Effect of calcium ions on staphylococcal alpha-toxin-induced hemolysis of rabbit erythrocytes.

Calcium in millimolar concentrations protected rabbit erythrocytes from hemolysis caused by staphylococcal alpha-toxin. This effect was maximal at 30 mM CaCl2 and required the continued presence of calcium. The protection was not absolute and could be overcome by increased concentrations of alpha-toxin. Calcium did not block the binding of alpha-toxin to erythrocytes but inhibited the alpha-toxin-induced release of small ions from the cell as measured by 86Rb release. The transient removal of calcium was sufficient to abrogate its protective effect, suggesting that its action involves a reversible alteration in the state of the membrane. The three steps of the alpha-toxin-induced hemolytic sequence are: (i) binding to specific receptors, (ii) formation of transmembrane pores, and (iii) cell lysis. We concluded that calcium acted at step ii by impeding the lateral movement of alpha-toxin necessary to form the transmembrane hexamer pores.     

Purification and characterization of alpha-toxin produced by Clostridium novyi type A.

Our study describes the production, purification, and properties of alpha-toxin from Clostridium novyi type A 19402. The bacterium produced maximal amounts of alpha-toxin when grown at 37 degrees C for 72 h in dialysis flask cultures containing brain heart infusion supplemented with 0.75% Tween 80 and 2% glycerol. The alpha-toxin was purified by precipitation with polyethylene glycol 6000, followed by chromatography on Q-Sepharose, phenyl-agarose, and Mono-Q. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the toxin exhibited a single band with an M(r) of 200,000. The toxin also exhibited a single immunoprecipitin arc by crossed immunoelectrophoresis with antiserum against crude toxin. It was stable when stored at 4 degrees C and also following exposure to buffers with pHs in the range of 4 to 7. The toxin had a minimum lethal dose in mice of 5 to 10 ng, caused rounding of a variety of cells in tissue culture, and was negative in the rabbit ileal loop assay. The cytotoxic activity was inhibited by agents that affect receptor-mediated processes, and the toxin was less active on a CHO mutant cell line that is defective in endosomal acidification. The analysis of the amino acid composition revealed an unusually high proline content. The N-terminal sequence is Met-Leu-Ile-Thr-Arg-Glu-Gln-Leu-Met-Lys.     

Susceptibility to staphylococcal alpha-toxin of Friend virus-infected murine erythroblasts during differentiation.

Splenic erythroblasts obtained from BALB/c mice infected with the anemia strain of Friend virus were compared with "matured" cells and adult erythrocytes for their sensitivity to staphylococcal alpha-toxin. Matured cells were obtained by treating erythroblasts in culture with erythropoietin for 48 h. Sensitivity to staphylococcal alpha-toxin, measured both by release of 86Rb and by cell lysis, failed to demonstrate significant differences among the cell types. Since maturation of erythroblasts to matured cells or erythrocytes is associated with synthesis of band 3, hemoglobin, and spectrin and the loss of transferrin receptors, we conclude that none of these compounds serves as the specific receptor for staphylococcal alpha-toxin in BALB/c mice.     

Specific binding of staphylococcal alpha-toxin to isolated rabbit vagus nerves in vitro.

The binding of staphylococcal [125I]alpha-toxin to rabbit vagus nerves in vitro was a saturable process. The radiolabeled alpha-toxin binding was reduced by the coaddition of added navive alpha-toxin, indicating that the binding is specific. Sucrose gradient analysis of detergent-extracted complexes of [125I]alpha-toxin-rabbit vagus nerves showed both high and low S-value peaks analogous to those observed with similarly treated alpha-toxin-rabbit erythrocyte preparations (P. Cassidy and S. Harshman, Biochemistry, in press).     

Hyperproduction of alpha-toxin by Staphylococcus aureus results in paradoxically reduced virulence in experimental endocarditis: a host defense role for platelet microbicidal proteins.

Staphylococcal alpha-toxin targets several cell types which are important components of cardiac vegetations in endocarditis, including platelets, erythrocytes, and endothelial cells. We evaluated the in vivo role of Staphylococcus aureus alpha-toxin in experimental endocarditis by using isogenic strains differing in the capacity to produce functional alpha-toxin, including 8325-4 (wild-type strain), DU-1090 (a mutant strain with allelic replacement of the alpha-toxin gene [hla]), DU1090(pH35L) (a mutant strain producing a target cell-binding but nonlytic toxin), DU1090(pDU1212) (a variant of DU1090 carrying the cloned hla gene on a multicopy plasmid), and DU1090(pCL84::hla) (a variant of DU1090 with a single copy of the hla gene cloned into the chromosomal lipase locus). In vitro, wild-type alpha-toxin (from parental strain 8325-4) extensively lysed both erythrocytes and platelets. In contrast, mutant alpha-toxin [from strain DU1090(pH35L)] lysed neither cell type. Following exposure to the wild-type alpha-toxin, platelet lysates were found to contain microbicidal activity against Bacillus subtilis (but not against Micrococcus luteus), as well as against the parental and alpha-toxin variant S. aureus strains noted above. Furthermore, lysate microbicidal activity was heat stable, neutralized by polyanionic filters or compounds, and recoverable from anionic filter membranes by hypertonic saline elution. These characteristics are consistent with those of cationic platelet microbicidal proteins (PMPs). Reverse-phase high-pressure liquid chromatography and polyacrylamide gel electrophoresis confirmed the presence of three distinct PMPs (1, 2, and 3) in platelet lysates. In experimental endocarditis, the two variant staphylococcal strains producing either minimal alpha-toxin or nonlytic alpha-toxin in vitro [strains DU1090 and DU1090(pH35L), respectively] exhibited significantly lower virulence in vivo than the parental strain (decreased intravegetation staphylococcal densities). Paradoxically, the two variant staphylococcal strains producing alpha-toxin at supraparental levels in vitro [strains DU1090(p1212) and DU1090(pCL84::hla)] also exhibited significantly decreased induction rates and intravegetation staphylococcal densities in experimental endocarditis versus the parental strain. The reduced in vivo virulence of the latter variant staphylococcal strains could not be explained by differences in bacteremic clearance or initial adherence to sterile vegetations (compared to the parental strain). These findings suggest that the reduced virulence exhibited by the variant staphylococcal strains in this model was related to pathogenetic events subsequent to bacterial adherence to the damaged endocardium. Excess intravegetation secretion of alpha-toxin, leading to increased PMP release (secondary to either increased platelet secretion or lysis), may well explain the reduced virulence observed in experimental endocarditis.     

Corneal virulence of Staphylococcus aureus: roles of alpha-toxin and protein A in pathogenesis.

Staphylococcus aureus produces a variety of proteins, including alpha-toxin and protein A, that could contribute to corneal tissue damage during keratitis. We examined corneal infections produced by intrastromal injection of four S. aureus strains--three isogenic mutants, one lacking alpha-toxin (Hly- Spa+), one lacking protein A (Hly+ Spa-), and one lacking both alpha-toxin and protein A (Hly- Spa-), and the wild type (Hly+ Spa+)--in a rabbit model of experimental keratitis. Rabbit corneas were injected intrastromally with 100 CFU of one of the four strains, and the eyes were examined by slit lamp biomicroscopy over a 25-h period. Corneal homogenates were used for determination of CFU and neutrophil myeloperoxidase activity at 5-h intervals. All strains had the same logarithmic growth curve from 0 to 10 h postinfection, after which CFU remained constant at 10(7) CFU per cornea. By 15 h postinfection, slit lamp examination scores were significantly higher for eyes infected with Hly+ strains than for Hly(-)-infected eyes. At this time, distinct epithelial erosions were seen in Hly(+)-infected eyes but not in Hly(-)-infected eyes. Myeloperoxidase activity was significantly greater for Hly(+)-infected corneas than for Hly(-)-infected corneas at both 20 and 25 h postinfection. Spa(+)- and Spa(-)-infected eyes showed no differences in slit lamp examination scores or myeloperoxidase activities. These results suggest that alpha-toxin, but not protein A, is a major virulence factor in staphylococcal keratitis, mediating the destruction of corneal tissue in eyes infected with this bacterial pathogen.     

备用背根猫脊髓背核组织提取液对鸡胚背根节神经突起生长的影响

应用Liu和Chambers创立的备用背根模型,切除成年雄猫(5只)一侧的L_1～L_5、L_7～S_2背根节,保留L_6背根为备用背根,术后动物存活5d。分别制备脊髓T_(12)～L_3节段手术侧(实验组)、非手术侧(对照组)背核组织及其条件培养液。以Hanks平衡盐溶液的条件培养液作为参照组。用实验组、对照组以及参照组条件培养液对Hamburger 35期Leghorn鸡胚腰段背根部进行悬滴法培养,每只动物进行一批实验。于培养24h,48h观察测量各个背根节神经突起的平均长度。在各组背根节从培养24h到48h神经突起明显增长的基础上,求出每批培养物实验组、对照组背根节平均突起长度与参照组平均突起长度的比值以及5批培养物之平均比值。比较实验组、对照组平均比值在同一观测时间内的差异,发现两个观测时间实验组平均比值都明显大于对照组者。结果提示,猫脊髓经部分去后肢背根传入后,背核组织提取液促进神经突起生长的作用增强。     

Reaction of staphylococcal alpha-toxin with peptide-induced antibodies.

Two peptides representing separate 13-amino-acid sequences of staphylococcal alpha-toxin have been synthesized and acrylamide gel-purified alpha-toxin monomer and hexamer forms have been prepared and used to produce antisera in rabbits. We report here that each synthetic peptide, P-I and P-II, induces the formation of a specific precipitating antiserum. Moreover, these sera also react with the toxin monomer and sometimes with the hexamer, indicating that each peptide has more than one epitope. The purified toxin monomer can induce antibodies to fragments of toxin but is significantly less potent than the hexamer in inducing antibodies to the toxin monomer and almost not effective in inducing a response to the toxin hexamer. The purified toxin hexamer induces responses that are almost the reciprocals of the monomers, with the antihexamer and -monomer responses dominating and almost no responses to fragments of toxin being induced. These responses are interpreted in terms of the stability of the toxin hexamer to proteolytic degradation, compared with the relative sensitivity of the monomer to proteases. In assays of toxin-neutralization activity, only those sera containing antihexamer antibodies can block toxin hemolytic activity. This is true for both peptide- and toxin-induced antisera. The basis for this apparent association between toxin-neutralizing potency and antihexamer reactivity is being studied. Peptide P-I contains the uniquely reactive tyrosine residue and may be involved in monomer-to-monomer associations required to form hexamers. Peptide P-II is near the carboxyl terminus of alpha-toxin and may be involved in the binding of toxin to membranes. In a study of the ability of each peptide to inhibit the rate of hexamer formation induced by membrane lipoprotein, peptide P-I (as expected) proves to be more efficient than peptide P-II. Finally, one rabbit immunized with the toxin hexamer produces antibodies to peptides P-I and P-II. This finding suggests that the two synthetic peptides selected for study are relevant to the in vivo immunoprocessing of staphylococcal alpha-toxin.     

Antagonism of the ruthenium red-induced paralysis in mice by 4-aminopyridine, guanidine and lanthanum

4-Aminopyridine and guanidine were administered intraperitoneally to mice during the complete flaccid paralysis induced by treatment with ruthenium red (RuR). At 1-8 min after 4-aminopyridine or guanidine the animals had recovered completely from the paralysis, whereas the control mice injected only with RuR remained paralytic for at least 60 min. Intraperitoneal injections of LaCl3 had no apparent effects on animal motility and did not reverse the paralysis produced by RuR. However, when La3 + was administered 30 min prior to RuR the occurrence of flaccid paralysis was totally prevented. The results obtained are discussed in terms of the possible antagonist effects of the compounds used on acetylcholine release at neuromuscular junctions.     

Passive immunization with antiserum to a nontoxic alpha-toxin mutant from Staphylococcus aureus is protective in a murine model.

A nonhemolytic, nonlethal variant of Staphylococcus aureus alpha-toxin constructed via oligonucleotide-directed mutagenesis and containing a single amino acid substitution (H-35 to L) was used to immunize a rabbit. The resulting antiserum was cross-reactive with wild-type alpha-toxin and neutralized its hemolytic activity in vitro. Passive immunization of mice with rabbit antiserum conferred protection against lethal challenge with wild-type alpha-toxin and against acute lethal challenge with a high-alpha-toxin -producing S. aureus strain. H35L alpha-toxin may be useful as a protective immunogen in S. aureus vaccine studies.     

新生大鼠大脑皮质、纹状体及中脑黑质器官型脑片培养

目的探索大脑皮质、纹状体及黑质密部器官型脑片的体外培养。方法选出生2d内的W istar乳鼠,取大脑皮质、纹状体及黑质致密部,切成300μm厚的脑片,共同转至带有M illicell微孔膜插件的培养皿中。分别培养0d、10d、20d和30d,倒置显微镜观察,酪氨酸羟化酶(TH)免疫荧光染色及激光共聚焦显微镜下检测脑片摄取溴化已啶(EB)能力。结果黑质密部多巴胺能神经元(TH阳性)及突起逐渐长入纹状体中,约20d左右,纹状体及黑质致密部脑片长在一起,脑片中所有细胞EB染色呈阴性,说明无坏死细胞。培养30d时,脑片约10%神经元呈EB染色阳性,细胞变性坏死。结论培养20d内脑片已经形成多巴胺能神经元从黑质密部到纹状体通路,可用于帕金森病等神经退行性疾病研究的组织模型。     

NUMBER AND MORPHOLOGY OF MECHANORECEPTORS IN THE MYOTENDINOUS JUNCTION OF PARALYSED HUMAN MUSCLE

The mechanoreceptor system of the myotendinous junction (MTJ) of human palmaris longus muscle obtained at autopsy was studied histologically from six patients with flaccid paralysis (complete acute tetraplegia 4–6 weeks before the autopsy, due to a spinal cord injury), eight patients with spastic paralysis (chronic hemiplegia due to cerebral stroke) and ten neurologically normal controls. Four types of nerve endings, Ruffini and Pacini corpuscles, Golgi tendon organs, and free nerve endings, could be identified in the MTJs of the controls. In the MTJs of the patients with flaccid and spastic paralysis, the free nerve endings were not present and the mechanoreceptors that were found were few in number, degenerated, fibrotic, and atrophic. These mechanoreceptors had lost their connection with the muscle fibres and tendon bundles and were frequently located within pathological accumulations of fatty tissue in the myotendinous region. The number and distribution of mechanoreceptors in the MTJ were almost identical in patients with flaccid and spastic paralysis.     

Quantitative analysis of the binding and oligomerization of staphylococcal alpha-toxin in target erythrocyte membranes.

The binding of staphylococcal alpha-toxin to rabbit and human erythrocytes was quantitated over a wide range of toxin concentrations (3 x 10(-11) to 3 x 10(-6) M) with the use of an enzyme-linked immunosorbent assay that permitted simultaneous quantitation of monomeric and oligomeric toxin forms. Three basic observations were made. First, in no range of concentrations did the binding of alpha-toxin to rabbit erythrocytes display characteristics of a receptor-ligand interaction. Net binding to rabbit cells was nil at sublytic concentrations (10(-10) M or 3 ng/ml). The onset of binding occurred at around 10 ng/ml and remained fairly constant and ineffective (5 to 8% of toxin offered) over a wide concentration range (up to 10 micrograms/ml). Second, hemolysis of rabbit and human erythrocytes at 37 degrees C was always accompanied by the formation of toxin oligomers in the membrane. Third, overall toxin binding at 0 degree C followed a pattern similar to that at 37 degrees C. However, oligomer formation and cell lysis were retarded (but not totally inhibited) at 0 degree C. When rabbit erythrocytes were incubated with low levels of toxin at 0 degree C (0.5 microgram/ml) for 30 min, the toxin became bound exclusively in monomer form, and no lysis occurred. When cells thus treated were washed and suspended at 37 degrees C, lysis rapidly ensued, and native monomeric toxin was replaced by oligomeric toxin. The collective results directly support the oligomer pore concept of toxin action and also indicate that toxin oligomers form by lateral aggregation of bound monomers in the bilayer. They speak against the existence of specific binding sites for alpha-toxin on rabbit erythrocytes.     

Infection of mice with Neospora caninum (Protozoa: Apicomplexa) does not protect against challenge with Toxoplasma gondii.

Neospora caninum and Toxoplasma gondii are structurally related protozoal parasites of mammals that may cause abortion and neonatal morbidity and mortality. Groups of mice were subcutaneously inoculated with 10(5) live zoites of the NC-1 or NC-3 isolates of N. caninum and reinoculated with an identical number of live zoites 2 weeks later. Groups of mice which were injected subcutaneously with Hanks balanced salt solution served as controls. Three weeks after the final N. caninum inoculation, one-half of the mice were inoculated subcutaneously with 2.5 x 10(4) zoites of the RH isolate of T. gondii and the other half were inoculated subcutaneously with 2.5 x 10(4) zoites of the GT-1 isolate of T. gondii. Serum samples taken from mice on the day of T. gondii inoculation were negative for specific antibodies to T. gondii, but mice inoculated with N. caninum had reciprocal titers of greater than or equal to 800 to this protozoan. All of the mice died after challenge with T. gondii, and no significant differences (P greater than 0.05) between the survival times of mice inoculated with either isolate of N. caninum and those of control mice were seen. This study indicates that N. caninum and T. gondii are distinct biologic entities and not closely related isolates.     

Effect of pH and osmolality on in vitro phagocytosis and killing by neutrophils in urine.

Phagocytosis and intracellular killing of two strains of Escherichia coli and a Staphylococcus saprophyticus by polymorphonuclear neutrophils (PMN) in pooled sterile urine at three osmolalities (800, 485, and 200 mosM/kg of H2O) between pHs 5 and 8 was investigated. Urine at 800 mosM virtually abolished phagocytosis of both E. coli strains, regardless of pH, and reduced the phagocytosis of S. saprophyticus to 30%; no killing of any organisms took place at this osmolality. On the other hand, phagocytosis was a good in urine as in Hanks balanced salt solution at both 485 and 200 mosM between pHs 6 and 8. Phagocytosis of all three strains was virtually abolished at pH 5. Killing of the strains by PMN was optimal between pHs 6.5 and 7.5 in urine at 485 mosM (being at least 90% of the control values in Hanks balanced salt solution), whereas at 200 mosM killing was reduced to 50 to 70% of these values. Reduced killing of all three strains occurred at pH 8, whereas at pH 6 only S. saprophyticus was killed. Thus, the bactericidal activity of PMN in urine was more sensitive than phagocytic function to alterations in pH. The dominant modulating factor affecting PMN function in urine of 500 mosM or less was pH, but osmolality had a greater influence at 800 mosM. Thus, raising the pH of urine and reducing the osmolality may increase the ability of natural defense mechanisms to eliminate infecting organisms.     

Mechanism of leukotriene generation in polymorphonuclear leukocytes by staphylococcal alpha-toxin

The effects of staphylococcal alpha-toxin on arachidonic acid metabolism in rabbit polymorphonuclear leukocytes (PMNs) were investigated and compared with those of the ionophore A23187 and the chemotactic tripeptide formylmethionyl-leucyl-phenylalanine (fMLP). Sublytic amounts of alpha-toxin stimulated the release of leukotriene B4 (LTB4) in PMNs in a dose-dependent manner. The toxin was several times more potent than fMLP but was not as effective as the ionophore. Preincubation of the toxin with neutralizing antibodies abolished the effect. Extracellular calcium was strictly required for eliciting LTB4 generation. Verapamil, a calcium channel blocker, inhibited fMLP-mediated LTB4 generation but had no effect on alpha-toxin- or A23187-exposed PMNs. Agents such as trifluoperazine and N-6(aminohexyl)-5-chloro-1-naphthalene sulfonamid that interfered with calmodulin activity, however, inhibited LTB4 generation in all cases. One minute after the addition of alpha-toxin, PMNs exhibited a severalfold enhancement in passive permeability to 45Ca2+. In addition, these cells became permeable to sucrose but not to inulin or dextran. The influx pattern was consistent with the previous observation that alpha-toxin creates discrete transmembrane channels in erythrocytes with an effective internal diameter of 2 to 3 nm. The results suggest that alpha-toxin triggers the arachidonic acid pathway in PMNs by facilitating calcium influx into the cells, possibly via transmembrane toxin pores that serve as calcium gates. Generation of arachidonic acid metabolites in PMNs by sublytic amounts of alpha-toxin may represent an important cellular reaction that generally occurs during infections with Staphylococcus aureus.     

The neuropathology observed in wild-type mice inoculated with human poliovirus mirrors human paralytic poliomyelitis

Human paralytic poliomyelitis results from the destruction of spinal cord anterior horn motor neurons by human poliovirus (PV). CNS disease pathology similar to human poliomyelitis has been observed in experimentally infected chimpanzees, monkeys and wild-type mice. In this study we present a detailed examination of the clinical and histopathological features in the wild-type mouse after intracranial (i.c.) and novel intramuscular (i.m.) injection of poliovirus. Either route of poliovirus administration results in a clinical disease characterized predominately by flaccid paralysis. The observed histopathological features are compared with the histopathology reported for human paralytic poliomyelitis, experimentally infected chimpanzees, monkeys and transgenic mice expressing the human poliovirus receptor (hPVR). The observation of flaccid paralysis and anterior horn motor neuron destruction mirrors what is observed in human paralytic poliomyelitis. Our results suggest that the neuropathology observed in the wild-type mouse model is similar to what has been observed in both the human disease and in other experimental animal models, with the possible exception of the transgenic mouse model. The observed neuropathology of the wild-type mouse model more closely reflects what has been observed in human poliomyelitis, as well as in experimentally infected chimpanzees and monkeys, than does the hPVR transgenic mouse model. The previously reported poliovirus-induced white matter demyelinating disease was not observed.     

Interferons increase cell resistance to Staphylococcal alpha-toxin

Many bacterial pathogens, including Staphylococcus aureus, use a variety of pore-forming toxins as important virulence factors. Staphylococcal alpha-toxin, a prototype β-barrel pore-forming toxin, triggers the release of proinflammatory mediators and induces primarily necrotic death in susceptible cells. However, whether host factors released in response to staphylococcal infections may increase cell resistance to alpha-toxin is not known. Here we show that prior exposure to interferons (IFNs) prevents alpha-toxin-induced membrane permeabilization, the depletion of ATP, and cell death. Moreover, pretreatment with IFN-α decreases alpha-toxin-induced secretion of interleukin 1β (IL-1β). IFN-α, IFN-β, and IFN-γ specifically protect cells from alpha-toxin, whereas tumor necrosis factor alpha (TNF-α), IL-6, and IL-4 have no effects. Furthermore, we show that IFN-α-induced protection from alpha-toxin is not dependent on caspase-1 or mitogen-activated protein kinases, but requires protein synthesis and fatty acid synthase activity. Our results demonstrate that IFNs may increase cell resistance to staphylococcal alpha-toxin via the regulation of lipid metabolism and suggest that interferons play a protective role during staphylococcal infections.     

Early events in the action of staphylococcal alpha-toxin on the plasma membrane of adrenocortical Y1 tumor cells.

The early events in staphylococcal alpha-toxin action on mouse adrenocortical (Y1) tumor cells were studied. Cell-bound toxin could be partially neutralized by anti-alpha-toxin and inactivated by trypsin added within 10 min at 37 degrees C after the end of the binding step. Likewise, cell-bound toxin was capable of lysing rabbit erythrocytes (RRBC) added to the cells within 10 min after binding at 37 degrees C. After this time, the Y1 cells could not be rescued from intoxication by antibodies or trypsin, and the toxin was not accessible for lysis of RRBC. However, at 0 to 4 degrees C, the cell-bound toxin remained accessible to antibodies for at least 4 h. CaCl2 (30 mM) did not affect binding of the toxin to Y1 cells but completely prevented the intoxication if added within 10 min at 37 degrees C after the end of the binding step. The intoxication was independent of metabolic energy, active receptor clustering on the cell surface, and endocytosis of the toxin. Therefore, alpha-toxin interacted with the Y1 cell membrane in at least three separable steps: binding, a conformational change at the cell surface, and membrane damage. These early events appear to be similar to those occurring on RRBC treated with alpha-toxin.     

The in vitro : Uptake of Rabbit Antibody by Chopped Guinea-pig Lung and its Relationship to Anaphylactic Sensitization

With the use of chopped guinea-pig lung, previously perfused with Tyrode's solution, the rate and extent of uptake of antibody γ globulin has been measured using ¹³¹I-labelled rabbit antibody. The reversibility of uptake was studied by soaking the tissue in Tyrode's solution or in a solution of unlabelled γ globulin. Uptake and wash-off of antibody resembled a process of reversible adsorption, and occurred both at low temperatures and when dead tissue was used. Wash-off of labelled antibody was more rapid in the presence of added normal rabbit γ globulin than in its absence. Simultaneous measurements were made of the degree of sensitization, defined as the ability of the tissue to release histamine on subsequent contact with antigen for a standard period of time at 37°. It was found that a plateau level of sensitization had been reached in a few minutes, when only a small fraction of the total antibody eventually taken up had been adsorbed. Under the conditions of our experiments, it was not possible to wash off the antibody sufficiently to de-sensitize the tissue. Evidence was obtained that antigen was most effective in releasing histamine when added over a broad but definite range of antigen excess.

By pre-incubation of the labelled antibody with lung tissue, or by `screening' in a living guinea pig, it was shown to be unlikely that sensitization was due to taking up by the lung of a specific readily adsorbed fraction of antibody. When the antibody preparation was separated by chromatography into physico-chemically distinct fractions, there were quantitative differences, but all were taken up and caused sensitization of the tissue. The mechanism of sensitization of guinea-pig lung by rabbit antibody is discussed, and is considered to be one of reversible adsorption of γ globulin at unidentified sites.