A new system for lineage ablation is based on transgenic expression of a diphtheria toxin receptor (DTR) in mouse cells and application of diphtheria toxin (DT). To streamline this approach, we generated Cre-inducible DTR transgenic mice (iDTR) in which Cre-mediated excision of a STOP cassette renders cells sensitive to DT. We tested the iDTR strain by crossing to the T cell- and B cell-specific CD4-Cre and CD19-Cre strains, respectively, and observed efficient ablation of T and B cells after exposure to DT. In MOGi-Cre/iDTR double transgenic mice expressing Cre recombinase in oligodendrocytes, we observed myelin loss after intraperitoneal DT injections. Thus, DT crosses the blood-brain barrier and promotes cell ablation in the central nervous system. Notably, we show that the developing DT-specific antibody response is weak and not neutralizing, and thus does not impede the efficacy of DT. Our results validate the use of iDTR mice as a tool for cell ablation in vivo.

Successful transplantation of allogeneic organs is an important objective in modern medicine. However, sophisticated immune defense mechanisms, primarily evolved to combat infections, often work against medical transplantation. To investigate the roles of natural and adaptive immune responses in transplant rejection, we functionally inactivated key effector systems of the innate (NK cells) and the adaptive immune system (CD28-mediated costimulation of T cells) in mice. Neither of these interventions alone led to acceptance of allogeneic vascularized cardiac grafts. In contrast, inhibition of NK-receptor-bearing cells combined with CD28-costimulation blockade established long-term graft acceptance. These results indicate a concerted interplay between innate and adaptive immune surveillance for graft rejection. Thus we suggest that inactivation of NK-receptor-bearing cells could be a new strategy for successful survival of solid-organ transplants.

Containment of intracellularly viable microorganisms requires an intricate cooperation between macrophages and T cells, the most potent mediators known to date being IFN-γ and TNF. To identify novel mechanisms involved in combating intracellular infections, experiments were performed in mice with selective defects in the lymphotoxin (LT)/LTβR pathway. When mice deficient in LTα or LTβ were challenged intranasally with Mycobacterium tuberculosis, they showed a significant increase in bacterial loads in lungs and livers compared with wild-type mice, suggesting a role for LTαβ heterotrimers in resistance to infection. Indeed, mice deficient in the receptor for LTα1β2 heterotrimers (LTβR-knockout (KO) mice) also had significantly higher numbers of M. tuberculosis in infected lungs and exhibited widespread pulmonary necrosis already by day 35 after intranasal infection. Furthermore, LTβR-KO mice were dramatically more susceptible than wild-type mice to i.p. infection with Listeria monocytogenes. Compared with wild-type mice, LTβR-KO mice had similar transcript levels of TNF and IFN-γ and recruited similar numbers of CD3+ T cells inside granulomatous lesions in M. tuberculosis-infected lungs. Flow cytometry revealed that the LTβR is expressed on pulmonary macrophages obtained after digestion of M. tuberculosis-infected lungs. LTβR-KO mice showed delayed expression of inducible NO synthase protein in granuloma macrophages, implicating deficient macrophage activation as the most likely cause for enhanced susceptibility of these mice to intracellular infections. Since LIGHT-KO mice proved to be equally resistant to M. tuberculosis infection as wild-type mice, these data demonstrate that signaling of LTα1β2 heterotrimers via the LTβR is an essential prerequisite for containment of intracellular pathogens.

Until recently, IgE-activated mast cells have been regarded merely as effector cells of adaptive immune responses, involved in allergic reactions and mucosal immunity to parasites. Herein, we report that murine dermal mast cells, activated by local administration of a cream containing the synthetic TLR7 ligand imiquimod, are essential to initiate an early inflammatory reaction. The mast-cell-derived cytokines TNF-␣ and IL-1␤ play an important role in this process. Furthermore, TLR7-activated mast cells are also able to promote the emigration of Langerhans cells, which partly depends on the expression of mast-cell-derived IL-1␤. We have previously shown that TLR7 ligation enhances transcutaneous immunization evoked by topical application of vaccine antigens to the skin, a procedure that directly targets skin-resident antigen-presenting cells.Consequently, we now demonstrate here that the capacity to mount a peptidespecific cytotoxic T-lymphocyte response following transcutaneous immunization using imiquimod as adjuvant is severely impaired in mast-cell-deficient mice. IntroductionIgE-dependent reactions including dysregulated allergic responses to environmental antigens and mucosal immunity to some extracellular parasites were believed to be the hallmarks of mast-cell immunology for several decades.We know by now that mast cells, predominantly localized at the interface between host and environment (ie, skin and mucosal surfaces), are in addition able to perceive a variety of allergens and invading pathogens. Based on 2 groundbreaking publications using murine models of acute bacterial infection, mast cells were found to be critical effectors of innate immunity. In these studies, mastcell-deficient mice were highly susceptible to induced septic peritonitis 1 and Klebsiella pneumoniae instillation. 2 In these settings, the unique ability of mast cells to secrete preformed TNF-␣ within minutes following IgE-independent stimulation enables the host to mount an early and protective neutrophil response to bacterial challenge. Evidence further accumulates that mast cells are implicated in host defense against a still increasing range of clinically relevant bacterial infections. [3][4][5] Furthermore, mast cells are also thought to participate in response to viruses, but this is less understood and deserves further research. 5,6 With respect to IgEindependent activation, both rodent and human mast cells were found to express a variety of Toll-like receptors, some of which are expressed only on certain mast cell subsets. [5][6][7] However, several murine in vivo models clearly showed the importance of TLRmediated mast cell activation, for example, for the clearance of enterobacterial infection by triggering TLR4 8 and for the recruitment of CD8 ϩ T cells into the peritoneal cavity following ligation of TLR3 with poly(I:C), which mimics viral dsRNA. 9 It is also increasingly being appreciated that mast cells are able to finely control the magnitude of the secretory response following activation. 10 Thus, IgE-independent ...