Macrophages respond to infection with Legionella pneumophila by the induction of inflammatory mediators, including type I Interferons (IFN-Is). To explore whether the bacterial second messenger cyclic 3’-5’ diguanylate (c-diGMP) activates some of these mediators, macrophages were infected with L. pneumophila strains in which the levels of bacterial c-diGMP had been altered. Intriguingly, there was a positive correlation between c-diGMP levels and IFN-I expression. Subsequent studies with synthetic derivatives of cdiGMP, and newly described 3’-5’ diadenylate (c-diAMP), determined that these molecules activate overlapping inflammatory responses in human and murine macrophages. Moreover, UV cross-linking studies determined that both dinucleotides physically associate with a shared set of host proteins. Fractionation of macrophage extracts on a biotin-c-diGMP affinity matrix led to the identification of a set of candidate host binding proteins. These studies suggest that mammalian macrophages can sense and mount a specific inflammatory response to bacterial dinucleotides.

Major- and minor-group human rhinoviruses (HRV) enter their host by binding to the cell surface molecules ICAM-1 and LDL-R, respectively, which are present on both macrophages and epithelial cells. Although epithelial cells are the primary site of productive HRV infection, previous studies have implicated macrophages in establishing the cytokine dysregulation that occurs during rhinovirus-induced asthma exacerbations. Analysis of the transcriptome of primary human macrophages exposed to major- and minor-group HRV demonstrated differential gene expression. Alterations in gene expression were traced to differential mitochondrial activity and signaling pathway activation between two rhinovirus serotypes, HRV16 (major-group) and HRV1A (minor-group), upon initial HRV binding. Variances in phosphorylation of kinases (p38, JNK, ERK5) and transcription factors (ATF-2, CREB, CEBP-alpha) were observed between the major- and minor-group HRV treatments. Differential activation of signaling pathways led to changes in the production of the asthma-relevant cytokines CCL20, CCL2, and IL-10. This is the first report of genetically similar viruses eliciting dissimilar cytokine release, transcription factor phosphorylation, and MAPK activation from macrophages, suggesting that receptor use is a mechanism for establishing the inflammatory microenvironment in the human airway upon exposure to rhinovirus.

Although rhinoviral infections, a major cause of asthma exacerbations, occur predominantly in upper airway bronchial epithelial cells, monocytic-lineage cells are implicated in establishing the inflammatory microenvironment observed during the disease. Human rhinovirus (HRV) is unique in that nearly genetically identical viruses bind either the ICAM-1 or low-density lipoprotein receptor (LDL-R). Within minutes of binding, HRV is capable of eliciting a signaling response in both epithelial cells and monocyte-derived macrophages. It is unclear whether this signaling response is important to the subsequent release of inflammatory mediators, particularly in cells not capable of supporting viral replication. We show here that the small molecular mass G-protein Rac is activated following exposure of macrophages to HRV serotypes known to be ICAM-1- and LDL-R-tropic. We demonstrate that inhibiting Rac resulted in the upregulation of TLR3 in macrophages exposed to major- and minor-group HRV, and resulted in increased release of IFN-α. Furthermore, inhibiting Rac in HRV-exposed macrophages attenuated activation of the stress kinase p38 and release of the pro-inflammatory cytokine CCL2, but inhibiting Rac did not affect release of the pro-inflammatory cytokine CCL5. These findings suggest that Rac is an important regulator in establishing the inflammatory microenvironment that is initiated in the human airway upon exposure to rhinovirus.

Viral respiratory infections are a major cause of asthma exacerbations and can contribute to the pathogenesis of asthma. Major‐ and minor‐group human rhinoviruses (HRV) enter cells by binding to the cell surface molecules ICAM‐1 and LDL‐R that are present on epithelial cells and macrophages. The focus of the resulting viral infection is in bronchial epithelia. Despite this, previous studies of HRV infection and subsequent cytokine dysregulation have implicated macrophages as playing a role in establishing the inflammatory environment seen in HRV infection and asthma exacerbation. We demonstrate that the small molecular‐weight G‐protein Rac is differentially activated by the binding of major‐ and minor‐group rhinovirus to macrophages, that MCP‐1 release differs between the two viruses, and that inhibition of Rac attenuates the activation of the stress kinase p38 and the release of MCP‐1. Interestingly, RANTES release is not affected by Rac inhibition. This is the first report of a relationship involving major‐ or minor‐group HRV exposure, small molecular‐weight G‐protein activation, MCP‐1 release and macrophages, suggesting that Rac plays a role in establishing the inflammatory microenvironment initiated in the human airway upon exposure to rhinovirus. This work was supported by the Wriston scholarship, NIH R15 AI065505‐01A1 and NSF 0521112.