CD28 and infection: To assess the implications of loss of CD28 after T cell activation, we infected Cd28flox/floxOx40cre/+ and heterozygous control mice intranasally (I.N.)with influenza A virus (HKx31), which causes an acute localized infection in the respiratory tract that generates robust anti-viral T and B cell responses in the draining mediastinal lymph node (medLN) and the lungs.