Calcium Oxalate Stone Fragment and Crystal Phagocytosis by Human Macrophages

In murine and human hyperoxaluric conditions macrophages can be seen surrounding renal calcium oxalate crystal deposits. We hypothesized that macrophages have a role in degrading and destroying these deposits. We investigated the inflammatory response and phagocytic mechanisms when macrophages were exposed to human kidney stones and inorganic crystals.

Human monocytes were differentiated into resting, fully differentiated macrophages by treatment with recombinant human macrophage colony-stimulating factor (M-CSF) or GM-CSF (granulocyte M-CSF) for 6 days. After confirming phenotype by flow cytometry the macrophages were exposed for 20 hours to fragments of sterile human calcium oxalate stones or calcium oxalate crystals. Crystal uptake was determined, and supernatant cytokine and chemokine profiles were analyzed using antibody arrays. Quantitative reverse transcriptase-polymerase chain reaction was done to validate mRNA profile expression.

Under direct vision fluorescence microscopy activated human macrophages were noted to surround stone fragments and synthesized crystals, and destroy them in a step-by-step process that involved clathrin mediated endocytosis and phagocytosis. An inflammatory cascade was released by macrophages, including the chemokines chemokine ligand (CCL)2, CCL3, interleukin (IL)-1 receptor antagonist (IL-1ra), complement component C5/C5a and IL-8. Response patterns to stone and crystal material depended on macrophage phenotype and activation status.

In our in vitro study macrophages differentiated with M-CSF showed greater ability to phagocytize crystal deposits than those treated with GM-CSF. Following clathrin mediated endocytosis macrophages released a number of cytokines that are crucial for the inflammatory immune response. This suggests that tissue macrophages have an important role in preventing kidney stone disease by removing and digesting interstitial renal crystal deposits.