Cutting edge: re-evaluating the in vivo cytokine responses of CD8+ T cells during primary and secondary viral infections.

Virus-specific CD8(+) T cells produce IFN-gamma after Ag contact and, in the absence of this cytokine, the host often cannot eradicate infection. However, our ability to identify cells that are actively expressing this critical effector function in vivo is limited, because the protein is rapidly secreted. In this study, we describe a simple approach that circumvents the need for ex vivo Ag stimulation and allows the enumeration of CD8(+) T cells that are actively synthesizing IFN-gamma in vivo during primary and secondary virus infections. The proportion of Ag-specific primary CD8(+) T cells producing IFN-gamma peaks at 5 days postinfection, when the T cell population is still expanding exponentially. In vivo IFN-gamma synthesis by memory cells is explosive, peaking at approximately 12 h after secondary infection and terminating hours thereafter. This technique will be useful when evaluating in vivo immune cell activity in many situations, including a variety of noninfectious (e.g., autoimmune) diseases.