Ageing of lymphocytes and lymphocytes in the aged

T cell subsets and mortality in older community-dwelling women

The relationship between specific T cell subset alterations and mortality has not been well characterized in older adults. The specific aim was to determine whether specific T cell subsets are associated with an increased risk of death. We conducted a case-control study of T cell subsets (CD4+ and CD8+ T cells, and subsets of these cells defined by expression or non-expression of CD28, CD45RA, and CD45RO) nested within two complementary prospective cohorts of women aged 65 and over living in the community, the Women's Health and Aging Studies (WHAS). Cases consisted of 61 women who died during 5 years of follow-up, and controls consisted of 61 women matched by age, frailty, and morbidities who survived during 7 years of follow-up. There were no significant differences between cases and controls in any of the T cell subsets studied. When analyses were stratified by frailty status, these data suggest that CD8+CD28- lymphocyte counts were significantly higher among women who were frail compared with pre-frail and non-frail women.

Lessons from the study of T-cell differentiation in persistent human virus infection

Confusion surrounds the current classification of memory and effector T-cell subsets and there is a lack of consistency in the use of these terms between human and murine studies. The development of peptide-HLA tetrameric complexes ("tetramers") that accurately identify virus-specific T cells and can be used with a range of cell surface and intra-cellular markers has provided further insights in our understanding of the process of T-cell differentiation, or post-thymic development. We propose that T-cell differentiation subsets in human viral infection should be regarded as distinct from the current definitions of memory and effector cells; further work is needed to reveal the role of the differentiation process in anti-viral immunity.

Immunity & Ageing: a new journal looking at ageing from an immunological point of view

In the elderly, many alterations of both innate and clonotypic immunity have been described. Alterations to the immune system in the elderly are generally viewed as a deterioration of immunity, leading to the use of the term immunosenescence. However, although many immunological parameters are often notably reduced in the elderly, retained function of both innate and clonotypic immunity in the elderly is tightly correlated to health status. Recognising the important role of the immune system in ageing, over the last few years, journals oriented towards gerontology and geriatric sciences have increasingly published articles dealing with the immunology of ageing, but a specialised journal in this area does not exist. Immunity & Ageing is a new Open Access, peer reviewed journal that aims to cover all the topics dealing with innate and clonotypic immunity which are relevant to ageing. The journal will provide an opportunity to focus on this topic, which is emerging as one of the critical mechanisms of ageing. Furthermore, as an online, Open Access journal, Immunity & Ageing will promote immediate accessibility to research, which is generally not possible for articles published in printed journals. We hope this forum, concentrating on the themes of ageing and immunology with a strong focus on human studies, will create a new perspective for viewing a world that is inevitably becoming older.

Moderate exercise may attenuate some aspects of immunosenescence

BACKGROUND

Immunosenescence is related to the deterioration of many immune functions, which may be manifested in increased susceptibility to infection, cancer, and autoimmunity. Lifestyle factors, such as diet or physical activity, may influence the senescence of the immune system. It is widely accepted that moderate physical activity may cause beneficial effects for physical and psychological health as well as for the immune system activity in aged people.

METHODS

Thirty elderly women aged 62 to 86 were subjected to a two-years authorized physical activity program. Peripheral blood lymphocytes distribution and the production of cytokines involved in the immune response development and regulation (IL-2, IL-4 and IFN-gamma) were investigated. The same parameters were evaluated in two control groups of women: a sedentary group of 12 elderly women selected for the second round of the physical activity program and in a group of 20 sedentary young women. Flow cytometry methods were used for the examination of surface markers on peripheral blood lymphocytes and intracellular cytokines expression.

RESULTS

The distribution of the main lymphocytes subpopulations in the peripheral blood of elderly women did not show changes after long-term moderate physical training. The percentage of lymphocytes expressing intracellular IL-2 was higher in the group of women attending 2-years physical activity program than in the control group of elderly sedentary women, and it was similar to the value estimated in the group of young sedentary women. There was no difference in the intracellular expression of IL-4 and IFN-gamma between the active and elderly sedentary women.

CONCLUSIONS

Our results suggest that moderate, long-term physical activity in elderly women may increase the production of IL-2, an important regulator of the immune response. This may help ameliorate immunosenescence in these women.

Cellular cholesterol enrichment impairs T cell activation and chemotaxis

Human aging is associated with an increase in immune cell cholesterol levels, independent of circulating cholesterol levels. The effects of such an increase in membrane cholesterol on lipid raft-associated immune cell function have not been investigated. We sought to examine the effects of in vitro cholesterol loading on two known lipid raft-associated pathways of T cells, namely T cell activation and chemokine stimulation. Using beta-cyclodextrin (BCD) as a vehicle, we were able to rapidly load cholesterol onto human T cell lines and primary peripheral blood T cells without inducing significant cell toxicity. Loading of cholesterol to four-fold that of normal levels induced significant inhibition of intracellular calcium mobilization by both alphaCD3 and SDF-1alpha. Cholesterol-loaded peripheral T cells were completely unresponsive to alphaCD3/alphaCD28 stimulation, demonstrating no increase in IL-2, GM1 expression or cell size. T cell polarization of lipid rafts to alphaCD3/alphaCD28 beads was also impaired. In addition, cholesterol loading potently inhibited SDF-1alpha-induced chemotaxis. We propose that excess membrane cholesterol could potentially disrupt raft-related cell functions downstream of receptor triggering and that the loss of cholesterol regulation of aging immune cells could contribute to immune cell senescence.

Factors affecting reconstitution of the T cell compartment in allogeneic haematopoietic cell transplant recipients

The factors affecting T cell reconstitution post haematopoietic cell transplantation (HCT) are not well characterised. We carried out a longitudinal analysis of T cell reconstitution in 32 HCT recipients during the first 12 months post transplant. We analysed reconstitution of naïve, memory and effector T cells, their diversity and monitored thymic output using TCR rearrangement excision circles (TRECs). Thymic-independent pathways were responsible for the rapid reconstitution of memory and effector T cells less than 6 months post HCT. Thymic-dependent pathways were activated between 6 and 12 months in the majority of patients with naïve T cell numbers increasing in parallel with TREC levels. Increasing patient age, chronic GVHD and T cell depletion (with or without pretransplant Campath-1H) predicted low TREC levels and slow naïve T cell recovery. Furthermore, increasing patient age also predicted high memory and effector T cell numbers. The effects of post HCT immunosuppression, total body irradiation, donor leucocyte infusions, T cell dose and post HCT infections on T cell recovery were also analysed. However, no effects of these single variables across a variety of different age, GVHD and T cell depletion groups were apparent. This study suggests that future analysis of the factors affecting T cell reconstitution and studies aimed at reactivating the thymus through therapeutic intervention should be analysed in age-, GVHD- and TCD-matched patient groups.

Establishment and characterization of a bank of cytotoxic T lymphocytes for immunotherapy of epstein-barr virus-associated diseases

Adoptive immunotherapy using Epstein-Barr virus (EBV)-specific cytotoxic T lymphocytes (CTL) generated ex vivo can be an effective treatment of EBV-positive posttransplantation lymphoproliferative disease (PTLD). We describe the establishment of a cryopreserved repository of allogeneic virus-specific CTL lines, to our knowledge the first of its kind in the world. CTL lines were grown by weekly stimulation with autologous EBV immortalized lymphoblastoid cell lines (LCLs) from 96 EBV-seropositive blood donors. Analysis of 60 CTL lines grown continuously for 7 to 10 weeks showed an average proportional weekly increase in cell numbers of 1.4, with an overall increase ranging from 1.1 to 83.4. The greatest increase occurred during the early culture period. After four rounds of stimulation, killing of autologous LCLs was generally high (mean 48%); however, most lines required 9 or 10 stimulations to reduce the killing of nonspecific targets. Overall, 79% of CTLs generated showed acceptable levels of specific killing. Phenotypically, the CTL lines consisted of TCRalpha beta+, CD8+ T cells (medians 97% and 90% respectively) with a minority population of CD4+ T cells (median 2%). Most cells expressed the activation and differentiation markers, HLA-DR, CD26, CD45RO, CD69, and CD150. Favorable results have been obtained in an open trial using partially HLA-matched, allogeneic CTLs from this bank to treat PTLD patients. This now represents a single resource that can provide therapeutic CTLs rapidly on a countrywide basis, superseding the time-consuming, expensive practice of generating autologous CTLs from each patient requiring treatment. Additionally, other patient groups, such as those with EBV-positive Hodgkin disease, may benefit from CTL treatment.

Extrathymic T cells expand in nude mice following different allogeneic stimuli

We studied extrathymic lymphocyte populations expanded in nude mice after allogeneic stimuli. These were either cells from different tissues or Immunoglobulin (Ig). Although the cells transferred, obtained from Thy-1.1+ donors, were able to induce similar increase in the nude host Thy-1.2+ population, the expanded populations could be qualitatively distinguished from each other by their different expression of mature T cell molecules and by their functional profile. The extrathymic lymphocytes expanded in animals receiving allogeneic fetal thymocytes (FT) were preferentially CD4+ cells and could confer a functional immunocompetent system to the nude host, able to reject allogeneic skin grafts. In contrast, allogeneic adult red blood cells (RBC) led to the expansion of a CD8+ population and to an auto-reactive profile, resulting in the rejection of syngeneic skin grafts by most of the nude hosts. Neither of these profiles was achieved with the other stimuli. These findings support the view that different activation pathways and/or regulatory interactions may lead to the development of distinct extrathymic populations.

Ageing, autoimmunity and arthritis: senescence of the B cell compartment - implications for humoral immunity

Immunosenescence is associated with a decline in both T and B lymphocyte function. Although aged individuals have normal numbers of B cells in the periphery and are capable of mounting robust humoral responses, the antibodies produced are generally of lower affinity and are less protective than those produced by young animals. Here we review multiple studies that address the mechanisms that contribute to this decline. Taken together, these studies suggest that age-associated loss of the ability to generate protective humoral immunity results in part from reduced B lymphopoiesis. As the output of new, naïve B cells declines, homeostatic pressures presumably force the filling of the peripheral B cell pool by long-lived antigen-experienced cells. Because the antibody repertoire of these cells is restricted by previous antigenic experience, they make poor quality responses to new immunologic insults.

Influence of increased age on the development of herpes stromal keratitis

Herpes stromal keratitis (HSK) is the leading infectious cause of blindness in the United States and is a consequence of events following HSV-1 infection of the eye. The pathology of the disease is currently thought to be caused by a destructive, CD4(+) T helper 1 (Th1) type inflammatory immune response within the cornea rather than a cytopathic response elicited by the virus. A large percentage of people can become infected with HSV-1 as children whereas some studies have concluded that many others do not become infected with HSV-1 until much later in life. In this paper we investigate the role of increasing age on ocular HSV-1 infection. Following an ocular infection of mice with HSV-1 we observed greater pathology in the cornea during both early and late time-points in adult mice when compared to young animals. No significant differences in viral titers were observed in either the eyes or trigeminal ganglia from infected mice, regardless of age, suggesting that increased viral load may not be responsible for the ocular pathology in the adult mice. We hypothesize that age-related changes in the immune response may predispose adult animals to HSK disease.

The effect of age and gender on cytokine production by human peripheral blood mononuclear cells and markers of bone metabolism

BACKGROUND

Aging has been associated with various alterations of immune functions, the musculoskeletal system and a decline of sex hormone levels. Estradiol has a central role in the regulation of bone turnover and also modulates the production of cytokines such as interleukin-1 and -6 and tumor necrosis factor-alpha. We therefore studied the effect of age and gender on cytokine production by mononuclear cells and markers of bone metabolism.

METHODS

Peripheral blood mononuclear cells were isolated from young and elderly subjects; intracellular detection of cytokine production after stimulation with ionomycine and PMA (T cells) or LPS (monocytes) was performed by four color flow cytometry. Sex hormone levels and markers of bone metabolism were measured by RIA or ELISA:

RESULTS

When we compared elderly to young women we found an increased proportion of T cells that were positive for interferon-gamma, interleukin-2, -4, -10 and -13. Also the percentage of cells producing interleukin-4 or interferon-gamma within the CD8(+) population was higher in the group of elderly women. In contrast, proportionally fewer monocytes of elderly women were positive for tumor necrosis factor-alpha or interleukin-6 than those of young women. In elderly men a higher percentage of T cells produced interleukin-2, -4 and -13. In the group of aged men we found a higher frequency of cells that produced interleukin-4 within the CD4(+) or CD8(+) population. Moreover, within monocytes of elderly men we found an increased percentage of cells positive for both interleukin-1beta and tumor necrosis factor-alpha. The data on markers of bone metabolism indicated an increase of bone turnover in old age.

CONCLUSION

Our data demonstrate that aging is associated with significant alterations of bone metabolism and cytokine production by T cells and monocytes. For particular cytokines (interferon-gamma and interleukin-10 in T cells, interleukin-6 and tumor necrosis factor-alpha in monocytes) these changes are gender specific.

Paradoxical age-related cell cycle quickening of human CD4+ lymphocytes: a role for cyclin D1 and calpain

Precise determination of cell cycle length and G(0)-->G(1) transition time of CD4(+) lymphocytes in relation to age was never done before. We show that the cell cycle of healthy elderly donors' CD4(+) cells is significantly shorter, while time to the first division (G(0)-->G(1)) extended compared to cells of young people. The G(0)-->G(1) time inversely correlates with cycle length and the number of CD28 molecules. Quickening of elderly CD4(+) cell divisions depends on overexpression of cyclin D1, possibly related to lowered proteolytic degradation by calpain. Apoptosis eliminates most of responding cells after only one or two divisions, especially in older donors.

Immune activation and CD8+ T-cell differentiation towards senescence in HIV-1 infection

Progress in the fight against the HIV/AIDS epidemic is hindered by our failure to elucidate the precise reasons for the onset of immunodeficiency in HIV-1 infection. Increasing evidence suggests that elevated immune activation is associated with poor outcome in HIV-1 pathogenesis. However, the basis of this association remains unclear. Through ex vivo analysis of virus-specific CD8(+) T-cells and the use of an in vitro model of naïve CD8(+) T-cell priming, we show that the activation level and the differentiation state of T-cells are closely related. Acute HIV-1 infection induces massive activation of CD8(+) T-cells, affecting many cell populations, not only those specific for HIV-1, which results in further differentiation of these cells. HIV disease progression correlates with increased proportions of highly differentiated CD8(+) T-cells, which exhibit characteristics of replicative senescence and probably indicate a decline in T-cell competence of the infected person. The differentiation of CD8(+) and CD4(+) T-cells towards a state of replicative senescence is a natural process. It can be driven by excessive levels of immune stimulation. This may be part of the mechanism through which HIV-1-mediated immune activation exhausts the capacity of the immune system.

T-cell dynamics during acute SIV infection

OBJECTIVES

To delineate T-cell dynamics during acute SIV infection, particularly of phenotypically defined memory T cell subsets.

DESIGN

T cells are a heterogeneous mix of naive and memory subsets delineated by simultaneously measuring CD4, CD8, CD45RA/RO, CD11a, CD28, and CD27. The effects of SIV infection on these subsets was measured to evaluate the impact of changes in functionally distinct cell types during pathogenesis.

METHODS

Peripheral blood was obtained from six SIV-infected macaques at multiple times before and after SIV infection and analyzed using 12-color flow cytometry.

RESULTS

Acute infection was characterized by an initial lymphopenia caused by a decline in B cells. Total T-cell counts remained steady during the early acute phase; however, CD4 cell counts declined while CD8 T cells increased. The decline in CD4 T cells was a result of a decline in both naive and memory cells. CCR5+ or CD103+ subsets of CD4 T cells were depleted but only partially accounted for the decline of CD4 memory T cells, suggesting that acute infection was associated with a rapid redistribution of T cells from the periphery. Naive CD8 cell counts declined while memory CD8 cell counts increased. The increase coincided with declines in plasma viremia and was made up initially of CD27-CD28- (effector) cells; subsequently, the predominant phenotype became CD27+CD28-, akin to central memory cells.

CONCLUSIONS

A complete understanding of the T-cell dynamics during acute SIV or HIV infection requires the simultaneous evaluation of a broad spectrum of T-cell subsets. Changes in homeostasis and associated immunopathogenesis can no longer be accurately described simply by measuring naive and memory T-cell subsets.

A retrospective study of the association between haematological and biochemical parameters and morphine intolerance in patients with cancer pain

BACKGROUND

Morphine is the strong opioid of choice for the treatment of moderate to severe cancer pain according to guidelines of the World Health Organization (WHO). However, a minority of patients do not receive the desired analgesic effect or suffer intolerable side effects from morphine, and are switched to alternative opioids.

METHODS

The aim of this retrospective study was to identify factors that might be associated with morphine intolerance. Data were analysed from 100 controls who tolerated morphine and 77 patients who were switched to an alternative opioid. We investigated whether currently logged data could fully explain the need to switch. Demographic details, cancer type (histological diagnosis) and markers related to organ function were included in an analysis of biochemical and haematological parameters.

RESULTS

Patients over 78 years (P = 0.03), or with a high white cell (P = 0.002) or high platelet count (P = 0.003), were more likely to switch. Although our numbers were small, patients with severe organ impairment were more likely to switch. However, a model including white cell count, platelet count, age, serum albumin and alkaline phosphatase, accurately separated switchers and controls in only 68% of cases. There was no significant difference between the two groups in terms of the numbers of patients having cytotoxic drugs in the two weeks prior to the haematological and biochemical analysis. Similarly, there were no significant differences in histological diagnoses between groups.

CONCLUSIONS

The white cell count was the strongest single effect observed and, as such, warrants further investigation. Further studies are needed in order to accurately define a model that will predict those patients likely to be intolerant of morphine.

Effect of age on immune parameters and the immune response of dogs to vaccines: a cross-sectional study

The evaluation of anti-aging intervention strategies in dogs would benefit from reliable quantitative biomarkers of aging. In the present study, the expression of various immune parameters was measured in young and old dogs to identify potential biomarkers of aging. The second goal of the study was to determine the effect of age on the immune response to vaccines. The immune function, including the antibody response to vaccines, was determined in 32 young adult (3.15+/-0.8 years of age) and 33 old dogs (12.1+/-1.3 years of age) of various breeds. Old dogs had a significantly lower lymphocyte proliferative response and a lower percentage of CD4+ T cells and CD45R+/CD4+ T cells, and a higher percentage of CD8+ T cells and a higher concentration of serum and salivary IgA. The most significant differences (P<0.001) occurred in the lymphocyte proliferative responses to ConA and PHA, the CD4:CD8 ratio, and the percentage of CD45R+/CD4+ T cells suggesting that these parameters are potential biomarkers of aging. There was no difference in the percentage of total T and B lymphocytes and the concentration of serum IgM and IgG. Both groups of dogs had protective titers against distemper virus, parvovirus and rabies virus before annual revaccination. The pre-vaccination titer against rabies virus was higher in the old dogs than in the young dogs, and there were no differences in post-vaccination titers against any of the viruses. This suggests that annual vaccination protocols provide adequate protection for old dogs.

Follicular dendritic cells in aging, a "bottle-neck" in the humoral immune response

Senescence leads to the appearance of atrophic follicular dendritic cells (FDCs) that trap and retain little immune complexes (IC), generate few memory B cells, and induce a reduced number of germinal centers (GC). Deficiencies in antibody responses to T cell dependent exogenous antigens such as pneumonia and influenza vaccines may reflect intrinsic FDC defects or altered FDC-B cell interactions. We recently studied antigen handling capacity and co-stimulatory activity of old FDCs and determined age-related changes in the expression or function of FcgammaRII or CR1 and 2 on FDCs. Here, we present an overview of FDC function in recall responses with known deficiencies in FDCs and GC development. Then, we review our recent work on aged FDCs and discuss age-related changes in molecular interactions between FDCs and B cells. We also discuss the causes underlying the impaired humoral immune response with respect to age-related molecular changes in FDC and B cell interactions. In vitro evidence suggests that FcgammaRII on aged FDCs is regulated abnormally and this in turn might cause the development of a defective FDC-network (reticulum) that retains few ICs, promotes ITIM signaling, prevents B cell proliferation and GC formation, and antibody production.

Age-associated alterations in the recruitment of signal-transduction proteins to lipid rafts in human T lymphocytes

Aging is associated with a decline in T cell activation and proliferation, but the underlying mechanisms are not fully understood. Recent findings suggest that lipid rafts act as a platform in the initiation of T cell activation by selectively recruiting signaling proteins associated with formation of the initial complex of signal transduction. We tested the hypothesis that lipid raft properties are altered in T lymphocytes from elderly, healthy individuals in comparison with young subjects. Results showed that the cholesterol content of lipid rafts derived from these cells was consistently higher in the case of elderly donors and that membrane fluidity was decreased. In addition, lipid rafts coalescence to the site of T cell receptor engagement was impaired in T cells from elderly donors. The recruitment of p56(lck), linker of activated T cells, and their tyrosine-phosphorylated forms to lipid rafts was decreased in activated T cells from aged individuals. CD45 was not recruited to the lipid raft fractions in either group of subjects. Our data suggest that some properties of lipid rafts are altered in aging, and this finding may be part of the causes for the decline in T cell functions that are observed in elderly individuals.

In vitro senescence of immune cells

Immune cells are eminently suitable model systems in which to address the possible role of replicative senescence during in vivo aging. Since there are more than 10(8) unique antigen specificities present within the total T lymphocyte population of each individual, the immune response to any single antigen requires massive clonal expansion of the small proportion of T cells whose receptors recognize that antigen. The Hayflick Limit may, therefore, constitute a barrier to effective immune function, at least for those T cells that encounter their specific antigen more than once over the life course. Application of the fibroblast replicative senescence model to the so-called cytotoxic or CD8 T cell, the class of T cells that controls viral infection and cancer, has revealed certain features in common with other cell types as well as several characteristics that are unique to T cells. One senescence-associated change that is T cell-specific is the complete loss of expression of the activation signaling surface molecule, CD28, an alteration that enabled the documentation of high proportions of senescent T cells in vivo. The T cell model has also provided the unique opportunity to analyze telomere dynamics in a cell type that has the ability to upregulate telomerase yet nevertheless undergoes senescence. The intimate involvement of the immune system in the control of pathogens and cancer as well as in modulation of bone homeostasis suggests that more extensive analysis of the full range of characteristics of senescent T cells may help elucidate a broad spectrum of age-associated physiological changes.

Ineffective humoral immunity in the elderly

As individuals age, dysfunction of the immune system leads to an increased incidence of infectious disease. Due to the complex network of cellular interactions and the multi-factorial process of aging, numerous impairments in humoral immunity have been reported. Advances in technology have allowed scientists to begin to identify the molecular mechanisms behind the age-associated decline.

Molecular biology of hematopoietic stem cells

Human CD34+ hematopoietic stem and progenitor cells are capable of maintaining a life-long supply of the entire spectrum of blood cells dependent on systemic needs. Recent studies suggest that hematopoietic stem cells are, beyond their hematopoietic potential, able to differentiate into nonhematopoietic cell types, which could open novel avenues in the field of cellular therapy. Here, we concentrate on the molecular biology underlying basic features of hematopoietic stem cells. Immunofluorescence analyses, culture assays, and transplantation models permit an extensive immunological as well as functional characterization of human hematopoietic stem and progenitor cells. New methods such as cDNA array technology have demonstrated that distinct gene expression patterns of transcription factors and cell cycle genes molecularly control self-renewal, differentiation, and proliferation. Furthermore, several adhesion molecules have been shown to play an important role in the regulation of hematopoiesis and stem cell trafficking. Progress has also been made in elucidating molecular mechanisms of stem cell aging that limit replicative potential. Finally, more recent data provide the first molecular basis for a better understanding of transdifferentiation and developmental plasticity of hematopoietic stem cells. These findings could be helpful for non-hematopoietic cell therapeutic approaches.

Dramatic increase of telomerase activity during dendritic cell differentiation and maturation

Telomerase, the reverse transcriptase that maintains telomere DNA, is usually undetectable in most adult tissues but is positive in embryonic tissues and in cancers. In addition, freshly islolated or in vitro-activated lymphocytes were shown to express high levels of telomerase activity, although its expression in myeloid cells including dendritic cells (DCs) is largely unknown. Here, we investigated telomerase activity during the differentiation and maturation process of DCs. In vitro culture of bone marrow (BM) cells with granulocyte macrophage-colony stimulating factor and interleukin-4 induced a dramatic increase of telomerase activity accompanied with their differentiation into DCs. Furthermore, stimulation with microbial components such as lipopolysaccharide (LPS), which triggers maturation of DCs, augmented the activity. In vivo responses of telomerase activity were also observed in splenic DCs by injection of LPS intraperitoneally. It is interesting that in old mice, telomerase activity of splenic DCs was significantly higher than young mice but rather decreased after LPS stimulation. By measuring expression of cell-surface activation markers, splenic DCs of old mice responded poorly to LPS stimulation. Such poor responses to LPS were also observed in BM-derived DCs. These different features of DCs between young and old mice may contribute to a pathogenesis to microbial infections.

Factors influencing Leishmania major infection in IL-4-deficient BALB/c mice

The outcome of Leishmania major infection in IL-4-deficient BALB/c mice has been a controversial subject. We have shown that IL-4-deficient BALB/c mice infected with Leishmania major developed progressive lesions and could not contain the replication of the parasites, whereas other studies have reported that IL-4-deficient mice were able to resist infection. Therefore, we examined different factors that can influence the course of Leishmania major infection. We tested different lines of IL-4-deficient BALB/c mice and show that the reported differences in the outcome of infection were not due to the different genetic origin of the embryonic stem cells used to disrupt the IL-4 gene. In addition, we infected IL-4-deficient mice with different isolates of L. major parasites and show that none of the parasite strains tested were cleared, although some of them caused milder pathology. Interestingly, this milder pathology was paralleled by a reduced arginase activity of the parasites. We also tested the influence of age on the course of Leishmania major infection in IL-4-deficient BALB/c mice and show that older mice express a transient resistance. Thus, we conclude that differences in the age of the mice and in the arginase activity of the different isolates of parasites are factors that can influence the non-healing phenotype of IL-4-/- BALB/c mice.

The role of stem cells in aging

The objectives of this review were first to critically review what is known about the effects of aging on stem cells in general, and hematopoietic stem cells in particular. Secondly, evidence is marshalled in support of the hypothesis that aging stem cells play a critical role in determining the effects of aging on organ function, and ultimately on the lifespan of a mammal. Aging has both quantitative and qualitative effects on stem cells. On balance, the qualitative changes are the more important since they affect the self-renewal potential, developmental potential, and interactions with extrinsic signals, including those from stroma. Although hematopoiesis is generally maintained at normal and life-supporting levels during normal aging, diminished function is acutely apparent when old stem cells are subjected to stress. There is ample evidence of diminished self-renewal capacity, restriction of the breadth of developmental potency, and decreased numbers of progeny of old stem cells subjected to hematopoietic demands. The prediction is made that whatever plasticity in developmental potential possessed by a young stem cell is lost during aging. Those parts of the world enjoying an ever-increasing standard of living are also inhabited by an increasingly elderly population. The effects of age on many physiological functions are not well studied or appreciated. A public health challenge to provide increased quality of life for this growing segment of the population requires more attention to the variable of age in experimental studies. Stem cell populations are likely to be a fruitful subject for studies of this type.

Identification of immune responses against aldehyde-modified peptide sequences in apoB associated with cardiovascular disease

OBJECTIVE

Atherosclerosis is associated with an immune response against oxidized LDL, which modulates the progression of the disease process.

METHODS AND RESULTS

Using a library of polypeptides covering the complete sequence of apoB-100, the only major protein of LDL, we have identified over 100 different human antibodies reacting against aldehyde-modified apoB-100 sequences. IgM antibody titer levels decreased with age and were associated with the intima-media thickness of the carotid artery in subjects younger than 60 years. There were also inverse associations between IgM levels and oxidized LDL in plasma. In prospective clinical studies, antibody levels against several aldehyde-modified apoB-100 sites were associated with cardiovascular disease in this age group. Whether this immune response is adaptive (protective) or maladaptive (causal) in atherosclerosis requires further investigation.

CONCLUSIONS

We have characterized a large number of epitopes within the apoB-100 component of oxidized LDL that provoke an immune response in humans. These findings will make it possible to study the role of immune responses against specific sites in oxidized LDL and to determine whether these immune responses influence the risk for future cardiac events.

CD28 loss in senescent CD4+ T cells: reversal by interleukin-12 stimulation

CD28 is the quintessential costimulatory molecule expressed on CD4(+) and CD8(+) T cells. During chronic infections and the normal aging process, CD28 expression is lost, compromising the functional activity of T cells. CD28 loss is promoted by replicative stress, particularly in the presence of tumor necrosis factor-alpha, owing to an inoperative CD28 initiator element. It is currently unknown whether CD28 loss is irreversible. The present study examined cytokines for their ability to reinduce CD28 expression. CD4(+)CD28(null) T cells constitutively expressed interleukin-12 (IL-12) alpha and beta receptors, which were functional and allowed for the up-regulation of the signal transducer and activator of transcription-4 (STAT-4)-dependent gene CD161. Costimulation of the T-cell and IL-12 receptors induced the transcription of CD28 in approximately 50% of CD4(+)CD28(null) T-cell clones and lines. IL-12 by itself did not restore CD28 expression. Up-regulation of CD28 after IL-12 exposure correlated with the reassembly of the CD28-initiator protein complex. The re-expressed CD28 was functional and restored the ability of CD4(+)CD28(null) T cells to express CD25 and CD40 ligand. Our data suggest that IL-12 may, in part, functionally rescue senescent CD4(+) T cells.

B cells in the aged: CD27, CD5, and CD40 expression

Ageing is characterized by numerous changes in lymphocyte subpopulations. In the present paper we have focused on B cells carrying the surface markers CD27, CD5 and CD40. CD27 is considered a marker of primed (memory) cells and its engagement promotes the differentiation of memory B cells into plasma cells. CD5 is expressed on B1 cells, which are considered to be responsible for T cell-independent antibody production other than autoantibodies. The CD40 molecule binds CD40L (CD154) and is necessary for T-dependent antibody responses. Here we show that the absolute number of CD5+ and CD40+ B cells is decreased in the elderly, while CD27+ B lymphocytes only marginally decrease in centenarians. However, there is a decrease of the percentage of CD5+ B cells, an increase of CD27+ B cells, while CD40 does not change significantly. These data, together with the increased number of NK cells during aging, suggest different regulation of antibody production in the elderly which might be another example of immune remodeling with aging, based on interactions between human B and NK cells.

Age related changes in population of peripheral T cells: towards a model of immunosenescence

In this paper, we presented the results of analysis of experimental evidence for the decline of the human immune system functioning with age using mathematical model of immunosenescence. The most prominent changes in this system are related to the decline in the T-cellular immunity. These include the decline in the nai;ve T cells generation rate, shrinkage of the volume of the peripheral lymphoid tissue, decline of absolute and relative concentrations of nai;ve T cells in the blood, shortening of the average telomere length of T cells. These alterations in the immune system are responsible for sharp increase of morbidity and mortality caused by infectious agents at old ages. Analysis shows that concentrations of memory and nai;ve T cells in peripheral lymphoid tissue are the key variables in this process. Simulation experiments with our model show that the average life span of memory T cells must grow with age, and that decreasing of antigenic load led to considerable increase in organism's resistance in middle ages, but only to slight increase in old ages. Restriction in the rate of thymus involution resulted in an increase of organism's resistance to infections in old ages. However, this growth is accompanied by the decline of concentration of memory T cells and shortening of their life span. The proposed model describes the trade-off between concentrations of nai;ve and memory T cells and their potential to proliferate in human organism.

Immune responses against allogeneic and syngeneic tumors in aged C57BL/6 mice

Aged C57BL/6 (B6) mice could reject allogeneic BALB/c RL male 1 tumor as efficiently as young B6 mice. However, in vitro analysis showed impaired generation of cytotoxic T cell response in aged B6 mice against allogeneic tumor. The reaction could be augmented by the addition of recombinant interleukin-2 (rIL-2). Enzyme-linked immunospots (ELISPOT) produced by CD8+ T cells purified from spleen cells showed no reduction in aged mice. The findings suggested that the number of CD8+ T cells capable of reacting against allogeneic H-2 antigens was similar in young and aged B6 mice. Low cytotoxic T lymphocyte (CTL) responsiveness in aged B6 mice appeared to have resulted from low responsiveness of CD4+ T cells producing IL-2. Although CTL generation was apparently impaired, strong multiple antigenicity of allogeneic tumor evoked a rejection response in aged B6 mice. On the other hand, no rejection response was observed against syngeneic EL4 tumor in aged B6 mice even after depletion of CD4+ CD25+ immunoregulatory cells. Depletion of CD4+ CD25+ cells caused rejection of EL4 tumor in young B6 mice. The findings suggested that aged B6 mice were incapable of inducing effector cells against weak tumor antigens. Only marginal CTL response and small number of ELISPOTs were generated in young but not aged B6 mice against EL4. Addition of rIL-2 to the culture augmented EL4 killing and ELISPOTs in spleen cells from young and aged B6 mice.

Pathways to a robust immune response in the elderly

Circumstantial evidence suggests that infectious disease is the major cause of morbidity and mortality in the elderly, and immune-system dysfunction may contribute to this finding. Because innate and humoral immunity seem to be relatively unaffected by aging and because the T-cell compartment shows marked age-associated alterations, this article focuses on the association between T cells and aging. Longitudinal studies suggest that immune parameters, which predominantly are related to T cells, can be clustered to yield an IRP that is predictive of mortality in the elderly. Determining the IRP also may be helpful in younger individuals, particularly those under chronic antigenic stress (eg, patients with cancer or chronic infections) who experience premature aging of the immune system. Some changes in T cells can be modeled in clonal cultures in vitro to discover new biomarkers of immune aging. These biomarkers, which need to be validated in vivo, could be used to refine IRP. Interventions to selectively target changes that are identified as part of IRP may improve the health and quality of life of the elderly, reduce healthcare costs, and avoid potential unwanted side effects of global intervention approaches, such as triggering or exacerbating autoimmunity and inflammation.

Distinct TCRAV and TCRBV repertoire and CDR3 sequence of T lymphocytes clonally expanded in blood and GVHD lesions after human allogeneic bone marrow transplantation

Acute graft-versus-host disease (GVHD) is a disorder involving the skin, gut and liver that is caused by mismatches of major and/or minor histocompatibility antigens between the HLA-identical donor and recipient. If T lymphocytes infiltrating GVHD lesions recognize antigens expressed in these organs, T cell clones should expand in inflammatory tissues. We previously reported that recipients of allogeneic bone marrow grafts have clonally expanded TCRalphabeta(+) T lymphocytes soon after transplantation, which leads to a skew of TCR repertoires. To establish whether or not the same antigens cause clonal expansion of T lymphocytes in both blood and GVHD tissues, we examined the usage of TCR alpha and beta chain variable regions (TCRAV and TCRBV) and determined the complementarity-determining region 3 (CDR3) of T lymphocytes clonally expanded in circulating blood and GVHD lesions. We found that the repertoires and CDR3 diversity of TCRAV and TCRBV differed between the GVHD lesions and circulating blood, suggesting the selective recruitment of antigen-specific T cells into GVHD tissues. We also found that the usage of TCRAV and TCRBV by the clonally expanded T lymphocytes and their CDR3 sequences differed between the GVHD tissues and blood. These results suggest that the antigen specificity of TCRalphabeta(+) T lymphocytes clonally expanded in blood and GVHD lesions is different.

Premature ageing of the immune system: the cause of AIDS?

The reasons for the failure of the immune system to control HIV-1 infection, and the resulting immunodeficiency, remain unclear. HIV-1 persists in its host despite vigorous immune responses, including a strong, and probably functional, HIV-specific cytotoxic T-lymphocyte response. Interestingly the immunological features of HIV-1-infected individuals show many similarities to those seen in elderly people without HIV infection. We propose that, through a process of continuous immune activation, HIV-1 infection leads to an acceleration of the adaptive immune system ageing process, resulting in premature exhaustion of immune resources, which participates in the onset of immunodeficiency. This hypothesis might shed new light on HIV-1 pathogenesis and could suggest the need to reconsider current immunotherapeutic strategies to fight the virus.

Circulating, interferon-producing plasmacytoid dendritic cells decline during human ageing

Increased frequency and severity of infections in the elderly have been taken as indicative of declining immune function. Dendritic cells (DCs), the most important antigen-presenting cells, play a central role in initiating and modulating immune responses. One type, DC2, arises from precursor plasmacytoid DCs (pDCs), a rare population of circulating blood cells, whose hallmark function is rapid and copious production of interferon-alpha (IFN-alpha) upon microbial challenge. We found significant decreases of the circulating pDCs during ageing in healthy adult humans, as defined both by flow cytometry and IFN-alpha generation. Mean pDC/mm3 in peripheral blood declined from 7.8 for the youngest age group (18-39 years) to 4.2 for the oldest (60-91 years; P = 0.017). IFN-alpha generation declined similarly, from 3537 to 1201 IU/ml, respectively (P = 0.006). There was also a slight decline over the age range in the amount of IFN generated per pDC (slope = -0.0087; P = 0.046). CD4+ T cells decreased by approximately 20% over the same age range (P = 0.001), while there was no change in the total lymphocyte or monocyte counts.

Molecular and flow cytometric characterization of the CD4 and CD8 T-cell repertoire in patients with myelodysplastic syndrome

We studied 18 patients with myelodysplastic syndrome (MDS), measuring clonality and T-cell receptor Vbeta (TCRBV) expression of CD4 and CD8 T cells by polymerase chain reaction and by flow cytometric analysis of TCRBV families. The CD4 and CD8 T-cell repertoire in most MDS patients is characterized by an abnormal TCRBV-restricted expansion of T cells in CD4 and CD8 cells, and increased expression of the CD8 effector marker CD57 of multiple TCRBV in CD8 cells. Clonality analysis of CD4 and CD8 cells showed that seven of 10 patients analysed had a major clone in the CD8 cells but not in CD4 cells. Furthermore, in one patient we found that both the CD57- and CD57+ fraction contained the clone (which was absent from the TCRBV-negative fraction). These data suggest that, in MDS, multiple T-cell expansions can be found in both helper and cytotoxic T cells, and that, in the CD8 cells, T cells functionally differentiate in vivo from memory to effector T cells. Together, these data support the hypothesis of the involvement of T cells in the pathogenesis of MDS.

The assessment of antigen-specific CD8+ T cells through the combination of MHC class I tetramer and intracellular staining

Peptide-bound histocompatibility leukocyte antigen (HLA) class I tetramers enable a precise identification of antigen-specific CD8+ T cells using flow cytometry. The combination of this technology with intracellular staining techniques opens up significantly better ways of studying these cells than previously possible, allowing immunologists to look at their life cycle (activation and proliferation), manner of death (aging and apoptosis) and effector function (cytotoxic potential and cytokine production). In this review, we hope to provide an overview of these possibilities, as well as making specific suggestions about the use of intracellular staining techniques in the study of antigen-specific T cells. Understanding how antigen-specific cells develop and function in different circumstances and pathologies will be the key to unravelling the secrets of our cellular immune system.

Age-related increase of protein glycation in peripheral blood lymphocytes is restricted to preferential target proteins

Advanced glycation end products (AGE) have been analyzed in aging human peripheral blood lymphocytes since protein glycation and glycoxidation are believed to contribute to the intracellular age-related accumulation of damaged proteins, a process that has been associated with the cellular functional deficits that occur with age. The appearance of AGE in cell lysates was monitored with an enzyme-linked immunosorbent assay using an anti-AGE antibody raised against glycated RNAse. When lymphocyte cytosolic extracts from old donors (86-91 years old) were compared with those from young donors (20-25 years old), a small but significant 40% increase of protein glycation was observed. In both age groups, further analysis of the pattern of glycated proteins by two-dimensional gel electrophoresis followed by western blotting with the same anti-AGE antibody, showed that the protein silver stain and the immunoblot patterns were not superimposable indicating that glycoxidative modifications are targeting only a restricted set of proteins. Among these preferential protein targets, seven of them exhibited a significant age-related increased immunoreactivity with the anti-AGE antibody suggesting that the corresponding modified proteins might serve as biomarkers of aging lymphocytes.

Epstein-Barr virus-specific CD8(+) T cells that re-express CD45RA are apoptosis-resistant memory cells that retain replicative potential

During acute infection, latent and lytic Epstein-Barr virus (EBV) epitope-specific CD8(+) T cells have a CD45RO(+) CD45RA(-) phenotype. However, after resolution of the infection, a large proportion of these cells, particularly those specific for lytic viral epitopes, re-express the CD45RA molecule. The role of CD8(+) CD45RA(+) T cells in ongoing immunity to EBV and other viruses is unknown. We now demonstrate that, relative to their CD45RO(+) counterparts, the EBV-specific CD8(+) T cells that revert to CD45RA expression after acute infectious mononucleosis are not in cell cycle, have longer telomeres, and are more resistant to apoptosis partly because of increased Bcl-2 expression. However, the EBV-specific CD8(+) CD45RA(+) T cells have shorter telomeres than the total CD8(+) CD45RA(+) T-cell pool and predominantly express low levels of the CCR7 chemokine receptor, indicating that they are not naive cells. In addition, EBV-specific CD8(+) CD45RA(+) T cells can be induced to proliferate and exhibit potent cytotoxic activity against target cells loaded with specific peptide. Our results strongly suggest, therefore, that EBV-specific CD8(+) CD45RA(+) T cells represent a stabilized virus-specific memory pool and not terminally differentiated effector cells. The identification of mechanisms that enable stable virus-specific CD8(+) T cells to persist after acute infection may lead to the enhancement of antiviral immunity in immunocompromised and elderly persons.

Influenza A virus specific T cell immunity in humans during aging

To study the decreasing responsiveness of the immune system during aging, influenza virus specific cellular immunity was investigated in a cohort of healthy blood donors between 18 and 70 years of age. The percentage of influenza A virus specific T cells was determined by flow cytometry and found not to change during aging. After stimulation with phorbol 12-myristate 13-acetate and ionomycin, an increase in the percentage of IFN-gamma and IL-4 producing CD8(+) T cells was observed during aging. In addition, the cytotoxic T lymphocyte (CTL) activity was investigated in two additional groups of five donors, 18-20 and 68-70 years of age. The lytic capacity of purified CD8(+) T cells, after in vitro stimulation of peripheral blood mononuclear cells with influenza A virus, seemed lower in 68- to 70-year-old donors than in 18- to 20-year-old donors. Therefore we conclude that the reduced CTL activity in the elderly is not the result of a lower frequency of virus-specific T cells, but more likely the result of impaired antigen-specific proliferation or lower lytic capacity of these cells.

Identification of altered integrin alpha/beta chain expression on T cells from old mice infected with Mycobacterium tuberculosis

During the natural aging process there is a gradual acquisition of T cell surface markers normally associated with cellular activation and memory. Because of this it is difficult to identify cells which are capable of responding to new antigenic challenge in older individuals, which therefore hinders the study of the immune response to infectious diseases. In this study we demonstrate that during natural aging there was an expansion in T cells that expressed the beta1 and beta2 integrins. Fewer CD8 T cells from old mice expressed beta7 integrins, however, they remained unchanged on CD4 T cells. In this study we also measured the expression of alphabeta integrin chains on the surface of T cells from mice of increasing age in response to an infection with Mycobacterium tuberculosis. We found that VLA-2 expression was increased on CD8 T cells from old mice however, the majority of integrins were unchanged in response to infection, in consequence of the increased expression of these molecules normally found in un-infected old mice. These findings are consistent with the hypothesis that age-associated changes occur in the number of cells that express molecules that allow T cells to traffic to inflammatory sites.

Phagocytosis by invertebrate hemocytes: causes of individual variation in Panorpa vulgaris scorpionflies

An in vitro phagocytosis assay, adjusted to as little as 1 microL of insect hemolymph, enables the microscopic determination of phagocytosis for single individuals of small insects. Even repeated determination over the lifetime of individuals is possible. This method makes it feasible to study individual variation in invertebrate phagocytic capacity. Possible sources of such variation are reviewed in this article: genetic differences, development, aging, reproduction, presence of parasites, and diverse environmental influences are natural sources of individual variation in phagocytosis. However, the methods used for phagocytosis and microscopic evaluation are also (unwelcome) sources of variation. To optimize incubation time for in vitro phagocytosis, time courses were determined. Furthermore, the reliability of visual counting and image analysis for the microscopic quantification of phagocytosis are compared. The influences of larval development and adult aging on phagocytosis by Panorpa vulgaris hemocytes are subsequently demonstrated. During development, a decrease in hemocyte numbers but a simultaneous increase in the proportion of phagocytosing hemocytes was observed when larvae reached pupation. On the other hand, adults showed a dramatic decrease in phagocytic capacity with age, while cell numbers remained fairly constant. The results show that individual variation in phagocytosis can be determined accurately in small invertebrates and related to its causes. This might be especially interesting in the context of studies relating individual immunocompetence to ecology, life history variation, and behavior.

Lymphocyte apoptosis and cell replacement in human liver allografts

BACKGROUND

Apoptosis of graft-infiltrating T cells has been described after rodent liver transplantation. The aim of this study was to assess lymphocyte apoptosis in human allografts. Additionally, kinetics of leukocyte turnover were studied to determine whether apoptotic cells were likely to be of donor or recipient origin.

METHODS

Liver biopsy specimens (n=36) taken between days 3 and 1855 were stained with terminal deoxynucleotidyl transferase dUTP nick end-labeling and anti-CD3 to detect apoptotic lymphocytes. Renal allograft and hepatitis C biopsy specimens served as controls. Donor cell turnover was studied in sex-mismatched grafts using Y-chromosome in situ hybridization to detect recipient cells and double immunostaining for leukocyte phenotyping.

RESULTS

T-cell apoptosis was prominent in hepatic sinusoids (72% of biopsy specimens) as early as day 3. It ranged from 0% to 18.2% of CD3+ cells (mean 5.28+/-0.82%) and persisted for >14 days, including time points >1 year. There was no difference between biopsy specimens with or without rejection (6.34+/-1.14% and 4.61+/-1.13%, P=NS). Apoptotic cells in portal tracts were less frequent (33% of biopsy specimens) and less abundant (1.13+/-0.36%, P<0.0001). No lymphocyte apoptosis was seen in renal allograft biopsy specimens or hepatitis C biopsy specimens, indicating that it is a distinctive feature of the liver allograft. Persisting lymphocyte apoptosis even after donor lymphocytes had been replaced suggests that recipient lymphocyte deletion must occur. Donor Kupffer cells persisted for many months.

CONCLUSIONS

Our results suggest that the sinusoidal microenvironment promotes recipient lymphocyte apoptosis, which may account for the improved outcome of liver grafts compared with other organ allografts.

Characterization of CD4(+) CTLs ex vivo

The cytotoxic potential of CD8(+) T cells and NK cells plays a crucial role in the immune response to pathogens. Although in vitro studies have reported that CD4(+) T cells are also able to mediate perforin-mediated killing, the in vivo existence and relevance of cytotoxic CD4(+) T cells have been the subject of debate. Here we show that a population of CD4(+) perforin(+) T cells is present in the circulation at low numbers in healthy donors and is markedly expanded in donors with chronic viral infections, in particular HIV infection, at all stages of the disease, including early primary infection. Ex vivo analysis shows that these cells have cytotoxic potential mediated through the release of perforin. In comparison with more classical CD4(+) T cells, this subset displays a distinct surface phenotype and functional profile most consistent with end-stage differentiated T cells and include Ag experienced CD4(+) T cells. The existence of CD4(+) cytotoxic T cells in vivo at relatively high levels in chronic viral infection suggests a role in the immune response.

[Immunological aging]

Immune responses decline with age, as assessed by the increasing role of infections in elderly subjects. This decline is, however, very progressive, and usually only clear-cut after 80 years of age. Major difficulties have been encountered to delineate the cellular basis of this immunodeficiency, which is subtle and inconsistent, at the origin of contradictory reports. This is, however, an important challenge in view of the possible therapeutic perspectives of immunotherapy of aged subjects.

Aging-dependent exclusion of antigen-inexperienced cells from the peripheral B cell repertoire

Aging is accompanied by greatly reduced B cell production in the bone marrow, yet peripheral B cell numbers do not decline. We hypothesize that this may reflect filling of the peripheral pool with B cells that are long-lived as a consequence of specificity for, and chronic stimulation by, environmental Ags. To begin to explore this possibility, we analyzed the effects of aging on B cell population dynamics in the anti-H2(k/b) 3-83 mu-delta Ig-transgenic mouse. We predicted that, because they presumably do not bind environmental Ags, B cells bearing the transgenic receptor may be lost in aged animals. As seen in nontransgenic animals, total splenic B cell numbers remained constant with age in the Ig-transgenic animals despite reduced B cell production. Importantly, although the few newly produced B cells in the bone marrow of aged mice are 3-83 positive, the peripheral compartment of these mice is dominated by B cells that express endogenous Ig genes rather than the transgenes. This population includes large numbers of marginal zone-like and CD21(low/-)CD23(low/-)IgM(low) B cells, as well as elevated numbers of CD5+ B cells. Many of these cells express only non-B220 CD45 isoforms, suggesting that they may be memory cells. A significant proportion of aged transgenic animals produce autoantibodies that are reactive with ssDNA, dsDNA, or histones. Results support the hypothesis that, in the face of severely reduced production with age, B cells are selected based on reactivity to environmental Ags, accumulate, and display activated phenotypes. Cells bearing 3-83-transgenic receptors are excluded from this population due to their specificity. Beyond their importance in aging, these findings define a novel form of receptor revision in which B cells are selected rather than deleted based on Ag reactivity.

Modulating airway defenses against microbes

Prevention and treatment of respiratory infections remain an important health care challenge as the US population ages, contains more susceptible or high-risk people, and encounters new pathogens or antibiotic resistant bacteria. Reasonably protective vaccines against very common microbes are available for childhood and adult immunization, but, generally, these are underutilized. A broader definition of higher risk individuals is evolving, which will include more for immunization. Different approaches to vaccine development through design of new component vaccines are necessary. This review has updated host defense mechanisms at three levels in the human respiratory tract: naso-oropharynx (upper airways), conducting airways, and alveolar space. Examples of representative pathogenic microbes have been inserted at the respective airway segment where they may colonize or create infection (influenza, measles virus, Porphyromonas gingivalis causing periodontitis, Bordetella pertussis, Chlamydia pneumoniae, Streptococcus pneumoniae, and Bacillus anthracis ). Hopefully, microbe-host interactions will suggest new approaches for preventing these kinds of infections.