T cell based influenza vaccines offer the potential for cross protective immunity to multiple clades of influenza virus. Here we explored the effect of increasing CD8 T cell responses during intranasal vaccination by incorporating a T cell costimulator, 4-1BBL. Inclusion of 4-1BBL in an influenza nucleoprotein (NP)-containing adenoviral vector increased the number of NP-specific CD8 T cells and lowered the vaccine dose required for short-term protection from influenza-induced disease in mice. At higher vaccine doses, the inclusion of 4-1BBL increased the duration of protection of mice from influenza-induced mortality. Bone marrow chimera experiments revealed that the major effects of 4-1BBL were directly on αβ T cells with minor additional effects through cells other than αβ T cells. The implications of these findings are that including 4-1BBL or adjuvants that induce 4-1BBL expression may be of benefit in a vaccine setting for enhancing the magnitude and duration of T cell responses to influenza virus