TABLE 2 Dextran endocytosis

CME, clathrin-mediated endocytosis; DFR, DC-SIGN (dendritic cell–specific intercellular adhesion molecule [ICAM]-3-grabbing nonintegrin) family receptors;FPE, fluid-phase endocytosis; L-SIGN, liver/lymph node-specific intercellular adhesion molecule (ICAM)-3-grabbing nonintegrin; MDDC, monocyte-derived dendritic cell; MR, mannose receptor.

Use of dextran as a marker for different endocytosis processes requires the discrimination between CME, phagocytosis, and FPE. In CME the uptake of dextran can be dependent on receptors including MR, DC-SIGN (human), L-SIGN (human), SIGN-R1 (mouse), SIGN-R3 (mouse), and langerin. CME is available for particles up to 200 nm (72). Uptake of small particles via CME (and other endocytosis mechanisms) is sometimes called phagocytosis. This term has specific implications. Phagocytosis indeed uses the machinery of different types of endocytosis at the initial stage. However, owing to the initiation of additional mechanisms, it allows uptake of much bigger particles of 500 to 2000 nm or more in diameter. Phagocytosis of dextran-based or dextran-covered particles can be dependent on the same receptors as CME (MRs, DFRs, langerin). Dextrans dissolved in media can be taken up by FPE mechanisms independent of ligand recognition. In the case of FPE, potential mechanisms include macropinocytosis or cdc42-dependent―so-called CLIC/GEEC―pinocytosis. The main molecules participating in this process are clathrin-independent carriers (CLICs) and glycosylphosphatidylinositol-enriched endocytic compartments (GEECs). Different endocytosis mechanisms may be activated simultaneously.

Fluorescently labeled dextrans became quite popular in endocytosis studies when Schröder et al. first developed fluorescently labeled dextran (fluorescein isothiocyanate, FITC-dextran) in 1976 (73). Ohkuma and Poole published their classical work on lysosomal acidification control using FITC-dextran in 1978 (74). In recent decades the labeled dextrans have been used extensively as lysosomal markers (75). They were used to evaluate FPE (76), endocytic activity in general (77), phagocytosis (78, 79), macropinocytosis (80), and macropinocytosis plus MR-mediated uptake (19). They were also applied as the ligands of MR (81), SIGN-R1 (35), and as the ligand of MR and DC-SIGN simultaneously (57). All the terms clathrin-mediated endocytosis, phagocytosis, fluid-phase endocytosis, and macropinocytosis applied to dextran (or dextran-containing particles) as an endocytotic or lysosomal marker are applicable, but in different cases: dependent on cell types and phenotypes.

When clinical dextran is injected into the bloodstream, one part is taken up by cells, another part is excreted by the kidney and a third part is retained in the bloodstream. Ratio of these parts depends on the molecular weight and the dose (for more specific data see (39, 82, 83)). The main organs of dextran uptake are liver, spleen, lung, and kidney. From the blood, dextran can enter into interstitial fluid, then the lymph, and then back to the bloodstream. Hepatocytes are able to transport small amounts of dextran to the bile (39, 84-87). Kidney filtration of dextran is dependent on the molecular mass/size: molecules smaller than ~50 kDa are excreted quickly, whereas larger ones stay in the blood longer (Figure 3A and B) (85, 88). Cells that take up dextran are able to metabolize it slowly into glucose by acid and neutral a‑glucosidases expressed in all cell types (89-92). These glucose molecules participate in glucose metabolism and can yield dextran-derived exhaled carbon dioxide (93, 94).

Figure 3. A) Dextran metabolism and excretion pathways. Dextran from the blood circulates in the interstitial fluid and lymph ducts and interacts with most cell types. The main organs of active dextran uptake are the liver, spleen, and lungs. Kidney cells take up dextran via pinocytosis and do not metabolize it, providing only temporarily retention. B) Time dependence of clinical dextran excretion and metabolism.After dextran injection, kidneys excrete the fractions with low molecular mass. Heavier fractions circulate in the body fluids or are taken up into the endosomes. Endosomal compartment volume is limited and some injected dextran may remain in the circulation. In the endosomes, dextrans are metabolized to glucose or excreted by transcytosis. Owing to metabolism, new endosomal volume becomes available and can be filled with dextran molecules from the blood. Thus the dextran endosomal pool depletes when dextran concentration in the blood does not provide its renewal. LSEC, liver sinusoidal endothelial cells.

DEXTRAN DERIVATIVES IN TUBERCULOSIS, CANDIDIASIS, AND INFLUENZA MODELS

Dextran has shown to be inert to DC cytokine reactions while the ligands of pathogens binding to MR and DFRs restrict Th1 response (12, 13). The studies of dextran or dextran-drug conjugates in models of bacterial, fungal, and viral infections that are dependent on dextran-binding receptors (Table 3) are of great interest. In such models, the dextran core is able to interfere with pathogen–macrophage and pathogen-DC interaction. Possible inhibition of pathogen uptake or changes in immune response by dextran should influence infection outcomes and several studies confirm this notion.