The molecular weight of protein A, isolated from Staphylococcus aureus by lysostaphin digestion, was found to be 42 000 by sedimentation equilibrium analyses and by gel chromatography on Sepharose 6B in 6 M guanidine hydrochloride. Hydrodynamic studies revealed a frictional ratio of 2.1-2.2 and an intrinsic viscosity of 29 ml/g. Both these parameters suggest that protein A is not a typical globular protein but rather has a markedly extended shape. Molecular weights were determined by sedimentation equilibrium using a rapid long-column meniscusdepletion method [5] in conjunction with the interference optical system. Measurements were performed in 0.1 M Tris-HC1 pH 7.2 containing 0.05 M sodium chloride or in 6 M guanidine hydrochloride (Heico Inc., Delaware Water Gap, Pa, U.S.A.) adjusted to pH 7.0. The initial protein concentrations were between 0.4 and 1.0 mg/ml. Prior to the experiments, protein solutions were dialyzed for 24 h a t 5 "C against the Tris buffer or for 72-80 h a t room temperature against 6 M guanidine * HC1. A 2.5-deg double-sector synthetic-boundary cell with sapphire windows was used and the final column height after layering was 6-7 mm. Attainment of equilibrium was checked by measuring the fringe displacements a t several radial distances of two successive exposures taken 2-4 h apart. Molecular weights were calculated in the manner described by Yphantis [6]. The value for the partial specific volume was estimat-

Histamine release from normal human leukocytes can be induced by anti‐IgE or protein A from Staphylococcus aureus. After incubation in buffer at 37°C for various time intervals or repeated washings with buffer, or both procedures, the leukocytes lose most of their reactivity to protein A, whereas the reactivity to anti‐IgE is unaltered. Cells deprived of their protein A reactivity can be induced to release histamine by IgG complexed with protein A. Maximal release (23%–81%) from 0.6–1.0 × 107 leukocytes per ml is obtained if the mixture contains 1–2 μg protein A and 8–16 μg IgG per ml. The ratio between protein A and IgG in the most active mixtures is 1:8 or 1:16 on a weight basis, which corresponds to 2–4 IgG molecules per protein A molecule. Heat treatment does not destroy the capacity of IgG to mediate histamine release. Mixtures of protein A and the Fc part of IgG can also initiate the release. Furthermore, it is shown that the protein A‐IgG mixture and anti‐IgE induce cross‐desensitization to each other. This indicates that, like cell‐bound IgG, IgG in complex with protein A triggers partially the same reaction sequence as IgE.

Pretreatment of human leukocytes with the new xanthine compound, 3,7-dihydro-1,8-dimethyl-3-phenyl-1H-purine-2,6-dione (D 4026), induced a dose-dependent and statistically significant inhibition of immunoglobulin E-mediated histamine release in the concentration interval 0.1-1000 microM. Histamine release elicited with suboptimum amounts of the triggering agent (anti-IgE or antigen) was inhibited to a greater extent than a release initiated with optimum amounts. At a concentration of 10 microM, D 4026 had at least the same inhibitory effect as 100 microM theophylline. When leukocytes were incubated simultaneously with D 4026 and a histamine H-2 receptor-stimulating drug (histamine or clonidine), the two drugs combined induced an inhibition significantly greater than the sum of their individual inhibitory effects. Only pure additive inhibitory effects were, however, obtained during simultaneous treatment of leukocytes with theophylline and histamine.

Antigen-induced histamine release from passively sensitized human lung tissue was inhibited in the presence of the beta 2-adrenoceptor agonist, terbutaline. A sustained and statistically significant suppression was detected in the concentration interval 3 X 10(-8)-1 X 10(-6) M. Fifty per cent inhibition IC50, was obtained at an interpolated concentration of 5.3 X 10(-8) +/- 0.4 X 10(-8) M (n = 13), when the histamine secretion was elicited with optimum concentration of antigen. Histamine release induced with a suboptimum concentration of antigen was inhibited to a greater extent than release initiated with optimum concentration. The data in the present investigation support the concept that terbutaline-induced inhibition of mediator release from human lung tissue can contribute to the clinical effectiveness of the drug during treatment of allergic asthma.

Protein A from Staphylococcus aureus has been found to react with all human leukocyte preparations tested. In 70 percent of the experiments the reaction leads to histamine release. Furthermore, protein A treatment of cells at 37 degrees C, both in complete and Ca-2+-free medium, results in the inhibition of anti-IgE-induced histamine release in all cell preparations, indicating that protein A and anti-IgE antibodies release histamine from the same cells. This inhibition seems to be due to the blocking or exhaustion of a step in the biochemical pathway, leading to histamine release activated by both protein A and anti-IgE. In some cell preparations desensitization but no histamine liberation is induced by protein A. No inhibition occurs if the protein A treatment is performed at 4 degrees C. It is concluded that protein A elicits histamine liberation and desensitization by acting on IgG present on the surface of the basophil granulocytes. Treatment of leukocytes at 37 degrees C with anti-IgE antibodies, or F(ab)2 fragments from such antibodies, also results in an inhibition of a subsequent anti-IgE-induced histamine release. Desensitization with low doses of anti-IgE results in an inhibition of the same type as that obtained with protein A. Supraoptimum amounts of anti-IgE or high amounts of monovalent Fab fragments from anti-IgE immunoglobulin G give an inhibition that could be due to a competition between the sensitizing and the challenging agents for combining with cell fixed IgE molecules. This inhibition is independent of temperature and calcium concentration.