Role of cell-to-cell variability in activating a positive feedback antiviral response in human dendritic cells.

In the first few hours following Newcastle disease viral infection of human monocyte-derived dendritic cells, the induction of IFNB1 is extremely low and the secreted type I interferon response is below the limits of ELISA assay. However, many interferon-induced genes are activated at this time, for example DDX58 (RIGI), which in response to viral RNA induces IFNB1. We investigated whether the early induction of IFNBI in only a small percentage of infected cells leads to low level IFN secretion that then induces IFN-responsive genes in all cells. We developed an agent-based mathematical model to explore the IFNBI and DDX58 temporal dynamics. Simulations showed that a small number of early responder cells provide a mechanism for efficient and controlled activation of the DDX58-IFNBI positive feedback loop. The model predicted distributions of single cell responses that were confirmed by single cell mRNA measurements. The results suggest that large cell-to-cell variation plays an important role in the early innate immune response, and that the variability is essential for the efficient activation of the IFNB1 based feedback loop.

A common polymorphism in the caspase recruitment domain of RIG-I modifies the innate immune response of human dendritic cells.

Infection of human dendritic cells (DCs) by negative-strand RNA viruses, such as Newcastle disease virus, leads to the induction of the IFNbeta gene, IFNB1, through the activation of the RNA helicase RIG-I, which is encoded by DDX58. Expression levels of IFNB1 and DDX58 in infected DCs showed positive correlations at the population and the single-cell levels. DDX58 has a common and potentially functional single nucleotide polymorphism, rs10813831 (A/G), encoding an Arg7Cys amino acid change in the RIG-I protein caspase recruitment domain (CARD). Quantitative RT-PCR analysis on Newcastle disease virus-infected primary DCs from 130 individuals revealed a significant association of the Arg7Cys single nucleotide polymorphism with increased IFNB1 and DDX58 transcription. Allelic imbalance analysis ruled out allele-specific DDX58 message levels and suggested that the observed association between Arg7Cys and IFNB1 and DDX58 transcription originated from a functional change in RIG-I due to the amino acid substitution in the CARD. DDX58 transfection experiments in 293T cells confirmed a biological functional difference between RIG-I 7Cys and the more common RIG-I 7Arg. Taken together, these data indicate that the innate immune response to viral infection of human cells is modified by a functional polymorphism in the RIG-I CARD.

Induction of autophagy in axonal dystrophy and degeneration.

Autophagy is a highly regulated cellular mechanism for the bulk degradation of cytoplasmic contents. It has been implicated in a variety of physiological and pathological conditions relevant to neurological diseases. However, the regulation of autophagy in neurons and its role in neuronal and axonal pathology are not yet understood. Using transgenic mice producing green fluorescent protein-tagged autophagic marker microtubule-associated protein light chain 3 (GFP-LC3), we provide molecular evidence for the induction of autophagy in axonal dystrophy and degeneration in Purkinje cells of the Lurcher mice, a model for excitotoxic neurodegeneration. We show that the excitotoxic insult of Lurcher mutation triggers an early response of Purkinje cells involving accumulation of GFP-LC3-labeled autophagosomes in axonal dystrophic swellings (a hallmark of CNS axonopathy). In brain, LC3 interacts with high affinity with the microtubule-associated protein 1B (MAP1B). We show that MAP1B binds to LC3 of both cytosolic form (LC3I) and lipidated form (LC3II). Moreover, in cell culture, overexpression of MAP1B results in reduced LC3II levels and number of GFP-LC3-labeled autophagosomes; phosphorylated MAP1B is associated with GFP-LC3-labeled autophagosomes. Furthermore, in brain, phosphorylated MAP1B accumulates in axonal dystrophic swellings of degenerating Purkinje cells and binds to LC3 at increased level. Therefore, the MAP1B-LC3 interaction may participate in regulation of LC3-associated autophagosomes in neurons, in particular at axons, under normal and pathogenic conditions. We propose that induction of autophagy serves as an early stress response in axonal dystrophy and may participate in the remodeling of axon structures.