Mycobacterium tuberculosis heparin-binding haemagglutinin adhesin (HBHA) triggers receptor-mediated transcytosis without altering the integrity of tight junctions.

Mycobacterium tuberculosis, the etiologic agent of tuberculosis, adheres to, invades and multiplies in both professional phagocytes and epithelial cells. Adherence to epithelial cells is predominantly mediated by the 28-kDa heparin-binding haemagglutinin adhesin (HBHA), which is also required for the extrapulmonary dissemination of the bacilli. To study the cellular mechanisms that might result in HBHA-mediated extrapulmonary dissemination, we used a transwell model of cellular barrier and fluorescence microscopy and found that HBHA induces a reorganization of the actin filament network in confluent endothelial cells, but does not affect the tight junctions that link them. When coupled to colloidal gold particles, HBHA-mediated a rapid attachment of the particles to the membrane of human laryngeal epithelial cells (non polarized HEp-2 cells) and human type II pneumocytes (polarized A-549 pneumocytes). After attachment, the particles were internalized in membrane-bound vacuoles that migrated across the polarized pneumocytes to reach the basal side. Attachment of the HBHA-coated particles was not observed when the epithelial cells were pretreated with heparinase III, a lyase that specifically cleaves the heparan sulfate chains borne by the proteoglycans. Furthermore, no binding was observed when the gold particles were coated with HBHA lacking its C-terminal heparin-binding domain. These observations indicate that HBHA induces receptor-mediated endocytosis through the recognition of heparan sulfate-containing proteoglycans by the heparin-binding domain of the adhesin. In addition, the transcellular migration of the endocytic vacuoles containing HBHA-coated particles suggests that HBHA induces epithelial transcytosis, which may represent a macrophage-independent extrapulmonary dissemination mechanism leading to systemic infection by M. tuberculosis.