CD4+ T Cell Help Has an Epitope-Dependent Impact on CD8+ T Cell Memory Inflation during Murine Cytomegalovirus Infection

Abstract

Murine CMV (MCMV) establishes a systemic, low-level persistent infection resulting in the accumulation of CD8+ T cells specific for a subset of viral epitopes, a process called memory inflation. Although replicating virus is rarely detected in chronically infected C57BL/6 mice, these inflationary cells display a phenotype suggestive of repeated Ag stimulation, and they remain functional. CD4+ T cells have been implicated in maintaining the function and/or number of CD8+ T cells in other chronic infections. Moreover, CD4+ T cells are essential for complete control of MCMV. Thus, we wondered whether CD4+ T cell deficiency would result in impaired MCMV-specific CD8+ T cell responses. Here we show that CD4+ T cell deficiency had an epitope-specific impact on CD8+ T cell memory inflation. Of the three codominant T cell responses during chronic infection, only accumulation of the late-appearing IE3-specific CD8+ T cells was substantially impaired in CD4+ T cell-deficient mice. Moreover, the increased viral activity did not drive increased CD8+ T cell division or substantial dysfunction in any MCMV-specific population that we studied. These data show that CD4+ T cell help is needed for inflation of a response that develops only during chronic infection but is otherwise dispensable for the steady state maintenance and function of MCMV-specific CD8+ T cells.