Prolonged Toll-Like Receptor Signaling byMycobacterium tuberculosisand Its 19-Kilodalton Lipoprotein Inhibits Gamma Interferon-Induced Regulation of Selected Genes in Macrophages

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Abstract
Infection of macrophages with Mycobacterium tuberculosis or exposure to M. tuberculosis 19-kDa lipoprotein for >16 h inhibits gamma interferon (IFN-γ)-induced major histocompatibility complex class II (MHC-II) expression by a mechanism involving Toll-like receptors (TLRs). M. tuberculosis was found to inhibit murine macrophage MHC-II antigen (Ag) processing activity induced by IFN-γ but not by interleukin-4 (IL-4), suggesting inhibition of IFN-γ-induced gene regulation. We designed an approach to test the ability of M. tuberculosis-infected cells to respond to IFN-γ. To model chronic infection with M. tuberculosis with accompanying prolonged TLR signaling, macrophages were infected with M. tuberculosis or incubated with M. tuberculosis 19-kDa lipoprotein for 24 h prior to the addition of IFN-γ. Microarray gene expression studies were then used to determine whether prolonged TLR signaling by M. tuberculosis broadly inhibits IFN-γ regulation of macrophage gene expression. Of 347 IFN-γ-induced genes, M. tuberculosis and 19-kDa lipoprotein inhibited induction of 42 and 36%, respectively. Key genes or gene products were also examined by quantitative reverse transcription-PCR and flow cytometry, confirming and extending the results obtained by microarray studies. M. tuberculosis inhibited IFN-γ induction of genes involved in MHC-II Ag processing, Ag presentation, and recruitment of T cells. These effects were largely dependent on myeloid differentiation factor 88, implying a role for TLRs. Thus, prolonged TLR signaling by M. tuberculosis inhibits certain macrophage responses to IFN-γ, particularly those related to MHC-II Ag presentation. This inhibition may promote M. tuberculosis evasion of T-cell responses and persistence of infection in tuberculosis.