Airways infection with virulent <i>Mycobacterium tuberculosis</i> delays the influx of dendritic cells and the expression of costimulatory molecules in mediastinal lymph nodes

Despite tuberculosis resurgence and extensive dendritic cell (DC) research, there are no in vivo studies evaluating DC within regional lymphoid tissue during airways infection with virulent Mycobacterium tuberculosis (Mtb) H37Rv. Using DC-specific antibodies, immunocytochemistry, flow cytometry and Ziehl-Neelsen (ZN) for bacilli staining, we searched for Mtb and DC changes within mediastinal lymph nodes, after intratracheal (ITT) inoculation of virulent Mtb. ZN and immunocytochemistry in frozen and paraffin sections of mediastinal lymph nodes identified Mtb until day 14 after ITT inoculation, associated with CD11c(+) and Dec205(+) DC. Analysing CD11c, MHC-CII, and Dec205 combinations by flow cytometry in MLN suspensions revealed that CD11c(+)/MHC-CII(+) and CD11c(+)/Dec205(+) DC did not increase until day 14, peaked on day 21, and sharply declined by day 28. No changes were seen in control, saline-inoculated animals. The costimulatory molecules evaluated in CD11c(+) DCs followed a similar trend; the CD80 increase was negligible, slightly surpassed by CD40. CD86 increased earlier and the three markers peaked at day 21, declining by day 28. While antigen-specific proliferation was not evident for MLN CD4(+) T cells at 2 weeks postinfection, delayed-type hypersensitivity responses upon ITT inoculation revealed that, as early as day 3 and 7, both the priming and peripheral systemic immune responses were clearly established, persisting until days 14-21. While airways infection with virulent Mtb triggers an early, systemic peripheral response maintained for three weeks, this seems dissociated from regional events within mediastinal lymph nodes, such as antigen-specific T-cell reactivity and a delay in the influx and local activation of DC.