Biomarker discovery

Identification of albumin receptors and characterization of albumin intracellular trafficking in human peripheral blood mononuclear cells

The issue

Several studies showed a correlation between serum albumin concentration and the severity of hepatic and septic disease severity and mortality rates, leading to the indication of parenteral albumin therapy in the case of decompensated liver diseases and septic shock. The exact mechanisms by which albumin supplementation may improve the clinical outcome are not known. However, several laboratories have demonstrated immunomodulatory and anti-inflammatory effects of albumin therapy in complication of cirrhosis. In this context, we proposed to identify the main cellular population in the blood that internalize the albumin, the receptors leading to albumin intracellular storage and the endocytic mechanisms by which albumin enter in the cells.

The approach

This project addresses three different objectives related with the trafficking of the albumin molecule in peripheral immune cells: 

  • Identify the peripheral human blood cells that most efficiently internalize the albumin molecule;
  • Identify the albumin receptor(s) in this process; and
  • Investigate the intracellular trafficking pathway of albumin after binding to its receptor(s) following their internalization.

Using peripheral blood samples from healthy donors and cirrhotic patients, we identified that classical monocytes, which are the most abundant cell population in the blood, are the most efficient cells in albumin up-take. Of interest, during systemic inflammation, classical monocytes are crucial players in the production of pro-inflammatory mediators.

By co-immunoprecipitations and mass-spectrometry analysis of immunoprecipitated material, we identified the neonatal Fc receptor (FcRn) and CD44 as potential albumin receptors in classical monocytes. Whether CD44 serves as an albumin receptor remains to be investigated. However, FcRn depletion by lentiviral shRNA confirmed that in the absence of FcRn, classical monocyte ability to store the albumin is decreased. Finally, by cell biology methods, such as protein depletion via shRNA, confocal microscopy and flow cytometry, we identified that after internalization, the albumin is first targeted to early endosomes and arrive in lysosomes, a cellular compartment from where endosomal Toll-like receptors trigger the inflammatory response. The endocytic pathway by which albumin is internalized in early endosomes is clathrin-independent and involves dynamin-2, ARF1 and ARF6.

Considering that FcRn, dynamin-2, ARF1 and ARF6 expression levels are essential for albumin internalization by the classical monocytes, these proteins and the number of classical monocytes might be useful for predicting the patient response to albumin infusions.