Dendritic cells in the development of antimicrobial immunity : Studies on Epstein-Barr virus and the Haemophilus ducreyi cytolethal distending toxin

Abstract: Dendritic cell (DC) plays a pivotal role in both innate and adaptive immunity, we studied the inhibitory effect of pathogens on the development and maturation of monocyte derived DCs, with special emphasis on the effect of Epstein-Barr virus (EBV) and Haemophilus ducreyi cytolethal distending toxin (HdCDT). Results demonstrate that EBV infection of monocytes inhibits their development into dendritic cells by interfering their responsiveness to granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4. The inhibition is achieved through the induction of apoptosis in the developing DC population. The pro-apoptotic activity was not affected by UV-inactivation and was neutralised by EBV antibody positive human sera indicating that binding of the virus to monocytes is sufficient to alter their responses to the cytokines. Experiments with the relevant blocking antibodies or mutated EBV strains lacking either the EBV envelope glycoprotein gp42 or gp85 demonstrated that interaction of the viral fusion complex, gp25/gp42/gp85, with the monocyte membrane is required for the effect. The role of the fusion complex in monocyte/EBV interactions is further supported by the demonstration of a putative gp85 binding receptor on the surface of developing DCs. This data provide the first evidence that EBV can prevent DC development through a mechanism that appears to bypass the requirement for viral gene expression and suggest a new strategy for interference with DC function during the initiation and maintenance of virus specific immune responses. In addition, we studied the effect of EBV produced from different cellular background on the inhibition of DC development. Results demonstrate that the cells supporting EBV replication determines the composition of viral fusion complex and the capacity to inhibit dendritic cell development. As compared to virus assembled in B-cells, EBV produced from MHC class II negative epithelial cells has a relatively higher content of the trimolecular gp85/gp42/gp24 complex and exhibits a significantly increased inhibitory activity on DC development. Thus the site of viral replication appears to determine the immunomodulating activity of the virus. This phenomenon may explain the absence of inflammation in epithelial foci of EBV replication in vivo and contribute to the generally asymptomatic course of primary EBV infection and should be considered in assessing the role of EBV in the pathogenesis of different diseases. Our studies on HdCDT demonstrate that the purified toxin possesses DNase activity and the HdCdtB subunit is the enzymatic component. Intoxication with HdCDT activates sensors of DNA damage, the histone H2AX, and re-localizes the DNA repair complex Mre11 in a functional Ataxia Telangiectasia mutated (ATM) kinase dependent manner. Intoxication of terminal differentiated, non-proliferating dendritic cells reveals similar cellular responses as in proliferating cells, and results in apoptotic cell death. The pro-apoptotic activity is differentiation dependent, as LPS-induced maturation of dendritic cells renders its resistance to HdCDT intoxication. The data highlight several novel aspects of CDTs biology and demonstrate that induction of apoptotic death of immature DCs by HdCDT may represent a previously unknown mechanism of immune evasion by CDT-producing microbes. In conclusion, our study demonstrates that targeting dendritic cell function appear to represent a common strategy utilized by different microbes, either virus or bacteria, to manipulate the immune system. Studies in this area may expand our knowledge of DC/pathogen interactions and lead to the development of new approaches for prophylaxes and treatment of infectious disease.