(493) Early establishment of Virtual memory T cells in the neonatal lung

Introduction/Rationale: CD8⁺ T virtual memory (TVM) cells are antigen-inexperienced, memory-like cells that arise early in life. To date, most TVM studies have focused on adult, splenic TVM populations and failed to consider TVM function at their origin and in mucosal sites. Here, we sought to determine how TVM populate the lung during early life and may impact subsequent immunity.

Methods: TVM in the lung tissue versus the circulation were studied using a novel intravascular staining protocol via the retro-orbital sinus that we optimized for use in neonates. Lungs were subsequently phenotyped and analyzed by flow cytometry for CD8⁺CD44⁺CD122⁺CD49d⁻ TVM across postnatal (P) days P3–P14. To explore the contribution of cytokine signaling in neonatal lung TVM establishment and localization, parallel analyses were performed in IL-15 knockout neonates.

Results: In naïve neonatal mice, TVM were detected in the lung parenchyma as early as P3. While total numbers of overall lung TVM increased with age, this expansion was primarily driven by circulating TVM. While lung-resident TVM remained numerically stable, expression of Ki-67 suggested local proliferation. In P5 naïve IL-15 knockout neonates, lung-resident TVM were completely absent, accompanied by approximately 70% reduction in circulating lung TVM and ~50% reduction in splenic TVM numbers compared to wild-type controls.

Conclusion: Our findings demonstrate that T virtual memory cells emerge in the neonatal lungs shortly after birth, establishing a stable resident pool capable independent of infection. Moreover, IL-15 may specifically contribute to the early establishment and maintenance of the TVM niche in the lung parenchyma, with circulating and splenic niches only partially dependent on IL-15. Together, these data suggest that neonatal TVM are regulated distinctly from adults, with IL-15 being requisite for maintaining TVM at the respiratory barrier early in life.