CD4 Effector TCR Avidity for Peptide on APC Determines the Level of Memory Generated

Abstract

Initial TCR affinity for peptide Ag is known to impact the generation of memory; however, its contributions later, when effectors must again recognize Ag at 5–8 d postinfection to become memory, is unclear. We examined whether the effector TCR affinity for peptide at this “effector checkpoint” dictates the extent of memory and degree of protection against rechallenge. We made an influenza A virus nucleoprotein (NP)-specific TCR transgenic mouse strain, FluNP, and generated NP-peptide variants that are presented by MHC class II to bind to the FluNP TCR over a broad range of avidity. To evaluate the impact of avidity in vivo, we primed naive donor FluNP in influenza A virus–infected host mice, purified donor effectors at the checkpoint, and cotransferred them with the range of peptides pulsed on activated APCs into second uninfected hosts. Higher-avidity peptides yielded higher numbers of FluNP memory cells in spleen and most dramatically in lung and draining lymph nodes and induced better protection against lethal influenza infection. Avidity determined memory cell number, not cytokine profile, and already impacted donor cell number within several days of transfer. We previously found that autocrine IL-2 production at the checkpoint prevents default effector apoptosis and supports memory formation. Here, we find that peptide avidity determines the level of IL-2 produced by these effectors and that IL-2Rα expression by the APCs enhances memory formation, suggesting that transpresentation of IL-2 by APCs further amplifies IL-2 availability. Secondary memory generation was also avidity dependent. We propose that this regulatory pathway selects CD4 effectors of highest affinity to progress to memory.

CD4 memory has a hierarchical structure created by requirements for infection-derived signals at an effector checkpoint

“…Moreover, the impact of avidity on responding effectors was already apparent in their survival after only 3 dpi and was also proportional to the amount of IL-2 produced by the responding effectors, which was strictly driven by avidity and peptide dose. This supports the concept that autocrine IL-2 acts directly to support effector survival and explains how the strength of TCR signals determines the extent of CD4 memory generation ( 21 ).…”

“…We pulsed activated APC with high concentrations of each peptide to evaluate the impact of avidity on NP-specific CD4 T cells. We found that higher TCR avidity for peptide-MHC-II proportionally increased memory generation ( 21 ). Moreover, the impact of avidity on responding effectors was already apparent in their survival after only 3 dpi and was also proportional to the amount of IL-2 produced by the responding effectors, which was strictly driven by avidity and peptide dose.…”

“…We analyzed the impact of the avidity of TCR:peptide/MHC-II interaction at the checkpoint for memory generation. We developed a new CD4 TCR transgenic mouse that recognizes a dominant influenza nucleoprotein (NP) determinant and made a series of NP peptides with a broad range of avidities for the TCR ( 21 ). We pulsed activated APC with high concentrations of each peptide to evaluate the impact of avidity on NP-specific CD4 T cells.…”

“…We found that IAV infection induces expression of the IL-2 receptor β and γ chains on CD4 effectors, but many effectors did not express IL-2Rα (22). However, in vitro TLR agonist-activated APC and in vivo APC 6 dpi in infected mice did upregulate IL-2Rα, and APC that could not express IL-2Rα supported less memory generation than wild-type APC ( 21 ). These results suggest that IL-2 is trans-presented to the effectors by activated APC.…”

“…Trans-presentation of IL-15 is a common mechanism ( 24 ), but IL-2 trans-presentation has rarely been identified ( 25 ). We suggest that the APC trans-presentation of IL-2 to the responding effectors amplifies the effective concentration of IL-2 during APC : CD4 effector cognate interaction ( 21 ). If so, the activation of IL-2Rα on APC by PR pathways is another mechanism by which infection-stimulated PR pathways contribute to CD4 memory generation.…”