Inflammatory mediators facilitate the maturation of dendritic cells (DC), enabling them to induce the activation, proliferation and differentiation of cognate T cells. The role of CD40 on DC and CD154 on T cells has been studied by the co‐adoptive transfer of antigen‐pulsed DC and TCR‐transgenic (Tg) T cells in vivo. It is shown that in the absence of CD40‐CD154 interactions, initial Tg T cell expansion occurs in vivo, but over time, T cell expansion cannot be sustained. The basis for the demise of the T cell population is likely due to the disappearance of the antigen‐pulsed DC in the draining lymph nodes when CD154‐CD40 interactions are interrupted. These findings show that both T cell and DC persistence in vivo is dependent on CD40‐CD154 interactions. In addition to the physical persistence of the DC, CD40 triggering of DC also greatly increases the period for which they can productively present antigen to Tg T cells. Hence DC persistence and antigen‐presenting cell capacity are both dependent on CD40 signaling. While TNF‐α can mature DC as measured by a variety of criteria, the unique capacity of CD40 signaling to sustain T cell responses and induce DC maturation is underscored by the inability of TNF‐α to rescue the immune deficiency of CD40^–/– DC. Hence, the profound impact of CD154 deficiency on cell‐mediated immunity may be due to its ability to limit the duration of antigen presentation in vivo and cause the premature demise of antigen‐specific T cells.