CD8⁺ T cells are a biologically relevant source of macrophage inflammatory protein-1α in vivo

Chemokines are small proteins that direct the migration of leukocytes to inflammatory foci. Many cell types, including macrophages, fibroblasts, endothelial cells, and lymphocytes, produce chemokines in vitro, but biologically relevant sources of chemokines in vivo have not been well characterized. To investigate the pertinent sources of macrophage inflammatory protein-1α (MIP-1α) in vivo, we used MIP-1α-deficient (MIP- 1α(-/-)) mice as donors and as recipients in adoptive transfer experiments after a lethal infection with Listeria monocytogenes (LM). Unexpectedly, we found that the production of MIP-1α by CD8⁺ T cells was critical in this system, as the cells from MIP-α(-/-) mice primed with LM were significantly less effective in protecting naive mice against a lethal infection by LM than were the CD8⁺ T cells from wild-type (wt) mice. This requirement for donor T cell production of MIP-1α was confirmed by the observation that wt donor T cells do not mediate protection when coadministered with an anti-MIP-1α polyclonal antiserum. Production of MIP-1α by the recipient mice was not required for protection, because wt and MIP-1α(-/-) recipients were equally well protected by wt T cells. A 2- to 3-fold decrease in the number of transferred lymphocytes was seen in the spleens of mice receiving T cells from MIP-1α(-/-) mice compared with those receiving wt T cells, in addition, CD8⁺ T cells from MIP-1α(-/-) mice had a reduced ability to kill LM- infected target cells in vitro. These findings demonstrate that T cell production of MIP-1α is required for clearance of an intracellular pathogen in vivo.