SCID-hu mice: a model for studying disseminated HIV infection

Humanized mice recapitulate key features of HIV-1 infection: a novel concept using long-acting anti-retroviral drugs for treating HIV-1

BACKGROUND

Humanized mice generate a lymphoid system of human origin subsequent to transplantation of human CD34+ cells and thus are highly susceptible to HIV infection. Here we examined the efficacy of antiretroviral treatment (ART) when added to food pellets, and of long-acting (LA) antiretroviral compounds, either as monotherapy or in combination. These studies shall be inspiring for establishing a gold standard of ART, which is easy to administer and well supported by the mice, and for subsequent studies such as latency. Furthermore, they should disclose whether viral breakthrough and emergence of resistance occurs similar as in HIV-infected patients when ART is insufficient.

METHODS/PRINCIPAL FINDINGS

NOD/shi-scid/γ(c)null (NOG) mice were used in all experimentations. We first performed pharmacokinetic studies of the drugs used, either added to food pellets (AZT, TDF, 3TC, RTV) or in a LA formulation that permitted once weekly subcutaneous administration (TMC278: non-nucleoside reverse transcriptase inhibitor, TMC181: protease inhibitor). A combination of 3TC, TDF and TMC278-LA or 3TC, TDF, TMC278-LA and TMC181-LA suppressed the viral load to undetectable levels in 15/19 (79%) and 14/14 (100%) mice, respectively. In successfully treated mice, subsequent monotherapy with TMC278-LA resulted in viral breakthrough; in contrast, the two LA compounds together prevented viral breakthrough. Resistance mutations matched the mutations most commonly observed in HIV patients failing therapy. Importantly, viral rebound after interruption of ART, presence of HIV DNA in successfully treated mice and in vitro reactivation of early HIV transcripts point to an existing latent HIV reservoir.

CONCLUSIONS/SIGNIFICANCE

This report is a unique description of multiple aspects of HIV infection in humanized mice that comprised efficacy testing of various treatment regimens, including LA compounds, resistance mutation analysis as well as viral rebound after treatment interruption. Humanized mice will be highly valuable for exploring the antiviral potency of new compounds or compounds targeting the latent HIV reservoir.

Coming of age: reconstruction of heterosexual HIV-1 transmission in human ex vivo organ culture systems

Heterosexual transmission of human immunodeficiency virus-1 (HIV-1), from men to women, involves exposure to infectious HIV-1 in semen. Therefore, the cellular and molecular processes that underlie HIV-1 transmission are closely interconnected with fundamental principles of human reproductive biology. Human ex vivo organ culture systems allow experimental reconstruction of HIV-1 transmission, using human semen and premenopausal cervicovaginal mucosal tissue, with specific emphasis on the progression from exposure to development of primary HIV-1 infection. Clearly, an isolated piece of human tissue cannot duplicate the full complexity of events in natural infections, but with correct observation of conventional medical and ethical standards, there is no opportunity to study HIV-1 exposure and primary infection in young women. Human mucosal organ cultures allow direct study of HIV-1 infection in a reproducible format while retaining major elements of complexity and variability that typify community-based HIV-1 transmission. Experimental manipulation of human mucosal tissue both allows and requires acquisition of new insights into basic processes of human mucosal immunology. Expanding from the current foundations, we believe that human organ cultures will become increasingly prominent in experimental studies of HIV-1 transmission and continuing efforts to prevent HIV-1 infection at human mucosal surfaces.

The utilization of humanized mouse models for the study of human retroviral infections

The development of novel techniques and systems to study human infectious diseases in both an in vitro and in vivo settings is always in high demand. Ideally, small animal models are the most efficient method of studying human afflictions. This is especially evident in the study of the human retroviruses, HIV-1 and HTLV-1, in that current simian animal models, though robust, are often expensive and difficult to maintain. Over the past two decades, the construction of humanized animal models through the transplantation and engraftment of human tissues or progenitor cells into immunocompromised mouse strains has allowed for the development of a reconstituted human tissue scaffold in a small animal system. The utilization of small animal models for retroviral studies required expansion of the early CB-17 scid/scid mouse resulting in animals demonstrating improved engraftment efficiency and infectivity. The implantation of uneducated human immune cells and associated tissue provided the basis for the SCID-hu Thy/Liv and hu-PBL-SCID models. Engraftment efficiency of these tissues was further improved through the integration of the non-obese diabetic (NOD) mutation leading to the creation of NODSCID, NOD/Shi-scid IL2rγ^-/-, and NOD/SCID β2-microglobulin^null animals. Further efforts at minimizing the response of the innate murine immune system produced the Rag2^-/-γ_c^-/- model which marked an important advancement in the use of human CD34+ hematopoietic stem cells. Together, these animal models have revolutionized the investigation of retroviral infections in vivo.

Novel mouse models for understanding HIV-1 pathogenesis

Small animal models in which in vivo HIV-1 infection, pathogenesis, and immune responses can be studied would permit both basic research on the biology of the disease, as well as a system to rapidly screen developmental therapeutics and/or vaccines. To date, the most widely-used models have been the severe combined immunodeficient (SCID)-hu (also known as the thy/liv SCID-hu) and the huPBL-SCID mouse models. Recently three new models have emerged, i.e., the intrasplenic huPBL/SPL-SCID model, the NOD/SCID/IL2Rgamma(null) mouse model, and the Rag2(-/-)gamma(c) (-/-) mouse model. Details on the construction, maintenance and HIV-1 infection of these models are discussed.

Interleukin-4-transgenic hu-PBL-SCID mice: a model for the screening of antiviral drugs and immunotherapeutic agents against X4 HIV-1 viruses

CXCR4-tropic (X4) human immunodeficiency virus type 1 (HIV-1) does not efficiently infect and replicate in severe combined immunodeficiency (SCID) mice reconstituted with human peripheral blood mononuclear cells, termed "hu-PBL-SCID mice," due to, at least in part, relatively low levels of expression of the CXCR4 coreceptor. To overcome this limitation, interleukin (IL)-4-transgenic hu-PBL-SCID mice were derived that spontaneously synthesized human IL-4, which has been shown to enhance CXCR4 expression and promote X4 virus infection in vitro. Experiments reported here show that (1) synthesis of human IL-4 in vivo augmented CXCR4 expression on human CD4(+) lymphocytes and importantly led to productive infection of not only X4 HIV-1(NL4-3) but also multidrug-resistant primary clinical isolates and that (2) the in vivo infection could be significantly blocked by the administration of a CXCR4 antagonist. Altogether, IL-4-transgenic hu-PBL-SCID mice provide a useful model for X4 HIV-1 study and testing/screening of anti-X4 viral drugs.

Targeted killing of virally infected cells by radiolabeled antibodies to viral proteins

BACKGROUND

The HIV epidemic is a major threat to health in the developing and western worlds. A modality that targets and kills HIV-1-infected cells could have a major impact on the treatment of acute exposure and the elimination of persistent reservoirs of infected cells. The aim of this proof-of-principle study was to demonstrate the efficacy of a therapeutic strategy of targeting and eliminating HIV-1-infected cells with radiolabeled antibodies specific to viral proteins in vitro and in vivo.

METHODS AND FINDINGS

Antibodies to HIV-1 envelope glycoproteins gp120 and gp41 labeled with radioisotopes bismuth 213 ((213)Bi) and rhenium 188 ((188)Re) selectively killed chronically HIV-1-infected human T cells and acutely HIV-1-infected human peripheral blood mononuclear cells (hPBMCs) in vitro. Treatment of severe combined immunodeficiency (SCID) mice harboring HIV-1-infected hPBMCs in their spleens with a (213)Bi- or (188)Re-labeled monoclonal antibody (mAb) to gp41 resulted in a 57% injected dose per gram uptake of radiolabeled mAb in the infected spleens and in a greater than 99% elimination of HIV-1-infected cells in a dose-dependent manner. The number of HIV-1-infected thymocytes decreased 2.5-fold in the human thymic implant grafts of SCID mice treated with the (188)Re-labeled antibody to gp41 compared with those treated with the (188)Re-control mAb. The treatment did not cause acute hematologic toxicity in the treated mice.

CONCLUSIONS

The current study demonstrates the effectiveness of HIV-targeted radioimmunotherapy and may provide a novel treatment option in combination with highly active antiretroviral therapy for the eradication of HIV.

Naïve T Cells Are Maintained in the Periphery During the First3 Months of Acute HIV-1 Infection: Implications for Analysis of Thymus Function

A key determinant of T cell dynamics in HIV-1 infection is the status of thymic function. To date, most studies of the impact of HIV-1 on the thymus during early HIV-1 infection have been done in samples collected in the interval of 3-12 months after infection. In this study, we have probed the status of thymic function and peripheral naive T cells in patients with acute HIV-1 infection diagnosed 18-72 days after the onset of symptoms. We found that peripheral CD4 and CD8 T cell proliferation was initially elevated, then waned over time. The fall in T cell proliferation correlated with a reduction in HIV-1 viral RNA levels and a rise in peripheral blood CD4+ CD25+ T cells. In spite of elevated T cell proliferation early on in primary HIV-1 infection, levels of naive phenotype CD4 and CD8 T cells and T cell receptor excision circle positive cells (sjTREC(+)) remained constant. Taken together with the observation that T cell proliferation normally dilutes peripheral T cell episomal sjTREC levels, these data suggested that thymopoiesis contributes to maintenance of the naive T cell pool during the earliest stages of HIV-1 infection (18-72 days).

Human periprosthetic tissues implanted in severe combined immunodeficient mice respond to gene transfer of a cytokine inhibitor

BACKGROUND

Periprosthetic tissue formation and local inflammation that are associated with wear debris contribute to the pathogenesis of aseptic loosening of a prosthesis. This study evaluated a retrovirus-mediated gene therapy with use of a novel xenograft-based animal model.

METHODS

Human periprosthetic tissues obtained from patients during revision arthroplasty performed because of aseptic loosening of a prosthetic joint were transplanted into the left quadriceps and paravertebral muscles of severe combined immunodeficient (SCID) mice. The engrafted tissues were recovered seven, fifteen, or thirty days after implantation for histological and molecular analyses. The periprosthetic tissues were incubated with retroviruses encoding for human interleukin-1 receptor antagonist (hIL-1Ra) or bacteria beta-galactosidase (LacZ) at 37 degrees C for three hours prior to implantation to evaluate their responses to gene modification.

RESULTS

The human periprosthetic tissues were well accepted in SCID mice for up to thirty days, with angiogenesis occurring in the majority of the implanted tissue sections. The histological appearance was consistent between the recovered graft tissue and the original donor tissue. Strong expression of interleukin-1, tumor necrosis factor, and interleukin-6 was detected in the xenografts with use of immunohistochemical stains. Histological analysis revealed that interleukin-1 receptor antagonist gene modification significantly decreased the total number of inflammatory cells (p < 0.01) in engrafted human tissue containing implant wear debris. Real-time reverse transcription-polymerase chain reaction and immunohistochemical staining showed declining expression levels of interleukin-1 and tumor necrosis factor following interleukin-1 receptor antagonist gene transfer in comparison with LacZ-transduced or virus-free controls.

CONCLUSIONS

Human periprosthetic tissue can survive in the SCID mouse host for up to thirty days and responds to the interleukin-1 receptor antagonist gene transfer with the amelioration of inflammation.

Cetacean-reconstituted severe combined immunodeficient (SCID) mice respond to vaccination with canine distemper vaccine

Morbillivirus infections have been responsible for mass mortalities in several species of marine mammals. Nevertheless, relatively little is known on the pathogenesis of the disease and the immune response to the agent, especially in cetaceans, hindering the treatment of individuals and the development of appropriate vaccines, given the difficulty of performing experimental work in marine mammals. The reconstitution of severe combined immunodeficient (SCID) mice, which do not have the ability to reject grafts, with lymphocytes from different species has been used with increasing success as a surrogate species model to study the immune system. We injected NOD/SCID mice with lymphocytes from different species of cetaceans and further vaccinated those mice with a commercial canine distemper virus (CDV) vaccine to develop a practical model to study cetacean immune response to a morbillivirus. Reconstitution was detected in 10/20 mice reconstituted with harbor porpoise spleen, 6/10 mice reconstituted with harbor porpoise lymph node cells, 8/10 mice reconstituted with fresh beluga PBMCs and none of the mice reconstituted with neonate bottlenose dolphin spleen or thymus cells when assessed 42-63 days after reconstitution. While a humoral immune response was detected in none of the reconstituted mice, a cell-mediated immune response to the CDV vaccine was detected in 6/15 (40%) and 2/18 (11%) of the SCID mice after reconstitution with cetacean immune cells after a single or booster vaccination, respectively, for a combined total of 8/33 (24%). This represents the first demonstration of successful reconstitution of SCID mice with marine mammal cells, and to the authors' knowledge, the first direct demonstration of a primary antigen-specific cell-mediated immune response in reconstituted SCID mice. This model will be useful for further research on the physiology of the marine mammal immune system and its response to infectious agents and vaccines, with possible important outcomes in conservation issues.

RNA-based anti-HIV-1 gene therapeutic constructs in SCID-hu mouse model

Effective suppression of HIV-1 replication requires inhibition of critical viral target molecules. Tat and Rev are indispensable regulatory factors for HIV-1 replication, whereas Env mediates virus entry by direct interaction with surface receptor CD4 and coreceptor CCR5 or CXCR4. Anti-HIV-1 tat-rev and env ribozymes and Rev aptamers were previously demonstrated to provide relatively long-term protection against HIV-1 infection in vitro. However, further improvements in these constructs for clinical application in a stem-cell-based gene therapy setting requires in vivo characterization. Toward this end, we introduced these constructs into CD34(+) hematopoietic progenitor cells by retrovirus-mediated gene transduction. Ribozyme- and aptamer-transduced CD34(+) cells differentiated normally into multiple lineages of erythroid and myeloid progenies in a colony-forming unit assay. Macrophages that differentiated from the transduced CD34(+) cells expressed anti-tat-rev and -env ribozymes and Rev aptamers and displayed their normal characteristic surface markers CD14, CD4, and CCR5. Using the SCID-hu mouse in vivo human thymopoiesis model, we demonstrated that ribozyme- and aptamer-transduced CD34(+) cells retained their normal capacity to reconstitute human fetal thymus and liver tissue (thy/liv) grafts. Reconstitution by ribozyme- and aptamer-transduced CD34(+) cells reached levels of up to 87% based on HLA surface marker staining. Differentiated thymocytes derived from reconstituted grafts expressed anti-tat-rev and -env ribozymes and Rev aptamers and showed significant resistance to HIV-1 infection upon challenge. Analysis of reconstituted thymocytes by hybridization revealed an average of 0.4 to 2 copies of vector sequences per cell. Southern analysis of proviral integration junctions in progeny thymocytes demonstrated that the human thy/liv grafts were reconstituted by a few primitive hematopoietic stem cells. These results highlight the utility of RNA-based anti-HIV-1 gene therapeutic approaches and their preclinical testing in a surrogate animal model harboring human tissue.

Human Trichuris-specific antibody responses in vaccinated hu-PBL-SCID mice

Trichuris trichiura is a highly prevalent intestinal helminth of humans with a well-characterized animal model, Trichuris muris in the mouse. Relating the murine work back to the human infection has been difficult, however, as many of the questions addressed in the mouse cannot be asked in humans. The ability to reconstitute a mouse with a human immune system could help bridge this gap, allowing a human immune response to be studied under a controllable laboratory environment. In this study, we demonstrate that severe combined immunodeficient mice engrafted with naïve human peripheral blood lymphocytes are capable of mounting a Trichuris specific human antibody response after vaccination with T. muris antigens. The phenotype of the response depended on the vaccinating antigen with excretory/secretory antigens eliciting a human immunoglobulin IgG2 response, and whole worm homogenate stimulating IgG1 and IgG2 responses. Vaccination with homogenate also enhanced a human IgG response against a 66-kDa component of T. muris homogenate in a donor-dependent manner. This work shows the potential of using the humanized mouse model for studying the immune responses of humans living in T. trichiura endemic areas.

Thy/Liv-SCID-Hu mice implanted with human intestine: an in vivo model for investigation of mucosal transmission of HIV

Mucosal transmission is a major route by which individuals become infected with HIV. Investigation into the mechanism by which mucosal transmission of HIV occurs would be greatly facilitated by the development of a small animal model infectible with HIV by the mucosal route. We have previously described a SCID-hu mouse model, in which human thymic and liver tissues are implanted under both kidney capsules (thy/liv-SCID-hu mice), which are populated in the periphery with high numbers of human T cells and that develop disseminated HIV-1 infection after intraperitoneal injection. To expand further the usefulness of the thy/liv-SCID-hu mouse as a model for studying mucosal transmission of HIV, thy/liv-SCID-hu mice were subcutaneously implanted with human intestinal tissue in a manner that maintained the lumen. Four months later, the histological appearance of the implanted intestine resembled that of normal human bowel tissue and the lamina propria was populated with human T cells. Six weeks after introduction of HIV into the lumen of the intestinal implant, the mice developed disseminated HIV infection. Scattered HIV-infected cells were detected in the lamina propria of the implant, indicating that HIV infection in these mice was mediated by transmission of the virus across the mucosa of the human intestinal implant. Thus, our modified thy/liv-SCID-hu mice transplanted with human bowel tissue should provide a novel model for investigating mucosal transmission of HIV.