Absence of T Lymphocyte-Derived Cytokines Fails to Diminish Macrophage 12/15-Lipoxygenase Expression In Vivo

Abstract

IL-4 and IL-13 are the only known activators of 15-lipoxygenase (LO) expression in cultured macrophages. To determine whether these lymphocyte-derived cytokines regulate 15-LO expression in vivo, the abundance of the murine homologue (12/15-LO) was assessed in peritoneal macrophages from immune-deficient strains of mice. Macrophages were harvested from recombinase activator gene (RAG)-2−/− mice that do not develop mature lymphocytes and cannot secrete activation-dependent cytokines. Unexpectedly, 12/15-LO protein and activity were significantly increased in peritoneal macrophages from RAG-2−/− mice compared with strain-matched controls. This increase was related to phenotypic differences between cells from RAG-2+/+ and RAG-2−/− mice. After 3 h in culture, RAG-2+/+ macrophages were of two distinct sizes, with only the larger cells immunostaining for 12/15-LO. However, all RAG-2−/− cells were distributed in the large size range, and all were immunoreactive for the enzyme. The activation of 12/15-LO expression appears to be related to prolonged residence within the peritoneum, since there were fewer resident peritoneal macrophages in RAG-2−/− than in RAG-2+/+ mice, and newly recruited macrophages elicited by the administration of Sephacryl to RAG-2−/− mice did not immunostain for 12/15-LO. To determine whether 12/15-LO expression was due to IL-4 or IL-13 from nonlymphoid cells, the abundance of the enzyme was quantified in peritoneal macrophages from STAT6−/− mice that have attenuated responses to both cytokines. STAT6 deficiency did not influence the abundance of the protein in macrophages. Therefore, neither IL-4 nor IL-13 secretion is a requirement for macrophage 15-LO expression in vivo.