The project goal is to understand how age-associated changes to the lymph nodes contribute to decreased
survival of naïve T cells, a hallmark of immune aging. Naïve T cells are essential for resolving infections
caused by newly encountered pathogens, but their number and frequency within the T cell population
decrease dramatically with age. This contributes to reduced magnitude and delayed timing of the aged immune
response against infectious disease. Naïve T cell survival requires constant signaling from the tissue
environment, and it has been well-supported that the aged environment is defective in its ability to maintain
naïve T cell homeostasis. Naïve T cells primarily interact with the tissues of the lymph nodes, organized tissue
compartments strategically located throughout the body that serve as coordination centers for the immune
response. Broad changes to the size and structure of the lymph nodes have been described with age, for both
mice and humans; however, the mechanisms driving these changes, and how they impact naïve T cells, are
unknown. Our data have revealed changes with age to the lymph node stromal cell type known as the
fibroblastic reticular cell (FRC). Naïve T cells adhere to FRCs and crawl along the FRC network in order to
navigate the lymph nodes. FRCs differentiate in situ in a process that requires continuous lymphotoxin-beta
(LTβ) signaling. Mature FRCs express homeostatic cytokines that are essential for naive T cell survival and
chemokines that promote interactions and motility within the lymph nodes. Homeostatic chemokines such as
CCL21 have well-appreciated roles in promoting naïve T cell recruitment to and migration within the lymph
nodes, but how their expression and distribution are impacted by age is not clear. In our preliminary work,
we determined that lymph node stromal expression of Ltbr (lymphotoxin-beta receptor) and Ccl21 were
significantly decreased with age. We also found evidence that FRC differentiation is impaired with age, in that
early FRC precursors were decreased in frequency, and that there was an accumulation intermediate FRC
precursors. The perturbation to FRC precursors was correlated to decreased expression of LTBR,
suggesting that aged FRC precursors were unable to receive the LTβ signals necessary to complete
maturation. We hypothesize that with impaired FRC differentiation, the organization of the FRC network and
its expression of homeostatic chemokines is impaired with age, which inhibits the interactions necessary
for naïve T cell survival. In this project we will determine the role of LTβ signaling in FRC decline with age (Aim
1), and whether altering the architecture of the lymph node microenvironment contributes to decreased naïve T
cell survival (Aim 2). Our experimental approach uses live-cell 2-photon microscopy, which enables us to
visualize the interaction of naïve T cells with FRCs in real time within intact tissues. The completion of the
project will support a mechanism by which the aged tissue microenvironment drives the attrition of naïve T
cells. Establishing this link can lead to development of therapeutics to restore the aged lymph node stroma to
promote the survival and function of naïve T cells in the elderly.