Changes of CD14 and CD1a expression in response to IL-4 and granulocyte-macrophage colony-stimulating factor are different in cord blood and adult blood monocytes

Neonates are relatively immature in their immune response; thus, to further clarify the differences of monocyte function and differentiation between neonates and adults, we investigated their CD14(+)CD4(+) and CD14(+)CD16(+) monocyte subpopulations, production of IL-1beta and tumor necrosis factor-alpha induced by lipopolysaccharide, and their CD14 and CD1a phenotypic changes in response to IL-4 and granulocyte-macrophage colony-stimulating factor. Our results showed that 1) the expression of CD14 in cord blood monocytes was significantly lower than that in adult peripheral blood monocytes; 2) both the percentages of CD14(+)CD4(+) cells and CD14(+)CD16(+) cells among CD14(+) monocytes were also significantly lower in cord blood; 3) after stimulation by lipopolysaccharide for 72 h, production of both IL-1beta and tumor necrosis factor-alpha was lower in cord blood than that in adult peripheral blood; and 4) in response to IL-4 or GM-CSF, the phenotype development of CD14 and CD1a in cord blood and adult peripheral blood was different. Down-regulation of CD14 expression in response to IL-4 and GM-CSF was slower in cord blood monocytes than that in adult peripheral blood monocytes. After 9 d of culture in the presence of IL-4 and GM-CSF, the percentage of CD1a(+) monocytes was significantly more increased in cord blood than that in adult peripheral blood. The reduced expression of CD14 and other mature phenotype markers such as CD16 and CD4 as well as the reduced IL-1beta and tumor necrosis factor-alpha production may contribute to the impaired immune response of neonates. Slower down-regulation of CD14 by IL-4 and GM-CSF suggests that differential properties of cord blood monocytes in response to cellular stress signals take a longer time than those of adult peripheral blood monocytes.     

Monocyte switch in neonates: high phagocytic capacity and low HLA-DR expression in VLBWI are inverted during gestational aging

Pre-term neonates are at high risk to develop early-onset sepsis which possibly is caused by an immature immune system. Monocytes play a pivotal role as professional phagocytic and antigen-presenting cells in the innate immunity. In the present study, we investigated in monocytes from cord blood the expression of human leukocyte antigen (HLA)-DR as a marker for antigen-presenting capability, the expression of the high-affinity receptor for IgG (FcgammaRI/CD64), and the capacity to phagocytize non-opsonized Escherichia coli. We compared 70 infants in three groups according to their gestational age (group I: 20 very low birth weight infants (VLBWI), 24-31 weeks of gestation; group II: 25 pre-term infants, 32-36 weeks of gestation, and group III: 25 term neonates). The expression of CD64 as well as the phagocytic capacity of monocytes from cord blood were highest in VLBWI (p < 0.05 and p < 0.01, respectively). In contrast, HLA-DR expression was significantly (p < 0.05) diminished in VLBWI, which possibly leads to a reduced antigen-presenting capacity. We conclude that monocytes have different functional properties during gestational aging, which perhaps participate in the high incidence of infections in VLBWI.     

Developmental biology of the dendritic cell system

Aim : To determine whether an imbalance of dendritic cell subsets might contribute to diminished adaptive host responses observed in newborn infants. It was hypothesized that the proportion of lymphoid dendritic cells would be greater than that of myeloid dendritic cells in cord blood. Methods : To investigate this, dendritic cell subsets were evaluated in whole cord blood by flow cytometry. Circulating dendritic cells were also isolated from cord blood based on CD1c and BDCA-2 expression. Myeloid dendritic cells were also obtained by culturing cord and adult blood monocytes. Surface phenotypes of these cells were determined by flow cytometry using monoclonal antibodies directed against lineage, major histocompatibility, adhesion, co-stimulation and cytokine receptor molecules. Antigen-presenting functions of dendritic cell subsets were determined by mixed leukocyte reactions. Results : Circulating myeloid dendritic cells were higher in cord blood than previously reported in adult blood, whereas lymphoid dendritic cell numbers were similar between cord and adult blood. Expression of CD11c, CD45RA and CD45RO did not accurately differentiate between dendritic cell subsets circulating in cord blood. Fresh and culture-derived cord blood myeloid dendritic cells stimulated adult allogeneic leukocyte proliferation, while lymphoid dendritic cells were less effective inducers of an adult allogeneic leukocyte response. Culture-derived dendritic cells induced modest autologous cord blood leukocyte proliferation, but freshly isolated myeloid and lymphoid dendritic cells did not stimulated autologous leukocytes.
Conclusion : Contrary to the hypothesis, an imbalance in the ratio of circulating myeloid to lymphoid dendritic cell subsets does not exist and, therefore, does not contribute to diminished adaptive immune responses in newborn infants.     

Developmental biology of the dendritic cell system

AIM: To determine whether an imbalance of dendritic cell subsets might contribute to diminished adaptive host responses observed in newborn infants. It was hypothesized that the proportion of lymphoid dendritic cells would be greater than that of myeloid dendritic cells in cord blood. METHODS: To investigate this, dendritic cell subsets were evaluated in whole cord blood by flow cytometry. Circulating dendritic cells were also isolated from cord blood based on CD1c and BDCA-2 expression. Myeloid dendritic cells were also obtained by culturing cord and adult blood monocytes. Surface phenotypes of these cells were determined by flow cytometry using monoclonal antibodies directed against lineage, major histocompatibility, adhesion, co-stimulation and cytokine receptor molecules. Antigen-presenting functions of dendritic cell subsets were determined by mixed leukocyte reactions. RESULTS: Circulating myeloid dendritic cells were higher in cord blood than previously reported in adult blood, whereas lymphoid dendritic cell numbers were similar between cord and adult blood. Expression of CD11c, CD45RA and CD45RO did not accurately differentiate between dendritic cell subsets circulating in cord blood. Fresh and culture-derived cord blood myeloid dendritic cells stimulated adult allogeneic leukocyte proliferation, while lymphoid dendritic cells were less effective inducers of an adult allogeneic leukocyte response. Culture-derived dendritic cells induced modest autologous cord blood leukocyte proliferation, but freshly isolated myeloid and lymphoid dendritic cells did not stimulated autologous leukocytes. CONCLUSION: Contrary to the hypothesis, an imbalance in the ratio of circulating myeloid to lymphoid dendritic cell subsets does not exist and, therefore, does not contribute to diminished adaptive immune responses in newborn infants.     

Review of Human Dendritic Cells: Isolation and Culture from Precursors

There is no single characteristic or marker that identifies a dendritic cell. This review of the methods used for dendritic cell identification stresses that changes occur over the lifetime and changing functional status of the cell. Human dendritic cell populations have been obtained from adult peripheral blood, umbilical cord blood, bone marrow, thymus, and monocytes as starting substrates. Most recently dendritic cell populations have been grown from separated hematopoietic precursors, suggesting that there is a common granulocyte-monocyte-dendritic cell progenitor. Whether monocytes and dendritic cells can be modulated back and forth remains an open question. Granulocyte-monocyte colony stimulating factor (GM-CSF) appears to be critical to the development and function of these enriched and cultered cells. The characteristics of the cells produced by these various methods are tabulated. Important to the understanding of Langerhans cell biology is the variation in CD1 expression under different circumstances.     

Cytokine expression of cord and adult blood mononuclear cells in response to Streptococcus agalactiae

Neonatal bacterial sepsis is often characterized by a fulminant clinical course and highly elevated plasma levels of proinflammatory cytokines. To evaluate in vitro activation of the neonatal immune system by specific infectious stimuli, cord blood cells from healthy neonates were examined for expression of tumor necrosis factor-alpha (TNF-alpha), IL-1beta, IL-6, and IL-8 in response to Streptococcus agalactiae (GBS), lipopolysaccharide (LPS), and lipoteichoic acid (LTA). Cytokine-expression was compared in mononuclear cells from cord and adult peripheral blood. TNF-alpha and IL-6 levels in the supernatant of cord blood cell cultures were significantly higher after stimulation with heat-killed GBS (10(7)/mL) than with LPS (2 microg/mL) or LTA (2 microg/mL) (TNF-alpha: 2215 versus 267.5 versus 40 pg/mL, p = 0.001; IL-6: 9667 versus 4909 versus 919 pg/mL, p = 0.006). mRNA expression of TNF-alpha, IL-1beta, IL-6, and IL-8 was equally pronounced after stimulation with either GBS, LPS, or LTA in cord or adult blood cells at various times. A MAb directed against the monocyte receptor molecule CD14 did not inhibit the release of cytokines in cord blood mononuclear cells after stimulation with GBS. In summary, activation of cord blood cells by infectious stimuli is comparable to the adult immune response in terms of expression of proinflammatory cytokines. GBS in particular proves to be a potent activator of the neonatal immune system when compared with LPS and LTA. CD14 seems not to be a crucial molecule for activation of cord blood cells by GBS.     

树突状细胞在小儿哮喘发病机制中的作用

目的　探讨树突状细胞 (DC)在小儿哮喘发病中的可能作用及机制。方法　以Thomas法 ,采用粒 巨噬细胞集落刺激因子 ,白细胞介素 4 (IL 4 )和肿瘤坏死因子α(TNF α)联合培养诱导哮喘患儿外周血DC细胞 ,进而检测DC协同刺激分子B7 1(CD80 )、B7 2 (CD86)的表达 ,DC诱导自体T淋巴细胞的增殖反应 ,以及其分泌的IL 10、IL 12等水平的变化。结果　哮喘患儿外周血DC诱导自体T淋巴细胞增殖反应明显高于对照组 (cpm值分别为 115 6 0± 12 6和 6 5 39± 12 6 ,t =12 1 6 96 ) ,差异有非常显著意义 (P <0 0 1) ;其分泌IL 10水平 [( 3 6± 0 3) μg/L]明显低于对照组 [( 6 9± 0 8) μg/L],差异有非常显著意义 (t=2 0 6 0 8,P <0 0 1) ;分泌IL 12水平 [( 2 9 7± 8 4 )ng/L]也明显低于对照组 [( 4 5 2± 9 8)ng/L],差异有非常显著意义 (t=5 979,P <0 0 1) ;同时 ,哮喘患儿外周血人类白细胞抗原DR( 19 9± 1 3)和协同刺激分子B7 2 (CD86)的表达水平 ( 36 3± 14 0 )明显高于对照组( 10 6± 1 5和 2 4 7± 8 5 ) ,差异有非常显著意义 (t分别 =2 3 30 1和 3 314 ,P均 <0 0 1)。结论　DC可能通过诱导TH1/TH2细胞分化在小儿哮喘发作时起重要作用。其可能的机制系通过DC分泌的细胞因子而起作用。     

Interleukin-3, interleukin-6, granulocyte-macrophage colony-stimulating factor and erythropoietin cord blood levels of preterm and term neonates

The cascade of known haematopoietic growth factors controlling granulomonopoiesis and erythropoiesis includes interleukin-3 (IL-3), interleukin-6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (EPO). Elevated endogenous IL-3 and IL-6 cord blood levels in infection-free premature and mature neonates may reflect their possible role for expansion of haematopoietic progenitor cells, granulocytes and monocytes. Within the erythroid lineage a synergistic action of IL-3, IL-6 and EPO can be assumed. To identify the regulatory role in fetal haematopoietic expansion cord blood plasma levels of these haematopoietic growth factors were assessed in 19 premature and 20 mature infants using commercial enzyme-linked immunosorbent assay and enzyme-amplified sensitivity immuno assay test kits. Peripheral blood IL-3, GM-CSF and EPO were studied in 5 and 10 premature infants respectively. Compared with cord blood levels we found a decline in EPO levels but no decrease of IL-3 and GM-CSF during the 1st month of life. We conclude that postnatal decrease in plasma burst-promoting activity levels in preterm infants is mainly explained by low postnatal EPO levels.     

Association of cord blood cytokines with prematurity and cerebral palsy

BACKGROUND: Many studies have shown that certain cytokines in amniotic fluids are correlated to premature labor and neonatal brain insults. AIMS: We investigated whether different fetal phagocyte and vascular mediators including IL-8, myeloperoxidase (MPO), PGE(2) and soluble vascular cell adhesion molecule-1 (sVCAM-1) levels were correlated to prematurity and cerebral palsy (CP) of premature infants. SUBJECTS: Umbilical cord blood samples from 96 preterm babies from 2250 cord blood collections were studied. METHOD: The enzyme-linked immunoassay (ELISA) technique was used to determine concentrations of cord blood IL-8, MPO, PGE(2) and sVCAM-1. RESULTS: Preterm babies with gestational age (GA) < or =32 weeks had significant higher cord blood IL-8, MPO and PGE(2) levels than full-term babies. These mediators, however, were not correlated to gestational age, suggesting that increases of these mediators are more related to preterm delivery but not fetal maturation. Further analysis showed that IL-8, a mediator mainly from monocytes, but not MPO, another mediator mostly from granulocytes, was correlated to cerebral palsy. CONCLUSIONS: Taken together, these results suggest that both premature monocytes and granulocytes activation are involved in preterm delivery, but maybe only monocytes activation is correlated to premature infants' cerebral palsy. Selective manipulation of monocytes activation may be useful to prevent premature-related cerebral palsy.     

Regulation of CD31 expression and interleukin-4 production by human cord blood CD4+ T cells with interleukin-4 and interleukin-7

BACKGROUND: T helper 2 cell type cytokines, such as interleukin (IL)-4 and IL-5, play pivotal roles in the development of allergic diseases. However, the mechanism by which naive CD4+ T cells acquire the ability to produce these cytokines remains unclear. Recently, it was reported that IL-7 induces the ability to produce IL-4 as well as interferon (IFN)-gamma and IL-5 in naive CD4+ T cells without TCR stimulation. To further analyze the mechanism of acquiring IL-4-producing ability by naive CD4+ T cells, the effects of IL-7 on human cord blood CD4+ T cells were compared with those of IL-4, which induced the ability to produce IFN-gamma but not IL-4. RESULTS: Interleukin-7 preserved the population of CD4+CD31- T cells in cord blood and induced their IL-4-producing ability without T cell receptor (TCR) stimulation, while IL-4 induced CD31 on CD31- T cells and could not induce their IL-4-producing ability. Both the CD31-inducing effect and the inhibitory priming effect for IL-4-production by IL-4 were also observed after cord blood CD4+ T cells had been primed with IL-7 and acquired the IL-4-producing ability. CONCLUSIONS: Interleukin-7 induced the IL-4-producing ability in naive CD4+CD31- T cells without TCR stimulation, suggesting that the signal transduction via CD31 may have an inhibitory effect on the acquisition of the IL-4-producing ability by cord blood CD4+ T cells in the absence of TCR stimulation.     

The effect of labor on neonatal T-cell phenotype and function

With increasing interest in the role of fetal programming in child and adulthood diseases, and therefore interest in the measurement of various factors at birth, it is essential to ascertain whether the factors of interest show any gestation- or parturition-associated changes. We have investigated whether mode of delivery influenced T-cell phenotype and function (CD4+) as has been described for monocytes and neutrophils. Interferon-gamma production in response to either the mitogen phytohemagglutinin or anti-CD2/CD3/CD28 F(ab')3 was significantly reduced by neonatal mononuclear cells compared with adult cells but did not differ with mode of delivery at term (normal vaginal delivery versus elective lower-segment cesarean section). Likewise, anti-CD2/CD3/CD28-stimulated IL-2 production by the neonate was lower than adult levels but did not differ with mode of delivery. The expression of common T-cell activation markers (CD25, MHC class II, CD69, CD62L, CD11a, CD44, and CD49d) was examined. Only CD62L (L-selectin) expression was significantly different, with fewer adult T cells expressing this surface antigen compared with neonatal T cells (p < 0.0003), and significantly more T cells from lower-segment cesarean section than normal vaginal delivery were positive for CD62L (p = 0.012). sCD62L levels were significantly lower in cord plasma compared with adult plasma but did not differ with mode of delivery. Thus the phenotype and function of cord blood T cells did not differ greatly with mode of delivery, but possible differences for the marker of interest should always be assessed. Furthermore, although there was no significant difference with mode of delivery for all markers, except CD62L, the variation in the normal vaginal delivery samples, as for the adults, was greater than in the lower-segment cesarean section samples, indicating that the effects of length of labor and stress at delivery may well be relevant.     

Postnatal development of monocyte cytokine responses to bacterial lipopolysaccharide

Early childhood is a period of heightened susceptibility to infection due to immaturity of the immune system, and the nature of these developmental deficiencies is only partially understood. In this study, we focused on the ontogeny of the innate immune system by investigating the capacity of mononuclear cells to secrete a wide spectrum of inflammatory cytokines in response to interferon (IFN)-gamma priming and lipopolysaccharide (LPS) stimulation, namely IL-6, IL-10, IL-12, IL-18, IL-23, tumor necrosis factor (TNF)-alpha, and myxovirus resistance protein A, induced by type-I IFN, at several time points between birth (cord blood) and adulthood. Competence to produce all these cytokines followed a similar developmental pattern, with slow postnatal up-regulation from the response observed in cord blood. Unexpectedly, IL-6, IL-10, TNF-alpha, and IFN-gamma showed slow postnatal up-regulation but also elevated cord blood responses equal to or greater than the adult level. This was transient and not observed at 2 mo of age, and was not related to predelivery stress of the newborns. Variations in Toll-like receptor (TLR)4 function may account for these age related differences in cytokine responses, as TLR4 expression on neonatal monocytes post LPS stimulation was elevated and sustained relative to infants and adults.     

The effect of interleukin-12 and interleukin-15 on CD69 expression of T-lymphocytes and natural killer cells from umbilical cord blood

We investigated the effect of two immunoregulatory cytokines interleukin-12 (IL-12) and IL-15, alone or in combination, on CD69 expression of mononuclear cells (MNCs) obtained from umbilical cord blood (CB) and adult peripheral blood (APB). We established that (1) CD3/69, but not CD16/69, expression on CB MNCs could be increased with IL-12, IL-15 or both in 18-hour cultures, but to a lesser degree compared to that on corresponding APB MNCs, (2) CD3/69 expression on CB MNCs was significantly increased after 1 week's culture with IL-12, especially with IL-15, exceeding that on APB MNCs similarly activated and (3) CD16/69 expression on CB MNCs, but not APB MNCs, was greatly increased after 1 week's culture with IL-15. The combination of IL-12 + IL-15 resulted in greater CB CD3/69 expression than individual cytokines, while producing less of an effect on CD16/CD69 expression as compared to IL-15 alone. The results of our study indicate that neonatal T and NK cells readily respond to cytokine stimulation by upregulating CD69 expression, with a greater effect achieved using IL-15 compared to IL-12.     

Granulocyte functions and changes in ability with age in newborns; Report no. 2: Activation of granulocyte functions by cytokines

BACKGROUND: The aim of the present study was to examine the function of granulocytes in newborns from the perspective of granulocyte activation by cytokines. METHODS: Granulocytes were stimulated with tumor necrosis factor-alpha, granulocyte-macrophage colony stimulating factor (GM-CSF) or granulocyte colony stimulating factor, and the reactivity of granulocytes to these substances was then compared between newborns (umbilical cord blood, peripheral blood obtained at age 5 days and at age 1 month) and peripheral blood obtained from adults. In addition, the expression of cytokine receptors on the surface of granulocytes was measured for each age group. RESULTS: The amplification of CD11b expression on the surface of granulocytes and suppression of l-selectin expression were weaker for cord blood regardless of which cytokine was added. In addition, the increases in the activity of intracellular elastase when stimulated with tumor necrosis factor-alpha or GM-CSF were significantly lower for cord blood. Moreover, the expression of GM-CSF receptors and granulocyte colony stimulating factor receptors on the surface of granulocytes was lower for cord blood, and this expression approached the level found in adults as age increased. CONCLUSION: Granulocytes during early infancy exhibit low reactivity to inflammatory cytokines, and this was considered to be one of the factors contributing to the higher incidence of serious bacterial infections in infants.     

人脐血移植重症联合免疫缺陷小鼠的实验研究

为了探讨人脐血移植到小鼠体内的生长特性，将人脐血细胞注入经亚致死剂量照射后的严重联合免疫缺陷（ＳＣＩＤ）小鼠。１０只移植人脐血的小鼠中５只存活达２８天，其造血和部分免疫功能得到恢复。利用人粒－单系祖细胞培养和ＣＤ＋４５细胞流式细胞仪检测及人ＨＬＡ－ＤＰＢ基因聚合酶链反应－寡聚核苷酸单链构像多态性分析等方法，检测出了植入ＳＣＩＤ小鼠骨髓中的人脐血细胞。提示，人脐血细胞可以植入ＳＣＩＤ小鼠并重建其造血和免疫功能。     

Human congenital infection with Trypanosoma cruzi induces phenotypic and functional modifications of cord blood NK cells

We studied the phenotype and activity of cord blood natural killer (NK) cells in newborns congenitally infected with Trypanosoma cruzi. We found that the proportion of CD56(bright) NK cells was significantly decreased in cord blood from these newborns, suggesting they may have been recruited to secondary lymphoid organs. The remaining CD56(bright) NK cells exhibited a defective ability in the production of interferon (IFN)-gamma following in vitro activation with interleukin (IL)-12 + IL-2 or IL-12 + IL-15 cytokines, as compared with NK cells from uninfected newborns. In addition, cord blood NK cells from congenitally infected newborns stimulated with cytokines have a decreased release of granzyme B (GrB) when incubated with K562 target cells. This defect in cytotoxic effector function is associated with a reduced surface expression of activating NK receptors (NKp30, NKp46, and NKG2D) on CD56(dim) NK cells compared with uninfected newborns. These alterations of fetal NK cells from congenitally infected newborns may reflect a down-regulation of the NK cell response after an initial peak of activation and could also be the result of T. cruzi modulating the immune response.     

脐血造血细胞含量与白血病脐血移植疗效分析

目的分析4711份库存脐血造血细胞含量及探讨脐血造血细胞含量与白血病脐血移植疗效的关系。方法分析4711例库存脐血总有核细胞数(TNC)和CD34+细胞数分布情况,探讨不同的造血细胞输入量、供受者HLA不相合数、受者性别、年龄、体重和疾病类型间植入率和生存率的差异。结果 4711例库存脐血TNC和CD34+细胞中位数分别为1.14×109/kg和4.06×106/kg,按3.7×107/kg有效TNC输入量计算,93.2%脐血可供体重50 kg以下受者移植。89例白血病患者移植后植入75例,植入率为84.3%。中性粒细胞绝对值≥0.5×109/L、血小板≥20×109/L和≥50×109/L的时间分别为移植后17、34和46 d。75例植入病例中,长期无病存活47例,死亡26例,2例复发;急性移植物抗宿主病(GVHD)Ⅰ～Ⅱ度、Ⅲ～Ⅳ度和慢性GVHD发生率分别为54.7%、20.0%、9.3%。影响移植植入率的因素包括受者年龄、TNC和CD34+细胞输入量;影响生存率的因素包括受者年龄、体重和输入CD34+细胞数。结论在无法找到HLA全相合骨髓供者时,可选择脐血作为替代骨髓的造血干细胞来源治疗儿童与成人白血病,TNC和CD34+细胞数仍是选择脐血移植物的参考指标。     

Increased respiratory burst and increased expression of complement receptor-3 (CD11b/CD18) and of IL-8 receptor-A in neutrophil granulocytes from newborns after vaginal delivery

To study neutrophil activation in cord blood as a function of the mode of delivery, we performed analysis of the function of neutrophil granulocytes by assessing their ability to produce reactive oxygen products (ROP) as well as neutrophil cell surface expression of CD11b/CD18 and interleukin (IL)-8 receptors quantified with flow cytometry. Plasma levels of granulocyte colony-stimulating factor (G-CSF) were measured using an immunoassay. Neutrophil granulocytes were derived from cord blood of term newborns delivered vaginally (n = 20) and by cesarean section (n = 10), and, for comparison, from adult peripheral blood (n = 15). Blood neutrophil counts and the capacity of neutrophil granulocytes to produce ROP in response to stimulation with Escherichia coli was increased in newborns after vaginal delivery as compared to newborns delivered by cesarean section. The level of expression of the adhesion molecule/complement receptor CD11b/CD18 and the chemokine receptor IL-8 RA was also higher after vaginal delivery. Plasma concentrations of G-CSF in cord blood of newborns were higher than those of adults with no difference detectable between vaginal delivery and cesarean section. The data demonstrate a higher functional responsiveness and a higher expression level of functionally important receptors in neutrophils after vaginal delivery possibly due to activation of neutrophil granulocytes during labor.     

Two different maturational stages of natural killer lymphocytes in human newborn infants.

The objective of this study was to assess the basis for the diminished natural killer (NK) lymphocyte activity of neonates. We found either severely reduced (63% of 68 neonates) or normal (similar to healthy adult) levels of NK activity. The percentages of cord blood mononuclear cells from the two groups of infants that expressed CD16, a differentiation antigen found in NK cells, were similar and within the range found in peripheral blood mononuclear cells of adults. However, infants with low NK activity had reduced numbers of cells in the CD16+56+ subpopulation, whereas the number of these effector cells present in cord blood mononuclear cells from infants with normal NK activity was within the range found in adults. Recombinant interleukin-2, but not recombinant interferon-gamma, normalized the low NK activity of infants in a dose- and time-dependent manner. Analysis of the pattern of target cell susceptibility to lysis, together with the CD16+CD3- phenotype of the precursor and effector lymphocytes, demonstrated that the induced cytotoxicity was mediated by NK cells. In contrast, NK cells from infants with normal cytotoxic levels exhibited a functional response to interleukin-2 and interferon-gamma similar to that of adults. Our results indicate that NK cells in human neonates go through two different maturational stages.