Summary Zwitterionic capsular polysaccharides (ZPS) are bacterial products that modulate T cells, including inducing anti-inflammatory IL-10-secreting T regulatory cells (Tregs). However, only a few diverse bacteria are known to modulate the host immune system via ZPS. We present a genomic screen for bacteria encoding ZPS molecules. We identify diverse host-associated bacteria, including commensals and pathogens with known anti-inflammatory properties, with the capacity to produce ZPSs. Human mononuclear cells stimulated with lysates from putative ZPS-producing bacteria induce significantly greater IL-10 production and higher proportions of Tregs than lysates from non ZPS-encoding relatives or a commensal strain of Bacteroides cellulosilyticus in which a putative ZPS biosynthetic operon was genetically disrupted. Similarly, wild-type B. cellulosilyticus DSM 14838, but not a close relative lacking a putative ZPS, attenuated experimental colitis in mice. Collectively, this screen identifies bacterial strains that may use ZPS to interact with the host as well as those with potential probiotic properties.

Human cytomegalovirus (HCMV) is a member of the Herpesviridae family that manipulates host immune responses and establishes life-long latent infection, in part through mimicry of cytokines, chemokines, and chemokine receptors. The HCMV US27 gene product is a putative chemokine receptor with no known ligands. We generated a stable US27 cell line to screen for chemokine ligands but unexpectedly found that US27 potentiated the activity of an endogenous human chemokine receptor, CXCR4. Cells expressing both US27 and CXCR4 exhibited greater calcium mobilization and enhanced chemotaxis in response to CXCL12/ SDF-1α than controls. Quantitative RT-PCR revealed a significant increase in CXCR4 expression when US27 was present, and elevated CXCR4 receptor levels were detected via flow cytometry, western blot, and immunofluorescence microscopy. Potentiation of CXCR4 signaling by US27 could represent a novel strategy by which HCMV targets virus-infected cells to the bone marrow in order to expand the reservoir of latently infected cells.

BackgroundHuman cytomegalovirus (HCMV) is ubiquitous in the population but generally causes only mild or asymptomatic infection except in immune suppressed individuals. HCMV employs numerous strategies for manipulating infected cells, including mimicry of G-protein coupled receptors (GPCRs). The HCMV US27 gene product is a putative GPCR, yet no ligand or signaling has been identified for this receptor. In the present study, immunofluorescence microscopy was used to examine the cellular distribution of wild type US27, as well as US27 deletion mutants and chimeric receptors.ResultsIn transiently transfected cells, wild type US27 was found primarily in intracellular compartments, in striking contrast to the cell surface distribution seen for the human cellular chemokine receptor CXCR3. When the N-terminal extracellular domains of the two receptors were swapped, no change in protein localization was observed. However, swapping of the C-terminal intracellular domains resulted in a significant change in receptor distribution. A chimera that contained US27 fused to the C-terminal intracellular tail of CXCR3 exhibited surface distribution similar to that of wild-type CXCR3. When the C-terminal domain of US27 was fused to CXCR3, this chimeric receptor (CXCR3/US27-CT) was found in the same intracellular pattern as wild-type US27. In addition, a US27 mutant lacking the C-terminus (US27ΔCT) failed to accumulate inside the cell and exhibited cell surface distribution. Co-localization with organelle-specific markers revealed that wild-type US27 was found predominantly in the Golgi apparatus and in endosomal compartments, whereas the US27/CXCR3-CT chimera, US27ΔCT and US27Δ348 mutants were not localized to endosomal compartments.ConclusionsThe results indicate that the C-terminal domain of the HCMV US27 protein, which contains a di-leucine endocytic sorting motif, is both necessary and sufficient for intracellular localization, which may also help explain why no cellular ligands have yet been identified for this viral receptor.

Viruses are dependent on their hosts for replication and dispersal in the environment; thus, the most successful viruses are those that co-evolve with their hosts. CXCR4 is a cellular chemokine receptor that plays central roles in development, hematopoiesis, and immune surveillance through signaling induced by its ligand, CXCL12. The CXCR4-CXCL12 axis has been besieged by many pathogens that employ a range of strategies to modify or exploit CXCR4 activity. While CXCR4 was identified as a critical co-factor for entry of HIV into CD4+ T cells early on, other viruses may utilize CXCR4 to gain cell entry as well. Moreover, several viruses have been found to modulate CXCR4 expression or alter its functional activity, with direct effects on cell trafficking, immune responses, cell proliferation, and cell survival. Because CXCR4 is targeted by a diverse group of viral pathogens, modification of host CXCR4 signaling activity is emerging as a common theme in virus persistence and is likely to be important for subversion of the host immune system. This review highlights major viral pathogens that use and abuse CXCR4 and explores the possible reasons why this chemokine receptor has become “a virus’s best friend”.

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