Investigating the role of BHLHE40 in host responses to Mycobacterium tuberculosisinfection and myeloid cell development.

Abstract

Abstract We currently lack a complete understanding of the immune responses required to control Mycobacterium tuberculosis (Mtb) infection. Our lab has shown that the transcription factor BHLHE40 is essential during Mtb infection. Deletion of Bhlhe40 in CD11c +lung macrophages and dendritic cells (DCs) is sufficient to increase mouse susceptibility to Mtb infection, though the role of BHLHE40 in these cells during infection is unknown. Murine bone marrow cultured in the presence of GM-CSF, an abundant cytokine in the lung, develop into populations of CD11c +macrophages, DCs, and granulocytes, all of which express Bhlhe40. Using this system, we examined cytokine and transcriptional responses before and after exposure to heat-killed Mtb to understand how BHLHE40 impacts cell development and inflammatory responses. We found that in WT mice, GM-CSF-cultured cells develop into 9 distinct populations and expression of Bhlhe40 was required for GM-CSF-mediated pro-inflammatory cytokine and transcriptional programs. Additionally, loss of Bhlhe40 led to decreased DCs accompanied by upregulation of the transcription factor C/EBPβ in DC progenitor populations, which negatively impacts DC development. ChIP-seq analysis revealed BHLHE40 binds directly to C/EBPβ’s gene locus, indicating that BHLHE40 may directly repress Cebpb. In contrast, loss of Bhlhe40 did not affect macrophage development, but led to a transcriptional shift towards an anti-inflammatory state. These findings demonstrate that in the presence of GM-CSF, BHLHE40 is required for DC development and inflammatory macrophage polarization. Defects in either of these aspects could contribute to the essential role for BHLHE40 in lung macrophages and DCs during Mtb infection in vivo.