Albert C. Shaw

Aging of the innate immune system

The innate immune system is composed of a network of cells including neutrophils, NK and NKT cells, monocytes/macrophages, and dendritic cells that mediate the earliest interactions with pathogens. Age-associated defects are observed in the activation of all of these cell types, linked to compromised signal transduction pathways including the Toll-like Receptors. However, aging is also characterized by a constitutive pro-inflammatory environment (inflamm-aging) with persistent low-grade innate immune activation that may augment tissue damage caused by infections in elderly individuals. Thus, immunosenescence in the innate immune system appears to reflect dysregulation, rather than exclusively impaired function.

Age-associated decrease in TLR function in primary human dendritic cells predicts influenza vaccine response.

We evaluated TLR function in primary human dendritic cells (DCs) from 104 young (age 21–30 y) and older (≥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-α, IL-6, and/or IL-12 (p40) production in mDCs and in TNF-α and IFN-α 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.

Human innate immunosenescence: causes and consequences for immunity in old age

The past decade has seen an explosion in research focusing on innate immunity. Through a wide range of mechanisms including phagocytosis, intracellular killing and activation of proinflammatory or antiviral cytokine production, the cells of the innate immune system initiate and support adaptive immunity. The effects of aging on innate immune responses remain incompletely understood, particularly in humans. Here we review advances in the study of human immunosenescence in the diverse cells of the innate immune system, including neutrophils, monocytes, macrophages, natural killer and natural killer T (NKT) cells and dendritic cells-with a focus on consequences for the response to infection or vaccination in old age.

Age-dependent dysregulation of innate immunity

As we age, the innate immune system becomes dysregulated and is characterized by persistent inflammatory responses that involve multiple immune and non-immune cell types and that vary depending on the cell activation state and tissue context. This ageing-associated basal inflammation, particularly in humans, is thought to be induced by several factors, including the reactivation of latent viral infections and the release of endogenous damage-associated ligands of pattern recognition receptors (PRRs). Innate immune cell functions that are required to respond to pathogens or vaccines, such as cell migration and PRR signalling, are also impaired in aged individuals. This immune dysregulation may affect conditions associated with chronic inflammation, such as atherosclerosis and Alzheimers disease.

Age-Associated Defect in Human TLR-1/2 Function

The effects of aging on human TLR function remain incompletely understood. We assessed TLR function and expression in peripheral blood monocytes from 159 subjects in 2 age categories, 21–30 and >65 years of age, using a multivariable mixed effect model. Using flow cytometry to assess TLR-induced cytokine production, we observed a substantial, highly significant defect in TLR1/2-induced TNF-α (p = 0.0003) and IL-6 (p < 0.0001) production, in older adults compared with young controls. In contrast to findings in aged mice, other TLR (including TLR2/6)-induced cytokine production appeared largely intact. These differences were highly significant even after correcting for covariates including gender, race, medications, and comorbidities. This defect in TLR1/2 signaling may result from alterations in baseline TLR1 surface expression, which was decreased by 36% in older adults (p < 0.0001), whereas TLR2 surface expression was unaffected by aging. Production of IL-6 (p < 0.0001) and TNF-α (p = 0.003) after stimulation by N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-Cys-[S]-Ser1-[S]-Lys(4) trihydrochloride was strongly associated with TLR1 surface expression. Diminished TLR1/2 signaling may contribute to the increased infection-related morbidity and mortality and the impaired vaccine responses observed in aging humans.

Circulating Monocytes from Systemic Sclerosis Patients with Interstitial Lung Disease Show an Enhanced Profibrotic Phenotype

Profibrotic cells derived from circulating CD14+ monocytes include fibrocytes and alternatively activated macrophages. These cells are associated with interstitial lung disease (ILD) and are implicated in the pathogenesis of systemic sclerosis (SSc); however, the simultaneous presence of profibrotic cells and their associated mediators in the circulation of these patients has not been defined. We hypothesized that monocytes from patients with SSc-related ILD (SSc-ILD) would show profibrotic characteristics when compared with normal controls. We recruited patients with SSc-ILD (n=12) and normal controls (n=27) and quantified circulating collagen-producing cells by flow cytometry for CD45 and pro-collagen I. The in vitro activation potential of CD14+ monocytes in response to lipopolysaccharide was assessed using flow cytometry for CD163, and by ELISA for CCL18 and IL-10 secretion. Profibrotic mediators in plasma were quantified using Luminex-based assays. The concentration of circulating collagen-producing cells was increased in the SSc-ILD patients when compared with controls. These cells were composed of both CD34+ fibrocytes and a population of CD34+CD14+ cells. Cultured CD14+ monocytes from SSc-ILD patients revealed a profibrotic phenotype characterized by expression of CD163 and by enhanced secretion of CCL18 and IL-10 in response to proinflammatory activation. Plasma levels of IL-10, MCP-1, IL-1RA, and TNF levels were significantly elevated in the plasma of the SSc-ILD cohort. Subgroup analysis of the normal controls revealed that unlike the subjects ≤35 years, subjects ≥60 years old showed higher levels of circulating CD34+CD14+ cells, collagen-producing CD14+ monocytes, CD163+ monocytes, IL-4, IL-10, IL-13, MCP-1, and CCL18. These data indicate that the blood of patients with SSc-ILD and of healthy aged controls is enriched for fibrocytes, profibrotic monocytes, and fibrosis-associated mediators. Investigations defining the factors responsible for this peripheral blood profile may provide new insight into SSc-ILD as well as the pathophysiology of aging.

Prevaccine Determination of the Expression of Costimulatory B7 Molecules in Activated Monocytes Predicts Influenza Vaccine Responses in Young and Older Adults

BACKGROUND Innate immunity, including Toll-like receptor (TLR)-mediated expression of the B7 costimulatory molecules CD80 and CD86, is critical for vaccine immunity. We examined whether CD80 and CD86 expression vary with aging and predict response to the trivalent inactivated influenza vaccine. METHODS One hundred sixty-two subjects between 21 and 30 years of age (the young group) or > or =65 years of age (the older group) enrolled before vaccination. We determined TLR-induced monocyte CD80/CD86 expression by flow cytometry and vaccine antibody responses by hemagglutination inhibition. RESULTS The mean increase in TLR-induced CD80(+) monocytes was reduced in older, compared with young, adults by 68% (P=.0002), and each decile increase of CD80(+) cells was associated with an 8.5% increase in mean number of vaccine strains with a > or =4-fold titer increase (P=.01) and a 3.8% increase in mean number of strains with a postvaccine titer > or =1 : 64 (P=.037). Each decile decrease of CD86(+) cells was associated with an 11% increase in the mean number of strains with a 4-fold increase (P=.002) and a 3.9% increase in the mean number of strains with a postvaccine titer > or =1 : 64 (P=.07). CONCLUSIONS CD80 and CD86 expression on activated monocytes is highly associated with influenza vaccine response. This approach prospectively identifies adults unlikely to respond to immunization who may benefit from alternative vaccines or antiviral prophylaxis during influenza outbreaks.

Dysregulation of human Toll-like receptor function in aging.

Studies addressing immunosenescence in the immune system have expanded to focus on the innate as well as the adaptive responses. In particular, aging results in alterations in the function of Toll-like receptors (TLRs), the first described pattern recognition receptor family of the innate immune system. Recent studies have begun to elucidate the consequences of aging on TLR function in human cohorts and add to existing findings performed in animal models. In general, these studies show that human TLR function is impaired in the context of aging, and in addition there is evidence for inappropriate persistence of TLR activation in specific systems. These findings are consistent with an overarching theme of age-associated dysregulation of TLR signaling that likely contributes to the increased morbidity and mortality from infectious diseases found in geriatric patients.

Toll‐Like Receptors in Older Adults

Toll‐like receptors (TLRs) recognize a limited number of conserved elements in pathogens and, by activating antigen‐presenting cells such as dendritic cells and monocytes and macrophages, play a crucial role in the immune response to infection and vaccination. Most data on TLR function in the context of human aging focus on responses to lipopolysaccharide, an integral component of gram‐negative bacteria, which signals through TLR4. However, such studies have not led to a consensus conclusion and are limited by differences in epidemiological and laboratory methods. A recent comprehensive evaluation of TLR function in monocytes from older adults was conducted using a multivariable mixed statistical model to account for covariates. It was found that cytokine production after TLR1/2 engagement, which is essential for the recognition of triacylated lipopeptides found in a variety of bacteria, is substantially lower in monocytes from older adults. The upregulation of costimulatory proteins such as CD80, essential for optimal activation of T cells, on monocytes from older adults was less for all TLR ligands tested than for cells from young individuals, and the extent of CD80 upregulation predicted subsequent antibody response to influenza immunization. These and other consequences of aging on human TLR function may impair activation of the immune response and contribute to poorer vaccine responses and greater morbidity and mortality from infectious diseases in older adults. Such age‐associated alterations have particular relevance in view of the interest in TLR agonists as therapeutic agents not only for infections, but also for allergic, autoimmune, and malignant disease.

Discs large (Dlg1) complexes in lymphocyte activation

T cell antigen recognition involves the formation of a structured interface between antigen-presenting and T cells that facilitates the specific transmission of activating and desensitizing stimuli. The molecular machinery that organizes the signaling molecules and controls their disposition in response to activation remains poorly understood. We show here that in T cells Discs large (Dlg1), a PDZ domain-containing protein, is recruited upon activation to cortical actin and forms complexes with early participants in T cell activation. Transient overexpression of Dlg1 attenuates basal and Vav1-induced NFAT reporter activation. Reduction of Dlg1 expression by RNA interference enhances both CD3- and superantigen-mediated NFAT activation. Attenuation of antigen receptor signaling appears to be a complex, highly orchestrated event that involves the mutual segregation of important elements of the early signaling complex.