SMAD4 promotes formation of terminally differentiated CTLs that localize in the microvasculature of the lungs but are excluded from the lumen of the airways

Cytotoxic T lymphocytes (CTLs) circulate around the body searching for infected and transformed cells, that undergo apoptosis when lytic granules are delivered into the cytoplasm. To find pathogens that propagate in different tissues, naive CD8 T cells differentiate into heterogeneous populations of effector (TEFF) and memory CD8 T cells with different migratory properties. Several subsets can be identified using antibodies that recognize surface receptors that are expressed at specific stages during CD8 T cell differentiation. Although flow cytometry is a powerful method for tracking antigen specific CTLs during a dynamic immune response, the data provide little information about the distribution of cells in specific anatomical compartments. In this study, confocal imaging was used to explore how signaling via SMAD4 influenced the tissue-tropism of antigen specific CTLs during respiratory infection. During microbial infection, wildtype CTLs gave rise to terminally differentiated TEFF cells that expressed KLRG1 and CX3CR1 at high levels and localized in the microvasculature of the lungs. However, both markers were expressed at reduced levels on SMAD4-deficient CTLs, which preferentially entered the lumen of the airways. These disparate homing properties emphasize the important contributions of SMAD signaling pathways to cell-mediated immunity.