Differences in carbohydrate specificities and complement-activating capacity of guinea pig and human antibodies to neuraminidase-treated autologous erythrocytes

Binding of serum autoantibodies to sialidase-treated tracheal epithelial cells. Determination of autoantibodies isotypes in normal and influenza virus infected guinea pig sera

Cultured epithelial cells isolated from guinea pig trachea were treated with Vibrio cholerae sialidase. The treatment was not cytotoxic and resulted in membrane desialylation as assessed by measurement of sialic acids released, along with an increased fixation of the galactose-specific lectin peanut agglutinin. After incubation in serum from normal guinea pigs, membrane-bound immunoglobulins were detected using peroxidase-labelled antibodies. Sialidase-treated cells bound significantly more IgM than controls (P < 0.0005), whereas binding of IgG was not significantly different between treated and untreated cells (0.1 < P < 0.375); IgA were never detected. In influenza-infected guinea-pigs, as assessed by reactivity with peanut agglutinin, the tracheal and lung epithelium, as well as alveolar cells were hyposialylated. In these animals, the level of serum IgG autoantibodies capable to bind sialidase treated cultured cells increased, while the level of IgM autoantibodies did not change. These autoantibodies may participate in cellular dysfunctions and modified bronchoreactivity that occur during infection of the respiratory tract by sialidase-producing microorganisms, either through activation of the complement system, or subsequently to their reaction with cells expressing membrane complement and/or Fc receptors.

An enzyme-linked lectin assay for sialidase

A procedure for the detection of low activities of sialidase (= neuraminidase) is described. Natural substrates for sialidase (human erythrocytes, fetuin or gangliosides) were coated onto the wells of microplates and incubated at 37 degrees C with the enzyme. Sialidase-induced desialylation of these natural substrates unmasks saccharides that are specifically recognized by the peanut agglutinin lectin (PNA). The use of a peroxidase-conjugated PNA (Po-PNA) allowed the binding of the lectin to the desialylated substrate to be quantified. The amount of bound Po-PNA correlated directly with the amount of sialic acid released from the substrate, and therefore with the sialidase activity. With this method, it was possible to detect sialidase activity associated with bacteria, myxoviruses and cells from higher organisms. This method may have important clinical implications as the use of ELISA allows automation and concurrent analysis of numerous samples.

Measurement of anti-influenza neuraminidase antibody using a peroxidase-linked lectin and microtitre plates coated with natural substrates

Neuraminidase-induced removal of sialic acid from natural substrates (desialylation) unmasks saccharides that are specifically recognized by the lectin peanut agglutinin (PNA). We demonstrate that, when a neuraminidase substrate is coated on to the wells of a microplate, it is possible to quantitate the binding of PNA to the desialylated substrate using a peroxidase-conjugated PNA (Po-PNA). The amount of bound PNA correlated directly with the amount of sialic acid removed from the substrate and therefore with the neuraminidase activity. By reacting with specific epitopes that are located near to the enzyme active site, anti-neuraminidase antibodies are capable of inhibiting the virus-induced desialylation of the substrate. Such antibodies therefore reduce the binding of Po-PNA. The advantage of this assay is that since different natural substrates for neuraminidase (erythrocytes, fetuin or gangliosides) can be used to coat the microplates, the capacity of anti-neuraminidase antibody to inhibit the neuraminidase activity towards different types of sialoglycoconjugates can be evaluated. Anti-hemagglutinin or non-specific anti-neuraminidase antibody have no interfering reactivity.

Anticarbohydrate autoantibodies to sialidase-treated erythrocytes and thymocytes in serum from patients with pulmonary sarcoidosis

PURPOSE

The presence of immunoglobulins and complement in sarcoid granulomata and bronchoalveolar lavage from patients with sarcoidosis suggests that humoral mechanisms may be of importance in granuloma formation. To test this hypothesis, we examined the possibility that antibodies to specific tissue carbohydrates causing alterations and/or dysfunction of immunocompetent cells might be present during sarcoidosis. Because we had previously shown the presence of sialidase activity in bronchoalveolar lavage from these patients, we have looked for the presence of antibodies that recognize sialidase-treated erythrocytes (mostly antigalactose) in the serum of patients with sarcoidosis. Since thymocytes are spontaneously recognized by peanut agglutinin, a lectin that binds galactose, the reactivity of serum from sarcoidosis patients with normal or neuraminidase-treated thymocytes has also been studied.

PATIENTS AND METHODS

Serum samples were obtained from the venous blood of patients with biopsy-proven sarcoidosis, most of whom had no extrathoracic symptoms. The mean patient age was 31 years, with a range from 21 to 57 years. There were 12 women and 19 men, and 10% of the patients were smokers. Sarcoidosis was classified as recent if symptoms had been present for less than 1 year and chronic if symptoms had been present for longer than this. Control serum samples were obtained from patients with idiopathic pulmonary fibrosis (n = 9) and from healthy volunteers (n = 15). Furthermore, serum from patients who had previously had sarcoidosis but in whom cures had been achieved was also studied (n = 6).

RESULTS

Sialidase-treated erythrocytes were lysed in autologous serum upon incubation at 37 degrees C providing that the serum came from a patient with active disease. Serum from either normal volunteers or patients with resolved sarcoidosis had no significant cytotoxic activity. Lysis proceeded through activation of the classical complement pathway following fixation of autoantibodies. These antibodies were predominantly of the IgM class. They were able to agglutinate neuraminidase-treated thymocytes, whereas untreated thymocytes did not fix the antibodies. Carbohydrate inhibition experiments demonstrated that these antibodies are mostly galactose specific. As this sugar is located immediately below the sialic acid residues in the carbohydrate moiety of membrane glycoconjugates, it is unmasked following sialidase treatment.

CONCLUSION

Since galactose has been shown to be present on the membrane of certain subsets of immunocompetent cells (e.g., lymphocytes and macrophages either spontaneously or after stimulation), it is possible that antigalactose antibodies may affect the metabolism of these cells, leading to some of the immune dysfunctions that are observed during sarcoidosis.

Modifications of sialidase activity during the monocyte-macrophage differentiation in vitro

Human mononuclear cells were isolated from peripheral blood by centrifugation over Ficoll Hypaque, followed by adherence to plastic dishes. Monocyte-derived macrophages were obtained after culture for 3 or 5 days of the adherent cells in RPMI medium containing 20% heat-inactivated foetal calf serum. The sialidase activities were assayed in the whole homogenate using sodium 4-methyl-umbelliferyl-alpha-D-neuraminate as substrate, at various pHs, ranging from 3.6 to 6. The in vitro differentiation of monocytes into macrophages from day 0 up to day 5 was accompanied by a significant (P less than or equal to 0.01) increase in the sialidase activity on both a per-cell (+360%) and a per-mg protein in the homogenate (+125%) basis.

Carbohydrate interference of complement-dependent cell lysis

The antibody-mediated cytotoxicity of three autoreactive sera, an allogeneic hyperimmune serum and a xenogeneic hyperimmune serum was abrogated by the presence of either glucosamine, galactosamine, lactulose or lactose. This inhibition could be overcome in a dose-dependent fashion by increasing the amount of complement in the cytotoxicity assay, but not by increasing the amount of antibody. Furthermore, the inhibition was specific for these sugars in that isomers and N-acetylated derivatives were not inhibitory. The results suggest that these sugars directly blocked events of the complement cascade.

Lysosomal and cytosolic sialidases in rabbit alveolar macrophages: demonstration of increased lysosomal activity after in vivo activation with bacillus Calmette-Guerin

Sialidase activity was assayed in homogenized rabbit alveolar macrophages using a fluorogenic substrate: sodium 4-methylumbelliferyl-alpha-D-neuraminate. After differential centrifugation one acid-active enzyme (optimum pH 4.2) was detected in the 16,000 X g pellet that contained lysosomes, mitochondria and peroxisomes. A second activity, with an optimum pH of 5.4, was found in the cytosolic fraction. The acid-active sialidase accounted for more than 95% of the total sialidase activity in crude homogenate. When alveolar macrophages were collected from rabbits stimulated with bacillus Calmette-Guerin (BCG), the acid-active sialidase specific activity was increased 2.5-fold whereas other lysosomal enzymes such as N-acetylglucosaminidase and beta-galactosidase were stable. The cytosolic sialidase activity did not change.