Developmental maturation of millimeter-scale functional networks across brain areas.

Interacting with the environment to process sensory information, generate perceptions, and shape behavior engages neural networks in brain areas with highly varied representations, ranging from unimodal sensory cortices to higher-order association areas. Recent work suggests a much greater degree of commonality across areas, with distributed and modular networks present in both sensory and non-sensory areas during early development. However, it is currently unknown whether this initially common modular structure undergoes an equally common developmental trajectory, or whether such a modular functional organization persists in some areas-such as primary visual cortex-but not others. Here we examine the development of network organization across diverse cortical regions in ferrets of both sexes using in vivo widefield calcium imaging of spontaneous activity. We find that all regions examined, including both primary sensory cortices (visual, auditory, and somatosensory-V1, A1, and S1, respectively) and higher order association areas (prefrontal and posterior parietal cortices) exhibit a largely similar pattern of changes over an approximately 3 week developmental period spanning eye opening and the transition to predominantly externally-driven sensory activity. We find that both a modular functional organization and millimeter-scale correlated networks remain present across all cortical areas examined. These networks weakened over development in most cortical areas, but strengthened in V1. Overall, the conserved maintenance of modular organization across different cortical areas suggests a common pathway of network refinement, and suggests that a modular organization-known to encode functional representations in visual areas-may be similarly engaged in highly diverse brain areas.

Coordination of gaze and action during high-speed steering and obstacle avoidance.

When humans navigate through complex environments, they coordinate gaze and steering to sample the visual information needed to guide movement. Gaze and steering behavior have been extensively studied in the context of automobile driving along a winding road, leading to accounts of movement along well-defined paths over flat, obstacle-free surfaces. However, humans are also capable of visually guiding self-motion in environments that are cluttered with obstacles and lack an explicit path. An extreme example of such behavior occurs during first-person view drone racing, in which pilots maneuver at high speeds through a dense forest. In this study, we explored the gaze and steering behavior of skilled drone pilots. Subjects guided a simulated quadcopter along a racecourse embedded within a custom-designed forest-like virtual environment. The environment was viewed through a head-mounted display equipped with an eye tracker to record gaze behavior. In two experiments, subjects performed the task in multiple conditions that varied in terms of the presence of obstacles (trees), waypoints (hoops to fly through), and a path to follow. Subjects often looked in the general direction of things that they wanted to steer toward, but gaze fell on nearby objects and surfaces more often than on the actual path or hoops. Nevertheless, subjects were able to perform the task successfully, steering at high speeds while remaining on the path, passing through hoops, and avoiding collisions. In conditions that contained hoops, subjects adapted how they approached the most immediate hoop in anticipation of the position of the subsequent hoop. Taken together, these findings challenge existing models of steering that assume that steering is tightly coupled to where actors look. We consider the study's broader implications as well as limitations, including the focus on a small sample of highly skilled subjects and inherent noise in measurement of gaze direction.

Common modular architecture across diverse cortical areas in early development.

In order to deal with a complex environment, animals form a diverse range of neural representations that vary across cortical areas, ranging from largely unimodal sensory input to higher-order representations of goals, outcomes, and motivation. The developmental origin of this diversity is currently unclear, as representations could arise through processes that are already area-specific from the earliest developmental stages or alternatively, they could emerge from an initially common functional organization shared across areas. Here, we use spontaneous activity recorded with two-photon and widefield calcium imaging to reveal the functional organization across the early developing cortex in ferrets, a species with a well-characterized columnar organization and modular structure of spontaneous activity in the visual cortex. We find that in animals 7 to 14 d prior to eye-opening and ear canal opening, spontaneous activity in both sensory areas (auditory and somatosensory cortex, A1 and S1, respectively), and association areas (posterior parietal and prefrontal cortex, PPC and PFC, respectively) showed an organized and modular structure that is highly similar to the organization in V1. In all cortical areas, this modular activity was distributed across the cortical surface, forming functional networks that exhibit millimeter-scale correlations. Moreover, this modular structure was evident in highly coherent spontaneous activity at the cellular level, with strong correlations among local populations of neurons apparent in all cortical areas examined. Together, our results demonstrate a common distributed and modular organization across the cortex during early development, suggesting that diverse cortical representations develop initially according to similar design principles.

Estimating curvilinear self-motion from optic flow with a biologically inspired neural system.

Optic flow provides rich information about world-relative self-motion and is used by many animals to guide movement. For example, self-motion along linear, straight paths without eye movements, generates optic flow that radiates from a singularity that specifies the direction of travel (heading). Many neural models of optic flow processing contain heading detectors that are tuned to the position of the singularity, the design of which is informed by brain area MSTd of primate visual cortex that has been linked to heading perception. Such biologically inspired models could be useful for efficient self-motion estimation in robots, but existing systems are tailored to the limited scenario of linear self-motion and neglect sensitivity to self-motion along more natural curvilinear paths. The observer in this case experiences more complex motion patterns, the appearance of which depends on the radius of the curved path (path curvature) and the direction of gaze. Indeed, MSTd neurons have been shown to exhibit tuning to optic flow patterns other than radial expansion, a property that is rarely captured in neural models. We investigated in a computational model whether a population of MSTd-like sensors tuned to radial, spiral, ground, and other optic flow patterns could support the accurate estimation of parameters describing both linear and curvilinear self-motion. We used deep learning to decode self-motion parameters from the signals produced by the diverse population of MSTd-like units. We demonstrate that this system is capable of accurately estimating curvilinear path curvature, clockwise/counterclockwise sign, and gaze direction relative to the path tangent in both synthetic and naturalistic videos of simulated self-motion. Estimates remained stable over time while rapidly adapting to dynamic changes in the observer's curvilinear self-motion. Our results show that coupled biologically inspired and artificial neural network systems hold promise as a solution for robust vision-based self-motion estimation in robots.

A Dynamic Efficient Sensory Encoding Approach to Adaptive Tuning in Neural Models of Optic Flow Processing.

This paper introduces a self-tuning mechanism for capturing rapid adaptation to changing visual stimuli by a population of neurons. Building upon the principles of efficient sensory encoding, we show how neural tuning curve parameters can be continually updated to optimally encode a time-varying distribution of recently detected stimulus values. We implemented this mechanism in a neural model that produces human-like estimates of self-motion direction (i.e., heading) based on optic flow. The parameters of speed-sensitive units were dynamically tuned in accordance with efficient sensory encoding such that the network remained sensitive as the distribution of optic flow speeds varied. In two simulation experiments, we found that model performance with dynamic tuning yielded more accurate, shorter latency heading estimates compared to the model with static tuning. We conclude that dynamic efficient sensory encoding offers a plausible approach for capturing adaptation to varying visual environments in biological visual systems and neural models alike.

Effects of Buprenorphine Treatment on Influenza Pathogenesis in the Ferret (Mustela putorius furo).

Ferrets are the gold-standard model for influenza A virus (IAV) research due to their natural susceptibility to human and zoonotic IAV, comparable respiratory anatomy and physiology to humans, and development of clinical signs similar to those seen in infected people. Because the presence and progression of clinical signs can be useful in infectious disease research, uncertainty in how analgesics alter research outcomes or compromise characteristics of disease progression have outweighed the concern regarding animal discomfort from these symptoms. Nonetheless, the principles of animal research require consideration of refinements for this important model for IAV research. Opioids offer a possible refinement option that would not directly affect the inflammatory cascade involved in IAV infection. Mirroring pathogenicity studies that use ferrets, 12 ferrets were inoculated intranasally with the A(H3N2) IAV A/Panama/2007/1999 and divided into 3 treatment groups ( n = 4 each), of which 2 groups received buprenorphine treatments on different schedules and the third received a saline control. The duration and location of viral replication, lymphohematopoietic changes, and clinical signs were comparable across all groups at all time points. High quantities of infectious virus in nasal wash specimens were detected in ferrets from all groups through day 5 after inoculation, and peak viral titers from the upper respiratory tract did not differ between ferrets receiving buprenorphine treatments on either schedule. Compared with the saline group, ferrets receiving buprenorphine exhibited transient weight loss and pyrexia, but all groups ultimately achieved similar peaks in both of these measurements. Collectively, these findings support the continued evaluation of buprenorphine as a refinement for IAV-challenged ferrets.

Affordance-based versus current-future accounts of choosing whether to pursue or abandon the chase of a moving target.

Affordance-based control and current-future control offer competing theoretical accounts of the visual control of locomotion. The aim of this study was to test predictions derived from these accounts about the necessity of self-motion (Experiment 1) and target-ground contact (Experiment 2) in perceiving whether a moving target can be intercepted before it reaches an escape zone. We designed a novel interception task wherein the ability to perceive target catchability before initiating movement was advantageous. Subjects pursued a target moving through a field in a virtual environment and attempted to intercept the target before it escaped into a forest. Targets were catchable on some trials but not others. If subjects perceived that they could not reach the target, they were instructed to immediately give up by pressing a button. After each trial, subjects received a point reward that incentivized them to pursue only those targets that were catchable. On the majority of trials, subjects either pursued and successfully intercepted the target or chose not to pursue at all, demonstrating that humans are sensitive to catchability while stationary. Performance also degraded when the target was floating rather than in contact with the ground. Both findings are incompatible with the current-future account and support the affordance-based account of choosing whether to pursue moving targets.

Pharmacokinetic Profiles of Gabapentin after Oral and Subcutaneous Administration in Black-tailed Prairie Dogs (Cynomys ludovicianus).

In veterinary and human medicine, gabapentin (a chemical analog of γ-aminobutyric acid) is commonly prescribed to treat postoperative and chronic neuropathic pain. This study explored the pharmacokinetics of oral and subcutaneous administration of gabapentin at high (80 mg/kg) and low (30 mg/kg) doses as a potential analgesic in black-tailed prairie dogs (Cynomys ludovicianus; n = 24). The doses (30 and 80 mg/kg) and half maximal effective concentration (1.4 to 16.7 ng/mL) for this study were extrapolated from pharmacokinetic efficacy studies in rats, rabbits, and cats. Gabapentin in plasma was measured by using an immunoassay, and data were evaluated using noncompartmental analysis. The peak plasma concentrations (mean ±1 SD) were 42.6 ±14.8 and 115.5 ±15.2 ng/mL, respectively, after 30 and 80 mg/kg SC and 14.5 ±3.5 and 20.7 ±6.1 ng/mL after the low and high oral dosages, respectively. All peak plasma concentrations of gabapentin occurred within 5 h of administration. Disappearance half-lives for the low and high oral doses were 7.4 ± 6.0 h and 5.0 ± 0.8 h, respectively. The results of this study demonstrate that oral administration of gabapentin at low (30 mg/kg) doses likely would achieve and maintain plasma concentrations at half maximum effective concentration for 12 h, making it a viable option for an every 12-h treatment.

Sterile field contamination from powered air-purifying respirators (PAPRs) versus contamination from surgical masks.

BACKGROUND: Currently, powered air-purifying respirators (PAPRs) are not recommended for usage in close proximity to sterile fields owing to concerns that exhaled, unfiltered air potentially may cause contamination; however, this has not been confirmed by experimental study. METHODS: After establishing background levels of airborne contamination, our team placed settling plates in a sterile field and collected contamination from participants who were performing particulate-generating actions. Participants performed the actions while wearing various forms of respiratory protection, including: (1) a full facepiece PAPR, (2) a full facepiece PAPR with a shoulder-length hood, (3) a surgical mask, and (4) no facial covering (as a positive control to determine contamination-reduction effectiveness). Specimens were collected at the end of a 10-minute sampling time frame. After incubation at 36.5˚C for 72 hours, we tabulated colony forming units as a marker of contamination. RESULTS: Surgical masks and the 2 PAPR configurations all drastically reduced aerosolized droplet contamination. Surgical masks reduced contamination by 98.48%, and both PAPRs reduced contamination by 100% (compared with the usage of no facial covering). There was no statistical difference between their effectiveness (surgical mask vs both PAPRs, P value = .588 and no hood PAPR vs hood PAPR, P value >.999). DISCUSSION/CONCLUSIONS: Based on these findings, the tested PAPR configurations are effective at reducing aerosolized droplet contamination into a sterile field, and further testing is warranted to assess other PAPR configurations as well as PAPR suitability in an operating room.

Development of an Occupational Risk Assessment Tool for Laboratory Animal Facilities.

Introduction: Laboratory animal facilities aim to provide excellence in animal care and welfare and support scientific research. Critical to these goals is to ensure a safe work environment for personnel comprising veterinary and animal care, laboratory research, and maintenance staff. Objective: Thus, performing occupational risk assessments allows for evaluation of risks from identified hazards associated with a variety of tasks ongoing in laboratory animal facilities. Methods: Herein, we present the development of an occupational risk assessment tool purposed to capture the dynamics of work performed in laboratory animal facilities, calculate and prioritize identified risks associated with procedures and processes, and inform and evaluate risk mitigations. Results: We also discuss a risk assessment for refining sharps use in nonhuman primate husbandry and care to demonstrate the utility of this tool to improve occupational safety in our animal facility. Conclusion: This tool and framework evolve into a holistic occupational risk management system that identifies, evaluates, and mitigates occupational risks; determines risk acceptability; consistently ensures communication and consultation with frontline personnel, stakeholders, senior leadership, and subject matter experts in biosafety, science, and animal care and welfare; and continuously strives to improve and enhance the operations of laboratory animal facilities.

The effects of repeated automated plasmapheresis in goats (Capra hircus) in response to vaccination with purified influenza hemagglutinin proteins.

Seasonal influenza is a contagious respiratory illness that annually affects millions of people worldwide. To identify currently circulating influenza virus subtypes, the Centers for Disease Control and Prevention's International Reagent Resource distributes the World Health Organization (WHO) influenza reagent kits, which are used globally by testing laboratories for influenza surveillance. The data generated by the kits aid in strain selection for the influenza vaccine each season. The use of animals to produce high quality and quantities of antibodies is critical to the production of these kits. In this study, we assessed the effects and efficacy of repeated sampling from automated plasmapheresis in goats. Analysis of blood samples demonstrated that repeated automated plasmapheresis procedures did not adversely affect the immediate or long-term health of goats. Further, our results indicate that repeated plasmapheresis in goats was capable of generating 2 liters of antibody-rich plasma per goat per week. This volume is sufficient to produce enough WHO influenza kits to conduct over 1 million tests. Thus, we have shown that the rapid production of plasma in goats can positively impact the public health preparedness and response to influenza.

Pharmacokinetic Profiles of Meloxicam and Sustained-release Buprenorphine in Prairie Dogs (Cynomys ludovicianus).

In this study, we evaluated the pharmacokinetic profiles of meloxicam and sustained-release (SR) buprenorphine in prairie dogs. The 4 treatment groups were: low-dose meloxicam (0.2 mg/kg SC), high-dose meloxicam (4 mg/kg SC), low-dose buprenorphine SR (0.9 mg/kg SC), and high-dose buprenorphine SR (1.2 mg/kg SC). The highest plasma concentrations occurred within 4 h of administration for both meloxicam treatment groups. The therapeutic range of meloxicam in prairie dogs is currently unknown. However, as compared with the therapeutic range documented in other species (0.39 - 0.91 µg/mL), the mean plasma concentration of meloxicam fell below the minimal therapeutic range prior to 24 h in the low-dose group but remained above therapeutic levels for more than 72 h in the high-dose group. These findings suggest that the current meloxicam dosing guidelines may be subtherapeutic for prairie dogs. The highest mean plasma concentration for buprenorphine SR occurred at the 24-h time point (0.0098 µg/mL) in the low-dose group and at the 8-h time point (0.015 µg/mL) for the high-dose group. Both dosages of buprenorphine SR maintained likely plasma therapeutic levels (0.001 µg/mL, based on previous rodent studies) beyond 72 h. Given the small scale of the study and sample size, statistical analysis was not performed. The only adverse reactions in this study were mild erythematous reactions at injection sites for buprenorphine SR.

Assessment of Domestic Goats as Models for Experimental and Natural Infection with the North American Isolate of Rickettsia slovaca.

Rickettsia slovaca is a tick-borne human pathogen that is associated with scalp eschars and neck lymphadenopathy known as tick-borne lymphadenopathy (TIBOLA) or Dermacentor-borne necrosis erythema and lymphadenopathy (DEBONEL). Originally, R. slovaca was described in Eastern Europe, but since recognition of its pathogenicity, human cases have been reported throughout Europe. European vertebrate reservoirs of R. slovaca remain unknown, but feral swine and domestic goats have been found infected or seropositive for this pathogen. Recently, a rickettsial pathogen identical to R. slovaca was identified in, and isolated from, the American dog tick, Dermacentor variabilis. In previous experimental studies, this organism was found infectious to guinea pigs and transovarially transmissible in ticks. In this study, domestic goats (Capra hircus) were experimentally inoculated with the North American isolate of this R. slovaca-like agent to assess their reservoir competence-the ability to acquire the pathogens and maintain transmission between infected and uninfected ticks. Goats were susceptible to infection as demonstrated by detection of the pathogen in skin biopsies and multiple internal tissues, but the only clinical sign of illness was transient fever noted in three out of four goats, and reactive lymphoid hyperplasia. On average, less than 5% of uninfected ticks acquired the pathogen while feeding upon infected goats. Although domestic goats are susceptible to the newly described North American isolate of R. slovaca, they are likely to play a minor role in the natural transmission cycle of this pathogen. Our results suggest that goats do not propagate the North American isolate of R. slovaca in peridomestic environments and clinical diagnosis of infection could be difficult due to the brevity and mildness of clinical signs. Further research is needed to elucidate the natural transmission cycle of R. slovaca both in Europe and North America, as well as to identify a more suitable laboratory model.

Representational changes of latent strategies in rat medial prefrontal cortex precede changes in behaviour.

The ability to change behavioural strategies in the face of a changing world has been linked to the integrity of medial prefrontal cortex (mPFC) function in several species. While recording studies have found that mPFC representations reflect the strategy being used, lesion studies suggest that mPFC is necessary for changing strategy. Here we examine the relationship between representational changes in mPFC and behavioural strategy changes in the rat. We found that on tasks with a forced change in reward criterion, strategy-related representational transitions in mPFC occurred after animals learned that the reward contingency had changed, but before their behaviour changed. On tasks in which animals made their own strategic decisions, representational transitions in mPFC preceded changes in behaviour. These results suggest that mPFC does not merely reflect the action-selection policy of the animal, but rather that mPFC processes information related to a need for a change in strategy.

Assessment of penile erection methods in rhesus macaques to model pharmacokinetics of antiretroviral drugs and penile infection with simian immunodeficiency virus.

BACKGROUND: An established macaque model to assess HIV interventions against penile transmission is currently not available. Physiological changes during penile erections may affect susceptibility to infection and drug pharmacokinetics (PK). Here, we identify methods to establish erections in macaques to evaluate penile transmission, PK, and efficacy under physiologic conditions. METHODS: Penile rigidity and length were evaluated in eight rhesus macaques following rectal electrostimulation (RES), vibratory stimulation (VS), or pharmacological treatment with Sildenafil Citrate (Viagra) or Alprostadil. RESULTS: Rectal electrostimulation treatment increased penile rigidity (>82%) and length (2.5 ± 0.58 cm), albeit the response was transient. In contrast, VS alone or coupled with Viagra or Alprostadil failed to elicit an erection response. CONCLUSION: Rectal electrostimulation treatment elicits transient but consistent penile erections in macaques. High rigidity following RES treatment demonstrates increased blood flow and may provide a functional model for penile PK evaluations and possibly simian immunodeficiency virus (SIV) transmission under erect conditions.

A combined oral contraceptive affects mucosal SHIV susceptibility factors in a pigtail macaque (Macaca nemestrina) model.

BACKGROUND: Injectable hormonal contraception may increase women's risk of HIV acquisition and can affect biological risk factors in animal models of HIV. We established, for the first time, a model to investigate whether combined oral contraceptives (COC) alter SHIV susceptibility in macaques. METHODS: Seven pigtail macaques were administered a monophasic levonorgestrel (LNG)/ethinyl estradiol (EE) COC at 33% or 66% of the human dose for 60 days. Menstrual cycling, vaginal epithelial thickness, and other SHIV susceptibility factors were monitored for a mean of 18 weeks. RESULTS: Mean vaginal epithelial thicknesses were 290.8 µm at baseline and 186.2 µm during COC (P = 0.0141, Mann-Whitney U-test). Vaginal pH decreased from 8.5 during treatment to 6.5 post-treatment (0.0176 two-tailed t-test). Measured microflora was unchanged. CONCLUSIONS: COC caused thinning of the vaginal epithelium and vaginal pH changes, which may increase SHIV susceptibility. 0.033 mg LNG + .0066 mg EE appeared effective in suppressing ovulation.

Gradient rotating outer volume excitation (GROOVE): A novel method for single-shot two-dimensional outer volume suppression.

PURPOSE: To introduce a new outer volume suppression (OVS) technique that uses a single pulse and rotating gradients to accomplish frequency-swept excitation. This new technique, which is called gradient rotating outer volume excitation (GROOVE), produces a circular or elliptical suppression band rather than suppressing the entire outer volume. METHODS: Theoretical and k-space descriptions of GROOVE are provided. The properties of GROOVE were investigated with simulations, phantom, and human experiments performed using a 4T horizontal bore magnet equipped with a TEM coil. RESULTS: Similar suppression performance was obtained in phantom and human brain using GROOVE with circular and elliptical shapes. Simulations indicate that GROOVE requires less SAR and time than traditional OVS schemes, but traditional schemes provide a sharper transition zone and less residual signal. CONCLUSION: GROOVE represents a new way of performing OVS in which spins are excited temporally in space on a trajectory that can be tailored to fit the shape of the suppression region. In addition, GROOVE is capable of suppressing tailored regions of space with more flexibility and in a shorter period of time than conventional methods. GROOVE provides a fast, low SAR alternative to conventional OVS methods in some applications (e.g., scalp suppression).

Complex neural codes in rat prelimbic cortex are stable across days on a spatial decision task.

The rodent prelimbic cortex has been shown to play an important role in cognitive processing, and has been implicated in encoding many different parameters relevant to solving decision-making tasks. However, it is not known how the prelimbic cortex represents all these disparate variables, and if they are simultaneously represented when the task requires it. In order to investigate this question, we trained rats to run the Multiple-T Left Right Alternate (MT-LRA) task and recorded multi-unit ensembles from their prelimbic regions. Significant populations of cells in the prelimbic cortex represented the strategy controlling reward receipt on a given lap, whether the animal chose to go right or left on a given lap, and whether the animal made a correct decision or an error on a given lap. These populations overlapped in the cells recorded, with several cells demonstrating differential firing to all three variables. The spatial and strategic firing patterns of individual prelimbic cells were highly conserved across several days of running this task, indicating that each cell encoded the same information across days.

MRI by steering resonance through space.

PURPOSE: This work introduces a technique to excite MR signals locally and to steer this localized region over the object in a spatiotemporal manner. The purpose is to demonstrate the feasibility of MRI with multidimensional spatiotemporal-encoding in a way that provides the ability to compensate extreme field inhomogeneity. METHODS: The method is called steering resonance over the object (STEREO). A modulated gradient is applied in concert with a frequency-modulated pulse to steer a resonant region through space and thus produce sequential excitation and echo formation. Images are reconstructed using exclusively an inverse problem solution. RESULTS: Images of phantoms and human brain were produced to demonstrate the feasibility of the STEREO sequence and image reconstruction. Simulations support the postulated capability to compensate for extreme field inhomogeneity. CONCLUSION: STEREO represents a substantial departure from conventional MRI in which spins contained in the sample, slab, or slice are excited synchronously. By exciting spins sequentially along a curved spatial trajectory, STEREO in principle affords a unique opportunity to adjust for spatial variations in static and radiofrequency fields. By adjusting field amplitudes and frequencies in a temporal manner in STEREO, in future works it should be possible to perform MRI with highly inhomogeneous fields.

Evaluation of pigtail macaques as a model for the effects of copper intrauterine devices on HIV infection.

BACKGROUND: Long-acting, hormonal contraception may increase HIV risk. Copper intrauterine devices (IUDs) could serve as non-hormonal alternatives. We pilot a pigtail macaque model for evaluating HIV susceptibility factors during copper IUD use. METHODS: Frameless and flexible GyneFix(®) copper IUDs were surgically implanted into three SHIVSF 162p3 -positive macaques via hysterotomy and monitored for up to 4 months. Four macaques served as non-IUD controls. RESULTS: All animals retained the devices without complications. No consistent change in vaginal viral RNA or inflammatory cytokines was seen. Two animals had altered menstrual cycles and experienced marked thinning of vaginal epithelium after IUD insertion. Histological examination of uterine tissue at necropsy revealed endometrial ulceration and lymphocytic inflammation with glandular loss at sites of direct IUD contact. CONCLUSIONS: Although the need for insertion surgery could limit its usefulness, this model will allow studies on copper IUDs and SHIV shedding, disease progression, and HIV susceptibility factors.