The coxsackievirus-adenovirus receptor (CAR) is the described primary receptor for adenovirus serotype 5 (Ad5), a common human pathogen that has been exploited as a viral vector for gene therapy and vaccination. This study showed that monocytes and dendritic cells (DCs), such as freshly isolated human blood myeloid DCs, plasmacytoid DCs and monocyte-derived DCs, are susceptible to recombinant Ad5 (rAd5) infection despite their lack of CAR expression. Langerhans cells and dermal DCs from skin expressed CAR, but blocking CAR only partly decreased rAd5 infection, together suggesting that other receptor pathways mediate viral entry of these cells. Lactoferrin (Lf), an abundant protein in many bodily fluids known for its antiviral and antibacterial properties, promoted rAd5 infection in all cell populations except plasmacytoid DCs using a CAR-independent process. Lf caused phenotypic differentiation of the DCs, but cell activation played only a minor role in the increase in infection frequencies. The C-type lectin receptor DC-SIGN facilitated viral entry of rAd5-Lf complexes and this was dependent on highmannose-type N-linked glycans on Lf. These results suggest that Lf present at high levels at mucosal sites can facilitate rAd5 attachment and enhance infection of DCs. A better understanding of the tropism and receptor mechanisms of Ad5 may help explain Ad5 pathogenesis and guide the engineering of improved rAd vectors. INTRODUCTIONAdenoviruses are frequent causes of acute upper respiratory tract infections and can be responsible for ocular and gastrointestinal illnesses in humans. They have also been exploited for the development of replication-incompetent viral vectors because their genomes allow large inserts of expression cassettes, thereby offering efficient gene delivery. Recombinant adenoviruses (rAds), serotype 5 (rAd5) in particular, are commonly used vectors for gene therapy and vaccination trials (Barouch & Nabel, 2005;McConnell & Imperiale, 2004;Tatsis & Ertl, 2004). The primary receptor described for infection of most Ad species (A, C, D, E and F) is the coxsackievirus and adenovirus receptor (CAR) (Bergelson et al., 1997;Roelvink et al., 1998;Tomko et al., 1997). As currently understood, infection of Ad5 (species C) involves binding of the viral fiber knob to CAR on the target cells (Roelvink et al., 1996), followed by an interaction between the viral penton base with a v integrins on the cell surface (Wickham et al., 1993). This allows virus-receptor complexes to enter clathrin-coated pits via endocytosis Wang et al., 1998;Wickham et al., 1993). Efficient infection of Ad5 via CAR has mainly been demonstrated in vitro using immortalized cell lines with well-exposed CAR. However, CAR is naturally expressed in tight junctions between cells and therefore et al., 2007). In this study, we investigated the involvement of CAR and the influence of Lf in rAd5 infection in relevant primary human APC populations from both blood and skin. METHODSIsolation of blood DCs. This study was approved by the ethical commi...

Plasmacytoid dendritic cells (pDCs) are rarely present in normal skin but have been shown to infiltrate lesions of infections or autoimmune disorders. Here, we report that several DC subsets including CD123(+) BDCA-2/CD303(+) pDCs accumulate in the dermis in indurations induced by the tuberculin skin test (TST), used to screen immune sensitization by Mycobacterium tuberculosis. Although the purified protein derivate (PPD) used in the TST did not itself induce pDC recruitment or IFN-α production, the positive skin reactions showed high expression of the IFN-α-inducible protein MxA. In contrast, the local immune response to PPD was associated with substantial cell death and high expression of the cationic antimicrobial peptide LL37, which together can provide a means for pDC activation and IFN-α production. In vitro, pDCs showed low uptake of PPD compared with CD11c(+) and BDCA-3/CD141(+) myeloid DC subsets. Furthermore, supernatants from pDCs activated with LL37-DNA complexes reduced the high PPD uptake in myeloid DCs, as well as decreased their capacity to activate T-cell proliferation. Infiltrating pDCs in the TST reaction site may thus have a regulatory effect upon the antigen processing and presentation functions of surrounding potent myeloid DC subsets to limit potentially detrimental and excessive immune stimulation.

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