Surface antigen expression in spleen cells of C57B1/6 mice during ageing: influence of sex and parity

Frailty and the risk of infection-related hospitalizations in older age: Differences by sex

OBJECTIVES

To investigate the extent to which frailty is associated with infection-related hospitalizations in older men and women, and to explore whether, among women, previous exposure to endogenous estrogens in terms of age at menopause and number of pregnancies modify such a relationship.

STUDY DESIGN

The sample comprised 2784 participants in the Progetto Veneto Anziani aged ≥65 years. At baseline and after 4.4 years, frailty was identified according to the presence of three or more of the following: weakness, exhaustion, weight loss, low physical activity, and low walking speed. A passive follow-up on infection-related hospitalizations and mortality was performed for 10 years of observation through linkage with regional registers.

MAIN OUTCOME MEASURES

The association between frailty and infection-related hospitalizations was assessed through mixed-effects Cox regressions.

RESULTS

Frailty was significantly associated with a 78 % higher risk of infection-related hospitalization, with stronger results in men (hazard ratio = 2.32, 95 % confidence interval 1.63-3.30) than in women (hazard ratio = 1.54, 95 % confidence interval 1.18-2.02). Focusing on women, we found a possible modifying effect for the number of pregnancies but not menopausal age. Women who had experienced one or no pregnancy demonstrated a higher hazard of infection-related hospitalization as a function of frailty (hazard ratio = 3.00, 95 % confidence interval 1.58-5.71) than women who had experienced two or more pregnancies (hazard ratio = 1.68, 95 % confidence interval 1.18-2.39).

CONCLUSION

Frailty in older age increases the risk of infection-related hospitalizations, especially in men. The "immunologic advantage" of the female sex in younger age seems to persist also after menopause as a function of the number of pregnancies a woman has experienced.

Pregnancy, postpartum and parity: Resilience and vulnerability in brain health and disease

Risk and resilience in brain health and disease can be influenced by a variety of factors. While there is a growing appreciation to consider sex as one of these factors, far less attention has been paid to sex-specific variables that may differentially impact females such as pregnancy and reproductive history. In this review, we focus on nervous system disorders which show a female bias and for which there is data from basic research and clinical studies pointing to modification in disease risk and progression during pregnancy, postpartum and/or as a result of parity: multiple sclerosis (MS), depression, stroke, and Alzheimer's disease (AD). In doing so, we join others (Shors, 2016; Galea et al., 2018a) in aiming to illustrate the importance of looking beyond sex in neuroscience research.

Double deletion of tetraspanins CD9 and CD81 in mice leads to a syndrome resembling accelerated aging

Chronic obstructive pulmonary disease (COPD) has been recently characterized as a disease of accelerated lung aging, but the mechanism remains unclear. Tetraspanins have emerged as key players in malignancy and inflammatory diseases. Here, we found that CD9/CD81 double knockout (DKO) mice with a COPD-like phenotype progressively developed a syndrome resembling human aging, including cataracts, hair loss, and atrophy of various organs, including thymus, muscle, and testis, resulting in shorter survival than wild-type (WT) mice. Consistent with this, DNA microarray analysis of DKO mouse lungs revealed differential expression of genes involved in cell death, inflammation, and the sirtuin-1 (SIRT1) pathway. Accordingly, expression of SIRT1 was reduced in DKO mouse lungs. Importantly, siRNA knockdown of CD9 and CD81 in lung epithelial cells additively decreased SIRT1 and Foxo3a expression, but reciprocally upregulated the expression of p21 and p53, leading to reduced cell proliferation and elevated apoptosis. Furthermore, deletion of these tetraspanins increased the expression of pro-inflammatory genes and IL-8. Hence, CD9 and CD81 might coordinately prevent senescence and inflammation, partly by maintaining SIRT1 expression. Altogether, CD9/CD81 DKO mice represent a novel model for both COPD and accelerated senescence.

The Severe Deficiency of the Somatotrope GH-Releasing Hormone/Growth Hormone/Insulin-Like Growth Factor 1 Axis of Ghrh-/- Mice Is Associated With an Important Splenic Atrophy and Relative B Lymphopenia

A debate is still open about the precise control exerted by the somatotrope GH-releasing hormone (GHRH)/growth hormone (GH)/insulin-like growth factor 1 axis on the immune system. The objective of this study was to directly address this question through the use of Ghrh^-/- mice that exhibit a severe deficiency of their somatotrope axis. After control backcross studies and normalization for the reduced global weight of transgenic mice, no difference in weight and cellularity of the thymus was observed in Ghrh^-/- mice when compared with C57BL/6 wild-type (WT) control mice. Similarly, no significant change was observed in frequency and number of thymic T cell subsets. In the periphery, Ghrh^-/- mice exhibited an increase in T cell proportion associated with a higher frequency of sjTREC and naïve T cells. However, all Ghrh^-/- mice displayed an absolute and relative splenic atrophy, in parallel with a decrease in B cell percentage. GH supplementation of transgenic mice for 6 weeks induced a significant increase in their global as well as absolute and relative splenic weight. Interestingly, the classical thymus involution following dexamethasone administration was shown to recover in WT mice more quickly than in mutant mice. Altogether, these data show that the severe somatotrope deficiency of Ghrh^-/- mice essentially impacts the spleen and B compartment of the adaptive immune system, while it only marginally affects thymic function and T cell development.

Impact of Immune Deficiency on Remodeling of Maternal Resistance Vasculature 4 Weeks Postpartum in Mice

Pregnancy manifests changes in the vascular and immune systems that persist postpartum (PP), have important implications for future pregnancies, and may modify responses to cardiovascular stress in late life. The association between immune and vascular function and the generation or progression of cardiovascular disease beg the question of whether altered immunity modifies pregnancy-induced changes in the vasculature. Our objective was to compare changes in the function and remodeling of systemic resistance vessels 4 weeks PP in normal C57BL/6 (B6), and immunodeficient mice recombinase 1-deficient/B6 ( Rag1^-/-). Immune deficiency did not change the responsiveness to acetylcholine (ACh) and phenylephrine at baseline but decreased arterial distensibility and increased stiffness PP. Adoptive transfer of CD8 T cells into Rag1^-/- mice decreased the response to ACh while increasing distensibility and wall thickness. When compared to PP Rag1^-/-, vessels from PP CD4-deficient mice, which have B cells and CD8 T cells, but no CD4 cells, show increased distensibility and decreased responsiveness to ACh in a pattern similar to that seen in Rag1^-/- given CD8 T cells prior to mating. These studies suggest a key role for T cell, particularly CD8 T cell, associated factors in the PP remodeling of maternal resistance vessels.

Tripartite motif-containing protein 30 modulates TCR-activated proliferation and effector functions in CD4+ T cells

To avoid excessive activation, immune signals are tightly controlled by diverse inhibitory proteins. TRIM30, a tripartite motif (TRIM)-containing protein is one of such inhibitors known to function in macrophages. To define the roles of TRIM30, we generated Trim30 knockout (Trim30^−/−) mice. Trim30 deletion caused no major developmental defects in any organs, nor showed any discernable defect in the activation of macrophages. But, Trim30^−/− mice showed increased CD4/CD8 ratio when aged and Trim30^−/− CD4⁺ T cells exhibited an abnormal response upon TCR activation, in particular in the absence of a costimulatory signal. Adoptive transfer of wild-type and Trim30^−/− CD4⁺ T cells together into lymphopenic hosts confirmed higher proliferation of the Trim30^−/− CD4⁺ T cells in vivo. Despite the enhanced proliferation, Trim30^−/− T cells showed decreased levels of NF-κB activation and IL-2 production compared to wild-type cells. These results indicate a distinct requirement for TRIM30 in modulation of NF-κB activation and cell proliferation induced by TCR stimulation.

Age-associated alterations in sympathetic noradrenergic innervation of primary and secondary lymphoid organs in female Fischer 344 rats

Normal aging processes, as well as, psychological stress affect the immune system; each can act alone, or interact with each other, to cause dysregulation of immune function substantially altering physical and mental health. The sympathetic nervous system (SNS), a major mediator of stress effects on immune function, is significantly affected by normal aging process, and stress can affect aging of the SNS. Previously, we have shown age-associated changes in sympathetic noradrenergic (NA) innervation of lymphoid organs in male rodents that affect immune regulation. The purpose of this study was to investigate sympathetic innervation of lymphoid organs and associated alterations in immune responses in young and aging female Fischer 344 (F344) rats. Histofluorescence and immunocytochemistry for NA innervation, and neurochemistry for norepinephrine (NE) levels were performed in the thymus, spleen, and mesenteric lymph nodes (MLN) isolated from 3-month-old young (normal estrous cycle), 8- to 9-month-old (onset of irregular estrous cycling), and 24-25 month, and 30-31 month female F344 rats (acyclic) at diestrus based on vaginal smears. Age-related alterations in natural killer (NK) cell activity, interleukin-2 (IL-2) and interferon-γ (IFN-γ) production, T and B lymphocyte proliferation were examined in splenocytes. Sympathetic NA innervation and NE levels increased with aging in the thymus, declined in spleen and MLN, and was accompanied by significant reductions in NK cell activity, IL-2 and IFN-γ production, and T and B cell proliferation in old female rats. In 8-9 mo rats, NE levels in the hilar region of the spleen and IFN-γ production were unaltered, while NE levels in the end region of the spleen and IL-2 production were reduced. Collectively, these results suggest that aging is characterized by significant alterations in sympathetic NA innervation in the thymus, spleen, and MLN associated with immunosuppression, and that there is a marked shift in NA activity and immune reactivity occurring during middle-aged female rats.

Fetal cells in the pregnant mouse are diverse and express a variety of progenitor and differentiated cell markers

To better understand fetomaternal cell trafficking during pregnancy, we used a mouse model to determine the cell surface markers expressed on fetal cells, based on the hypothesis that fetal progenitor cells have the capacity to repair maternal organs, whereas more differentiated cells might initiate graft versus host disease. Wild-type females were mated to either homozygous or hemizygous transgenic males and euthanized in the peripartum period. Using dual color flow cytometry, we analyzed fetal transgene positive cells for the presence of nine markers (ITGAM, ITGB1, PECAM, CD34, CD44, PTPRC, ENG, SLAMF1, and CXCR4) to begin to identify the phenotype and degree of differentiation of fetal cells in nine maternal organs (lung, liver, spleen, blood, bone marrow, kidney, heart, thymus, and brain). Fetal cells were found in all maternal organs following either type of mating, albeit always at a higher frequency following mating with homozygous males. Some organs (e.g., lung and liver) had a wide variety of fetal cell markers present, while other organs (e.g., bone marrow and spleen) had a skewed distribution of fetal cell markers. Fetal cells in the murine pregnant female are diverse. Our results suggest that the fetal cells comprise a mixed population of progenitor and differentiated cells, with different relative proportions in different maternal organs. Future studies will address whether fetal cells cross the placental barrier in a differentiated state or as a homogenous population and subsequently differentiate in target maternal organs.

Neutrophil Infiltration in Exercise-Injured Skeletal Muscle

Neutrophils have not consistently been detected in exercise-injured skeletal muscle and, therefore, neutrophil infiltration in this muscle has become a controversial issue. Thirty-eight animal and human studies that assessed injured muscle for neutrophils and employed acute exercise (e.g. level, uphill or downhill running, eccentric contractions, or swimming) were analysed to help clarify the relationship between neutrophil infiltration and exercise-induced muscle injury. Findings from nearly three-quarters of the reviewed studies suggest that neutrophil accumulation follows exercise-induced muscle injury. Intramuscular neutrophil infiltration was present in 85% and 55% of the animal and human studies, respectively. However, no consistent relationship between the potential damaging effect of the exercise type and neutrophil infiltration can be conclusively established from these studies. Specific animal-related factors that could influence these results include age, animal strain, catecholamines, corticosterone, acute stressors and muscle type, whereas a specific human-related influencing factor is physical activity status. Factors affecting both animal and human studies could include sex hormones, muscle sampling techniques and neutrophil detection approaches. General categories of methods that have been used to detect neutrophil infiltration are microscopy, myeloperoxidase (MPO) biochemical assay, antibody staining and white blood cell radionuclide imaging. Only studies employing white blood cell radionuclide imaging have consistently detected neutrophil infiltration. However, antibody staining with a quantitative analysis is currently the most feasible, valid and sensitive method. Research recommendations, therefore, are warranted to resolve the neutrophil infiltration controversy. We propose two approaches for animal studies. The first approach encompasses (i) studying or measuring factors that could influence neutrophil infiltration; (ii) using quantitative antibody staining analysis (in all studies and employing a panel of anti-neutrophil antibodies); (iii) examining the relationship between fibre morphological changes and neutrophil antigen expression; and (iv) developing a neutrophil antibody-radionuclide imaging technique. The second approach will yield animal findings complementing or addressing the gaps from the human exercise studies. For human studies, we suggest that (i) physical activity status is investigated; (ii) quantitative antibody staining analysis is performed (including staining injured muscle with a panel of antibodies such as anti-elastase, anti-MPO, anti-CD11b and anti-CD15 or assessing injured muscle using both immunohistochemistry and the MPO biochemical assay); and (iii) the relationship between fibre morphological changes and neutrophil antigen expression is examined. Studies that incorporate these recommendations could lead to an increased understanding of whether neutrophils are essential for the recovery from an exercise-induced muscle injury.

A major role for memory CD4 T cells in the control of lymphopenia-induced proliferation of naive CD4 T cells

In a state of lymphopenia, naive and memory CD4 T cells compete with each other for expansion at the expense of naive T cells. This competition prevents the proliferation as well as the phenotypic and functional conversion of naive T cells to "memory-like" T cells and may consequently prevent immune pathology frequently associated with lymphopenia-induced proliferation of naive cells. However, in T cell replete mice, memory T cells do not compete with naive T cells, indicating independent homeostatic control of naive and memory CD4 T cells in conditions that do not involve profound lymphopenia. Moreover, within the memory compartment, subsequent generation of new memory T cells precludes the survival of memory-like T cells. Thus, memory T cells have a major role in the control of lymphopenia-induced proliferation of naive cells because they inhibit both the generation of memory-like T cells and their persistence within the memory compartment.

Recent thymic emigrants (CD4+) continuously migrate through lymphoid organs: within the tissue they alter surface molecule expression

T-cell progenitors migrate from bone marrow (BM) into the thymus. After maturation they are released as recent thymic emigrants (RTE) into the periphery ensuring the diversification of the T-cell repertoire. Both the kinetics with which RTE migrate through the periphery and the surface molecules they express are still unclear. In 1- and 18-month-old Lewis rats CD4+ RTE were identified in blood, spleen, lymph node, and thoracic duct lymph by flow cytometry (CD45RC- and CD90+), were differentiated from CD4+ naive (CD45RC+) and memory T cells (CD45RC-CD90-), and were characterized regarding the expression of surface molecules. Both in 1- and 18-month-old animals the percentage of RTE among the CD4+ population in blood was comparable to that in all other compartments. Surprisingly, RTE expressed alpha4-integrin, LFA-1, and interleukin (IL)-2 receptor at a significantly higher level than naive T cells and more comparable to memory T cells. Within lymphoid tissues RTE, naive, and memory T cells significantly upregulated the expression of CD44 and ICAM-1, and downregulated the expression of L-selectin. These changes were reversed before the cells re-entered the blood. Thus, our data indicate that CD4+ RTE travel through the periphery of young and old rats like mature T cells, continuously modulating their surface molecule expression.

Aged mice exhibit in vivo defective peripheral clonal deletion of D(b)/H-Y reactive CD8(+) T cells

We previously reported that T cells from aged mice were resistant to activation-induced cell death (AICD) in vitro. To determine whether the presence of AICD-resistant T cells is associated with defects in age-related peripheral clonal deletion in vivo, congenic male SCID mice were reconstituted with T cells from aged or young female D(b)/H-Y TCR (Tg71) transgenic mice. Compared with recipients of young cells, the recipients of T cells from aged mice exhibited a 3-fold increase in the percentage of autoreactive CD8(+) H-Y antigen-reactive T cells as defined by the clonotypic antibody, M33. There were significantly increased sera levels of interferon-gamma, a significantly decreased expression of FasL by M33(+)CD8(+) T cells, and significantly decreased apoptosis by DNA fragmentation staining of the spleen of mice reconstituted with T cells from aged mice compared to those from young mice. By day 21, the recipients of T cells from aged mice but not young mice, exhibited infiltration of CD3(+) cells into the non-lymphoid organs. These results indicate that there is defective peripheral deletion of the self-reactive T cells derived from aged female Tg71 mice, and that failure to delete these cells is associated with the defective T-cell clonal deletion in the recipient mice.

Bilateral dissected spleens and thymuses in rodents exhibit homogeneity in leukocyte markers

Histopathologic evaluation and/or archiving of sections of spleen or thymus from all study animals may be mandated by study protocol (e.g., Toxic Substances Control Act-compliant studies). In such cases, whole spleen or thymus is not available for immunophenotyping. It has not been previously demonstrated that immunologic data representative of whole organs can be reliably obtained using a section of the spleen or using one thymic lobe. Light-scatter characteristics and immune cell-surface antigen expression were therefore compared in the right and left halves of the spleen and in the right and left thymic lobes of young adult female C57B1/6 mice and Sprague-Dawley rats. Antigens compared were: mouse spleen - CD11b, CD45R, CD90; rat spleen - CD11b, CD45RA, Pan-T/Ox-52; mouse and rat thymus - CD4, CD8a. There were no significant differences in distribution of cells by size or by expression level for any of these antigens when the right part of the organs was compared to the left part. These data indicate that use of entire spleen or both thymic lobes is not required to reliably quantify resident immune cell subpopulations.

Pregnancies modulate B lymphopoiesis and myelopoiesis during murine ageing

We recently reported that pregnancy affects age-related changes in the distribution of lymphoid and macrophage populations in the spleen of C57Bl/6 mice. In the present study, we examined the influence of pregnancies on the generation of various developmental B-cell subsets and granulocyte/macrophage lineage cells during murine ageing. Using flow cytometry, changes in lymphoid (mature and early B-cell precursors: B220high, B220low, surface immunoglobulin M (sIgM) mu chain +/-) and myeloid (monocyte/macrophage Mac-1/CD11b, granulocyte Gr-1/Ly-6G) compartments were monitored in the bone marrow of young (2 months) and 15- and 23-month-old mice including male, multiparous and virgin female mice. Pregnancies delayed the age-related decline in murine B lymphopoiesis and maintained B-cell reserve capacity during ageing. We also found an increased production of myeloid cells induced by pregnancies at middle (15 months) and advanced (23 months) ages. This comparative study provides new information on changes in marrow lymphopoiesis and myelopoiesis with age. Our data emphasizes that the onset, magnitude and kinetics of age-related changes in the haematopoietic marrow are parity dependent. These changes could influence the incidence of age-related diseases and may account for the greater longevity of females.

A central role for thymic emigrants in peripheral T cell homeostasis

After initial seeding by thymic emigrants, homeostatic regulation of the T cell pool has been thought to occur entirely within the periphery. Here we report that the degree of thymic emigration directly affects the number and the CD4/CD8 ratio of peripheral T cells. We demonstrate that the increase in T cell pool size caused by the engraftment of 2, 6, or 9 thymic lobes correlates almost exactly with the number of emigrants exported from those grafts in the previous 3 weeks, regardless of how long the graft has been in place. The extent of the increase supports the concept of a 3-week period after thymic export in which emigrant T cells are exempt from peripheral T cell homeostasis. This apparent exclusion of recent thymic emigrants from the niche-based regulation of peripheral T cell numbers ensures repertoire turnover throughout adult life and provides the basis of a direct and previously unrecognized role for the thymus in the regulation of peripheral T cell homeostasis.

Nutrition and immunity in the elderly

Immune function declines with age, leading to increased infection and cancer rates in aged individuals. In fact, recent progress in the study of immune ageing has introduced the idea that rather than a general decline in the functions of the immune system with age, immune ageing is mainly characterized by a progressive appearance of immune dysregulation throughout life. Changes appear earlier in life for cell-mediated immunity than for humoral immunity. Thus, age-related modifications in cell-mediated immunity, i.e. changes in naive : memory T-cells, mature : immature T-cells, T-helper 1 : T-helper 2 cells are more important in the elderly than changes in humoral immunity, i.e. CD5 : CD5+ cells or length of antibody responses. Such evolution of the immune system has been linked to declining thymus function and to accumulative antigenic influence over the lifespan. In contrast, innate immunity (macrophage functions) is preserved or even increased during the ageing process. This finding shows that the 'primitive' immune system is less affected by the ageing process than the sophisticated specific immune system. The present review focuses on innate and cell-mediated immune changes with ageing. It provides evidence that primary changes (intrinsic modifications in the immune system) and secondary changes (resulting from environmental influences during the lifespan) exert different influences on the immune system. Primary changes, occurring in healthy individuals, seem less important nowadays than they were considered to be previously. For example, interleukin 2 secretion in some very healthy aged individuals is comparable with that in younger adults. Primary immune changes may not explain the increased incidence and severity of infections observed in the elderly population. Secondary immunological changes are far more frequent and are certainly responsible for most of the immune modifications observed in the elderly population. Environmental factors leading to secondary immune dysfunctions include not only antigenic influence, which is a reflection of diseases experienced over the lifespan, but also many other factors such as drug intake, physical activity and diet; factors for which important changes occur in the elderly population. Nutritional factors play a major role in the immune responses of aged individuals and the present review shows that nutritional influences on immune responses are of great consequence in aged individuals, even in the very healthy elderly.

Effect of multiparity on T-cell proliferation response to mitogen stimulation in elderly women

The purpose of the present study was to investigate whether multiparity in women prevents some age-related changes in their cell function, as is known for age and sex. 49 women from the same village, free of acute inflammation or neoplastic diseases, were selected and divided into the following three groups: nulliparous 1935 years old, nulliparous 65 92 years old and multiparous (3-9 pregnancies) 65-88 years old. The response of their blood-lymphocytes to phytohaemagglutinin-P was estimated by thymidine uptake, number of mitoses and number of sister-chromatid exchanges. The stimulation index, mitotic index and proliferative rate in the aging nulliparous women were significantly lower than in the younger nulliparous women, but the latter group (young nulliparous) did not differ in their lymphocyte parameters assayed, from the values in the aging multiparous women breast-feeding their newborn. It is concluded that when the proliferative capacity to mitogen in elderly multiparous women (after 3-9 pregnancies) is related to breast feeding, it is similar to the proliferative capacity in young women. We suggest therefore, that breast feeding is beneficial not only to the newborns, but also to their mothers.

Immune response during disease and recovery in the elderly

The present article reviews immune ageing and its relationship with nutritional ageing, with a particular insight into the influences of disease on both ageing processes. Immune ageing can be described primarily as the progressive appearance of immune dysregulations, mainly acquired immunity (mature: immature, naive: memory T lymphocyte subset decreases) leading to gradual increases in T-helper 2: T-helper 1 cells. This change is due initially to decreased thymic function, and later to accumulative antigen pressure over the lifespan. In contrast, innate immunity (macrophage functions) is preserved during the ageing process and in the elderly this leads to macrophage-lymphocyte dysequilibrium, which is particularly critical during on-going disease. Indeed, any disease induces long-lasting acute-phase reactions in aged patients and leads to body nutritional reserve (mainly protein) losses. Episodes of disease in the aged patient progressively deplete body nutritional reserves and lead to protein-energy malnutrition, undernutrition-associated immunodeficiency, and finally cachexia. Undernutrition is a common symptom in the elderly; protein-energy malnutrition is found in more than 50% of hospitalized elderly patients and in most elderly diseased subjects. In addition, micronutrient deficit or low levels are common in home-living self-sufficient apparently-healthy elderly subjects. All these nutritional deficits induce decreased immune responses, and micronutrient deficits are now thought to be partly responsible for the decreased immune responses (immune ageing?) observed in the apparently-healthy elderly. Indeed, several studies have shown that micronutrient supplements induce increased immune responses in the healthy elderly. The progression of infectious diseases depends on immune responses and on nutritional status before the onset of illness in aged subjects. In addition, recovery depends on the intensity of acute-phase responses in the undernourished elderly. In fact, chronic acute-phase responses, commonly associated with diseases in aged patients, lead to progressive lowering of metabolic responses in the undernourished elderly. This can be quantified by increased production of free radicals during treatment and these increases may explain the difficulty in successfully treating aged patients. Nutritive therapy in order to improve metabolic processes and also to maintain body reserves should be considered as a necessary adjuvant therapy in the treatment of elderly patients.

Nutritional influences on immune response in healthy aged persons

Healthy elderly (80+/-5 years) with different nutritional status were compared to young healthy adults (25+/-5 years) to quantify the relative influences of aging and nutrition on immune response. Aged persons, without alteration of their nutritional status, had lower CD3+, CD8+, and CD45RA+ as well as higher CD2+CD3-, CD2+CD4-CD8-, and CD45RO+ T cell subsets and IL-6 release than their younger counterparts. T cell proliferation and IL-2 production were comparable in the two healthiest groups. Aged subjects with low nutritional status expressed similar but more marked changes in immune response while nutritional status did not influence the immune response in young subjects. Furthermore, lower nutritional status was associated with lower CD4+ counts and lower T cell functions in aged persons. These results indicate that the influences of aging and undernutrition in humans are cumulative and suggest that some changes in immune response that have been attributed to aging may, in fact, be related to nutrition and not aging.

The role of the thymus and recent thymic migrants in the maintenance of the adult peripheral lymphocyte pool

The thymus is essential for the initial seeding of T cells to the periphery, but its role in maintaining the adult T cell pool remains poorly defined. We investigated whether changes to the rate of T cell export could form part of the mechanism(s) controlling the homeostatic regulation of the size and composition of the peripheral T cell pool. Using neonatal thymi grafted under the kidney capsule, we found that irrespective of whether the pool was oversupplied (by thymic grafts) or undersupplied (due to neonatal thymectomy), the thymic export rate was constant from both the host and graft thymus, and the periphery remained constant in size. Recent thymic emigrants (RTE) were also tracked to determine the extent of their acceptance into the T cell pool of a normal mouse. As a population, RTE are phenotypically mature, but were distinct from resident T cells in the periphery, being released in a CD4/CD8 ratio approximately twice that of established peripheral T cells. This export ratio is similar to that of T cells in the mature thymic compartment, but soon after entry into the periphery, the ratio falls, indicating separate thymic and peripheral regulation of the CD4/CD8 ratio. RTE may also be preferentially incorporated into the periphery, causing displacement of resident T cells, thus maintaining the size of the peripheral pool. Although not vital for the maintenance of a functional T cell pool, the acceptance of RTE in a "full" peripheral pool would ensure that the T cell receptor repertoire is kept diverse and that the T cell population encompasses a broad range of naive as well as memory T cells.