An In vitro Assessment of Retention Force of ZrO2, Polyether Ether Ketone, and ZrO2-Polyether Ether Ketone Telescopic Attachment for Mandibular Overdentures.

Objectives: The objective of this study was to compare ZrO2, polyether ether ketone (PEEK), and ZrO2-PEEK telescopic attachments in terms of retention of overdenture. Methodology: Forty-five acrylic resin models of the lower arch were divided into 3 groups of 15 each. In Group I, primary and secondary crowns were constructed from all zirconia (ZrO2), whereas all PEEK was used for Group II and Group III was made up of ZrO2 PEEK. Results: The mean retention value in Group I was 14.12 ± 3.4 N, in Group II was 15.86 ± 5.1, and in Group III was 22.40 ± 10.3 N. The mean final initial retention value in Group I was 14.50 ± 6.1 N, in Group II was 14.97 ± 8.2, and in Group III was 17.21 ± 9.3 N. A significant difference was observed in intergroup comparison (P < 0.05). Conclusion: Zirconia resulted in maximum retention as compared to other telescopic crown materials.

Protein 4.1R-deficient mice are viable but have erythroid membrane skeleton abnormalities.

A diverse family of protein 4.1R isoforms is encoded by a complex gene on human chromosome 1. Although the prototypical 80-kDa 4.1R in mature erythrocytes is a key component of the erythroid membrane skeleton that regulates erythrocyte morphology and mechanical stability, little is known about 4.1R function in nucleated cells. Using gene knockout technology, we have generated mice with complete deficiency of all 4.1R protein isoforms. These 4.1R-null mice were viable, with moderate hemolytic anemia but no gross abnormalities. Erythrocytes from these mice exhibited abnormal morphology, lowered membrane stability, and reduced expression of other skeletal proteins including spectrin and ankyrin, suggesting that loss of 4. 1R compromises membrane skeleton assembly in erythroid progenitors. Platelet morphology and function were essentially normal, indicating that 4.1R deficiency may have less impact on other hematopoietic lineages. Nonerythroid 4.1R expression patterns, viewed using histochemical staining for lacZ reporter activity incorporated into the targeted gene, revealed focal expression in specific neurons in the brain and in select cells of other major organs, challenging the view that 4.1R expression is widespread among nonerythroid cells. The 4.1R knockout mice represent a valuable animal model for exploring 4.1R function in nonerythroid cells and for determining pathophysiological sequelae to 4.1R deficiency.

Comutagenic effects of 3-aminobenzamide in Chinese hamster ovary cells.

Inhibition of poly (ADP-ribose) synthesis by agents such as 3-aminobenzamide (3-AB) potentiates the cytotoxic, carcinogenic, and clastogenic effects of certain DNA-damaging agents. Experiments were carried out in Chinese hamster ovary cells to compare chromosome aberration production and cytotoxicity with the induction of somatic mutations at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and sodium-potassium ATPase loci after treatment with 3-AB in combination with certain monofunctional alkylating agents. On its own, 1 to 10 mM concentrations of 3-AB were not mutagenic, reduced plating efficiencies only slightly, and produced a small elevation in the frequency of chromatid aberrations. In combination with ethyl methanesulfonate (EMS), 3-AB increased cytotoxicity and the frequency of alkylation-induced chromatid aberrations. 3-AB also increased the frequency of EMS and N-methyl-N'-nitro-N-nitrosoguanidine-induced 6-thioguanine-resistant cells (a marker for the HGPRT- phenotype). It had no effect on the frequency of EMS-induced ouabain-resistant cells (a marker for ATPase mutations). All the effects were dose dependent. Larger absolute increases were found with 10 mM 3-AB as compared with 1 mM 3-AB and with 2 mM EMS as compared to 1 mM EMS. The 3-AB-mediated increases in 6-thioguanine-resistant cells, which are often deletion mutations, and the lack of any increase in the frequency of ouabain-resistant cells, which can only arise through point mutation induction, along with the increases in chromosome aberration frequency, suggests that 3-AB increases the frequency of deletion mutations by increasing the frequency and duration of DNA strand breaks.

Increased ultraviolet sensitivity and chromosomal instability related to P53 function in the xeroderma pigmentosum variant.

The xeroderma pigmentosum (XP) variant (XPV) is a form of XP that has normal excision repair but shows defective DNA replication after UV irradiation. In developing various transformed fibroblast cell lines from these patients, we have found that there are significant phenotypic changes in transformed cells that seem to correlate with inactivation of p53. After transformation with SV40, XPV cell lines are only slightly UV sensitive, like their primary counterparts, but their sensitization with caffeine and the induction of sister chromatid exchanges (SCEs) by UV irradiation are greatly enhanced. After transformation by HPV16 E7, which targets the retinoblastoma cell cycle regulatory gene, there is no change in the UV sensitivity of XPV cells; but, when transformed by HPV16 E6 or E6 and E7 combined, there is a large increase in UV sensitivity and in the induction of SCEs. These changes are not associated with any detectable changes in the reactivation of an externally irradiated luciferase expression vector, the excision of cyclobutane pyrimidine dimers from bulk DNA, or unscheduled DNA synthesis and, therefore, do not involve excision repair. We suggest that if SCEs represent homologous recombination between sister chromatids, then in the absence of p53 function, the DNA chain arrest typical of UV-damaged XPV cells initiates strand exchange during recovery. In untransformed cells with normal p53, the preferred mode of recovery would then be replication bypass. The symptoms of elevated solar carcinogenesis in XPV patients may, therefore, be associated with increased genomic instability in cells of the skin in which p53 is inactivated by UV-induced mutations.

Variation in normal tissue responsiveness to WR-2721.

The radioprotection afforded normal tissues by WR-2721 or S-2-[3-aminopropylamino]ethylphosphorothioic acid varies widely, with some of the most responsive tissues showing low levels of absorbed drug and vice versa. While the oxygen tension of the respective tissues may be contributing to this variation, it can be shown that the drug is not homogeneously distributed within each tissue, and that its location within the sub-cellular compartments can vary widely. Recognition of this variability, when combined with the newly developed HPLC assays for drug quality, should provide new insights into the sources of variation in normal tissue responsiveness to these agents.

Indications of repair of radon-induced chromosome damage in human lymphocytes: an adaptive response induced by low doses of X-rays.

Naturally occurring radon is a relatively ubiquitous environmental carcinogen to which large numbers of people can be exposed over their lifetimes. The accumulation of radon in homes, therefore, has led to a large program to determine the effects of the densely ionizing alpha particles that are produced when radon decays. In human lymphocytes, low doses of X-rays can decrease the number of chromatid deletions induced by subsequent high doses of clastogens. This has been attributed to the induction of a repair mechanism by the low-dose exposures. Historically, chromosome aberrations induced by radon have been considered to be relatively irreparable. The present experiments, however, show that if human peripheral blood lymphocytes are irradiated with low doses of X-rays (2 cGy) at 48 hr of culture, before being exposed to radon at 72 hr of culture, the yield of chromatid deletions induced by radon is decreased by a factor of two. Furthermore, the numbers of aberrations per cell do not follow a Poisson distribution but are overdispersed, as might be expected because high-linear energy transfer (high LET) alpha particles have a high relative biological effectiveness compared to low-LET radiations such as X-rays or gamma rays. Pretreatment with a low dose of X-rays decreases the overdispersion and leads to a greater proportion of the cells having no aberrations, or lower numbers of aberrations, than is the case in cells exposed to radon alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitroxyl spin label contrast enhancers for magnetic resonance imaging. Studies of acute toxicity and mutagenesis.

Two nitroxyl spin label (NSL) compounds that are used experimentally as in vivo contrast enhancers in magnetic resonance (MR) imaging were tested for acute toxicity in rats and for genotoxic effects in cell cultures. These compounds, 2,2,5,5-tetramethyl-1-pyrrolidinyl-oxy-3-carboxylic acid (PCA) and 2,2,6,6-tetramethyl-1-oxido-4-piperidinyl-1-succinic acid (TES) and their hydroxylamine and amine derivatives did not induce sister chromatid exchanges or mutations in Chinese hamster ovary cells at the HGPRT or Na+/K+ ATPase loci. The acute LD50 doses in rats for PCA and TES are 15.1 mmol/kg or greater, suggesting relatively high tolerance.

Characterization of the adaptive response to ionizing radiation induced by low doses of X rays to human lymphocytes.

In previous studies we have shown that low doses of radiation from incorporated tritiated thymidine can make human lymphocytes less susceptible to the genetic damage manifested as chromatid breakage induced by a subsequent high dose of X rays. We have also shown that this adaptive response to ionizing radiation can be induced by very low doses of X rays (0.01 Gy; i.e., 1 rad) delivered during S phase of the cell cycle. To see if a low dose of X rays could induce this response in cells at other phases of the cell cycle, human lymphocytes were irradiated with 0.01 or 0.05 Gy before stimulation by phytohemagglutinin (G0) or with 0.01 Gy at various times after stimulation (G1), followed by 1.5 Gy (150 rad) at G2 phase. Although G0 lymphocytes failed to exhibit an adaptive response, G1 cells irradiated as early as 4 h after stimulation did show the response. Experiments were also carried out to determine how long the adaptive response induced by 0.01 Gy could persist. A 0.01-Gy dose was delivered to lymphocytes in the first S phase, followed by 1.5 Gy in the same or subsequent cell cycles. Lymphocytes receiving a 1.5-Gy dose at 40, 48, or 66 h after stimulation exhibited an adaptive response, whereas those receiving a 1.5-Gy dose at 90 or 114 h did not. Duplicate cultures containing bromodeoxyuridine showed that at 40 h all the lymphocytes were in their first cell cycle after stimulation, at 48 h half of the lymphocytes were in their first cell cycle and half in their second, and at 66 h 80% of the lymphocytes were in their third cell cycle. Thus the adaptive response persists for at least three cell cycles after it is induced by 0.01 Gy of X rays. In other experiments, the time necessary for maximal expression of the adaptive response was determined by delivering 0.01 Gy at hourly intervals 1-6 h before the 1.5-Gy dose. While a 4-h interval was enough for expression of the adaptive response, shorter intervals were not.

Depletion of cellular polyamines by alpha-difluoromethylornithine increases 1,3-bis(2-chloroethyl)-1-nitrosourea-induced sister-chromatid exchanges.

Treatment of cells with alpha-difluoromethylornithine (DFMO), which leads to the intracellular depletion of the polyamines putrescine and spermidine, is known to increase the cell killing induced by the DNA cross-linking chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). This has been attributed to a change in DNA conformation that is related to the intracellular polyamine content and a concomitant change in accessibility of BCNU to reactive sites in DNA. Cytogenetic experiments were previously carried out showing that DFMO pretreatment also increased sister chromatid exchanges (SCEs) induced by BCNU. Subsequently, there was difficulty in repeating this phenomenon, which led to retraction of the results. More recent information indicates, however, that a change in protocol occurred between the time of the original experiments and the repeat experiments. Because cell killing by genotoxic agents, such as X-rays, can be related to cytogenetic damage, and agents that cause deformation of DNA do lead to SCEs, experiments have now been carried out to see whether or not the subtle deformation caused by polyamine depletion could, indeed, lead to the induction of increased SCEs by BCNU, as had been originally postulated. When the experiments were carried out with the original protocols, pretreatment with DFMO did increase BCNU-induced SCEs. Modification of the protocols, especially washing of the cells before the addition of BCNU, which reduces the toxicity, reduced the DFMO effect and in some experiments even obliterated it.

Cultured human lymphocytes proliferate faster in medium lacking fetal calf serum and antibiotics.

When human whole blood was cultured in medium containing neither fetal calf serum nor antibiotics, more peripheral lymphocytes initiated DNA synthesis at about 24 h of culture than did lymphocytes in blood cultured in complete medium. Furthermore, the cells proliferated faster, so that by 48 h in culture approximately 20% of the metaphase cells in medium lacking fetal calf serum and antibiotics were in their second division. Such rapid cell proliferation can affect quantitative studies of chromosome aberrations, which should be observed in their first mitosis in culture, before cell death can cause a diminution in aberration yields. The rapid appearance of second-division metaphases should also be advantageous for the measurement of sister-chromatid exchanges (SCEs), which require two cell divisions for their observation. In contrast to cultures with complete medium, cells grown without fetal calf serum and antibiotics had fewer SCEs in second-division cells that divided later in the culture period.

Adaptive response of human lymphocytes for the repair of radon-induced chromosomal damage.

In human lymphocytes low doses of X-rays can decrease the number of chromatid deletions induced by subsequent high doses of sparsely ionizing X-rays. Because of the concern with the carcinogenic effects of low doses of alpha-particles from radon in homes, experiments were carried out to see if low doses of X-rays could also decrease the yield of chromosomal aberrations induced by subsequent exposure to radon. Human peripheral blood lymphocytes were irradiated with low doses of X-rays (2 cGy) at 48 h of culture, exposed to radon at 72 h of culture, and analyzed for the presence of chromatid aberrations at subsequent intervals. The frequency of chromatid aberrations induced by radon alone increased with time after exposure, indicating exaggerated differences in the stage sensitivity of cell cycle stages to high-LET radiation. Furthermore, the numbers of aberrations per cell did not follow a Poisson distribution but were over dispersed, as might be expected since high-LET radiations have a high relative biological effectiveness compared with low-LET radiations. Nevertheless, lymphocytes exposed to 2 cGy of X-rays before radon exposure contained approximately one-half the number of chromatid deletions compared with lymphocytes treated with radon alone and analyzed at the same time. Thus, the putative chromosomal repair mechanism induced by low doses of sparsely ionizing radiation is also effective in reducing chromosomal aberrations induced by radon, which hitherto had been thought to be relatively independent of repair processes.

Segregation of DNA polynucleotide strands into sister chromatids and the use of endoreduplicated cells to track sister chromatid exchanges induced by crosslinks, alkylations, or x-ray damage.

The method of Matsumoto and Ohta [Matsumoto, K. & Ohta, T. (1992) Chromosoma 102, 60-65; Matsumoto, K. & Ohta, T. (1995) Mutat. Res. 326, 93-98] to induce large numbers of endoreduplicated Chinese hamster ovary cells has now been coupled with the fluorescence-plus-Giemsa method of Perry and Wolff [Perry, P. & Wolff, S. (1974) Nature (London) 251, 156-158] to produce harlequin endoreduplicated chromosomes that after the third round of DNA replication are composed of a chromosome with a light chromatid and a dark chromatid in close apposition to its sister chromosome containing two light chromatids. Unless the pattern is disrupted by sister chromatid exchange (SCE), the dark chromatid is always in the center, so that the order of the chromatids is light-dark light-light. The advent of this method, which permits the observation of SCEs in endoreduplicated cells, makes it possible to determine with great ease in which cell cycle an SCE occurred. This now allows us to approach several vexing questions about the induction of SCEs (genetic damage and its repair) after exposure to various types of mutagenic carcinogens. The present experiments have allowed us to observe how many cell cycles various types of lesions that are induced in DNA by a crosslinking agent, an alkylating agent, or ionizing radiation, and that are responsible for the induction of SCEs, persist before being repaired and thus lose their ability to inflict genetic damage. Other experiments with various types of mutagenic carcinogens and various types of cell lines that have defects in different DNA repair processes, such as mismatch repair, excision repair, crosslink repair, and DNA-strand-break repair, can now be carried out to determine the role of these types of repair in removing specific types of lesions.

Evidence that the [3H]thymidine-induced adaptive response of human lymphocytes to subsequent doses of X-rays involves the induction of a chromosomal repair mechanism.

When human lymphocytes are treated with [3H]thymidine ([ 3H]dThd), the observed number of chromosomal aberrations induced by [3H]dThd and subsequent doses of X-rays is less than the number induced by X-rays alone. Experiments in which cells were examined at various times after exposure to the X-rays showed that this phenomenon, termed an adaptive response to the endogenous radiation from tritium, is not an artefact caused by radiation-induced mitotic delays, which could result in the sampling of metaphase cells that were irradiated in different parts of the G2 phase of the cell cycle, where sensitivity to X-rays changes dramatically. Reconstruction experiments in which labelled female cells were co-cultured with unlabelled male cells now show that labelled and unlabelled cells progress to metaphase equally, and therefore that the adaptive response is not the result of selection against a radiosensitive population of cells that have incorporated [3H]dThd. Measurements of chromosomal aberrations induced in the labelled female cells and unlabelled male cells that had been co-cultured show that the adaptive response is restricted to those cells exposed to radiation from the incorporation of [3H]dThd and that diffusible factors are not involved. The results are consistent with the proposal that this adaptive response is the result of the induction of a hitherto unknown chromosomal repair mechanism. It has now been found that this repair mechanism is induced at levels of radiation from [3H]dThd that in themselves are too low to induce any discernible chromosomal aberrations and that its activity is dependent on the enzyme poly(ADP-ribose) polymerase, because 3-aminobenzamide, an inhibitor of poly(ADP-ribosyl)ation, prevents the adaptive response.

Antipain-mediated suppression of X-ray-induced chromosomal aberrations in human lymphocytes.

The protease inhibitor antipain is known to modulate the number of chromosomal aberrations induced by the S-phase-dependent alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine. Experiments have now been carried out to see if antipain might also effect the yield of aberrations induced by X-rays, which are S-independent and thus produce chromosomal aberrations by a different mechanism. The results show that human lymphocytes exposed to 0.4 or 1.5 Gy of X-rays at 48 h of culture and fixed at 3, 6, 8, 10 or 12 h thereafter contain 27-52% fewer chromatid breaks if the cells are also treated with antipain before irradiation. Because previous studies postulated that antipain could affect the induction of chromosomal aberrations by suppressing free radical reactions within cells, we also tested whether antipain affects X-ray-induced aberrations when present only during the time of irradiation, as is the case for free radical scavengers, such as L-cysteine. The results indicate that, in contrast to L-cysteine, antipain can suppress the induction of X-ray-induced aberrations even when administered as late as 2 h after irradiation, suggesting that the effects of antipain on aberrations are not attributable to its interference with short-lived radicals within the cells. Although the exact mechanism whereby antipain decreases the yield of chromosome aberrations induced by the S-independent agent X-rays is unknown, these data indicate that the formation of chromosome aberrations by S-independent agents too can involve an antipain-sensitive process.

Sister chromatid exchanges in cells defective in mismatch, post-replication and excision repair.

Three processes associated with DNA damage and genomic instability have been defined experimentally as operating during or soon after DNA replication: mismatch repair, post-replication repair and sister chromatid exchange. All these processes appear to operate on damage and/or errors in newly replicated DNA. Both mismatch repair and post-replication repair involve resynthesis of up to 1 kb of newly synthesized DNA: mismatch repair operates on single-base or slippage errors; post-replication repair operates on persistent gaps in newly synthesized DNA caused by damage on parental strands. Using colon cancer cells with different mismatch repair capacity, together with normal cells and excision-repair-defective and post-replication-repair-defective xeroderma pigmentosum (XP) cells, we analysed possible interactions between these processes. No evidence for overlap of mismatch repair with excision or post-replication repair was found. However, post-replication-repair-defective XP variant cells that were SV40 transformed showed higher UV-induced sister chromatid exchange frequencies than did untransformed cells. This suggests that sister chromatid exchanges in the XP variant are closely involved with UV-induced replication errors that are enhanced by transformation.

Human lymphocytes exposed to low doses of ionizing radiations become refractory to high doses of radiation as well as to chemical mutagens that induce double-strand breaks in DNA.

Human lymphocytes exposed to low doses of ionizing radiation from incorporated tritiated thymidine or from X-rays become less susceptible to the induction of chromatid breaks by high doses of X-rays. This response can be induced by 0.01 Gy (1 rad) of X-rays, and has been attributed to the induction of a repair mechanism that causes the restitution of X-ray-induced chromosome breaks. Because the major lesions responsible for the induction of chromosome breakage are double-strand breaks in DNA, attempts have been made to see if the repair mechanism can affect various types of clastogenic lesions induced in DNA by chemical mutagens and carcinogens. When cells exposed to 0.01 Gy of X-rays or to low doses of tritiated thymidine were subsequently challenged with high doses of tritiated thymidine or bleomycin, which can induce double-strand breaks in DNA, or mitomycin C, which can induce cross-links in DNA, approximately half as many chromatid breaks were induced as expected. When, on the other hand, the cells were challenged with the alkylating agent methyl methanesulfonate (MMS), which can produce single-strand breaks in DNA, approximately twice as much damage was found as was induced by MMS alone. The results indicate that prior exposure to 0.01 Gy of X-rays reduces the number of chromosome breaks induced by double-strand breaks, and perhaps even by cross-links, in DNA, but has the opposite effect on breaks induced by the alkylating agent MMS. The results also show that the induced repair mechanism is different from that observed in the adaptive response that follows exposure to low doses of alkylating agents.

Chromatid exchanges may be induced by damage in sites of transcriptional activity.

A conditional expression system has allowed us to vary the expression level of the xeroderma pigmentosum group A (XPA) photoproduct-specific DNA-binding protein in human cells and so control the response of cells to damage by UV light. Using a form of XPA that contains a single missense mutation (R207G) enabled us to study a lower range of function than that obtained with the wild-type sequence. This form of XPA has been previously shown to stimulate pyrimidine dimer excision preferentially in actively transcribed genes. We found that UV resistance increased as a linear function of XPA expression levels. Excision of (6-4) pyrimidine-pyrimidone photoproducts in the whole genome increased to a maximum at about the haploid level of XPA expression, but there was little pyrimidine dimer excision from the whole genome. SCE frequency induced by UV light was high in cells with no SPA expression and fell rapidly with increasing levels of SPA expression within 0-50% of the haploid level of expression. No further reduction in SCE frequency was produced at the highest levels of XPA expression, when repair replication extended to the overall genome. We speculate that a low level of repair, especially that occurring in actively transcribed genes, may selectively eliminate photoproducts that are particularly important in causing cell killing and SCEs.

Magnetic resonance imaging: absence of in vitro cytogenetic damage.

Human lymphocytes and Chinese hamster ovary (CHO) cells in culture were exposed for 12 1/2 hours to a magnetic resonance imaging apparatus with a 2.35-Tesla magnet and 100-MHz radio frequency emission. The cells were examined for cytogenetic damage manifested either as chromosome aberrations or sister chromatid exchanges (SCEs), which constitute very sensitive measures of genetic and cellular damage. In either unstimulated or stimulated human lymphocytes, as well as in exponentially growing CHO cells, no increase in either chromosome aberrations or SCEs was found as a result of exposure to these MR conditions. The data indicate that long-term exposure to MR imaging conditions far exceeding those to be found in the clinical situation does not cause cytogenetic damage.

Regulation and activity of the human ABCA1 gene in transgenic mice.

The ABCA1 transporter is one of the limiting steps in cellular cholesterol efflux. To study the expression and activity of the human ABCA1 gene in vivo we have examined mice containing two human BAC transgenes with different 5' ends. Mice containing a 255-kilobase (kb) BAC transgene, including 70 kb upstream of the previously defined exon 1, demonstrated a pattern of tissue-specific expression mimicking that of the endogenous mouse gene. Compared with macrophages from control mice, macrophages from these transgenics had increases in apoA-I cholesterol efflux heightened in response to increases in cell cholesterol content. The observed increase in macrophage apoA-I-mediated cholesterol efflux was not accompanied by alterations in plasma high density lipoprotein in the transgenics. Although mice containing a smaller 171-kb human BAC transgene, lacking the previously described exon 1 and ABCA1 promoter, did not express human ABCA1 in macrophages, they did express the human transgene in liver at levels comparable with those of the orthologous mouse gene. Analysis by 5' rapid amplification of cDNA ends of liver mRNA from these animals revealed a new ABCA1 exon 1 (exon 1A) and a previously unrecognized promoter. Analysis of human tissue revealed that exon 1A containing transcripts accounted for a high proportion of the ABCA1 mRNAs present in human liver. This analysis of ABCA1 transgenics showed that the expression of human ABCA1 transgenes can result in increased cholesterol efflux from macrophages, unaccompanied by changes in plasma high density lipoprotein, and identified a new ABCA1 promoter in humans.

Deletion of a coordinate regulator of type 2 cytokine expression in mice.

Mechanisms that underlie the patterning of cytokine expression in T helper (T(H)) cell subsets remain incompletely defined. An evolutionarily conserved approximately 400-bp noncoding sequence in the intergenic region between the genes Il4 and Il13, designated conserved noncoding sequence 1 (CNS-1), was deleted in mice. The capacity to develop T(H)2 cells was compromised in vitro and in vivo in the absence of CNS-1. Despite the profound effect in T cells, mast cells from CNS-1(-/-) mice maintained their capacity to produce interleukin 4. A T cell-specific element critical for the optimal expression of type 2 cytokines may represent the evolution of a regulatory sequence exploited by adaptive immunity.