Hardy Kornfeld

Virulent Mycobacterium tuberculosis Strains Evade Apoptosis of Infected Alveolar Macrophages

Human alveolar macrophages (AMφ) undergo apoptosis following infection with Mycobacterium tuberculosis in vitro. Apoptosis of cells infected with intracellular pathogens may benefit the host by eliminating a supportive environment for bacterial growth. The present study compared AMφ apoptosis following infection by M. tuberculosis complex strains of differing virulence and by Mycobacterium kansasii. Avirulent or attenuated bacilli (M. tuberculosis H37Ra, Mycobacterium bovis bacillus Calmette-Guérin, and M. kansasii) induced significantly more AMφ apoptosis than virulent strains (M. tuberculosis H37Rv, Erdman, M. tuberculosis clinical isolate BMC 96.1, and M. bovis wild type). Increased apoptosis was not due to greater intracellular bacterial replication because virulent strains grew more rapidly in AMφ than attenuated strains despite causing less apoptosis. These findings suggest the existence of mycobacterial virulence determinants that modulate the apoptotic response of AMφ to intracellular infection and support the hypothesis that macrophage apoptosis contributes to innate host defense in tuberculosis.

Autophagy controls IL-1beta secretion by targeting pro-IL-1beta for degradation.

Autophagy is a key regulator of cellular homeostasis that can be activated by pathogen-associated molecules and recently has been shown to influence IL-1β secretion by macrophages. However, the mechanisms behind this are unclear. Here, we describe a novel role for autophagy in regulating the production of IL-1β in antigen-presenting cells. After treatment of macrophages with Toll-like receptor ligands, pro-IL-1β was specifically sequestered into autophagosomes, whereas further activation of autophagy with rapamycin induced the degradation of pro-IL-1β and blocked secretion of the mature cytokine. Inhibition of autophagy promoted the processing and secretion of IL-1β by antigen-presenting cells in an NLRP3- and TRIF-dependent manner. This effect was reduced by inhibition of reactive oxygen species but was independent of NOX2. Induction of autophagy in mice in vivo with rapamycin reduced serum levels of IL-1β in response to challenge with LPS. These data demonstrate that autophagy controls the production of IL-1β through at least two separate mechanisms: by targeting pro-IL-1β for lysosomal degradation and by regulating activation of the NLRP3 inflammasome.

Nitric oxide controls the immunopathology of tuberculosis by inhibiting NLRP3 inflammasome–dependent processing of IL-1β

Interleukin 1 (IL-1) is an important mediator of innate immunity but can also promote inflammatory tissue damage. During chronic infections such as tuberculosis, the beneficial antimicrobial role of IL-1 must be balanced with the need to prevent immunopathology. By exogenously controlling the replication of Mycobacterium tuberculosis in vivo, we obviated the requirement for antimicrobial immunity and discovered that both IL-1 production and infection-induced immunopathology were suppressed by lymphocyte-derived interferon-γ (IFN-γ). This effect was mediated by nitric oxide (NO), which we found specifically inhibited assembly of the NLRP3 inflammasome via thiol nitrosylation. Our data indicate that the NO produced as a result of adaptive immunity is indispensable in modulating the destructive innate inflammatory responses elicited during persistent infections.

Processing and Activation of Pro-Interleukin-16 by Caspase-3

Interleukin-16, a proinflammatory cytokine produced in CD8+ lymphocytes, is synthesized as a precursor protein (pro-IL-16). It is postulated that the C-terminal region of pro-IL-16 is cleaved, releasing bioactive IL-16. To characterize IL-16 cleavage, we transfected COS cells with a cDNA encoding a ∼50-kDa form of pro-IL-16. Transfected COS cells released a ∼20-kDa IL-16 cleavage product shown to consist of the 121 C-terminal residues of pro-IL-16 by immunoblotting and amino acid sequencing. Cleaved IL-16, but not pro-IL-16, exhibited lymphocyte chemoattractant activity. A C-terminal ∼20-kDa IL-16 polypeptide was also released when pro-IL-16 was treated with concanavalin A-stimulated CD8+ lymphocyte lysate. Cleavage occurred after an Asp, suggesting involvement of a caspase (interleukin-1β-converting enzyme/CED-3) family protease. Using recombinant caspases and granzyme B, we determined that pro-IL-16 cleavage is mediated only by caspase-3. Relevance to pro-IL-16 processing in primary lymphocytes was supported by identifying the p20 subunit of activated caspase-3 in stimulated CD8+ lymphocytes and by inhibition of CD8+lymphocyte lysate-mediated cleavage with Ac-DEVD-CHO. Pro-IL-16 is a substrate for caspase-3, and cleavage by this enzyme releases biologically active IL-16 from its inactive precursor.

Identification of HIV-1 Envelope Glycoprotein in the Serum of AIDS and ARC Patients

Binding of the human immunodeficiency virus type 1 (HIV-1) external envelope glycoprotein (gp120) has been reported to alter the function and surface antigen expression of lymphocytes and monocytes in vitro. To determine whether these in vitro findings could be relevant in vivo, we searched for the presence of this antigen in the serum of patients with AIDS and the AIDS-related complex (ARC). Using an antigen capture enzyme-linked immunosorbent assay (ELISA) with polyclonal anti-gp120 antibody, we detected envelope antigens (gp160/120) in serum of 22 of 32 AIDS patients. In contrast, an ELISA using solid-phase recombinant CD4 to capture gp160/120 failed to detect any positives. A modification of the anti-gp120-based ELISA identified gp160/120-IgG immune complexes in all of 11 AIDS patients tested and in 4 ARC patients who were negative for gp160/120 antigen. We conclude that gp160/120, predominantly in the form of immune complexes, can be identified as circulating antigen in patients with AIDS. The potential pathogenic consequences of this antigenemia, its relation to soluble CD4 therapy, and its application as a clinical marker of disease merit further study.

Macrophage Apoptosis in Response to High Intracellular Burden of Mycobacterium tuberculosis Is Mediated by a Novel Caspase-Independent Pathway

We previously reported that macrophage exposure to attenuated strains of pathogenic mycobacteria at multiplicities of infection (MOI) ≤ 10 triggers TNF-α-mediated apoptosis which reduces the viability of intracellular bacilli. Virulent strains were found to suppress macrophage apoptosis, and it was proposed that apoptosis is an innate defense against intracellular Mycobacterium tuberculosis analogous to apoptosis of virus-infected cells. The potential similarity of host cell responses to intracellular infection with mycobacteria and viruses suggests that M. tuberculosis might lyse infected macrophage when that niche is no longer needed. To investigate this question, we challenged murine macrophages with high intracellular bacillary loads. A sharp increase in cytolysis within 24 h was observed at MOI ≥ 25. The primary death mode was apoptosis, based on nuclear morphology and phosphatidyl serine exposure, although the apoptotic cells progressed rapidly to necrosis. Apoptosis at high MOI differs markedly from low MOI apoptosis: it is potently induced by virulent M. tuberculosis, it is TNF-α-independent, and it does not reduce mycobacterial viability. Caspase inhibitors failed to prevent high MOI apoptosis, and macrophages deficient in caspase-3, MyD88, or TLR4 were equally susceptible as wild type. Apoptosis was reduced in the presence of cathepsin inhibitors, suggesting the involvement of lysosomal proteases in this novel death response. We conclude that the presence of high numbers of intracellular M. tuberculosis bacilli triggers a macrophage cell death pathway that could promote extracellular spread of infection and contribute to the formation of necrotic lesions in tuberculosis.

CD4+ regulatory T cells require CTLA-4 for the maintenance of systemic tolerance

Cytotoxic T lymphocyte antigen-4 (CTLA-4) plays a critical role in negatively regulating T cell responses and has also been implicated in the development and function of natural FOXP3+ regulatory T cells. CTLA-4–deficient mice develop fatal, early onset lymphoproliferative disease. However, chimeric mice containing both CTLA-4–deficient and –sufficient bone marrow (BM)–derived cells do not develop disease, indicating that CTLA-4 can act in trans to maintain T cell self-tolerance. Using genetically mixed blastocyst and BM chimaeras as well as in vivo T cell transfer systems, we demonstrate that in vivo regulation of Ctla4−/− T cells in trans by CTLA-4–sufficient T cells is a reversible process that requires the persistent presence of FOXP3+ regulatory T cells with a diverse TCR repertoire. Based on gene expression studies, the regulatory T cells do not appear to act directly on T cells, suggesting they may instead modulate the stimulatory activities of antigen-presenting cells. These results demonstrate that CTLA-4 is absolutely required for FOXP3+ regulatory T cell function in vivo.

THP-1 Cell Apoptosis in Response to Mycobacterial Infection

ABSTRACT We previously reported that Mycobacterium tuberculosis infection primes human alveolar macrophages (HAM) for tumor necrosis factor alpha (TNF-α)-mediated apoptosis and that macrophage apoptosis is associated with killing internalized bacilli. Virulent mycobacterial strains elicit much less apoptosis than attenuated strains, implying that apoptosis is a defense against intracellular infection. The present study evaluated the potential for phorbol myristate acetate-differentiated THP-1 cells to mimic this response of primary macrophages. Consistent with the behavior of alveolar macrophages, attenuated M. tuberculosis H37Ra and Mycobacterium bovis BCG strongly induce THP-1 apoptosis, which requires endogenous TNF. THP-1 apoptosis is associated with reduced viability of infecting BCG. In contrast, virulent wild-type M. tuberculosis H37Rv and M. bovis do not increase THP-1 apoptosis over baseline. BCG induced early activation of caspase 10 and 9, followed by caspase 3. In contrast, wild-type M. bovis infection failed to activate any caspases in THP-1 cells. BCG-induced THP-1 apoptosis is blocked by retroviral transduction with vectors expressing crmA but not bcl-2. We conclude that differentiated THP-1 cells faithfully model the apoptosis response of HAM. Analysis of the THP-1 cell response to infection with virulent mycobacteria suggests that TNF death signals are blocked proximal to initiator caspase activation, at the level of TNF receptor 1 or its associated intracytoplasmic adaptor complex. Interference with TNF death signaling may be a virulence mechanism that allows M. tuberculosis to circumvent innate defenses leading to apoptosis of infected host cells.

Mycobacterium tuberculosis blocks crosslinking of annexin-1 and apoptotic envelope formation on infected macrophages to maintain virulence

Macrophages infected with attenuated Mycobacterium tuberculosis strain H37Ra become apoptotic, which limits bacterial replication and facilitates antigen presentation. Here we demonstrate that cells infected with H37Ra became apoptotic after the formation of an apoptotic envelope on their surface was complete. This process required exposure of phosphatidylserine on the cell surface, followed by deposition of the phospholipid-binding protein annexin-1 and then transglutaminase-mediated crosslinking of annexin-1 through its amino-terminal domain. In macrophages infected with the virulent strain H37Rv, in contrast, the amino-terminal domain of annexin-1 was removed by proteolysis, thus preventing completion of the apoptotic envelope, which resulted in macrophage death by necrosis. Virulent M. tuberculosis therefore avoids the host defense system by blocking formation of the apoptotic envelope, which leads to macrophage necrosis and dissemination of infection in the lung.

Human eosinophils express functional interleukin 2 receptors.

Because T cell-derived cytokines may affect the functioning of eosinophils, we have investigated the capacity of human eosinophils to respond to IL-2. IL-2 was a potent chemoattractant with ED50 of 10(-12) M with eosinophils from all normal and eosinophilic donors tested. The monoclonal antibodies anti-Tac and TU27 against p55 (Tac/CD25) and p75 receptor subunits, respectively, each inhibited IL-2-dependent eosinophil migration. The molar potency of IL-2 and the inhibitory activity of each of the above antibodies suggest that high affinity heterodimeric IL-2 receptor complexes mediated the migration responses of eosinophils to IL-2. Binding of monoclonal antibody against p75 was not detectable by flow cytometry, and high affinity binding sites for 125I-IL-2 were below the limits of quantitation on eosinophils from individuals with eosinophilia. Expression of p55 (Tac/CD25) by eosinophils, without requirement for in vitro activation, was demonstrable by flow cytometry, radioimmunoprecipitation, and Northern blotting for mRNA. Surface expression of p55 on eosinophils from normal or eosinophilic individuals increased during culture for 24-48 h with a biologic activity purified from stimulated U937 cells and to a lesser extent with granulocyte-macrophage CSF or lymphocyte chemoattractant factor but not with nine other cytokines. These studies indicate that blood eosinophils respond to IL-2 and identify one mechanism whereby activation of T lymphocytes may influence the function of eosinophils.