Binding, Neutralization and Internalization of the Interleukin-13 Antibody, Lebrikizumab

INTRODUCTION

IL-13 is the primary upregulated cytokine in atopic dermatitis (AD) skin and is the pathogenic mediator driving AD pathophysiology. Lebrikizumab, tralokinumab and cendakimab are therapeutic monoclonal antibodies (mAb) that target IL-13.

METHODS

We undertook studies to compare in vitro binding affinities and cell-based functional activities of lebrikizumab, tralokinumab and cendakimab.

RESULTS

Lebrikizumab bound IL-13 with higher affinity (as determined using surface plasma resonance) and slower off-rate. It was more potent in neutralizing IL-13-induced effects in STAT6 reporter and primary dermal fibroblast periostin secretion assays than either tralokinumab or cendakimab. Live imaging confocal microscopy was employed to determine the mAb effects on IL-13 internalization into cells via the decoy receptor IL-13Rα2, using A375 and HaCaT cells. The results showed that only the IL-13/lebrikizumab complex was internalized and co-localized with lysosomes, whereas IL-13/tralokinumab or IL-13/cendakimab complexes did not internalize.

CONCLUSION

Lebrikizumab is a potent, neutralizing high-affinity antibody with a slow disassociation rate from IL-13. Additionally, lebrikizumab does not interfere with IL-13 clearance. Lebrikizumab has a different mode of action to both tralokinumab and cendakimab, possibly contributing to the clinical efficacy observed by lebrikizumab in Ph2b/3 AD studies.

The blood proteomic signature of prurigo nodularis reveals distinct inflammatory and neuropathic endotypes: A cluster analysis

BACKGROUND

Prurigo nodularis (PN) is an extremely pruritic, chronic inflammatory skin disease. Little is known about systemic inflammation in PN.

OBJECTIVE

To characterize plasma inflammatory biomarkers in patients with PN and investigate the presence of disease endotypes.

METHODS

In this cross-sectional study, Olink proteomic analysis was performed on plasma samples from patients with PN (n = 29) and healthy controls (n = 18).

RESULTS

Patients with PN had increased levels of 8 circulating biomarkers compared to controls, including tumor necrosis factor, C-X-C Motif Chemokine Ligand 9, interleukin-12B, and tumor necrosis factor receptor superfamily member 9 (P < .05). Two PN clusters were identified in cluster 1 (n = 13) and cluster 2 (n = 16). Cluster 2 had higher levels of 25 inflammatory markers than cluster 1. Cluster 1 had a greater percentage of patients with a history of myelopathy and spinal disc disease compared with cluster 2 (69% vs 25%, P = .03). Patients in cluster 2 were more likely to have a history of atopy (38% in cluster 2 vs 8% in cluster 1, P = .09).

LIMITATIONS

Small sample size precludes robust subgroup analyses.

CONCLUSION

This study provides evidence of neuroimmune-biased endotypes in PN and can aid clinicians in managing patients with PN that are nonresponsive to traditional therapies.

Serum Interleukin-13 and Caspase 8 are Elevated in Prurigo Nodularis

Abstract is missing (Short communication)

Circulating plasma IL-13 and periostin are dysregulated type 2 inflammatory biomarkers in prurigo nodularis: A cluster analysis

Importance

Prurigo nodularis (PN) is a chronic heterogeneous inflammatory skin disease.

Objective

To elucidate which components of type 2 inflammation are dysregulated systemically in PN.

Design

Whole blood was obtained from PN patients with uncontrolled disease and control patients without pruritus. Plasma was assayed for IL-4, IL-5, IL-13, IgE, and periostin. ANOVA was utilized to compare PN and control patients and multiple-hypothesis adjusted p-value was calculated with the significance threshold at 0.05. Clustering was performed using K-means clustering.

Participants

PN patients (n = 29) and controls (n = 18) from Johns Hopkins Dermatology had similar age sex, and race distributions.

Results

Single-plex assays of the biomarkers demonstrated elevated circulating plasma IL-13 (0.13 vs. 0.006 pg/mL, p = 0.0008) and periostin (80.3 vs. 60.2 ng/mL, p = 0.012) in PN compared to controls. IL-4 (0.11 vs. 0.02 pg/mL, p = 0.30) and IL-5 (0.75 vs. 0.40 pg/mL, p = 0.10) were not significantly elevated, while IgE approached significance (1202.0 vs. 432.7 ng/mL, p = 0.08). Clustering of PN and control patients together revealed two clusters. Cluster 1 (n = 36) consisted of 18 PN patients and 18 controls. Cluster 2 (n = 11) consisted entirely of PN patients (p < 0.01). Cluster 2 had higher levels of IL-13 (0.33 vs. 0.008 pg/mL, p = 0.0001) and IL-5 (1.22 vs. 0.43 pg/mL, p = 0.03) compared to cluster 1.

Conclusion and relevance

This study demonstrates elevation of IL-13 and periostin in the blood of PN patients, with distinct clusters with varying degrees of type 2 inflammation. Given this heterogeneity, future precision medicine approaches should be explored in the management of PN.

Generation and Characterization of Torudokimab (LY3375880): A Monoclonal Antibody That Neutralizes Interleukin-33

Background

Interleukin-33 (IL-33) is an alarmin that is released following cellular damage, mechanical injury, or necrosis. It is a member of the IL-1 family and binds to a heterodimer receptor consisting of ST2 and IL-1RAP to induce the production of a wide range of cellular mediators, including the type 2 cytokines IL-4, IL-5 and IL-13. This relationship has led to the hypothesis that the IL-33/ST2 pathway is a driver of allergic disease and inhibition of the IL-33 and ST2 association could have therapeutic benefit.

Methods

In this paper, we describe the selection of a phage antibody through the ability to bind human IL-33 and block IL-33/ST2 interaction. This hit antibody was then affinity matured by site-directed mutagenesis of the antibody complementarity-determining regions (CDRs). Further characterization of a fully human monoclonal antibody (mAb), torudokimab (LY3375880) included demonstration of human IL-33 neutralization activity in vitro with an NFκB reporter assay and IL-33 induced mast cell cytokine secretion assay, followed by an in vivo IL-33-induced pharmacodynamic inhibition assay in mice that used IL-5 production as the endpoint.

Results

Torudokimab is highly specific to IL-33 and does not bind any of the other IL-1 family members. Furthermore, torudokimab binds human and cynomolgus monkey IL-33 with higher affinity than the binding affinity of IL-33 to ST2, but does not bind mouse, rat, or rabbit IL-33. Torudokimab’s half-life in cynomolgous monkey projects monthly dosing in the clinic.

Conclusion

Due to torudokimab’s high affinity, its ability to completely neutralize IL-33 activity in vitro and in vivo, and the observed cynomolgus monkey pharmacokinetic properties, this molecule was selected for clinical development.

Interleukin-33 Contributes Toward Loss of Tolerance by Promoting B-Cell-Activating Factor of the Tumor-Necrosis-Factor Family (BAFF)-Dependent Autoantibody Production

Breaking tolerance is a key event leading to autoimmunity, but the exact mechanisms responsible for this remain uncertain. Here we show that the alarmin IL-33 is able to drive the generation of autoantibodies through induction of the B cell survival factor BAFF. A temporary, short-term increase in IL-33 results in a primary (IgM) response to self-antigens. This transient DNA-specific autoantibody response was dependent on the induction of BAFF. Notably, radiation resistant cells and not myeloid cells, such as neutrophils or dendritic cells were the major source of BAFF and were critical in driving the autoantibody response. Chronic exposure to IL-33 elicited dramatic increases in BAFF levels and resulted in elevated numbers of B and T follicular helper cells as well as germinal center formation. We also observed class-switching from an IgM to an IgG DNA-specific autoantibody response. Collectively, the results provide novel insights into a potential mechanism for breaking immune-tolerance via IL-33-mediated induction of BAFF.

Human mast cells release the migraine-inducing factor pituitary adenylate cyclase-activating polypeptide (PACAP)

Background Many patients with migraines suffer from allergies and vice versa, suggesting a relationship between biological mechanisms of allergy and migraine. It was proposed many years ago that mast cells may be involved in the pathophysiology of migraines. We set out to investigate the relationship between mast cell activation and known neurogenic peptides related to migraine. Methods Cultured human mast cells were assayed for the presence of neuropeptides and their receptors at the RNA and protein level. Immunohistochemistry analyses were performed on tissue resident and cultured mast cells. Mast cell degranulation assays were performed and pituitary adenylate cyclase-activating polypeptide (PACAP) activity was measured with a bioassay. Results We found that cultured and tissue resident human mast cells contain PACAP in cytoplasmic granules. No other neurogenic peptide known to be involved in migraine was detected, nor did mast cells express the receptors for PACAP or other neurogenic peptides. Furthermore, mast cell degranulation through classic IgE-mediated allergic mechanisms led to the release of PACAP. The PACAP released from mast cells was biologically active, as demonstrated using PACAP receptor reporter cell lines. We confirmed existing literature that mast cell degranulation can also be induced by several neurogenic peptides, which also resulted in PACAP release. Conclusion Our data provides a potential biological explanation for the association between allergy and migraine by demonstrating the release of biologically active PACAP from mast cells.

Elevated levels of Interleukin (IL)-33 induce bone pathology but absence of IL-33 does not negatively impact normal bone homeostasis

Interleukin (IL)-33 is a member of the IL-1 family. IL-33 effects are mediated through its receptor, ST2 and IL-1RAcP, and its signaling induces the production of a number of pro-inflammatory mediators, including TNFα, IL-1β, IL-6, and IFN-γ. There are conflicting reports on the role of IL-33 in bone homeostasis, with some demonstrating a bone protective role for IL-33 whilst others show that IL-33 induces inflammatory arthritis with concurrent bone destruction. To better clarify the role IL-33 plays in bone biology in vivo, we studied IL-33 KO mice as well as mice in which the cytokine form of IL-33 was overexpressed. Mid-femur cortical bone mineral density (BMD) and bone strength were similar in the IL-33 KO mice compared to WT animals during the first 8months of life. However, in the absence of IL-33, we observed higher BMD in lumbar vertebrae and distal femur in female mice. In contrast, overexpression of IL-33 resulted in a marked and rapid reduction of bone volume, mineral density and strength. Moreover, this was associated with a robust increase in inflammatory cytokines (including IL-6 and IFN-γ), suggesting the bone pathology could be a direct effect of IL-33 or an indirect effect due to the induction of other mediators. Furthermore, the detrimental bone effects were accompanied by increases in osteoclast number and the bone resorption marker of C-terminal telopeptide collagen-I (CTX-I). Together, these results demonstrate that absence of IL-33 has no negative consequences in normal bone homeostasis while high levels of circulating IL-33 contributes to pathological bone loss.

IL-33 released by alum is responsible for early cytokine production and has adjuvant properties

Human vaccines have used aluminium-based adjuvants (alum) for >80 years despite incomplete understanding of how alum enhances the immune response. Alum can induce the release of endogenous danger signals via cellular necrosis which elicits inflammation-associated cytokines resulting in humoral immunity. IL-33 is proposed to be one such danger signal that is released from necrotic cells. Therefore, we investigated whether there is a role for IL-33 in the adjuvant activity of alum. We show that alum-induced cellular necrosis results in elevated levels of IL-33 following injection in vivo. Alum and IL-33 induce similar increases in IL-5, KC, MCP-1, MIP-1α and MIP-1β; many of which are dependent on IL-33 as shown in IL-33 knockout mice or by using an IL-33-neutralizing recombinant ST2 receptor. Furthermore, IL-33 itself functions as an adjuvant that, while only inducing a marginal primary response, facilitates a robust secondary response comparable to that observed with alum. However, IL-33 is not absolutely required for alum-induced antibody responses since alum mediates similar humoral responses in IL-33 knockout and wild-type mice. Our results provide novel insights into the mechanism of action behind alum-induced cytokine responses and show that IL-33 is sufficient to provide a robust secondary antibody response independently of alum.

LIGHT-deficiency impairs CD8+ T cell expansion, but not effector function

LIGHT, a newly identified member of the tumor necrosis factor (TNF) family, is expressed on activated T lymphocytes. To evaluate how LIGHT contributes to T cell functions, we generated LIGHT-deficient (LIGHT(-/-)) mice using gene targeting. Disruption of LIGHT significantly reduced CD8(+) T cell-cycle progression, leading to reduced proliferation to anti-CD3, anti-CD3/anti-CD28 or allogeneic stimulation, whereas proliferation of CD4(+) T cells remained unchanged. In contrast to the observed proliferative defects, isolated CD8(+) T cells from LIGHT(-/-) mice displayed normal cytotoxic effector function development when compared to wild-type CD8(+) T cells. Underlying a potential mechanism of reduced CD8(+) T cell proliferation, LIGHT(-/-) CD8(+) T cells displayed reduced surface levels of CD25 and a diminished ability to proliferate in response to exogenous IL-2. Furthermore, addition of IL-12 to LIGHT(-/-) CD8(+) T cell cultures could not ameliorate this proliferative defect. These results reveal a potential mechanism of action for LIGHT as a positive regulator of CD8(+) T cell expansion, but not lytic effector function development.

A p74 common gamma receptor chain isoform facilitates IL-2 and IL-15 responses by the myelomonocytic cell line Tf-1beta2

Functional forms of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors require the gamma c receptor component. We have described previously a myeloid cell line called Tf-1beta, which binds IL-2 with intermediate-affinity and proliferates in response to IL-2. In this study, we characterize gamma c expression on Tf-1beta2 cells, a derivative of Tf-1beta cells stimulated exclusively with IL-2. Although Tf-1beta2 cells bind IL-2 with intermediate-affinity and proliferate in response to IL-2, this cell line does not express the p64 gamma c chain at the protein level. This result was surprising because prior studies suggest these cells should not be expected to proliferate in response to IL-2 or IL-15 in the absence of the p64 gamma c chain. A p74 protein was detected by western blot following immunoprecipitation with an anti-gamma c polyclonal antibody, and a p74 protein was identified consistently in complex with IL-2 and IL-15 on these cells. However, the gamma c gene in these Tf-1beta2 cells shows no evidence of mutation by sequence analysis. Furthermore, inhibition of glycosylation of these Tf-1beta2 cells by tunicamycin treatment yields a standard 39-kDa molecule recognized on western blot with anti-gamma c antibody, as seen for the standard 64-kDa isoform of gamma c. These results demonstrate that a 74-kDa gamma c receptor isoform was involved in the response of the Tf-1beta2 cells to cytokines which normally interact with the 64-kDa gamma c chain.

Cervical dorsal rhizotomy increases brain-derived neurotrophic factor and neurotrophin-3 expression in the ventral spinal cord

Although neurotrophic factors have been implicated in several forms of neuroplasticity, little is known concerning their potential role in spinal plasticity. Cervical dorsal rhizotomy (CDR) enhances serotonin terminal density near (spinal) phrenic motoneurons and serotonin-dependent long-term facilitation of phrenic motor output (Kinkead et al., 1998). We tested the hypothesis that selected neurotrophic factors change in a manner consistent with an involvement in this model of spinal plasticity. Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), glial cell line-derived neurotrophic factor (GDNF), and transforming growth factor-beta(1) (TGF-beta(1)) concentrations were measured (ELISA) in three regions of interest to respiratory control: (1) ventral cervical spinal segments associated with the phrenic motor nucleus (C3-C6), (2) ventral thoracic spinal segments associated with inspiratory intercostal motor output (T3-T6) and (3) the diaphragm. Tissues were harvested from rats 7 d after bilateral CDR and compared with sham-operated and unoperated control rats. CDR increased BDNF (110%; p = 0.002) and NT-3 (100%; p = 0.002) in the cervical and NT-3 in the thoracic spinal cord (98%; p = 0.009). GDNF and TGF-beta(1) were not altered by CDR in any tissue. Immunohistochemistry localized BDNF and NT-3 to motoneurons and interneurons of the ventral spinal cord. These studies provide novel, suggestive evidence that BDNF and NT-3, possibly through their trophic effects on serotonergic neurons and/or motoneurons, may underlie serotonin-dependent plasticity in (spinal) respiratory motor control after CDR.

Facile generation and use of immunogenic polypeptide fusions to a sparingly soluble non-antigenic protein carrier

Many researchers attempt to prepare antipeptide antibodies by immunizing animals with preparations of fusion proteins or conjugates between the target peptide and a larger protein (such as GST or KLH). Often, the immune response to the larger protein dominates. We have engineered a protein to be sparingly soluble in aqueous solution and nonantigenic, and show that fusions of this sparingly soluble non-antigenic protein (SSNAP) to target peptide sequences can be purified easily to a point suitable for immunizations. When animals are immunized with such fusion proteins, the majority of the immune response is to the target peptide. In all three cases tested, the peptide-specific immune response generated using the SSNAP carrier was greater than that obtained with peptides chemically linked to BSA or KLH, or expressed as fusion proteins to GST. The SSNAP carrier induced a very early IgG response with all classes of IgG well represented in the specific antibody response. All of the SSNAP fusion peptide-derived antibodies were capable of recognizing the full-length target protein in both ELISA and Western analysis. Based on the superior performance of the SSNAP antigens, these studies suggest this novel strategy will have broad utility for the generation of peptide antibodies.

Elevated tryptase, nerve growth factor, neurotrophin-3 and glial cell line-derived neurotrophic factor levels in the urine of interstitial cystitis and bladder cancer patients

PURPOSE

The 2 prominent features of interstitial cystitis are pain and increased numbers of mast cells in the bladder. In this pilot study we determined the concentration of soluble mediators associated with activation of sensory neurons and/or mast cells that were present in the urine.

MATERIALS AND METHODS

The study groups included 4 interstitial cystitis patients, 7 kidney donors with no history of bladder disease as negative controls, 6 bladder cancer patients and 7 patients with urinary tract infection as reference controls. Urine samples were assayed for different soluble mediators using immunoassays for tryptase (a marker for mast cell activation), neurotrophic factors (markers of neuronal plasticity) and chemokines (markers of inflammatory cell activity). Results were normalized based on creatinine concentration.

RESULTS

There was a marked increase in the average amounts of tryptase and 3 neurotrophic factors in patient urine. Interestingly, the mediator profile in the urine of bladder cancer patients was indistinguishable from that of interstitial cystitis patients with respect to these same 4 proteins. There was no difference between normal control and urinary tract infection urine samples.

CONCLUSIONS

These findings may account for several clinical and pathological features found in interstitial cystitis and bladder cancer. Although preliminary due to the limited numbers of patients, they also suggest that increased levels of neurotrophin-3, nerve growth factor, glial cell line-derived neurotrophic factor and tryptase in the urine could serve as a basis for adjunct diagnosis, monitoring and treatment of interstitial cystitis.

GDNF is abundant in the adult rat gut

Glial derived neurotrophic factor (GDNF) is essential for the development of the enteric nervous system (ENS). Although previous work has measured GDNF mRNA levels, little is known about the concentration of GDNF protein produced in developing or adult tissues. The aim of this study was to quantitate the concentration of GDNF protein in various tissues of the developing and adult rat and in adult human gut. A two site antibody immunoassay was used to quantitate GDNF using recombinant rat GDNF as a standard. In the adult rat gastrointestinal tract the intestine contained the highest concentration of GDNF while the stomach and esophagus have the lowest concentrations. The isolated muscular wall of the intestine has approximately four times the GDNF concentration of the intact intestine. Other tissues with smooth muscle such as the aorta and urinary bladder contain moderate GDNF concentrations. In contrast, GDNF is barely detectable in the adult kidney and liver. High concentrations of GDNF were also detected in human colon and jejunum. As development proceeds in the rat, there is a tendency for the concentration of GDNF to increase in the intestine but decrease in other tissues. Treatment of the jejunum with the cationic surfactant benzyldimethyltetradecylammonium chloride (BAC) results in an increase in the number of smooth muscle cells, a decrease in myenteric neurons, and an increase in the concentration of GDNF in homogenates of intestine. The observations that GDNF concentrations are high in the adult intestine suggest that this growth factor may be important for the maintenance of the adult ENS.

An acid-treatment method for the enhanced detection of GDNF in biological samples

Glial cell line-derived neurotrophic factor (GDNF), a distant member of the transforming growth factor-beta (TGFbeta) family, is a protein that is essential for the survival of dopaminergic, motor, and peripheral neurons. To facilitate its study, we and others have developed sensitive (low pg/ml) enzyme-linked immunosorbant assays (ELISA) to quantitate endogenous concentrations of GDNF, along with neurotrophin-3 (NT-3) and nerve growth factor (NGF). However, endogenous tissue levels of GDNF in adult animals are not readily detected by ELISA and do not correlate well with message RNA. Based upon previously described methods for the extraction of TGFbeta from tissue samples, we have developed an acid-treatment procedure to allow the quantification of total endogenous GDNF. This procedure also was evaluated for use when measuring total endogenous levels of NT-3 and NGF from biological samples. The acid-treatment procedure increases the detectable amounts of GDNF, NT-3, and NGF in all tissue samples and most of the serum samples tested. Moreover, these values were as much as 35 times greater than those detected using traditional extraction buffers. Such elevated concentrations likely resulted from the acid treatment promoting the dissociation of ligands from receptors or binding proteins, thereby making more of the analyte available to be measured in the ELISA. These findings indicate that appropriate sample treatment is essential for the measurement of total endogenous neurotrophic factors.

Autologous mixed leucocyte reaction and the polyclonal activation of bovine gamma/delta T cells

Bovine gamma/delta T cells proliferate in response to stimulation with gamma-irradiated autologous monocytes in the autologous mixed leukocyte reaction (AMLR). Flow cytometric analyses indicated that the proliferating cells included three major subpopulations of bovine gamma/delta T cells, distinguished by the differential expression of the gamma/delta T cell receptor epitopes N6 and N7. Interleukin-2 and acid-labile interferon were produced in AMLR cultures but the cultured cells did not lyse any of a large variety of target cells, including monocytes, allogeneic lymphoblasts, transformed bovine B cells (BL3), bovine fibroblast and the natural killer cell targets D17 and K562, even in the presence of lectins or after co-stimulation in the AMLR with antibodies to WC1, the gamma/delta T cell lineage-specific cell-surface differentiation antigen. Ex vivo gamma/delta T cells did not display lymphokine-activated killing whereas populations of peripheral blood mononuclear cells containing alpha/beta T cells did.

Bovine gamma/delta T-cell proliferation is associated with self-derived molecules constitutively expressed in vivo on mononuclear phagocytes

Bovine gamma/delta T cells have been shown previously to proliferate when cocultured with gamma-irradiated bovine monocytes in the 'autologous mixed leucocyte reaction' (AMLR). It was suggested that the response may be to culture-derived or culture-induced antigenic epitopes. Data presented here indicate that the gamma/delta T-cell stimulatory activity is attributable to a self-derived cell-surface molecule of mononuclear phagocytes that is constitutively expressed in vivo. The ability to induce an AMLR did not require in vitro culture or stress associated with in vitro isolation of cells or increased temperature since it could be induced by monocytes fixed by paraformaldehyde during blood collection from normal animals. Furthermore, stimulation by monocytes did not depend upon secreted molecules since fixed monocytes that had been incubated overnight at 37 degrees to allow secretion of preformed molecules, or subjected to hypotonic shock in H2O for 10 min before addition to the cultures, induced an AMLR as did plasma membranes prepared from ex vivo monocytes. In contrast, enzymatic treatment of monocytes to digest surface molecules followed by fixation destroyed their ability to stimulate an AMLR. The ability of monocytes to stimulate proliferation of gamma/delta T cells was distinguishable from their ability to stimulate alpha/beta T cells, since the former was destroyed by glutaraldehyde fixation whereas stimulation of alpha/beta T cells by major histocompatibility complex (MHC)-presented antigenic epitopes is not. Moreover, induction of proliferation of bovine gamma/delta T cells was not MHC-restricted. Finally, bovine alveolar macrophages, sheep monocytes and transformed bovine monocytes stimulated proliferation of bovine gamma/delta T cells whereas none of the following did so: human monocytes, murine macrophages, bovine myeloid cells other than mononuclear phagocytes, other nucleated cells found in bovine blood including activated MHC class II-bearing B cells, and a variety of species of bacteria. Thus, the stimulatory epitope is unique to and conserved among mononuclear phagocytes of ruminants. Demonstration of stimulation of bovine gamma/delta T cells by self-derived molecules is consistent with reports for murine gamma/delta T cells.

Monocytes control gamma/delta T-cell responses by a secreted product

Gamma-irradiated ex vivo bovine monocytes induce proliferation of gamma/delta T cells in the autologous mixed lymphocyte reaction (AMLR), whereas when not irradiated they prevent this response. In contrast, non-irradiated autologous monocytes have no effect on bovine alpha/beta T-cell proliferation in the allogenic MLR suggesting that the regulation is specific for gamma/delta T-cell responses. Here, we showed that the inhibition was not mediated by inducing cell death and that the ability of ex vivo monocytes to prevent proliferation of gamma/delta T cells was not generalized in that gamma/delta T cells still responded to mitogenic stimulation. Inhibition of the AMLR by non-irradiated monocytes could not be overcome by addition of interleukin-2 to the cultures or by costimulation with antibodies to WC1, a gamma/delta T-cell-specific cell-surface differentiation antigen shown elsewhere by us to be involved in activation of gamma/delta T cells. Furthermore, we showed that monocytes inhibited gamma/delta T-cell responses via a soluble product since inhibition occurred even when monocytes and gamma/delta T cells were separated by membranes of transwells or when supernatants from monocyte cultures were added to AMLR cultures. Maximal secretion of the inhibitory product by the monocytes occurred during the first 6 hr of in vitro culture at 37 degrees, rapidly decreased thereafter, and did not occur when monocytes were incubated at 4 degrees. The inhibition was not attributable to nitric oxide, reactive oxygen intermediates, prostaglandin E2 or transforming growth factor-beta (TGF-beta) but the ability of monocyte supernatants to mediate inhibition was sensitive to heating at 65 degrees. Lipopolysaccharide and granulocyte-macrophage colony-stimulating factor activation of monocytes temporarily abrogated their ability to inhibit proliferation. In contrast, heat-shocking had no effect on their ability to inhibit. We hypothesize that non-irradiated monocytes produce the inhibitory material in vivo in order to regulate gamma/delta T-cell responses to self-derived monocyte membrane components, but that when monocytes are altered by infection, transformation, irradiation, or cytokine activation, production of the inhibitor is temporarily suspended allowing stimulation of gamma/delta T cells to occur.