Lymphoid homeostasis is required to ensure immune responsiveness and to prevent immunodeficiency. As such, the immune system must maintain distinct populations of naïve T cells that are able to respond to new antigens as well as memory T cells specific to those antigens it has already encountered. Though both naïve and memory T cells reside in and traffic through secondary lymphoid organs, there is growing evidence that the two populations may be regulated differently. We show here that naïve T cell survival and memory T cell survival have different requirements for cytokines (including the interleukins IL-2, IL-4, IL-7, IL-9 and IL-15) that use the common cytokine receptor gamma chain (gamma c). Using monoclonal populations of antigen-specific CD4+ T cells, we found that naïve T cells cannot survive without gamma c, whereas memory T cells show no such requirement. In contrast, neither naïve nor gamma c-deficient memory T cells were impaired in their ability to proliferate and produce cytokines in response to in vivo antigenic stimulation. These data call into question the physiological role of gamma c-dependent cytokines as T cell growth factors and show that naïve and memory CD4+ T cell survival is maintained by distinct mechanisms.
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