Rapid growth of the geriatric population has been made possible with advancements in pharmaceutical and health sciences. Hence, age-associated diseases are becoming more common. Aging encompasses deterioration of the immune system, known as immunosenescence. Dysregulation of the immune cell production, differentiation, and functioning lead to a chronic subclinical inflammatory state termed inflammaging. The hallmarks of the aging immune system are decreased naïve cells, increased memory cells, and increased serum levels of pro-inflammatory cytokines. Mesenchymal stem cell (MSC) transplantation is a promising solution to halt immunosenescence as the cells have excellent immunomodulatory functions and low immunogenicity.
1. Introduction
Frailty can be defined as a decline in physiological reserve across organ systems; it afflicts geriatric subjects above the age of 65. Immunosenescence is the term used to refer to profound changes in the immune system related to age. Immunosenescence involves dysregulation of the immune functions at both cellular and serological levels
[1]. As a result of degenerating immunity, older age groups are more susceptible to severe infections with poor prognoses
[2]. The risk of contracting community-acquired pneumonia increased by 21% in older adults of 65–74 years compared to younger patients, with an even higher incidence in older adults 85 years and above
[3][4][3,4]. Bacteremia and sepsis are also more prevalent among older adults. More troubling—older adults have a higher risk of morbidity and developing cognitive decline post-infection
[5][6][5,6]. Interestingly, older adults have higher autoimmunity, but not autoimmune disorders. The autoimmunity in older adults is associated with the high levels of circulating T-regulatory cells (Treg) and reduced CD4/CD8 ratio. Subsequently, this predisposes the aging host to infection and cancer
[7]. The increasing age also hampers the effects of vaccination unless the vaccine is developed to bypass this concern, i.e., conjugating it with an adjuvant. Hence, immunization for the aging population is limited
[1][8][9][10][1,8,9,10].
As a direct result of senescence, the immune system is in a constant subclinical inflammatory state known as ‘inflammaging’. Inflammaging is conjectured to be a consequence of activation of innate immunity and declination of adaptive immunity without exogenous stimuli. This state is associated with the cytokine’s milieu that skews towards a pro-inflammatory phenotype. The exact relationship between inflammaging and the disease state is yet to be elucidated
[11]. However, most age-related degenerative diseases share similar inflammatory pathogenesis to which inflammaging may further exacerbate the disease process and its morbidity. The common inflammatory disease includes cardiovascular disease (myocardial infarction, hypertension, atherosclerosis), cognitive impairments (Alzheimer’s disease, Parkinson’s disease), rheumatoid arthritis, and metabolic diseases (type II diabetes)
[11][12][13][11,12,13].
The population aging is rapidly accelerating. The United Nations speculates that the number of people aged 65 years old and older will double between 2019 and 2050. In 30 years, one out of six people worldwide will be categorized in the “older adult” age bracket
[14]. The older adult population is accompanied by a state of physiological vulnerability and declining ability to maintain homeostasis and respond to stress. This clinical expression of age-related decline is also known as frailty. Frailty and inflammation are strongly correlated where the serum levels of inflammatory markers are significantly higher in the older age group compared to the younger age group
[15]. Frailty includes functional and structural alterations in multiple organ systems and impaired immune responses, which predispose to a plethora of disorders
[16][17][16,17]. Consequently, the older adult population is a significant financial burden to the healthcare system
[18]. Although the presentation of diseases may be incited by other risk factors, aging is a significant contributing mechanism due to the inevitable frailty development.
Currently, there are a few measures that may delay frailty onset and improve the morbidity of age-associated disease. The management of geriatric patients includes implementing calorie restriction, exercise regimes, and hormonal supplementations
[12][19][20][12,19,20]. Diets high in n-3 polyunsaturated fatty acids and vitamin D have positive outcomes in reducing circulating levels of inflammatory molecules, namely C-reactive protein (CRP) and interleukin (IL)-6, as well as lower the mortality of the inflammatory diseases
[21][22][21,22]. Zhang et al. showed that physical exercise can delay cognitive impairment while Ng et al. reported that cognitive training can improve physical mobility and strength
[23][24][23,24]. The studies also showed that a mix of interventions (exercise and/or nutrition and/or cognitive training) would have better results than just either one
[25]. Frailty is a complex condition that is unique to every individual; these clinical treatments require personalization to directly intercept immunological frailty. Moreover, Zhang et al. have found that the frailty index scoring system does not necessarily reflect the conditions the subject is facing. Some elderly may still be classified as pre-frail due to the cut-off score, but were experiencing frailty in different domains, be it cognitive or functional
[23]. In the systemic review composed by Apostolo et al., the current personalized approach to manage disease-associated frailty has failed to produce consistent results
[25]. Hence, there is yet an exact solution to frailty.
Mesenchymal stem cells (MSCs) are multipotent progenitor cells that can be isolated from the bone marrow, adipose tissue, dental tissues, skin, salivary gland, limb buds, menstrual blood, and perinatal tissues
[26][27][28][29][26,27,28,29]. MSCs can differentiate into adipocytes, osteoblasts, and chondrocytes. Although MSCs do not differentiate into immune cells, MSCs provide a supporting microenvironmental niche for hematopoietic stem cells (HSCs) to differentiate into myeloid and lymphoid cells, which are essentially the immune cells. This specialized environment plays an important role to maintain the longevity of HSCs by controlling their proliferation and apoptotic activities
[30].
One of the speculated theories of declining immunity as the host ages is the MSC senescence. Subsequently, the functions and structures of MSCs, which are significant in maintaining the immune system, diminishes
[31]. Although they are multipotent, mesenchymal progenitors exist in a small population, only consisting of 0.001% to 0.01% bone marrow mononuclear cells. Therefore, ex vivo expansion of MSCs and subsequent administration of optimized dosage is necessary to maintain and boost the effects of MSCs in vivo
[32]. Furthermore, numerous in vivo and in vitro studies have proven that MSCs have low immunogenicity, excellent immunomodulatory function, and homing capability to regenerate damaged tissues through multipotent differentiation and paracrine secretion
[11][33][34][35][36][11,33,34,35,36]. Despite that, the current studies are not primarily focused on aging or the restoration of the immune system. There have been extensive studies done on pathological conditions than actual aging itself. Aging and MSC were studied separately, but the similarities of the immune markers involved may come into convergence. The proliferative capacity and immunomodulatory function of MSCs could aid in the restoration of the immune cells and reduce the pro-inflammatory markers since these parameters are observed in aging as well. It is imperative to discuss the papers based on the aspects related to immunosenescence and inflammaging.
2. Mesenchymal Stem Cell Therapy to Reverse Aging Effects on the Immune System
According to The International Society for Cellular Therapy (ISCT) 2006, there are three minimal criteria for defining MSCs: “(i) MSC must be plastic-adherent when maintained in standard culture conditions. (ii) ≥95% of the MSC population must express CD105, CD73 and CD90, and ≤2% of the MSC population express of CD45, CD34, CD14 or CD11b, CD79a or CD19 and HLA Class II surface molecules. (iii) MSC must differentiate to osteoblasts, adipocytes and chondroblasts in vitro.”
[37][141].
MSCs lack MHC class II, which translates to low immunogenicity and allows both allogeneic and autologous MSC transplantation. Standardization and commercialization of allogeneic MSCs would be readily available at a reduced cost due to the lack of need to create a personalized cell therapy from the autologous source and the potential of expanding the cells in large-scale
[38][142]. To date, MSCs have been used in clinical trials for osteoarthritis, spinal cord injury, diabetes mellitus, autoimmune disease (Crohn’s disease, multiple sclerosis, systemic lupus erythematosus, and systemic sclerosis), and systemic diseases such as graft-versus-host diseases and sepsis
[30][39][40][41][30,143,144,145]. Additionally, MSCs also have been used to treat neonatal diseases, i.e., intraventricular hemorrhage, bronchopulmonary dysplasia, and necrotizing enterocolitis
[42][146].
2.1. Mechanism of MSCs Action on Immune System
Some evidences showed that the ameliorating effects of MSCs on the immune system are not due to direct engraftment and cell replacement, but rather paracrine manner and direct cell-to-cell contact
[26][43][26,147]. MSCs secrete soluble paracrine factors including TGF-β, prostaglandin E2 (PGE2), indoleamine 2,3-dioxygenase (IDO), hepatocyte growth factor (HGF), nitric oxide (NO), interferon-gamma (IFN-γ), IL-2, and IL-10, which produce an immunomodulatory effect. They also express FasL and PD-L1 for contact-dependent inhibition to induce T cell apoptosis
[20][26][20,26]. MSCs express IL-10, which is an anti-inflammatory and immunoregulatory cytokine. Furthermore, they produce IL-6 and IL-8, which are known to be associated with MSC tissue repair potential
[44][148]. Subsequently, MSCs control the inflammatory state as evidence of the reduced expression of proinflammatory cytokines such as TNF-α, IL-1β, IL-6, and CRP
[45][140]. Then, the STAT6 pathway is activated by IL-4, which then stimulates the MSCs to secrete TGF-β. This promotes the development of CD8+ T cells and Treg cells while suppressing the Th1
[46][47][48][49][50][51][149,150,151,152,153,154]. Moreover, MSC-secreted TGF-β has a role in macrophage polarization towards the M2 phenotype. These M2 macrophages stimulate the expression of IL-10, which alleviates inflammation. The macrophage phagocytic ability is also enhanced by TGF-β through Akt-FoxO1 pathway
[36][52][36,119]. shows the list of potential markers involved in inflammaging, which may be useful to determine the efficacy of MSC therapy.
Table 1. The potential ‘inflammaging markers’ related to inflammatory diseases and aging. These markers may be used to validate the efficacy of MSC treatment. (‘↓’ = decrease; ‘↑’ = increase; ‘-‘ = no change).
Potential ‘Inflammaging Markers’ |
Status in Inflammaging |
References |
MSC and Dosage |
Results (Related to Immune Cells and Inflammatory Markers) |
Additional Notes |
IGF-1 |
↓ |
[17][53][54][17,155,156] |
Golpanian et al. (2017) [88][175] |
An average age of 78.4 ± 4.7 years and Clinical Frailty Score of 4–6 |
Group 1 = 20 × 106 allo-hBM-MSCs, IV injection |
Group 2 and Group 3 showed significant decrease in TNF-α, whereas Group 1 showed moderate reduction. |
100 × 106 cells is the optimal dose level. |
CD4+ T cells |
↓ |
[19][55][56][57][19,40,81,98] |
Group 2 = 100 × 106 allo-hBM-MSCs, IV injection |
CD28+ T cells |
↓ |
No significant changes were seen in CRP, IL-6, fibrinogen, D-dimer, and white blood cell counts. |
No additional benefit or loss of effect when 200 × 106 cell dose was used. | [11][58][59][11,157,158] |
CD19+ B cells |
↓ |
[60][61][88,114] |
Group 3 = 200 × 106 allo-hBM-MSCs, IV injection |
IL-10 |
↓/- |
[2 |
Tompkins et al. (2017) [89][138] | ] | [35][62][63][2,35,39,50] |
Age ≥60 and ≤95 years with Clinical Frailty Score of 4–7 |
Group 1 = 100 × 10 | 6 allo-hBM-MSCs, IV injection |
Decreased serum TNF-α levels in Group 1. |
No therapy-related side effects occurred. |
TGF-β |
↓ |
[33][54][64][65][33,156,159,160] |
IL-2 |
- |
[66][161] |
IFN-γ |
↑ |
[66][67][161,162] |
TNF-α |
↑ |
[66][68][69][161,163,164] |
] | [ | 56] |
Decreased B cell intracellular TNF-α in both Group 1 and Group 2. |
IL-6 |
↑ |
[15][[ |
Decreased early CD 69 and late activated CD25 T cells in both Group 1 and Group 2. |
36 | ][54]57 |
Decreased CD8 in Group 2. |
Group 2 = 200 × 106 allo-hBM-MSCs, IV injection |
No changes in CD4 in both Group 1 and Group 2. |
[70][71][15,36,156,165,166] |
CD4/CD8 ratio increased in Group 2. |
WBC |
↑ |
[17] |
No significant changes noted in IL-6, CRP, D-dimer, CBC, and fibrinogen in both Group 1 and Group 2. |
CD8+ T cells |
↑ |
[19] |
Chin et al. (2020) [90][176] | [ | ] |
Healthy, non-frail subjects with mean age of 55 ± 13 years | 55[72] |
Group 1: 65 × 106 | [58][73][19,40,81,98,103,157,167] |
allo-hUC-MSCs, IV injection |
In Group 1, no significant changes were noted in the serum levels of IL-10, IL-1RA, IL-6, PGE2, and TNF-α. |
No therapy-related side effects occurred. |
CD56+ NK cells |
↑ |
[74][75][76][72][77][78][86,96,97,103,126,168] |
In Group 2, the serum IL-1RA level was significantly increased for at least 6 months post-infusion. |
IL-1β |
↑/- |
[36][69][36,164] |
The serum IL-6 level throughout the 6 months monitoring period was higher in Group 2 than in Group 1. |
The immunoglobulin E (IgE) level remained low within the normal range which indicated that there were no hypersensitivity reaction post-infusion. |
IL-15 |
↑ |
[69][164] |
Group 2: 130 × 106 allo-hUC-MSCs, IV injection |
The serum TNF-α level was significantly lower at day 2 in Group 2 than Group 1. |
IL-18 |
↑ |
[69][164] |
Both Group 1 and Group 2 observed a significant increase in C-reactive protein at day 2 post-infusion, which then dropped continuously over 6 months. |
No significant changes in total white cell count or its subfractions post-infusion. |
CD68 |
↑ |
[68][163] |
MCP-1 |
The albumin/globulin ratio was higher in Group 2 than in Group 1 at 6 months. |
No significant changes in the lung function tests (FEV1 and FEV1/FVC levels) post-infusion. |
↑ |
[68][163] |
No significant changes in the growth factors (VEGF, TGF-β, and HGF) level post-infusion. |
IL-17 |
↑ |
Hashemian et al. (2021) [91][177] | [ | 34] |
11 patients diagnosed with COVID-19-induced ARDS who were admitted to the intensive care unit, age range was 42–66 years old |
3 × IV injections (200 × 10 | 6 cells) every other day for a total of 600 × 106 hUC-MSCs (6 cases) or PL-MSCs (5 cases). |
Significant reductions in serum levels of TNF-α, IL-8 and CRP were seen in all six survivors. |
IL-8 (CXCL8) |
↑ |
[11][74][11,86] |
CXCL10 |
↑ |
[79][169[80],170] |
CCL2 |
↑ |
[80][81][170,171] |
The study of MSC effects on the immune system is largely focused on T cells rather than B cells, as its effects are more prominent in the former. Rosado et al. suggested that the prerequisite of MSCs to exert effects on B cells is a functional T cell population. Cell-to-cell contact between MSCs and T cells inhibit the proliferation and antibody production of B cells, which in turn, may aid in the management of autoimmune conditions and graft rejections
[82][139]. Moreover, Lee et al. noted that the xenogeneic transplantation of human MSCs (hMSCs) in SLE mice models only inhibited the T cells but not the B cells. However, hMSCs that are primed with IFN-γ have increased CXCL10 and IDO expression, which effectively attracts B cells for contact inhibition
[45][140].
In a study by Shin et al., they found that adipose tissue-derived MSCs (AT-MSCs) treatment successfully prevented the ill-effects of sepsis by mitigating the systemic inflammation and multi-organ damage. They observed the drop in pro-inflammatory markers namely IL-6 and TNF-α and reduced damage in kidney, lungs, and liver
[35]. During the treatment with MSCs, there is an increased expression in inflammatory cytokines including IL-1α, IL-1β, and IL-6. It is important to note that this increase is not associated with the severity of inflammation, but it is to prime the MSCs for a sustained immunosuppression
[44][148].
The mechanism of action of MSCs on the immune system is not constitutively inhibitory, but is acquired after exposure to the inflammatory environment with IFN-γ. IFN-γ is one of the cytokines released by T cytotoxic cells during inflammation. Therefore, in Th17 centered inflammatory response, MSC treatment would require the addition of Treg to successfully regulate the inflammation
[45][83][140,172]. Lim et al. found that combination of MSCs and Treg has shown promising results in IFN-γ knockout mice with reduced inflammation and IL-7 production
[83][172]. Additionally, Fan et al. divulged that the IFN-γ stimulation could also induce a higher expression of galectin-9 (Gal-9) in the umbilical cord-derived MSCs (UC-MSCs) through the signal transducer and activator of transcription (STAT) and c-Jun N-terminal kinase (JNK) signaling pathways. Gal-9 is one of the constitutively expressed immunomodulatory components of MSCs, which acts by suppressing CD4+ T helper cells (Th1 and Th17) and CD8+ T cytotoxic cells and regulates the suppressive activity of Treg. Even so, when Gal-9 production is inhibited, MSCs could still exert its immunosuppressive function through paracrine manner
[83][172]. Roux et al. also observed a significant reduction in the population of both CD4+ and CD8+ T lymphocytes post-treatment with human iPSC-derived MSCs. The immunosuppression on T cells by MSCs was further substantiated with the increased expression of LAG3 and CTLA4, and cytokines including IL-10, TGF-β, and LIF
[44][148]. Li et al. observed a significant increase in CXCR3+ Tregs in the lungs and lymphoid tissues post-MSC infusion. MSCs also increased the production of CXCL9 and CXCL10 produced by lung phagocytes which mediate the recruitment of Tregs
[34].
Anderson et al.’s experiment on mice has also shown that murine AT-MSCs reduced the severity of experimental autoimmune encephalomyelitis (EAE) in mice. It is achievable due to the inhibition of the autoimmune T cell response with no increase in foxp3 Tregs. Moreover, MSCs inhibited the maturation of DCs in vitro via COX-1/2 activity and also lowered the amount of activated DCs in the lymph nodes of EAE mice
[84][173]. DCs from the older adults have increased reactivity to self-antigen, hence their constantly activated state produces proinflammatory cytokines and stimulates the proliferation of T cells
[85][174]. Through the inhibition of DC maturation, the inflammatory state of EAE was managed. Moreover, a study by Liu et al. transplanting human UC-MSCs into mice model showed significant improvements in the EAE pathogenesis in which the transplantation stimulated spinal cord remyelination and induced a shift of Th1 to Th2
[86][137]. Another study by Donders et al. using Wharton’s jelly-derived MSCs (WJ-MSCs) also found reduction in signs and severity of EAE in rats. Nevertheless, they found that the ameliorating effects of MSCs were only temporary, and the transplanted rats will clinically deteriorate again. Although repeated dosages of MSCs were administered, the disease pathogenesis of EAE did not improve
[87][134]. This contradicting data calls for more research data on the extent of MSC regenerative capability in clinical use. shows the effect of MSC on the immune system in human clinical studies.
Table 2. A summary of clinical studies of MSC effects on the immune system from 2017–2021.
References |
Human Subjects |
All six survivors were well with no complaints of dyspnea on day 60 post-infusion. |
IL-6 levels decreased in five patients. |
Radiological parameters of the lung CT scans showed great signs of recovery. |
IFN-γ levels decreased in four patients. |
Four patients who had signs of multi-organ failure or sepsis died in average 10 days after the first MSC infusion. |
IL-4 and IL- 10 levels increased in four cases, but the differences were not statistically significant. |