Reduced primary antigen-specific T-cell precursor frequencies in neonates is associated with deficient interleukin-2 production

A Review of the Immunomodulating Components of Maternal Breast Milk and Protection Against Necrotizing Enterocolitis

Breast milk contains immunomodulating components that are beneficial to newborns during maturation of their immune system. Human breast milk composition is influenced by an infant's gestational and chronological age, lactation stage, and the mother and infant's health status. Major immunologic components in human milk, such as secretory immunoglobulin A (IgA) and growth factors, have a known role in regulating gut barrier integrity and microbial colonization, which therefore protect against the development of a life-threatening gastrointestinal illness affecting newborn infants called necrotizing enterocolitis (NEC). Breast milk is a known protective factor in the prevention of NEC when compared with feeding with commercial formula. Breast milk supplements infants with human milk oligosaccharides, leukocytes, cytokines, nitric oxide, and growth factors that attenuate inflammatory responses and provide immunological defenses to reduce the incidence of NEC. This article aims to review the variety of immunomodulating components in breast milk that protect the infant from the development of NEC.

Cutaneous infectious diseases: Kids are not just little people

The changes in immune response that occur with age play a significant role in disease presentation and patient management. Evolution of the innate and adaptive immune systems throughout life, influenced partly by hormonal changes associated with puberty, plays a role in the differences between pediatric and adult response to disease. We review a series of manifestations of dermatologic infectious diseases spanning bacterial, viral, and fungal origins that can be seen in both pediatric and adult age groups and highlight similarities and differences in presentation and disease course. Therapeutic options are also discussed for these infectious diseases, with particular attention to variations in management between these population subgroups, given differences in pharmacokinetics and side effect profiles.

Deficiencies in the CD4+ T-Helper Cell Arm of the Immune System of Neonates and Young Children

Newborns and young children rely on innate immunity to protect against infections until the adaptive immune system matures. Immunization helps facilitate protection, but multiple doses are needed to establish sufficient antibody levels and T-cell-facilitated immune memory. Deficient T-cell activation and function among neonates and young children are primarily present in the CD4⁺ compartment, whereas CD8⁺ T-cell function is at par with adults. CD4⁺ T cells in neonates and young children produce low levels of IFNγ, interleukin (IL)-2, IL-13, IL-5, and IL-17. This inherent deficiency in neonatal and young child CD4⁺ T-cell functionality has been linked to several mechanistic failures: (1) lower sensitivity to T-cell receptor stimulation, (2) increased apoptosis after proliferation, (3) unavailability of antigen for T-cell priming, and (4) inefficient stimulation by relatively immature antigen-presenting cells. In this review, we discuss evidence from infection and vaccination responses that shed light on the various checkpoints possibly involved in delayed maturation of CD4⁺ T-cell activation and function in newborns and young children.

Characterization of the immune response of human cord-blood derived gamma/delta T cells to stimulation with aminobisphosphonate compounds

Vγ9Vδ2 T lymphocytes have been shown to respond to a variety of non-peptide antigens including alkylamines and phosphoantigens. Recently, aminobisphosphonates have also been shown to stimulate this subset of γδ+ T cells. In this study we analyzed the proliferative responses of freshly isolated γδ T lymphocytes obtained from human cord blood when challenged with pyrophosphomonoesters or aminobisphosphonates. Nitrogen-containing aminobisphopsphonates, in contrast to phoshoantigens, readily stimulated expansion of Vδ2Vγ9 cells in human cord blood. Expanded cells displayed an activated mature phenotype, and were capable of producing TNFalpha and IFNgamma but not perforin following secondary stimulation, consistent with the development of a regulatory, as opposed to cytotoxic, phenotype. This approach may provide a useful strategy for a new approach to the treatment of neonatal pathologies.

Natural killer T cells constitutively expressing the interleukin-2 receptor α chain early in life are primed to respond to lower antigenic stimulation

Invariant natural killer T (iNKT) cells are known to constitutively express the high affinity interleukin-2 receptor α chain (CD25) in neonates, but the functional consequence of this phenotype is unknown. Here, we show that high numbers of CD25-expressing iNKT cells are present early in gestation and represent a significant proportion of the developing immune system. Despite their activated phenotype, neonatal iNKT cells express high levels of the Krüppel-like factor-2, a transcription factor associated with quiescent T cells, and require de novo T-cell receptor and CD28 co-stimulation to proliferate. In contrast to bona fide CD4/CD25-expressing regulatory T cells, neonatal iNKT cells do not suppress T-cell responses, indicating that they do not represent an immunosuppressive cell subset. Evidence that neonatal iNKT cells respond to dramatically reduced amounts of CD1d-restricted antigen compared with adult iNKT cells or T cells, and that their proliferation can be induced in the absence of early interleukin-2 suggest that constitutive expression of CD25 'primes' neonatal iNKT cells to respond rapidly to low amounts of antigen. This unique phenotype, which is distinct from adult iNKT cells, as well as other CD25-expressing activated T or regulatory T cells, may be important to ensure stability of a structurally limited peripheral iNKT-cell repertoire early in life.

Immunological response to cytomegalovirus in congenitally infected neonates

Cytomegalovirus (CMV) is the most common cause of congenital infection worldwide and occurs as a result of transplacental transmission of the virus. The human neonate is highly susceptible to infection due to a combination of immaturity of the immune system and antigenic inexperience. This study uses the in vivo model of congenital CMV to examine both the humoral and cell-mediated immune responses in vertically infected neonates and their mothers. Ten pairs of matched neonates and their mothers were evaluated for specific IgM responses to three immunodominant CMV antigens: pp38 (pUL80a), pp52 (pUL44) and pp150 (pUL32). In contrast to conventional enzyme immunoassay (EIA) testing for CMV-specific IgM, which found five of the mothers and four of the neonates to be positive, Western immunoblotting showed all 10 adults and nine newborns to be positive. Eight mothers and nine newborns had serological evidence of primary infection. All neonates showed a response to pp38, an assembly protein, nine responded to the pp52 immediate early antigen but only four had reactivity to the pp150 tegument associated protein. Of the mothers, eight had pp38 reactivity, 10 showed a response to the pp52 antigen and seven to the pp150 antigen. T cell-mediated immunity was assessed by measuring cytokines using a multiplex microarray assay. Levels of interferon (IFN)-gamma were high in both groups [mean +/- standard error of the mean (s.e.m.): neonates = 657 +/- 238 pg/ml, mothers = 1072 +/- 677 pg/ml, pNS]; however, neonates had significantly higher levels of interleukin (IL)-8 (316 +/- 136 pg/ml versus 48 +/- 28 pg/ml, P < 0.005). Similar levels of IL-2, IL-7, IL-10 and IL-12 were measured in both groups, but levels of IL-1alpha, IL-1beta, IL-4, IL-6 and tumour necrosis factor (TNF)-alpha were either absent or low. In response to CMV, neonates and adults mount a predominant T helper 1 (Th1) response, as evidenced by the presence of IL-2, IL-8, IL-12 and IFN-gamma with concomitant lack of IL-4. These findings suggest that the neonate, when presented with infection in utero, is capable of mounting an individual response; however, the lower IFN-gamma and higher IL-8 levels suggest reduced immune responsiveness when compared to their adult counterparts.

Ex vivo expansion of umbilical cord blood stem cells for transplantation: growing knowledge from the hematopoietic niche

Umbilical cord blood transplantation (UCBT) in adults is limited by the small number of primitive hematopoietic stem cells (HSC) in each graft, resulting in delayed engraftment post transplant, and both short- and long-term infectious complications. Initial efforts to expand UCB progenitors ex vivo have resulted in expansion of mature rather than immature HSC, confounded by the inability to accurately and reliably measure long-term reconstituting cells. Ex vivo expansion of UCB HSC has failed to improve engraftment because of resulting defects that promote apoptosis, disrupt marrow homing and initiate cell cycling. Here we discuss the future of ex vivo expansion, which we suggest will include the isolation of immature hematopoietic progenitors on the basis of function rather than surface phenotype and will employ both cytokines and stroma to maintain and expand the stem cell niche. We suggest that ex vivo expansion could be enhanced by manipulating newly discovered signaling pathways (Notch, Wnt, bone morphogenetic protein 4 and Tie2/angiopoietin-1) and intracellular mediators (phosphatase and tensin homolog and glycogen synthase kinase-3) in a manner that promotes HSC expansion with less differentiation. Improved methods for ex vivo expansion will make UCBT available to more patients, decrease engraftment times and allow more rapid immune reconstitution post transplant.

Interleukin-2 in human milk: a potential modulator of lymphocyte development in the breastfed infant

Development of lymphocyte subpopulations and response to antigen exposure will be influenced by the limited ability of neonates to produce cytokines. In the case of cytokines such as interleukin (IL)-2 which are potent T lymphocyte regulators but poorly produced by newborn infants, the supply of cytokines through human milk could alleviate an immunological deficit and potentially aid the maturation of the immune system. We analysed human milk from 52 mothers (15-357 days postpartum) by ELISA to determine levels of aqueous IL-2, as well as production by human milk cells. IL-2 was detectable (>8 pg/mL) in the aqueous phase of 81% of all day 1 samples with no significant difference found in the mean concentration over 3 consecutive days. IL-2 was produced constitutively at detectable levels by 57% of milk cell samples and production was significantly increased by stimulation with Con A (380%). No correlation was found between aqueous and cellular IL-2, however there was a significant correlation between milk aqueous IL-2 and serum IL-2. This is the first report of IL-2 in the aqueous phase of human milk. A supply of exogenous IL-2 in human milk may provide the suckling infant with important immunological signals during a significant stage of T cell development.

Interleukin-15 enhances cytotoxicity, receptor expression, and expansion of neonatal natural killer cells in long-term culture

Newborn infants have a higher susceptibility to various pathogens due to developmental defects in their host defense system, including deficient natural killer (NK) cell function. In this study, the effects of interleukin-15 (IL-15) on neonatal NK cells was examined for up to 12 weeks in culture. The cytotoxicity of fresh neonatal mononuclear cells (MNC) as assayed by K562 cell killing is initially much less than that of adult MNC but increases more than eightfold after 2 weeks of culture with IL-15 to a level equivalent to that of adult cells. This high level of cytotoxicity was maintained for up to 12 weeks. In antibody-dependent cellular cytotoxicity (ADCC) assays using CEM cells coated with human immunodeficiency virus gp120 antigen, IL-15 greatly increased ADCC lysis by MNC from cord blood. IL-15 increased expression of the CD16+ CD56+ NK markers of cord MNC fivefold after 5 weeks of incubation. Cultures of neonatal MNC with IL-15 for up to 10 weeks resulted in a unique population of CD3- CD8+ CD56+ cells (more than 60%), which are not present in fresh cord MNC. These results show that IL-15 can stimulate neonatal NK cells and sustain their function for several weeks, which has implications for the clinical use of IL-15.

Cord blood immunology and stem cell transplantation

Allogeneic stem cell transplantation can be curative in a variety of malignant and nonmalignant disorders. Unfortunately, more than 75% of potential recipients lack a matched family donor. Although 50% of these recipients may find a matched unrelated adult stem cell donor from one of the worldwide registries, the other 50% have had no other viable donor alternatives. Cord blood cellular immunity is immature at birth and allows for a greater human leukocyte antigen disparity between a cord blood donor and recipient after an unrelated cord blood transplant. More than 25 cord blood banks have been developed worldwide to support the growing clinical needs of unrelated cord blood transplantation. Standard operating procedures have been developed for maternal donor screening and consent, cord blood collection, processing, cryopreservation, characterization, shipping, and thawing. Cord blood transplantation after myeloablative and reduced-intensity conditioning has been successfully demonstrated, resulting in long-term full donor chimerism, decreased Grade 3/4 acute graft-versus-host disease and improvements in overall survival. Several areas of ongoing research include ex vivo expansion of cord blood hematopoietic progenitor cells to enhance the rapidity of engraftment and isolation and activation of select immune cell populations for prevention or treatment of acute graft-versus-host disease, infectious complications, and tumor reoccurrence.

Equivalent functional nuclear factor of activated T cell 1 mRNA and protein expression in cord blood and adult T cells

Cord blood (CB) cells are now an important source of stem cells for bone marrow reconstitution as a result of the decreased risk of graft-versus-host disease that was attributed to previously reported absent or diminished nuclear factor of activated T (NFAT) cell 1 expression by CB T cells. This study reexamines NFAT1 expression in CB T cells. CB T cells were examined for NFAT1 mRNA and protein expression. These cells were then stimulated, and NFAT1 translocation and interleukin-2 production were assessed. Our results show that resting CB CD4+ T cells express NFAT1 mRNA and protein at levels similar to their adult counterparts. On stimulation, NFAT1 is translocated into the nucleus where DNA binding can occur, whereas calcineurin inhibition prevents interleukin-2 production. We conclude that differential responsiveness of CB T cells cannot be attributed to differences at the level of NFAT1 expression or its ability to function in these cells but may represent an altered sensitivity to stimulation.

Polyunsaturated fatty acids and T-cell function: implications for the neonate

Infant survival depends on the ability to respond effectively and appropriately to environmental challenges. Infants are born with a degree of immunological immaturity that renders them susceptible to infection and abnormal dietary responses (allergies). T-lymphocyte function is poorly developed at birth. The reduced ability of infants to respond to mitogens may be the result of the low number of CD45RO+ (memory/antigen-primed) T cells in the infant or the limited ability to produce cytokines [particularly interferon-y, interleukin (IL)-4, and IL-10. There have been many important changes in optimizing breast milk substitutes for infants; however, few have been directed at replacing factors in breast milk that convey immune benefits. Recent research has been directed at the neurological, retinal, and membrane benefits of adding 20:4n-6 (arachidonic acid; AA) and 22:6n-3 (docosahexaenoic acid; DHA) to infant formula. In adults and animals, feeding DHA affects T-cell function. However, the effect of these lipids on the development and function of the infant's immune system is not known. We recently reported the effect of adding DHA + AA to a standard infant formula on several functional indices of immune development. Compared with standard formula, feeding a formula containing DHA + AA increased the proportion of antigen mature (CD45RO+) CD4+ cells, improved IL-10 production, and reduced IL-2 production to levels not different from those of human milk-fed infants. This review will briefly describe T-cell development and the potential immune effect of feeding long-chain polyunsaturated fatty acids to the neonate.

Deficient IL-12(p35) gene expression by dendritic cells derived from neonatal monocytes

To gain insight into the defects responsible for impaired Th1 responses in human newborns, we analyzed the production of cytokines by dendritic cells (DC) derived from cord blood monocytes. We observed that neonatal DC generated from adherent cord blood mononuclear cells cultured for 6 days in the presence of IL-4 and GM-CSF show a phenotype similar to adult DC generated from adherent PBMC, although they express lower levels of HLA-DR, CD80, and CD40. Measurement of cytokine levels produced by neonatal DC upon stimulation by LPS, CD40 ligation, or poly(I:C) indicated a selective defect in the synthesis of IL-12. Determination of IL-12(p40) and IL-12(p35) mRNA levels by real-time RT-PCR revealed that IL-12(p35) gene expression is highly repressed in stimulated neonatal DC whereas their IL-12(p40) gene expression is not altered. The addition of rIFN-gamma to LPS-stimulated newborn DC restored their expression of IL-12(p35) and their synthesis of IL-12 (p70) up to adult levels. Moreover, we observed that neonatal DC are less efficient than adult DC to induce IFN-gamma production by allogenic adult CD4(+) T cells. This defect was corrected by the addition of rIL-12. We conclude that neonatal DC are characterized by a severe defect in IL-12(p35) gene expression which is responsible for an impaired ability to elicit IFN-gamma production by T cells.

Lower proportion of CD45R0+ cells and deficient interleukin-10 production by formula-fed infants, compared with human-fed, is corrected with supplementation of long-chain polyunsaturated fatty acids

BACKGROUND

The immune consequences of adding 20:4n-6 and 22:6n-3 fatty acids to preterm infant formula are not known.

METHODS

The effect of feeding preterm infants (14-42 days of age) human milk (Human Milk group), infant formula (Formula group), or formula with added long-chain polyunsaturated fatty acids 20:4n-6 and 22:6n-3 (Formula + LCP group) on isolated peripheral blood lymphocytes (by flow cytometry) and lipid composition (by gas-liquid chromatography) was determined. Lymphocytes were stimulated in vitro with phytohemagglutinin to measure soluble interleukin (sIL)-2R and IL-10 production (by enzyme-linked immunosorbent assay).

RESULTS

With age, the percentage of CD3+ CD4+ T cells and the percentage of CD20+ cells increased in the Human Milk and Formula + LCP groups (P < 0.05), but not in the unsupplemented Formula group. Compared with the Formula group, CD4+ cells from the Formula + LCP and Human Milk groups expressed more CD45R0 (antigen mature) and less CD45RA (antigen naive) at 42 days of age (P < 0.05). At 42 days, IL-10 production was lower (P < 0.05) in cells of the Formula group than in cells of the Human Milk group. Production of IL-10 by the cells of the Formula + LCP group was not different from that produced by the Human Milk group cells. An age-related decrease (P < 0.05) in sIL-2R production by Formula + LCP lymphocytes was observed, but sIL-2R production at 42 days in the Formula + LCP group did not differ significantly from that in the Human Milk group. Compared with Formula alone, adding LCP to formula resulted in a lower C18:2n-6 and higher C20:4n-6 content in lymphocyte phospholipids (P < 0.05).

CONCLUSIONS

Adding LCP to a preterm infant formula resulted in lymphocyte populations, phospholipid composition, cytokine production, and antigen maturity that are more consistent with that in human milk-fed infants. This may affect the ability of the infant to respond to immune challenges.

CD4(+) Epstein-Barr virus-specific cytotoxic T-lymphocytes from human umbilical cord blood

Umbilical cord blood (CB) is increasingly used for allogeneic hematopoietic stem cell transplantation. To determine whether viral antigen-specific cytotoxic T-lymphocytes (CTL) could be generated from the predominantly naive T-cell populations in CB, CB-derived mononuclear cells were stimulated with autologous Epstein-Barr virus (EBV) transformed B-lymphoblastoid cell lines over several weeks in the presence of recombinant human interleukin-2 (IL-2). By 28 days of culture, T-lymphocytes from all six CB that had been treated with IL-2 displayed EBV-specific cytotoxicity. These cells were largely CD4(+), with complete inhibition of cytotoxicity by anti-CD3 and variable inhibition by anti-HLA DR monoclonal antibodies. The EBV-specific effectors were cloned by limiting dilution, and most of the CTL clones were CD4(+). The cytotoxicity of the CB-derived CD4(+) CTL clones was inhibited by EGTA but not by anti-Fas ligand mAb, suggesting that this cytotoxicity was mediated by perforin/granzyme B. These data indicate that virus-specific CTL can be cultivated and cloned from CB, a human T-cell source that may not have prior in vivo antigenic exposure or reactivity. This finding may have applications in adoptive immunotherapy to recipients of CB transplants.

Difference in response of NK cell activity in newborns and adult to IL-2, IL-12 and IL-15

The killing activity of cord blood mononuclear cells (cMNC) against cytomegalovirus (CMV)-uninfected and -infected fibroblasts was comparable to that of adult peripheral blood mononuclear cells (aPBMC). The killing activity of cMNC against K562 cells was significantly lower compared with that of aPBMC. Treatment of cMNC and aPBMC with interleukin-2 (IL-2), IL-12 or IL-15 significantly enhanced killing activity against K562 cells and CMV-uninfected and -infected cells. By comparison of cMNC with aPBMC, killing activity against the K562 cells of cMNC was augmented to the level of aPBMC when cultured with IL-2, IL-12 or IL-15. The killing activity of cMNC against CMV-uninfected and -infected fibroblasts did not increase to the level of adult PBMC by treatment with IL-2, IL-12 or IL-15. These data suggest that cord blood contains a functionally different NK cell subpopulation than that among adult NK cells.

Cellular immune responses to Neisseria meningitidis in children

There is an urgent need for effective vaccines against serogroup B Neisseria meningitidis. Current experimental vaccines based on the outer membrane proteins (OMPs) of this organism provide a measure of protection in older children but have been ineffective in infants. We postulated that the inability of OMP vaccines to protect infants might be due to age-dependent defects in cellular immunity. We measured proliferation and in vitro production of gamma interferon (IFN-gamma), tumor necrosis factor alpha, and interleukin-10 (IL-10) in response to meningococcal antigens by peripheral blood mononuclear cells (PBMCs) from children convalescing from meningococcal disease and from controls. After meningococcal infection, the balance of cytokine production by PBMCs from the youngest children was skewed towards a TH1 response (low IL-10/IFN-gamma ratio), while older children produced more TH2 cytokine (higher IL-10/IFN-gamma ratio). There was a trend to higher proliferative responses by PBMCs from older children. These responses were not influenced by the presence or subtype of class 1 (PorA) OMP or by the presence of class 2/3 (PorB) or class 4 OMP. Even young infants might be expected to develop adequate cellular immune responses to serogroup B N. meningitidis vaccines if a vaccine preparation can be formulated to mimic the immune stimulus of invasive disease, which may include stimulation of TH2 cytokine production.

Low CD3+CD28-induced interleukin-2 production correlates with decreased reactive oxygen intermediate formation in neonatal T cells

The capacity of neonatal T cells to secrete interleukin-2 (IL-2) has been reported to be variable. We analysed IL-2 production in purified neonatal and adult T cells using polyclonal activator phorbol ester + calcium ionophore (PDBu + iono) or receptor-mediated anti-CD3/anti-CD3+ anti-CD28 stimulation. PDBu + iono induced equally high IL-2 levels in both groups and, when stimulated with plate-bound anti-CD3 monoclonal antibody (mAb), the IL-2 secretion by neonatal cells was undetectable and adult cells produced low amounts of IL-2 (mean 331 +/- 86 pg/ml). The addition of anti-CD28 mAb to anti-CD3-stimulated cells markedly increased IL-2 production in both cell types, but levels of IL-2 in neonatal T cells remained clearly lower than those of adult T cells (respective mean values: 385 +/- 109 pg/ml and 4494 +/- 1199 pg/ml). As NF-kappa B is a critical transcription factor in the control of IL-2 expression, we next analysed its nuclear translocation in neonatal and adult T cells using the electrophoretic mobility shift assay and, because induction of reactive oxygen intermediates (ROI) is required for the activation of NF-kappa B, we also analysed levels of intracellular ROI in these cells using the ROI-reactive fluorochrome DCFH-DA and flow cytometry. In neonatal T cells NF-kappa B activation and ROI formation after anti-CD3 stimulation were low compared with adult T cells and, although addition of anti-CD28 mAb increased induction of NF-kappa B and ROI formation, levels similar to those of adults were not achieved. After PDBu + iono stimulation, the cells showed similar ROI formation and IL-2 secretion. Our results suggest that reduced IL-2 production by neonatal T cells is specific for anti-CD3 and anti-CD3+ anti-CD28-mediated stimulation and that these activators cannot effectively activate the ROI-NF-kappa B signalling pathway in neonatal T cells.

Interleukin (IL)-15 enhances antibody-dependent cellular cytotoxicity and natural killer activity in neonatal cells

Interleukin (IL)-15 is a novel cytokine that is very similar to IL-2 in receptor specificity and biological activities. We compared the ability of IL-15 and IL-12 to enhance the cytotoxicity of neonatal (cord blood) and adult mononuclear cells (MNC) in both natural killer (NK) and antibody-dependent cellular cytotoxicity (ADCC) assays. Incubation with IL-15 (10 ng/ml) or IL-12 (1 ng/ml) for 18 h enhanced the NK activity (using K562 target cells) of both cord and adult MNC, increasing cord cell cytotoxicity threefold. Similar enhancement was seen in ADCC assays using erythrocyte targets and NK-resistant CEM cells coated with HIV gp-120 antigen. Incubation of cord cells with IL-15 or IL-12 for 1 week increased both NK and ADCC, although the combination produced less of an effect than either cytokine alone. IL-15 also increased the percentage of CD16+/CD56+ cells after 1 week incubation. This enhancement of NK and ADCC activities and the number of NK cells by IL-15 suggests it may be clinically useful in treating immunodeficient patients.

Enhancement of antibody-dependent cellular cytotoxicity of neonatal cells by interleukin-2 (IL-2) and IL-12

Newborn infants are more susceptible to infections due in part to deficiencies in the cytotoxic functions of their lymphocytes. We investigated the ability of interleukin-2 (IL-2) and IL-12 to enhance the cytotoxicity of neonatal (cord blood) and adult mononuclear cells (MNCs) in both natural killer (NK) cell and antibody-dependent cellular cytotoxicity (ADCC) assays. The cytotoxic activity of cord blood MNCs was less than 50% that of adult MNCs in most assays prior to exposure to cytokines. Incubation with IL-2 (100 U/ml) or IL-12 (1 ng/ml) for 18 h increased the NK cell activity (using K562 target cells) of both cord blood and adult MNCs, and the combination of IL-2 and IL-12 increased cord blood cytotoxicity threefold, making the cytotoxicity of cord blood cells equivalent to that of adult cells treated with the same cytokines. In ADCC assays with chicken erythrocyte targets, the combination of IL-2 and IL-12 increased the cytotoxicities of both cord blood and adult MNCs, with greater enhancement again seen with cord blood cells. In assays with NK cell-resistant CEM cells coated with human immunodeficiency virus (HV) gp120 antigen in the presence of hyperimmune anti-HIV immunoglobulin, ADCC of cord blood MNCs was about 50% that of adult MNCs; ADCC of cord blood MNCs increased two- to threefold with the addition of IL-2 and IL-12, whereas ADCC of adult MNCs did not increase. Incubation of cord blood cells, but not adult cells, with IL-2 or IL-12 for 1 week increased the percentage of CD16+/CD56+ cells two- to fivefold and enhanced ADCC activity. Thus, IL-2 and IL-12 greatly enhance both the NK cell and ADCC activities of neonatal MNCs and increase the number of NK cells in longer-term culture.

Cytokine production differs in children and adults

The susceptibility of normal, healthy children to infection has long been recognized, but the underlying mechanisms are poorly understood. As adequate cytokine production is crucial for optimal immune responses, we assessed antigen and mitogen-induced cytokine production in healthy children. Our results demonstrate that healthy children differ markedly compared with adults in their ability to produce cytokines (IL-2, interferon-gamma, IL-4, and IL-6). Maximal stimulation with mitogen demonstrated impaired cytokine production with markedly lower levels of all four cytokines produced compared with adult levels. When stimulated with antigens, median levels of IL-2 and IL-4 remained lower than adult values, IL-6 production was increased as was interferon-gamma, albeit not significantly. Although the study was carried out on peripheral blood mononuclear cells that represent a restricted compartment of the immune system, these data suggest that, in healthy children, cytokine production is decreased and/or altered and could result in a suboptimal immune response, which could be one of the factors underlying increased susceptibility to infection in children.

Maturation of the mitogen responsiveness, and IL2 and IL6 production by neonatal swine leukocytes

The maturation of the immune system of neonatal piglets was studied by following changes in the phenotypic composition and function of blood-borne leukocytes. The proportion of mature T and B lymphocytes decreased in the first week of birth and the circulating cells had poorly developed capacities to respond to mitogens and to secrete interleukins. From the end of the first week, however, there was a steady increase in the proportion of mature T cells (CD4+ and CD8+) and B cells in blood until 6-7 weeks after birth, when the study was ended. By 3-4 weeks, the relative proportions of different lymphocyte subsets resembled an adult-type composition. As they increased in prevalence, lymphocytes also developed capacities to proliferate and secrete interleukins. Proliferative responses to T-cell and B-cell mitogens reached adult levels within 2 weeks and 4-5 weeks, respectively. Blood leukocytes produced large quantities of IL6 by 1-2 weeks after birth and IL2 by 2-3 weeks. In contrast to lymphocyte patterns, the myeloid and granulocyte lineages were dominant at birth but then declined steadily. Unlike lymphocytes, the monocytes, macrophages and granulocytes appeared to be fully functional from the time of birth and exhibited a strong oxidative burst after appropriate stimulations. The magnitude of this response remained constant over the first 6-7 weeks. These results indicate that the first 3-4 weeks of post-natal life are a particularly susceptible interval for newborn piglets because constitutive and functional components necessary for specific cellular immune responses remain immature. This deficit may be offset by non-specific cellular mechanisms and maternally derived antibodies.

Cord blood CD4+ CD45RA+ T cells achieve a lower magnitude of activation when compared with their adult counterparts

Highly purified CD4+ CD45RA+ cells from cord blood and peripheral blood from healthy adults were studied. The levels of expression of the CD2, CD3, CD4 and CD28 antigens were similar; however, CD45 and CD45RA antigen expression were slightly lower in cord cells. The reduced expression of the CD45RA antigen on cord CD4+ T cells was confirmed in whole blood. Functional assessment revealed deficiencies in cord CD4+ CD45RA+ T cells. Interleukin-2 (IL-2) production in response to specific triggering via CD2 monoclonal antibody (mAb) alone, or CD2 mAb in combination with CD28 mAb showed marked underproduction (about 10% of adult production). When CD25 expression was examined, it was observed that the proportion of activated CD4+ CD45RA+ T cells in cord blood was lower than in adult (about 20% of adult expression). Proliferation to CD2 mAbs or CD2 + 28 mAbs of cord blood native cells was similarly depressed. Investigation of IL-2 mRNA expression under these stimulatory conditions paralleled the results observed for CD25 expression, IL-2 production and proliferation. When phorbol 12-myristate 13-acetate (PMA) was added to the cells triggered with CD2 + 28mAbs, the responses examined were enhanced in both cord and adult blood with no significant differences between the groups. These findings suggest that under identical conditions of stimulation, purified cord blood CD4+ CD45RA+ T cells do not acquire similar activation status as their adult counterparts. These findings may help in understanding the reduced graft-versus-host disease (GVHD) observed in cord blood stem cell transplantation.