In vivo binding and effects of concanavalin A (Con A) on rat and mouse pulmonary alveolar epithelial cells and macrophages

Glycan-directed SARS-CoV-2 inhibition by leek extract and lectins with insights into the mode-of-action of Concanavalin A

Four years after its outbreak, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains a global challenge for human health. At its surface, SARS-CoV-2 features numerous extensively glycosylated spike proteins. This glycan coat supports virion docking and entry into host cells and at the same time renders the virus less susceptible to neutralizing antibodies. Given the high genetic plasticity of SARS-CoV-2 and the rapid emergence of immune escape variants, targeting the glycan shield by carbohydrate-binding agents emerges as a promising strategy. However, the potential of carbohydrate-targeting reagents as viral inhibitors remains underexplored. Here, we tested seven plant-derived carbohydrate-binding proteins, called lectins, and one crude plant extract for their antiviral activity against SARS-CoV-2 in two types of human lung cells: A549 cells ectopically expressing the ACE2 receptor and Calu-3 cells. We identified three lectins and an Allium porrum (leek) extract inhibiting SARS-CoV-2 infection in both cell systems with selectivity indices (SI) ranging between >2 and >299. Amongst these, the lectin Concanavalin A (Con A) exerted the most potent and broad activity against a panel of SARS-CoV-2 variants. We used multiplex super-resolution microscopy to address lectin interactions with SARS-CoV-2 and its host cells. Notably, we discovered that Con A not only binds to SARS-CoV-2 virions and their host cells, but also causes SARS-CoV-2 aggregation. Thus, Con A exerts a dual mode-of-action comprising both, antiviral and virucidal, mechanisms. These results establish Con A and other plant lectins as candidates for COVID-19 prevention and basis for further drug development.

Lectin-induced nitric oxide production

Considering that nitric oxide (NO) may be involved in anti-tumoral and anti-parasite lectin effects, in this report we investigated whether lectin induces NO production. Lectins from Canavalia brasiliensis, Dioclea grandiflora, Pisum arvense (PAA), and concanavalin A induced murine peritoneal cells to produce NO in vitro. PAA induced similar levels to that obtained with lipopolysaccharide plus interferon-gamma. NO production by adherent cells was significantly lower than that of unfractionated cells, suggesting a combination of lectin stimuli directly on macrophages and via lymphocyte stimulation. Ex vivo experiments showed that cells stimulated in vivo could maintain NO production in vitro without further stimuli. NO synthesis blockage in vivo can significantly increase cell numbers in draining lymph nodes after lectin injection compared to unblocked controls, suggesting an in vivo association of lectin stimuli and NO production. Taken together these data show that lectins can induce NO production both in vitro and in vivo.

Analysis of differentiation and transformation of cells by lectins

During differentiation cells are known to change their biological behavior according to their genotype. This is thought to be accompanied by a modulation of cell surface determinants expressed on the outer cell membrane. Vice versa, cell surface molecules are suggested to mediate extracellular signals to the genome. Most of these molecules integrated in the cell membrane have been proven to be glycoconjugates. The carbohydrate moieties of these molecules can be detected by means of lectins that are characterized by their ability to react specifically with distinct terminal sugar sequences. Thus, lectins have been used as appropriate tools for studying the modulation of functionally important membrane-associated molecules during the differentiation of cells, in particular of B- and T-lymphocytes. Moreover, lectins have been proven to distinguish between differentiated cells and malignant cell clones, according to the hypothesis that transformed cells possess a glycoconjugate profile that corresponds to the stage of differentiation at which they are arrested. Since lectins, like monoclonal antibodies, make it possible to study functionally important molecules that are associated with differentiation and malignancy, they might be of value for diagnostic purposes and, moreover, for analyzing malignant transformation.

Processing of concanavalin A-receptor complexes by rat osteoclasts in vitro

The osteoclast is a large multinucleate cell that is widely accepted as the primary effector cell responsible for normal bone resorption. In a previous study, we demonstrated that concanavalin A (con A) has a dose-dependent biphasic effect on the bone-resorbing capacity of osteoclasts, using a 45Ca bone-organ culture system; bone resorption was stimulated at low concentrations and inhibited at high concentrations. The mitogenic property of con A in lymphocyte cultures is well documented; therefore con A has been used extensively to study the manner in which lymphocytes and other mononuclear cells process the cell-bound lectin. In this study, we have investigated the processing of con A-receptor complexes by osteoclasts in culture, using con A-FITC to evaluate the redistribution of cell-bound con A via epifluorescence microscopy and using con A-ferritin to determine whether the lectin receptor complexes are internalized. The osteoclasts were obtained from the long bones of newborn rats and allowed to attach to glass coverslips at 37 degrees C. Following attachment, the nonadherent cells were removed by rinsing. The adherent osteoclasts were preincubated in 50 micrograms/ml con A-FITC or con A-ferritin at 4 degrees C for 10 min, washed to remove unbound con A, and incubated at 37 degrees C for 15 or 30 min in the absence of con A. Positive controls were fixed immediately after preincubation at 4 degrees C; negative controls were preincubated in con A-FITC and alpha-methyl mannoside, the haptenic inhibitor of con A binding. The results demonstrate that redistribution and endocytosis of con A-receptor complexes occurs within 30 min. These findings confirm the hypothesis that cell-bound con A can alter the structure and activity of osteoclast membrane components in a manner similar to that observed in mononuclear cell cultures. The internalization of con A may be important in altering osteoclastic activity by mediating intracellular mechanisms involved in the bone-resorbing process.

Lectin binding and surface glycoprotein pattern of human macrophage populations

In the present study unstimulated and stimulated human blood monocytes, untreated and phorbol ester treated U-937 cells, as well as human peritoneal and alveolar macrophages were studied with respect to their surface membrane properties. Binding of different lectins and electrophoretic patterns of tritium labeled surface glycoproteins were compared. The analysis of surface glycoproteins could be interpreted as evidence for a common origin of the analysed cell populations. Furthermore, banding patterns of glycoproteins might be useful to define certain activation states within monocyte/macrophage differentiation. In contrast, lectin binding pattern did not clearly discriminate macrophage subpopulations.

The effects of lectins on the interaction between macrophages and bone in vitro. A morphological and functional study

Recent studies have demonstrated that the attachment of elicited rat macrophages to bone is mediated by specific saccharides located on the cell and/or bone surfaces. We have used a macrophage-bone culture system to study the effects of two lectins, concanavalin A (con A) and soybean agglutinin (SBA), on the morphology of macrophage attachment to a devitalized bone surface and subsequent functional activity. Macrophages were obtained from 3- to 4-week-old rats by peritoneal lavage and the adherent pool was used to prepare cell suspensions. Con A-treated, SBA-treated or control cell suspensions were aliquoted onto the endocranial surface of devitalized rat calvariae. The cells were allowed to attach for 1 h at 37 degrees C, after which, the bone samples were removed from culture and prepared for scanning electron microscopy (SEM). The morphology of con A-treated macrophages attached to bone was markedly different from that of control or SBA-treated cells. Con A altered the attachment and subsequent spreading of macrophages on bone as visualized by SEM. Furthermore, the number of con A-treated cells that attached to bone and the average surface area of cell membrane apposed to the matrix was significantly different from that of control or SBA-treated cells. A 45Ca bone-release assay was performed to evaluate the functional significance of the morphological findings. Lectin-treated or control cell suspensions were allowed to attach to the endocranial surface of 45Ca pre-labeled calvariae for 1 h. Following attachment, the samples were cultured for 72 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum antibody response and germinal centre reaction in the upper respiratory tract after antigen challenge in mice

In order to establish an experimental model for the study of immunological reactions to exogenic immunogens in the upper and lower respiratory system NMRI mice were subjected to inhalation of the mitogenic and inflammatory response inducing lectin concanavalin A (Con A) and bovine serum albumin (BSA). Immunological reactions were induced in the upper respiratory tract and in the lungs. Serologically, the course of the antibody titre was measured by radio-immunoassay over several weeks. Germinal centre reactions in the nose and paranasal sinus equivalents were the morphological correlate. The lungs showed an increase in the number of macrophages and a thickening of the alveolocapillary membrane, but no germinal centre reactions were observed. These morphological and serological results may indicate a priming of the immune response in the upper respiratory tract leading to a subsequent production of antibodies in the lung.