Abstract
Previously, we discovered that influenza-generated CD4 effectors must recognize cognate Ag at a defined effector checkpoint to become memory cells. Ag recognition was also required for efficient protection against lethal influenza infection. To extend these findings, we investigated if vaccine-generated effectors would have the same requirement. We compared live infection with influenza to an inactivated whole influenza vaccine. Live infection provided strong, long-lasting Ag presentation that persisted through the effector phase. It stimulated effector generation, long-lived CD4 memory generation, and robust generation of Ab-producing B cells. In contrast, immunization with an inactivated virus vaccine, even when enhanced by additional Ag-pulsed APC, presented Ag for 3 d or less and generated few CD4 memory cells or long-lived Ab-producing B cells. To test if checkpoint Ag addition would enhance this vaccine response, we immunized mice with inactivated vaccine and injected Ag-pulsed activated APC at the predicted effector checkpoint to provide Ag presentation to the effector CD4 T cells. This enhanced generation of CD4 memory, especially tissue-resident memory in the lung, long-lived bone marrow Ab-secreting cells, and influenza-specific IgG Ab. All responses increased as we increased the density of peptide Ag on the APC to high levels. This suggests that CD4 effectors induced by inactivated vaccine require high levels of cognate Ag recognition at the effector checkpoint to most efficiently become memory cells. Thus, we suggest that nonlive vaccines will need to provide high levels of Ag recognition throughout the effector checkpoint to optimize CD4 memory generation.