CD4⁺ T cell-dependent IFN-γ production by CD8⁺ effector T cells in Mycobacterium tuberculosis infection

Both CD4⁺ and CD8⁺ T cells contribute to immunity to tuberculosis, and both can produce the essential effector cytokine IFN-γ. However, the precise role and relative contribution of each cell type to in vivo IFN-γ production are incompletely understood. To identify and quantitate the cells that produce IFN-γ at the site of Mycobacterium tuberculosis infection in mice, we used direct intracellular cytokine staining ex vivo without restimulation. We found that CD4⁺ and CD8⁺ cells were predominantly responsible for production of this cytokine in vivo, and we observed a remarkable linear correlation between the fraction of CD4⁺ cells and the fraction of CD8⁺ cells producing IFN-γ in the lungs. In the absence of CD4⁺ cells, a reduced fraction of CD8 ⁺ cells was actively producing IFN-γ in vivo, suggesting that CD4⁺ effector cells are continually required for optimal IFN-γ production by CD8⁺effector cells. Accordingly, when infected mice were treated i.v. with an MHC-II-restricted M. tuberculosis epitope peptide to stimulate CD4⁺ cells in vivo, we observed rapid activation of both CD4⁺ and CD8⁺ cells in the lungs. Indirect activation of CD8⁺cells was dependent on the presence of CD4⁺ cells but independent of IFN-γ responsiveness of the CD8⁺ cells. These data provide evidence that CD4⁺ cell deficiency impairs IFN-γ production by CD8⁺ effector cells and that ongoing cross-talk between distinct effector T cell types in the lungs may contribute to a protective immune response against M. tuberculosis. Conversely, defects in these interactions may contribute to susceptibility to tuberculosis and other infections.