Epithelial injury, alternative macrophage accumulation, and fibroproliferation coexist in the lungs of patients with idiopathic pulmonary fibrosis (IPF). Chitinase 3–like 1 (CHI3L1) is a prototypic chitinase-like protein that has been retained over species and evolutionary time. However, the regulation of CHI3L1 in IPF and its ability to regulate injury and/or fibroproliferative repair have not been fully defined. We demonstrated that CHI3L1 levels were elevated in patients with IPF. High levels of CHI3L1 are associated with progression—as defined by lung transplantation or death—and with scavenger receptor–expressing circulating monocytes in an ambulatory IPF population. In preterminal acute exacerbations of IPF, CHI3L1 levels were reduced and associated with increased levels of apoptosis. We also demonstrated that in bleomycin-treated mice, CHI3L1 expression was acutely and transiently decreased during the injury phase and returned toward and eventually exceeded baseline levels during the fibrotic phase. In this model, CHI3L1 played a protective role in injury by ameliorating inflammation and cell death, and a profibrotic role in the repair phase by augmenting alternative macrophage activation, fibroblast proliferation, and matrix deposition. Using three-dimensional culture system of a human fibroblast cell line, we found that CHI3L1 is sufficient to induce low grade myofibroblast transformation. In combination, these studies demonstrate that CHI3L1 is stimulated in IPF, where it represents an attempt to diminish injury and induce repair. They also demonstrate that high levels of CHI3L1 are associated with disease progression in ambulatory patients and that a failure of the CHI3L1 antiapoptotic response might contribute to preterminal disease exacerbations.

It is widely believed that innate immune responses to Borrelia burgdorferi (Bb) are primarily triggered by the spirochete's outer membrane lipoproteins signaling through cell surface TLR1/2. We recently challenged this notion by demonstrating that phagocytosis of live Bb by peripheral blood mononuclear cells (PBMCs) elicited greater production of proinflammatory cytokines than did equivalent bacterial lysates. Using whole genome microarrays, we show herein that, compared to lysates, live spirochetes elicited a more intense and much broader transcriptional response involving genes associated with diverse cellular processes; among these were IFN-β and a number of interferon-stimulated genes (ISGs), which are not known to result from TLR2 signaling. Using isolated monocytes, we demonstrated that cell activation signals elicited by live Bb result from cell surface interactions and uptake and degradation of organisms within phagosomes. As with PBCMs, live Bb induced markedly greater transcription and secretion of TNF-α, IL-6, IL-10 and IL-1β in monocytes than did lysates. Secreted IL-18, which, like IL-1β, also requires cleavage by activated caspase-1, was generated only in response to live Bb. Pro-inflammatory cytokine production by TLR2-deficient murine macrophages was only moderately diminished in response to live Bb but was drastically impaired against lysates; TLR2 deficiency had no significant effect on uptake and degradation of spirochetes. As with PBMCs, live Bb was a much more potent inducer of IFN-β and ISGs in isolated monocytes than were lysates or a synthetic TLR2 agonist. Collectively, our results indicate that the enhanced innate immune responses of monocytes following phagocytosis of live Bb have both TLR2-dependent and -independent components and that the latter induce transcription of type I IFNs and ISGs.

We examined the interactions of live and lysed spirochetes with innate immune cells. THP-1 monocytoid cells were activated to comparable extents by live Borrelia burgdorferi and by B. burgdorferi and Treponema pallidum lysates but were poorly activated by live T. pallidum. Because THP-1 cells poorly internalized live spirochetes, we turned to an ex vivo peripheral blood mononuclear cell system that would more closely reflect spirochetemononuclear phagocyte interactions that occur during actual infection. In this system, B. burgdorferi induced significantly greater monocyte activation and inflammatory cytokine production than did borrelial lysates or T. pallidum, and only B. burgdorferi elicited gamma interferon (IFN-␥) from NK cells. B. burgdorferi was phagocytosed avidly by monocytes, while T. pallidum was not, suggesting that the enhanced response to live B. burgdorferi was due to phagocytosis of the organism. When cytochalasin D was used to block phagocytosis of live B. burgdorferi, cytokine production decreased to levels comparable to those induced by B. burgdorferi lysates, while the IFN-␥ response was abrogated altogether. In the presence of human syphilitic serum, T. pallidum was efficiently internalized and initiated responses resembling those observed with live B. burgdorferi, including the production of IFN-␥ by NK cells. Depletion of monocytes revealed that they were the primary source of inflammatory cytokines, while dendritic cells (DCs) directed IFN-␥ production from innate lymphocytes. Thus, phagocytosis of live spirochetes initiates cell activation programs in monocytes and DCs that differ qualitatively and quantitatively from those induced at the cell surface by lipoprotein-enriched lysates. The greater stimulatory capacity of B. burgdorferi versus T. pallidum appears to be explained by the successful recognition and phagocytosis of B. burgdorferi by host cells and the ability of T. pallidum to avoid detection and uptake by virtue of its denuded outer membrane rather than by differences in surface lipoprotein expression.

The role of CD4 T cells in providing help to CD8 T cells in primary and secondary responses to infection remains controversial. Using recombinant strains of virus and bacteria expressing the same Ag, we determined the requirement for CD4 T cells in endogenous CD8 T cell responses to infection with vesicular stomatitis virus and Listeria monocytogenes (LM). Depletion of CD4 T cells had no effect on the frequency of primary or secondary vesicular stomatitis virus-specific CD8 T cells in either lymphoid or nonlymphoid tissues. In contrast, the primary LM-specific CD8 T cell response was CD4 T cell dependent. Surprisingly, the LM-specific CD8 T cell recall response was also CD4 T cell dependent, which correlated with a requirement for CD40/CD40L interactions. However, concomitant inhibition of CD40L and CD4 T cell removal revealed that these pathways may be operating independently. Importantly, despite the absence of CD4 T cells during the recall response or throughout the entire response, CD8 memory T cells were functional effectors and proliferated equivalently to their “helped” counterparts. These data call into question the contention that CD4 T cells condition memory CD8 T cells during the primary response and indicate that the principal role of CD4 T cells in generating CD8 memory cells after infection is augmentation of proliferation or survival through costimulatory signals.

We have previously demonstrated that phagocytosed Borrelia burgdorferi induces activation programs in human peripheral blood mononuclear cells that differ qualitatively and quantitatively from those evoked by equivalent lipoprotein-rich lysates. Here we report that ingested B. burgdorferi induces significantly greater transcription of proinflammatory cytokine genes than do lysates and that live B. burgdorferi, but not B. burgdorferi lysate, is avidly internalized by monocytes, where the bacteria are completely degraded within phagolysosomes. In the course of these experiments, we discovered that live B. burgdorferi also induced a dose-dependent decrease in monocytes but not a decrease in dendritic cells or T cells and that the monocyte population displayed morphological and biochemical hallmarks of apoptosis. Particularly noteworthy was the finding that apoptotic changes occurred predominantly in monocytes that had internalized spirochetes. Abrogation of phagocytosis with cytochalasin D prevented the death response. Heat-killed B. burgdorferi, which was internalized as well as live organisms, induced a similar degree of apoptosis of monocytes but markedly less cytokine production. Surprisingly, opsonophagocytosis of Treponema pallidum did not elicit a discernible cell death response. Our combined results demonstrate that B. burgdorferi confined to phagolysosomes is a potent inducer of cytosolic signals that result in (i) production of NF-B-dependent cytokines, (ii) assembly of the inflammasome and activation of caspase-1, and (iii) induction of programmed cell death. We propose that inflammation and apoptosis represent mutually reinforcing components of the immunologic arsenal that the host mobilizes to defend itself against infection with Lyme disease spirochetes.