Update on animal models for HIV research

Opioid abuse and SIV infection in non-human primates

Human immunodeficiency virus (HIV) and drug abuse are intertwined epidemics, leading to compromised adherence to combined antiretroviral therapy (cART) and exacerbation of NeuroHIV. As opioid abuse causes increased viral replication and load, leading to a further compromised immune system in people living with HIV (PLWH), it is paramount to address this comorbidity to reduce the NeuroHIV pathogenesis. Non-human primates are well-suited models to study mechanisms involved in HIV neuropathogenesis and provide a better understanding of the underlying mechanisms involved in the comorbidity of HIV and drug abuse, leading to the development of more effective treatments for PLWH. Additionally, using broader behavioral tests in these models can mimic mild NeuroHIV and aid in studying other neurocognitive diseases without encephalitis. The simian immunodeficiency virus (SIV)-infected rhesus macaque model is instrumental in studying the effects of opioid abuse on PLWH due to its similarity to HIV infection. The review highlights the importance of using non-human primate models to study the comorbidity of opioid abuse and HIV infection. It also emphasizes the need to consider modifiable risk factors such as gut homeostasis and pulmonary pathogenesis associated with SIV infection and opioid abuse in this model. Moreover, the review suggests that these non-human primate models can also be used in developing effective treatment strategies for NeuroHIV and opioid addiction. Therefore, non-human primate models can significantly contribute to understanding the complex interplay between HIV infection, opioid abuse, and associated comorbidities.

Long-term hepatitis B virus infection of rhesus macaques requires suppression of host immunity

Hepatitis B virus has infected a third of the world's population, and 296 million people are living with chronic infection. Chronic infection leads to progressive liver disease, including hepatocellular carcinoma and liver failure, and there remains no reliable curative therapy. These gaps in our understanding are due, in large part, to a paucity of animal models of HBV infection. Here, we show that rhesus macaques regularly clear acute HBV infection, similar to adult humans, but can develop long-term infection if immunosuppressed. Similar to patients, we longitudinally detected HBV DNA, HBV surface antigen, and HBV e antigen in the serum of experimentally infected animals. In addition, we discovered hallmarks of HBV infection in the liver, including RNA transcription, HBV core and HBV surface antigen translation, and covalently closed circular DNA biogenesis. This pre-clinical animal model will serve to accelerate emerging HBV curative therapies into the clinic.

Tackling HIV and AIDS: contributions by non-human primate models

During the past three decades, non-human primate (NHP) models have gained an increasing importance in HIV basic and translational research. In contrast to natural host models, infection of macaques with virulent simian or simian-human immunodeficiency viruses (SIV, SHIV) results in a disease that closely resembles HIV infection and AIDS. Although there is no perfect animal model, and each of the available models has its benefits and limitations, carefully designed NHP studies with selection of experimental variables have unraveled important questions of basic pathogenesis and have provided the tools to explore and screen intervention strategies. For example, NHP studies have advanced our understanding of the crucial events during early infection, and have provided proof-of-concept of antiretroviral drug treatment and prevention strategies such as pre-exposure prophylaxis (PrEP) regimes that are increasingly used worldwide, and upon overcoming further barriers of implementation, have the potential to make the next generation AIDS-free. Remaining goals include the pursuit of an effective HIV vaccine, and HIV cure strategies that would allow HIV-infected people to ultimately stop taking antiretroviral drugs. Through a reiterative process with feed-back from results of human studies, NHP models can be further validated and strengthened to advance our scientific knowledge and guide clinical trials.

Engineering Recombinant Reoviruses To Display gp41 Membrane-Proximal External-Region Epitopes from HIV-1

Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

The gp41 membrane-proximal external region (MPER) is a target for broadly neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1). However, replication-defective virus vaccines currently under evaluation in clinical trials do not efficiently elicit MPER-specific antibodies. Structural modeling suggests that the MPER forms an α-helical coiled coil that is required for function and immunogenicity. To maintain the native MPER conformation, we used reverse genetics to engineer replication-competent reovirus vectors that displayed MPER sequences in the α-helical coiled-coil tail domain of viral attachment protein σ1. Sequences in reovirus strain type 1 Lang (T1L) σ1 were exchanged with sequences encoding HIV-1 strain Ba-L MPER epitope 2F5 or the entire MPER. Individual 2F5 or MPER substitutions were introduced at virion-proximal or virion-distal sites in the σ1 tail. Recombinant reoviruses containing heterologous HIV-1 sequences were viable and produced progeny yields comparable to those with wild-type virus. HIV-1 sequences were retained following 10 serial passages in cell culture, indicating that the substitutions were genetically stable. Recombinant viruses engineered to display the 2F5 epitope or full-length MPER in σ1 were recognized by purified 2F5 antibody. Inoculation of mice with 2F5-containing vectors or rabbits with 2F5- or MPER-containing vectors elicited anti-reovirus antibodies, but HIV-1-specific antibodies were not detected. Together, these findings indicate that heterologous sequences that form α-helices can functionally replace native sequences in the α-helical tail domain of reovirus attachment protein σ1. However, although these vectors retain native antigenicity, they were not immunogenic, illustrating the difficulty of experimentally inducing immune responses to this essential region of HIV-1.

IMPORTANCE Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

Latest animal models for anti-HIV drug discovery

INTRODUCTION

HIV research is limited by the fact that lentiviruses are highly species specific. The need for appropriate models to promote research has led to the development of many elaborate surrogate animal models.

AREAS COVERED

This review looks at the history of animal models for HIV research. Although natural animal lentivirus infections and chimeric viruses such as chimera between HIV and simian immunodeficiency virus and simian-tropic HIV are briefly discussed, the main focus is on small animal models, including the complex design of the 'humanized' mouse. The review also traces the historic evolution and milestones as well as depicting current models and future prospects for HIV research.

EXPERT OPINION

HIV research is a complex and challenging task that is highly manpower-, money- and time-consuming. Besides factors such as hypervariability and latency, the lack of appropriate animal models that exhibit and recapitulate the entire infectious process of HIV, is one of the reasons behind the failure to eliminate the lentivirus from the human population. This obstacle has led to the exploitation and further development of many sophisticated surrogate animal models for HIV research. While there is no animal model that perfectly mirrors and mimics HIV infections in humans, there are a variety of host species and viruses that complement each other. Combining the insights from each model, and critically comparing the results obtained with data from human clinical trials should help expand our understanding of HIV pathogenesis and drive future drug development.

Technical advance: liposomal alendronate depletes monocytes and macrophages in the nonhuman primate model of human disease

Nonhuman primates are critical animal models for the study of human disorders and disease and offer a platform to assess the role of immune cells in pathogenesis via depletion of specific cellular subsets. However, this model is currently hindered by the lack of reagents that safely and specifically ablate myeloid cells of the monocyte/macrophage Lin. Given the central importance of macrophages in homeostasis and host immunity, development of a macrophage-depletion technique in nonhuman primates would open new avenues of research. Here, using LA at i.v. doses as low as 0.1 mg/kg, we show a >50% transient depletion of circulating monocytes and tissue-resident macrophages in RMs by an 11-color flow cytometric analysis. Diminution of monocytes was followed rapidly by emigration of monocytes from the bone marrow, leading to a rebound of monocytes to baseline levels. Importantly, LA was well-tolerated, as no adverse effects or changes in gross organ function were observed during depletion. These results advance the ex vivo study of myeloid cells by flow cytometry and pave the way for in vivo studies of monocyte/macrophage biology in nonhuman primate models of human disease.

Stem memory T cells (TSCM)-their role in cancer and HIV immunotherapies

Stem memory T cells (TSCM) have been described in mice, non-human primates and in humans, constituting approximately 2-4% of the total CD4(+) and CD8(+) T-cell population in the periphery. TSCM represent the earliest and long-lasting developmental stage of memory T cells, displaying stem cell-like properties, and exhibiting a gene profile between naïve and central memory T cells. Their self-renewal capacity and long-term survival has sparked interest in the cancer and human immunodeficiency virus (HIV) fields. How and when the formation of TSCM occurs during the immune response to pathogens and the therapeutic potential of these cells are currently being investigated. This review will explore the potential role of TSCM to be used as, or targeted by, immunotherapies and vaccines for treatment of cancer and HIV.

Viral RNA levels and env variants in semen and tissues of mature male rhesus macaques infected with SIV by penile inoculation

HIV is shed in semen but the anatomic site of virus entry into the genital secretions is unknown. We determined viral RNA (vRNA) levels and the envelope gene sequence in the SIVmac 251 viral populations in the genital tract and semen of 5 adult male rhesus monkeys (Macaca mulatta) that were infected after experimental penile SIV infection. Paired blood and semen samples were collected from 1–9 weeks after infection and the monkeys were necropsied eleven weeks after infection. The axillary lymph nodes, testes, epididymis, prostate, and seminal vesicles were collected and vRNA levels and single-genome analysis of the SIVmac251 env variants was performed. At the time of semen collection, blood vRNA levels were between 3.09 and 7.85 log10 vRNA copies/ml plasma. SIV RNA was found in the axillary lymph nodes of all five monkeys and in 3 of 5 monkeys, all tissues examined were vRNA positive. In these 3 monkeys, vRNA levels (log10 SIVgag copies/ug of total tissue RNA) in the axillary lymph node (6.48±0.50) were significantly higher than in the genital tract tissues: testis (3.67±2.16; p<0.05), epididymis (3.08±1.19; p<0.0001), prostate (3.36±1.30; p<0.01), and seminal vesicle (2.67±1.50; p<0.0001). Comparison of the SIVmac251 env viral populations in blood plasma, systemic lymph node, and genital tract tissues was performed in two of the macaques. Visual inspection of the Neighbor-Joining phylograms revealed that in both animals, all the sequences were generally distributed evenly among all tissue compartments. Importantly, viral populations in the genital tissues were not distinct from those in the systemic tissues. Our findings demonstrate striking similarity in the viral populations in the blood and male genital tract tissues within 3 months of penile SIV transmission.

Nonpathogenic simian immunodeficiency virus infections

The simian immunodeficiency viruses (SIVs) are a diverse group of viruses that naturally infect a wide range of African primates, including African green monkeys (AGMs) and sooty mangabey monkeys (SMs). Although natural infection is widespread in feral populations of AGMs and SMs, this infection generally does not result in immunodeficiency. However, experimental inoculation of Asian macaques results in an immunodeficiency syndrome remarkably similar to human AIDS. Thus, natural nonprogressive SIV infections appear to represent an evolutionary adaptation between these animals and their primate lentiviruses. Curiously, these animals maintain robust virus replication but have evolved strategies to avoid disease progression. Adaptations observed in these primates include phenotypic changes to CD4(+) T cells, limited chronic immune activation, and altered mucosal immunity. It is probable that these animals have achieved a unique balance between T-cell renewal and proliferation and loss through activation-induced apoptosis, and virus-induced cell death. A clearer understanding of the mechanisms underlying the lack of disease progression in natural hosts for SIV infection should therefore yield insights into the pathogenesis of AIDS and may inform vaccine design.

Animal Models of Human Viral Diseases

As the threat of exposure to emerging and reemerging viruses within a naive population increases, it is vital that the basic mechanisms of pathogenesis and immune response be thoroughly investigated. By using animal models in this endeavor, the response to viruses can be studied in a more natural context to identify novel drug targets, and assess the efficacy and safety of new products. This is especially true in the advent of the Food and Drug Administration's animal rule. Although no one animal model is able to recapitulate all the aspects of human disease, understanding the current limitations allows for a more targeted experimental design. Important facets to be considered before an animal study are the route of challenge, species of animals, biomarkers of disease, and a humane endpoint. This chapter covers the current animal models for medically important human viruses, and demonstrates where the gaps in knowledge exist.

Single-nucleotide polymorphisms reveal patterns of allele sharing across the species boundary between rhesus (Macaca mulatta) and cynomolgus (M. fascicularis) macaques

Both phenotypic and genetic evidence for asymmetric hybridization between rhesus (Macaca mulatta) and cynomolgus (Macaca fascicularis) macaques has been observed in the region of Indochina where both species are sympatric. The large-scale sharing of major histocompatibility complex (MHC) class II alleles between the two species in this region supports the hypothesis that genes, and especially genes involved in immune response, are being transferred across the species boundary. This differential introgression has important implications for the incorporation of cynomolgus macaques of unknown geographic origin in biomedical research protocols. Our study found that for 2,808 single-nucleotide polymorphism (SNP) markers, the minor allele frequencies (MAF) and observed heterozygosity calculated from a sample of Vietnamese cynomolgus macaques was significantly different from those calculated from samples of both Chinese rhesus and Indonesian cynomolgus macaques. SNP alleles from Chinese rhesus macaques were overrepresented in a sample of Vietnamese cynomolgus macaques relative to their Indonesian conspecifics and located in genes functionally related to the primary immune system. These results suggest that Indochinese cynomolgus macaques represent a genetically and immunologically distinct entity from Indonesian cynomolgus macaques.

The use of nonhuman primate models of HIV infection for the evaluation of antiviral strategies

Several nonhuman primate models are used in HIV/AIDS research. In contrast to natural host models, infection of macaques with virulent simian immunodeficiency virus (SIV) isolates results in a disease (simian AIDS) that closely resembles HIV infection and AIDS. Although there is no perfect animal model, and each of the available models has its limitations, a carefully designed study allows experimental approaches that are not feasible in humans, but that can provide better insights in disease pathogenesis and proof-of-concept of novel intervention strategies. In the early years of the HIV pandemic, nonhuman primate models played a minor role in the development of antiviral strategies. Since then, a better understanding of the disease and the development of better compounds and assays to monitor antiviral effects have increased the usefulness and relevance of these animal models in the preclinical development of HIV vaccines, microbicides, and antiretroviral drugs. Several strategies that were first discovered to have efficacy in nonhuman primate models are now increasingly used in humans. Recent trends include the use of nonhuman primate models to explore strategies that could reduce viral reservoirs and, ultimately, attempt to cure infection. Ongoing comparison of results obtained in nonhuman primate models with those observed in human studies will lead to further validation and improvement of these animal models so they can continue to advance our scientific knowledge and guide clinical trials.

4040 SNPs for genomic analysis in the rhesus macaque (Macaca mulatta)

Although the rhesus macaque (Macaca mulatta) is commonly used for biomedical research and becoming a preferred model for translational medicine, quantification of genome-wide variation has been slow to follow the publication of the genome in 2007. Here we report the properties of 4040 single nucleotide polymorphisms discovered and validated in Chinese and Indian rhesus macaques from captive breeding colonies in the United States. Frequency-matched measures of linkage disequilibrium were much greater in the Indian sample. Although the majority of polymorphisms were shared between the two populations, rare alleles were over twice as common in the Chinese sample. Indian rhesus had higher rates of heterozygosity, as well as previously undetected substructure, potentially due to admixture from Burma in wild populations and demographic events post-captivity.

Functional analysis of frequently expressed Chinese rhesus macaque MHC class I molecules Mamu-A1*02601 and Mamu-B*08301 reveals HLA-A2 and HLA-A3 supertypic specificities

The Simian immunodeficiency virus (SIV)-infected Indian rhesus macaque (Macaca mulatta) is the most established model of HIV infection and AIDS-related research, despite the potential that macaques of Chinese origin is a more relevant model. Ongoing efforts to further characterize the Chinese rhesus macaques' major histocompatibility complex (MHC) for composition and function should facilitate greater utilization of the species. Previous studies have demonstrated that Chinese-origin M. mulatta (Mamu) class I alleles are more polymorphic than their Indian counterparts, perhaps inferring a model more representative of human MHC, human leukocyte antigen (HLA). Furthermore, the Chinese rhesus macaque class I allele Mamu-A1*02201, the most frequent allele thus far identified, has recently been characterized and shown to be an HLA-B7 supertype analog, the most frequent supertype in human populations. In this study, we have characterized two additional alleles expressed with high frequency in Chinese rhesus macaques, Mamu-A1*02601 and Mamu-B*08301. Upon the development of MHC-peptide-binding assays and definition of their associated motifs, we reveal that these Mamu alleles share peptide-binding characteristics with the HLA-A2 and HLA-A3 supertypes, respectively, the next most frequent human supertypes after HLA-B7. These data suggest that Chinese rhesus macaques may indeed be a more representative model of HLA gene diversity and function as compared to the species of Indian origin and therefore a better model for investigating human immune responses.

The most common Chinese rhesus macaque MHC class I molecule shares peptide binding repertoire with the HLA-B7 supertype

Of the two rhesus macaque subspecies used for AIDS studies, the Simian immunodeficiency virus-infected Indian rhesus macaque (Macaca mulatta) is the most established model of HIV infection, providing both insight into pathogenesis and a system for testing novel vaccines. Despite the Chinese rhesus macaque potentially being a more relevant model for AIDS outcomes than the Indian rhesus macaque, the Chinese-origin rhesus macaques have not been well-characterized for their major histocompatibility complex (MHC) composition and function, reducing their greater utilization. In this study, we characterized a total of 50 unique Chinese rhesus macaques from several varying origins for their entire MHC class I allele composition and identified a total of 58 unique complete MHC class I sequences. Only nine of the sequences had been associated with Indian rhesus macaques, and 28/58 (48.3%) of the sequences identified were novel. From all MHC alleles detected, we prioritized Mamu-A1*02201 for functional characterization based on its higher frequency of expression. Upon the development of MHC/peptide binding assays and definition of its associated motif, we revealed that this allele shares peptide binding characteristics with the HLA-B7 supertype, the most frequent supertype in human populations. These studies provide the first functional characterization of an MHC class I molecule in the context of Chinese rhesus macaques and the first instance of HLA-B7 analogy for rhesus macaques.

Animal models in infection and inflammation - chance and necessity

In biomedical research, to understand pathogenesis and test innovative therapeutic strategies, animal models of human disease are a bare necessity. We cannot do without them, but could we do better with them? In this issue of the European Journal of Immunology a series of Viewpoints discusses the pros and cons of currently available animal models that address the clinical challenges of immunology in infection and inflammatory diseases.