The main regulators of leukocyte trafficking during inflammatory responses are chemokines. However, another class of recently identified chemotactic agents is extracellular cyclophilins, the proteins mostly known as receptors for the immunosuppressive drug, cyclosporine A. Cyclophilins can induce leukocyte chemotaxis in vitro and have been detected at elevated levels in inflamed tissues, suggesting that they might contribute to inflammatory responses. We recently identified CD147 as the main signaling receptor for cyclophilin A. In the current study we examined the contribution of cyclophilin-CD147 interactions to inflammatory responses in vivo using a mouse model of acute lung injury. Blocking cyclophilin-CD147 interactions by targeting CD147 (using anti-CD147 Ab) or cyclophilin (using nonimmunosuppressive cyclosporine A analog) reduced tissue neutrophilia by up to 50%, with a concurrent decrease in tissue pathology. These findings are the first to demonstrate the significant contribution of cyclophilins to inflammatory responses and provide a potentially novel approach for reducing inflammation-mediated diseases.

Extracellular cyclophilins have been well described as chemotactic factors for various leukocyte subsets. This chemotactic capacity is dependent upon interaction of cyclophilins with the cell surface signaling receptor CD147. Elevated levels of extracellular cyclophilins have been documented in several inflammatory diseases. We propose that extracellular cyclophilins, via interaction with CD147, may contribute to the recruitment of leukocytes from the periphery into tissues during inflammatory responses. In this study, we examined whether extracellular cyclophilin-CD147 interactions might influence leukocyte recruitment in the inflammatory disease allergic asthma. Using a mouse model of asthmatic inflammation, we show that 1) extracellular cyclophilins are elevated in the airways of asthmatic mice; 2) mouse eosinophils and CD4+ T cells express CD147, which is up-regulated on CD4+ T cells upon activation; 3) cyclophilins induce CD147-dependent chemotaxis of activated CD4+ T cells in vitro; 4) in vivo treatment with anti-CD147 mAb significantly reduces (by up to 50%) the accumulation of eosinophils and effector/memory CD4+ T lymphocytes, as well as Ag-specific Th2 cytokine secretion, in lung tissues; and 5) anti-CD147 treatment significantly reduces airway epithelial mucin production and bronchial hyperreactivity to methacholine challenge. These findings provide a novel mechanism whereby asthmatic lung inflammation may be reduced by targeting cyclophilin-CD147 interactions.

Summary CD147 is a type I transmembrane glycoprotein expressed on a wide variety of cell types, including all leucocytes. While CD147 is best known as a potent inducer of matrix metalloproteinases, it can also function as a regulator of leucocyte migration through its cell surface interaction with chemotactic extracellular cyclophilins. A potential role for CD147–cyclophilin interactions during inflammatory diseases, including rheumatoid arthritis (RA), is suggested from several studies. For example, CD147 expression is increased on reactive leucocytes in the synovial fluid and tissues of patients with arthritis. In addition, the synovial fluid of patients with RA contains high levels of extracellular cyclophilin A. In the current studies we investigated the contribution of the chemotactic function of CD147–cyclophilin interactions to joint inflammation using the mouse model of collagen‐induced arthritis. Our data demonstrate that proinflammatory leucocytes, specifically neutrophils, monocytes and activated CD4+ T cells, lose their ability to migrate in response to cyclophilin A in vitro when treated with anti‐CD147 monoclonal antibody. Furthermore, in vivo treatment with anti‐CD147 monoclonal antibody can reduce the development of collagen‐induced arthritis in mice by > 75%. Such findings suggest that CD147–cyclophilin interactions might contribute to the pathogenesis of RA by promoting the recruitment of leucocytes into joint tissues.

Chemokines contribute to inflammatory responses by inducing leukocyte migration and extravasation. Extracellular cyclophilins are a group of alternative chemokines that can be elevated during inflammation, and we have previously shown that their inhibition reduces neutrophil infiltration by >40% in a mouse model of LPS-induced acute lung inflammation. Given the presence of many other chemokines during this inflammatory response, the observed potent contribution of cyclophilins led us to investigate whether they might function in a synergistic partnership with concurrent chemokines. To test whether cyclophilin A (CypA) can work with other chemokines in vivo, recombinant extracellular CypA was co-delivered with intranasal LPS and lung neutrophil numbers were examined at a time point when two classical chemokines, MIP-2 and KC, but not CypA, are present. The addition of CypA induced a significant increase in early neutrophil recruitment that was greater than the additive recruitment induced by LPS and CypA alone. Boyden chamber chemotaxis assays in which purified mouse neutrophils were incubated with various combinations of CypA plus MIP-2 or KC were used to show that CypA had the capacity to synergize with MIP-2, but not KC, to induce augmented neutrophil migration. These findings provide a potential explanation for why the inhibition of cyclophilin activity in vivo has such a great impact on cell recruitment.

Cyclophilins can function extracellularly as chemotactic agents for leukocytes. Elevated levels of extracellular cyclophilins have been observed in many inflammatory diseases. Previous studies in our laboratory suggest that extracellular cyclophilins contribute to asthmatic lung inflammation, although the mechanism of contribution is unknown. The objectives of the current study were to establish: (1) the cell source of extracellular cyclophilins and when they play a role in inflammation, (2) the impact of blocking cyclophilins using NIM811 (Novartis), a non‐immunosuppressive form of cyclosporine A, and (3) the mechanism(s) by which cyclophilins contribute to inflammation. Using a mouse model of acute allergic asthma, elevated levels of cyclophilins were observed throughout the asthmatic response, potentially from infiltrating leukocytes. NIM811 treatment resulted in a 30–50% reduction in numbers of eosinophils and effector CD4+ T cells within lung tissues and airways, suggesting that extracellular cyclophilins function by promoting leukocyte recruitment. Blocking extracellular cyclophilins may provide a novel approach for reducing the pathology associated with allergic asthma. Funded by NIH AI‐059208.