Molecular mechanisms of mucosal immunity using avian infectious bronchitis virus (IBV) and CpG oligodeoxynucleotides (ODN) as model systems.

Despite the recent recognition that mucosal immune responses are critical for protection of hosts from clinical illness and even mortality caused by mucosal pathogens, the molecular mechanism of mucosal immunity, which is regulated independent of systemic immune system, remains elusive. The goal of this study was to better understand the underlying mechanisms of respiratory mucosal immunity by defining the molecular markers of protective immunity as well as by characterizing key regulatory molecules in triggering efficient mucosal immunity. A number of key signature molecules in driving the induction and development of mucosal immunity were identified through profiling the transcriptiome at respiratory tract with a well-characterized mucosal pathogen, infectious bronchitis virus (IBV). These included the well-characterized toll-like receptor 3 (TLR3), interleukin-1beta (IL-1beta), complement 3a (C3a), Fas, myxovirus resistance protein (Mx), caspase 3/6, granzyme A/K, major histocompatibility complex (MHC)-I/II, cluster of differentiation 3 (CD3), retinoic-acid-inducible gene I (RIG-I), matrix metalloproteinase 3 (MMP3) and others. Both innate immunity and cytotoxic T lymphocytes (CTLs) contributed to the initial virus clearance from the local infection sites. Strong mucosal IgG response, but not innate, T-helper (Th1) adaptive immunity, CTL and secretory IgA (sIgA) was induced by a secondary exposure to IBV, suggesting the role of mucosal IgG in protecting host against re-entry of virus. MMP3 is one of the key signature molecules we identified, but its role in respiratory immunity is not well characterized. MMP3 has been implicated in specific intestinal immune responses against bacterial infections. We sought to investigate the possible role of MMP3 in respiratory mucosal immunity using a well-characterized mucosal adjuvant CpG ODN. Our result showed that wild-type (WT) mice responded to CpG ODN stimulation with an increase in CD4+, CD8+ T cells, CD11c+ antigen-presenting cells (APCs) in lungs compared to PBS controls. In contrast, MMP3 knockout (KO) mice did not respond to CpG ODN, as demonstrated by a comparable of immune cells in CpG ODN-stimulated mice and PBS control mice. Surprisingly, significantly more T cells and APCs were seen in PBS-treated KO mice than PBS-treated WT mice. Further cytokine/chemokine profiling indicated MMP3 upregulated expression of IL-1beta, macrophage inflammatory protein-1beta (MIP-1beta), and regulated on activation, normal T cell expressed and secreted (Rantes). Altered cytokines and chemokines might be the driving forces for the changes in T cell responses. In addition, expression of other MMPs, including MMP2, 9, 11, and 12, was negatively regulated by MMP3, suggesting that MMP3 might be required for their induction. It was highly possible that these MMPs also contributed indirectly to MMP3-mediated mucosal immunity in lung tissues. Overall, our results suggested a possible role of MMP3 in immune regulation at the respiratory tract. Further studies are needed to examine in greater details how MMPs contribute to immune cell trafficking and regulation of cytokines/chemokines.