Down Syndrome: Is It Really Characterized by Precocious Immunosenescence?

Abstract

The immune system declines with aging, leading to an increased susceptibility to infections and higher incidence and progression of autoimmune phenomena and neoplasia. Down syndrome prematurely shows clinical manifestations that are normally seen with aging. This review provides a concise overview of abnormalities in the adaptive immune system of Down syndrome in comparison to normal and precocious (Progeria syndromes) aging. Clinical signs and immunological changes are reviewed. We challenge the hypothesis that the immunological abnormalities in Down syndrome should be interpreted as precocious immunosenescence.

In humans, as in all species, a progressive functional decline occurs with aging in all organ systems due to loss and instability of genetic material. To protect the genome, chromosomes are capped by so-called telomeres which prevent degradation of genes near the ends of the chromosome. But with each cell division a small part of the telomere is lost. This proces is limited, but not prevented, by the enzyme telomerase which increases the length of the telomeres. Therefore, cumulative cell divisions ultimately result in genome instability and apoptosis of cells [1].

The immune system is essential for detection and elimination of pathogens and thereby for prevention of damage and degeneration of the organism. Lymphocytes continuously undergo proliferation [2]. So, especially lymphocytes are highly susceptible to telomeric shortening, leading to aging of the adaptive immune system, which is often referred to as ‘immunosenescence’. As a result, elderly people are more susceptible to infections. Also, malignancies and autoimmune phenomena are more common with aging due to failing immune surveillance secondary to immunosenescence [3, 4].

Down syndrome is the most frequent genetic cause of mental retardation in man; it is caused by an extra chromosome 21. People with Down syndrome (DS) prematurely show signs that are normally seen with aging; for example, adults with DS show early signs of Alzheimer’s disease [5]. Also, clinical features reminiscent of immunosenescence are seen, with higher rates of infections, malignancies, and autoimmune phenomena. It is therefore not unlogical to hypothesize that the immune system in DS shows accelerated aging as well [6].

This review provides a concise overview of the abnormalities found in the DS immune system in comparison to normal immunosenescence and the immunological findings in the precocious aging or Progeria syndromes (PS) [7]. The scope of this article is not exhaustive with respect to this complex topic, but attempts to show that the immune alterations in DS should – in spite of previous publications on the subject – not be interpreted as precocious immunosenescence [8].

The adaptive immune system

The immune system defends the human body against invading micro-organisms. It consists of two parts: the innate and the adaptive immune system. The innate immune system provides a primary immune response. Although a fast reaction takes place within minutes to hours, no memory is generated. The innate immune system therefore provides only short-term solutions. The adaptive immune system plays a pivotal role for long-term survival [9]. An essential difference between innate and adaptive immune cells is that the latter react specifically to a myriad of antigens while providing long-term memory as well. Since immunosenescence primarily affects the adaptive immune system, that system will be the focus of this review.

Key players in the adaptive immune response are B- and T-lymphocytes. B-lymphocytes are responsible for humoral immunity by producing specific antibodies. T-lymphocytes are accountable for cellular immune responses by helping other immunological cells through cytokine production and stimulation, and by direct cytotoxicity. Both T- and B-lymphocyte precursors are generated from hematopoietic stem cells in the bone marrow. While B-lymphocytes fully develop in the bone marrow, T-cell-precursors migrate to the thymus for further proliferation and development. In the secondary lymphoid organs (spleen, tonsils, lymph nodes) antigens are collected and presented. Also, T- and B-lymphocytes migrate there, and proliferate and differentiate into different effector and memory subsets after stimulation.

Within the thymus, T-cell-precursors can only survive if their T-cell receptors can interact with self major histocompatibility complexes (MHC) expressed on cell membranes, so-called positive selection. Too strong binding to self-antigens leads to cell death by negative selection, no binding at all results in cell death by neglect. Thymocytes binding to MHC-type II differentiate into helper-T-lymphocytes (Th), thymocytes binding to MHC-type I differentiate into cytotoxic-T-lymphocytes (Tc). As only antigen-presenting cells such as B-lymphocytes, dendritic cells and phagocytes express MHC-type II molecules, Th can only interact with these types of cells. Th are responsible for coordination and communication with both innate and adaptive immune cells; they serve as immunoregulators. Tc interact with MHC-type I expressing cells, which almost all human cells are, and can act directly as “killing machines” after activation and proliferation. Tc are especially suitable for strong cellular immune responses against tumour cells and intracellular pathogens such as viruses, whereas Th can help both humoral and cellular immune responses. The continuous generation of new Th and Tc from the thymus is crucial to maintain a functional immune system. Recent thymic emigrants all carry T-cell receptor rearrangement excision circles (TREC) as a by-product of DNA recombination processes. TRECs are not replicated and therefore diluted in the progeny that is formed after cell division. The TREC content can therefore be used to estimate the thymic output and also – indirectly – the thymic involution with aging.

Primary B-cell development takes place in the bone marrow. A unique B-cell antigen receptor is created on each B-lymphocyte membrane through gene rearrangements without previous antigen-exposure. B-lymphocytes do not need MHC for antigen recognition and can respond not only to peptides, but also to polysaccharides. Naive B-lymphocytes react to antigen exposure by producing immunoglobulins (Igs), primarily IgM. Extracellular pathogens such as bacteria are the main focus for these Igs. T-lymphocytes and T-lymphocyte-derived factors are necessary for further B-lymphocyte development. With the help of Th, B-lymphocytes can class-switch to the production of IgG, IgA and IgE, with altered effector function while maintaining antigen specificity. Repeated exposure to T-lymphocyte dependent antigens activates selected clones of memory B-lymphocytes to undergo somatic hypermutation (SHM) leading to higher affinity Igs.

The net result of all these processes is a broad diversity of B- and T-lymphocytes, which can survive for many years and provide resistance against the pathogens attacking the human body.

Down syndrome compared to normal aging

A comparison between the adaptive immune systems of DS, normal aging and PS is summarized in Table 1.

Table 1.

The adaptive immune system in normal aging, Progeria syndromes and Down syndrome

	Normal aging	Progerias	Down syndrome
References	[1,3,4,22–24,29]	[7,37–43]	[5,6,8,10–13,19–21,25–28,30–36]
T-lymphocytes
CD3+ T-lymphocytes	Decreased (abs)	HGPS: decr/normal (abs) CS, Werner, XP: normal (abs/rel)	Decr/normal (abs)
CD3+CD4+ Th	Decreased (abs)	HGPS: decr/normal (abs) CS, XP: normal (abs)	Decreased (abs)
CD3+CD8+ Tc	Decreased (abs)	Normal (abs)	Decr/normal (abs)
CD4+CD45RA+ (Naive Th)	Decreased (abs)		Decreased (abs)
CD4+CD45RO+ (Memory Th)	Increased (abs)		Normal (abs)
CD8+CD45RA+CD27+ (Naive Tc)	Decreased (abs)		Incr/normal (abs)
CD8+CD45RA+CD27−(Term Diff Tc)	Increased (abs)		Normal (abs)
T-lymphocyte proliferation	Decreased	CS:normal HGPS:decr/normal Werner:decr/normal XP:decr/normal	Decr/normal
Thymus	Abnormal structure		Smaller, abnormal structure
TREC count	Decreased		Decreased
Th1/Th2 ratio	Normal		Increased
CD4/CD8 ratio	Normal	Decreased	Decreased
B-lymphocytes
CD19+ B-lymphocytes	Normal (abs)	HGPS: incr/normal (abs) Werner: normal (rel)	Decreased (abs)
CD19+CD27+ (Naive B)	Decreased (abs)		Decreased (abs)
CD19+CD27− (Memory B)	Increased (abs)		Normal (abs)
B-lymphocyte proliferation (PWM)	Decreased	HGPS: incr/normal Werner: decr/normal	Decreased
Immunoglobulins
IgG	Decr/normal	HGPS: decr/normal Werner: normal	Increased >2yr
IgM	Normal	Normal	Decreased
IgA	Decr/normal	HGPS: decr/normal Werner: normal	Normal
IgG1	Normal		Increased > 3yr
IgG2	Normal		Decreased
IgG3	Normal		Incr/normal
IgG4	Normal		Decreased
Auto-antibodies	Increased	HGPS: no increase	Increased
Oligo/monoclonal antibodies	Increased		No increase
Escherichia coli antigen	Decreased response		Decreased response
Response to vaccines
Tetanus	Decreased		Decreased avidity
Pneumococcal polysaccharide vaccine	Decreased		Decr/normal
Meningococcal conjugate vaccine	Decreased		Decreased
Pertussis vaccine (acellular)	Decreased		Decreased
Hepatitis B vaccine	Decreased		Decr/normal
Hepatitis A vaccine	Decreased		Normal
Influenza vaccine	Decr/normal		Decr/normal
Polio vaccine (oral)	Normal		Decreased
Response to viruses
CMV	Increased specific T-subsets, increased reactivation		Normal
RSV	Increased risk		Increased risk and severity
Response to bacteria
Bacterial sepsis	Increased rate	HGPS, Werner: increased rate	Increased rate
Mortality/morbidity	Increased rate	Increased rate	Increased rate
Auto-immune diseases
Thyroid	No increase	No increase	Increased
Diabetes Mellitus	Increased type 2	Incr/normal type 2	Increased type 1
Celiac disease	No increase		Increased
Malignancies
Hematological	Increased	XP: possible increase	Increased
Non-hematological	Increased	Werner: increased XP: increased skin cancer	No increase
Age-related disease
Osteoporosis	Increased	Increased	No increase
Atherosclerosis	Increased	Increased	No increase
Alzheimer disease	Increased	Increased	Increased
Periodontal disease	Increased		Increased

Abs = absolute counts; CD = cluster of differentiation; CMV= cytomegalovirus; CS= Cockayne syndrome; decr= decreased; HGPS = Hutchinson-Gilford progeria syndrome; Ig = immunoglobulin; Incr=increased; PWM = pokeweed mitogen; rel= relative counts; RSV = respiratory syncytial virus; TCR = T-cell-receptor; Tc= cytotoxic-T-lymphocyte; Term Diff = terminally differentiated; Th = helper-T-lymphocyte; TREC = T-cell receptor rearrangement excision circles; XP= xeroderma pigmentosum; yr = years. Empty column = not described in current literature.

T-lymphocytes

With aging the renewal capacity of stem cells declines, the hematopoietic tissue in the bone marrow decreases, and thymic involution with low peripheral blood TREC counts ensues [1]. T-lymphocytes can influence their own differentiation and proliferation process in the thymus and periphery by cross-talk and feedback-mechanisms. Decreased output of thymic emigrants can therefore normally be compensated in aging individuals by an increase in effector and memory Th and Tc numbers. In this way, total T-lymphocyte counts remain relatively stable in aging adults despite decreasing naive counts, as effector and memory subsets fill up the T-lymphocyte pool [3, 4]. However, these T-lymphocytes are continuously antigen-driven and eventually accumulate in “dead end” T-lymphocyte subsets such as terminally differentiated (TD) Tc. In other words, the T-lymphocyte pool becomes more experienced but less flexible with aging, and the cells show a restricted repertoire and reduced proliferative response [3, 4].

People with DS show T-lymphocyte abnormalities from an early age onwards. Newborns and fetuses with DS already show an altered thymic anatomy with impaired thymic output and lower TREC counts [10]. Some interesting candidate genes influencing thymocyte production by altered cross-talk and feedback-mechanisms can be found on chromosome 21. For example, increased expression of DS-cell-adhesion molecules on thymic epithelia and thymocytes can cause abnormal T-lymphocyte maturation and inefficient T-lymphocyte release to the peripheral blood [11]. Naive Tc and Th are decreased from birth [18]. Apart from decreased production, proliferation is impaired in DS as well. Tc and Th lack the normal antigen-driven expansion in the first years of life. Memory Tc show a gradual increase over time, but in contrast to normal aging memory Th do not. The expansion of TD Tc with normal aging is not seen in DS either [12].

The continuous DNA rearrangement processes make the lymphocyte pool extremely vulnerable for DNA errors. Lymphocytes are capable of upregulating telomerase and thereby can prolong their lifespan [1, 2]. The constant microbial pressure throughout life leads to an ongoing proliferative demand, which ultimately results in genome instability, senescence and apoptosis of cells. Enhanced cell death by apoptosis could play an extra role in DS besides decreased T-lymphocyte production and proliferation. Increased telomere shortening is found in DS T-lymphocytes, which could lead to higher apoptosis rates [13–16]. Recent studies however did not find increased apoptosis in peripheral T-lymphocytes, despite increased apoptosis markers on T-lymphocytes in earlier reports [10,17,18].

Functional impairment in DS T-lymphocytes is supported by decreased proliferative and antigen T-cell responses [10]. Functional T-lymphocyte impairment could explain the increased incidence of hematological malignancies.

B-lymphocytes

With aging, fewer B-lymphocytes are produced in the bone marrow. Total peripheral B-lymphocyte numbers do not decline with age, but the composition of the peripheral B-lymphocyte compartment changes: antigen-experienced memory B-lymphocytes increase and naive B-lymphocytes decrease in number. Memory B-lymphocytes with a decreased susceptibility to apoptosis accumulate in elderly persons, leading to clonal expansions of certain B-lymphocyte specificities, which may limit the diversity of the repertoire [3, 4].

Alterations of the B-lymphocyte compartment in DS are on the contrary present from birth onwards: newborns show decreased naive B-lymphocyte numbers, early expansion is absent, and memory B-lymphocytes do not increase with age. This results in extremely low total B-lymphocyte counts [19, 20].

Although serum immunoglobulin levels remain stable during normal aging, antibodies generated in old age are of lower affinity because of an age-associated decrease in somatic hypermutation due to decreasing help from Th, and restricted B-lymphocyte repertoire due to clonal expansions.

In DS, despite the low B-lymphocyte numbers, a profound hypergammaglobulinemia develops from around 3 years of age onwards, without evidence of mono- or oligoclonality [21]. This hypergammaglobulinemia with increased total IgG, IgG₁ and IgG₃, but decreased IgM, IgG₂ and IgG₄ serum levels [21] is more suggestive of dysregulation in class-switching and somatic hypermutation of B-lymphocytes within the germinal centers than of an impaired B-lymphocyte production in the bone marrow. An altered Ig-pattern can result from both an intrinsic defect in DS B-lymphocyte activation and proliferation or B-T-miscommunication in the periphery. Decreased numbers of Th type 2 – essential for the humoral immune response – have been reported in patients with DS [10] and could be associated with this altered Ig-pattern through impaired SHM and class-switching.

Immunizations

An indirect way to look at B-T-lymphocyte communication and the functional capacity of B- and T-lymphocytes is by studying vaccination responses. The specific antibody response to T-cell dependent protein antigens requires combined T-lymphocyte function, T-B interaction and B-lymphocyte function, whereas the response to T-cell independent polysaccharide antigens is largely determined by B-lymphocyte function alone.

The age-related decreased output and functional deficiency of the T- and B-lymphocyte pool hampers the adaptive immune response to both T-cell dependent and independent booster- and neo-vaccinations in the elderly [4]. Quantitative antibody responses are lower, decline faster and the affinity of the antibodies is diminished [4, 22–24].

Impaired specific antibody responses to both T-cell-dependent (e.g. Tetanus [25], influenza [26]) and T-cell-independent (e.g. Pneumococcal polysaccharide [27]) vaccines [28] are repeatedly reported in DS as well, which suggests both altered T- and B-lymphocyte function and communication.

Clinical relevance

The restricted B- and T-lymphocyte repertoire in the elderly, with antibodies of lower affinity, leads to immunodeficiency and immunodysregulation, resulting in a trias of higher infection and malignancy rates and more auto-immune phenomena such as rheumatoid arthritis (RA) [29]. This clinical trias is seen in DS as well. The increased susceptibility to respiratory tract infections in DS could – at least in part – be explained by a combination of anatomic and functional ear-nose-throat abnormalities, hypotonia, mental retardation and increased incidence of gastro-oesophageal reflux, but these cannot explain the increased auto-immune phenomena and hematological malignancies [30]. Although patients with DS have the same clinical trias of increased infections, malignancies and auto-immune diseases as elderly people, the actual pathogens and disease-burden are to some extent different [31]. In DS a higher frequency of mainly hypothyroidism [33], celiac disease [34] and diabetes mellitus type 1 [35] is seen. Malignancies in DS consist mostly of hematological ones in contrast to elderly with an increase in non-hematological malignancies as well [36].

Progeria syndromes

Progeria syndromes (PS) are very rare diseases, leading to precocious and/or accelerated aging [7]. PS form a clinically and genetically heterogenous group. Most PS are only segmental in nature; they do not cause early or accelerated aging in all human cell lines. Also, humans with PS can present with symptoms that are not common during normal aging [7].

PS can be further subdivided on the basis of the underlying genetic defect [7]. Hutchinson-Gilford progeria syndrome (HGPS) is the first described PS; LMNA gene mutation results in accumulation of lamin A protein in the cell nucleus. Werner syndrome – also called adult-onset progeria – is caused by a mutation in ATP dependent helicase leading to repair defects in DNA-double-strand breaks, but with slower onset than HGPS. Cockayne syndrome (CS) and xeroderma pigmentosum (XP) are examples of DNA-repair defect disorders causing progeria-like disease. Because of the segmental nature of most PS, it is not always clear whether and to what extent hematological cell lines are influenced [7]. The immune system in these progerias has only been studied in case reports and small cohort studies. In a case report of a child with HGPS [37] increased B-lymphocyte numbers with severely decreased IgG and IgA serum levels were seen. In 12 adults with HGPS however normal levels of Igs and normal B- and T-lymphocyte counts were found (including Th and Tc) [38]. Five adults with Werner syndrome showed normal immunoglobulin levels, normal relative B-lymphocyte and normal to decreased relative T-lymphocyte counts [39]. Neoplasms and infections were found to be more frequent in Werner syndrome, which could be interpreted as clinically suggestive of immunosenescence. However, these neoplasms (e.g. sarcomas and melanomas) do not always overlap with commonly occurring neoplasms in aging [40].

XP patients have normal T-lymphocyte (including Tc and Th) counts, but decreased natural killer (NK) activity [41–43]. CS patients show normal T-lymphocyte counts; their NK-activity is normal [41]. XP patients have an increased risk for skin cancer, but apparently no increased risk for infections and auto-immune diseases. CS patients have no associated increased cancer risk, possibly due to normal NK-cell and T-lymphocyte related immune surveillance [41].

More research is needed, but it seems that the picture in progeria and progeria-like syndromes is not comparable to normal ‘immunosenescence’; it is not the same in the different subtypes of PS and moreover is very different from the picture in DS.

Conclusion

At first sight, the DS profile seems to fit in with precocious immunosenescence, as thymic involution with low thymic output and T-lymphocyte dysregulation - resulting in higher rates of infections, malignancies and auto-immune disease - occur both in DS and normal aging. Appearances however can be deceptive.

The decreased naive B- and T-lymphocyte production from birth onwards combined with the lack of compensatory memory cell expansion and proliferation do not support precocious immunosenescence in DS; these findings fit intrinsic immunodeficiency better. The hypergammaglobulinemia in combination with decreased specific antibody responses support the theory that patients with DS harbor a combined T- and B-lymphocyte immunodeficiency with different mechanisms involved than in normal aging. Progeria syndromes are not comparable to DS either. PS show segmental aspects of accelerated/precocious aging at most reflected in minor lymphocyte changes. Also, the PS clinical profile differs from DS with respect to the occurrence of malignancies, infections and auto-immune diseases.

Therefore, we challenge the hypothesis that the immunological abnormalities in Down syndrome should be interpreted as precocious immunosenescence. More genetic and immunological research is needed to study the true nature of the effect of an extra chromosome 21 on the immune system in DS.