It is well-established that adaptive immunity changes with increasing age. The literature is increasingly clear that the innate immune cells called macrophages also demonstrate agerelated alterations. Numerous macrophage functions are affected: microbicidal and tumoricidal capabilities are decreased, the immune-suppressing production of prostaglandin E₂ (PGE₂) is increased, and antigen presentation to T cells is diminished. Since macrophages promote profound modulations of the adaptive arm of immunity and are among the first responders to invading pathogens, the immunosenescent alterations in macrophage functions are worthy of further investigation.

This dissertation proposes that relative to those from young, macrophages from aged mice produce lower levels of pro-inflammatory cytokines when stimulated with lipopolysaccharide (LPS) in vitro. This diminished pro-inflammatory response is a result of altered intracellular signaling that is specific to mitogen-activated protein kinase (MAPK) pathways. This proposal is demonstrated in three aims: (1) to compare the LPS-stimulated production of pro-inflammatory cytokines by macrophages from young and aged mice; (2) to determine if an age-related alteration in pro-inflammatory cytokine production is specific to stimulation with LPS; (3) to determine if an age-related alteration in LPS-stimulated macrophage pro-inflammatory cytokine production is due to altered intracellular signaling.

In the studies described herein, macrophages from aged mice produced less tumor necrosis factor-α (TNFα) following LPS stimulation than macrophages from young animals. This correlated with decreased levels of phosphorylated and total p38 and c-jun N-terminal kinase (JNK) MAPKs. Additionally, zymosan-stimulated, but not interleukin-2 (IL-2)-induced, TNFα and IL-6 production is attenuated in splenic macrophages from aged mice compared to young. These changes did not correlate with alterations in the cell-surface expression of Toll-like receptor-2 (TLR2), TLR4, or IL-2Rβ. Macrophages from aged mice demonstrated lower p38 MAPK and MAPK-activated protein kinase-2 (MAPK-APK-2) activation. Protein expression of p38, but not MAPK-APK-2, was reduced with age. Nuclear factor κB (NFκB) activation was also significantly decreased in macrophages from aged mice after exposure to LPS, but not IL-2. These data indicate that age-associated macrophage signaling alterations are pathway-specific and suggest that TLR-mediated pathways are impaired with age at the level of MAPK expression.