Age-related refractoriness of PHA-induced lymphocyte transformation. I. Comparable sensitivity of spleen cells from young and old mice to culture conditions

Immunopharmacological actions of an extract isolated from inflamed skin of rabbits inoculated with Vaccinia virus (Neurotropin)--II. Restorative effect on immune responses through the recovery of IL-2 production in aging mice

To examine the immunopharmacological actions of an extract isolated from inflamed skin of rabbits inoculated with Vaccinia virus (Neurotropin), its effect on the immune responses in aging BALB/c mice was examined. Neurotropin clearly restored the decreasing T-cell-dependent immune responses such as delayed-type hypersensitivity (DTH) response and plaque-forming cells (PFC) response to sheep red blood cells (SRBC) when administered i.p. from 13 months old (mo) to 16 mo. However, Neurotropin administration from 2 to 5 mo had no effect on the immune responses of young animals. Neurotropin administration from 13 to 16 mo restored not only the T-cell proliferation of spleen cells induced by concanavalin A (Con A) and phytohemagglutinin (PHA), but also the interleukin-2 (IL-2) production by spleen cells activated with Con A. However, Neurotropin did not affect the responsiveness of Con A-activated spleen cells to exogenous recombinant IL-2. An absence of suppressor cells capable of inhibiting the IL-2 production in the spleens was confirmed in the 16 mo mice. Neurotropin administration also restored IL-1 production by peritoneal macrophages stimulated with lipopolysaccharide (LPS). These results suggest that long-term administration of Neurotropin restores the decreasing T-cell-dependent immune responses through the recovery of IL-2 and in part IL-1 production, but not the responsiveness to IL-2 in aging BALB/c mice.

Factors that influence formation of sister chromatid exchanges in human blood lymphocytes

Sister chromatid exchange (SCE) reflects an interchange of DNA sequences between helices in a replicating chromosome. This was initially accomplished by Taylor and colleagues (1957) using tritiated thymidine incorporation followed by autoradiography. The development of an elegant technique for differential staining of sister chromatids by incorporating a thymidine analog, 5-bromodeoxyuridine (BrdU) has greatly simplified the detection of SCEs in metaphase chromosomes. In recent years, the analysis of SCE has been considered to be a highly sensitive and additional (i.e., with chromosome aberrations) end point for measuring mutagenic/carcinogenic potential of various environmental agents and is increasingly being used to detect and differentiate among chromosome fragility human diseases that predispose to neoplasia. Attention has been focused to see if the induction of SCEs in lymphocyte cultures can be used as a reliable "biological dosimeter" for genetic risk assessment and to monitor the exposed populations. Several physical or preparatory as well as biological factors that modify the response and formation of SCEs make the monitoring difficult. The purpose of this article is to review and analyze these factors to facilitate an effective development of a standard protocol for SCE testing and for appropriate evaluation of test results. This may also provide clues to understand the yet unknown molecular mechanism(s) and biological significance of SCE formation.

Metabolic breakdown of [3H]thymidine and the inability to measure human lymphocyte proliferation by incorporation of radioactivity

Quantitative measurement of the incorporation of tritiated thymidine into cultures of phytohemagglutinin-stimulated lymphocytes is routinely used as an indication of the immunocompetence of the cells and of their proliferation. The present experiments show that several components of human blood catabolize nucleosides, including thymidine, extensively. Most of the radioactivity from tritiated thymidine, for example, is quickly rendered unincorporable as the compound is metabolized to thymine and further breakdown products. Thus, cells continue to proliferate without incorporating radioactivity from the medium. Furthermore, variability in the degree of catabolism has been found from person to person, so that neither measurement of the depletion of radioactivity from the medium nor measurement of the amount of label incorporated into the cultures can be used as a quantitative indicator of cell proliferation or immunocompetence.

The effect of aging on the expression of interleukin 2 messenger ribonucleic acid

It is well documented that the production of interleukin 2 (IL-2) by lymphocytes declines with increasing age. Using a cDNA probe to IL-2, the effect of age on the expression of IL-2 mRNA was studied on concanavalin A-stimulated lymphocytes. The induction of IL-2 mRNA by concanavalin A was maximum after 20 hr of incubation. Northern blots demonstrated that the cDNA probe to IL-2 hybridized to an 11 S to 12 S RNA species. No age-related change in the size of the IL-2 mRNA was observed. In addition, there was no evidence of an age-related change in IL-2 mRNA degradation or the post-transcriptional processing of IL-2 mRNA. The induction of IL-2 mRNA decreased 85% between 5 and 29 months and paralleled the decline in IL-2 production and lymphocyte proliferation. Therefore, the age-related decline in the ability of lymphocyte population to produce IL-2 arises from a decrease in the ability of the lymphocytes to genetically express IL-2 mRNA.

Expression of intracellular biochemical defects of lymphocytes in aging: proposal of a general aging mechanism which is not cell-specific

There is a decline in immune capacity with age which is expressed on the organismic level by association with numerous immune-related diseases, on the cellular level by impaired mitogenesis, on the biochemical level by impaired metabolic pathways, and on the molecular level by decreased protein synthesis and degradation. Defects in various cofactors such as calcium and several nucleotides also occur and may be related to the impaired enzyme function during mitogenesis in the aged. The central cause for decreased mitogenesis in the aged could be a decrease in protein synthesis which appears to cause impaired enzyme induction. This impaired enzyme induction accounts in part for the decreased glycolytic flux and DNA synthesis in these cells. Decreased protein synthesis also has been associated with a decreased synthesis of lymphokines which help these cells to proliferate. Numerous other intracellular age-related defects of lymphocytes also occur which may collectively play important interdependent roles in the impaired lymphocyte function of the aged. A potential general underlying mechanism of cellular senescence is proposed based on a genetic "slowing-cycle" effect of transcription, translation, and enzyme induction with immunosenescence presented as an example of an expression of these basic defects.

Decreased protein synthesis of transforming lymphocytes from aged humans: relationship to impaired mitogenesis with age

Recent studies have indicated that the decline in mitogenesis during the aging process may be related to intracellular defects that become apparent when the cells are subjected to the metabolic stress of cell transformation. We provide the first report of an age-related decline in protein synthesis in human lymphocytes exposed to phytohemagglutinin. This impairment in protein synthetic capacity from aged subjects' transforming cells is apparent from 24 to 72 h of culture. By 72 h of culture the incorporation of radiolabeled leucine in stimulated cells from elderly subjects is about half that for the young. However, cells from the aged have an increased protein content in the face of decreased synthesis suggesting a degradation defect. Since protein synthesis may be necessary for the induction of key effectors which activate glycolysis (which is necessary for transformation), we sought to relate the impaired protein synthesis to the impaired glycolytic enzyme induction with age. Cycloheximide totally inhibited glycolytic enzyme induction as well as cell transformation. A defect in protein synthesis with age may interfere with new enzyme synthesis which is necessary to activate requisite pathways for mitogenesis such as glycolysis.

Interleukin-1 synthesis and activity in aged mice

Recent studies have provided evidence that deficient interleukin-2 (IL-2) production by helper T cells contributes to the impaired T-cell-mediated functions observed in aged mice. Since most of these responses depend upon the presence of macrophages, a deficit in the functional capacity or in cell cooperation of macrophages may result in a decrease in immune reactivity. We found in the present study, that in vitro the cytostatic activity of macrophages from aged C57BL/6 (B6) mice is affected only slightly, but that in vivo their number increases with age. The synthesis of IL-1 is reduced when macrophages from aged mice are stimulated in vitro by lipopolysaccharide, but addition of exogenous IL-1 apparently does not restore either the mixed lymphocyte reaction or cytotoxic T lymphocyte generation. Co-cultures of young splenic macrophages with aged T lymphocytes do not restore to normal level the impaired proliferative response to T mitogens of aged B6 mice, but aged splenic macrophages provide a full accessory help for mitogenesis of young T cells. Thus, absorption of IL-1 by phytohemagglutinin-activated T cells is slightly altered in aged mice. IL-2 responsive T cells are not altered since exogenous IL-2 supply in vitro completely reconstitutes cytotoxic T lymphocyte generation after an allogeneic stimulation. Moreover, the number of Lyt 1+ cells is not modified in aged B6 mice. These results suggest that the impaired capacity of macrophages to release IL-1 and of blast T cells to bind IL-1 may contribute to the depression of cell-mediated immune reactivity associated with aging but also that the main defect is a functional lesion of IL-2 production by Lyt 1+ helper T cells.

Influence of age on the frequency of sister-chromatid exchanges and X-ray-induced chromosome aberrations in muntjac

The BrdU-differential staining technique was used in a study of the frequency of sister-chromatid exchanges (SCEs) and X-ray-induced chromosome aberrations in peripheral blood lymphocytes of the same individual muntjacs. Blood was collected periodically from immediately after birth (1 day old) to the adult stage (1 year). The results showed that both the frequency of base-line SCEs and induced chromosome aberrations changed as a function of age. At a young age, the frequency of SCEs was significantly low, whereas a high frequency of chromosome aberrations was observed. But with increase in age of the individuals, an enhanced frequency of SCEs and a decreased frequency of induced chromosome aberrations were observed; and as the age advanced further, the frequencies of both SCEs and chromosomal aberrations came to a steady level.

Mechanism of the age-related decline in lymphocyte proliferation: role of IL-2 production and protein synthesis

Although an age-related decline in mitogen-induced proliferation in spleen lymphocytes has been reported by numerous investigators, the molecular mechanism responsible is unknown. In this study, we compared the mitogen-induced proliferation, IL-2 production, and protein synthesis in spleen lymphocytes isolated from 4, 12, 20, and 30 month-old male Fischer F344 rats. IL-2 production by Con A-stimulated lymphocytes, as determined by the ability of the culture supernatants to support the growth of cultured T cells, declined over 72% between 4 and 30 months of age. This decline in IL-2 production paralleled a similar decrease in proliferation. Early protein synthesis by Con A-stimulated spleen lymphocytes was determined by measuring the incorporation of [3H]-valine into acid insoluble material, and this dropped 74% between 4 and 30 months of age. There was a strong correlation between the age-related decline in the three parameters tested. Based on these results, we propose that the age-related decline in protein synthesis may be the molecular basis for the similar decrease in IL-2 production and mitogenesis.

Age-related changes in responsiveness of various rat tissue lymphocytes to mitogens

The effect of age on the mitogen responses of rat lymphoid tissues was investigated. Evaluation was based on using the in vitro proliferative response of lymphocytes from various tissues of Fischer-344 rats (2, 7, 13, 19 and 25 months) using concanavalin A (Con A), phytohemagglutinin-P (PHA-P), pokeweed mitogen (PWM) and Mycoplasma neurolyticum. The stimulation index (S.I.) for cervical, mesenteric, thymic and splenic lymphocytes treated with Con A and PHA-P was greatest for 2-month-old rats and lowest for 19-month-old rats; however, no age-related change was observed with either PWM or M. neurolyticum. The levels of mitogenic responses for lymph nodes in the two different anatomical sites paralleled one another, with the cervical lymphocytes showing a greater response. The splenic lymphocytes responded less than either lymph node lymphocyte population. When PHA-P treatment of splenic lymphocytes followed the removal of the plastic adherent population, the S. I. of the resulting non-adherent population was comparable to the S. I. of other tissue lymphocytes; however, an age-related decrease was still observed. The PHA-P proliferative response of either the 7- or 19-month non-adherent population was suppressed by the 7-month adherent population and not by the 19-month adherent population i.e., adherent population interaction with non-adherent population decreases with age.

Nature of the decline in antigen-induced humoral immunity with age

An attempt has been made to summarize our present knowledge of the influence of age on humoral immunity, as judged by the ability of an individual to generate antibody in response to antigenic stimulation. In general, humoral immunity declines with age, and the onset can occur as early as when an individual reaches sexual maturity. The decline is due to changes in the immune cells and their milieu. Cell loss, shift in the proportion of subpopulations, and qualitative cellular changes have all been detected. The most prominent cellular target of aging appears to be T cells involved in the regulation of humoral response. Since the changes are closely associated with the involution and atrophy of the thymus, an understanding of its changes could be the key to understanding immunosenescence.

Cyclic nucleotide levels in resting and mitogen-stimulated spleen cell suspensions from young and old mice

The levels of cyclic adenosine 3', 5'-monophosphate (cAMP) in suspensions of unstimulated spleen cells from tumor-free 30-month old (C57BL/10Sn X C3H/HeDiSn)F1 hybrid mice averaged only 14% of that of 6-month old mice. By contrast, the level of cyclic guanosine 3', 5'-monophosphate (cGMP) in spleen cell suspensions from old mice was about 270% that of young mice. The cAMP/cGMP ratio for the unstimulated (resting) state showed a decline by 30 months to about 5% of its 6-month value. Cyclic nucleotide levels were also measured in cell suspensions from old and young mice at intervals over a two hour period following in vitro stimulation with the plant mitogens phytohemagglutinin, concanavalin-A and pokeweed mitogen. Quantitative and in some instances qualitative differences in responses were noted. These results might conceivably reflect either age-related changes in the splenic lymphoid cell subpopulations or intrinsic cellular alterations or both. It is unlikely that changes of this degree could be wholly explained by population shifts. An imbalance in cyclic nucleotide levels in both resting and stimulated lymphoid cells in older animals might contribute to the immune dysfunction known to occur with normal aging.

Cytochemical study of human lymphocytes stimulated by PHA in function of donor age

The 3H-AM binding reflects the structural changes involved in the cellular differentiation. This parameter was studied during blastic transformation of human lymphocytes, in relation to the age of the donor. Although they are individual variations, the 3H-AM binding is higher in the young group than in the aged subjects, as well as the blast transformation score. These results indicate that the weak lymphocyte response to the mitogen lectine (PHA) stimulation could be related to some age-induced structural alterations of the chromatin, resulting in an irreversible blockage in G1, at least in some of the T lymphocytes.

A multiple-parameter comparison of immunocompetence and tumor resistance in aged BALB/c mice

Several parameters of cell-mediated and humoral immunity were measured in young-adult and aged BALB/c mice and compared with resistance to the ascites form of intraperitoneal induced P815 mastocytoma. It was found that the age-related decline of in vitro phytohemagglutinin (PHA) responsiveness by spleen cells approximated the marked decrease of old mice to tumor-cell challenge and approached that characteristically seen in humoral immunity. Thus, decreased PHA responsiveness of splenic lymphocytes provided as sensitive an estimate of the age-related decline of immunocompetence in old mice as other classical parameters of cell-mediated immunity (e.g. graft-vs-host reaction or in vivo cellular proliferation of parental spleen cells in lethally-irradiated F1 recipients). Results could be interpreted to represent a decreased ability of noncycling T-cells to be released to a functional cycling state.

Age-related changes in ConA- and LPS-induced lymphocyte transformation. I. Effect of culture conditions on mitogen responses of blood and spleen lymphocytes from young and aged rats

Age-related changes in LPS- and ConA-responsiveness of rat spleen lymphocytes as judged by 14C-TdR incorporation were studied. It was found that responses to both mitogens decreased with advancing age. This report shows that the reduced 14C-TdR incorporation could not be attributed to decreased cell survival or viability of spleen lymphocytes from old rats, to delayed proliferation of the old lymphocytes, or to differences in minimum mitogen doses required for optimal stimulation. The results suggest that the observed decrease is due to a decrease in the number of mitogen responsive cells. The response to LPS was even more depressed than was the response to ConA. The response to ConA in whole blood is also shown to decline with ageing at multiple mitogen doses.