Molecular identification of the skin transformation center of anuran larval skin using genes of Rana adult keratin (RAK) and SPARC as probes

Anuran larval skin undergoes a process of metamorphosis into pre-adult and adult skin. Basal skein, larval basal and adult basal cells are basement membrane-attaching cells in the larval, pre-adult and adult epidermis, respectively, and are identified as cells expressing genes of RLK (Rana larval keratin), both RLK and RAK (Rana adult keratin), and RAK. Larval to pre-adult skin conversion takes place in the histological entity called the skin transformation center (STC). The present study performed a cDNA subtractive gene screening on cDNA of the larval and the pre-adult skin, and cloned the secreted protein acidic and rich in cysteine (SPARC) gene as an upregulated gene in the larva to pre-adult skin conversion. RAK gene-positive basal skein cells and fibroblasts in and around the STC were weakly and strongly sparc-positive, respectively. Using sparc and rak, we redefined the STC and visualized it on a histological section as an approximately 150 microm-long region that contained about 20 rak-negative and weakly sparc-positive basal cells. Intense sparc expression was observed in basal skein cells, but not in larval basal cells, suggesting that SPARC acts as a suppressor of rak during epidermal differentiation. This suggestion was tested by investigating the effect of SPARC on cultured larval basal cells. We observed that SPARC suppressed the expression of rak, but not rlk.     

T3-hydrocortisone synergism on adult-type erythroblast proliferation and T3-mediated apoptosis of larval-type erythroblasts during erythropoietic conversion in Xenopus laevis

 The conversion of an erythropoietic system from larval to adult type in anuran amphibia may possibly come about through cell replacement. The hormonal regulation of apoptosis of larval-type precursor cells and adult-type cell proliferation has yet to be examined in detail. In amphibians, corticoids synergize T₃ action during metamorphosis. In the present study, examination was made of the process of larval-to-adult conversion in the liver erythropoietic site of Xenopus laevis, with special attention to how these metamorphic hormones, T₃ and corticoid, regulate programmed cell death specific for larval erythroblasts and the proliferation of adult cells. Immunohistochemical analysis of liver sections indicates that the number of larval erythroblasts decreased to less than 50% at the early climax stage (stages 59–60) of metamorphosis. Overall liver morphology greatly changed subsequent to the climax stage from the three-lobe to the two-lobe shape. The addition of T₃ (10^-8 M) to premetamorphic tadpoles induced considerable liver morphological change and a 50% decrease in larval-type erythroblasts. These erythroblast decreases seem to take place through the apoptotic process, since double-staining experiments with in situ DNA nick-end labeling (TUNEL) and hemoglobin immunostaining revealed that DNA breakage of nuclei, a well-known feature of apoptosis, occured specifically in larval erythroblasts during prometamorphosis. Hydrocortisone (HC), which modulates T₃ action during metamorphosis, was found not to be a factor in larval cell decrease. But adult erythroblasts increased by 8 times as much through the action of T₃ and 32 times as much by the action of T₃ plus HC, indicating the important action of T₃–HC synergism. It thus follows that the erythropoietic system is converted during metamorphosis effectively by two distinct hormonal mechanisms, T₃–HC synergism on adult erythroblast proliferation and T₃-mediated programmed death of larval precursor cells. Accepted: 14 January 1999     

Thermal evolution of rate of larval development in Drosophila melanogaster in laboratory and field populations

The duration of Drosophila melanogaster larval and pupal periods was measured in laboratory thermal lines and in populations collected along a latitudinal transect in eastern Australia. In replicated laboratory lines kept for 9 years at 16.5° C or 25° C the duration of larval development had continued to diverge compared with 4 and 5 years previously, with more rapid larval development, and hence reduced total duration of pre-adult development, in the low temperature lines at both experimental temperatures. After 4 years of separate evolution, lines derived from the 25° C lines and subsequently cultured at 29° C showed no evidence of significant divergence in the duration of any part of the pre-adult period. The geographic populations showed a decrease in the duration of larval development, and hence of the total pre-adult period, with increasing latitude. In both laboratory and field populations, evolution at lower temperature was associated with more rapid larval development to a larger adult body size, the opposite genetic correlation between these traits to that found within a single temperature. The indications are that lower temperatures may be permissive of more efficient growth in D. melanogaster. It will be important to discover if evolution in response to temperature induces similar correlations in other ectotherms.     

Adult-type pigment cells,which color the ocular sides of flounders at metamorphosis,localize as precursor cells at the proximal parts of the dorsal and anal fins in early larvae

Flounders form left-right asymmetry in body coloration during metamorphosis through differentiation of adult-type melanophores and xanthophores on the ocular side. As the first step in investigating the formation of flounder body coloration asymmetry, in this study, we aimed to determine where the precursors of adult-type chromatophores distribute in larvae before metamorphosis. In Paralichthys olivaceus and Verasper variegatus, GTP cyclohydrolase 2 (gch2), a common marker of melanoblasts and xanthoblasts, was found to be transiently expressed in cells located along the bilateral skeletal muscles at the basal parts of the dorsal and anal fins of premetamorphic larvae. When V. variegatus larvae were fed with a strain of Artemia collected in Brazil, this gch2 expression was abolished and the differentiation of adult-type melanophores was completely inhibited, while the density of larval melanophores was not affected. In a cell trace test in which the cells at the basal part of the dorsal fin were labeled with DiI at the premetamorphic stage, adult-type melanophores labeled with DiI were found in the skin on the ocular side after metamorphosis. These data suggest that, in flounder larvae, adult-type melanophores are distributed at the basal parts of the dorsal and anal fins as unpigmented precursor cells.     

Body-specific proliferation of adult precursor cells in Xenopus larval epidermis

Cell proliferation was examined in the back and tail epidermis of larval Xenopus laevis using bromodeoxyuridine (BrdU). The BrdU labeling index of the back epidermis increased temporally at stage 59, followed by a rapid decrease to the same level as at stage 51. The temporal increase in cell proliferation of the back epidermis produced a new epidermal layer composed of basal cells. In vitro analysis showed that tri-iodothyronine (T₃) promotes cell proliferation of basal cells but suppresses that of skein cells. Immunohistochemical studies showed that the newly formed basal cell layer functions as adult precursor cells which produce the adult epidermal cells. In contrast to the back epidermis, the labeling index of the tail epidermis decreased from stage 57. However, when the tail skin was transplanted to the back area, cell proliferation in the tail epidermis increased to the same level as that of the normal back epidermis. Cell proliferation of the back epidermis was not suppressed by transplanting the skin to the tail area. These results suggest that some promoting factors are produced in the body region and regulate the number of adult precursor cells, which determine the developmental fate of the larval skin.     

Conversion of a postocellar into an ocellar region as a transdetermination event occurring in adult ribbonworms

Transections and grafting experiments performed in Lineus ruber rostral ends allowed us to generate ribbonworms with a duplication of the postocellar region combined with a deletion of the ocellar region. In such homeotically reconstructed animals, the syngeneic postocellar region transdifferentiated into an ocellar region with functional eyes while the allogeneic postocellar region underwent no transformation. In this case, transdifferentiation is a morphogenetic process leading to the restoration of the normal antero-posterior (A-P) axis pattern in adult worms. This regulative conversion of one adult body region into another, which so far has not been described in any bilaterian animal, is comparable with transdetermination of larval imaginal discs in Drosophila. Under certain conditions, Drosophila, wing imaginal disc cells express the eyeless master control gene and give rise to eyes. Here, we show in Lineus that the transposition of postocellar tissue into the ocellar location causes expression of the eyeless ortholog LsPax-6 and results in eye development. Our results in Lineus clearly suggest that transdifferentiation of adult body regions moved to a different position along the A-P axis is similar to transdetermination of the larval imaginal disc cells which are determined, but not yet differentiated.     

Pigment cell localizations in anuran ventral skin at climactic metamorphosis.

In anuran amphibians, the specific color pattern of the skin is expressed after metamorphosis, and its formation involves pigment cell migrations. Pigment cells are differently distributed in the tadpole, larval, and froglet skin. To learn more about their fate during metamorphic climax and in the young froglet, we focused our attention on the different localizations of larval melanophores and iridophores in the ventral skin of Rana esculenta before and during skin homing. Localizations of melanophores and iridophores can be elucidated at the developmental stages suggested by Taylor and Kollros (TK stages). At TK stage II (during early premetamorphosis), large melanophores beneath the larval skin are detected. At TK stage X, dispersed melanophores lie under bundles of muscular striated fibrils near the larval skin; they are also observed at the vascular level. At TK stage XVII (prometamorphosis), melanophores are extended on the inner side of the basement lamellar collagen. At the end of prometamorphosis, iridophores are located with melanophores in the separating space between attached basement collagen and derived basement collagen. At TK stage XX (earlier climax), melanophores and iridophores are detected inside the upper extremities of fractures opened in the derived basement collagen. At TK stage XXIV (later climax), both types of larval pigment cells are observed in the inner extremities of breaks derived from the fractures. During climax, these pigment cells occupy the well-formed breaks. At TK stage XXV in young froglet, the pigment cells remain alone in the breaks formed in the derived basement collagen. Briefly, breaks in the basement lamellar collagen are opened by invading cell processes of mesenchymal cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

The developing visual system and metamorphosis in the lamprey

Metamorphosis of the sea lamprey, Petromyzon marinus, is a true metamorphosis. The larval lamprey is a filter-feeder who dwells in the silt of freshwater streams and the adult is an active predator found in large lakes or the sea. The transformation usually occurs in the fifth or sixth year of life. Enlargement of the eye has been long accepted as a distinctive indication of metamorphosis in the sea lamprey, but it had been thought that this was because eye development in the larva was arrested after the formation of only the small central region. Recent studies indicate that all of the retina begins its development in the larva and that ganglion, amacrine, and horizontal cells differentiate in the peripheral retina of the larva. Retinal development is arrested during the premetamorphic period, to be resumed during metamorphosis. Metamorphic contributions include the differentiation of photoreceptor and bipolar cells. With the early appearance of ganglion cells, retinal pathways to the thalamus and tectum are established in larvae, as is a centripetal pathway. Tectal development spans the larval period but a spurt in tectal growth and differentiation is correlated with the completion of the retinal circuitry late in metamorphosis. The metamorphic changes in retina and tectum complete the functional development of the visual system and provide for the adult lamprey's predatory and reproductive behavior.     

Hemoglobin transition in relation to metamorphosis in normal and isogenicXenopus

Summary Electrophoretic separation of hemoglobins of normalXenopus laevis and of isogenic animals derived from female hybrids ofXenopus laevis×Xenopus gilli revealed 5–9 components in premetamorphic larvae, and 3–4 components in adult toads. InXenopus laevis the number of larval hemoglobin components showed considerable variation, but this variation was absent in isogenic tadpoles, suggesting a genetic basis for hemoglobin polymorphism in larvae.Electrophoretic separation of larval and adult hemoglobins at different concentrations of acrylamide and treatment of these solutions with mercaptoethanol revealed that larval hemoglobin components are charge isomers, whereas adult hemoglobin was found to contain a minor dimeric component.Estimation of hemoglobin components showed that the main increase in adult hemoglobin, i.e from 30–90% of total hemoglobin, occurs within 4 weeks after completion of metamorphosis. By incroporation of³H amino acids in vivo a switch to preferential synthesis of adult hemoglobin and a corresponding decrease in larval hemoglobin production could be demonstrated during early climax stages. This suggests that thyroid hormones are involved in the hemoglobin transition. Yet chemical inhibition of the larval thyroid by thiourea resulted in a delayed but complete hemoglobin transition without morphological transformation. It is concluded that hemoglobin transition and morphological transformation of theXenopus tadpole require different concentrations of thyroid hormones.Abbreviations Hb hemoglobin - Hb_A adult hemoglobin - Hb_L larval hemoglobin     

Evolutionary responses of Drosophila melanogaster life history to differences in larval density

The study examined the effects of evolution at two different larval densities on pre-adult and adult fitness traits. Five replicate selection lines each were cultured at either 50 or 150 larvae per vial, avoiding selection on development time, age at breeding or for adaptation to adult density, one or more of which factors has been a confounding variable in previous studies. Low density selection lines evolved extended development times at both growth densities. The extended development times were associated with greater adult body size at the lower growth density only, and particularly in females. The lines did not differ significantly in larval competitive ability at either growth density. At neither growth density did the early adult fertility of females or the lifespan of either sex differ between the lines from the two selection regimes, but at the lower growth density the late fertility of low density line females was significantly enhanced. The results suggest that larval density does have important effects on the expression and resolution of life history trade-offs in Drosophila melanogaster, but that these may be somewhat different from those reported in previous studies.     

Distribution and localization of immunoreactive FMRFamide-like peptides in the lancelet.

Immunofluorescence was used to study the distribution of FMRFamide (Phe-Met-Arg-Phe-NH2) in premetamorphic larvae and adults of the lancelet, Branchiostoma lanceolatum. In the larvae, FMR-Famide-containing presumably neuronal perikarya and fibers were limited to the anterior third of the dorsal nerve cord. Throughout this region, most of the immunoreactive perikarya and fibers were located ventrolaterally and ventrally within the nerve cord; in addition, in the caudal part of the cerebral vesicle, some of the immunofluorescent cells projected cytoplasmic extensions across the slot-like neural canal. In adult lancelets, immunofluorescence was detected in cells of the Hatschek's pit (a probable homologue of the anterior hypophysis of vertebrates); however, no immunofluorescence was detected in the larval preoral pit, which is the ontogenetic precursor of Hatschek's pit. Moreover, the FMR-Famide-containing elements do not show immunoreactivity to other peptides of the FaRPs family such as pancreatic polypeptide (PP). The results suggest that FMRF-amide may be involved in neuroendocrine functions of lancelets.     

Temperature dependent larval resource allocation shaping adult body size in Drosophila melanogaster

Geographical variation in Drosophila melanogaster body size is a long-standing problem of life-history evolution. Adaptation to a cold climate invariably produces large individuals, whereas evolution in tropical regions result in small individuals. The proximate mechanism was suggested to involve thermal evolution of resource processing by the developing larvae. In this study an attempt is made to merge proximate explanations, featuring temperature sensitivity of larval resource processing, and ultimate approaches focusing on adult and pre-adult life-history traits. To address the issue of temperature dependent resource allocation to adult size vs. larval survival, feeding was stopped at several stages during the larval development. Under these conditions of food deprivation, two temperate and two tropical populations reared at high and low temperatures produced different adult body sizes coinciding with different probabilities to reach the adult stage. In all cases a phenotypic trade-off between larval survival and adult size was observed. However, the underlying pattern of larval resource allocation differed between the geographical populations. In the temperate populations larval age but not weight predicted survival. Temperate larvae did not invest accumulated resources in survival, instead they preserved larval biomass to benefit adult weight. In other words, larvae from temperate populations failed to re-allocate accumulated resources to facilitate their survival. A low percentage of the larvae survived to adulthood but produced relatively large flies. Conversely, in tropical populations larval weight but not age determined the probability to reach adulthood. Tropical larvae did not invest in adult size, but facilitated their own survival. Most larvae succeeded in pupating but then produced small adults. The underlying physiological mechanism seemed to be an evolved difference in the accessibility of glycogen reserves as a result of thermal adaptation. At low rearing temperatures and in the temperate populations, glycogen levels tended to correlate positively with adult size but negatively with pupation probability. The data presented here offer an explanation of geographical variation in body size by showing that thermal evolution of resource allocation, specifically the ability to access glycogen storage, is the proximate mechanism responsible for the life-history trade-off between larval survival and adult size.     

Neurometamorphosis of the ear in the gypsy moth,Lymantria dispar,and its homologue in the earless forest tent caterpillar moth,Malacosoma disstria

The adult gypsy moth, Lymantria dispar (Lymantriidae: Noctuoidea) has a pair of metathoracic tympanic ears that each contain a two-celled auditory chordotonal organ (CO). The earless forest tent caterpillar moth, Malacosoma disstria (Lasiocampidae: Bombycoidea), has a homologous pair of three-celled, nonauditory hindwing COs in their place. The purpose of our study was to determine whether the adult CO in both species arises from a preexisting larval organ or if it develops as a novel structure during metamorphosis. We describe the larval metathoracic nervous system of L. dispar and M. distria, and identify a three-celled chordotonal organ in the anatomically homologous site as the adult CO. If the larval CO is severed from the homologue of the adult auditory nerve (IIIN1b1) in L. dispar prior to metamorphosis, the adult develops an ear lacking an auditory organ. Axonal backfills of the larval IIIN1b1 nerve in both species reveal three chordotonal sensory neurons and one nonchordotonal multipolar cell. The axons of these cells project into tracts of the central nervous system putatively homologous with those of the auditory pathways in adult L. dispar. Following metamorphosis, M. disstria moths retain all four cells (three CO and one multipolar) while L. dispar adults possess two cells that service the auditory CO and one nonauditory, multipolar cell. We conclude that the larval IIIN1b1 CO is the precursor of both the auditory organ in L. dispar and the putative proprioceptor CO in M. disstria and represents the premetamorphic condition of these insects. The implications of our results in understanding the evolution of the ear in the Lepidoptera and insects in general are discussed. © 1996 John Wiley & Sons, Inc.     

Developmental Sequence of Chloride Cells in the Body Skin and Gills of Japanese Flounder (Paralichthys olivaceus) Larvae

The developmental sequence of chloride cells was examined in both the body skin and gills of Japanese flounder (Paralichthys olivaceus) larvae by whole-mount immunocytochemistry using an antiserum specific for Na(+),K(+)-ATPase. In premetamorphic larvae at 0 and 4 days after hatching (days 0 and 4), immunoreactive chloride cells were distributed only in the yolk-sac membrane and body skin. Premetamorphic larvae at days 8-18 possessed both cutaneous and branchial chloride cells. Large chloride cells in the skin of premetamorphic larvae often formed multicellular complexes, suggestive of their ion-secreting function. Cutaneous chloride cells decreased in size and density at the beginning of metamorphosis (days 21 and 24), and disappeared at the metamorphic climax (days 28 and 33). In contrast, branchial chloride cells first appeared at day 8, and increased during metamorphosis. These results indicate that the site for ion secretion in seawater may shift from cutaneous to branchial chloride cells during metamorphosis. The appearance of branchial chloride cells before the differentiation of gill lamellae suggests that the primary function of the gills during the early development is ion regulation rather than gas exchanges.     

Larval juvenile hormone treatment affects pre-adult development,but not adult age at onset of foraging in worker honey bees (Apis mellifera)

Previous research has shown that juvenile hormone (JH) titers increase as adult worker honey bees age and treatments with JH, JH analogs and JH mimics induce precocious foraging. Larvae from genotypes exhibiting faster adult behavioral development had significantly higher levels of juvenile hormone during the 2nd and 3rd larval instar. It is known that highly increased JH during this period causes the totipotent female larvae to differentiate into a queen. We treated third instar larvae with JH to test the hypothesis that this time period may be a developmental critical period for organizational effects of JH on brain and behavior also in the worker caste, such that JH treatment at a lower level than required to produce queens will speed adult behavioral development in workers. Larval JH treatment did not influence adult worker behavioral development. However, it made pre-adult development more queen-like in two ways: treated larvae were capped sooner by adult bees, and emerged from pupation earlier. These results suggest that some aspects of honey bee behavioral development may be relatively insensitive to pre-adult perturbation. These results also suggest JH titer may be connected to cues perceived by the adult bees indicating larval readiness for pupation resulting in adult bee cell capping behavior.     

Phenotypic plasticity of starvation resistance in the butterfly <Emphasis Type="Italic">Bicyclus anynana</Emphasis>

Starvation resistance is an important trait related to survival in many species and often involves dramatic changes in physiology and homeostasis. The tropical African butterfly Bicyclus anynana lives in two seasonal environments and has evolved phenotypic plasticity. The contrasting demands of the favourable, wet season and the harsh, dry season have shaped a remarkable life history, which makes this species particularly interesting for investigating the relationship between starvation resistance, metabolism, and its environmental modulation. This study reports on two laboratory experiments to investigate the effects of pre-adult and adult temperatures that mimic the seasonal environments, on starvation resistance and resting metabolic rate (RMR) in adult B. anynana. In addition, we investigate starvation resistance in wet and dry seasonal form genotypes; artificial selection on eyespot size has yielded lines that only produce one or the other of the seasonal forms across all rearing environments. As expected, the results show a large effect of adult temperature. More relevant, we show here that both pre-adult temperature and genetic background also influence adult starvation resistance, showing that phenotypic plasticity in this species includes starvation resistance. The dry season form genotype has a higher starvation resistance when developed at dry season temperatures, indicating a genetic modulation of starvation resistance in relation to temperature. Paradoxically, dry season pre-adult temperatures reduce starvation resistance and raise RMR. The high overall association of RMR and starvation resistance in our experiments suggests that energy expenditure and survival are linked, but that they may counteract each other in their influence on fitness in the dry season. We hypothesize that metabolism is moderating a trade-off between pre-adult (larval) survival and adult survival in the dry season.     

Developmental expression of drop-dead is required for early adult survival and normal body mass in Drosophila melanogaster

In Drosophila melanogaster, mutations in the gene drop-dead (drd) result in early adult lethality, with flies dying within 2 weeks of eclosion. Additional phenotypes include neurodegeneration, tracheal defects, starvation, reduced body mass, and female sterility. The cause of early lethality and the function of the drd protein remain unknown. In the current study, the temporal profiles of drd expression required for adult survival and body mass regulation were investigated. Knockdown of drd expression by UAS-RNAi transgenes and rescue of drd expression on a drd mutant background by a UAS-drd transgene were controlled with the Heat Shock Protein 70 (Hsp70)-Gal4 driver. Flies were heat-shocked at different stages of their lifecycle, and the survival and body mass of the resulting adult flies were assayed. Surprisingly, the adult lethal phenotype did not depend upon drd expression in the adult. Rather, expression of drd during the second half of metamorphosis was both necessary and sufficient to prevent rapid adult mortality. In contrast, the attainment of normal adult body mass required a different temporal pattern of drd expression. In this case, manipulation of drd expression solely during larval development or metamorphosis had no effect on body mass, while knockdown or rescue of drd expression during all of pre-adult (embryonic, larval, and pupal) development did significantly alter body mass. Together, these results indicate that the adult-lethal gene drd is required only during development. Furthermore, the mutant phenotypes of body mass and lifespan are separable phenotypes arising from an absence of drd expression at different developmental stages.     

Dynamics of body mass and oxygen consumption in the ontogeny of the Spanish ribbed newt (<Emphasis Type="Italic">Pleurodeles waltl</Emphasis>): 2. Larval stage

The correlation between characteristics of growth and energy metabolism during the larval stage of development of the Spanish ribbed newt (Pleurodeles waltl) has been studied. During this period, its body mass is found to increase 140 times and the oxygen consumption rate, 77 times. The highest rate of specific body mass increase and oxygen consumption rate are noted in the early larval stage. Later, these characteristics decrease except for a brief period before completion of metamorphosis when the rate specific body mass increase rises. Comparison of the studied characteristics allows us to note a similar pattern in changes of the specific growth rate and the oxygen consumption rate during the premetamorphic development of the Spanish ribbed newt.     

Larval antigen molecules recognized by adult immune cells of inbred Xenopus laevis: partial characterization and implication in metamorphosis

It has been shown that larval skin (LS) grafts are rejected by an inbred strain of adult Xenopus, which suggests a mechanism of metamorphosis by which larval cells are recognized and attacked by the newly differentiating immune system, including T lymphocytes. In an attempt to define the larval antigenic molecules that are targeted by the adult immune system, anti-LS antibodies (IgY) were produced by immunizing adult frogs with syngeneic LS grafts. The antigen molecules that reacted specifically with this anti-LS antiserum were localized only in the larval epidermal cells. Of 53 and 59-60 kDa acidic proteins that were reactive with anti-LS antibodies, a protein of 59 kDa and with an isoelectric point of 4.5 was selected for determination of a 19 amino acid sequence (larval peptide). The rat antiserum raised against this peptide was specifically reactive with the 59 kDa molecules of LS lysates. Immunofluorescence studies using these antisera revealed that the larval-specific molecules were localized in both the tail and trunk epidermis of premetamorphic larvae, but were reduced in the trunk regions during metamorphosis, and at the climax stage of metamorphosis were detected only in the regressing tail epidermis. Culture of splenocytes from LS-immunized adult frogs in the presence of larval peptide induced augmented proliferative responses. Cultures of larval tail pieces in T cell-enriched splenocytes from normal frogs or in natural killer (NK)-cell-enriched splenocytes from early thymectomized frogs both resulted in significant destruction of tail pieces. Tissue destruction in the latter was enhanced when anti-LS antiserum was added to the culture. These results indicate that degeneration of tail tissues during metamorphosis is induced by a mechanism such that the larval-specific antigen molecules expressed in the tail epidermis are recognized as foreign by the newly developing adult immune system, and destroyed by cytotoxic T lymphocytes and/or NK cells.