Regulation of myeloid cell survival and homeostasis by c-FLIP
Macrophages play vital roles in pathogen clearance, initiation of immune responses, and maintenance of immune homeostasis; however, current understanding of the contributions of macrophages to these processes <italic>in vivo</italic> has been limited by the reagents and animal models available. Of the many macrophage-deficient mouse models that have been reported, none have specific, long-term loss of distinct macrophage populations, and most develop infections that complicate the study of immune homeostasis. By conditionally deleting the anti-apoptotic protein cellular FLICE-like inhibitory protein (c-FLIP) in myeloid cells, we have generated a novel mouse model that has proven useful in studying both the <italic>in vivo</italic> functions of macrophages under steady-state conditions and the requirements for macrophage survival at steady-state and during inflammation.
c-FLIP<super>f/f</super> Lysm-Cre mice specifically lack bone marrow macrophages and splenic marginal zone macrophages and develop severe neutrophilia, splenomegaly, extramedullary hematopoiesis, decreased body weight, and increased production of G-CSF and IL-1β, but not IL-17 secondary to the loss of these macrophage populations. c-FLIP<super>f/f</super> Lysm-Cre mice exhibit delayed clearance of circulating neutrophils, suggesting that failure of macrophages to efficiently clear apoptotic neutrophils causes production of cytokines that drive excess granulopoiesis. Further, blocking G-CSF, but not IL-1R signaling in vivo rescues this neutrophilia, suggesting that a G-CSF-dependent, IL-1β-independent pathway plays a role in promoting neutrophil production in mice with defective clearance of apoptotic cells.
Furthermore, using mice expressing only one c-FLIP isoform in myeloid cells (c-FLIPS or c-FLIPL), we have shown that although either isoform is sufficient to promote survival of macrophages under steady-state conditions, both c-FLIPS and c-FLIPL required for macrophage survival during inflammation. In contrast, c-FLIPL is sufficient to promote survival of eosinophils in inflammatory conditions in the absence of c-FLIPS. These data demonstrate distinct requirements for myeloid cell survival in the presence and absence of inflammation and point to a mechanism by which pathogenic organisms may target macrophages to evade the immune response.
Together, these findings demonstrate a critical role for c-FLIP in promoting macrophage survival, which is in turn required for neutrophil homeostasis, and provide an <italic>in vivo</italic> model system for continuing studies of the non-redundant functions of c-FLIPS and c-FLIPL in myeloid cells during infection and inflammation.
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