Induction of mesangial interleukin-6 synthesis by apoptotic U937 cells and monocytes

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.