Distinct critical behaviors from the same state in quantum spin and population dynamics perspectives.

There is a deep connection between the ground states of transverse-field spin systems and the late-time distributions of evolving viral populations-within simple models, both are obtained from the principal eigenvector of the same matrix. However, that vector is the wave-function amplitude in the quantum spin model, whereas it is the probability itself in the population model. We show that this seemingly minor difference has significant consequences: Phase transitions that are discontinuous in the spin system become continuous when viewed through the population perspective, and transitions that are continuous become governed by new critical exponents. We introduce a more general class of models that encompasses both cases and that can be solved exactly in a mean-field limit. Numerical results are also presented for a number of one-dimensional chains with power-law interactions. We see that well-worn spin models of quantum statistical mechanics can contain unexpected new physics and insights when treated as population-dynamical models and beyond, motivating further studies.

Clustering of Nonergodic Eigenstates in Quantum Spin Glasses.

The two primary categories for eigenstate phases of matter at a finite temperature are many-body localization (MBL) and the eigenstate thermalization hypothesis (ETH). We show that, in the paradigmatic quantum p-spin models of the spin-glass theory, eigenstates violate the ETH yet are not MBL either. A mobility edge, which we locate using the forward-scattering approximation and replica techniques, separates the nonergodic phase at a small transverse field from an ergodic phase at a large transverse field. The nonergodic phase is also bounded from above in temperature, by a transition in configuration-space statistics reminiscent of the clustering transition in the spin-glass theory. We show that the nonergodic eigenstates are organized in clusters which exhibit distinct magnetization patterns, as characterized by an eigenstate variant of the Edwards-Anderson order parameter.

Major histocompatibility complex class I and II expression on macrophages containing a virulent strain of Brucella abortus measured using green fluorescent protein-expressing brucellae and flow cytometry.

Immune responses appropriate for control of an intracellular pathogen are generated in mice infected with Brucella abortus, shown by the ability of T cells to adoptively transfer resistance to naive mice. The infection nevertheless persists for months. It was hypothesized that one factor in maintaining the infection despite the presence of immune T cells was suboptimal expression of major histocompatibility complex (MHC) molecules on macrophages containing brucellae. This would allow B. abortus to elude detection by the host's immune system. To test this, B. abortus organisms expressing green fluorescent protein (GFP-Brucella) were constructed and three-color flow cytometry used to evaluate MHC expression on macrophages following in vitro or in vivo infection. When infected in vitro, the levels of MHC class I and class II expression on J774 macrophages containing GFP-Brucella were the same or higher than on macrophages without GFP-Brucella in the same cultures. Similarly, the MHC expression was higher on GFP(+) peritoneal exudate cells following infection or phagocytosis of heat-killed GFP-Brucella than it was on uninfected peritoneal exudate cells. Following in vivo infection of mice the level of MHC class I and II expression on GFP(+) cells in their spleens (the main site of infection) also tended to be as high as or higher than that on the GFP-negative cells. The only in vivo GFP(+) cells that showed a decreased MHC expression was a population of splenic Mac1(+) cells recovered from interferon-gamma gene-disrupted mice at the time of their death due to an overwhelming number of bacteria per spleen. Overall, it was concluded that decreased MHC expression is not a general principle associated with brucella infection of macrophages and thus not likely to contribute to maintenance of the chronic infection.

Activation of bovine peripheral blood gammadelta T cells for cell division and IFN-gamma production.

Bovine peripheral blood gammadelta T cells have been evaluated for effector function (IFN-gamma production) and clonal expansion in a variety of systems including following activation by mitogens, IL-12, and stimulation, through the T cell receptor (TCR) with anti-CD3 monoclonal antibody (mAb), a cell-bound molecule and a soluble antigenic extract. To evaluate cell division, carboxyfluorescein succinimidyl ester (CFSE) loading of cells and flow cytometric analysis were used, while IFN-gamma production was evaluated by intracytoplasmic staining. It was found that bovine gammadelta T cells produced IFN-gamma and clonally expanded when stimulated through the TCR/CD3 complex by a cell-associated autologous molecule on monocyte, by bacterial components following in vivo sensitization of gammadelta T cells with a leptospira vaccine or by anti-CD3 mAb. In addition, gammadelta T cells were activated efficiently for effector function but not clonal expansion by culturing with IL-12. In contrast, stimulation by Con A or PMA/ionomycin induced efficient replication but only low level IFN-gamma production which was not enhanced by the presence of IL-12. In several systems the amount of IFN-gamma produced per cell by gammadelta T cells was less than that produced by CD4 T cells in the same cultures.

Immune control of Brucella abortus 2308 infections in BALB/c mice.

BALB/c mice infected with Brucella abortus strain 2308 have 10-fold higher levels of bacteria during the plateau phase of infection (the time period when the number of colony-forming units in vivo remains consistent) than the more resistant C57BL/10 mice. This is due to a cessation of interferon-gamma (IFN-gamma) production that begins after the first week of infection and continues until the end of the plateau phase at least 6 weeks post infection. Despite the lack of IFN-gamma production during this time BALB/c mice are able to prevent an increase in bacterial colony-forming units. Here it was shown that both tumor necrosis factor (TNF)-alpha and CD8 T cells were involved in controlling bacterial numbers in BALB/c mice during this time. That is, neutralization of TNF-alpha or depletion of CD8 T cells with monoclonal antibodies resulted in a significant increase in the number of splenic colony-forming units recovered at 3 weeks post infection. In the absence of CD8 T cells there was also a significant increase in splenic macrophages. The role of TNF-alpha may depend upon the presence of interferon-gamma early in the infection since when TNF-alpha was neutralized in interferon-gamma gene knockout mice there was a marked increase in splenic macrophages, NK cells and neutrophils but not a significant increase in colony-forming units.

Protective killed Leptospira borgpetersenii vaccine induces potent Th1 immunity comprising responses by CD4 and gammadelta T lymphocytes.

Leptospira borgpetersenii serovar hardjo is the most common cause of bovine leptospirosis and also causes zoonotic infections of humans. A protective killed vaccine against serovar hardjo was shown to induce strong antigen-specific proliferative responses by peripheral blood mononuclear cells (PBMC) from vaccinated cattle by 2 months after the first dose of vaccine. This response was absent from nonvaccinated control cattle. The mean response peaked by 2 months after completion of the two-dose vaccination regimen, and substantial proliferation was measured in in vitro cultures throughout the 7 months of the study period. Variations in magnitude of the response occurred among the vaccinated animals, but by 7 months postvaccination there was a substantial antigen-specific response with PBMC from all vaccinated animals. Up to one-third of the PBMC from vaccinated animals produced gamma interferon (IFN-gamma) after 7 days in culture with antigen, as ascertained by flow cytometric analysis, and significant levels of IFN-gamma were measured in culture supernatants by enzyme-linked immunosorbent assay. Two-color immunofluorescence revealed that one-third of the IFN-gamma-producing cells were gammadelta T cells, with the remaining cells being CD4(+) T cells. The significance of this study is the very potent Th1-type immune response induced and sustained following vaccination with a killed bacterial vaccine adjuvanted with aluminum hydroxide and the involvement of gammadelta T cells in the response. Moreover, induction of this Th1-type cellular immune response is associated with the protection afforded by the bovine leptospiral vaccine against L. borgpetersenii serovar hardjo.

Interferon-gamma is crucial for surviving a Brucella abortus infection in both resistant C57BL/6 and susceptible BALB/c mice.

Brucella abortus is an intracellular bacterial pathogen that causes chronic infections in humans and a number of agriculturally important species of animals. It has been shown that BALB/c mice are more susceptible to infections with virulent strains of Brucella abortus than C57BL/6 or C57BL/10 strains. In experiments described here, gene knock-out mice were utilized to elucidate some of the salient components of resistance. Resistant C57BL/6 mice with gene deletions or disruptions in the interferon-gamma (IFN-gamma), perforin or beta(2)-microglobulin genes had decreased abilities to control intracellular infections with B. abortus strain 2308 during the first week after infection. However, only the IFN-gamma knock-out mice had a sustained inability to control infections and this resulted in death of the mice at approximately 6 weeks post-infection. These mice had a continual increase in the number of bacterial colony-forming units (CFU) in their spleens until death. When BALB/c mice with the disrupted IFN-gamma gene were infected they had more splenic CFU at one week post-infection than control mice but the increase was not statistically significant and by 3 weeks they did not have more CFU than control mice. Moreover, the number of splenic bacteria did not increase in the BALB/c IFN-gamma knock-out mice between 6 and 10.5 weeks, although they died at 10.5 weeks, the time by which normal BALB/c mice were clearing the infection. Death in both strains of IFN-gamma gene disrupted mice coincided with symptoms of cachexia and macrophages comprised > or= 75% of the splenic leucocytes.

Gender differences in suicide attempts from adolescence to young adulthood.

OBJECTIVE: To examine associations of age, gender, and psychosocial factors during adolescence with risk of suicide attempt between ages 19 and 23 years. METHOD: Initial assessments were conducted with 1,709 adolescents (aged 14-18) in western Oregon between 1987 and 1989. One year later, 1,507 participants returned for a second assessment. A subset of participants (n = 941; 57.2% women) had a third diagnostic assessment after turning 24 (between 1993 and 1999). Information on suicidal behavior, psychosocial risk factors, and lifetime DSM-III-R psychiatric diagnosis was collected at each assessment. RESULTS: The suicide attempt hazard rate for female adolescents was significantly higher than for male adolescents (Wilcoxon chi 2(1)[n = 941] = 12.69, p < .001). By age 19, the attempt hazard rate for female adolescents dropped to a level comparable with that of male adolescents. Disappearance of the gender difference for suicide attempts by young adulthood was not paralleled by a decrease in the gender difference for major depression. Adolescent suicidal behavior predicted suicide attempt during young adulthood for female, but not male, participants. Adolescent psychosocial risk factors for suicide attempt during young adulthood were identified separately for girls and boys. CONCLUSIONS: Unlike depression, the elevated incidence rate of suicide attempts by adolescent girls is not maintained into young adulthood. Screening and prevention implications are discussed.

Evaluation of cell replication by bovine T cells in polyclonally activated cultures using carboxyfluorescein succinimidyl ester (CFSE) loading and flow cytometric analysis.

Studies reported here demonstrated that carboxyfluorescein succinimidyl ester (CFSE) loading of lymphocytes and flow cytometric analysis is a powerful assay to assess the kinetics and extent of cellular replication by bovine T-cell subpopulations in heterogeneous cultures of peripheral blood mononuclear cells (PBMC) where subpopulation interactions can occur. As CFSE analysis allows determination of the proportion of lymphocytes that divided, as well as the number of cell divisions each cell underwent, distinctions in responses among mitogen-stimulated cultures could be made even when(3)H-thymidine incorporation was equivalent. When combined with surface staining for detection of differentiation antigens, differences among T-cell subpopulations with regard to the number of divisions their members had undergone, were found. Anti-CD3 mAb stimulated both CD8(+)and CD4(+)T cells to undergo several cell divisions in 72 hours, while there was essentially no division by gamma delta T cells. In contrast, in concanavalin A-stimulated cultures, all T-cell subpopulations had divided.

Rapid changes occur in the percentage of circulating bovine WC1(+)gamma delta Th1 cells.

gamma delta T cells found in the peripheral blood of cattle include a major subpopulation distinguished by expression of WC1. These cells are distinct from the WC1(-)gamma delta T cell population based on T cell receptor gene usage. We documented that a group of 6-month-old calves allowed free-range grazing and access to their mothers had a significantly greater proportion of total gamma delta T cells in their blood, attributable to the WC1(+)gamma delta T cell subpopulation, compared to age and breed-matched calves held in conventional housing. When the animals with the greater proportion of gamma delta T cells were transferred to conventional housing there was a decrease in the WC1(+)population so that by 3 weeks after transfer there was no longer a significant difference between the two groups. To investigate the biological activities of WC1(+)gamma delta T cells, the cells were purified by flow cytometric sorting. In vitro, they responded to stimulation by irradiated monocytes in autologous mixed leukocyte reaction (AMLR) cultures but not to direct stimulation through the T cell receptor (T c R) by anti-delta monoclonal antibody. After stimulation in the AMLR, WC1(+)gamma delta T cells had a Th1 cytokine profile characterised by production of IFN -gamma and lack of IL -4. Thus we propose that higher levels of the WC1(+)gamma delta T cells may provide calves with a mechanism to produce Th1 cytokines and that the level of these cells may be modulated according to environment or stress since both groups of calves were apparently disease-free.

Growth of Brucella abortus in macrophages from resistant and susceptible mouse strains.

C57Bl/10 mice have a superior ability to control chronic infections with virulent strains of the intracellular bacteria Brucella abortus compared with BALB/c mice. While a number of differences in the cytokines produced by lymphocytes following infection of these two strains of mice have been shown, macrophages have not been evaluated for their role in conveying relative resistance. The importance of macrophages in control of brucella infections is demonstrated by the observations that intracellular survival of various strains of B. abortus directly correlates with their virulence in vivo, and the ability of macrophages to control brucellae in vitro has been shown to correlate with a brucella-resistant phenotype in ruminants. While both BALB/c and C57Bl are Nramp-susceptible mouse strains, additional differences in macrophage function outside of the Nramp1 gene effects could influence susceptibility to brucellosis. The studies conducted here comparing the ability of macrophages from C57Bl/10 and BALB/c mice indicate that the macrophages from resistant mice did not control intracellular growth of B. abortus strain 2308 more efficiently than those from the susceptible mice, either in the absence of, or following, interferon-gamma activation or iron supplementation. A number of different conditions for culturing macrophages were evaluated to rule out the influence of antibiotics on the conclusions drawn from the results.

The siderophore 2,3-dihydroxybenzoic acid is not required for virulence of Brucella abortus in BALB/c mice.

2,3-Dihydroxybenzoic acid (DHBA) is the only siderophore described for Brucella, and previous studies suggested that DHBA might contribute to the capacity of these organisms to persist in host macrophages. Employing an isogenic siderophore mutant (DeltaentC) constructed from virulent Brucella abortus 2308, however, we found that production of DHBA is not required for replication in cultured murine macrophages or for the establishment and maintenance of chronic infection in the BALB/c mouse model.

Brucella abortus siderophore 2,3-dihydroxybenzoic acid protects brucellae from killing by macrophages.

Addition of 2,3-dihydroxybenzoic acid, a siderophore produced by Brucella abortus, to macrophage cultures prevented intracellular killing of brucellae during the first 12 h after infection and increased the number of intracellular brucellae recovered at 48 h after infection. The protective effect could be demonstrated with inflammatory macrophages, interferon-gamma-activated macrophages and with macrophages supplemented with iron, shown elsewhere to facilitate killing of B abortus.

Comparison of T cell cytokines in resistant and susceptible mice infected with virulent Brucella abortus strain 2308.

C57BL/10 and BALB/c mice differ in their abilities to clear infections with the intracellular bacterium Brucella abortus strain 2308. We have previously reported that in vivo of IL-10 in the susceptible BALB/c mice results in significantly fewer bacteria in their spleens 1 week after infection with 5 x 10(3) colony forming units (CFU) of 2308. Here we extend those studies and report a similar effect when IL-4 is neutralized. In contrast, in the more resistant C57BL/10 mice infected with 5 x 10(3) CFU, neither neutralization of IL-4 significantly decreased the level of infection nor did it in either BALB/c or C57BL/10 mice infected with a 1000-fold higher dose of strain 2308. While splenocytes from the later mentioned groups of mice produced IL-10 in response to stimulation with brucella antigen, they also produced higher levels of interferon (IFN)-gamma than those from BALB/c mice with the low challenge dose of 5 x 10(3) CFU. Results of in vivo neutralization of IFN-gamma by monoclonal antibodies (MAb) reported here and elsewhere indicated that IFN-gamma is important for control; thus, we postulate that the higher levels of IFN-gamma override the detrimental effects of Th2 cytokines. In vitro studies also showed that macrophages from the more resistant C57BL/10 mice were less susceptible to the ability of IL-10 to decrease anti-brucella activities than were BALB/c macrophages. CD4+ T cells were principally responsible for the production of IL-10 in BALB/c but not C57BL/10 splenocyte populations. C57BL/10 splenocytes produced more IFN-gamma than those from BALB/c mice in response to stimulation with brucella antigens. These differences between BALB/c and C57BL/10 mice may contribute to the superior capacity of C57BL/10 mice to control infections with B. abortus strain 2308.

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.

Modulation of WC1, a lineage-specific cell surface molecule of gamma/delta T cells augments cellular proliferation.

WC1, also known as T19, is the only unique gamma/delta T-cell differentiation antigen described to date other than the gamma/delta T-cell receptor. We present evidence that modulation of WC1 results in augmented proliferation of gamma/delta T cells. Immobilized IL-A29, a monoclonal antibody (mAb) specific for WC1, augmented proliferation of gamma/delta T cells in the autologous mixed leucocyte reaction (AMLR) as well as proliferation induced by either anti-CD3 or anti-CD5 mAb. In contrast, anti-CD5 mAb did not increase proliferation in the AMLR even though both CD5 and WC1 are members of the scavenger receptor cysteine-rich family of proteins and are expressed by bovine peripheral blood gamma/delta T cells. IL-A29 did not induce proliferation when assessed alone or in the presence of either phorbol myristate acetate (PMA) or interleukin-2. IL-A29 also did not induce detectable calcium mobilization when evaluated in the presence of monocytes, PMA, or following cross-linking of IL-A29 with anti-immunoglobulin antibody. We conclude that WC1 is a gamma/delta T-cell lineage-specific cell-surface differentiation antigen which is involved in activation of gamma/delta T cells using an as yet unidentified pathway.

Toxicokinetic analysis of losartan during gestation and lactation in the rat.

Previous developmental and reproductive toxicity studies in rats with losartan, a potent AT1-selective angiotensin II (AII) receptor antagonist, correlated maternal treatment during gestation day (GD) 15-20 with irreversible renal abnormalities in the F1 generation (Spence et al., '95a,b). Continued treatment through lactation was also associated with increases in pup mortality and decreases in pup body weights that persisted through weaning. The studies presented here were undertaken to quantify fetal and neonatal exposure to losartan when administered to the dam by oral gavage during early gestation, late gestation, and lactation. Following daily oral dosing of 135 mg/kg/day on GD6-15, fetal drug levels were negligible. However, losartan and its active metabolite, EXP3174 (L-158,641) were readily detectable in fetal plasma on GD 20 (estimated AUC values, 50.70 and 167.70 micrograms/hr/ml, respectively) and maternal milk during lactation (1.61 and 1.67 micrograms/ml, respectively). These studies suggest that the relative increased sensitivity of the fetus as compared to the neonate for losartan-induced renal lesions is related to the degree of exposure which is dependent on the time of administration (early gestation vs. late gestation/lactation) and the route of exposure (transplacental or through the milk). Furthermore, the maximum exposure to losartan and EXP3174 correlates with the ontogeny of the renin angiotensin system on approximately GD 17 and the critical period for losartan-induced renal lesions (GD15-20). The data support the hypothesis that the observed adverse fetal and neonatal effects are pharmacologically mediated, presumably through the lack of AT1 receptor stimulation.

Cloning, sequencing and expression of the bovine CD3 epsilon and TCR-zeta chains, two invariant components of the T-cell receptor complex.

CD3 epsilon and the zeta-chain of the bovine T-cell receptor (TCR) are two invariant molecules with an important role in signal transduction via the TCR/CD3 complex. The nucleotide sequence of a bovine CD3 epsilon cDNA clone containing the complete coding sequence was determined and the deduced amino acid (aa) sequence compared to that of other species. The cytoplasmic domains of the different CD3 epsilon clearly show a higher degree of conservation than the extracellular domains. Bovine CD3 epsilon produced in Escherichia coli using different bacterial expression vectors was recognised by antibodies (Ab) directed against the intracytoplasmic domain of human CD3 epsilon. A partial bovine TCR zeta-chain cDNA was generated by the polymerase chain reaction (PCR) using primers that were based on sequences that are conserved between different species; 3' and 5' RACE-PCR were carried out to obtain the complete TCR zeta-chain cDNA sequence. A comparison of the predicted TCR zeta-chain aa sequence reveals that the GDP/GTP-binding motif, which is conserved in other species, shows marked differences in the bovine and ovine TCR zeta-chains. In contrast to CD3 epsilon, the short extracellular domain of the TCR zeta-chain is 100% conserved between the different species and the transmembrane domain also shows a high degree of identity. Ab were raised against the TCR zeta-chain, produced as a glutathione S-transferase fusion protein in E. coli, and were used in Western blot analysis to further characterise TCR zeta-chain expression in T-cells. The regents provide valuable tools for the study of signal transduction pathways in normal and transformed bovine T-cells.

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.