Visualization of polyoma virus-specific CD8⁺ T cells in vivo during infection and tumor rejection

T cells are critical for clearing infection and preventing tumors induced by polyoma virus, a natural murine papovavirus. We previously identified the immunodominant epitope for polyoma virus-specific CTL in tumor-resistant H-2(k) mice as the D(k)restricted peptide, MT389-397, derived from the polyoma middle T oncoprotein. In this study, we developed tetrameric D(k) complexes containing the MT389-397 peptide to directly visualize and enumerate MT389-397-specific CTL during polyoma virus infection. We found that D(k)/MT389 tetramer⁺CD8⁺ T cells undergo a massive expansion during primary infection such that by day 7 postinfection these Ag-specific CD8⁺ T cells constitute ~20% of the total and ~40% of the activated CD8⁺ T cells in the spleen. This expansion of D(k)/MT389 tetramer⁺CD8⁺ T cells parallels the emergence of MT389-397-specific ex vivo cytolytic activity and clearance of polyoma virus. Notably, D(k)/MT389 tetramer⁺CD8⁺ T cells are maintained in memory at very high levels. The frequencies of D(k)/MT389 tetramer⁺CD8⁺ effector and memory T cells in vivo match those of CD8⁺ T cells producing intracellular IFN-γ after 6-h in vitro stimulation by MT389-397 peptide. Consistent with preferential Vβ6 expression by MT389-397-specific CD8⁺CTL lines and clones, D(k)/MT389 tetramer⁺CD8⁺ T cells exhibit biased expression of this Vβ gene segment. Finally, we show that D(k)/MT389 tetramer⁺CD8⁺ T cells efficiently infiltrate a polyoma tumor challenge to virus-immune mice. Taken together, these findings strongly implicate virus- induced MT389-397-specific CD8⁺ T cells as essential effectors in eliminating polyoma-infected and polyoma-transformed cells in vivo.