Amphidial neurons ADL and ASH initiate sodium dodecyl sulphate avoidance responses in the infective larva of the dog hookworm Anclyostoma caninum.

Ablations of specific amphidial neuron pairs with a laser microbeam were conducted to understand better the neurological basis of the behaviours of larval parasitic nematodes. To date, the functions of the amphidial neurons of Caenorhabditis elegans and their counterparts in parasitic nematodes have been found to be remarkably conserved allowing the possibility to predict the relationships between neurons and their functions. Therefore, we anticipated that ablation of neuron pairs ASH and ASK would abrogate avoidance of sodium dodecyl sulphate (SDS) by infective larvae (L3i) of Anclyostoma caninum. Instead, we have found that laser microbeam ablation of these neuron pairs did not eliminate SDS avoidance in A. caninum, but that neuron pairs ASH and ADL are the amphidial neurons responsible for SDS repulsion. When a droplet of the repellent is placed in the direct path of a normal A. caninum L3i, a strong backward avoidance response is triggered. However, when the ASH and ADL neurons are ablated, the nematodes demonstrate the opposite reaction, increasing their movement in a forward direction.

Vertical migratory behavior of the infective third-stage larvae of Oesophagostomum dentatum.

The vertical migratory behavior of third-stage infective larvae (L3i) of Oesophagostomum dentatum was investigated using upright truncated agarose cones and equivalent conical depressions in agarose. Geotactic response varied with the age of the infective larvae. Four-day-old L3i showed no preference for the sloping surfaces of either indented or upright cones, while the 8-day-old L3i showed a positive geotactic reaction, migrating down the sloping surface of the depressions.

Chemoattraction and chemorepulsion of Strongyloides stercoralis infective larvae on a sodium chloride gradient is mediated by amphidial neuron pairs ASE and ASH, respectively.

Depending on its concentration, sodium chloride acts as either an attractant or a repellant to the infective larvae (L3i) of Strongyloides stercoralis. On a concentration gradient, L3i are attracted to 0.05 M NaCl, but repelled by 2.8M. To test the hypothesis that amphidial neurons ASE and ASH might mediate attraction and repulsion, respectively, these neurons, and control neurons as well, were ablated in hatchling larvae with a laser microbeam. After the larvae attained infectivity (L3i), they were tested on a NaCl gradient. When placed at low salinity, 73.5% of normal controls migrated "up" the gradient, while 26.4% crawled randomly. In contrast, only 20.6% of ASE-ablated L3i migrated "up" the gradient, while 79.4% migrated randomly. Ablation-control ASK-ablated L3i (58.8%) migrated "up" the gradient while 41.1% crawled randomly. When placed at a region of high salinity, 100% of normal control L3i migrated "down" the gradient, whereas 62.5% of ASH-ablated L3i migrated randomly, the remaining 37.5% migrating "down" the gradient. In sharp contrast with ASH-ablated L3i, 94.1% of ablation-control larvae, i.e. ASK-ablated L3i, migrated "down" the gradient. Migration behavior of ASE- and ASH-ablated L3i was significantly different (P < 0.001) from that of ASK-ablated L3i and normal controls. It is noteworthy that 87.5% of ASE-ablated L3i that failed to exhibit chemoattractive behavior were actively chemorepelled from high salinity. Also, 70.0% of ASH-ablated L3i that failed to be chemorepelled from high salinity were capable of chemoattractive behavior, indicating that the worms had retained their behavioral responses except for those associated with the targeted neurons.

The amphidial neuron pair ALD controls the temperature-sensitive choice of alternative developmental pathways in the parasitic nematode, Strongyloides stercoralis.

The parasitic nematode Strongyloides stercoralis, has several alternative developmental pathways. Upon exiting the host (humans, other primates and dogs) in faeces, 1st-stage larvae (L1) can enter the direct pathway, in which they moult twice to reach the infective 3rd-stage. Alternatively, if they enter the indirect pathway, they moult 4 times and become free-living adults. The choice of route depends, in part, on environmental cues. In this investigation it was shown that at temperatures below 34 degrees C the larvae enter the indirect pathway and develop to free-living adulthood. Conversely, at temperatures approaching body temperature (34 degrees C and above), that are unfavorable for the survival of free-living stages, larvae develop directly to infectivity. The time-period within the L1's development during which temperature influenced the choice of the pathway depended on the temperature, but, at any given temperature, occurred approximately in the middle of the time-span spent in the L1 stage, which varied inversely with temperature. This critical period was associated with the time-interval in which the number of cells in the genital primordium began to increase, thus providing a morphological marker for the pathway decision in individual worms. Sensing the environment is the function of the amphidial neurons, and therefore we examined the role of individual amphidial neurons in controlling entry into the direct pathway to infectivity. The temperature-sensitive developmental switch is controlled by the neuron pair ALD (which also controls thermotaxis), as seen by the loss of control when these neurons are ablated. Thus, in S. stercoralis a single amphidial neuron pair controls both developmental and behavioural functions.

Chemotactic behaviour of Strongyloides stercoralis infective larvae on a sodium chloride gradient.

Chemotactic responses of Strongyloides stercoralis infective larvae (L3) to sodium chloride (NaCl) were investigated by recording larval tracks on a saline gradient in agarose. On agarose, larvae migrated randomly, whereas when placed at 0.01 m NaCl larvae moved to approximately 1.1 m NaCl where they turned, headed down the gradient and eventually remained circling at a favoured salinity (0.03-0.07 m). Conversely, when placed at 2.85 m NaCl, the L3 larvae moved unidirectionally to lower, more favoured salt concentrations. Here they circled, changing directions frequently while making 'loop-like' tracks. Larvae were immobilized within 5 min at salt concentrations exceeding 3 m NaCl. When placed at 0.01 m NaCl, 51.1% +/- 26.9 migrated to 1.1 m NaCl after 2 min, and 80% +/- 18.7 did so after 8 min, at an average velocity of 4.1 +/- 1.4 mm/min. Larvae (53.6% +/- 21.6) were repelled from 2.85 m NaCl to lower concentrations after 2 min. After 8 min, 95% +/- 11.1 were repelled, moving at an average velocity of 6.2 +/- 1.1 mm/min. Using this bioassay, the influence of neuronal control over chemotactic behaviour of S. stercoralis and other parasitic nematodes can be elucidated.

Vertical migration by the infective larvae of three species of parasitic nematodes: is the behaviour really a response to gravity?

Vertical migration by infective larvae (L3) of 3 species of nematodes was investigated. Upright truncated agarose cones were used to test upward migration, and comparable truncated cone-shaped agarose hollows were used to test downward migration. Flat agarose plates were control surfaces. When placed at the bases of upright cones, 74% of Ancylostoma caninum L3 migrated up, whereas only 16.5% migrated down the indented cones; this latter value was not significantly different from larval migration on flat plates. Strongyloides stercoralis L3 also migrated upward in significant numbers (80%). These larvae also failed to migrate downward under normal conditions. However, when the bottoms of the indented cones were 3-5 degrees C warmer than the tops, 75.5% of S. stercoralis L3 migrated down. In contrast, Haemonchus contortus L3 showed no tendency to crawl up or down cones, when compared with normal crawling behaviour on flat plates. Thus, L3 of A. caninum and S. stercoralis exhibited negative geotaxis, crawling against the pull of gravity, while H. contortus did not. The biology of these parasites may explain these behavioural differences.

Effect of chronic ethanol consumption on protective T-helper 1 and T-helper 2 immune responses against the parasites Leishmania major and Strongyloides stercoralis in mice.

BACKGROUND: Chronic alcohol consumption has been associated with significant increases in the prevalence of infectious diseases, and it has been suggested that these increases are caused by a direct effect of ethanol on the immune response. The objective of this study was to determine whether chronic ethanol consumption would affect the development of protective immunity to Leishmania major, which is controlled by the T-helper 1 (Th1) subset of CD4 cells, and Strongyloides stercoralis, which is controlled by the Th2 subset. METHODS: Mice were fed ethanol-containing liquid diet (25% ethanol-derived calories), liquid isocaloric diet without ethanol, or solid chow and then exposed to either of the two parasites. The ability of the mice chronically consuming alcohol to eliminate the infections was determined, as were the levels of parasite-specific humoral and cellular immune responses. RESULTS: Mice chronically consuming alcohol were capable of eliminating both of these infections in a manner identical to the control mice. In addition, splenocytes from mice chronically consuming alcohol infected with L. major produced nitric oxide at the same levels as in control mice. Antibody responses were altered in a manner suggesting an increase in Th2 immunity and a decrease in Th1 immunity in the mice chronically consuming alcohol. In mice chronically consuming alcohol that were infected with S. stercoralis, eosinophils migrated to the parasite's microenvironment, and antibodies were produced at levels equivalent to those seen in control mice. CONCLUSIONS: Mice maintained on an ethanol-containing liquid diet had some alteration in their ability to produce Th1 and Th2 immune responses yet were capable of generating unimpaired protective Th1 and Th2 responses.

Ancylostoma caninum: the finger cell neurons mediate thermotactic behavior by infective larvae of the dog hookworm.

Bhopale, V. M., Kupprion, E. K., Ashton, F. T., Boston, R., and Schad, G. A. 2001. Ancylostoma caninum: The finger cell neurons mediate thermotactic behavior by infective larvae of the dog hookworm. Experimental Parasitology 97, 70-76. In the amphids (anteriorly positioned, paired sensilla) of the free-living nematode Caenorhabditis elegans, the so-called finger cells (AFD), a pair of neurons, each of which ends in a cluster of microvilli-like projections, are known to be the primary thermoreceptors. A similar neuron pair in the amphids of the parasitic nematode Haemonchus contortus is also known to be thermoreceptive. The hookworm of dogs, Ancylostoma caninum, has apparent structural homologs of finger cells in its amphids. The neuroanatomy of the amphids of A. caninum and H. contortus is strikingly similar, and the amphidial cell bodies in the lateral ganglia of the latter nematode have been identified and mapped. When the lateral ganglia of first-stage larvae (L1) of A. caninum are examined with differential interference contrast microscopy, positional homologs of the recognized amphidial cell bodies in the lateral ganglia of H. contortus L1 are readily identified in A. caninum. The amphidial neurons in A. caninum were consequently given the same names as those of their apparent homologs in H. contortus. It was hypothesized that the finger cell neurons (AFD) might mediate thermotaxis by the skin-penetrating infective larvae (L3) of A. caninum. Laser microbeam ablation experiments with A. caninum were conducted, using the H. contortus L1 neuronal map as a guide. A. caninum L1 were anesthetized and the paired AFD class neurons were ablated. The larvae were then cultured to L3 and assayed for thermotaxis on a thermal gradient. L3 with ablated AFD-class neuron pairs showed significantly reduced thermotaxis compared to control groups. The thermoreceptive function of the AFD-class neurons associates this neuron pair with the host-finding process of the A. caninum infective larva and shows functional homology with the neurons of class AFD in C. elegans and in H. contortus.

Sensory neuroanatomy of a passively ingested nematode parasite, Haemonchus contortus: amphidial neurons of the third-stage larva.

The sensory neuronal ultrastructure of the amphids of the infective larva (L3) of Haemonchus contortus was analyzed, compared, and contrasted with that of the first-stage larva (L1). As in L1, each amphid of the L3 is innervated by 12 neurons. Thirteen ciliated dendritic processes of 10 neurons, 3 with double processes, lie in each amphidial channel. The dendritic process of each finger cell neuron ends in a large number of digitiform projections or "fingers," many more than in the L1. Processes of another pair of specialized neurons, probable homologs of wing cells in Caenorhabditis elegans, extend into the extreme anterior tip of the larva; they are much longer than those in L1. In L3, the neurons exit through the posterior wall of the amphidial chamber individually rather than in a bundle, as in L1. Cell constancy between L1 and L3 was confirmed, and the neurons were individually identified. Significant neuron-specific variations, presumably related to functional differences between the 2 stages were observed. In contrast, species-specific differences are surprisingly small. Haemonchus contortus is closely related to hookworms and has amphidial structure nearly identical to that in hookworms and similar to that in C. elegans, to which it is also closely related.

Infection of mice with the helminth Strongyloides stercoralis suppresses pulmonary allergic responses to ovalbumin.

Asthma and helminth infections induce similar immune responses characterized by the presence of peripheral blood eosinophilia and elevated serum IgE levels. Epidemiological surveys have reported either increases or decreases in the development of atopic diseases and asthma based on the prevalence of helminth infections in the population. The aim of this study was to determine if a pre-existing helminth infection would increase or decrease subsequent allergic responses to an unrelated allergen in the lungs. BALB/cByJ mice were infected with the nematode parasite Strongyloides stercoralis prior to ovalbumin (OVA) immunization and intratracheal challenge. Bronchoalveolar lavage (BAL) and fluid (BALF) were collected 3 days post-challenge and cellular and humoral immune responses were measured. Intracellular cytokine staining revealed increased IL-4 and IL-5 producing cells in BAL from mice infected with S. stercoralis before OVA sensitization. Increased IL-5 protein levels and decreased IFN-gamma protein levels were also observed in the BALF. There was, however, no increase in airway eosinophil accumulation in mice infectd with parasites before sensitization with OVA as compared to mice exposed to OVA alone. Furthermore, eotaxin levels in the lungs induced by OVA was suppressed in mice infected with the parasite before OVA sensitization. The development of OVA specific IgE responses in BALF was also impaired in mice infected with the parasite before sensitization with OVA. These results suggest that a pre-existing helminth infection may potentiate a systemic Type 2-type response yet simultaneously suppress in the lungs allergen-specific IgE responses and eotaxin levels in response to subsequent exposure to allergens.

Role of IL-5 in innate and adaptive immunity to larval Strongyloides stercoralis in mice.

Protective immunity to Strongyloides stercoralis infective larvae in mice has been shown to be dependent on IL-5 based on mAb depletion studies. The goal of this study was to determine the functional role of IL-5 during the innate and adaptive immune response to larval S. stercoralis in mice. In these studies, three strains of mice were used: wild-type C57BL/6J (WT), IL-5 knockout (KO), and IL-5 transgenic (TG). Innate responses to the larvae indicated that there was enhanced survival in the KO animals and decreased survival in the TG animals compared with WT. Furthermore, killing of larvae in TG mice was associated with eosinophil infiltration and degranulation. In studying the adaptive immune response, it was observed that immunization of KO mice did not lead to the development of protective immunity. Experiments were then performed to determine whether KO mice reconstituted with Abs or cells could then develop protective immunity. KO mice displayed protective immunity via a granulocyte-dependent mechanism following injection of purified IgM from immune wild-type animals. Immunity in KO mice could also be reconstituted by the injection of eosinophils at the time of immunization. These eosinophils did not participate in actively killing the challenge infection, but rather were responsible for the induction of a protective Ab response. We conclude that IL-5 is required in the protective immune response for the production of eosinophils, and that eosinophils were involved in larval killing during innate immunity and in the induction of protective Abs in the adaptive immune response.

The neurons of class ALD mediate thermotaxis in the parasitic nematode, Strongyloides stercoralis.

Strongyloides stercoralis, a skin-penetrating nematode parasite of homeotherms, migrates to warmth. In nematodes, the amphids, anteriorly positioned, paired sensilla, each contain a bundle of sensory neurons. In the amphids of the free-living nematode Caenorhabditis elegans, a pair of neurons, each of which ends in a cluster of microvilli-like projections, are known to be the primary thermoreceptors, and have been named the finger cells (class AFD). A similar neuron pair in the amphids of the parasite Haemonchus contortus is also known to be thermosensory. Strongyloides stercoralis lacks finger cells but, in its amphids, it has a pair of neurons whose dendrites end in a multi-layered complex of lamellae, the so-called lamellar cells (class ALD). Consequently, it was hypothesised that these lamellar cells might mediate thermotaxis by the skin-penetrating infective larva of this species. To investigate this, first stage S. stercoralis larvae were anaesthetised and the paired ALD class neurons were ablated with a laser microbeam. The larvae were then cultured to the infective third stage (L3) and assayed for thermotaxis on a thermal gradient. L3 with ablated ALD class neuron pairs showed significantly reduced thermotaxis compared with control groups. The thermoreceptive function of the ALD class neurons (i) associates this neuron pair with the host-finding process of S. stercoralis and (ii) demonstrates a functional similarity with the neurons of class AFD in C. elegans. The structural and positional characteristics of the ALD neurons suggest that these neurons may, in fact, be homologous with one pair of flattened dendritic processes known as wing cells (AWC) in C. elegans, while their florid development and thermosensory function suggest homology with the finger cells (AFD) of that nematode.

Growth of the genital primordium as a marker to describe a time course for the heterogonic larval development in Strongyloides stercoralis.

A time course for the heterogonic development of Strongyloides stercoralis is described and a method for distinguishing the early larval stages of this nematode is proposed. The number of cells in the developing gonad were counted at various time intervals of incubation, along with the percentage of larvae in molt at each interval. The time course of growth of the gonad follows a pattern comparable to that reported for body length in an idealized general nematode. A model for the heterogonic development of S. stercoralis is proposed, which, although similar to other nematode developmental models, is stage specific for S. stercoralis, allowing the otherwise morphologically similar rhabditiform stages (L1, L2) to be distinguished.

Thermotaxis and thermosensory neurons in infective larvae of Haemonchus contortus, a passively ingested nematode parasite.

As a basis for studies of thermal behavior of infective larvae (L3) of Haemonchus contortus resulting from ablation of amphidial neurons, the locations of the amphidial cell bodies in the hatchling larva (L1) were compared with their locations in the L3. We sought to verify that killing each targeted cell body in L1 destroys the putative corresponding dendrite of the L3. These comparisons confirmed the predicted cell body-to-dendrite connections, as well as similarities in the general amphidial structure of the two stages. We then conducted a series of studies using laser microbeam ablation of amphidial cell bodies in the L1 to determine the role of specific neurons in the thermal behavior of the L3. In a thermal gradient, normal L3 of H. contortus migrate to the temperature at which they were cultured and/or maintained. Larvae grown at 16 degrees or 26 degrees C migrate appropriately to either of these temperatures. Larvae grown to the L3 stage at 16 degrees C and then moved to 26 degrees C become acclimated to this temperature and thereafter migrate to it. However, when the putative thermosensory neurons, the finger cell neurons (AFD), were ablated in hatchling larvae with a laser microbeam, and these were grown to the L3 stage and tested on a radial thermal gradient, they failed to migrate to their culture temperature. Instead, they moved actively and continuously over much of the assay plate surface, with no obviously oriented cryo- or thermotactic movement. Ablation-control larvae, those in which putatively chemosensory neuron classes ASE or AWC were killed, migrated normally to their culture temperature. When the RIA interneurons (identified by positional homology with those of Caenorhabditis elegans) were ablated, the operated larvae moved actively, but circled near the initial placement point; control larvae, in which other nonamphidial neurons were killed, migrated normally. These results indicate that the finger cell neurons (AFD) are the primary thermosensory class in H. contortus. The RIA-class neurons integrate thermal responses in H. contortus, as do their putative structural homologs in C. elegans, but the behavior of H. contortus subsequent to RIA ablation is strikingly different.

Sensory neuroanatomy of a passively ingested nematode parasite, Haemonchus contortus: amphidial neurons of the first stage larva.

When infective larvae of Haemonchus contortus (a highly pathogenic, economically important, gastric parasite of ruminants) are ingested by grazing hosts, they are exposed to environmental changes in the rumen, which stimulate resumption of development. Presumably, resumption is controlled by sensory neurons in sensilla known as amphids. Neuronal function can be determined by ablation of specifically recognized neurons in hatchling larvae (L1) in which neuronal cell bodies are easily visualized using differential interference microscopy. Using three-dimensional reconstructions from electron micrographs of serial transverse sections, amphidial structure of the L1 is described. Each amphid of H. contortus is innervated by 12 neurons. The ciliated dendritic processes of 10 neurons lie in the amphidial channel. Three of these end in double processes, resulting in 13 sensory cilia in the channel. One process, that of the so-called finger cell, ends in a number of digitiform projections. Another specialized dendrite enters the amphidial channel, but leaves it to end within the sheath cell, a hollow, flask-shaped cell that forms the base of the amphidial channel. Although not flattened, this process is otherwise similar to the wing cells in Caenorhabditis elegans; we consider it AWC of this group. Two other neurons, ASA and ADB, appear to be homologs of wing cells AWA and AWB in C. elegans, although they end as ciliated processes in the amphidial channel, rather than as flattened endings seen in C. elegans. Each of the 12 amphidial neurons was traced to its cell body in the lateral ganglion, posterior to the worm's nerve ring. The positions of these bodies were similar to their counterparts in C. elegans; they were named accordingly. A map for identifying the amphidial cell bodies in the living L1 was prepared, so that laser microbeam ablation studies can be conducted. These will determine which neurons are involved in the infective process, as well as others important in establishing the host-parasite relationship.

Strongyloides stercoralis: oral transfer of parasitic adult worms produces infection in mice and infection with subsequent autoinfection in gerbils.

Oral transfer of parasitic adult Strongyloides stercoralis produced patent infections in gerbils, C57BL/6J and SCID mice. In gerbils receiving adult worms, 7.3% of the transferred worms established and autoinfective L3 were found beginning on day 5 post-transfer, with peak numbers seen on days 6 and 7 post-transfer and few seen by 9 days post-transfer. These results suggest that development of autoinfective L3 in the gerbil is limited by the immune response of the host. When given orally to mice, between 7.2% (C57BL/6J) and 19.5% (SCID) of the adult worms established. These levels are higher than those previously obtained by the subcutaneous infection of SCID mice with infective larvae. No autoinfective larvae were found in infected mice and the ratio of L1/adult worms was small compared with that seen in gerbils. Thus, mice infected orally can be used as a model to study the interaction between the adult worm and the host, and since autoinfection has not been seen in the murine model, as developed to date, orally infected mice may be useful as a model to study mechanisms preventing autoinfection.

Chemo- and thermosensory neurons: structure and function in animal parasitic nematodes.

Nematode parasites of warm-blooded hosts use chemical and thermal signals in host-finding and in the subsequent resumption of development. The free-living nematode Caenorhabditis elegans is a useful model for investigating the chemo- and thermosensory neurons of such parasites, because the functions of its amphidial neurons are well known from laser microbeam ablation studies. The neurons found in the amphidial channel detect aqueous chemoattractants and repellants; the wing cells-flattened amphidial neurons-detect volatile odorants. The finger cells-digitiform amphidial neurons-are the primary thermoreceptors. Two neuron classes, named ADF and ASI, control entry into the environmentally resistant resting and dispersal dauer larval stage, while the paired ASJ neurons control exit from this stage. Skin-penetrating nematode parasites, i.e. the dog hookworm Ancylostoma caninum, and the threadworm, Strongyloides stercoralis, use thermal and chemical signals for host-finding, while the passively ingested sheep stomach worm, Haemonchus contortus, uses environmental signals to position itself for ingestion. Amphidial neurons presumably recognize these signals. In all species, resumption of development, on entering a host, is probably triggered by host signals also perceived by amphidial neurons. In the amphids of the A. caninum infective larva, there are wing- and finger-cell neurons, as well as neurons ending in cilia-like dendritic processes, some of which presumably recognize a sequence of signals that stimulate these larvae to attach to suitable hosts. The functions of these neurons can be postulated, based on the known functions of their homologs in C. elegans. The threadworm, S. stercoralis, has a complex life cycle. After leaving the host, soil-dwelling larvae may develop either to infective larvae (the life-stage equivalent of dauer larvae) or to free-living adults. As with the dauer larva of C. elegans, two neuron classes control this developmental switch. Amphidial neurons control chemotaxis to a skin extract, and a highly modified amphidial neuron, the lamellar cell, appears to be the primary thermoreceptor, in addition to having chemosensory function. The stomach worm, Haemonchus contortus, depends on ingestion by a grazing host. Once ingested, the infective larva is exposed to profound environmental changes in the rumen. These changes stimulate resumption of development in this species. We hypothesize that resumption of development is under the control of the ASJ neuronal pair. Identification of the neurons that control the infective process could provide the basis for entirely new approaches to parasite control involving interference with development at the time and place of initial host-contact.

Hyperinfective strongyloidiasis: Strongyloides stercoralis undergoes an autoinfective burst in neonatal gerbils.

One of the unusual aspect of the life cycle of Strongyloides stercoralis is the occurrence of autoinfective third-stage larvae (L3a). These are the causative agents of severe hyperinfective strongyloidiasis. When 6-wk-old gerbils are infected with 1,000 infective third-stage larvae (L3i), no L3a are seen during the course of the infection. However, in neonatal gerbils (1-13 days of age) infected with 1,000 L3i, a burst of autoinfection takes place between 15 and 30 days postinfection (PI). Only occasional L3a can be found in neonatally infected gerbils after 4 wk PI. This autoinfective burst is not seen in neonatal gerbils infected with 200 L3i.

Developmental switching in the parasitic nematode Strongyloides stercoralis is controlled by the ASF and ASI amphidial neurons.

Parasitic nematodes of the genus Strongyloides are remarkable for their ability to switch between alternative free-living developmental pathways in response to changing internal environmental conditions. After exiting the host, soil-dwelling larval stages may develop either to infectivity via 2 microbiverous stages (homogonic development) or to free-living adulthood via 4 microbiverous larval stages (heterogonic development). The progeny of these adults then give rise to the infective stage. In the latter case, free-living existence is extended in time and the number of infective larvae is greatly amplified. Anterior chemosensory neurons (amphidial neurons) are thought to respond to environmental cues and via signal transduction pathways control the direction of larval development. We now demonstrate by laser microbeam ablation that 2 classes of amphidial neurons (ASF and ASI), acting together, control the direction of free-living larval development. Larvae in which the neurons were killed developed to infectivity via the homogonic route rather than to adulthood via the otherwise predominant heterogonic route. These neurons are probable homologues of neurons ADF (=ASF) and ASI in Caenorhabditis elegans, suggesting the control of development at the cellular level is conserved among divergent taxa of nematodes. These observations also have important implications for the evolution of nematode parasitism and the design of new prophylactic measures against parasitic nematodes of medical and veterinary medical importance.

Strongyloides stercoralis host-adapted third-stage larvae are the target of eosinophil-associated immune-mediated killing in mice.

Host-adapted, transformed, Strongyloides stercoralis third-stage larvae (L3+) were previously found to be antigenically different from free-living, infective, third-stage larvae (L3). These antigenic differences were reproduced by transformation of free-living larvae in tissue culture medium at 37 C over 24 hr. Transformed L3 of both derivations were given as challenge infections in diffusion chambers to naive mice and mice immunized with S. stercoralis L3. Within 12 hr, the challenge infections were killed regardless of whether the L3+ were generated in vitro or in vivo. Eosinophils, previously found to be important in the immune response to S. stercoralis larvae, were recruited into the L3+ microenvironment within 12 hr of challenge infection in immune mice, which supports the previously proposed mechanisms of S. stercoralis larval killing. Thus, S. stercoralis L3+ appear to be targets of the immune response in mice instead of being involved in immune evasion.