Innate Immune Recognition Triggers Secretion of Lysosomal Enzymes by Macrophages

Abstract

Gamma interferon-induced lysosomal thiolreductase (GILT) is expressed constitutively in antigen-presenting cells, where it reduces disulfide bonds to facilitate antigen presentation. GILT is synthesized as an enzymatically active precursor protein and is processed in early endosomes to yield the mature enzyme. The exposure of the promonocytic cell line THP-1 to Escherichia coli causes a differentiation-dependent induction of GILT expression in which the majority of precursor GILT is secreted as active enzyme. We confirm this result in cultured primary monocytes and macrophages, and demonstrate, as an in vivo correlate of the phenomenon, upregulation of precursor GILT levels in the serum of mice injected with lipopolysaccharide. We show that macrophage differentiation is accompanied by a transcriptional downregulation of mannose-6-phosphorylation, which likely prevents the recognition and proper sorting of soluble lysosomal enzymes by the mannose-6-phosphate receptors. We provide evidence for a mechanism of generalized soluble lysosomal enzyme secretion through the constitutive secretory pathway.

Disulfide Reduction in the Endocytic Pathway: Immunological Functions of Gamma-Interferon-Inducible Lysosomal Thiol Reductase

“…GILT is constitutively expressed in most APCs, including monocytes=macrophages, B cells (primary and cell lines), and bone-marrow derived DCs (3,25,29,30,34,36). GILT is also constitutively expressed in thymocytes (37), mature T cells (5,37), and some fibroblasts (9,64).…”

“…IFN-g plays an important role in inducing expression of MHC class II and other components of the class II-restricted processing pathway, including Ii and HLA-DM (10). IFN-g rapidly induces GILT in many cell types, including immature monocytes and monocyte precursors, other fibroblasts, human umbilical vein endothelial cells, and melanoma cell lines, with mRNA detectable within 30 min to 2 h and maximal protein expression at 48 h (21,29,30,34). IFN-g signals through a specific cell surface receptor followed by activation of Janus kinases 1 and 2 and signal transducer and activator of transcription (STAT) 1.…”

“…Additionally, macrophage differentiation has been identified as a distinct pathway of induction of GILT expression. Treatment with phorbol-12-myristate-13-acetate or Toll-like receptor (TLR) 4 ligands such as lipopolysaccharide or whole E. coli bacteria induces differentiation of immature monocytes and monocyte precursors as demonstrated by adherence, secretion of proinflammatory cytokines and upregulation of antigen processing and presentation components, including GILT (29,30,34). TLR4-mediated induction of GILT protein continues to increase at 60 h, which is delayed in comparison to IFN-ginduced expression (29).…”

“…A portion of precursor GILT is secreted as an enzymatically active disulfide-linked dimer (29,30,44). In B cells constitutively synthesizing GILT or in IFN-g-treated monocytes, intracellular mature GILT is the dominant form (3,29).…”

“…In B cells constitutively synthesizing GILT or in IFN-g-treated monocytes, intracellular mature GILT is the dominant form (3,29). However, in TLR4-stimulated monocytes the majority of GILT generated is secreted as the precursor rather than being transported to and maturing in the lysosomes (29,30). Indeed, GILT levels are increased in the serum of mice following sublethal LPS injection (30).…”