Id3 expression identifies CD4⁺ memory Th1 cells

Memory CD4⁺ T cells play a pivotal role in mediating long-term protective immunity, positioning them as an important target in vaccine development. However, multiple functionally distinct helper CD4⁺ T-cell subsets can arise in response to a single invading pathogen, complicating the identification of rare populations of memory precursor cells during the effector phase of infection and memory CD4⁺ T cells following pathogen clearance and the contraction phase of infection. Furthermore, current literature remains unclear regarding whether a single CD4⁺ memory T-cell lineage gives rise to secondary CD4⁺ T helper subsets or if there are unique memory precursor cells within each helper lineage. A majority of T follicular helper (Tfh) cells, which have established memory potential, express Id3, an inhibitor of E protein transcription factors, following acute viral infection. We show that expression of Id3 definitively identified a subset of cells within both the CD4⁺ Tfh and T helper 1 (Th1) lineages at memory time points that exhibited memory potential, with the capacity for significant re-expansion in response to secondary infection. Notably, we demonstrate that a subset of Th1 cells that survive into the memory phase were marked by Id3 expression and possessed the potential for enhanced expansion and generation of both Th1 and Tfh secondary effector cell populations in a secondary response to pathogen. Additionally, these cells exhibited enrichment of key molecules associated with memory potential when compared with Id3^lo Th1 cells. Therefore, we propose that Id3 expression serves as an important marker to indicate multipotent potential in memory CD4⁺ T cells.