Down syndrome: immunologic and epidemiologic associations-enigmas remain

Changes in the peripheral blood cell count in pediatric patients with Down syndrome

Objectives

Down syndrome (DS) is associated with multiple complications, including a high risk of leukemia and thyroid dysfunction. This clinical study aimed to examine the complete blood cell count in patients with DS without leukemia or transient abnormal myelopoiesis. We also aimed to evaluate the effect of thyroid dysfunction on hematological anomalies in DS.

Methods

We analyzed the peripheral blood cell count in 23 pediatric patients with DS with and without thyroid dysfunction and in 17 pediatric patients without DS with thyroid dysfunction.

Results

Patients with DS showed greater neutrophilia and lymphopenia than did patients with DS and hypothyroidism and patients with hypothyroidism. Surprisingly, patients with DS showed a significant degree of eosinopenia in the peripheral blood. Interestingly, hypothyroidism had an attenuating effect on different lineages in the complete blood count. However, these anomalies were specific for DS.

Conclusions

Our clinical findings support previous data on DS-associated changes in the complete blood count. Our study also shows novel alterations in the complete blood count in leukemia-free patients with DS in association with hypothyroidism. The attenuating effect of thyroid dysfunction on changes in different lineages in the context of DS is novel and deserves further analysis in larger studies.

Haematopoietic development and leukaemia in Down syndrome

Children with constitutional trisomy 21 (cT21, Down Syndrome, DS) are at a higher risk for both myeloid and B-lymphoid leukaemias. The myeloid leukaemias are often preceded by a transient neonatal pre-leukaemic syndrome, Transient Abnormal Myelopoiesis (TAM). TAM is caused by cooperation between cT21 and acquired somatic N-terminal truncating mutations in the key haematopoietic transcription factor GATA1. These mutations, which are not leukaemogenic in the absence of cT21, are found in almost one-third of neonates with DS. Analysis of primary human fetal liver haematopoietic cells and of human embryonic stem cells demonstrates that cT21 itself substantially alters human fetal haematopoietic development. Consequently, many haematopoietic developmental defects are observed in neonates with DS even in the absence of TAM. Although studies in mouse models have suggested a pathogenic role of deregulated expression of several chromosome 21-encoded genes, their role in human leukaemogenesis remains unclear. As cT21 exists in all embryonic cells, the molecular basis of cT21-associated leukaemias probably reflects a complex interaction between deregulated gene expression in haematopoietic cells and the fetal haematopoietic microenvironment in DS.

Down syndrome and arterial ischemic stroke in childhood: a potential immunologic link with selective IgG4 subclass deficiency

We report four children with Down Syndrome (DS) without evidence of congenital heart disease who sustained cerebral infarction in the context of an infectious disease. In one child, stroke occurred in the context of acute infection with Mycoplasma pneumonia. In another child, stroke occurred in the context of Streptococcus oralis (viridans subgroup) infection. In two other children, stroke occurred in the context of a bibasilar pneumonia for which an etiologic agent was not found. All patients had evidence of selective IgG4 subclass deficiency. We followed 8 other children with down syndrome with infectious diseases, but without stroke and a control group of healthy children, and measured the value of IgG4 for each group. We found a statistical significant difference of levels of IgG4 subclass deficiency in the group of stroke, in comparison with the other two groups (P values <0.001). We, therefore, suggest an association between IgG4 subclass deficiency and stroke in DS patients. IgG4 subclass deficiency could conceivably play a role in the high frequency of para-infectious related stroke in this population.

Lymphocyte respiration in children with Trisomy 21

Background

This study measured lymphocyte mitochondrial O₂ consumption (cellular respiration) in children with trisomy 21.

Methods

Peripheral blood mononuclear cells were isolated from whole blood of trisomy 21 and control children and these cells were immediately used to measure cellular respiration rate. [O₂] was determined as a function of time from the phosphorescence decay rates (1/τ) of Pd (II)-meso-tetra-(4-sulfonatophenyl)-tetrabenzoporphyrin. In sealed vials containing lymphocytes and glucose as a respiratory substrate, [O₂] declined linearly with time, confirming the zero-order kinetics of O₂ conversion to H₂O by cytochrome oxidase. The rate of respiration (k, in μM O₂ min^-1), thus, was the negative of the slope of [O₂] vs. time. Cyanide inhibited O₂ consumption, confirming that oxidation occurred in the mitochondrial respiratory chain.

Results

For control children (age = 8.8 ± 5.6 years, n = 26), the mean (± SD) value of k_c (in μM O₂ per min per 10⁷ cells) was 1.36 ± 0.79 (coefficient of variation, Cv = 58%; median = 1.17; range = 0.60 to 3.12; -2SD = 0.61). For children with trisomy 21 (age = 7.2 ± 4.6 years, n = 26), the values of k_c were 0.82 ± 0.62 (Cv = 76%; median = 0.60; range = 0.20 to 2.80), p<0.001. Similar results (p<0.000) were obtained after excluding the five trisomy 21 children with elevated serum TSH (values >6.1 mU/L). Fourteen of 26 (54%) children with trisomy 21 had k_c values of 0.20 to 0.60 (i.e., <−2SD). The values of k_c positively correlated with body-mass index (BMI, R >0.302), serum creatinine (R >0.507), blood urea nitrogen (BUN, R >0.535) and albumin (R >0.446).

Conclusions

Children with trisomy 21 in this study have reduced lymphocyte bioenergetics. The clinical importance of this finding requires further studies.

Secondary immunodeficiencies, including HIV infection

Extrinsic factors can adversely affect immune responses, producing states of secondary immunodeficiency and consequent increased risk of infections. These immunodeficiencies, which can be encountered in routine clinical practice, arise from a number of conditions, such as treatment with glucocorticoids and immunomodulatory drugs, surgery and trauma, extreme environmental conditions, and chronic infections, such as those caused by HIV. The most common cause of immunodeficiency is malnutrition, affecting many communities around the world with restricted access to food resources. Protein-calorie deficiency and micronutrient deficiencies have been shown to alter immune responses; of note, recent progress has been made in the influence of vitamin D deficiency in causing failure of immune activation. Other categories of disease that might present with secondary immunodeficiency include metabolic diseases and genetic multisystemic syndromes. The immune defects observed in secondary immunodeficiency are usually heterogeneous in their clinical presentation, and their prognosis depends on the severity of the immune defect. Management of the primary condition often results in improvement of the immunodeficiency; however, this is sometimes not possible, and the risk of infections can be reduced with prompt antimicrobial treatment and prophylaxis.

T lymphocyte maturation is impaired in healthy young individuals carrying trisomy 21 (Down syndrome)

Cytokine production, immune activation, T lymphocytes maturation, and serum IL-7 concentration were examined in 24 youngsters with Down syndrome and no acquired diseases (healthy Down syndrome [12 prepubertal, 13 pubertal]) and 42 age- and gender-matched controls (20 prepubertal, 22 pubertal). Results showed that a complex immune and impairment is present in healthy individuals with Down syndrome in whom interferon gamma, interleukin (IL) IL-10 production, as well as serum IL-7 concentrations and activation markers-bearing T lymphocytes were significantly augmented. Additionally, a complex skewing of post-thymic lymphocyte maturation pathways was observed in patients: significant reduction of CD4+ and CD8+ naive (RA+CCR7+) lymphocytes, significant increase of CD4+ and CD8+ central memory (RA-CCR7+), and terminally differentiated (TD) (RA+CCR7-) lymphocytes.

Genetic susceptibility and resistance to influenza infection and disease in humans and mice

Although genetic risk factors for influenza infection have not yet been defined in people, differences in genetic background and related variation in the response to infection, as well as viral virulence, are all likely to influence both the likelihood of infection and disease severity. However, apart from characterization of viral binding sites in avian and mammalian hosts, relatively little investigation has focused on host genetic determinants of susceptibility or resistance to infection, or the severity of the associated disease in humans or other species. Similarly, the role of genetic background in the generation of an efficacious immune response to either infection or vaccination has not been extensively evaluated. However, genetic influences on susceptibility and resistance to numerous infectious agents and on the resultant host inflammatory and immune responses are well established in both humans and other animals. Mouse-adapted strains of human influenza viruses and the use of inbred strains of laboratory mice have supported extensive characterization of the pathogenesis and immunology of influenza virus infections. Like individual humans, inbred strains of mice vary in their reactions to influenza infection, particularly with regard to the inflammatory response and disease severity, supporting the potential use of these mice as a valuable surrogate for human genetic variation. Relying heavily on what we have learned from mice, this overview summarizes existing animal, human and epidemiologic data suggestive of host genetic influences on influenza infection.