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In summary, conventional CAPD fluids may interfere with most pathways of the intraperitoneal network of cytokines (Figure 4), potentially resulting in an impairment of important host defense functions. Thus alternative approaches are needed in order to improve dialysis fluid biocompatibility. Initial results indicate that pH neutralization and a restriction to moderate osmolality might constitute important determinants of modern dialysis fluid technology; however, clinical studies will have to prove that these fluids indeed improve the clinical outcome of the patients on long-term CAPD therapy.

Cytokine control of the synovial infiltrate is a central process in the development of inflammatory arthritis. In this study, we combine genetic approaches and intervention strategies to describe a fundamental requirement for IL-6-mediated STAT3 signaling in orchestrating the inflammatory infiltrate in monoarticular and systemic models of experimental arthritis. STAT3 activation via the common gp130 signal-transducing receptor for all IL-6-related cytokines led to increased retention of neutrophils and T cells within the inflamed synovium, which included STAT3-regulated IL-17A-secreting T cells. Control of leukocyte infiltration was reliant upon IL-6 signaling via its soluble receptor (termed IL-6 trans signaling), as evidenced by selective blockade of this alternative IL-6 signaling pathway using an engineered variant of soluble gp130 (sgp130Fc). This therapeutic intervention led to substantial clinical improvement in mice with emerging or established incidence of systemic arthritis. These data illustrate that IL-6 control of STAT3 is critical for regulating the synovial infiltrate in inflammatory arthritis, and suggest that selective inhibition of IL-6 trans signaling may provide a more refined intervention strategy for blocking IL-6-driven proarthritic activities.

Solutions were formulated to examine, independently, the roles of osmolality and glucose in the reduction of viability and inhibition of phagocyte function by dextrose-containing peritoneal dialysis fluids. The exposure of neutrophils (polymorphonuclear leukocytes) to test fluids containing > or = 2.7% (wt/vol) glucose resulted in significant cytotoxicity as assessed by the release of lactate dehydrogenase above control values (7.12 +/- 2.65%). At the highest concentration of glucose (4.5%), lactate dehydrogenase release was 15.83 +/- 0.49% (P < 0.05). These effects were directly related to the presence of D-glucose in the test fluids. In contrast, phagocytosis and the release of leukotriene B4 from PMN stimulated with serum-treated zymosan were significantly inhibited in an osmolality-, but not glucose-, dependent manner. The inhibition of tumor necrosis factor alpha and interleukin-6 release from mononuclear leukocytes was inhibited by a combination of osmolality and monosaccharide concentration. Under the same conditions, PMN respiratory burst activation remained unaffected irrespective of glucose concentration or fluid osmolality. These data indicate that, in addition to the low pH of peritoneal dialysis fluid and its high lactate concentration, its glucose content (either directly or as a consequence of the resulting hyperosmolality of the fluid) inhibits cell functional parameters. These findings suggest clinically significant inhibition of host defense mechanisms because, in high-glucose dialysis fluids, osmolality does not reach physiologic values, even during extended intraperitoneal dwell periods.

The successful resolution of inflammation is dependent upon the coordinated transition from the initial recruitment of neutrophils to a more sustained population of mononuclear cells. IL-6, which signals via the common receptor subunit gp130, represents a crucial checkpoint regulator of neutrophil trafficking during the inflammatory response by orchestrating chemokine production and leukocyte apoptosis. However, the relative contribution of specific IL-6-dependent signaling pathways to these processes remains unresolved. To define the receptor-mediated signaling events responsible for IL-6-driven neutrophil trafficking, we used a series of gp130 knockin mutant mice displaying altered IL-6-signaling capacities in an experimental model of acute peritoneal inflammation. Hyperactivation of STAT1 and STAT3 in gp130Y757F/Y757F mice led to a more rapid clearance of neutrophils, and this coincided with a pronounced down-modulation in production of the neutrophil-attracting chemokine CXCL1/KC. By contrast, the proportion of apoptotic neutrophils in the inflammatory infiltrate remained unaffected. In gp130Y757F/Y757F mice lacking IL-6, neutrophil trafficking and CXCL1/KC levels were normal, and this corresponded with a reduction in the level of STAT1/3 activity. Furthermore, monoallelic ablation of Stat3 in gp130Y757F/Y757F mice specifically reduced STAT3 activity and corrected both the rapid clearance of neutrophils and impaired CXCL1/KC production. Conversely, genetic deletion of Stat1 in gp130Y757F/Y757F mice failed to rescue the altered responses observed in gp130Y757F/Y757F mice. Collectively, these data genetically define that IL-6-driven signaling via STAT3, but not STAT1, limits the inflammatory recruitment of neutrophils, and therefore represents a critical event for the termination of the innate immune response.