Vitamin D3 replacement enhances antigen-specific immunity in older adults

Introduction

Ageing is associated with increased number of infections, decreased vaccine efficacy and increased systemic inflammation termed inflammageing. These changes are reflected by reduced recall responses to varicella zoster virus (VZV) challenge in the skin of older adults. Vitamin D deficiency is more common in the old and has been associated with frailty and increased inflammation. In addition, vitamin D increases immunoregulatory mechanisms and therefore has the potential to inhibit inflammageing.

Objectives

We investigated the use of vitamin D₃ replacement to enhance cutaneous antigen-specific immunity in older adults (≥65 years).

Methods

Vitamin D insufficient older adults (n = 18) were administered 6400IU of vitamin D₃/day orally for 14 weeks. Antigen-specific immunity to VZV was assessed by clinical score assessment of the injection site and transcriptional analysis of skin biopsies collected from challenged injection sites pre- and post-vitamin D₃ replacement.

Results

We showed that older adults had reduced VZV-specific cutaneous immune response and increased non-specific inflammation as compared to young. Increased non-specific inflammation observed in the skin of older adults negatively correlated with vitamin D sufficiency. We showed that vitamin D₃ supplementation significantly increased the response to cutaneous VZV antigen challenge in older adults. This enhancement was associated with a reduction in inflammatory monocyte infiltration with a concomitant enhancement of T cell recruitment to the site of antigen challenge in the skin.

Conclusion

Vitamin D₃ replacement can boost antigen-specific immunity in older adults with sub-optimal vitamin D status.

Human natural killer cells mediate adaptive immunity to viral antigens

Adaptive immune responses are defined as antigen sensitization-dependent and antigen-specific responses leading to establishment of long-lived immunological memory. Although natural killer (NK) cells have traditionally been considered cells of the innate immune system, mounting evidence in mice and nonhuman primates warrants reconsideration of the existing paradigm that B and T cells are the sole mediators of adaptive immunity. However, it is currently unknown whether human NK cells can exhibit adaptive immune responses. We therefore tested whether human NK cells mediate adaptive immunity to virally encoded antigens using humanized mice and human volunteers. We found that human NK cells displayed vaccination-dependent, antigen-specific recall responses in vitro, when isolated from livers of humanized mice previously vaccinated with HIV-encoded envelope protein. Furthermore, we discovered that large numbers of cytotoxic NK cells with a tissue-resident phenotype were recruited to sites of varicella-zoster virus (VZV) skin test antigen challenge in VZV-experienced human volunteers. These NK-mediated recall responses in humans occurred decades after initial VZV exposure, demonstrating that NK memory in humans is long-lived. Our data demonstrate that human NK cells exhibit adaptive immune responses upon vaccination or infection. The existence of human memory NK cells may allow for the development of vaccination-based approaches capable of establishing potent NK-mediated memory functions contributing to host protection.

Compartmentalized cytotoxic immune response leads to distinct pathogenic roles of natural killer and senescent CD8+ T cells in human cutaneous leishmaniasis

Cytotoxic activity mediated by CD8⁺ T cells is the main signature of the immunopathogenesis of cutaneous leishmaniasis (CL). Here, we performed a broad evaluation of natural killer (NK) cell phenotypic and functional features during cutaneous leishmaniasis. We demonstrate for the first time that CL patients present the accumulation of circulating NK cells with multiple features of replicative senescence including low proliferative capacity and shorter telomeres, elevated expression of CD57, KLRG1 but diminished CD27 stimulatory receptor expression. Moreover, they exhibited higher cytotoxic and inflammatory potential than age-matched controls. The accumulation of circulating senescent NK cells (CD56^dim  CD57^bright ) correlated positively with skin lesion size in the same patients, suggesting that they, like circulating senescent CD8⁺ T cells, may contribute to the immunopathology of CL. However, this senescent population had lower cutaneous lymphocyte antigen expression and so had diminished skin-homing potential compared with total or senescent CD8⁺ T cells. This was confirmed in CL skin lesions where we found a predominance of CD8⁺ T cells (both senescent and non-senescent) that correlated with the severity of the disease. Although there was also a correlation between the proportions of senescent NK cells (CD56⁺  CD57⁺ ) in the skin and lesion size, this was less evident. Collectively our results demonstrate first-hand that senescent cytotoxic cells may mediate skin pathology during human cutaneous leishmaniasis. However, as senescent cytotoxic CD8⁺ T cells predominate in the skin lesions, they may have a greater role than NK cells in mediating the non-specific skin damage in CL.

Skin barrier immunity and ageing

The skin is the outermost layer of the body with an extensive surface area of approximately 1·8 m² , and is the first line of defence against a multitude of external pathogens and environmental insults. The skin also has important homeostatic functions such as reducing water loss and contributing to thermoregulation of the body. The structure of the skin and its cellular composition work in harmony to prevent infections and to deal with physical and chemical challenges from the outside world. In this review, we discuss how the structural cells such as keratinocytes, fibroblasts and adipocytes contribute to barrier immunity. We also discuss specialized immune cells that are resident in steady-state skin including mononuclear phagocytes, such as Langerhans cells, dermal macrophages and dermal dendritic cells in addition to the resident memory T cells. Ageing results in an increased incidence of cancer and skin infections. As we age, the skin structure changes with thinning of the epidermis and dermis, increased water loss, and fragmentation of collagen and elastin. In addition, the skin immune composition is altered with reduced Langerhans cells, decreased antigen-specific immunity and increased regulatory populations such as Foxp3⁺ regulatory T cells. Together, these alterations result in decreased barrier immunity in the elderly, explaining in part their increased susceptiblity to cancer and infections.

Impact of Zostavax Vaccination on T-Cell Accumulation and Cutaneous Gene Expression in the Skin of Older Humans After Varicella Zoster Virus Antigen-Specific Challenge

Background

The live attenuated vaccine Zostavax was developed to prevent varicella zoster virus (VZV) reactivation that causes herpes zoster (shingles) in older humans. However, the impact of vaccination on the cutaneous response to VZV is not known.

Methods

We investigated the response to intradermal VZV antigen challenge before and after Zostavax vaccination in participants >70 years of age by immunohistological and transcriptomic analyses of skin biopsy specimens collected from the challenge site.

Results

Vaccination increased the proportion of VZV-specific CD4⁺ T cells in the blood and promoted the accumulation of both CD4⁺ and CD8⁺ T cells in the skin after VZV antigen challenge. However, Zostavax did not alter the proportion of resident memory T cells (CD4⁺ and CD8⁺) or CD4⁺Foxp3⁺ regulatory T cells in unchallenged skin. After vaccination, there was increased cutaneous T-cell proliferation at the challenge site and also increased recruitment of T cells from the blood, as indicated by an elevated T-cell migratory gene signature. CD8⁺ T-cell–associated functional genes were also highly induced in the skin after vaccination.

Conclusion

Zostavax vaccination does not alter the abundance of cutaneous resident memory T cells but instead increases the recruitment of VZV-specific T cells from the blood and enhances T-cell activation, particularly cells of the CD8⁺ subset, in the skin after VZV antigen challenge.

Senescent cells evade immune clearance via HLA-E-mediated NK and CD8+ T cell inhibition

Senescent cells accumulate in human tissues during ageing and contribute to age-related pathologies. The mechanisms responsible for their accumulation are unclear. Here we show that senescent dermal fibroblasts express the non-classical MHC molecule HLA-E, which interacts with the inhibitory receptor NKG2A expressed by NK and highly differentiated CD8+ T cells to inhibit immune responses against senescent cells. HLA-E expression is induced by senescence-associated secretary phenotype-related pro-inflammatory cytokines, and is regulated by p38 MAP kinase signalling in vitro. Consistently, HLA-E expression is increased on senescent cells in human skin sections from old individuals, when compared with those from young, and in human melanocytic nevi relative to normal skin. Lastly, blocking the interaction between HLA-E and NKG2A boosts immune responses against senescent cells in vitro. We thus propose that increased HLA-E expression contributes to persistence of senescent cells in tissues, thereby suggesting a new strategy for eliminating senescent cells during ageing.

Circulating Senescent T Cells Are Linked to Systemic Inflammation and Lesion Size During Human Cutaneous Leishmaniasis

Leishmania (Viannia) braziliensis induces American tegumentary leishmaniasis that ranges in severity from the milder form, cutaneous (CL) to severe disseminated cutaneous leishmaniasis. Patients with CL develop a cell-mediated Th1 immune response accompanied by production of inflammatory cytokines, which contribute to parasite control and pathogenesis of disease. Here, we describe the accumulation of circulating T cells with multiple features of telomere dependent-senescence including elevated expression of CD57, KLRG-1, and γH2AX that have short telomeres and low hTERT expression during cutaneous L. braziliensis infection. This expanded population of T cells was found within the CD45RA⁺CD27^- (EMRA) subset and produced high levels of inflammatory cytokines, analogous to the senescence-associated secretory profile (SASP) that has been described in senescent non-lymphoid cells. There was a significant correlation between the accumulation of these cells and the extent of systemic inflammation, suggesting that they are involved in the inflammatory response in this disease. Furthermore, these cells expressed high level of the skin homing receptor CLA and there was a highly significant correlation between the number of these cells in the circulation and the size of the Leishmania-induced lesions in the skin. Collectively our results suggest that extensive activation during the early stages of leishmaniasis drives the senescence of T cells with the propensity to home to the skin. The senescence-related inflammatory cytokine secretion by these cells may control the infection but also contribute to the immunopathology in the disease.

A Suction Blister Protocol to Study Human T-cell Recall Responses In Vivo

Cutaneous antigen-recall models allow for studies of human memory responses in vivo. When combined with skin suction blister (SB) induction, this model offers accessibility to rare populations of antigen-specific T-cells representative of the cellular memory response as well as the cytokine microenvironment in situ.

This report describes the practical procedure of a cutaneous recall, an SB induction, and a harvest of antigen-specific T-cells. To exemplify the method, the tuberculin skin test is used for antigenic recall in individuals who, prior to this study, underwent a Bacillus Calmette-Guérin vaccination against an infection with Mycobacterium tuberculosis. Finally, examples of multiplex and flow cytometric analyses of SB specimens are provided, illustrating high fractions of antigen-specific polyfunctional CD4+ T-cells available by this sampling method compared with cells isolated from the blood.

The method described here is safe and minimally invasive, provides a unique opportunity to study both innate and adaptive immune responses in vivo, and may be beneficial to a broad community of researchers working with cell-mediated immunity and human memory responses, in the context of vaccine development.

Enhancement of cutaneous immunity during aging by blocking p38 mitogen-activated protein (MAP) kinase-induced inflammation

Background

Immunity decreases with age, which leads to reactivation of varicella zoster virus (VZV). In human subjects age-associated immune changes are usually measured in blood leukocytes; however, this might not reflect alterations in tissue-specific immunity.

Objectives

We used a VZV antigen challenge system in the skin to investigate changes in tissue-specific mechanisms involved in the decreased response to this virus during aging.

Methods

We assessed cutaneous immunity based on the extent of erythema and induration after intradermal VZV antigen injection. We also performed immune histology and transcriptomic analyses on skin biopsy specimens taken from the challenge site in young (<40 years) and old (>65 years) subjects.

Results

Old human subjects exhibited decreased erythema and induration, CD4⁺ and CD8⁺ T-cell infiltration, and attenuated global gene activation at the site of cutaneous VZV antigen challenge compared with young subjects. This was associated with increased sterile inflammation in the skin in the same subjects related to p38 mitogen-activated protein kinase–related proinflammatory cytokine production (P < .0007). We inhibited systemic inflammation in old subjects by means of pretreatment with an oral small-molecule p38 mitogen-activated protein kinase inhibitor (Losmapimod; GlaxoSmithKline, Brentford, United Kingdom), which reduced both serum C-reactive protein levels and peripheral blood monocyte secretion of IL-6 and TNF-α. In contrast, cutaneous responses to VZV antigen challenge were increased significantly in the same subjects (P < .0003).

Conclusion

Excessive inflammation in the skin early after antigen challenge retards antigen-specific immunity. However, this can be reversed by inhibition of inflammatory cytokine production that can be used to promote vaccine efficacy and the treatment of infections and malignancy during aging.

CD4 T-Cell Dysregulation in Psoriatic Arthritis Reveals a Regulatory Role for IL-22

Dysregulation of interleukin-22 (IL-22) has been associated with autoimmune diseases but divergent effects upon inflammation have hampered efforts to define its contribution to pathogenesis. Here, we examined the role of IL-22 in patients with psoriatic arthritis (PsA). In the peripheral blood of PsA patients, there was a decrease in IL-22⁺CD4⁺ T cells compared with healthy controls resulting in a heightened CD4⁺ IFNγ⁺/IL-22⁺ ratio accompanied by diminished CCR6 expression. IL-22 expressing cells were depleted primarily from the central memory CD4 T-cell subset in PsA patients. Paradoxically IL-22 and particularly interferon-gamma (IFNγ) production were elevated within a CD4⁺ T-cell subset with phenotypic markers characteristic of naïve T cells (CD3⁺CD4⁺CD27⁺CD45RA⁺CCR7⁺CD95⁻IL-2Rβ⁻) from PsA patients with the highest IFNγ⁺/IL-22⁺ ratio of all the CD4 subsets. These unconventional “naïve” CD4⁺ T cells from PsA patients displayed some phenotypic and functional characteristics of memory cells including a marked proliferative response. Increased IFNγ production from these unconventional “naïve” T cells from PsA patients promoted greater expression of the chemo-attractant CXCL9 by HaCaT keratinocytes compared with their healthy counterparts. Treatment with anti-TNF therapy reversed these abnormalities in this T-cell subset though did not affect the frequency of IL-22⁺ T cells overall. Furthermore, blockade of IL-22 enhanced the IFNγ mediated release of CXCL-9. These results reveal CD4⁺ T-cell dysregulation in patients with PsA which can be reversed by anti-TNF and highlight the regulatory properties of IL-22 with important implications for therapeutic approaches that inhibit its production.

The Characterization of Varicella Zoster Virus-Specific T Cells in Skin and Blood during Aging

Reactivation of the varicella zoster virus (VZV) increases during aging. Although the effects of VZV reactivation are observed in the skin (shingles), the number and functional capacity of cutaneous VZV-specific T cells have not been investigated. The numbers of circulating IFN-γ-secreting VZV-specific CD4(+) T cells are significantly decreased in old subjects. However, other measures of VZV-specific CD4(+) T cells, including proliferative capacity to VZV antigen stimulation and identification of VZV-specific CD4(+) T cells with an major histocompatibility complex class II tetramer (epitope of IE-63 protein), were similar in both age groups. The majority of T cells in the skin of both age groups expressed CD69, a characteristic of skin-resident T cells. VZV-specific CD4(+) T cells were significantly increased in the skin compared with the blood in young and old subjects, and their function was similar in both age groups. In contrast, the number of Foxp3(+) regulatory T cells and expression of the inhibitory receptor programmed cell death -1 PD-1 on CD4(+) T cells were significantly increased in the skin of older humans. Therefore, VZV-specific CD4(+) T cells in the skin of older individuals are functionally competent. However, their activity may be restricted by multiple inhibitory influences in situ.

Investigation of the cutaneous response to recall antigen in humans in vivo

In this paper we provide a detailed description of an experimental method for investigating the induction and resolution of recall immune response to antigen in humans in vivo. This involves the injection of tuberculin purified protein derivative (PPD) into the skin, followed by inducing suction blisters at the site of injection, from which leucocytes and cytokines that are involved in the response can be isolated and characterized. Using this technique we found that although the majority of CD4(+) T cells in the skin that are present early in the response express cutaneous lymphocyte antigen (CLA), the expression of this marker is reduced significantly in later phases. This may enable these cells to leave the skin during immune resolution. Furthermore, interleukin (IL)-2 production can be detected both in CD4(+) T cells and also in the blister fluid at the peak of the response at day 7, indicating that mediators found in the blister fluid are representative of the cytokine microenvironment in vivo. Finally, we found that older humans have defective ability to respond to cutaneous PPD challenge, but this does not reflect a global immune deficit as they have similar numbers of circulating functional PPD-specific CD4(+) T cells as young subjects. The use of the blister technology enables further characterization of the skin specific defect in older humans and also general mechanisms that govern immune regulation in vivo.

Invariant natural killer T cells are enriched at the site of cutaneous inflammation in lupus erythematosus

BACKGROUND

Systemic lupus erythematosus (SLE) is associated with a numerical and functional reduction of peripheral blood (PB) invariant natural killer T (iNKT) cells. Limited information exists on the role of iNKT cells in the pathogenesis of lupus erythematosus.

OBJECTIVE

To investigate the frequency and phenotype of iNKT cells in PB and dermal infiltrates from patients with SLE, subacute-cutaneous lupus erythematosus (SCLE) and discoid lupus erythematosus (DLE).

METHODS

PB was obtained from 23 SLE, 6 SCLE, and 11 DLE patients, and from 30 healthy controls. iNKT cell frequency and CCR4/CCR6 surface expression were assessed by flow cytometry. The frequency and phenotype of skin infiltrating Vα24(+)Vβ11(+) iNKT cells were investigated by immunofluorescence in lesional biopsies from 20 patients, unaffected skin from 3 patients, and from 6 healthy controls.

RESULTS

Lupus erythematosus patients displayed significantly lower percentages of circulating CD3(+)6B11(+) iNKT cells compared to healthy controls. Whereas CCR6 expression on iNKT cells was enhanced in active SLE patients regardless of cutaneous involvement compared to healthy controls, CCR4 was exclusively increased in patients with active cutaneous lesions. Furthermore, iNKT cells were significantly enriched in lesional skin of SLE and DLE patients, but not in unaffected skin of lupus patients. The majority of lesional iNKT cells expressed IFN-γ and CCR4.

CONCLUSION

The deficiency in circulating iNKT cells in cutaneous lupus erythematosus is associated with an increase of iNKT cells at the site of cutaneous inflammation. These data underscore the importance of analyzing iNKT cells not only in PB, but also in the target tissues.

Varicella zoster-specific CD4+Foxp3+ T cells accumulate after cutaneous antigen challenge in humans

We investigated the relationship between varicella zoster virus (VZV)-specific memory CD4(+) T cells and CD4(+)Foxp3(+) regulatory T cells (Tregs) that accumulate after intradermal challenge with a VZV skin test Ag. VZV-specific CD4(+) T cells were identified with a MHC class II tetramer or by intracellular staining for either IFN-γ or IL-2 after Ag rechallenge in vitro. VZV-specific T cells, mainly of a central memory (CD45RA(-)CD27(+)) phenotype, accumulate at the site of skin challenge compared with the blood of the same individuals. This resulted in part from local proliferation because >50% of tetramer defined Ag-specific CD4(+) T cells in the skin expressed the cell cycle marker Ki67. CD4(+)Foxp3(+) T cells had the characteristic phenotype of Tregs, namely CD25(hi)CD127(lo)CD39(hi) in both unchallenged and VZV challenged skin and did not secrete IFN-γ or IL-2 after antigenic restimulation. The CD4(+)Foxp3(+) T cells from unchallenged skin had suppressive activity, because their removal led to an increase in cytokine secretion after activation. After VZV Ag injection, Foxp3(+)CD25(hi)CD127(lo)CD39(hi) T cells were also found within the VZV tetramer population. Their suppressive activity could not be directly assessed by CD25 depletion because activated T cells in the skin were also CD25(+). Nevertheless, there was an inverse correlation between decreased VZV skin responses and proportion of CD4(+)Foxp3(+) T cells present, indicating indirectly their inhibitory activity in vivo. These results suggest a linkage between the expansion of Ag-specific CD4(+) T cells and CD4(+) Tregs that may provide controlled responsiveness during Ag-specific stimulation in tissues.

Immune responses in the skin in old age

A marked increase in the susceptibility to cutaneous infections and malignancies has been observed in older humans indicating that cutaneous immunity becomes defective with age. In this review we will focus on recent developments in the understanding of age-related changes in immune function of the skin with a particular emphasis on how alterations in the interaction between cells involved in innate and adaptive immunity leads to decreased cutaneous antigen-specific T cell immunosurveillance.

Cytomegalovirus infection induces the accumulation of short-lived, multifunctional CD4+CD45RA+CD27+ T cells: the potential involvement of interleukin-7 in this process

The relative roles that ageing and lifelong cytomegalovirus (CMV) infection have in shaping naive and memory CD4+ T-cell repertoires in healthy older people is unclear. Using multiple linear regression analysis we found that age itself is a stronger predictor than CMV seropositivity for the decrease in CD45RA+ CD27+ CD4+ T cells over time. In contrast, the increase in CD45RA⁻ CD27⁻ and CD45RA+ CD27⁻ CD4+ T cells is almost exclusively the result of CMV seropositivity, with age alone having no significant effect. Furthermore, the majority of the CD45RA⁻ CD27⁻ and CD45RA+ CD27⁻ CD4+ T cells in CMV-seropositive donors are specific for this virus. CD45RA+ CD27⁻ CD4+ T cells have significantly reduced CD28, interleukin-7 receptor α (IL-7Rα) and Bcl-2 expression, Akt (ser473) phosphorylation and reduced ability to survive after T-cell receptor activation compared with the other T-cell subsets in the same donors. Despite this, the CD45RA+ CD27⁻ subset is as multifunctional as the CD45RA⁻ D27+ and CD45RA⁻ CD27⁻ CD4+ T-cell subsets, indicating that they are not an exhausted population. In addition, CD45RA+ CD27⁻ CD4+ T cells have cytotoxic potential as they express high levels of granzyme B and perforin. CD4+ memory T cells re-expressing CD45RA can be generated from the CD45RA⁻ CD27+ population by the addition of IL-7 and during this process these cells down-regulated expression of IL-7R and Bcl-2 and so resemble their counterparts in vivo. Finally we showed that the proportion of CD45RA+ CD27⁻ CD4+ T cells of multiple specificities was significantly higher in the bone marrow than the blood of the same individuals, suggesting that this may be a site where these cells are generated.

Different proliferative potential and migratory characteristics of human CD4+ regulatory T cells that express either CD45RA or CD45RO

Although human naturally occurring regulatory T cells (Tregs) may express either CD45RA or CD45RO, we find in agreement with previous reports that the ( approximately 80%) majority of natural Tregs in adults are CD45RO(+). The proportion of CD45RA(+) Tregs decreases, whereas CD45RO(+) Tregs increase significantly with age. Nevertheless, a small proportion of CD45RA(+) Tregs are found even in old (>80 y) adults and a proportion of these express CD31, a marker for recent thymic emigrants. We found that CD45RO(+) Tregs were highly proliferative compared with their CD45RA(+) counterparts. This was due in part to the conversion of CD45RA Tregs to CD45RO expression after activation. Another difference between these two Treg populations was their preferential migration to different tissues in vivo. Whereas CD45RA(+) Tregs were preferentially located in the bone marrow, associated with increased CXCR4 expression, CD45RO(+) Tregs were preferentially located in the skin, and this was associated with their increased expression of CLA and CCR4. Our studies therefore show that proliferation features strongly in maintenance of the adult Treg pool in humans and that the thymus may make a minor contribution to the maintenance of the peripheral pool of these cells, even in older adults. Furthermore, the different tissue compartmentalization of these cells suggests that different Treg niches exist in vivo, which may have important roles for their maturation and function.

Decreased TNF-alpha synthesis by macrophages restricts cutaneous immunosurveillance by memory CD4+ T cells during aging

Immunity declines during aging, however the mechanisms involved in this decline are not known. In this study, we show that cutaneous delayed type hypersensitivity (DTH) responses to recall antigens are significantly decreased in older individuals. However, this is not related to CC chemokine receptor 4, cutaneous lymphocyte-associated antigen, or CD11a expression by CD4⁺ T cells or their physical capacity for migration. Instead, there is defective activation of dermal blood vessels in older subject that results from decreased TNF-α secretion by macrophages. This prevents memory T cell entry into the skin after antigen challenge. However, isolated cutaneous macrophages from these subjects can be induced to secrete TNF-α after stimulation with Toll-like receptor (TLR) 1/2 or TLR 4 ligands in vitro, indicating that the defect is reversible. The decreased conditioning of tissue microenvironments by macrophage-derived cytokines may therefore lead to defective immunosurveillance by memory T cells. This may be a predisposing factor for the development of malignancy and infection in the skin during aging.

The kinetics of CD4+Foxp3+ T cell accumulation during a human cutaneous antigen-specific memory response in vivo

Naturally occurring CD4(+)CD25(hi)Foxp3(+) Tregs (nTregs) are highly proliferative in blood. However, the kinetics of their accumulation and proliferation during a localized antigen-specific T cell response is currently unknown. To explore this, we used a human experimental system whereby tuberculin purified protein derivative (PPD) was injected into the skin and the local T cell response analyzed over time. The numbers of both CD4(+)Foxp3(-) (memory) and CD4(+)Foxp3(+) (putative nTreg) T cells increased in parallel, with the 2 populations proliferating at the same relative rate. In contrast to CD4(+)Foxp3(-) T cell populations, skin CD4(+)Foxp3(+) T cells expressed typical Treg markers (i.e., they were CD25(hi), CD127(lo), CD27(+), and CD39(+)) and did not synthesize IL-2 or IFN-gamma after restimulation in vitro, indicating that they were not recently activated effector cells. To determine whether CD4(+)Foxp3(+) T cells in skin could be induced from memory CD4(+) T cells, we expanded skin-derived memory CD4(+) T cells in vitro and anergized them. These cells expressed high levels of CD25 and Foxp3 and suppressed the proliferation of skin-derived responder T cells to PPD challenge. Our data therefore demonstrate that memory and CD4(+) Treg populations are regulated in tandem during a secondary antigenic response. Furthermore, it is possible to isolate effector CD4(+) T cell populations from inflamed tissues and manipulate them to generate Tregs with the potential to suppress inflammatory responses.

Telomerase in T lymphocytes: use it and lose it?

The enzyme telomerase counteracts telomere loss in proliferating cells and extends their capacity for replication. The importance of telomerase is highlighted by the award of the 2006 Albert Lasker Prize for Basic Medical Research for its discovery. Malignant cells subvert telomerase induction to their advantage, and up-regulation of this enzyme confers these populations with unlimited proliferative potential with obvious detrimental consequences. However this enzyme is also essential for the lifelong maintenance of normal cell populations that have a high rate of turnover. Thymic involution in early adulthood dictates that memory T cell populations have to be maintained by continuous proliferation. This highlights the inherent paradox that telomerase down-regulation in T cells may protect against malignancy yet also lead to replicative exhaustion of repeatedly activated memory T cells. In this article, we review the data on telomerase regulation in T lymphocytes and the implications this has for the maintenance of T cell memory.

The dynamic co-evolution of memory and regulatory CD4+ T cells in the periphery

Whereas memory T cells are required to maintain immunity, regulatory T cells have to keep the immune system in check to prevent excessive inflammation and/or autoimmunity. Both cell types must be present during the lifetime of the organism. However, it is not clear whether both subsets are regulated in tandem or independently of each other, especially because thymic involution severely restricts the production of T-cell populations during ageing. In this Opinion article, we discuss recent evidence in both mice and humans that supports the hypothesis that some CD4(+)CD25(+)FOXP3(+)regulatory T cells can differentiate from rapidly proliferating memory T cells in the periphery.

Mantoux Test as a model for a secondary immune response in humans

The Mantoux Test (MT) is a classical delayed-type hypersensitivity (DTH) response to the intradermal injection of tuberculin purified protein derivative (PPD). It represents a cutaneous T cell mediated memory recall immune response. The test is typically used to determine immunity to tuberculosis in humans and positive reactions develop in individuals previously exposed to Mycobacterium tuberculosis, and those immunised with the Bacillus of Calmette and Guérin (BCG) vaccine. In view of its relative accessibility human skin represents a convenient tissue for the investigation of human immune responses. Using the MT, we have been able to determine that significant cellular proliferation and clonal expansion occur at the site of antigen deposition in the skin. Furthermore, cells undergoing proliferation in the skin also undergo accelerated differentiation. Taken together with other studies, in humans and in mice, these observations shed new light on the importance of the microenvironment at the site of the immune response for the proliferation and differentiation of memory T cells.

Human CD4+ CD25hi Foxp3+ regulatory T cells are derived by rapid turnover of memory populations in vivo

While memory T cells are maintained by continuous turnover, it is not clear how human regulatory CD4+ CD45RO+ CD25hi Foxp3+ T lymphocyte populations persist throughout life. We therefore used deuterium labeling of cycling cells in vivo to determine whether these cells could be replenished by proliferation. We found that CD4+ CD45RO+ Foxp3+ CD25hi T lymphocytes were highly proliferative, with a doubling time of 8 days, compared with memory CD4+ CD45RO+ Foxp3- CD25- (24 days) or naive CD4+ CD45RA+ Foxp3- CD25- populations (199 days). However, the regulatory population was susceptible to apoptosis and had critically short telomeres and low telomerase activity. It was therefore unlikely to be self regenerating. These data are consistent with continuous production from another population source. We found extremely close TCR clonal homology between regulatory and memory CD4+ T cells. Furthermore, antigen-related expansions within certain TCR Vbeta families were associated with parallel numerical increases of CD4+ CD45RO+ CD25hi Foxp3+ Tregs with the same Vbeta usage. It is therefore unlikely that all human CD4+ CD25+ Foxp3+ Tregs are generated as a separate functional lineage in the thymus. Instead, our data suggest that a proportion of this regulatory population is generated from rapidly dividing, highly differentiated memory CD4+ T cells; this has considerable implications for the therapeutic manipulation of these cells in vivo.

The Generation and Antigen-Specificity of CD4+CD25+ Regulatory T Cells

CD4+CD25+ regulatory T cells are essential components of the immune system. They help to maintain immune tolerance by exerting suppressive effects on cells of the adaptive and innate immune system. In the last few years there has been an abundance of papers addressing the suppressive effects of CD4+CD25+ regulatory T cells and their putative role in various experimental disease models and human diseases. Despite the enormous amounts of data on these cells a number of controversial issues still exists. CD4+CD25+ regulatory T cells were originally described as thymus-derived anergic/suppressive T cells. Recent papers however indicate that these cells might also be generated in the periphery. Due to the thymic development of CD4+CD25+ regulatory T cells it was thought that these cells were specific for self-antigens. Indeed it was shown that CD4+CD25+ regulatory T cells could be positively selected upon high affinity interaction with self-antigens. However, evidence is accumulating that these cells might also interact with non-self antigens. Finally, in the literature there is conflicting evidence regarding the role of soluble factors versus cell-contact in the mechanism of suppression. The aim of this review is to summarize the evidence supporting these opposing viewpoints and to combine them into a general model for the origin, function and antigen-specificity of CD4+CD25+ regulatory T cells.

Cytomegalovirus-specific CD4+ T cells in healthy carriers are continuously driven to replicative exhaustion

Repeated antigenic encounter drives proliferation and differentiation of memory T cell pools. An important question is whether certain specific T cells may be driven eventually to exhaustion in elderly individuals since the human life expectancy is increasing. We found that CMV-specific CD4+ T cells were significantly expanded in healthy young and old carriers compared with purified protein derivative-, varicella zoster virus-, EBV-, and HSV-specific populations. These CMV-specific CD4+ T cells exhibited a late differentiated phenotype since they were largely CD27 and CD28 negative and had shorter telomeres. Interestingly, in elderly CMV-seropositive subjects, CD4+ T cells of different specificities were significantly more differentiated than the same cells in CMV-seronegative individuals. This suggested the involvement of bystander-secreted, differentiation-inducing factors during CMV infection. One candidate was IFN-alpha, which induced loss of costimulatory receptors and inhibited telomerase in activated CD4+ T cells and was secreted at high levels by CMV-stimulated plasmacytoid dendritic cells (PDC). The CMV-specific CD4+ T cells in elderly subjects had severely restricted replicative capacity. This is the first description of a human memory T cell population that is susceptible to being lost through end-stage differentiation due to the combined effects of lifelong virus reactivation in the presence of bystander differentiation-inducing factors.

Relative impact of CD4+CD25+ regulatory T cells and tacrolimus on inhibition of T-cell proliferation in patients with atopic dermatitis

BACKGROUND

It has been established recently that CD4+CD25+ regulatory T cells (Tregs) play an important role in controlling various immune responses. Immunosuppressive drugs are often used to treat immune dysregulation but are frequently associated with undesirable side-effects.

OBJECTIVES

We examined the suppressive capacity of circulating Tregs in patients with atopic dermatitis (AD). Combined effects of Tregs and tacrolimus on the inhibition of T-cell proliferation in vitro were also assessed.

METHODS

CD4+CD25+ and CD4+CD25- T cells were isolated from peripheral blood mononuclear cells using immunomagnetic beads. CD4+CD25- T cells were stimulated with purified protein derivative (PPD) or house dust mite allergen (Der p1) for 6 or 7 days, respectively. A dose range of tacrolimus and CD4+CD25+ T cells were added separately, or together. Proliferation was measured by (3)H-thymidine incorporation.

RESULTS

CD4+CD25+ T cells from normal controls and patients with AD are anergic and inhibit the proliferation of CD4+CD25- T cells in response to PPD and Der p1 in vitro in a dose-dependent manner. Addition of tacrolimus and Tregs together showed significantly stronger inhibition of proliferation than either on their own. This was true for both antigens and both in normal controls and in patients with AD.

CONCLUSIONS

CD4+CD25+ T cells in patients with AD have normal suppressive activity compared with healthy controls. Tregs and tacrolimus have additive effects on the inhibition of proliferation in response to PPD and Der p1.

The immunomodulatory effects of regulatory T cells: implications for immune regulation in the skin

Regulatory T cells are thought to have a critical role in the suppression of immune responses. In addition to the prevention of the development of autoimmunity, they are also thought to have a role in the prevention of allergic responses to environmental allergens, immune responses to tumours and the development of memory responses to chronic infections. They have been isolated within the skin and have been shown to express surface markers that enable skin-specific migration, suggesting that regulatory T cells have a functional role in the skin. There is accumulating evidence to suggest that regulatory T cells may be involved in numerous skin disorders and may also be modified by various therapeutic agents used to treat these disorders. We review the evidence for the presence of this T-cell subset in humans, the suppressive effects of regulatory T cells, and their role in the skin.

Telomere erosion in memory T cells induced by telomerase inhibition at the site of antigenic challenge in vivo

The extent of human memory T cell proliferation, differentiation, and telomere erosion that occurs after a single episode of immune challenge in vivo is unclear. To investigate this, we injected tuberculin purified protein derivative (PPD) into the skin of immune individuals and isolated responsive T cells from the site of antigenic challenge at different times. PPD-specific CD4⁺ T cells proliferated and differentiated extensively in the skin during this secondary response. Furthermore, significant telomere erosion occurred in specific T cells that respond in the skin, but not in those that are found in the blood from the same individuals. Tissue fluid obtained from the site of PPD challenge in the skin inhibited the induction of the enzyme telomerase in T cells in vitro. Antibody inhibition studies indicated that type I interferon (IFN), which was identified at high levels in the tissue fluid and by immunohistology, was responsible in part for the telomerase inhibition. Furthermore, the addition of IFN-α to PPD-stimulated CD4⁺ T cells directly inhibited telomerase activity in vitro. Therefore, these results suggest that the rate of telomere erosion in proliferating, antigen-specific CD4⁺ T cells may be accelerated by type I IFN during a secondary response in vivo.

Immune regulation by CD4+CD25+ regulatory T cells: implications for transplantation tolerance

In transplantation research, the achievement of life-long tolerance for the graft without the need for immunosuppressive drugs, is a major goal. In the immune system various mechanisms are in place that help to prevent unwanted immunity. These mechanisms of peripheral tolerance include deletion, anergy, ignorance and suppression. In the last decade it has been demonstrated convincingly that a naturally occurring subset of CD4+ T cells, the so-called CD4+CD25+ regulatory T cells, play a key role in the suppression/regulation of immune responses. These cells have been shown to exist in mice, rats and humans, and can be found in thymus, peripheral blood, lymphoid organs and at sites of inflammation. CD4+CD25+ regulatory T cells can down-regulate the immune response by affecting T cell responses, antibody production, cytokine secretion and antigen-presenting cells. CD4+CD25+ regulatory T cells are generated in the thymus, but importantly recent evidence suggests that they can also be generated in the periphery. This latter finding is of particular importance for transplantation immunology, since it suggests that specific manipulation or induction of these cells is achievable in vivo. Here we review the recent developments on the CD4+CD25+ regulatory T cells and we discuss the potential use of these cells in transplantation immunology.

Antigen-specific T cell suppression by human CD4+CD25+ regulatory T cells

Anergic/suppressive CD4+CD25+ T cells have been proposed to play an important role in the maintenance of peripheral tolerance. Here we demonstrate that in humans these cells suppress proliferation to self antigens, but also to dietary and foreign antigens. The suppressive CD4+CD25+ T cells display a broad usage of the T cell receptor Vbeta repertoire,suggesting that they recognize a wide variety of antigens. They reside in the primed/memory CD4+CD45RO+CD45RB(low) subset and have short telomeres, indicating that these cells have the phenotype of highly differentiated CD4+ T cells that have experienced repeated episodes of antigen-specific stimulation in vivo. This suggests that anergic/suppressive CD4+CD25+ T cells may be generated in the periphery as a consequence of repeated antigenic encounter. This is supported by the observation that highly differentiated CD4+T cells can be induced to become anergic/suppressive when stimulated by antigen presented by non-professional antigen-presenting cells. We suggest that besides being generated in the thymus, CD4+CD25+ regulatory T cells may also be generated in the periphery. This would provide a mechanism for the generation of regulatory cells that induce tolerance to a wide array of antigens that may not be encountered in the thymus.

The balance of protein kinase C and calcium signaling directs T cell subset development

Development of naive T cells into type 1 (Th1, Tc1) or type 2 (Th2, Tc2) effector cells is thought to be under the control of cytokines. In this study, we show that when both IL-12 and IL-4 are present, murine and human T cell differentiation is regulated by the balance of protein kinase C (PKC) and calcium signaling within T cells. Although both biochemical signals were required for T cell activation via the TCR, altering the balance between them redirected type 1 cells to type 2 and vice versa. Stimulation of calcium signaling or inhibition of PKC favored type 1 differentiation, whereas stimulation of PKC or inhibition of calcineurin resulted in type 2 effectors. Altered peptide ligands induced distinct balances of PKC/calcium signaling and altered Tc1/Tc2 development in TCR-transgenic CD8 T cells. The data suggest novel strategies for manipulation of the immune response in vivo.

Human Tc1 and Tc2/Tc0 CD8 T-cell clones display distinct cell surface and functional phenotypes

It has recently become clear that distinct subsets of CD8 T cells, analogous to their CD4 counterparts, exist in rodents and humans. To examine functional differences between human CD8 T-cell subsets, we generated Tc1, Tc2, and Tc0 T-cell clones from the peripheral blood of healthy individuals. The majority of CD8 T-cell clones generated displayed a classic Tc1 phenotype, but 10% to 20% secreted interleukin (IL)-4 in addition to interferon-gamma (Tc0 phenotype). Generation of Tc2 clones was dependent on the use of anti-CD3 and anti-CD28 as the primary stimulus. The cytokine profiles of established clones remained susceptible to modification by the addition of IL-12 and IL-4. In addition, IL-12 enhanced and IL-4 inhibited the growth of Tc1 but not Tc2/0 CD8 T-cell clones. Significant functional differences were observed between the subsets. Tc2/0 clones expressed CD30 and CD40 ligand at a much higher level than Tc1 clones. Both Tc1 and Tc2/0 clones showed comparable cytotoxicity and produced similar levels of perforin and Fas L. However, Tc2 clones were much more resistant to activation-induced cell death and less susceptible to apoptosis by direct Fas ligation. Moreover, Tc1 and Tc2 clones had opposing effects on the development of CD4 effectors, promoting type 1 and type 2 responses, respectively. These data provide evidence for profound differences between human CD8 T-cell subsets that may be important in their functions as cytotoxic or immunoregulatory cells. (Blood. 2000;95:231-240)

CD8(+) T cell subsets and chronic obstructive pulmonary disease

COPD is a debilitating and progressive condition in which the airways become irreversibly obstructed and the lungs progressively damaged. Unlike asthma, we know little about the cells that initiate and drive this process. Research has shown that CD8(+) T cells are overrepresented in the lungs of patients with COPD and that they are inversely related to lung function. However, not all CD8(+) T cells are alike and subsets that make IFN-gamma but not IL-4 (Tc1), IL-4 but not IFN-gamma (Tc2) as well as those that make both (Tc0) have been described. This article focuses on the characteristics of CD8(+) T cell subsets and considers their potential contribution to chronic obstructive pulmonary disease (COPD). Kemeny DM, Vyas B, Vukmanovic-Stejic M, Thomas M, Noble A, Loh LC, O'Connor BJ. CD8(+) T cell subsets and chronic obstructive pulmonary disease.

IL-4 inhibits human CD8 T cell expression of the common IL-2 receptor gamma chain

Previous studies in both rats and humans have shown that interleukin (IL) 4 can suppress the generation of IL-2-producing CD8 T cells. In an attempt to elucidate the mechanism by which this suppression is brought about, we set out to investigate whether the IL-4 signal interferes with the IL-2 receptor system. It has already been reported that IL-2 can affect the expression of its own receptor and thus provide a means of controlling its own activities. In this study, we demonstrate that the IL-2 Ralpha- and the IL-2 Rgamma-chains are dramatically upregulated following stimulation of CD8 T cells, whereas lower levels of beta-chain are observed. IL-4 did not affect the expression of the alpha- or beta-chains, but was found to inhibit the generation of common gamma-chain-expressing cells. Moreover, CD4 T cells were found to express much lower levels of this subunit and appeared less sensitive to the effects of IL-4. We postulate that the differential expression of the gamma-chain subunit, in the presence and absence of IL-4, may provide a tool for identifying functionally distinct subpopulations of CD8 T cells.

Effect of IL-4, IFN-gamma and IL-12 on cytokine production from human CD45RA and CD45RO CD4 T cell precursors

The aim of this study was to compare the effects of IL-4, IL-12 and IFN-gamma on the production of T helper-1 (Th1) and T helper-2 (Th2)-type cytokines from human peripheral blood 'naive' CD45RA and 'memory' CD45RO CD4 T cells. CD45RA or CD45RO CD4 T cells were cultured for 4 days with phorbol myristate acetate (PMA) and ionomycin and either IL-4, IFN-gamma or IL-12 and their ability to proliferate and secrete IFN-gamma and IL-4 determined. Purified CD45RO CD4 T cells stimulated with PMA and ionomycin secreted higher levels of IL-4 and IFN-gamma, as measured by ELISA, than CD45RA CD4 T cells which secreted little IL-4 or IFN-gamma. However, CD45RA and CD45RO CD4 T cells proliferated to the same extent and IL-4, IFN-gamma and IL-12 had no effect on this. IL-12 and IFN-gamma had no effect on the amount of IL-4 secreted by PMA and ionomycin-stimulated CD45RO CD4 T cells, but culture with IL-4 enhanced IL-4 production 7-fold. IL-12 increased the amount of IFN-gamma produced by CD45RO CD4 T cells 2- to 3-fold. Small amounts of IFN-gamma production were induced in CD4 CD45RA T cells by IL-12 and IFN-gamma. These results indicate: (1) that CD45RA cells cannot make significant amounts of IL-4 under the conditions used, (2) that CD45RO cells can produce both Th1 and Th2 cytokines immediately upon restimulation, (3) that IL-12 favours Th1 cytokine production in both CD45RA and CD45RO CD4 T cells, and (4) that IFN-gamma favours IFN-gamma production in CD45RA but not CD45RO cells.