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Induction of mesangial interleukin-6 synthesis by apoptotic U937 cells and monocytes
S Heidenreich1, T Sato, M Schmidt
1Department of Nephrology, University Hospital Leiden, The Netherlands.
Abstract:
Infiltration of the glomerular mesangium by monocytes and macrophages is a central pathologic feature in various forms of glomerulonephritis. Dependent on the presence and activity of local survival factors, monocytes may undergo apoptosis. Therefore, we looked for the interaction between cultured human mesangial cells (HMC) and intact, necrotic or apoptotic monocytic cells with different stages of programmed cell death (U937 cells and blood-derived human monocytes) and the possible evoked secretory responses of HMC. Interleukin-6 (IL-6) synthesis of HMC after a two hour co-culture with late apoptotic U937 cells was significantly increased (505 +/- 55 pg/ml) as compared to intact U937 cells (349 +/- 27 pg/ml; HMC alone, 319 +/- 62 pg/ml), and was further elevated after 20 hours (815 +/- 108 pg/ml). U937 cells alone, after incubation in HMC-conditioned medium or after coincubation with HMC, did not produce any detectable IL-6. A high mesangial IL-6 synthesis in response to apoptotic U937 cells was dependent on the cellular contact between HMC and U937 and could not be mimicked by apoptotic U937 culture supernatants. Radiolabeling studies indicated that HMC bound (16.6 +/- 2.4%) and ingested (12.5 +/- 1.9%) apoptotic U937 cells to a much higher amount as compared to intact U937 (5.3 +/- 2.0% binding; 5.0 +/- 1.1% phagocytosis). Binding and ingestion of monocytic cells undergoing apoptosis was confirmed by morphologic studies using electron microscopy. Incubation of HMC with a blocker of the CD36/ vitronectin receptor (VnR) dependent recognition mechanism of phagocytes for apoptotic leukocytes (RGDS peptide) did not alter binding, phagocytosis or IL-6 synthesis of HMC in response to apoptotic U937. Phospho-L-serine as an antagonist of the phosphatidylserine (PS) mediated recognition pathway for apoptotic cell disposal was able to reduce binding and IL-6 production by HMC but not phagocytosis. Thus, binding of apoptotic monocytic cells by HMC rather than ingestion may be the prerequisite for a stimulated secretory response. To elucidate whether binding and phagocytosis of particles in general might stimulate HMC to produce IL-6, we looked for mesangial IL-6 production after binding and ingestion of opsonized zymosan particles. In this case, IL-6 synthesis was markedly down-regulated. Furthermore, HMC proliferated after zymosan treatment, whereas after apoptotic cell uptake the mesangial cell number remained constant. In conclusion, apoptotic monocytic cells provoked an enhanced mesangial IL-6 synthesis by a PS-dependent recognition mechanism. This secretory response may have secondary implications for humoral or cellular processes within the mesangium.
Insights
Human mesangial cells (HMC) increase interleukin-6 (IL-6) production when interacting with apoptotic monocytic cells via phosphatidylserine (PS) recognition. This interaction, crucial in glomerulonephritis, highlights a specific cellular communication pathway.
Area of Science:
- Nephrology and Immunology
- Cell Biology
- Pathophysiology of Glomerulonephritis
Background:
- Monocyte and macrophage infiltration of the glomerular mesangium is a key feature of glomerulonephritis.
- Monocytes can undergo apoptosis within the mesangium, influenced by local survival factors.
- Understanding the interaction between mesangial cells and apoptotic monocytes is crucial for deciphering inflammatory processes in the kidney.
Purpose of the Study:
- To investigate the interaction between human mesangial cells (HMC) and apoptotic monocytic cells.
- To determine the secretory responses of HMC, specifically Interleukin-6 (IL-6) production, upon encountering apoptotic cells.
- To elucidate the recognition mechanisms involved in HMC-apoptotic cell interactions.
Main Methods:
- Co-culture of cultured human mesangial cells (HMC) with intact, necrotic, and apoptotic U937 cells and primary human monocytes.
- Quantification of IL-6 synthesis in HMC using ELISA.
- Assessment of HMC binding and phagocytosis of monocytic cells using radiolabeling and electron microscopy.
- Inhibition studies using RGDS peptide (CD36/vitronectin receptor blocker) and phospho-L-serine (phosphatidylserine antagonist).
Main Results:
- HMC exhibited significantly increased IL-6 synthesis upon co-culture with late apoptotic U937 cells, dependent on cellular contact.
- HMC demonstrated higher binding and ingestion of apoptotic U937 cells compared to intact cells.
- Phosphatidylserine (PS) mediated recognition was identified as a key pathway for HMC binding and IL-6 production in response to apoptotic monocytic cells, while CD36/vitronectin receptor pathway was not involved.
- Binding and phagocytosis of zymosan particles by HMC led to down-regulated IL-6 synthesis and proliferation, contrasting with the response to apoptotic cells.
Conclusions:
- Apoptotic monocytic cells stimulate enhanced mesangial IL-6 synthesis through a phosphatidylserine (PS)-dependent recognition mechanism.
- The binding of apoptotic monocytic cells, rather than ingestion, appears to be the primary trigger for HMC's enhanced secretory response.
- This PS-dependent interaction may have significant implications for the inflammatory and cellular processes within the glomerulus during glomerulonephritis.