Valid Animal Model Research Articles

Immunization with amodiaquine-modified hepatic proteins prevents amodiaquine-induced liver injury

Amodiaquine (AQ) has been reported to cause severe idiosyncratic drug-induced liver injury (IDILI) in humans. There is evidence that AQ-induced idiosyncratic drug reactions are immune mediated, but their exact mechanism is not fully understood. AQ is oxidized to a reactive quinoneimine metabolite, and it has been suggested that covalent binding of this metabolite leads to an immune response and liver injury. A valid animal model would greatly facilitate mechanistic studies. This laboratory had previously reported that chronic treatment of C57BL/6 mice with AQ induced a delayed-onset mild liver injury that appeared to resolve with immune tolerance. The current study attempted to prevent immune tolerance by first immunizing mice with AQ-modified hepatic proteins. This study used a soluble adjuvant known as ‘covax’ that has been reported to produce a better immune response than Freund’s adjuvant. After immunization, the mice were treated with 0.2% AQ in food for 5 weeks, as previously done. Paradoxically, the immunization protected animals from AQ-induced liver injury instead of exacerbating it. Consistent with this protection, immunization also appeared to lead to a tolerogenic response with an increase in myeloid derived suppressor cells, M2 macrophages, and FoxP3+ T-cells. This attempt to develop an animal model of IDILI was unsuccessful and illustrated the complexity of the immune response and the difficulty of inducing a sustained immune response in the liver. It also reinforces the hypothesis that the key determinant of IDILI is immune tolerance.

Rodent models of treatment-resistant depression

Major depression is a prevalent and debilitating disorder and a substantial proportion of patients fail to reach remission following standard antidepressant pharmacological treatment. Limited efficacy with currently available antidepressant drugs highlights the need to develop more effective medications for treatment- resistant patients and emphasizes the importance of developing better preclinical models that focus on treatment- resistant populations. This review discusses methods to adapt and refine rodent behavioral models that are predictive of antidepressant efficacy to identify populations that show reduced responsiveness or are resistant to traditional antidepressants. Methods include separating antidepressant responders from non-responders, administering treatments that render animals resistant to traditional pharmacological treatments, and identifying genetic models that show antidepressant resistance. This review also examines pharmacological and non-pharmacological treatments regimes that have been effective in refractory patients and how some of these approaches have been used to validate animal models of treatment-resistant depression. The goals in developing rodent models of treatment-resistant depression are to understand the neurobiological mechanisms involved in antidepressant resistance and to develop valid models to test novel therapies that would be effective in patients that do not respond to traditional monoaminergic antidepressants.

A retrospective investigation on canine papillomavirus 1 (CPV1) in oral oncogenesis reveals dogs are not a suitable animal model for high-risk HPV-induced oral cancer.

CPV1 (also called COPV) is a papillomavirus responsible for oral papillomatosis in young dogs. The involvement of this viral type in oral oncogenesis has been hypothesized in oral squamous cell carcinomas (SCCs), but has never been investigated in other neoplastic and hyperplastic oral lesions of dogs. Aim of this study was to investigate the presence of CPV1 in different neoplastic and hyperplastic lesions in order to assess its role in canine oral oncogenesis; according to the results obtained, a second aim of the study was to define if the dog can be considered a valid animal model for oral high risk HPV-induced tumors. Eighty-eight formalin-fixed, paraffin-embedded (FFPE) canine oral lesions including 78 oral tumors (papillomas, SCCs, melanomas, ameloblastomas, oral adenocarcinomas) and 10 hyperplastic lesions (gingival hyperplasia) were investigated with immunohistochemistry for the presence of papillomavirus L1 protein and with Real-Time PCR for CPV1 DNA. RT-PCR for RNA was performed on selected samples. All viral papillomas tested were positive for immunohistochemistry and Real-time PCR. In 3/33 (10%) SCCs, viral DNA was demonstrated but no viral RNA could be found. No positivity was observed both with immunohistochemistry and Real-Time PCR in the other hyperplastic and neoplastic lesions of the oral cavity of dogs. Even though the finding of CPV1 DNA in few SCCs in face of a negative immunohistochemistry could support the hypothesis of an abortive infection in the development of these lesions, the absence of viral RNA points out that CPV1 more likely represents an innocent bystander in SCC oncogenesis. The study demonstrates a strong association between CPV1 and oral viral papillomas whereas viral contribution to the pathogenesis of other oral lesions seems unlikely. Moreover, it suggests that a canine model of CPV1 infection for HPV-induced oncogenesis could be inappropriate.

New ways of modeling bipolar disorder.

There is a well-known deficiency in valid animal models for bipolar disorder. Developing the single ideal model for the disorder-one that will represent its full scope-will probably not be possible until we have a much better understanding of the underlying pathology. Yet, intermediate models, even with partial validity, are critical in order to advance our knowledge and put us into position to develop even better models. The present article discusses the various efforts under way to develop the best models based on our current level of understanding. These efforts include (1) identifying new tests, (2) developing models based on the endophenotypes approach, (3) identifying the best rodent strains, (4) identifying the most appropriate species, (5) segregating susceptible versus resilient animals, and (6) segregating animals that respond or do not respond to treatment. It is suggested that a combined approach that includes these directions and others can result in better models with higher validity that will offer significant help in advancing research on bipolar disorder and developing new and better treatments.

Chronic stress reduces the number of GABAergic interneurons in the adult rat hippocampus, dorsal-ventral and region-specific differences.

Major depressive disorder is a common and complex mental disorder with unknown etiology. GABAergic dysfunction is likely to contribute to the pathophysiology since disrupted GABAergic systems are well documented in depressed patients. Here we studied structural changes in the hippocampal GABAergic network using the chronic mild stress (CMS) model, as one of the best validated animal models for depression. Rats were subjected to 9 weeks of daily stress and behaviorally characterized using the sucrose consumption test into anhedonic and resilient animals based on their response to stress. Different subtypes of GABAergic interneurons were visualized by immunohistochemistry using antibodies for parvalbumin (PV), calretinin (CR), calbindin (CB), cholecystokinin (CCK), somatostatin (SOM), and neuropeptide Y (NPY). We used an unbiased quantification method to systematically count labeled cells in different subareas of the dorsal and ventral hippocampus. Chronic stress reduced the number of specific interneurons in distinct hippocampal subregions significantly. PV+ and CR+ neurons were reduced in all dorsal subareas, whereas in the ventral part only the CA1 was affected. Stress had the most pronounced effect on the NPY+ and SOM+ cells and reduced their number in almost all dorsal and ventral subareas. Stress had no effect on the CCK+ and CB+ interneurons. In most cases the effect of stress was irrespective to the behavioral phenotype. However, in a few specific areas the number of SOM+, NPY+, and CR+ neurons were significantly reduced in anhedonic animals compared to the resilient group. Overall, these data clearly demonstrate that chronic stress affects the structural integrity of specific GABAergic neuronal subpopulations and this should also affect the functioning of these hippocampal GABAergic networks.

Chronic stress causes neuroendocrine-immune disturbances without affecting renal vitamin D metabolism in rats.

Vitamin D (VD) insufficiency has been repeatedly observed in the medical conditions associated with inflammation, such as cardiovascular disease, diabetes and depression. However, contrasting to the observational evidence, randomized trials of VD supplementation failed to demonstrate such link. Given the recent evidence that the inflammatory process can in turn alter VD metabolism, it has been hypothesized that the insufficient VD status could be the result rather than the cause of chronic inflammation involved in the onset of depression and other disease conditions. Chronic mild stress (CMS) is a valid animal model of depression that accompanied with neuroendocrine-immune disturbances. In the present research, we assessed serum VD concentrations and renal expression of the cytochromes P450 enzymes involved in VD activation (CYP27B1) and catabolism (CYP24A1) of rats following 8-week exposure to CMS. While CMS induced the rats to a depression-like state and increased serum levels of the proinflammatory cytokines and corticosterone, and the antidepressant, sertraline, mitigated depression-like behaviors and neuroendocrine-immune disturbances, neither the stress regimen nor sertraline significantly affected endocrine metabolism of VD. Our data suggest that the stress-induced neuroendocrine-immune disturbances may account for the development of depression, but are not responsible for the insufficient VD status that frequently observed in depressed patients.

P.2.a.022 Robust efficacy of 3β-methoxy-pregnenolone (MAP4343) in validated animal models of depression suggests a new antidepressant mechanism

P.2.a.022 Robust efficacy of 3β-methoxy-pregnenolone (MAP4343) in validated animal models of depression suggests a new antidepressant mechanism

Stable long-term mixed chimerism achieved in a canine model of allogeneic in utero hematopoietic cell transplantation

AbstractEvidence supporting the efficacy of in utero hematopoietic cell transplantation (IUHCT) in a valid large animal model is needed prior to clinical application. The objective of this study was to establish clinically relevant levels of hematopoietic chimerism in a canine model of maternal-to-fetal IUHCT. We first assessed immune and hematopoietic ontogeny relevant to IUHCT in the canine model and identified 40 days’ gestation (term 63 days) as a time point at the initiation of thymic selection, and prior to bone marrow hematopoiesis, that might be optimal for IUHCT. We next determined that intravascular administration of donor cells via intracardiac injection was far more efficient and resulted in much higher levels of donor cell engraftment than intraperitoneal injection. By applying these findings, we achieved stable long-term multilineage engraftment in 21 of 24 surviving recipients with an average level of initial chimerism of 11.7% (range 3% to 39%) without conditioning or evidence of graft-versus-host disease. Donor cell chimerism remained stable for up to 2 years and was associated with donor-specific tolerance for renal transplantation. The levels of donor cell chimerism achieved in this study would be therapeutic for many hematopoietic disorders and are supportive of a clinical trial of IUHCT.

Antipsychotic treatment modulates glutamate transport and NMDA receptor expression.

Schizophrenia patients often suffer from treatment-resistant cognitive and negative symptoms, both of which are influenced by glutamate neurotransmission. Innovative therapeutic strategies such as agonists at metabotropic glutamate receptors or glycin reuptake inhibitors try to modulate the brain's glutamate network. Interactions of amino acids with monoamines have been described on several levels, and first- and second-generation antipsychotic agents (FGAs, SGAs) are known to exert modulatory effects on the glutamatergic system. This review summarizes the current knowledge on effects of FGAs and SGAs on glutamate transport and receptor expression derived from pharmacological studies. Such studies serve as a control for molecular findings in schizophrenia brain tissue and are clinically relevant. Moreover, they may validate animal models for psychosis, foster basic research on antipsychotic substances and finally lead to a better understanding of how monoaminergic and amino acid neurotransmissions are intertwined. In the light of these results, important differences dependent on antipsychotic substances, dosage and duration of treatment became obvious. While some post-mortem findings might be confounded with multifold drug effects, others are unlikely to be influenced by antipsychotic treatment and could represent important markers of schizophrenia pathophysiology. In similarity to the convergence of toxic and psychotomimetic effects of dopaminergic, serotonergic and anti-glutamatergic substances, the therapeutic mechanisms of SGAs might merge on a yet to be defined molecular level. In particular, serotonergic effects of SGAs, such as an agonism at 5HT1A receptors, represent important targets for further clinical research.

G.P.82: Dystrophin-deficient pigs provide new insights into the hierarchy of physiological derangements of dystrophic muscle

Duchenne muscular dystrophy (DMD) is caused by mutations in the X-linked dystrophin (DMD) gene. The absence of dystrophin protein leads to progressive muscle weakness and wasting, disability and death. Existing animal models have been instrumental to understand the pathophysiology of DMD, but have limitations related to the type of mutation, the clinical phenotype, and the predictive value for molecular therapies. To establish a tailored large animal model of DMD, we deleted DMD exon 52 in male pig cells by gene targeting and generated offspring by nuclear transfer. DMD pigs exhibit absence of dystrophin in skeletal muscles, increased serum creatine kinase levels, progressive dystrophic changes of skeletal muscles, impaired mobility, muscle weakness, and a maximum life span of 3 months due to respiratory impairment. These findings render the DMD mutant pig a promising model for testing targeted genetic treatment approaches, such as exon skipping which partially restored in-frame dystrophin gene expression in our model. Regarding recent disappointing results of exon skipping in human DMD clinical trials, a valid animal model will be warranted.

Effects of chronic mild stress on the development of drug dependence in rats.

There is high comorbidity between depression and addiction. Features of addiction relevant to depression have been studied extensively, but less is known about features of depression relevant to addiction. Here, we have studied the effects of chronic mild stress (CMS), a valid animal model of depression, on measures of physical and psychological dependence resulting from subchronic treatment of rats with three drugs of abuse that act through disparate neurobiological mechanisms: morphine, nicotine and diazepam. In animals not treated subchronically with drugs of abuse, CMS increased the withdrawal-like effects of the opiate antagonist naloxone, but not those of the nicotinic antagonist mecamylamine or the benzodiazepine antagonist flumazenil. In animals treated subchronically with drugs of abuse, CMS exacerbated, precipitated and conditioned withdrawal effects associated with all three antagonists. CMS also potentiated withdrawal-induced and cue-induced place aversions associated with all three antagonists. All of the effects of CMS were reversed by chronic treatment with the specific serotonin reuptake inhibitor citalopram. These results suggest that treatment of comorbid depression, although not a primary treatment for addiction, may facilitate other treatments for addiction, by decreasing the severity of withdrawal symptoms and the likelihood of relapse.

Modelling Alzheimer-like cognitive deficits in rats using biperiden as putative cognition impairer

To enable the development of effective treatments for dementias such as Alzheimer's disease (AD), it is important to establish valid animal models of cognitive impairments. Scopolamine is widely used to induce cognitive deficits in animal models of AD, but also causes non-cognitive side effects. We assessed whether biperiden, a selective antagonist of M1 muscarinic receptors, which are predominantly expressed in brain areas involved in cognitive processes, causes cognitive deficits without inducing peripheral side-effects. Two different doses of biperiden (3 or 10mgkg−1) on the acquisition of a spatial cone field task were assessed in male Lister Hooded rats. This task measures, among others, spatial working (WM) – and reference memory (RM) simultaneously. Biperiden did not impair learning of the task. The animals reached asymptotic levels for all variables except reference memory and the number of rewards collected. However, the 10mgkg−1 dose decreased the tendency of rats to use searching strategies to solve the task and made them slower to start searching and completing the task. In conclusion, though no effects on WM and RM performance were seen, the present study cannot conclude that biperiden acts as a more selective cognition impairer than scopolamine in other rats strains and/or other doses than those tested.

51. Developing a novel animal model of depression to study the role of inflammation in disease progression and in the mechanism of antidepressant action

To study the role of inflammation in depression and in the mechanisms of antidepressant action a valid animal model is required. To develop such a model, we proposed to combine exposure to inflammatory stimulus modelled by peripheral lipopolysaccharide (LPS) injection with an environmental stress-based model of depression – unpredictable chronic mild stress (UCMS) to achieve an immunologically-relevant depression-like phenotype. In a pilot experiment, we treated 8 weeks old male BALB/c mice with 0.33 mg/kg LPS and subsequently exposed them to UCMS for 7 weeks. Fluoxetine-treated groups (10 mg/kg daily) were also included in our experimental design (N = 12). LPS and UCMS exposed mice displayed significant coat state deterioration (UCMS factor H (7) = 37.5, p < 0.0001) and locomotor hyperactivity in a novel arena (UCMS factor F (1,74) = 20.8, p < 0.0001). When coat state was assessed 4 days after the end of UCMS paradigm, the UCMS-only group already displayed signs of recovery with the coat deterioration score indistinguishable from control, while animals exposed to both LPS and UCMS did not show improvement of the coat state. Fluoxetine treatment of the LPS and UCMS exposed group promoted the recovery of the coat state with the coat deterioration score returning to control levels. Future assessment of the neurobiological phenotype (corticosterone response to stress, peripheral cytokines and hippocampal neurogenesis levels) will be presented to allow us to further unravel the neurobiological basis of the observed behavioural changes.

Modulation of mouse macrophage polarization in vitro using IL-4 delivery by osmotic pumps.

Modulation of macrophage polarization is emerging as promising means to mitigate wear particle-induced inflammation and periprosthetic osteolysis. As a model for continuous local drug delivery, we used miniature osmotic pumps to deliver IL-4 in order to modulate macrophage polarization in vitro from nonactivated M0 and inflammatory M1 phenotypes towards a tissue regenerative M2 phenotype. Pumps delivered IL-4 into vials containing mouse bone marrow macrophage (mBMM) media. This conditioned media (CM) was collected at seven day intervals up to four weeks (week 1 to week 4 samples). IL-4 concentration in the CM was determined by ELISA and its biological activity was assayed by exposing M0 and M1 mBMMs to week 1 or week 4 CM. The IL-4 concentration in the CM approximated the mathematically calculated amount, and its biological activity was well retained, as both M0 and M1 macrophages exposed to either the week 1 or week 4 CM assumed M2-like phenotype as determined by qRT-PCR, ELISA, and immunocytochemistry. The results show that IL-4 can be delivered using osmotic pumps and that IL-4 delivered can modulate macrophage phenotype. Results build a foundation for in vivo studies using our previously validated animal models and provide possible strategies to locally mitigate wear particle-induced macrophage activation and periprosthetic osteolysis.

Development of a novel mouse model of amodiaquine-induced liver injury with a delayed onset

Amodiaquine (AQ) treatment is associated with a high incidence of idiosyncratic drug-induced liver injury (IDILI) and agranulocytosis. Evidence suggests that AQ-induced IDILI is immune mediated. A significant impediment to mechanistic studies of IDILI is the lack of valid animal models. This study reports the first animal model of IDILI with characteristics similar to mild IDILI in humans. Treatment of female C57BL/6 mice with AQ led to liver injury with delayed onset, which resolved despite continued treatment. Covalent binding of AQ was detected in the liver, which was greater in female than in male mice, and higher in the liver than in other organs. Covalent binding in the liver was maximal by Day 3, which did not explain the delayed onset of alanine aminotransferase (ALT) elevation. However, coincident with the elevated serum ALT, infiltration of liver and splenic mononuclear cells and activation of CD8 T-cells within the liver were identified. By Week 7, when ALT levels had returned close to normal, down-regulation of several inflammatory cytokines and up-regulation of PD-1 on T-cells suggested induction of immune tolerance. Treatment of Rag1−/− mice with AQ resulted in higher ALT activities than C57BL/6 mice, which suggested that the adaptive immune response was responsible for immune tolerance. In contrast, depletion of NK cells significantly attenuated the increase in ALT, which implied a role for NK cells in mild AQ-induced IDILI. This is the first example of a delayed-onset animal model of IDILI that appears to be immune-mediated.

Differential effect of liver cirrhosis on the pregnane X receptor-mediated induction of CYP3A1 and 3A2 in the rat.

Conflicting results have been obtained by clinical studies investigating the effect of liver cirrhosis on enzyme induction. Because ethical concerns do not give consent for methodologically rigorous studies in humans, we addressed this question by examining the effect of the prototypical inducer dexamethasone (DEX) on the pregnane X receptor (PXR)-mediated induction of CYP3A1 and 3A2 in a validated animal model of liver cirrhosis obtained by exposure of rats to carbon tetrachloride. For this purpose, we assessed mRNA levels, protein expressions, and enzymatic activities of both CYP3A enzymes, as well as mRNA and protein expressions of PXR in rat populations rigorously stratified according to the severity of liver insufficiency. Constitutive mRNA and protein expressions of CYP3A1 and CYP3A2 and their basal enzyme activities were not affected by liver dysfunction. DEX treatment markedly increased steady-state mRNA level, protein content, and enzymatic activity of CYP3A1 in healthy and cirrhotic rats, irrespective of the degree of liver dysfunction. On the contrary, the inducing effect of DEX on gene and protein expressions and enzyme activity of CYP3A2 was preserved in moderate liver insufficiency, whereas it was greatly curtailed when liver insufficiency became severe. mRNA and protein expressions of PXR were neither reduced by liver dysfunction nor increased by DEX treatment. These results indicate that even the inducibility of cytochrome P450 isoforms under the transcriptional control of the same nuclear receptor may be differentially affected by cirrhosis and may partly explain why conflicting results were obtained by human studies.

An animal model of a newly emerging human ehrlichiosis.

Human ehrlichioses are emerging life-threatening diseases transmitted by ticks. Animal models have been developed to study disease development; however, there is no valid small animal model that uses a human ehrlichial pathogen. The objective of this study was to develop a mouse model for ehrlichiosis with the newly discovered human pathogen, Ehrlichia muris-like agent (EMLA). Three strains of mice were inoculated with different doses of EMLA by the intravenous, intraperitoneal, or intradermal route and evaluated for clinical and pathologic changes during the course of infection. EMLA infected C57Bl/6, BALB/c, and C3H/HeN mice and induced lethal or persistent infection in a route- and dose-dependent manner. The clinical chemistry and hematologic changes were similar to those of human infection by Ehrlichia chaffeensis or EMLA. Bacterial distribution in tissues differed after intradermal infection, compared with the distribution after intravenous or intraperitoneal injection. Lethal infection did not cause remarkable pathologic changes, but it caused fluid imbalance. EMLA infection of endothelium and mononuclear cells likely plays a role in the severe outcome. The EMLA mouse model mimics human infection and can be used to study pathogenesis and immunity and for development of a vector transmission model of ehrlichiosis.

Identifying novel interventional strategies for psychiatric disorders: integrating genomics, ‘enviromics’ and gene–environment interactions in valid preclinical models

Psychiatric disorders affect a substantial proportion of the population worldwide. This high prevalence, combined with the chronicity of the disorders and the major social and economic impacts, creates a significant burden. As a result, an important priority is the development of novel and effective interventional strategies for reducing incidence rates and improving outcomes. This review explores the progress that has been made to date in establishing valid animal models of psychiatric disorders, while beginning to unravel the complex factors that may be contributing to the limitations of current methodological approaches. We propose some approaches for optimizing the validity of animal models and developing effective interventions. We use schizophrenia and autism spectrum disorders as examples of disorders for which development of valid preclinical models, and fully effective therapeutics, have proven particularly challenging. However, the conclusions have relevance to various other psychiatric conditions, including depression, anxiety and bipolar disorders. We address the key aspects of construct, face and predictive validity in animal models, incorporating genetic and environmental factors. Our understanding of psychiatric disorders is accelerating exponentially, revealing extraordinary levels of genetic complexity, heterogeneity and pleiotropy. The environmental factors contributing to individual, and multiple, disorders also exhibit breathtaking complexity, requiring systematic analysis to experimentally explore the environmental mediators and modulators which constitute the 'envirome' of each psychiatric disorder. Ultimately, genetic and environmental factors need to be integrated via animal models incorporating the spatiotemporal complexity of gene-environment interactions and experience-dependent plasticity, thus better recapitulating the dynamic nature of brain development, function and dysfunction.

Sex differences in animal models of psychiatric disorders

Psychiatric disorders are characterized by sex differences in their prevalence, symptomatology and treatment response. Animal models have been widely employed for the investigation of the neurobiology of such disorders and the discovery of new treatments. However, mostly male animals have been used in preclinical pharmacological studies. In this review, we highlight the need for the inclusion of both male and female animals in experimental studies aiming at gender-oriented prevention, diagnosis and treatment of psychiatric disorders. We present behavioural findings on sex differences from animal models of depression, anxiety, post-traumatic stress disorder, substance-related disorders, obsessive-compulsive disorder, schizophrenia, bipolar disorder and autism. Moreover, when available, we include studies conducted across different stages of the oestrous cycle. By inspection of the relevant literature, it is obvious that robust sex differences exist in models of all psychiatric disorders. However, many times results are conflicting, and no clear conclusion regarding the direction of sex differences and the effect of the oestrous cycle is drawn. Moreover, there is a lack of considerable amount of studies using psychiatric drugs in both male and female animals, in order to evaluate the differential response between the two sexes. Notably, while in most cases animal models successfully mimic drug response in both sexes, test parameters and treatment-sensitive behavioural indices are not always the same for male and female rodents. Thus, there is an increasing need to validate animal models for both sexes and use standard procedures across different laboratories.

The immune space: a concept and template for rationalizing vaccine development.

Empirical testing of candidate vaccines has led to the successful development of a number of lifesaving vaccines. The advent of new tools to manipulate antigens and new methods and vectors for vaccine delivery has led to a veritable explosion of potential vaccine designs. As a result, selection of candidate vaccines suitable for large-scale efficacy testing has become more challenging. This is especially true for diseases such as dengue, HIV, and tuberculosis where there is no validated animal model or correlate of immune protection. Establishing guidelines for the selection of vaccine candidates for advanced testing has become a necessity. A number of factors could be considered in making these decisions, including, for example, safety in animal and human studies, immune profile, protection in animal studies, production processes with product quality and stability, availability of resources, and estimated cost of goods. The "immune space template" proposed here provides a standardized approach by which the quality, level, and durability of immune responses elicited in early human trials by a candidate vaccine can be described. The immune response profile will demonstrate if and how the candidate is unique relative to other candidates, especially those that have preceded it into efficacy testing and, thus, what new information concerning potential immune correlates could be learned from an efficacy trial. A thorough characterization of immune responses should also provide insight into a developer's rationale for the vaccine's proposed mechanism of action. HIV vaccine researchers plan to include this general approach in up-selecting candidates for the next large efficacy trial. This "immune space" approach may also be applicable to other vaccine development endeavors where correlates of vaccine-induced immune protection remain unknown.