Activation and robust expansion of naive T cells often require T cell costimulatory signals and T cell growth factors. However, the precise growth and costimulation requirements for activation and expansion of CD4+ and CD8+ T cells in vivo in allograft response are still not clearly defined. In the present study, we critically examined the role of CD28/CD40 ligand (CD40L) costimulation and the common γ-chain (γ c) signals, a shared signaling component by receptors for all known T cell growth factors (ie, IL-2, IL-4, IL-7, IL-9, IL-15, IL-21), in activation and expansion of CD4+ and CD8+ T cells in the allogeneic hosts. We found that CD28/CD40L costimulation and the γ c signals are differentially involved in proliferation and clonal expansion of CD4+ and CD8+ T cells in response to alloantigen stimulation. CD8+ T cells are highly dependent on the γ c signals for survival, expansion, and functional maturation, whereas in vivo expansion of alloreactive CD4+ T cells is largely γ c independent. T cell costimulation via CD28 and CD40L, however, is necessary and sufficient for activation and expansion of CD4+ T cells in vivo. In a skin transplant model, blocking both CD28/CD40L and the γ c pathways induced prolonged skin allograft survival. Our study provides critical insights that the CD4 and CD8 compartments are most likely governed by distinct mechanisms in vivo, and targeting both costimulatory and γ c signals may be highly effective in certain cytopathic conditions involving activation of both CD4+ and CD8+ T cells.