Instability of the HLA-E peptidome of HIV presents a major barrier to therapeutic targeting

Naturally occurring T cells that recognize microbial peptides via HLA-E, a nonpolymorphic HLA class Ib molecule, could provide the foundation for new universal immunotherapeutics. However, confidence in the biological relevance of putative ligands is crucial, given that the mechanisms by which pathogen-derived peptides can access the HLA-E presentation pathway are poorly understood. We systematically interrogated the HIV proteome using immunopeptidomic and bioinformatic approaches, coupled with biochemical and cellular assays. No HIV HLA-E peptides were identified by tandem mass spectrometry analysis of HIV-infected cells. In addition, all bioinformatically predicted HIV peptide ligands (>80) were characterized by poor complex stability. Furthermore, infected cell elimination assays using an affinity-enhanced T cell receptor bispecific targeted to a previously reported HIV Gag HLA-E epitope demonstrated inconsistent presentation of the peptide, despite normal HLA-E expression on HIV-infected cells. This work highlights the instability of the HIV HLA-E peptidome as a major challenge for drug development.

Rapid HIV disease progression following superinfection in an HLA-B*27:05/B*57:01-positive transmission recipient

Background

The factors determining differential HIV disease outcome among individuals expressing protective HLA alleles such as HLA-B*27:05 and HLA-B*57:01 remain unknown. We here analyse two HIV-infected subjects expressing both HLA-B*27:05 and HLA-B*57:01. One subject maintained low-to-undetectable viral loads for more than a decade of follow up. The other progressed to AIDS in < 3 years.

Results

The rapid progressor was the recipient within a known transmission pair, enabling virus sequences to be tracked from transmission. Progression was associated with a 12% Gag sequence change and 26% Nef sequence change at the amino acid level within 2 years. Although next generation sequencing from early timepoints indicated that multiple CD8+ cytotoxic T lymphocyte (CTL) escape mutants were being selected prior to superinfection, < 4% of the amino acid changes arising from superinfection could be ascribed to CTL escape. Analysis of an HLA-B*27:05/B*57:01 non-progressor, in contrast, demonstrated minimal virus sequence diversification (1.1% Gag amino acid sequence change over 10 years), and dominant HIV-specific CTL responses previously shown to be effective in control of viraemia were maintained. Clonal sequencing demonstrated that escape variants were generated within the non-progressor, but in many cases were not selected. In the rapid progressor, progression occurred despite substantial reductions in viral replicative capacity (VRC), and non-progression in the elite controller despite relatively high VRC.

Conclusions

These data are consistent with previous studies demonstrating rapid progression in association with superinfection and that rapid disease progression can occur despite the relatively the low VRC that is typically observed in the setting of multiple CTL escape mutants.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0390-9) contains supplementary material, which is available to authorized users.

Role of HIV-specific CD8+ T cells in pediatric HIV cure strategies after widespread early viral escape

Recent studies have suggested greater HIV cure potential among infected children than adults. A major obstacle to HIV eradication in adults is that the viral reservoir is largely comprised of HIV-specific cytotoxic T lymphocyte (CTL) escape variants. We here evaluate the potential for CTL in HIV-infected slow-progressor children to play an effective role in "shock-and-kill" cure strategies. Two distinct subgroups of children were identified on the basis of viral load. Unexpectedly, in both groups, as in adults, HIV-specific CTL drove the selection of escape variants across a range of epitopes within the first weeks of infection. However, in HIV-infected children, but not adults, de novo autologous variant-specific CTL responses were generated, enabling the pediatric immune system to "corner" the virus. Thus, even when escape variants are selected in early infection, the capacity in children to generate variant-specific anti-HIV CTL responses maintains the potential for CTL to contribute to effective shock-and-kill cure strategies in pediatric HIV infection.

Disease progression despite protective HLA expression in an HIV-infected transmission pair

Background

The precise immune responses mediated by HLA class I molecules such as HLA-B*27:05 and HLA-B*57:01 that protect against HIV disease progression remain unclear. We studied a CRF01_AE clade HIV infected donor-recipient transmission pair in which the recipient expressed both HLA-B*27:05 and HLA-B*57:01.

Results

Within 4.5 years of diagnosis, the recipient had progressed to meet criteria for antiretroviral therapy initiation. We employed ultra-deep sequencing of the full-length virus genome in both donor and recipient as an unbiased approach by which to identify specific viral mutations selected in association with progression. Using a heat map method to highlight differences in the viral sequences between donor and recipient, we demonstrated that the majority of the recipient’s mutations outside of Env were within epitopes restricted by HLA-B*27:05 and HLA-B*57:01, including the well-studied Gag epitopes. The donor, who also expressed HLA alleles associated with disease protection, HLA-A*32:01/B*13:02/B*14:01, showed selection of mutations in parallel with disease progression within epitopes restricted by these protective alleles.

Conclusions

These studies of full-length viral sequences in a transmission pair, both of whom expressed protective HLA alleles but nevertheless failed to control viremia, are consistent with previous reports pointing to the critical role of Gag-specific CD8+ T cell responses restricted by protective HLA molecules in maintaining immune control of HIV infection. The transmission of subtype CRF01_AE clade infection may have contributed to accelerated disease progression in this pair as a result of clade-specific sequence differences in immunodominant epitopes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0179-z) contains supplementary material, which is available to authorized users.

IVA: accurate de novo assembly of RNA virus genomes

Motivation: An accurate genome assembly from short read sequencing data is critical for downstream analysis, for example allowing investigation of variants within a sequenced population. However, assembling sequencing data from virus samples, especially RNA viruses, into a genome sequence is challenging due to the combination of viral population diversity and extremely uneven read depth caused by amplification bias in the inevitable reverse transcription and polymerase chain reaction amplification process of current methods.

Results: We developed a new de novo assembler called IVA (Iterative Virus Assembler) designed specifically for read pairs sequenced at highly variable depth from RNA virus samples. We tested IVA on datasets from 140 sequenced samples from human immunodeficiency virus-1 or influenza-virus-infected people and demonstrated that IVA outperforms all other virus de novo assemblers.

Availability and implementation: The software runs under Linux, has the GPLv3 licence and is freely available from http://sanger-pathogens.github.io/iva

Contact:iva@sanger.ac.uk

Supplementary information:Supplementary data are available at Bioinformatics online.

Programmed death-1 expression on HIV-1-specific CD8+ T cells is shaped by epitope specificity, T-cell receptor clonotype usage and antigen load

OBJECTIVES

Although CD8+ T cells play a critical role in the control of HIV-1 infection,their antiviral efficacy can be limited by antigenic variation and immune exhaustion.The latter phenomenon is characterized by the upregulation of multiple inhibitory receptors, such as programmed death-1 (PD-1), CD244 and lymphocyte activation gene-3 (LAG-3), which modulate the functional capabilities of CD8+ T cells.

DESIGN AND METHODS

Here, we used an array of different human leukocyte antigen(HLA)-B*15:03 and HLA-B*42:01 tetramers to characterize inhibitory receptor expression as a function of differentiation on HIV-1-specific CD8+ T-cell populations(n = 128) spanning 11 different epitope targets.

RESULTS

Expression levels of PD-1, but not CD244 or LAG-3, varied substantially across epitope specificities both within and between individuals. Differential expression of PD-1 on T-cell receptor (TCR) clonotypes within individual HIV-1-specific CD8+ T-cell populations was also apparent, independent of clonal dominance hierarchies. Positive correlations were detected between PD-1 expression and plasma viral load, which were reinforced by stratification for epitope sequence stability and dictated by effector memory CD8+ T cells.

CONCLUSION

Collectively, these data suggest that PD-1 expression on HIV-1-specific CD8+ T cells tracks antigen load at the level of epitope specificity and TCR clonotype usage. These findings are important because they provide evidence that PD-1 expression levels are influenced by peptide/HLA class I antigen exposure.

Fine motor control in rats is disrupted by delta-9-tetrahydrocannabinol

Evidence has suggested that cannabinoids such as THC, the active ingredient in marijuana, cause deficits in motor control and the production of movement. However, the specific components of motor control that are affected by cannabinoids have yet to be identified. The present study used an operant beam-press paradigm with a force criterion to determine the effects of THC on different parts of the force-time trajectory. Seven rats were trained to press a beam with at least 50 g of force to receive a sugar solution. THC was injected, as was apomorphine (APO), a selective dopamine D(2)/D(1) receptor agonist that acts as an antagonist at low doses. Low doses of APO, which have been found to cause deficits in motor execution, were used as a control for the effects of THC. Average peak force of a given press, as well as rate of rise of force, were significantly lowered by THC, as well as by apomorphine. Past research suggests that deficits in the rate of rise of force that can be attributed to depletions of dopamine in the nigrostriatal pathway, as in the case of low doses of APO, reflect failures of motor unit recruitment rather than of motor memory. Similarities in the motor effects of THC and APO suggest that THC plays a role in recruitment and synchronization of motor neurons appropriate for a given task.

A comparison of the effects of amphetamine and low doses of apomorphine on operant force production, inter-response times and response duration in rat

RATIONALE

Low doses of apomorphine (APO), a non-selective dopamine (DA) agonist, are thought to suppress motor activity via the preferential activation of DA autoreceptors, which effectively reduces DA tone.

OBJECTIVES

The suppressant effects on operant responding of low doses of apomorphine were explored and compared with the effects of amphetamine (AMP), an indirect DA agonist.

METHODS

In an operant task, rats were trained to press sequentially three separate beams under the following different behavioral requirements: low-force beam (1 g<force<3 g), high-force beam (force>50 g), and a long-duration beam (response duration>2 s). Inter-response times and kinetic measures, such as peak force, the rate of rise of force and response duration, were recorded. Following training, performance was assessed after systemic injection of low doses of APO (0.01, 0.03 and 0.1 mg/kg, s.c.) and AMP (0.1, 0.3 and 1.0 mg/kg, i.p.).

RESULTS

APO decreased peak force for the high-force and the long-duration beams by decreasing the rate of rise of force, but did not affect performance success on the low-force beam or response duration on the long-duration beam. This indicates that APO impaired the ability to generate high forces but did not interfere with the memory or execution of an overall motor plan. Low doses of APO also increased the times taken to switch from one response to the next and to visit the tray when food was present. In contrast, AMP at 1.0 mg/kg shortened both the time taken to switch between responses and the time spent visiting the food tray.

CONCLUSIONS

Low doses of APO interfered with response initiation and execution, suggesting that dopamine acts as a "gating" system, enabling certain processes to be carried out in an efficient and automated manner.

Control of operant response force

Differences in motor-control strategies (feedback or feedforward) engaged by rats to produce operant response force were investigated under 2 conditions of external feedback. In the immediate condition, liquid sucrose reinforcers were delivered as soon as each forelimb response met the force requirement, whereas under the terminal condition, reinforcers were delivered at response termination. When feedback control of response force was precluded by delivering reinforcers at response termination, force was adjusted by modulation of the rate of rise of force. However, under immediate reinforcer delivery, response force was controlled by adjustments of time to peak force. Such adjustments of response time to meet response requirements of increasing difficulty are consonant with expressions of the speed-accuracy tradeoff commonly observed in studies of human motor control.

Palpitations and cardiac awareness after heart transplantation

OBJECTIVE

The aim of this study was to examine the awareness of resting heartbeat in heart transplantation recipients, compare it with that found in other medical populations, and determine whether clinical characteristics are associated with accurate heartbeat awareness.

METHODS

Eligible patients underwent a research battery consisting of a heartbeat detection task and self-report questionnaires assessing cardiac symptoms, psychosocial variables, and cognitive function. The accurate awareness of resting heartbeat was determined by presenting the patients with auditory stimuli at each of six different delays following the R wave on the ECG. Patients then selected the tones that they thought coincided with the sensation they had of their heart beating. The patients' physicians rated their cardiac morbidity. The results were contrasted with comparable data obtained in previous work with other ambulatory medical populations.

RESULTS

Forty-one consecutive heart transplantation recipients who survived for at least 3 months after surgery were eligible. Thirty-four (82.9%) of them were studied and complete data were obtained on 26 (63.4%). Nine patients (34.6%) were reliably able to detect their resting heartbeat. When compared with the 17 patients who were not accurately aware of their heartbeat, the two groups did not differ significantly in cardiac morbidity, cognitive brain dysfunction, generalized psychiatric distress, depression, somatization, or hypochondriacal attitudes. A significantly higher proportion of heart transplantation recipients were accurately aware of their heartbeat than was found in a sample of general medical outpatients and in asymptomatic, nonpatient volunteers.

CONCLUSIONS

One-third of heart transplant recipients are accurately aware of resting heartbeat, despite the absence of cardiac innervation.

Dopamine depletion in nucleus accumbens influences locomotion but not force and timing of operant responding

This experiment examined the role of dopamine (DA) in the nucleus accumbens in regulating beam pressing and locomotor responses. Six rats were rewarded with sucrose on a partial reinforcement schedule for pressing force-sensitive beams. Open-field locomotor activity, and the force and timing characteristics of operant motor responses were recorded. It is known that low doses of apomorphine decrease DA tone through activating DA autoreceptors, resulting in suppression of both operant responses and locomotion. Our results showed that DA depletion in the nucleus accumbens, induced by bilateral injection of 6-hydroxydopamine, did not affect the force and timing of operant responses: neither did it reverse the suppressive effects of low doses of apomorphine on the force and timing of operant responses. However, accumbens DA depletion did block the suppressive effect of apomorphine on open-field locomotion. These results were interpreted as support for the hypothesis that the suppressive effects of low doses of apomorphine on locomotion, but not on operant beam pressing, are mediated mainly by DA autoreceptors in the mesolimbic pathway.

The antidepressant fluvoxamine increases natural killer cell counts in cancer patients

Knowing the negative effect of depression on lymphocyte number and activity in humans, we investigated the effect of antidepressant therapy on various lymphocyte subgroups. Cancer patients receiving treatment for at least 6 months were asked to take the antidepressant, fluvoxamine, for 28 days. Before and at the end of the study, physical and psychiatric examinations were performed, and the severity of depression was assessed by the Hamilton Scale for Depression (HAM-D). In addition, a sample of blood was withdrawn from the patients to quantify the following parameters: total leukocyte and lymphocyte counts, T4, T8, and Natural Killer (NK) cells, and lymphocyte response to the mitogens phytohemagglutinin (PHA) and pokeweed (PWM). Ten adult patients completed the study. Five of the 10 responded favorably to fluvoxamine treatment. Mean improvement was 50% from the score on day one. There was a significant correlation between the change in the HAM-D score of the "responders" and the change in NK cell counts (p = 0.02). The mean increment in NK cell number was 53%. In 4 of the 5 "non-responders", NK cell number dropped by 65% (mean). No correlation between the change in HAM-D score and any other immunological parameters was detected. Fluvoxamine increases NK cell counts in cancer patients, probably by its antidepressant effect.

Sensitivity to mechanical stimuli and the role of general sensory and perceptual processes in heartbeat detection

Sensitivity to heartbeat sensations is commonly assessed using tasks that require individuals to judge the simultaneity of heartbeats and tones. In two experiments, we investigated the suitability of this paradigm for examining cardioception. In the first experiment, participants judged the simultaneity of near-threshold vibrations and suprathreshold tones. Precision in judging vibration-tone simultaneity was directly related to the detectability of the mechanical stimuli, thereby supporting use of the simultaneity paradigm to assess heartbeat detection. In the second experiment, we examined the influences of sensitivity to mechanical stimuli and the ability to make intermodality simultaneity judgments on the precision of heartbeat detection. We measured participants' vibrotactile thresholds, precision in judging light-tone simultaneity, and precision in judging heartbeat-tone simultaneity. The ability to judge the simultaneity of lights and tones accounted for 24.3% of the variance in precision of heartbeat detection, and mechanical sensitivity accounted for a further 8.5%.

The work costs of earning food as a determinant of patch leaving

Foraging theorists have long emphasized the role of the energy (work) costs of food items on foraging behavior. However, few experiments have measured this variable or demonstrated that animals are indeed sensitive to work costs. Experiment 1 assessed whether rats (Long-Evans) can use the work costs of food to determine whether a food patch is exhausted. Rats performed a fixed amount of work for each food item (fixed-work [FW] schedule), but food was withheld unpredictably to simulate sudden patch depletion. It was found that rats left patches only when the work costs of unsuccessful searches (giving-up work) exceeded the prevailing work costs of food. The time and response costs of unsuccessful food searches (giving-up time and giving-up responses) were not predictive of patch leaving. Experiment 2 showed how rats regulated work in this paradigm by examining the role of exteroceptive stimuli connected with fulfilling the FW schedule.

The work costs of earning food as a determinant of patch leaving

Foraging theorists have long emphasized the role of the energy (work) costs of food items on foraging behavior. However, few experiments have measured this variable or demonstrated that animals are indeed sensitive to work costs. Experiment 1 assessed whether rats (Long-Evans) can use the work costs of food to determine whether a food patch is exhausted. Rats performed a fixed amount of work for each food item (fixed-work [FW] schedule), but food was withheld unpredictably to simulate sudden patch depletion. It was found that rats left patches only when the work costs of unsuccessful searches (giving-up work) exceeded the prevailing work costs of food. The time and response costs of unsuccessful food searches (giving-up time and giving-up responses) were not predictive of patch leaving. Experiment 2 showed how rats regulated work in this paradigm by examining the role of exteroceptive stimuli connected with fulfilling the FW schedule.

Influence of beliefs about heart rate and actual heart rate on heartbeat counting

The heartbeat counting task is used widely to assess the ability to perceive cardiac sensations. However, performance on this task may be determined by processes other than sensitivity to cardiac stimuli. Beliefs about the effects of postural and exercise challenges on heart rate were assessed by questionnaire prior to performing the heartbeat counting task, and then actual heart rate was manipulated during the counting task by changes in posture and exercise. On average, counted heart rates were closer to beliefs about heart rate than to actual heart rate. Hierarchical multiple regression analyses indicated that (a) believed heart rates predicted counted heart rates while controlling for actual heart rates and (b) actual heart rates also predicted counted heart rate while controlling for heart rate beliefs. These data suggest that the rate of heartbeat counting is influenced by beliefs about heart rate and by the processing of cardiac sensations.

Variation of Isometric Response Force in the Rat

Hungry, unrestrained rats (N = 7) were rewarded for pressing a response beam in excess of 11 different force requirements. Changes in peak force production as a function of peak force requirement were examined by analyses of the first four moments of distributions of peak response forces: constant error, the within-subject standard deviation, skewness, and kurtosis. Results were similar to those previously obtained with human subjects: Constant error was positive at low and negative at high force requirements, the within-subject standard deviation increased as a negatively accelerating function of force requirement, and skewness and kurtosis were positive at low force requirements and decreased to negative values at the highest increments. Additional analyses of response kinetics indicated that rats, like humans, meet increasing force requirements by altering the rate of rise of force. The performance similarities suggest that common processes are engaged by the human and rat motor control systems to solve the problem of generating forces that are appropriate to the prevailing environmental constraints.

Accurate awareness of heartbeat in hypochondriacal and non-hypochondriacal patients

We measured the accurate awareness of resting heartbeat in a sample of medical out-patients meeting DSM-III-R criteria for hypochondriasis (n = 60), and in a comparison group of non-hypochondriacal patients (n = 60) from the same general medical clinic. Patients also completed subjective self-report ratings of their sensitivity to benign bodily sensation and of functional somatic symptoms. Hypochondriacal patients did not differ significantly from non-hypochondriacal patients in their accurate awareness of heartbeat. They did, however, consider themselves more sensitive to benign bodily sensation and report more functional somatic symptoms. Within each sample, the only statistically significant association found was a negative correlation (r = -0.32, p = 0.025) between heartbeat awareness and the severity of hypochondriacal symptoms among the hypochondriacal patients. These results suggest that hypochondriacs may not be more accurately aware of normal cardiac activity, and therefore that hypochondriacal somatic complaints may not result from an unusually fine discriminative ability to detect normal physiological sensations that non-hypochondriacal individuals do not perceive.

Cardiac stimulus intensity and heartbeat detection: effects of tilt-induced changes in stroke volume

The effects of variations in stroke volume on the intensity of cardiac stimuli was examined in a series of three experiments that employed classical psychophysical methods to assess heartbeat detection. Stroke volume was manipulated by passive body tilt and recorded using impedance cardiography while subjects performed heartbeat detection tasks. The postural manipulation generated little or no change in contractility or momentum but did produce substantial changes in stroke volume. However, this potential source of variation in cardiac stimulus magnitude did not influence either the precision of heartbeat detection or the temporal location of heartbeat sensations. It is concluded that the intensity dimension of the heartbeat stimulus is not determined by stroke volume.

Effects of data limitations on heartbeat detection in the method of constant stimuli

In the method of constant stimuli applied to measuring heartbeat detection, subjects judge the simultaneity of heartbeats and exteroceptive comparison stimuli presented at various intervals after the R-wave (0, 100, 200, 300, 400, and 500 ms). Using versions of this procedure, investigators have found that between 20% (Yates, Jones, Marie, & Hogben, 1985) and 54% (Brener, Liu, & Ring, 1993) of subjects can detect heartbeat sensations. Whereas Yates et al. used a single comparison stimulus on each trial and Brener et al. used 10, the present study examined whether this disparity in heartbeat detection performance could be attributed to the number of comparison stimuli presented on each trial. In each of 360 trials, 30 subjects judged the simultaneity of heartbeat sensations and tones following 1, 5, or 10 comparison stimulus presentations. Significantly fewer subjects met the criterion for heartbeat detection with 1 tone presentation (13%) than with either 5 (43%) or 10 (47%) tone presentations. It is concluded that a single stimulus presentation imposes data limitations that result in underestimation of the accuracy of heartbeat detection. The presentation of at least 5 stimuli in each trial alleviates this limitation.

A method of constant stimuli for examining heartbeat detection: comparison with the Brener-Kluvitse and Whitehead methods

A method of constant stimuli (MCS) for measuring heartbeat detection was examined by comparing performance on this method with the Brener and Kluvitse (BK) method and a variant of the Whitehead (WH') method. All methods required subjects to judge the simultaneity of heartbeat sensations and tones, and each yielded measures of judgment precision and of the temporal location of heartbeat sensations relative to the electrocardiogram R-wave. Both measures were found to be significantly correlated across tasks. A greater proportion of subjects met criteria for classification as heartbeat detectors on the MCS (54%) and BK (50%) tasks than on the WH' task (33%). In the MCS and BK tasks, subjects judged tones presented 100-300 ms after the R-wave to be most simultaneous with heartbeat sensations. Intratask correlations showed that only the MCS procedure yielded stable measurements of both judgment precision and temporal location. The MCS procedure possesses commendable psychometric properties and provides a simpler means than the BK procedure of examining heartbeat detection.

The perception of cardiac activity in medical outpatients

This study compared several measures of cardiac perception and related them to patient' spontaneous reports of palpitations. One hundred and forty-five ambulatory patients referred for Holter monitoring for the evaluation of palpitations were compared with 70 asymptomatic nonpatients. Reports of palpitations during monitoring were compared with the ECG to determine whether they coincided with an arrhythmia. Subjects also completed a heartbeat detection task to determine whether they were accurately aware of cardiac systole while at rest. 20.7% of palpitation patients and 4.7% of asymptomatic controls demonstrated an accurate awareness of resting heartbeat (p = 0.01). Performance was unrelated to bodily amplification, somatization, hypochondriacal symptoms, ECG findings, or psychiatric morbidity. 34.3% of palpitation patients reported symptoms that consistently coincided with arrhythmias on ECG. These accurate patients had significantly lower levels of amplification, somatization, hypochondriacal symptoms, and psychiatric morbidity. Accuracy of symptom reporting and accuracy of heartbeat awareness were not statistically associated.(ABSTRACT TRUNCATED AT 250 WORDS)

The temporal locations of heartbeat sensations

This experiment examined individual differences in the temporal location of heartbeat sensations and the reliability of their detection. Following practice on a familiarization task in judging light-tone simultaneity, 50 subjects were studied for two sessions on a heartbeat detection task, which required judging the simultaneity of heartbeat sensations and tones presented at six intervals following the onset of ventricular contraction. Subjects inspected the heartbeat-tone intervals as frequently and for as long as desired before choosing the one in which heartbeat sensations and tones were most simultaneous. The temporal locations of heartbeat sensations in subjects classified as "good heartbeat perceivers" (n = 16) ranged from 131 and 363 ms following ventricular contraction (mean = 228 ms). Correlations between the familiarization and heartbeat detection tasks in the accuracy of simultaneity judgments and in the frequency and duration of interval inspections suggest that general perceptual abilities and strategies may underlie success in detecting heartbeat sensations.

Energetic and motor responses to increasing force requirements

The effects of increasing work (force) requirements on energy expenditure and response topography were examined in 7 rats pressing a beam to earn food. For the 1st 16 days, the force requirement was 5.52 x 10(-2) N (5.625 g). This increased by 4.91 x 10(-2) N (5 g) every 7th session until Ss had experienced 10 upward shifts. Following the 54.57 x 10(-2) N (55.625 g) condition, the original criterion was reinstated. During the augmented phase. Ss maintained stable reinforcement rates across conditions by increasing the peak force of beam pressing. These higher forces, occurring within 20 reinforcements of changing the force criterion, were produced primarily by increases in the rate of change of force (delta F/delta T). Also, while the rate of work performed on the beam increased, the overall energy expenditure fell. In contrast to these rapid adjustments, reinstating the original 5.52 x 10(-2) N (5.625 g) criterion resulted in only gradual alterations in motor performance.

Energetic and motor responses to increasing force requirements

The effects of increasing work (force) requirements on energy expenditure and response topography were examined in 7 rats pressing a beam to earn food. For the 1st 16 days, the force requirement was 5.52 x 10(-2) N (5.625 g). This increased by 4.91 x 10(-2) N (5 g) every 7th session until Ss had experienced 10 upward shifts. Following the 54.57 x 10(-2) N (55.625 g) condition, the original criterion was reinstated. During the augmented phase. Ss maintained stable reinforcement rates across conditions by increasing the peak force of beam pressing. These higher forces, occurring within 20 reinforcements of changing the force criterion, were produced primarily by increases in the rate of change of force (delta F/delta T). Also, while the rate of work performed on the beam increased, the overall energy expenditure fell. In contrast to these rapid adjustments, reinstating the original 5.52 x 10(-2) N (5.625 g) criterion resulted in only gradual alterations in motor performance.

Physiological correlates of schedule-induced activities in rats

Oxygen consumption and heart rate as well as a range of behavior variables were tracked continuously as rats adapted to a schedule of food delivery. Over 15 days of observation a majority of the subjects developed characteristic patterns of schedule-induced polydipsia (SIP) in which bouts of drinking reliably followed food delivery. Variations in food tray entries, oxygen consumption, heart rate, and, in the final stage of the experiment, rates of general activity were also time locked to food delivery in both rats exhibiting SIP and nondrinkers. However, the patterns of variation in these measures differed consistently between these two groups. Oxygen consumption varied over a wider range and reached higher levels in drinkers than nondrinkers. Additionally, heart rate was lower in the drinkers, which, in the final stage of the experiment, also exhibited depressed rates of food tray entries and general activity relative to the nondrinkers. Each of these between-subject differences was paralleled by differences within the drinking group between trials on which drinking occurred and trials on which it did not occur. The implications of these results on the utility of unidimensional energetic constructs are discussed.

Changes in energy expenditure and work during response acquisition in rats

The principle of least effort predicts that behavior will tend to maximum efficiency. To test this prediction, changes in the energy expended (VO2) and work performed per reinforcement were monitored continuously as rats learned to press a beam with a criterion force for liquid food rewards. All 12 subjects exhibited significant decreases in energy expended per reinforcement over the 16 days of observation. Of these, 10 subjects also decreased the work performed per reinforcement. Analyses of motor performance were undertaken to determine how motor programs for changing efficiency were generated. The 10 animals showing decreased work reinforcement also exhibited significant decreases in the variability of temporal and kinetic response features and in mean response magnitude (time integral of force or work per response) as a function of practice. Adjustments in work output were primarily accomplished by modifying temporal response features (response duration and, initially, interresponse time). The kinetic features (response recruitment and peak force) remained relatively constant for these animals. The remaining 2 subjects differed in that response recruitment increased after Day 9, resulting in progressively larger amounts of work being performed to earn each reinforcement, and the interval between successive reinforcements decreased.

Bovine babesiosis: severity and reproducibility of Babesia bovis infections induced by Boophilus microplus under laboratory conditions

A total of 61 intact Holstein-Friesian calves were exposed to Babesia bovis (= B argentina) by the injection of infected blood or the application of infected Boophilus microplus larvae. Tick-induced infections were uniformly severe, even when induced by relatively small numbers of infected ticks. In contrast, calves infected with carrier blood experienced mild, subclinical reactions despite detectable parasitaemia. The greater severity of tick-induced reactions appeared to be due to the large number of infective doses injected by each infected tick rather than greater virulence of tick versus blood-origin babesiae. The severity of tick-induced babesiosis was related to the age of the experimental calves, with more severe reactions and high mortality occurring among older animals. The mean daily temperature rise increased with age and was highest in 15 animals dying of babesiosis. No relationship could be found between peripheral blood parasitaemia, packed cell volume reduction and mortality in tick-induced infections. One thousand larvae from infected colonies induced babesiosis in all of 29 animals exposed. Although larval transmission of B bovis is difficult to quantitate, there would seem to be no objection to using a controlled tick-borne challenge in the laboratory for assessment of susceptibility.