype or immunoglobulin production, respectively, nor to expose these patients to infections or allergic diseases. Mast cells in mastocytosis cannot be said with certainty to be "normal" in all respects, however, and the failure to identify an effect of mast cells on either B-cell Ig production or T-cell cytokine profiles cannot be taken as absolute evidence that mast cell products have no influence on lymphocyte function, particularly at the local tissue level.

Delayed pressure urticaria histologically resembles cutaneous late-phase reactions.

In a recent study, all patients with delayed pressure urticaria (DPU) developed late cutaneous reaction (LCR) after intradermal injection of compound 48/80 and after skin testing with certain food antigens. In the present study, we analyzed the histologic changes in the pressure lesions and compared them with those found in normal skin injected with diluent and in LCR to 48/80. The study included five patients with DPU associated with chronic urticaria (CU) and four patients with CU but without DPU. Six to eight hours after pressure challenge and intradermal skin testing with 48/80 and diluent, skin biopsy specimens were obtained from the pressure lesions, the LCRs, and normal skin (diluent injection). Specimens were assessed by Giemsa staining of 1-micron sections and immunofluorescence of frozen sections. Total cells were counted in each specimen. Interstitial deposits of fibrin were observed by immunofluorescence in LCR and pressure lesions. The total numbers of infiltrating cells in the dermis among LCR sites and pressure lesions were not significantly different, while both LCR sites and pressure lesions contained significantly more infiltrating cells than did normal skin injected with saline diluent. The differential counts in LCR and DPU were mostly mononuclear cells. Infiltrates in the DPU and LCR were mostly perivascular. No histopathologic changes were seen at skin sites challenged with pressure in the control patients with CU without clinical manifestations of DPU. We conclude that lesions seen in DPU are morphologically similar to classic LCR.

Depressed IL-1 production by chronic GVHD dermal fibroblasts.

Chronic Graft-versus-Host disease (GVHD) is characterized by overt immunosuppression. In addition, the skin is a major anatomical site affected in chronic GVHD for reasons not yet known. Increased collagen deposition, a mononuclear cell infiltrate in the dermis as well as loss of fat and appendages, are observed in the skin. The inflammatory cytokine IL-1 was shown to affect fibroblast proliferation and secretory activities. In the present study, IL-1 generation by dermal fibroblasts, of chronic GVHD or control mice, was assessed. It was shown that two sequential signals are needed for IL-1 generation by dermal fibroblasts; priming by lymphokines/cytokines followed by a challenge with LPS. A variety of recombinant lymphokines and cytokines (G/M-CSF, IL-2, TNF, IL-1 beta and IFNs alpha, beta and gamma) were shown to be efficient in priming dermal fibroblasts for IL-1 generation. IL-1 activity in dermal fibroblasts, most probably of the IL-1 alpha species, was located in frozen-thawed cell lysates or associated to the cell membrane, though not secreted into the culture fluids. Dermal fibroblasts from chronic GVHD mice manifested a pronounced depression in IL-1 generation upon stimulation with exogenous lymphokines/cytokines and LPS. This was observed over a wide range of concentrations of lymphokines/cytokines and LPS. The depressed ability of chronic GVHD fibroblasts to generate IL-1 was pronounced even after few passages of the cells in vitro, and upon stimulation in culture outside the suppressive milieu of the animal.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunosuppression in vitro and in vivo by supernatants from murine lymphoma cell lines.

Supernatants from four murine B and one T-cell lymphoma cultures were tested for their suppressive effect on allogeneic response in vitro (MLR), on their effect on mitogenic response to ConA and LPS and on their effect in vivo on allogeneic stimulation of BALB/c mice by C57BL/6J spleen cells. Two out of four B-cell supernatants (WEHI-231 and 24-666) were also tested for their effect on induction of IL-2 like activity by ConA in BALB/c spleen cells and on the effect of their injection on subsequent allogeneic response of spleen cells from BALB/c mice immunized against C57BL/6J spleen cells. The supernatants from all lymphoma lines inhibited MLR but only supernatants from the B-cell lymphoma lines also inhibited mitogenic stimulation in vitro and allogeneic immunization. The two B-cell supernatants tested also inhibited induction of IL-2 like activity and decreased the ability of spleen cells from the injected mice to respond to allogeneic stimulation. Partial characterization of supernatants from WEHI-231 and 24-666 lymphoma cultures revealed the non-protein nature of the suppressor factor(s), indicated a molecular weight of less than 5 Kd and suggested a ganglioside or a sialyated glycoproteinic nature.