Dendritic cells efficiently immunoselect mycobacterial-reactive T cells in human blood, including clonable antigen-reactive precursors.

Given the persistence of tuberculosis throughout the world, the delineation of mechanisms that lead to protective immunity to Mycobacterium tuberculosis is important. We have evaluated the presenting function of human dendritic cells for mycobacterial antigens, since these antigen-presenting cells (APC) are particularly effective in initiating antigen-specific T-cell responses. Dendritic cells from blood prove to be active APC for mycobacteria-specific proliferative responses by CD4+ T cells from bacillus Calmette-Guérin (BCG)-vaccinated individuals. In the first 24-48 hr of the response, dendritic cells that have been pulsed with mycobacterial antigens, including live BCG, effectively bind T cells forming discrete cell clusters. The clusters represent about 1% of the applied T cells. Clusters are highly enriched in mycobacterial reactivity while the non-clusters are depleted. Clustered T cells can be used as a starting point to expand antigen-specific cell lines. Mitogen and allogeneic feeder cells were used as APC to expand the mycobacterial-reactive lines, because the antigen-specific T cells had been preselected by virtue of their binding to antigen-pulsed dendritic cells. We discuss the advantages of obtaining antigen-reactive T cells by using dendritic cells as immunoadsorbents. These lines should help delineate the range of mycobacterial antigens and T-cell responses that participate in host responses to mycobacteria.