Nonphagocytic dendritic cells are effective accessory cells for anti-mycobacterial responses in vitro.

The accessory cell requirements for a given T cell response may be examined in vitro by using highly purified lymph node T cells. We have examined the capacity of different antigen-presenting cells to stimulate proliferation of Mycobacterium tuberculosis-primed T cells when the antigenic challenge is either soluble or particulate in nature. By titrations of cell number and antigen concentration, it was shown that dendritic cells are not only extremely efficient at presenting soluble mycobacterial antigen compared with various macrophage populations, but also that they are capable of presenting whole mycobacteria. Because phagocytosis of mycobacteria does not occur with these cells, we suggest that processing of antigen by dendritic cells may be initiated at the plasma membrane. Because macrophages are not essential for this in vitro response, a role for dendritic cells in antibacterial immunity in vivo is implicated.