Age Associated Alterations in Toll-like Receptor Function of Human Dendritic Cells and Correlation With Influenza Vaccine Response

Older adults are more susceptible to infections and frequently respond poorly to vaccination. This is in part due to the gradual deterioration of the immune system precipitated by natural age advancement (immunosenescence). Studies addressing immunosenescence in the immune system have expanded to focus on the innate as well as the adaptive responses. Notably, aging results in alterations in the function of Toll-like receptors (TLRs), the first described pattern recognition receptor family of the innate immune system. Age-associated dysregulation of TLR signaling likely contributes to the increased morbidity and mortality from infectious diseases found in geriatric patients.;We evaluated TLR function in primary human dendritic cells (DCs) from 104 young (age 21-30 y) and older (> or =65 y) individuals. We used multicolor flow cytometry and intracellular cytokine staining of myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) and found substantial decreases in older compared with young individuals in TNF-alpha, IL-6, and/or IL-12 (p40) production in mDCs and in TNF-alpha and IFN-alpha production in pDCs in response to TLR1/2, TLR2/6, TLR3, TLR5, and TLR8 engagement in mDCs and TLR7 and TLR9 in pDCs. These differences were highly significant after adjustment for heterogeneity between young and older groups (e.g., gender, race, body mass index, number of comorbid medical conditions) using mixed-effect statistical modeling. Studies of surface and intracellular expression of TLR proteins and of TLR gene expression in purified mDCs and pDCs revealed potential contributions for both transcriptional and posttranscriptional mechanisms in these age-associated effects. Moreover, intracellular cytokine production in the absence of TLR ligand stimulation was elevated in cells from older compared with young individuals, suggesting a dysregulation of cytokine production that may limit further activation by TLR engagement. Our results provide evidence for immunosenescence in DCs; notably, defects in cytokine production were strongly associated with poor Ab response to influenza immunization, a functional consequence of impaired TLR function in the aging innate immune response.;Furthermore we established that decreased PRAT4a protein expression in aged monocytes contributes to a TLR1 trafficking defect and subsequent decreased TLR1 surface expression. This age associated decreased expression of PRAT4a is at least partially the result of increased oxidative stress in cells from older individuals causing PRAT4a degradation. Aged monocytes had more oxidative stress, and reducing oxidative stress in aged monocytes almost completely recovered the age-induced PRAT4a expression and partially recovered TLR1 expression and TNF-alpha and IL6 responses after PAM3Cys activation.;Moreover we examined the expression of TLR-induced B7-1 (CD80) and B7-2 (CD86) on DCs from this cohort. We found a generalized age-associated decrease in the upregulation of CD80 and CD86 on plasmacytoid DCs (pDCs) and CD80 on myeloid DCs (mDCs) for all TLR agonists evaluated, while TLR activation resulted in downregulation of CD86 in mDCs from older adults (age ≥ 65). The extent of TLR-induced expression of CD80 and CD86 in mDCs or pDCs assessed prior to immunization was strongly associated with antibody response to the seasonal trivalent inactivated influenza vaccine. In addition, in approximately 80% of young adults (age 21-30) CD86hi and CD86lo populations of mDCs were found; notably, such CD86hi mDCs were completely absent in older adults. Purified CD86hi mDCs expressed high levels of activation markers, but failed to produce cytokines and underwent apoptosis upon TLR stimulation. These findings suggest that such CD86hi cells represent previously activated mDCs that are now non-responsive to further TLR engagement. Their absence from the peripheral blood of older adults may indicate that mDCs from older individuals are more susceptible to TLR-induced apoptosis than mDCs from young adults, suggesting a defect in regulation of the duration of innate immune activation that could contribute to the overall age-associated impairment in TLR-mediated expression of B7 family proteins.;Our findings represent the most comprehensive evaluation of TLR function in human immune cells to date. They provide new insights into aging of the innate immune system, and will aid in the rational development of novel treatments and vaccines geared specifically towards older adults.