Impaired toll-like receptor 9 expression in alveolar macrophages with no sensitivity to CpG DNA

Lung dendritic cells have a potent capability to induce production of immunoglobulin A

The mucosal immune system provides the first line of defense against inhaled pathogens in the lung. This system is largely mediated by immunoglobulin A (IgA) locally produced by plasma cells, which originate from homing IgA-committed B cells. It has not been determined what types of antigen-presenting cells (APCs) primarily induce B cell differentiation for IgA production in the lung. In addition, although mucosal dendritic cells (DCs) are functionally distinct from DCs in other tissues, it is unclear whether IgA-inducing capability differs between mucosal lung DCs (LDCs) and nonmucosal DCs. The present study was conducted to identify APCs principally responsible for IgA induction in the lung, and to determine potential differences in IgA-inducing capacity between LDCs and nonmucosal DCs. We measured immunoglobulin and cytokine production in a coculture system containing naive IgD(+) B cells, naive T cells from ovalbumin-specific T cell-receptor transgenic mice, and APCs including LDCs, alveolar macrophages (AMs), or spleen DCs (SDCs). LDCs induced significantly greater levels of IgA, IgG1, IL-6, and TGF-beta than AMs and SDCs, whereas no differences were found in the production of IgM or IgG2a. In addition, the IgA percentage of total class-switched immunoglobulin was highest in cocultures with LDCs (38.4%) when compared with those with AMs (15.1%) and SDCs (22.7%). Neutralizing TGF-beta, but not IL-6, significantly decreased IgA induction by LDCs and SDCs, but not by AMs. This study suggests that LDCs are the primary APCs introducing IgA to the lung, and have a more potent IgA-inducing capacity than nonmucosal DCs.

Dendritic cells derived from murine colonic mucosa have unique functional and phenotypic characteristics

Dendritic cells (DCs) residing in different tissues and exposed to different organisms are likely to have different reactivities to their surrounding environment. Many studies use in vitro generated DCs to examine functions of these cells, but such cells may not truly reflect the nature of DCs and their in situ activities in vivo. We have used magnetic label-based technique to isolate colonic DCs to conduct derailed characterization of these cells. Colonic DCs comprise mainly CD11b+ DCs with few CD8alpha+ DCs or plasmacytoid DCs. Functionally, isolated colonic DCs are able to endocytose and process proteins, undergo maturation, and stimulate T cells to proliferate. Importantly, expression of TLRs by colonic DCs is significantly lower than that of their spleen counterparts; however, they appear to be as, or more, responsive to stimulation by oligodeoxynucleotides containing CpG motif based on their cytokine production. We speculate that colonic DCs have unique reactivities differing from DCs residing in other lymphoid tissues and are adapted for the unique microenvironment of the colonic mucosa and that these cells react uniquely to their environment.

Translational mini-review series on Toll-like receptors: networks regulated by Toll-like receptors mediate innate and adaptive immunity

The Toll-like receptor (TLR) family provide key components of mammalian immunity and are part of the earliest surveillance mechanisms responding to infection. Their activation triggers the innate immune response, and is crucial to the successful induction of Th1/Th2-phenotyped adaptive immunity. Innate immunity was long considered to be non-specific and somewhat simple compared to adaptive immunity, mediated via the engulfment and lysis of microbial pathogens by phagocytic cells such as macrophages and neutrophils, and involving no complex protein-protein interactions. The emergence of the TLR field has contributed to a revision of our understanding, and innate immunity is now viewed as a highly complex process, in line with adaptive immunity. This review will give a brief overview of our current knowledge of TLR biology, and will focus on TLRs as key components in complex networks that activate, integrate and select the appropriate innate and adaptive immune responses in the face of immunological danger.

Electroporation-mediated transfer of plasmids to the lung results in reduced TLR9 signaling and inflammation

Electroporation can deliver DNA efficiently and safely to tissues in live animals, including the lung where it causes little inflammation or lung injury. In contrast, cationic lipid-mediated gene transfer has been shown to induce an inflammatory response caused by unmethylated plasmid CpG residues, which activate the toll-like receptor (TLR9) signaling pathway. As TLR9 is located in the endosomal/lysosomal compartment, we hypothesized that plasmids do not activate TLR9 during electroporation because they enter the cytoplasm directly through transient pores in the plasma membrane. To test this, plasmids were transfected into kidney epithelial cells overexpressing TLR9 (HEK293-TLR9+) and cells lacking TLR9 (HEK293-TLR9-null). Interleukin (IL)-8 expression, an indicator of TLR9 activation, increased more than 10-fold at 24 h post-liposome transfection in HEK293-TLR9+ cells, but showed no significant increase in electroporated cells, compared with untransfected cells. In vivo liposome-mediated gene transfer caused increases in IL-6, IL-12, tumor necrosis factor alpha and interferon gamma in mouse bronchial alveolar lavage fluid, whereas the levels of these cytokines were more than 10-fold lower by comparison following electroporation. Depletion of alveolar macrophages suggested that this inflammatory response is mediated by resident pulmonary epithelial cells. These results suggest that electroporation-mediated gene transfer bypasses the TLR-9 pathway, thus accounting for the low levels of inflammation seen with this approach.

Synergistic activation of macrophages via CD40 and TLR9 results in T cell independent antitumor effects

We have previously shown that macrophages (Mphi) can be activated by CD40 ligation to become cytotoxic against tumor cells in vitro. Here we show that treatment of mice with agonistic anti-CD40 mAb (anti-CD40) induced up-regulation of intracellular TLR9 in Mphi and primed them to respond to CpG-containing oligodeoxynucleotides (CpG), resulting in synergistic activation. The synergy between anti-CD40 and CpG was evidenced by increased production of IFN-gamma, IL-12, TNF-alpha, and NO by Mphi, as well as by augmented apoptogenic effects of Mphi against tumor cells in vitro. The activation of cytotoxic Mphi after anti-CD40 plus CpG treatment was dependent on IFN-gamma but not TNF-alpha or NO, and did not require T cells and NK cells. Anti-CD40 and CpG also synergized in vivo in retardation of tumor growth in both immunocompetent and immunodeficient mice. Inactivation of Mphi in SCID/beige mice by silica treatment abrogated the antitumor effect. Taken together, our results show that Mphi can be activated via CD40/TLR9 ligation to kill tumor cells in vitro and inhibit tumor growth in vivo even in immunocompromised tumor-bearing hosts, indicating that this Mphi-based immunotherapeutic strategy may be appropriate for clinical testing.