No abstract

Inducible bronchus-associated lymphoid tissue (iBALT) is a tertiary lymphoid structure that resembles secondary lymphoid organs. iBALT is induced in the lung in response to Ag exposure. In some cases, such as infection with Mycobacterium tuberculosis, the formation of iBALT structure is indicative of an effective protective immune response. However, with persistent exposure to Ags during chronic inflammation, allergy, or autoimmune diseases, iBALT may be associated with exacerbation of inflammation. iBALT is characterized by well-organized T and B areas enmeshed with conventional dendritic cells, follicular dendritic cells, and stromal cells, usually located surrounding airways or blood vessels. Several of the molecular signals and cellular contributors that mediate formation of iBALT structures have been recently identified. This review will outline the recent findings associated with the formation and maintenance of iBALT and their contributions toward a protective or pathogenic function in pulmonary disease outcome.

C-C motif chemokine receptor 2 (CCR2) is a major chemokine axis that recruits myeloid cells including monocytes and macrophages. Thus far, CCR2 mice have not been found to be susceptible to infection with Mycobacterium tuberculosis (Mtb). Here, using a prototype W-Beijing family lineage 2 Mtb strain, HN878, we show that CCR2 mice exhibit increased susceptibility to tuberculosis (TB). Following exposure to Mtb HN878, alveolar macrophages (AMs) are amongst the earliest cells infected. We show that AMs accumulate early in the airways following infection and express CCR2. During disease progression, CCR2-expressing AMs exit the airways and localize within the TB granulomas. RNA-sequencing of sorted airway and non-airway AMs from infected mice show distinct gene expression profiles, suggesting that upon exit from airways and localization within granulomas, AMs become classically activated. The absence of CCR2 cells specifically at the time of AM egress from the airways resulted in enhanced susceptibility to Mtb infection. Furthermore, infection with an Mtb HN878 mutant lacking phenolic glycolipid (PGL) expression still resulted in increased susceptibility in CCR2 mice. Together, these data show a novel role for CCR2 in protective immunity against clinically relevant Mtb infections.

DNA that encodes tumor-specific antigens represents potential immunostimulatory agents. However, rapid enzymatic degradation and fragmentation of DNA during administration can result in limited vector expression and, consequently, poor efficacy. These challenges have necessitated the use of novel strategies for DNA delivery. Herein, we study the ability of cationic self-assembling peptide hydrogels to encapsulate plasmid DNA, and enhance its immunostimulatory potential in vivo. The effect of network charge on the gel's ability to retain the DNA was assessed employing three gel-forming peptides that vary systematically in formal charge. The peptide HLT2, having a formal charge of +5 at neutral pH, was optimal in encapsulating microgram quantities of DNA with little effect on its rheological properties, allowing its effective syringe delivery in vivo. The plasmid, DNA(TA), encapsulated within these gels encodes for a melanoma-specific gp100 antigen fused to the alarmin protein adjuvant HMGN1. Implantation of DNA(TA)-loaded HLT2 gels into mice resulted in an acute inflammatory response with the presence of polymorphonuclear cells, which was followed by infiltrating macrophages. These cellular infiltrates aid in the processing of encapsulated DNA, promoting increased lymphoproliferation and producing an enhanced immune response mediated by CD4+/IFNγ+ expressing Th1 cells, and complemented by the formation of gp100-specific antibodies.

The immune response to mycobacteria is characterized by granuloma formation, which features multinucleated giant cells as a unique macrophage type. We previously found that multinucleated giant cells result from Toll-like receptor-induced DNA damage and cell autonomous cell cycle modifications. However, the giant cell progenitor identity remained unclear. Here, we show that the giant cell-forming potential is a particular trait of monocyte progenitors. Common monocyte progenitors potently produce cytokines in response to mycobacteria and their immune-active molecules. In addition, common monocyte progenitors accumulate cholesterol and lipids, which are prerequisites for giant cell transformation. Inducible monocyte progenitors are so far undescribed circulating common monocyte progenitor descendants with high giant cell-forming potential. Monocyte progenitors are induced in mycobacterial infections and localize to granulomas. Accordingly, they exhibit important immunological functions in mycobacterial infections. Moreover, their signature trait of high cholesterol metabolism may be piggy-backed by mycobacteria to create a permissive niche.

No abstract

Mycobacterium tuberculosis (Mtb) is the causative agent of the infectious disease tuberculosis (TB), which is a leading cause of death worldwide. Approximately one fourth of the world's population is infected with Mtb. A major unresolved question is delineating the inducers of protective long-lasting immune response without inducing overt, lung inflammation. Previous studies have shown that the presence of inducible Bronchus-Associated Lymphoid Tissue (iBALT) correlate with protection from Mtb infection. In this study, we hypothesized that specific Mtb factors could influence the formation of iBALT, thus skewing the outcome of TB disease. We infected non-human primates (NHPs) with a transposon mutant library of Mtb, and identified specific Mtb mutants that were over-represented within iBALT-containing granulomas. A major pathway reflected in these mutants was Mtb cell wall lipid transport and metabolism. We mechanistically addressed the function of one such Mtb mutant lacking mycobacteria membrane protein large 7 (MmpL7), which transports phthiocerol dimycocerosate (PDIM) to the mycobacterial outer membrane (MOM). Accordingly, murine aerosol infection with the Mtb mutant mmpl7 correlated with increased iBALT-containing granulomas. Our studies showed that the mmpl7 mutant lacking PDIMs on the surface overexpressed diacyl trehaloses (DATs) in the cell wall, which altered the cytokine/chemokine production of epithelial and myeloid cells, thus leading to a dampened inflammatory response. Thus, this study describes an Mtb specific factor that participates in the induction of iBALT formation during TB by directly modulating cytokine and chemokine production in host cells.

CXCL17 is a novel mucosal chemokine that mediates myeloid cell recruitment and bactericidal activity and highly expressed in the respiratory tract. However, its role in tuberculosis (TB) immunopathogenesis or protection remains unknown. In this study, we evaluated the function of CXCL17 in a mouse model of aerosol infection with the clinical W-Beijing lineage Mycobacterium tuberculosis hypervirulent HN878 strain. Our results show that CXCL17 production increases in the lung of M. tuberculosis-infected mice during acute and chronic stages of infection. Moreover, in vitro M. tuberculosis infection of epithelial cells and myeloid cells induces production of CXCL17. In humans, lower serum CXCL17 levels are observed among active pulmonary TB patients when compared with subjects with latent TB infection and healthy controls, suggesting a protective role. However, mice treated with rCXCL17 show similar lung bacterial burden and inflammation compared with control animals, despite an increased lung myeloid cell accumulation. Finally, CXCL17 À/À mice are not more susceptible to TB than wild-type animals. These findings suggest that CXCL17 is induced in both murine epithelial and myeloid cells upon M. tuberculosis infection and increased expression during human latent TB infection. However, CXCL17 may have a dispensable role during pulmonary TB. ImmunoHorizons, 2021, 5: 752-759.