Overview

Overview        1

Innate Immune Activation and Recruitment        4

Summary        4

Age Related Phenotypes        4

Macrophage Polarization and Inflammation        4

Dendritic Cell Activation and Migration        5

Summary        5

Age Related Phenotypes        6

Recruitment of Monocytes and Monocyte Differentiation To Dendritic Cells        6

TLRs        8

Cytokines        11

Phagocytosis        11

Epigenetics        11

T-cell Activation, Differentiation, and Proliferation        12

Summary        12

Age Related Phenotypes        12

T-cell Activation (Dysregulated Signaling)        12

T-cell Activation (Poor Effector Functions)        14

T-cell Activation (Dendritic Cell Function)        16

T-cell Activation (Cytoskeletal Organization)        17

T-cell Epigenetics        18

Reduced T-cell Motility        19

T-cell Exhaustion        19

T-cell Metabolism        20

DNA Damage / Telomere length        20

Reduced naive CD8 T-cells        21

Repertoire        21

Chronic CMV Exposure        22

Reduction During Infection        22

B-cell Activation and Antibody Production        23

Summary        23

Age Related Phenotypes        23

Germinal Center Function        23

High Baseline HI        23

Tfh Activation        24

Tfh cell Frequency        24

TNF-a levels        25

AID expression        25

B-cell Activation (Cytoskeleton)        26

Reviews        26

T-cell        26

Dendritic Cells        26

Vaccination        26

Lymph Node        27

Mitochondria        27

Pathways        27

Expert Labs        27

Innate Immune Activation and Recruitment

Summary

Macrophages serve as the first line of immune system defense against pathogens that breach the epithelial barrier. They recognize molecular structures produced by microbial pathogens called pathogen-associated molecular patterns (PAMPs). Upon recognition, they initiate inflammation which results in many cellular and tissue changes in the surrounding area. Generally, it is accepted that directly after virus recognition, macrophages endocytose the invading pathogens and polarize to the M1-like phenotype. (Role of Human Macrophage Polarization in Inflammation during Infectious Diseases) One of the most important changes resulting from the inflammatory environment created by the macrophages is the rapid recruitment of circulating neutrophils and monocytes through the secretion of cytokines TNF, IL-1, and IL-6 and chemokines. The neutrophils are recruited to focus on phagocytosing the invaders while the monocytes enter an inflammatory state pushing them to differentiate into dendritic cells.

Age Related Phenotypes

Macrophage Polarization and Inflammation

  • The plasticity of macrophage polarization is dysfunctional in the elderly. Age-induced changes in macrophages are diverse and, in general, may represent pro-inflammatory activation of cells with an alternatively activated (M2-like) phenotype.

Dendritic Cell Activation and Migration

Summary

Monocyte derived dendritic cells start secreting inflammatory cytokines and phagocytosing pathogens in the inflamed area. The ingested pathogens are processed into antigens which are presented on MHC molecules. (Monocyte-derived dendritic cells: targets as potent antigen-presenting cells for the design of vaccines against infectious diseases) Once a monocyte derived dendritic cell is activated by an antigen, it migrates to the lymph nodes where the naive t-cells are waiting to be primed by the presented antigen. The robust activation requires the combination of signals from the antigen, costimulatory molecules, and cytokines. The t-cells role across the surrounding cells, interacting with many different APCs until a match is made between MHC molecules. This match forms an immunological synapse which, along with costimulatory signals, activates a targeted effector response in the t-cell. Other APCs such as monocytes, macrophages, and b-cells can present antigens to activate t-cells which drives t-cell helper functions. The specialization of APCs in response to different vaccines will have to be kept in mind when designing vaccines for the elderly. The response of different DC subsets to vaccine antigens and adjuvants may be determined to obtain information about the induction of adaptive immunity. For example, to improve vaccine response to fluzone in the elderly, it may be beneficial to design adjuvants that activate monocytes.

Age Related Phenotypes

Recruitment of Monocytes and Monocyte Differentiation To Dendritic Cells

  • In a Phase-3 efficacy trial, which included 7700 adult recipients of the vaccine with AS01 adjuvant and demonstrated 97.2% (95% CI 93.7–99.0) efficacy in preventing herpes zoster in adults ≥50 years of age. The ability of AS01 to improve adaptive immune responses, as has been demonstrated in clinical trials, is linked to a transient stimulation of the innate immune system leading to the generation of high number of efficient Ag-presenting dendritic cells. (Enhancement of adaptive immunity by the human vaccine adjuvant AS01 depends on activated dendritic cells, Adjuvant system AS01: helping to overcome the challenges of modern vaccines)
  • We demonstrated that the number of myeloid DCs progressively declines with age. This finding was accompanied by a decrease of CD34+ precursors and increase of circulating monocytes, suggesting that the entire differentiation process of antigen presenting cells is partially dysregulated in the elderly. (Peripheral blood dendritic cells and monocytes are differently regulated in the elderly)
  • MF59 promotes differentiation of Mo-DCs within dLNs from intranodal recruited monocytes and we suggest that this differentiation could take place in the medullary compartment of the LN. In addition we show that the Mo-DC subset represents the major source of antigen-loaded and activated APCs within the dLN when immunizing with MF59. Interestingly, this finding correlates with the enhanced triggering of antigen-specific CD4 T cell response induced by LN APCs. This study therefore demonstrates that MF59 is able to promote an immunocompetent environment also directly within the dLN, offering a novel insight on the mechanism of action of vaccine adjuvants based on emulsions. (Vaccine adjuvant MF59 promotes the intranodal differentiation of antigen-loaded and activated monocyte-derived dendritic cells)
  • A key element of the mechanism of action appears to be the creation of a transient ‘immunocompetent’ local environment at the injection site, resulting in the recruitment of key immune cells, which are able to take up antigen and adjuvant and transport them to the local lymph nodes, where the immune response is induced. This recruitment appears to be triggered by the induction of a chemokine driven gradient by the impact of MF59 on local cells, which are activated to secrete further chemokines, which are recruitment factors for more immune cells. (The mechanism of action of MF59 – An innately attractive adjuvant formulation)
  • Adjuvants such as MF59 may increase recruitment of immune cells into the injection site, accelerate and enhance monocyte differentiation into DCs, augment Ag uptake, and facilitate migration of DCs into tissue-draining lymph nodes to prime adaptive immune responses. (The Adjuvants Aluminum Hydroxide and MF59 Induce Monocyte and Granulocyte Chemoattractants and Enhance Monocyte Differentiation toward Dendritic Cells)
  • We show that IAV-infected monocytes from older humans have impaired antiviral interferon production but retain intact inflammasome responses. To understand the in vivo consequence, we used mice expressing a functional Mx gene encoding a major interferon-induced effector against IAV in humans. In Mx1-intact mice with weakened resistance due to deficiencies in Mavs and Tlr7, we found an elevated respiratory bacterial burden. Notably, mortality in the absence of Mavs and Tlr7 was independent of viral load or MyD88-dependent signaling but dependent on bacterial burden, caspase-1/11, and neutrophil-dependent tissue damage. Therefore, in the context of weakened antiviral resistance, vulnerability to IAV disease is a function of caspase-dependent pathology. (Mx1 reveals innate pathways to antiviral resistance and lethal influenza disease)

TLRs

Cytokines

Phagocytosis

Epigenetics

T-cell Activation, Differentiation, and Proliferation

Summary

Naive and memory T cells exist in a quiescent state, poised to proliferate and differentiate upon antigen stimulation during priming by APCs. Maintaining quiescence is vital to retain self-renewal potential and differentiation plasticity throughout life. In quiescence, cell division and growth are downregulated, cells are arrested in their cell cycle and they have low metabolic and mammalian target of rapamycin complex (mTORC) activity, resulting in reduced ribosome biogenesis and protein synthesis. Disruption of quiescence has been implicated in t-cell ageing. Additional disruption to proper activation and effector function of aged t-cells include signaling pathways, poor APC priming, improper cytoskeleton rearrangement, exhaustion, and reduced naive t-cells.  

Age Related Phenotypes

T-cell Activation (Dysregulated Signaling)

T-cell Activation (Poor Effector Functions)

  • Expansion / Proliferation
  • Memory and Virtual Memory T-cells
  • Cytokines

T-cell Activation (Dendritic Cell Function)

T-cell Activation (Cytoskeletal Organization)

T-cell Epigenetics

Reduced T-cell Motility

T-cell Exhaustion

T-cell Metabolism

DNA Damage / Telomere length

Reduced naive CD8 T-cells

Repertoire

  • T cell homeostasis is unique in that it needs to maintain not only population size but also high diversity of TCR specificities (Fig. 1). The notion that T cell specificities are lost with age, resulting in a contraction of the TCR repertoire, has been an attractive explanation for defective immune responses. However, there is little evidence that ‘holes’ in the repertoire occur in humans without extreme reduction in population sizes. In computer simulations of human T cell homeostasis, only minimal contraction in diversity was observed over 50 years of homeostatic proliferation despite the absence of thymic production; a marked reduction in diversity was not even observed when the size of the naive T cell compartment shrunk by >50%, as is regularly the case for CD8+ T cells44. Clonal extinction is more likely if the initial clonal size is very small, which does not appear to be the case in humans. Moreover, peripheral T cell selection implies that homeostatic proliferation occurs in TCR-specific niches, where clones compete for the recognition of self-antigen in the context of self-MHC; however, experimental evidence for the existence of such niches is lacking21. Given the enormous diversity of the human naive TCR repertoire, clonal size estimates are difficult. Theoretical estimates have been in the order of ten cells per clone45, which is likely at the lower end for humans, in particular for those clones that are generated early in life and have space to expand. Frequencies of naive T cells containing TCR excision circles in newborns suggest clonal sizes of 100, which would make complete extinction of a clone over a lifetime an unlikely event46. Several studies have used next-generation sequencing to estimate repertoire contraction with age. Due to undersampling and contamination with memory cells, estimates of naive T cell diversity are frequently misjudged47–50. In part circumventing these limitations by analysing replicate samples of purified T cell subsets, we estimated that up to 108 unique TCR β-chain sequences exist in young adults51. TCR β-chain diversity declined with age, but only by a factor of 3–5 in healthy elderly individuals, which still leaves a very diverse repertoire. Whether this decline is of functional significance is unclear. (Mechanisms underlying T cell ageing)

Chronic CMV Exposure

  • Repeated response to CMV infection leads to inflation of the memory compartment with CMV-specific clones and may place considerable limitations on the responsiveness of the CD8+ repertoire toward other antigens, as this oligoclonal expansion minimizes the space and resources necessary to maintain T cells with other specificities

Reduction During Infection

B-cell Activation and Antibody Production

Summary

The primary read-out for almost all vaccinations is the induction of protective antigen specific antibodies. The induction of vaccine-specific antibodies can be mediated by follicular or extrafollicular B cell responses, which provide long-term or short-term protection, respectively. Although short-term responses provide rapid antigen-specific antibody production, the cells generated from these interactions display poor survival. Thus, the generation of long-lived antibody-producing cells is essential for an effective vaccine response. In order to achieve long-lived protective antibody responses, B cells must undergo class switch recombination, somatic hypermutation and plasma cell differentiation, all of which require the help of a specialized T cell subset termed T follicular helper cells. These precursor TFH cells interact with local B cells to undergo full maturation into bona fide TFH cells, which again interact with B cells within germinal centers. This germinal center interaction induces the production of high affinity antibodies as well as the release of activated memory TFH cells from the tissue back into the blood. During aging, multiple changes in this pathway occur, including alterations naïve CD4 and TFH cell frequencies within the blood, reductions in TFH-B cell interactions and increases in TFR cells - which in turn lead to lower production of antigen-specific antibodies and activated TFH cells

Age Related Phenotypes

Germinal Center Function

High Baseline HI

  • a high baseline titer of hemagglutinin-inhibiting (HI) antibodies has been linked to lower fold-change in HI antibodies post-vaccination (Influence of pre-existing hemagglutination inhibition titers against historical influenza strains on antibody response to inactivated trivalent influenza vaccine in adults 50–80 years of age)
  • Due to their involvement in modulation of IgG effector mechanisms and their association with various diseases and disease risk factors, IgG glycans are a good predictor not only of chronological, but also of biological age. Being at an interface between our genomic sequence and environmental conditions, IgG glycans represent an excellent metric of healthy aging – the difference between chronological and IgG glycan-predicted age represents the consequences of environmental and life-style influences on individuals with different genetic make-up. However, unless the glycosylation pattern of antigen-specific IgG is determined, the IgG glycosylation pattern cannot be used as a stand-alone disease-specific biomarker, and is of more value as a biomarker of general immune activation. Since IgG glycosylation patterns in aging and many inflammatory and autoimmune diseases are very similar (decreased level of terminally galactosylated and sialylated structures, increased level of structures with bisecting GlcNAc), in order to assess a person’s biological age their IgG glycosylation pattern should always be compared to the healthy population of same age, sex and ethnicity. (Immunoglobulin G glycosylation in aging and diseases)

Tfh Activation

Tfh cell Frequency

TNF-a levels

  • The ability to generate a vaccine-specific antibody response is negatively correlated with levels of serum TNF-α. Moreover, human unstimulated B cells from elderly make higher levels of TNF-α than those from young individuals, and these positively correlate with serum TNF-α levels. These all negatively correlate with B cell function, measured by activation-induced cytidine deaminase, the enzyme of class switch recombination and somatic hypermutation (High TNF-α levels in resting B cells negatively correlate with their response.)

AID expression

B-cell Activation (Cytoskeleton)

Reviews

T-cell

Dendritic Cells

Vaccination

Lymph Node

Mitochondria

Pathways

  • BCR pathway: Increased SYK recruitment and activation of the BTK-BLNK-PLCγ2-complex leads to more Ca2+-release out of the endoplasmic reticulum. PKC. MAPK. NFkB.
  • IFN activation pathway including STAT1 and 3.
  • XBP1 stress response

Expert Labs

T-Cells

  • JJ Goronzy
  • Anis Larbi
  • Tamas Fulop
  • Laura Haynes

Lymph Node

  • Janko Nikolich-Žugich / Mladen Jergovic

B-Cells

  • Bonnie Blomber
  • Daniela Frasca

Monocytes / Dendritic Cells

  • Albert Chaw
  • Sudhir Gupta
  • Anshu Agrawal

Aging

  • Andrea Cossarizza