Bacterial/CpG DNA down-modulates colony stimulating factor-1 receptor surface expression on murine bone marrow-derived macrophages with concomitant growth arrest and factor-independent survival.

Unmethylated CpG motifs within bacterial DNA constitute a pathogen-associated molecular pattern recognized by the innate immune system. Many of the immunomodulatory functions of bacterial DNA can be ascribed to the ability to activate macrophages and dendritic cells. Here we show stimulatory DNA, like LPS, caused growth arrest of murine bone marrow-derived macrophages proliferating in CSF-1. Stimulatory DNA caused selective down-modulation of CSF-1 receptor surface expression. Flow cytometric analysis of CSF-1-deprived bone marrow-derived macrophages revealed that in contrast to the synchronous reduction of CSF-1 receptor upon CSF-1 addition, activating DNA (both bacterial DNA and CpG-containing oligonucleotide) caused rapid removal of receptor from individual cells leading to a bimodal distribution of surface expression at intermediate times or submaximal doses of stimulus. Despite causing growth arrest, both stimulatory DNA and LPS promoted factor-independent survival of bone marrow-derived macrophages, which was associated with phosphorylation of the mitogen-activated protein kinase family members, extracellular-regulated kinase 1 and 2. CSF-1 receptor down-modulation may polarize the professional APC compartment to the more immunostimulatory dendritic cell-like phenotype by suppressing terminal macrophage differentiation mediated by CSF-1.

The actions of bacterial DNA on murine macrophages.

Murine macrophages are able to distinguish bacterial from mammalian DNA. The response is mimicked by single-stranded oligonucleotides containing unmethylated CG dinucleotides ("CpG" motifs) in specific sequence contexts. The dose-response curve for activation is influenced by variation in the sequence flanking the core CpG motif. CpG or bacterial DNA activates several signaling pathways in common with bacterial lipopolysaccharide (LPS), leading to induction of cytokine genes such as tumor necrosis factor alpha. Pretreatment with LPS causes desensitization to subsequent activation by CpG DNA. Both stimuli also cause cell cycle arrest in macrophages proliferating in response to the macrophage growth factor colony-stimulating factor-1 (CSF-1), but prevent apoptosis caused by growth factor removal. In part, cell cycle arrest by CpG DNA and LPS may be linked to rapid down-modulation of the CSF-1 receptor from the cell surface, a response that occurs in an all-or-nothing manner. The response of macrophages to CpG DNA has aspects in common with the DNA damage response in other cell types, which may provide clues to the underlying mechanism.