The epithelial surface of the human small intestine is a barrier between the host and the microbial environment of the lumen. A human small intestine cDNA clone was found to encode a new member of the defensin family of antimicrobial peptides, named human defensin-6. Tissue expression of this mRNA is specific for the small intestine as determined by Northern blot analysis and polymerase chain reaction analysis. In situ hybridization demonstrated that human defensin-6 mRNA localizes to Paneth cells in the crypts of Lieberktihn. The finding of an abundant defensin mRNA in human Paneth cells supports the notion that these epithelial cells may play a key role in host defense of the human bowel. The results also strengthen the hypothesis that peptide-based host defenses are prevalent at mucosal surfaces in mammals.

Control of Leishmania infantum infection is dependent upon Th1 CD4+ T cells to promote macrophage intracellular clearance of parasites. Deficient CD4+ T cell effector responses during clinical visceral leishmaniasis (VL) are associated with elevated production of IL-10. In the primary domestic reservoir of VL, dogs, we define occurrence of both CD4+ and CD8+ T cell exhaustion as a significant stepwise loss of antigen-specific proliferation and IFNγ production, corresponding to increasing VL symptomatology. Exhaustion was associated with a fourfold increase in the population of T cells with surface expression of Programmed Death 1 (PD-1) between control and symptomatic populations. Importantly, exhausted populations of CD8+ T cells and to a lesser extent CD4+ T cells were present prior to onset of clinical VL. VL exhausted T cells did not undergo significant apoptosis ex vivo after antigen stimulation. Antibody block of PD-1 ligand, B7.H1, promoted return of CD4+ and CD8+ T cell function and dramatically increased reactive oxygen species production in co-cultured monocyte-derived phagocytes. As a result, these phagocytes had decreased parasite load. We demonstrate for the first time that pan-T cell, PD-1-mediated, exhaustion during VL influenced macrophage reactive oxygen intermediate production. Blockade of the PD-1 pathway improved the ability of phagocytes isolated from dogs presenting with clinical VL to clear intracellular parasites. T cell exhaustion during symptomatic canine leishmaniasis has implications for the response to vaccination and therapeutic strategies for control of Leishmania infantum in this important reservoir species.

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Leishmania amazonensis induces a nonhealing infection in C3H mice, whereas infection with Leishmania major is self-healing. We found that C3H mice infected with L. amazonensis exhibited decreased IL-12 production, which could account for the susceptibility to this organism. However, exogenous IL-12 administration failed to induce a healing immune response. The failure of L. amazonensis-infected C3H mice to respond to IL-12 was associated with a specific defect in IL-12 receptor β2 (IL-12Rβ2) mRNA expression by CD4+ T cells. Furthermore, decreased IL-12Rβ2 mRNA expression correlated with a decrease in the IL-12-signaling capacity of the lymph node (LN) cells. IL-4 did not contribute to susceptibility or down-regulation of the IL-12Rβ2 subunit, because IL-4−/− mice remained susceptible to L. amazonensis infection, even after IL-12 administration, and CD4+ cells from infected IL-4−/− mice also had reduced expression of IL-12Rβ2 mRNA. These results demonstrate that regulation of the IL-12 receptor, independent of IL-4, is a point of control for the immune response to leishmaniasis. In contrast to experimental L. major infections, where host genetics control susceptibility, these studies demonstrate that the lack of IL-12 responsiveness may be dictated by the pathogen, rather than the host.

We previously reported the isolation and characterization of a broad-spectrum antimicrobial peptide from the bovine tracheal mucosa, which we cafled tracheal antimicrobial peptide (TAP). We now show the TAP gene is expressed throughout the adult conducting airway, from nasal to bronchiolar tissue, but not in tissues other than airway mucosa, as determined by Northern blot analysis. In situ hybridization of airway sections localizes TAP mRNA to columnar cells of the pseudostratifled epithelium. We report the structural organization of the TAP gene and show that TAP is a member of a large family of related sequences with high nucleotide identity in the 5' exon. The data support the hypothesis that antimicrobial peptides contribute to host defense of the respiratory tract.