Early age-related atrophy of cutaneous lymph nodes precipitates an early functional decline in skin immunity in mice with aging

Secondary lymphoid organs (SLO; including the spleen and lymph nodes) are critical both for the maintenance of naïve T (TN) lymphocytes and for the initiation and coordination of immune responses. How they age, including the exact timing, extent, physiological relevance, and the nature of age-related changes, remains incompletely understood. We used time-stamping to indelibly mark cohorts of newly generated naïve T cells (a.k.a. recent thymic emigrants - RTE) in mice, and followed their presence, phenotype and retention in SLO. We found that SLO involute asynchronously. Skin-draining lymph nodes (LN) atrophied early (6-9 months) in life and deeper tissue-draining LN and the spleen late (18-20 months), as measured by the loss of both TN numbers and the fibroblastic reticular cell (FRC) network. Time-stamped RTE cohorts of all ages entered SLO and successfully completed post-thymic differentiation. However, in older mice, these cells were poorly retained, and those found in SLO exhibited an emigration phenotype (CCR7loS1P1hi). Transfers of adult RTE into recipients of different ages formally demonstrated that the defect segregates with the age of the SLO microenvironment and not with the age of T cells. Finally, upon intradermal immunization, RTE generated in mice as early as 6-7 months of age barely participated in de novo immune responses and failed to produce well-armed effector cells. These results highlight changes in structure and function of superficial secondary lymphoid organs in laboratory mice that are earlier than expected and are consistent with the long-appreciated and pronounced reduction of cutaneous immunity with aging.