Induced bronchus-associated lymphoid tissue serves as a general priming site for T cells and is maintained by dendritic cells

Mucosal vaccination via the respiratory tract can elicit protective immunity in animal infection models, but the underlying mechanisms are still poorly understood. We show that a single intranasal application of the replication-deficient modified vaccinia virus Ankara, which is widely used as a recombinant vaccination vector, results in prominent induction of bronchus-associated lymphoid tissue (BALT). Although initial peribronchiolar infiltrations, characterized by the presence of dendritic cells (DCs) and few lymphocytes, can be found 4 d after virus application, organized lymphoid structures with segregated B and T cell zones are first observed at day 8. After intratracheal application, in vitro–differentiated, antigen-loaded DCs rapidly migrate into preformed BALT and efficiently activate antigen-specific T cells, as revealed by two-photon microscopy. Furthermore, the lung-specific depletion of DCs in mice that express the diphtheria toxin receptor under the control of the CD11c promoter interferes with BALT maintenance. Collectively, these data identify BALT as tertiary lymphoid structures supporting the efficient priming of T cell responses directed against unrelated airborne antigens while crucially requiring DCs for its sustained presence.Bronchus-associated lymphoid tissue (BALT) is part of the mucosal immune system of the lung and is characterized by the aggregation of lymphoid cells at the bifurcations of the upper bronchioles (Bienenstock and Befus, 1984). Like other lymphoid follicles, BALT is composed of B cells surrounded by a parafollicular region of T cells (Sminia et al., 1989). Recirculating lymphocytes are believed to enter BALT via high endothelial venules and leave these structures by efferent lymphatics (Lührmann et al., 2002/2003; Xu et al., 2003). Although BALT is largely absent in normal mice, it spontaneously forms in mice deficient for the chemokine receptor CCR7 (Kocks et al., 2007). In humans, it is neither found at birth nor in healthy adults but transiently arises during childhood and adolescence (Tschernig and Pabst, 2000). In both humans and mice, pulmonary infection and inflammation can induce BALT (Moyron-Quiroz et al., 2004). Data derived from splenectomized lymphotoxin-α–deficient mice, which lack all secondary lymphoid organs but do develop BALT, suggest that BALT can serve as induction sites for adaptive immune responses to pathogens with lung tropism (Moyron-Quiroz et al., 2004). However, mechanisms that control the development and maintenance of BALT are largely unknown.Modified vaccinia virus Ankara (MVA) is a highly attenuated orthopoxvirus that lost its capacity to replicate in mammalian cells (Meyer et al., 1991). Recently, MVA was proposed to represent a useful agent for mucosal vaccination via the respiratory route in a nonhuman primate model (Corbett et al., 2008). In mice, MVA delivered via the intranasal (i.n.) route has been shown to induce long-lasting and protective antibody and T cell immune responses (Gherardi and Esteban, 2005; Kastenmuller et al., 2009). However, little is known about the immunological events after respiratory MVA infection.The present report demonstrates that a single i.n. application of the replication-deficient MVA is sufficient to induce the long-lasting presence of BALT and that the lung-specific depletion of DCs interferes with BALT maintenance. Ex vivo imaging of antigen-specific T cell–DC interactions within BALT by two-photon microscopy indicates that, independent of the specific antigenic challenge inducing its formation, BALT can function as a general priming site for T cell responses directed against antigens that reach the lower respiratory tract.