Role of innate immunity mechanisms in the development of normal and pathological aging

BACKGROUND.One of the key indicators of immunosenescence is inflammatory aging (inflammaging), which is primarily attributed to the mechanisms of innate immunity. The assumed cause of inflammaging is chronic activation of patternrecognizing receptors of the innate immunity and their signaling pathways by endogenous and exogenous danger signals. The major role in this process is held by Toll-like receptors (TLR) and NOD-like receptors (NLR). Their activation triggers the implementation of effector mechanisms of innate immune defense; in the context of aging, this contributes to the development of a pro-inflammatory phenotype. Inflammaging can lead to normal aging and longevity, as well as to pathological aging and age-associated pathology. Currently, several receptor and effector components of the innate immune system have been examined in the context of aging and various age-related diseases. However, few studies have been conducted on the contribution of innate immunity components to normal aging; no analysis of specific mechanisms of innate immunity in various aging phenotypes has been conducted. AIM. To conduct a complex study of the innate immunity system (signaling receptor system, inflammasome complex components, cytokines) in elderly individuals and long-living individuals with various aging phenotypes.

MATERIALS AND METHODS. A total of 100 elderly individuals, 100 longliving individuals and a comparison group consisting of 50 young people were examined. Within the elderly and long-living groups, physiological and pathological aging phenotype subgroups were divided based on the Charlson Comorbidity Index, the Short Physical Performance Battery and the Mini-Mental State Examination. Peripheral blood was used as a biomaterial for the study. The expression levels of the pattern recognition receptor genes (TLR2, TLR4) and inflammasome complex genes (NLRP3, CASP1) in leukocytes were assessed by real-time polymerase chain reaction (RT-PCR). Surface expression of TLR on monocytes was determined by flow cytometry. The concentration of cytokines in the blood serum and in the mononuclear cell culture (MNC) of the study groups was measured by enzyme immunoassay. The obtained data were assessed using the Statistica 14 software package (StatSoft, Europe) and GraphPad Prizm (Prizm, USA).

RESULTS. Analysis of the age-related changes in expression of pattern recognition receptors TLR2, TLR4 at the transcriptional and protein levels revealed an increase in TLR2 expression and a decrease in TLR4 expression in the group of elderly and long-living individuals compared to the young participants. In the normal aging phenotype, an increase in TLR2 expression was recorded in both the elderly and long-living individuals. In the pathological aging phenotype, TLR2 hyperexpression was observed in elderly individuals, and TLR2 expression was decreased in long-living individuals. No significant differences in the expression of the TLR4 gene and protein were found in older participants depending on the aging phenotype. Assessing the expression of the inflammasome complex genes demonstrated an increase in the expression of the NLRP3 gene in the peripheral blood leukocytes of elderly and long-living individuals by 20 and 23 times, respectively, compared to young donors. In the pathological aging phenotype, both the elderly and the long-livers showed hyperexpression of the inflammasome complex genes (NLRP3, CASP1), which may be associated with the risk of developing ageassociated pathology. Since the main result of NLRP3 inflammasome activation is the processing of proinflammatory cytokines to a biologically active form, the level of IL-1β and IL-18 cytokines in the blood serum and in the MNC culture of the studied age groups was assessed. In long-living individuals, an increase in the levels of IL-1β and IL-18 in the blood serum was observed compared to the elderly and young donors. No differences in the content of IL-1β in the blood serum were found between the normal and pathological aging groups. An elevation in the levels of IL-1β in the MNC culture was observed during aging, with a more pronounced increase in the pathological aging phenotype.

CONCLUSIONS. Therefore, an increase in TLR2 gene expression may be regarded as a marker of normal aging, while hyperexpression of the NLRP3 gene and elevated levels of IL-1β in the MNC culture can be viewed as markers of pathological aging in elderly and long-living individuals. The identified markers of aging phenotypes will enable prevention of age-associated pathologies, thereby promoting healthy longevity.