Pyrin Critical to Macrophage IL-1β Response to Francisella Challenge

Relative to monocytes, human macrophages are deficient in their ability to process and release IL-1 beta. In an effort to explain this difference, we used a model of IL-1 beta processing and release that is dependent upon bacterial escape into the cytosol. Fresh human blood monocytes were compared with monocyte-derived macrophages (MDM) for their IL-1 beta release in response to challenge with Francisella novicida. Although both cell types produced similar levels of IL-1 beta mRNA and intracellular pro-IL-1/3, only monocytes readily released processed mature IL-1 beta. Baseline mRNA expression profiling of candidate genes revealed a remarkable deficiency in the pyrin gene, MEFV, expression in MDM compared with monocytes. Immunoblots confirmed a corresponding deficit in MDM pyrin protein. To determine whether pyrin levels were responsible for the monocyte/MDM difference in mature IL-1 beta release, pyrin expression was knocked down by nucleofecting small interfering RNA against pyrin into monocytes or stably transducing small interfering RNA against pyrin into the monocyte cell line, THP-1. Pyrin knockdown was associated with a significant drop in IL-1 beta release in both cell types. Importantly, M-CSF treatment of MDM restored pyrin levels and IL-1 beta release. Similarly, the stable expression of pyrin in PMA-stimulated THP-1-derived macrophages induces caspase-1 activation, associated with increased IL-1 beta release after infection with F. novicida. In summary, intracellular pyrin levels positively regulate MDM IL-1 beta responsiveness to Francisella challenge. The Journal of Immunology, 2009, 182: 7982-7989.