Nitrogen dynamics of a mountain forest on dolomitic limestone--a scenario-based risk assessment.

The dominant nitrogen (N) fluxes were simulated in a mountain forest ecosystem on dolomitic bedrock in the Austrian Alps. Based on an existing small-scale climate model the simulation encompassed the present situation and a 50-yr projection. The investigated scenarios were current climate, current N deposition (SC1) and future climate (+2.5 degrees C and +10% annual precipitation) with three levels of N deposition (SC2, 3, 4). The microbially mediated N transformation, including the emission of nitrogen oxides, was calculated with PnET-N-DNDC. Soil hydrology was calculated with HYDRUS and was used to estimate the leaching of nitrate. The expected change of the forest ecosystem due to changes of the climate and the N availability was simulated with PICUS. The incentive for the project was the fact that forests on dolomitic limestone stock on shallow Rendzic Leptosols that are rich in soil organic matter are considered highly sensitive to the expected environmental changes. The simulation results showed a strong effect due to increased temperatures and to elevated levels of N deposition. The outflux of N, both as nitrate (6-25 kg N ha(-1)yr(-1)) and nitrogen oxides (1-2 kg N ha(-1)yr(-1)), from the forest ecosystem are expected to increase. Temperature exerts a stronger effect on the N(2)O emission than the increased rate of N deposition. The main part of the N emission will occur as N(2) (15 kg N ha(-1)yr(-1)). The total N loss is partially offset by increased rates of N uptake in the biomass due to an increase in forest productivity.

Interleukin-1 stimulation of human B-lymphoblast differentiation.

Transformed B-cell lines have provided useful tools for analyzing B-cell responses to growth and differentiation factors. SKW 6.4 is a human B-lymphoblastoid line that differentiates in response to B-cell differentiation factor (BCDF) but not to other T-cell-derived lymphokines. The present studies were undertaken to determine whether monocyte-derived factors such as interleukin-1 (IL-1) could also promote the maturation of SKW 6.4 cells. Cells were cultured with and without supernatants derived from adherent monocytes of normal individuals. The number of Ig-producing plaque-forming cells in control media cultures gradually increased over a period of 4 days. Monocyte supernatants caused a 3- to 10-fold further increase in the number of IgM-secreting cells over that found in control media cultures. This stimulation by monocytes supernatants was maximal on the third or fourth day of culture and was not accompanied by an increase in proliferation. Recombinant IL-1 added to cultures of SKW 6.4 also produced a dose-related increase in the number of IgM-secreting cells without stimulating proliferation. A polyclonal antiserum against IL-1 prevented the increase in IgM-secreting cells which occurred with the addition of monocyte supernatants. Finally, IL-1 activity was detected in unstimulated SKW 6.4 supernatants. We conclude from these studies that SKW 6.4 cells both secrete IL-1 and differentiate in response to exogenous IL-1. SKW 6.4 may therefore provide a useful tool for future studies on mechanisms of IL-1-induced B cell maturation. It is possible that the basal numbers of IgM-secreting cells seen in unstimulated cultures results from an autocrine mechanism of IL-1-induced differentiation. Finally, care should be taken to exclude IL-1 from supernatants that are being tested for BCDF activity using SKW 6.4 cells.

The effect of interleukin 1 on IgG synthesis in systemic lupus erythematosus.

The influence of interleukin 1 (IL-1) on IgG synthesis was studied in patients with systemic lupus erythematosus (SLE). Spontaneous IgG synthesis was assessed by culturing peripheral blood mononuclear cells in media for 7 days. The effect of adherent cell supernatants (ACS) on spontaneous IgG synthesis was also assessed, based upon previous studies showing that ACS contributes significantly more to IgG synthesis in SLE than it does in normals. Antiserum against IL-1 reduced spontaneous IgG synthesis by approximately 90% in SLE and normal mononuclear cell cultures. The antiserum caused a parallel reduction in the number of Ig-secreting B cells. Adsorption of SLE ACS with Sepharose-bound antiserum against IL-1 prevented ACS-induced increases in IgG synthesis. Affinity-purified or recombinant IL-1 added to media did not significantly stimulate IgG synthesis. Incubation of ACS with a monoclonal antibody against interferon-gamma had no effect on IgG synthesis, and no detectable interferon activity was found in ACS using a virus inhibition assay. Stimulation of IgG production by autologous mononuclear cell supernatants correlated directly with the presence of clinically active disease, suggesting that these in vitro observations may be important to the disease process. These findings demonstrate that hyperactive B cells from patients with SLE are regulated by factors released by adherent mononuclear cells, probably monocytes. The presence of IL-1 is crucial, but may not be sufficient, for spontaneous and ACS-induced IgG synthesis. Inhibiting or blocking the production of IL-1 may provide a means of reducing abnormal immunoglobulin synthesis in SLE.

Stimulation of B cell differentiation by adherent mononuclear cells in systemic lupus erythematosus.

We studied B cell proliferation and differentiation in response to factors released by adherent monocytes in patients with systemic lupus erythematosus (SLE). Adherent cell supernatants (ACS) were added to peripheral blood mononuclear cells, and the effects on IgG synthesis, the number of Ig-secreting cells (ISC), and proliferation were determined. Exposure of SLE mononuclear cells to autologous ACS caused an increase (approximately two-fold) in IgG production and ISC numbers. In contrast, exposure of normal mononuclear cells to autologous ACS did not significantly increase IgG production or ISC numbers. Addition of SLE ACS to cultures of normal mononuclear cells did not stimulate ISC production. There was no significant level of 3H-thymidine uptake by cultures of SLE or normal mononuclear cells in response to either SLE or normal ACS. In the presence of an excess number of autologous T cells, ACS stimulation of IgG synthesis was further enhanced. These findings indicate that adherent monocytes contribute to B cell hyperactivity in SLE by stimulating B cell differentiation. SLE mononuclear cells appear to be more responsive to ACS stimulation than are normal mononuclear cells.

The effect of adherent cell-derived factors on immunoglobulin and anti-DNA synthesis in systemic lupus erythematosus.

The role of adherent cells in the regulation of anti-DNA and immunoglobulin synthesis was investigated in patients with systemic lupus erythematosus (SLE). Peripheral blood mononuclear cells were cultured for 7 days, or were washed after 20 hr of incubation and recultured in fresh media. This washing resulted in a marked decrease in total IgG, IgM, and anti-DNA synthesis compared with unwashed cultures. Reculturing the washed cells in their original supernatant reconstituted Ig and anti-DNA synthesis. Hence it appeared that a supernatant factor, or factors, present in the first 20 hr of mononuclear cell cultures, was required for maximal Ig synthesis. Adherent cells were found to be the source of this Ig-stimulating activity. Moreover, adherent cell supernatants had no direct Ig-stimulating effect on B cells. Ig synthesis was stimulated, however, when T cells where present with the B cells at a 3:1 ratio. Autologous SLE mononuclear cell supernatants reconstituted Ig synthesis to a greater degree than did autologous normal supernatants. SLE adherent cell supernatants were fractionated on an HPLC sizing column. The fractions were tested for their ability to stimulate IgG synthesis by SLE mononuclear cells that had been washed after 20 hr of culture. A single peak of IgG-stimulating activity was found at approximately 14,000 Mr. A rabbit antiserum to interleukin-1 (IL-1) neutralized the Ig-stimulating activity in adherent cell supernatants. No correlations were found, however, between supernatant IL-1 levels assayed by C3H-HeJ mouse thymocyte proliferation and IgG stimulation in mononuclear cell cultures, suggesting that the effects of IL-1 on cell proliferation may not accurately reflect its effects on Ig synthesis. These observations suggest that in normal individuals and in patients with SLE in vitro polyclonal Ig and anti-DNA synthesis requires the presence of soluble adherent cell factors. The Ig-stimulating effect is facilitated by T cells and appears to be mediated at least in part by IL-1. This culture technique provides a new way of analyzing the role of soluble factors in autoantibody synthesis and suggests that IL-1 may be an important contributor to lupus B-cell hyperactivity.

Termination of the respiratory burst in human neutrophils.

Recent evidence has suggested that a particulate O(2) (-)-forming system is responsible for the respiratory burst in activated neutrophils. The respiratory burst is normally a transient event, lasting only 30-60 min. To investigate the mechanism by which the burst is terminated, we examined the O(2) (-)-forming activity of neutrophil particles as a function of time in the presence and absence of agents known to affect the function of intact cells. Measurements of the O(2) (-)-forming capacity of the particles against time of exposure of neutrophils to opsonized zymosan, a potent stimulating agent, revealed a rapid fall in activity when exposure was continued beyond 3 min. Exposure to zymosan under conditions in which the myeloperoxidase system was inactive (i.e., in the presence of myeloperoxidase inhibitors, or in the absence of oxygen) resulted in a substantial increase in the initial O(2) (-)-forming activity of particles from the zymosan-treated cells, but did not prevent the sharp fall in activity seen when zymosan exposure exceeded 10 min. The fall in activity was, however, prevented when activation took place in the presence of cytochalasin B (1.5 mug/ml), an agent thought to act largely by paralyzing the neutrophil through an interaction with its microfilament network.We conclude from these findings that the termination of the respiratory burst results at least in part from the inactivation of the particulate O(2) (-)-forming system. This inactivation involves at least two processes which probably act simultaneously. One is the destruction of the system through the action of myeloperoxidase. The other appears to require active cell motility and is independent of oxygen. The current view holds that the O(2) (-)-forming system of the neutrophil is located in the plasma membrane. It may be that the second process involves the internalization and degradation of this membrane-bound system.