Optimizing Lectin Staining Methodology to Assess Glycocalyx Composition of Legionella-Infected Cells

Introduction: Legionella is a gram-negative bacterium that replicates intracellularly within macrophages. Legionella utilizes effector proteins to hijack ER-Golgi vesicle trafficking to sustain proliferation in its intracellular niche. Legionella has a considerable influence on O-glycosylation but not N-glycosylation events in the Golgi of infected cells. This research aims to optimize the use of fluorescent lectins, which are proteins that bind carbohydrates, to effectively label host-cell glycocalyx during Legionella infection. Methods: Epifluorescence imaging or flow cytometry were used to optimize the lectin staining methodology. We noted that the most effective conditions for lectin-labeling were when live HeLa cells were incubated with lectins diluted in Hank’s balanced salt solution (HBSS) with 3% Bovine serum albumin (BSA) for 10-30 minutes at 4 °C. Results: Incubating suspended cells with lectins necessitated smaller lectin concentrations, whereas lectin labeling of adherent cells required considerably larger concentrations. Wheat germ agglutinin (WGA) lectin mean fluorescence intensity (MFI) was concentration-dependent, but Concanavalin A (ConA) and Maclura pomifera (MPA) MFIs did not alter substantially with increasing lectin concentrations. Discussion: The optimal lectin concentration required was lectin-specific and based on whether the lectin fluorescence was assessed using flow cytometry or epifluorescence. Furthermore, the use of phosphate-buffered saline (PBS) for lectin dilution, cell permeabilization for intracellular labelling, and incubation of lectins in fixed cells reduced productive labelling of lectins on cell surfaces because it inhibited the lectin's ability to effectively bind the associated carbohydrate structure. Conclusion: Further research using diverse lectins on U937 macrophages is necessary to reach a definitive conclusion on the effect of Legionella on the overall host-cell glycocalyx composition during infection of these relevant immune cells.