Clonotypic structure of the human CD4(+) memory T cell response to cytomegalovirus

High steady-state frequencies of CMV-specific CD4(+) memory T cells are mai ntained in CMV-exposed subjects, and these cells are thought to play a key role in the immunologic control of this permanent infection. However, the e ssential components of this response are poorly defined. Here, we report th e use of a step-wise application of flow cytometric and molecular technique s to determine the number and size of the TCR V beta -defined clonotypes wi thin freshly obtained CMV-specific CD4(+) memory T cell populations of four healthy, CMV-exposed human subjects. This analysis revealed a stable clono typic hierarchy in which 1-3 dominant clonotypes are maintained in concert with more numerous subdominant and minor clonotypes. These dominant clonoty pes accounted for 10-50% of the overall CMV response, and comprised from 0. 3 to 4.0% of peripheral blood CD4(+) T cells. Two subjects displayed immuno dominant responses to single epitopes within the CMV matrix phosphoprotein pp65; these single epitope responses were mediated by a single dominant clo notype in one subject, and by multiple subdominant and minor clonotypes in the other. Thus, the CMV-specific CD4(+) T cell memory repertoire in normal subjects is characterized by striking clonotypic dominance and the potenti al for epitope focusing, suggesting that primary responsibility for immunos urveillance against CMV reactivation rests with a handful of clones recogni zing a limited array of CMV determinants. These data have important implica tions for the understanding of mechanisms by which a genetically stable chr onic viral pathogen such as CMV is controlled, and offer possible insight i nto the failure of such control for a genetically flexible pathogen like HI V-1.