Protective heterologous immunity in aged mice correlates with the ability to develop a primary response to the NP epitope. Young and aged C57BL/6 mice were first intranasally infected with 300 EID50 PR8 (H1N1), and then were intranasally challenged with 30,000 EID50 x31 (H2N3) 30 d later. On day 7 after challenge, lung tissue was harvested from infected mice for measurement of viral titers, and splenic tissue was harvested for tetramer analyses, as described in the Materials and methods. Each symbol represents the frequency of splenic CD8 T cells specific for NP as determined for individual young (open circles) and aged (closed circles) mice that had either cleared or not cleared virus. The means of 18 young mice that cleared virus, 14 aged mice that cleared virus, and 7 aged mice that failed to clear virus are indicated.
