Background:  Our current understanding of sexual HIV transmission favors the initial uptake of virions by mucosal dendritic cells (DC), which then deliver HIV to CD4⁺ T cells.  This sequential model has arisen mainly from studies with monocyte-derived DC and epidermal Langerhans cells (LC) rather than direct observations in the human genital tract, which face technical and ethical limitations.  Two previous organ culture models of the human cervivovaginal mucosa have detected infection mostly in T cells and macrophages located beneath the epithelial layer. 

Methods:We introduce a novel ex vivo model that elucidates the initial intraepithelial transmission events leading up to this subepithelial infection.  Vaginal epithelial sheets were obtained without the use of digestive enzymes by gentle ex vivo suction blistering of surgically excised mucosa.  We then challenged these sheets with green-fluorescence protein (gfp)-tagged infectious virions by spinoculation. 

Results: Examination by confocal microscopy revealed that both R5-tropic HIV-1 JR-CSF and X4-tropic HIV-1 LAI simultaneously bound to intraepithelial vaginal T cells and LC.  Electron microscopy and blocking studies with monoclonal antibodies demonstrated that HIV-1 entered both cell types via CD4 and co-receptor-mediated fusion. 

Conclusions:  Although vaginal LC were able to also sequester intact virions in endosomes, our findings point to a parallel rather than a sequential mode of transmission, in which both intraepithelial LC and T cells become directly infected.  Additional blocking experiments with mannan suggested that in trans viral passage and C-type lectin receptors did not significantly contribute to this process.

Keywords: TRANSMISSION; MUCOSA; LANGERHANS CELLS