Heidi F. Horton

Transcriptional Mechanisms Underlying Lymphocyte Tolerance

In lymphocytes, integration of Ca2+ and other signaling pathways results in productive activation, while unopposed Ca2+ signaling leads to tolerance or anergy. We show that the Ca2+-regulated transcription factor NFAT has an integral role in both aspects of lymphocyte function. Ca2+/calcineurin signaling induces a limited set of anergy-associated genes, distinct from genes induced in the productive immune response; these genes are upregulated in vivo in tolerant T cells and are largely NFAT dependent. T cells lacking NFAT1 are resistant to anergy induction; conversely, NFAT1 induces T cell anergy if prevented from interacting with its transcriptional partner AP-1 (Fos/Jun). Thus, in the absence of AP-1, NFAT imposes a genetic program of lymphocyte anergy that counters the program of productive activation mediated by the cooperative NFAT:AP-1 complex.

Microbial Lipopeptides Induce the Production of IL-17 in Th Cells

Naive Th cells can be directed in vitro to develop into Th1 or Th2 cells by IL-12 or IL-4, respectively. In vivo, chronic immune reactions lead to polarized Th cytokine patterns. We found earlier that Borrelia burgdorferi, the spirochaete that causes Lyme disease, induces Th1 development in αβ TCR-transgenic Th cells. Here, we used TCR-transgenic Th cells and oligonucleotide arrays to analyze the differences between Th1 cells induced by IL-12 vs those induced by B. burgdorferi. Transgenic Th cells primed with peptide in the presence of B. burgdorferi expressed several mRNAs, including the mRNA encoding IL-17, at significantly higher levels than Th cells primed with peptide and IL-12. Cytometric single-cell analysis of Th cell cytokine production revealed that IL-17 cannot be categorized as either Th1 or Th2 cytokine. Instead, almost all IL-17-producing Th cells simultaneously produced TNF-α and most IL-17+ Th cells also produced GM-CSF. This pattern was also observed in humans. Th cells from synovial fluid of patients with Lyme arthritis coexpressed IL-17 and TNF-α upon polyclonal stimulation. The induction of IL-17 production in Th cells is not restricted to B. burgdorferi. Priming of TCR-transgenic Th cells in the presence of mycobacterial lysates also induced IL-17/TNF-α coproduction. The physiological stimulus for IL-17 production was hitherto unknown. We show here for the first time that microbial stimuli induce the expression of IL-17 together with TNF-α in both murine and human T cells. Chronic IL-17 expression induced by microbes could be an important mediator of infection-induced immunopathology.

CD1d on Myeloid Dendritic Cells Stimulates Cytokine Secretion from and Cytolytic Activity of Vα24JαQ T Cells: A Feedback Mechanism for Immune Regulation

The precise immunologic functions of CD1d-restricted, CD161+ AV24AJ18 (Vα24JαQ) T cells are not well defined, although production of IL-4 has been suggested as important for priming Th2 responses. However, activation of human Vα24JαQ T cell clones by anti-CD3 resulted in the secretion of multiple cytokines notably important for the recruitment and differentiation of myeloid dendritic cells. Specific activation of Vα24JαQ T cells was CD1d restricted. Expression of CD1d was found on monocyte-derived dendritic cells in vitro, and immunohistochemical staining directly revealed CD1d preferentially expressed on dendritic cells in the paracortical T cell zones of lymph nodes. Moreover, myeloid dendritic cells both activated Vα24JαQ T cells and were susceptible to lysis by these same regulatory T cells. Because myeloid dendritic cells are a major source of IL-12 and control Th1 cell differentiation, their elimination by lysis is a mechanism for limiting the generation of Th1 cells and thus regulating Th1/Th2 responses.

Human Alveolar Macrophage Gene Responses to Mycobacterium tuberculosis Strains H37Ra and H37Rv

H37Rv and H37Ra have been widely used as models of virulent and avirulent strains, respectively, of Mycobacterium tuberculosis. Since the sequencing of H37Rv, microarrays have been used to investigate gene expression of M. tuberculosis strains under various conditions, and to compare gene expression of specific isolates of the organism. Because differences in the virulence of these organisms could also be manifest via their differential induction of host genes, we used Affymetrix Human Genome Arrays U133A and U133B to evaluate human alveolar macrophage (AM) responses to infection with H37Rv and H37Ra. H37Rv altered expression of far more genes than did H37Ra. Moreover, the genes induced by H37Rv to a greater extent than by H37Ra were predominantly associated with the development of effective immunity. H37Rv markedly increased expression of IL-23 p19, whereas neither organism significantly induced IL-12 p35 expression. Quantitative PCR confirmed that H37Rv induced significantly more AM p19 expression than did H37Ra. After low-level infection of both AM and peripheral blood monocytes (MN) with H37Rv, neither cell type produced IL-12 (by ELISA). In contrast, AM displayed significant IL-23 production in response to H37Rv, whereas MN did not. Our findings thus suggest an important role for IL-23 in human host responses to pulmonary infection with M. tuberculosis, and are consistent with epidemiologic and genetic studies that imply that H37Rv may not have unusual capacity to cause human disease.