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Quantitating the Magnitude of the Lymphocytic Choriomeningitis Virus-Specific CD8 T-Cell Response: It Is Even Bigger than We Thought

David Masopust, Kaja Murali-Krishna, and Rafi Ahmed^*

Emory Vaccine Center and Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, Georgia 30322

Received 10 July 2006/ Accepted 21 November 2006

Measuring the magnitudes and specificities of antiviral CD8 T-cell responses is critical for understanding the dynamics and regulation of adaptive immunity. Despite many excellent studies, the accurate measurement of the total CD8 T-cell response directed against a particular infection has been hampered by an incomplete knowledge of all CD8 T-cell epitopes and also by potential contributions of bystander expansion among CD8 T cells of irrelevant specificities. Here, we use several techniques to provide a more complete accounting of the CD8 T-cell response generated upon infection of C57BL/6 mice with lymphocytic choriomeningitis virus (LCMV). Eight days following infection, we found that 85 to 95% of CD8 T cells exhibit an effector phenotype as indicated by granzyme B, 1B11, CD62L, CD11a, and CD127 expression. We demonstrate that CD8 T-cell expansion is due to cells that divide >7 times, whereas heterologous viral infections only elicited <3 divisions among bystander memory CD8 T cells. Furthermore, we found that approximately 80% of CD8 T cells in spleen were specific for ten different LCMV-derived epitopes at the peak of primary infection. These data suggest that following a single LCMV infection, effector CD8 T cells divide 15 times and account for at least 80%, and possibly as much as 95%, of the CD8 T-cell pool. Moreover, the response targeted a very broad array of peptide major histocompatibility complexes (MHCs), even though we examined epitopes derived from only two of the four proteins encoded by the LCMV genome and C57BL/6 mice only have two MHC class I alleles. These data illustrate the potential enormity, specificity, and breadth of CD8 T-cell responses to viral infection and demonstrate that bystander activation does not contribute to CD8 T-cell expansion.

Published ahead of print on 6 December 2006.

Present address: Department of Immunology, University of Washington, Seattle, WA 98195.

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