Setting: Dendritic cells (DC) could regulate between the protective and pathogenic immune responses following tuberculous infection. In this paper we investigated if their early infection in the lungs represents a plausible alternative to cross-priming with mycobacterial antigens acquired from infected macrophages.

Objective: To determine the extent and time course of infection of lung DCs following intranasal inoculation of BALB/c mice with green fluorescent protein (GFP) tagged Bacillus Calmette-Guerin (BCG).

Results: A fraction of GFP-BCG infected lung cells were classified as monocytic DCs with the CD11c⁺IA⁺33D1⁺CD8a⁻ phenotype. These cells represented 5–18% of the total GFP⁺ cells, the bulk of which were macrophages. The infected DCs could be separated by cell size into two fractions with similar cell surface staining properties during the 2–72h period after infection. An unexpected difference was observed for the time course of infection between DCs and macrophages: DC infection peaked at 48h followed by decline at 72h, while the proportion of infected macrophages remained steady during the same period.

Conclusion: The presented results are direct evidence that monocytic DCs are recruited to the lungs and take up live bacilli within 48h of intranasal infection with GFP-BCG. This finding is pertinent for the regulation of pulmonary and systemic immune responses and possibly for the dissemination of mycobacterial infection by DCs.