Does cytolysis by CD8+ T cells drive immune escape in HIV infection?

CD8(+) "cytotoxic" T cells are important for the immune control of HIV and the closely related simian models SIV and chimeric simian-human immunodeficiency virus (SHIV), although the mechanisms of this control are unclear. One effect of CD8(+) T cell-mediated recognition of virus-infected cells is the rapid selection of escape mutant (EM) virus that is not recognized. To investigate the mechanisms of virus-specific CD8(+) T cell control during immune escape in vivo, we used a real-time PCR assay to study the dynamics of immune escape in early SHIV infection of pigtail macaques. For immune escape mediated by cytolysis, we would expect that the death rate of wild type (WT) infected cells should be faster than that of EM-infected cells. In addition, escape should be fastest during periods when the total viral load is declining. However, we find that there is no significant difference in the rate of decay of WT virus compared with EM virus. Further, immune escape is often fastest during periods of viral growth, rather than viral decline. These dynamics are consistent with an epitope-specific, MHC class I-restricted, noncytolytic mechanism of CD8(+) T cell control of SHIV that specifically inhibits the growth of WT virus in vivo.