Amiela Globerson

The aging of hematopoietic stem cells

We have purified hematopoietic stem cells (HSCs) from the bone marrow of old mice and compared their properties to HSCs in young and middle–aged mice. Single, reconstituting HSCs (by limit dilution) from old and young mice exhibited indistinguishable progenitor activities in vivo. HSCs were five times as frequent in the bone marrow of old mice; however, HSCs from old mice were only one–quarter as efficient at homing to and engrafting the bone marrow of irradiated recipients. HSCs in young and middle–aged mice rarely were in the S/G2/M phases of the cell cycle, but HSCs in old mice were frequently in cycle. We speculate that the unexpected proliferation of HSCs in old mice might be related to the increased incidence of leukemia in old mice. HSCs change with age, but it is unknown whether these changes are determined intrinsically or caused by the aging of their environment.

T cells and aging, January 2002 update.

Age-related changes in the immune system may contribute to morbidity and mortality due to decreased resistance to infection and, possibly, certain cancers in the aged. Many studies mostly performed in mice, rats and man but also including monkeys and dogs have established that age-associated immune decline is characterized by decreases in both humoral and cellular responses. The former may be largely a result of the latter, because observed changes both in the B cell germline-encoded repertoire and the age-associated decrease in somatic hypermutation of the B cell antigen receptors are now known to be critically affected by helper T cell aging. As antigen presenting cell (APC) function appears to be well-maintained in the elderly, this review will focus on the T cell. Factors contributing to T cell immunosenescence may include a) altered production of T cell progenitors (stem cell defects, stromal cell defects), b) decreased levels of newly-generated mature T cells (thymic involution), c) aging of resting immune cells, d) disrupted activation pathways in immune cells (stimulation via the T cell receptor for antigen, costimulation, apoptosis control), e) replicative senescence of clonally expanding cells. This review aims to consider the current state of knowledge on the scientific basis for and potential clinical relevance of those factors in immunosenescence in humans. Experiments in other species will be touched upon with the proviso that there are clearly differences between them, especially between humans and rodents, but exactly what those differences are is not completely clear. Given its potential importance and the increasing proportion of elderly people the world over, coupled with the realisation that whereas mortality is decreasing, morbidity may not be decreasing in parallel (1), a better understanding of the causes and impact of immunosenescence may offer the possibility of identifying where prevention or delay of onset, as well as therapeutic intervention, might be beneficial. Amelioration of the effects of dysregulated immune responses in the elderly by replacement therapy, supplementation therapy or other approaches may result in an enhancement of their quality of life, and significant reductions in the cost of medical care in old age.

Ageing of lymphocytes and lymphocytes in the aged

Abstract Immunosenescence is a complex process involving multiple reorganizational and developmentally regulated changes, rather than simple unidirectional decline in all functions. Here, these compensatory modulations are reviewed with reference to the importance of integrating information on cellular alterations identified in vitro with data on global physiological changes in the aged environment with which these cells interact in vivo .

Correlations between membrane viscosity, serum cholesterol, lymphocyte activation and aging in man

Membrane microviscosity, serum cholesterol and lymphocyte stimulation in man were examined in relation to aging. A direct correlation was found between the mole ratio of cholesterol to phospholipids in the serum and the membrane microviscosity of the peripheral blood lymphocyte membranes, both of which increase progressively with age. The increase in membrane microviscosity with age is also concomitant with the decrease in response of the lymphocytes to concanavalin A stimulation. The data suggest that increase in lymphocyte membrane microviscosity is a prominent factor which may lead to the overt reduction of proliferative responses of lymphocytes with aging.

Viscosity of lymphocyte plasma membrane in aging mice and its possible relation to serum cholesterol.

Lymphocyte membrane fluidity was examined in aged mice and characterized as a qualitative and effective change which takes place in the aging process. Fluorescence polarization of diphenylhexatriene-labelled mouse spleen cells is substantially higher in cells from old mice (20--36 months) than young mice (2--7 months). A similar difference was also observed with isolated plasma membranes from spleen cells of old and young mice. The overall estimate is that the lipid microviscosity in the lymphocyte plasma membrane from old mice is about 20% higher than that of young mice. The cholesterol/phospholipid ratio determined for the isolated plasma membrane preparations was 0.68 and 0.9, respectively, which is probably the main cause for the difference in membrane viscosity. An elevated cholesterol/phospholipid ratio was also observed in the blood serum of old mice. It is plausible that the source of excess membrane cholesterol in the old mouse lymphocytes originates in the high serum cholesterol.

T Lymphocytes and Aging

Some of the key features of aging effects on T lymphocytes have been illuminated in the last few years from new angles. Experimental evidence indicates a profound increase in the proportion of memory versus naive types of T cells, a decline in the response to activation and in the capacity to enter the cycle, and decline in levels of IL-2, yet an increase in various other cytokines. At least part of these changes are based on altered patterns of T lymphocyte development. Long-term cultures of T cells from young donors show that certain manifestations of aging are acquired under such in vitro conditions. The T lymphocyte compartment thus offers a unique experimental model to investigate different checkpoints in development and function of the cells, as based on vulnerability to the effects of aging.

Age-related changes in the capacity of bone marrow cells to differentiate in thymic organ cultures

The capacity of the bone marrow to give rise to T cells in advanced age was studied in vitro by reconstituting fetal thymic lobes from 14-day C57BL/Ka (Thy-1.1) mice with bone marrow cells from old (24-month) or young (3-month) C57BL/6 (Thy-1.2) mice. The use of these congenic strains enabled distinguishing between donor and host contribution to the developing T cells. We found that bone marrow cells from aged mice maintained their capacity to reconstitute fetal thymic explants and to differentiate into various T-cell subsets as assessed by distinct T-cell-specific surface markers (Thy-1, Lyt-1, Lyt-2, and L3T4) and functions (concanavalin A-induced proliferative and cytotoxic responses). However, when mixtures of old and young bone marrow cells reconstituted fetal thymic explants, the cells of old mice were less efficient than those of young in their capacity to give rise to T cells. These results indicate that bone marrow cells from aged mice can reconstitute the thymus and differentiate into T cells; however, their reconstituting capacity is inferior to that of bone marrow cells from young mice.

Effects of growth hormone on thymocyte development from progenitor cells in the bone marrow

The effect of growth hormone (GH) on T cell differentiation was studied in young and old mice, employing in vivo and in vitro experimental approaches. Injections of GH during a period of 3 months to young and old mice resulted in a significant increase in the cell number and the percentage of CD3+ cells in the thymus of the old, but not in the young mice. Treatment of intact fetal thymus (FT) lobes with recombinant human GH (hGH) had no significant effect on cell numbers or on the values of CD4/CD8 thymocyte subsets. When partially depleted FT (10 Gy) were colonized with bone marrow (BM) cells and subsequently cultivated on monolayers of GH3, a GH-secreting cell line, the values of T cells deriving from the donor BM cells were elevated. Treatment with hGH to cocultures of lymphoid-depleted FT (dGUA) with BM lent further support to the idea that GH affects the newly emigrating BM cells, rather than the resident thymocytes. The results suggest that GH affects the thymocyte progenitors in the BM at the early stage of their development in the thymus.

Effect of aging on cytokine production in normal and experimental systemic lupus erythematosus afflicted mice

Aging mice of strains susceptible to the induction of experimental systemic lupus erythematosus (SLE) develop a milder disease than young animals. To find out whether the decrease in susceptibility to disease is due to age-associated changes in cytokine profile, we first examined the secretion of cytokines by healthy mice aged 2-15 months. A gradual age-related decline in the levels of interleukin (IL)-2 and interferon (IFN) gamma, and an increase in IL-4, IL-10, IL-1, and tumor necrosis factor (TNF) alpha were observed. Experimental SLE was induced in 2- and 10-month-old mice by immunization with the monoclonal anti-DNA antibody bearing the 16/6 Id. Early increased production of pro-inflammatory cytokines (TNFalpha and IL-1), followed by a peak of the Th1-type cytokines (IL-2, IFNgamma) were observed in young mice. The Th2-type cytokines (IL-4, IL-10) peaked later. In contrast, only a mild increase in all of the above cytokines was determined in 10-month immunized mice. It thus appears that the decline in susceptibility to SLE induction in older mice may be related to changes in the capacity to produce cytokines.

Ageing and the mismatch repair system.

Age-related accumulation of mutations has been extensively documented, and it has been proposed as one of the prominent causes of malignancies in old age. The present review is focused on the particular case of DNA mismatch repair system (MMR), that has drawn increased attention for its possible relevance to malignancy. We also report on our own observations on an age-associated genomic instability that develops with age in the MMR system. Our study was performed on DNA samples that were prepared from peripheral blood cells, obtained at a 10-year interval from young and old human subjects. The two DNA samples from each individual were examined comparatively. The older individuals showed a significantly higher rate of microsatellite instability (MSI) in several loci examined, whereas no difference was found between the paired samples of any of the young subjects. We suggest that this increase in MSI with age may indicate an overall genomic instability in the elderly.