Human Immune System Mice Research Articles

Partial clearance of a hepatotropic virus infection in HLA class I and II double transgenic human immune system mice (105.19)

Abstract Humanized mice have emerged as a promising model to study human immunity in vivo. While they are susceptible to many human lymphotropic pathogens, immune responses to infection remain functionally impaired in these mice. It has been shown that the expression of HLA class I or II improves human immunity in this model. However, little is known about the extent of functional human immune responses in non-lymphoid tissues, such as in the liver, and the role of HLA expression in this context. In this study, we analyzed human antiviral immune responses in humanized mice during a hepatotropic adenovirus infection. Specifically, we compared human immune responses of conventional humanized NSG (NOD-SCID IL2RγNULL) mice to those of a novel NSG strain transgenic for HLA-A*0201 and a chimeric HLA-DR*0101. Humanized mice were generated by transplantation of human CD34+ hematopoietic stem cells (HSC) into newborn, sublethally irradiated mice. The development of a human immune system was comparable in both mouse strains. However, the expression of HLA transgenes resulted in more efficient clearance of an adenovirus infection from the liver, which correlated with liver-infiltration and activation of T cells and the detection of antigen-specific adaptive immune responses. While additional modifications to the xeno-recipient may further improve human immunity this study provides first evidence for functional antiviral immunity against a hepatotropic pathogen

What we are learning on HTLV-1 pathogenesis from animal models

Isolated and identified more than 30 years ago, human T cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T cell leukemia/lymphoma, an aggressive lymphoproliferative disease of activated CD4+ T cells, and other inflammatory disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis. A variety of animal models have contributed to the fundamental knowledge of HTLV-1 transmission, pathogenesis, and to the design of novel therapies to treat HTLV-1-associated diseases. Small animal models (rabbits, rats, and mice) as well as large animal models (monkeys) have been utilized to significantly advance characterization of the viral proteins and of virus-infected cells in the early steps of infection, as well as in the development of leukemogenic and immunopathogenic processes. Over the past two decades, the creation of new immunocompromised mouse strains that are robustly reconstituted with a functional human immune system (HIS) after being transplanted with human tissues or progenitor cells has revolutionized the in vivo investigation of viral infection and pathogenesis. Recent observations obtained in HTLV-1-infected humanized HIS mice that develop lymphomas provide the opportunity to study the evolution of the proviral clonality in human T cells present in different lymphoid organs. Current progress in the improvement of those humanized models will favor the testing of drugs and the development of targeted therapies against HTLV-1-associated diseases.

Characteristics ofBorrelia hermsiiinfection in human hematopoietic stem cell-engrafted mice mirror those of human relapsing fever

Rodents are natural reservoirs for a variety of species of Borrelia that cause relapsing fever (RF) in humans. The murine model of this disease recapitulates many of the clinical manifestations of the human disease and has revealed that T cell-independent antibody responses are required to resolve the bacteremic episodes. However, it is not clear whether such protective humoral responses are mounted in humans. We examined Borrelia hermsii infection in human hematopoietic stem cell-engrafted nonobese diabetic/SCID/IL-2Rγ(null) mice: "human immune system mice" (HISmice). Infection of these mice, which are severely deficient in lymphoid and myeloid compartments, with B. hermsii resulted in persistent bacteremia. In contrast, this infection in HISmice resulted in recurrent episodes of bacteremia, the hallmark of RF. The resolution of the primary episode of bacteremia was concurrent with the generation of B. hermsii-specific human IgM. Remarkably, HISmice generated antibody responses to the B. hermsii outer-membrane protein Factor H binding protein A. Sera from humans infected by B. hermsii have a similar reactivity, and studies in mice have shown that this response is generated by the B1b cell subset. HISmice contain several B-cell subsets, including those with the phenotype CD20(+)CD27(+)CD43(+)CD70(-), a proposed human equivalent of mouse B1 cells. Reduction of B cells by administration of anti-human CD20 antibody resulted in diminished anti-B. hermsii responses and persistent bacteremia in HISmice. These data indicate that analysis of B. hermsii infection in HISmice will serve as a model in which to study the cellular and molecular mechanisms involved in controlling human RF.

Human immune system mice: current potential and limitations for translational research on human antibody responses

It has recently become possible to generate chimeric mice durably engrafted with many components of the human immune system (HIS mice). We have characterized the maturation and function of the B cell compartment of HIS mice. The antibody response of HIS mice to T cell-dependent B cell antigens is limited, and contributing factors may be the general immaturity of the B cell compartment, infrequent helper T cells selected on human MHC class II antigens, and incomplete reconstitution of secondary lymphoid organs and their microenvironments. In contrast, HIS mice generate protective antibody responses to the bacterium Borrelia hermsii, which acts as a T cell-independent antigen in mice, but do not respond to purified polysaccharide antigens (PPS). We speculate that the anti-B. hermsii response of HIS mice is derived from an abundant B cell subset that may be analogous to B1 B cells in mice. We suggest that failure of HIS mice to respond to PPS is due to the lack of a B cell subset that may originate from adult bone marrow and is highly dependent on human interleukin-7 for development.

Stem Cell Factor Consistently Improves Thymopoiesis after Experimental Transplantation of Murine or Human Hematopoietic Stem Cells in Immunodeficient Mice

Deficient thymopoiesis is a pivotal determinant of impaired immune competence following hematopoietic stem cell transplantation (HSCT). Stem cell factor (SCF) is essentially involved in early thymopoiesis. We evaluated whether SCF administration would improve recovery of thymopoiesis following HSCT in immunodeficient mice receiving: 1) bone marrow (BM) transplantation of congenic mice; or 2) human fetal liver HSCT in the human immune system mouse model. Following murine BM transplantation, SCF significantly enhanced thymopoiesis and peripheral T cell recovery in lymph nodes and spleen. SCF did not affect BM lymphoid progenitor recovery and/or expansion. Median thymic cellularity increased from 0.9 in PBS- to 266 × 10(4)/thymus in SCF-treated mice (p = 0.05). Following human HSCT in human immune system mice, higher thymic cellularity was observed in SCF-treated mice. Double-negative and early double-positive thymocyte subsets increased, but especially late double-positive, CD4 single-positive, and CD8 single-positive thymocyte subsets were significantly enhanced (p < 0.05). These results show that exogenous supply of SCF may significantly improve murine and human posttransplant thymopoiesis, for which the effect is probably exerted by directly promoting T cell development intrathymically rather than by enhanced entry of prethymically expanded lymphoid progenitors.

Autonomous and extrinsic regulation of thymopoiesis inhuman immune system (HIS) mice

Human Immune System (HIS) mice represent a novel biotechnology platform to dissect human haematopoiesis and immune responses. However, the limited human T-cell development that is observed in HIS mice restricts its utility for these applications. Here, we address whether reduced thymopoiesis in HIS mice reflects an autonomous defect in T-cell precursors and/or a defect in the murine thymic niche. Human thymocyte precursors seed the mouse thymus and their reciprocal interactions with murine thymic epithelial cells (TECs) led to both T-cell and TEC maturation. The human thymocyte subsets observed in HIS mice demonstrated survival, proliferative and phenotypic characteristics of their normal human counterparts, suggesting that the intrinsic developmental program of human thymocytes unfolds normally in this xenograft setting. We observed that exogenous administration of human IL-15/IL-15Rα agonistic complexes induced the survival, proliferation and absolute numbers of immature human thymocyte subsets, without any obvious effect on cell-surface phenotype or TCR Vβ usage amongst the newly selected mature single-positive (SP) thymocytes. Finally, when IL-15 was administered early after stem cell transplantation, we noted accelerated thymopoiesis resulting in the more rapid appearance of peripheral naïve T cells. Our results highlight the functional capacity of murine thymic stroma cells in promoting human thymopoiesis in HIS mice but suggest that the "cross-talk" between murine thymic stroma and human haematopoietic precursors may be suboptimal. As IL-15 immunotherapy promotes early thymopoiesis, this novel approach could be used to reduce the period of T-cell immunodeficiency in the post-transplant clinical setting.

Functional CD47/signal regulatory protein alpha (SIRPα) interaction is required for optimal human T- and natural killer- (NK) cell homeostasis in vivo

The homeostatic control mechanisms regulating human leukocyte numbers are poorly understood. Here, we assessed the role of phagocytes in this process using human immune system (HIS) BALB/c Rag2(-/-)IL-2Rγc(-/-) mice in which human leukocytes are generated from transplanted hematopoietic progenitor cells. Interactions between signal regulatory protein alpha (SIRPα; expressed on phagocytes) and CD47 (expressed on hematopoietic cells) negatively regulate phagocyte activity of macrophages and other phagocytic cells. We previously showed that B cells develop and survive robustly in HIS mice, whereas T and natural killer (NK) cells survive poorly. Because human CD47 does not interact with BALB/c mouse SIRPα, we introduced functional CD47/SIRPα interactions in HIS mice by transducing mouse CD47 into human progenitor cells. Here, we show that this procedure resulted in a dramatic and selective improvement of progenitor cell engraftment and human T- and NK-cell homeostasis in HIS mouse peripheral lymphoid organs. The amount of engrafted human B cells also increased but much less than that of T and NK cells, and total plasma IgM and IgG concentrations increased 68- and 35-fold, respectively. Whereas T cells exhibit an activated/memory phenotype in the absence of functional CD47/SIRPα interactions, human T cells accumulated as CD4(+) or CD8(+) single-positive, naive, resting T cells in the presence of functional CD47/SIRPα interactions. Thus, in addition to signals mediated by T cell receptor (TCR)/MHC and/or IL/IL receptor interactions, sensing of cell surface CD47 expression by phagocyte SIRPα is a critical determinant of T- and NK-cell homeostasis under steady-state conditions in vivo.

HTLV-1 propels thymic human T cell development in “human immune system” Rag2-/- IL-2R γc-/- Mice

Alteration of early haematopoietic development is thought to be responsible for the onset of immature leukemias and lymphomas. We have previously shown that HTLV-1 (Human T cell Leukemia Virus type 1) is able not only to infect immature thymocytes in vitro but also, through Tax expression, to alter the β-selection checkpoint critical for early T cell development. To further clarify the role of the natural HTLV-1 infection on human T-cell development, we developed an in vivo model by transplanting immunocompromised Rag2-/-γc-/- newborn mice with human cord blood CD34+ cells to obtain Human Immune System (HIS) mice. In these mice the development of human T cells in the thymus is fully developed within two months after human cell transplantation. Lethally irradiated HTLV-1 producing cells were then injected into these HIS mice. Herein we observed in the thymus of the infected animals an enlarged population of mature T cells when compared with the mock-infected mice. Furthermore, we noted an increased number of CD4+ cells expressing CD25. Infected animals also developed, several weeks after the infection, pathological features such as splenomegaly, adenopathy, thymomas, lymphomas and leukemias in which predominate human T cells, with a large proportion of CD25+ activated cells. Tax expression especially in the lymphomas and thymomas correlated with an up-regulation of NF-κB regulated genes. Altogether, these results underline that this HIS Rag2-/-γc-/- model might be of great interest to study the leukemogenic process induced by HTLV-1 as well as to validate new therapeutic approaches of ATL.

HTLV-1-infected HIS Rag2-/-γc-/- mice, a suitable model for in vivo investigating the effects of drugs in ATL treatment?

Adult T cell Leukemia, an aggressive T-cell malignancy linked to HTLV-1 infection, is resistant to chemotherapy. Recently, promising results were obtained with the combination of arsenic trioxide, interferon alpha and zidovudine. However, the cellular and molecular mechanisms of their anti-leukemia activity remain to be investigated. To that aim, we have relied on HIS (Human Immune System) Rag2-/-γc-/- mice. We have indeed observed that when infected with HTLV-1, these mice displayed, five months later, an elevated number of human Tax-expressing T cells in the thymus, the spleen and the mesenteric lymph nodes. Some of them also developed T lymphoproliferative diseases. To determine the effects of these three drugs on the apparition of these pathological features, Rag2-/-γc-/- mice were infected with HTLV-1 and, 16 weeks after infection, daily treated with the drugs for one week and sacrificed. Untreated HIS mice infected with HTLV-1 were used as control. Treatment resulted in a significant decrease in the spleen weight as compared to untreated controls. Interestingly, we also noted a decrease of the proviral load in thymocytes as well as a drop in the number of mature activated T cells in the spleen and in lymph nodes. These data clearly indicate that the HIS mouse model provides us with the opportunity not only for the pre-clinical evaluation of therapeutic approaches against the leukemogenic process associated with HTLV-1 infection, but also for unraveling the corresponding mechanisms.

Abstract 4745: TLR8 agonist and Doxil chemotherapy potently activate human antitumor immune response in a human immune system mouse model

Abstract Because of differences between mouse and human immune systems, many of the effects of immunomodulatory drugs cannot be fully studied in syngeneic mouse models. We thus generated a novel tumor-bearing mouse model with human immune system (HIS) to study interactions between chemotherapy and immune modulatory therapy. We tested the individual effects and the interactions between doxorubicin, a drug which induces immunogenic tumor cell death and activates antigen-presenting cells, and VTX-2337, a novel small-molecule TLR8 agonist, which induces potent activation and type 1 polarization of human myeloid DCs but is inactive on murine leukocytes. Nod/SCID/ILRγc knock out (NSG) mice were inoculated with human CD34+ cord blood cells from HLA-A2+ human donors; transplanted s.c. with human HLA-A2+ OVCAR5 ovarian cancer tumors; and treated with pegylated liposomal doxorubicin (Doxil); VTX-2337; or a combination. NSG-HIS mice exhibited a full human hematopoietic system, including human monocytes, macrophages and plasmacytoid and myeloid DCs as well as T and B cell subsets. In NSG-HIS mice, VTX-2337 induced dose-dependent activation of human Monocytes CD14+ and CD11c+ cells with upregulation of CD80, CD86, CD83, CD40 and MHCII within 6 hrs. Transient, dose-dependent upregulation of human Th1 cytokines but also IL-10 was observed in the plasma of treated mice, reaching peaks within 6 hrs and subsiding within 24 hrs. Doxil induced mild activation of CD11c+ DCs and mild upregulation of Th1 cytokines. The combination of two drugs induced potent activation of CD11c+ DCs and marked increase in Th1 cytokines with significant decrease of IL-10. HLA-A2+ OVCAR5 tumors were successfully engrafted, exhibiting infiltration by human leukocytes. VTX-2337 and Doxil treatment independently induced tumor-infiltrating lymphocytes and restricted growth of human ovarian tumor xenografts in a dose-dependent manner, while the combination of the two drugs induced the highest frequency of Leukocytes (hCD45) and potently restricted growth of ovarian tumors. Combined activation of innate and adaptive immunity by VTX-2337 and Doxil, as well sensitization of tumor cells by Doxil to adaptive and innate immune effector mechanisms was at the basis of the observed interactions suppressing tumor growth. We conclude that the NSG-HIS provided a suitable tool to establish potent interactions between TLR8 agonist and Doxil chemotherapy, which warrant clinical testing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4745. doi:10.1158/1538-7445.AM2011-4745

IL-15 transpresentation promotes both human T-cell reconstitution and T-cell–dependent antibody responses in vivo

Cytokine immunotherapies targeting T lymphocytes are attractive clinical interventions against viruses and tumors. In the mouse, the homeostasis of memory α/β CD8(+) T cells and natural killer (NK) cells is significantly improved with increased IL-15 bioavailability. In contrast, the role of "transpresented" IL-15 on human T-cell development and homeostasis in vivo is unknown. We found that both CD8 and CD4 T cells in human immune system (HIS) mice are highly sensitive to transpresented IL-15 in vivo, with both naïve (CD62L(+)CD45RA(+)) and memory phenotype (CD62L(-)CD45RO(+)) subsets being significantly increased following IL-15 "boosting." The unexpected global improvement in human T-cell homeostasis involved enhanced proliferation and survival of both naïve and memory phenotype peripheral T cells, which potentiated B-cell responses by increasing the frequency of antigen-specific responses following immunization. Transpresented IL-15 did not modify T-cell activation patterns or alter the global T-cell receptor (TCR) repertoire diversity. Our results indicate an unexpected effect of IL-15 on human T cells in vivo, in particular on CD4(+) T cells. As IL-15 promotes human peripheral T-cell homeostasis and increases the frequency of neutralizing antibody responses in HIS mice, IL-15 immunotherapy could be envisaged as a unique approach to improve vaccine responses in the clinical setting.

Lentiviral Vector Delivery of Human Interleukin-7 (hIL-7) to Human Immune System (HIS) Mice Expands T Lymphocyte Populations

Genetically modified mice carrying engrafted human tissues provide useful models to study human cell biology in physiologically relevant contexts. However, there remain several obstacles limiting the compatibility of human cells within their mouse hosts. Among these is inadequate cross-reactvitiy between certain mouse cytokines and human cellular receptors, depriving the graft of important survival and growth signals. To circumvent this problem, we utilized a lentivirus-based delivery system to express physiologically relevant levels of human interleukin-7 (hIL-7) in Rag2-/-γc-/- mice following a single intravenous injection. hIL-7 promoted homeostatic proliferation of both adoptively transferred and endogenously generated T-cells in Rag2-/-γc-/- Human Immune System (HIS) mice. Interestingly, we found that hIL-7 increased T lymphocyte numbers in the spleens of HIV infected HIS mice without affecting viral load. Taken together, our study unveils a versatile approach to deliver human cytokines to HIS mice, to both improve engraftment and determine the impact of cytokines on human diseases.

MicroRNAs enriched in hematopoietic stem cells differentially regulate long-term hematopoietic output

The production of blood cells depends on a rare hematopoietic stem-cell (HSC) population, but the molecular mechanisms underlying HSC biology remain incompletely understood. Here, we identify a subset of microRNAs (miRNAs) that is enriched in HSCs compared with other bone-marrow cells. An in vivo gain-of-function screen found that three of these miRNAs conferred a competitive advantage to engrafting hematopoietic cells, whereas other HSC miRNAs attenuated production of blood cells. Overexpression of the most advantageous miRNA, miR-125b, caused a dose-dependent myeloproliferative disorder that progressed to a lethal myeloid leukemia in mice and also enhanced hematopoietic engraftment in human immune system mice. Our study identifies an evolutionarily conserved subset of miRNAs that is expressed in HSCs and functions to modulate hematopoietic output.

CD34+CD31+ immature myeloid cells (IMC) as an inhibitor cell on the production of antimicrobial peptide: III. Translational studies of glycyrrhizin in humanized NOD-SCID IL-2rγ-/- mice (NOD-SCID mice) (89.30)

Abstract In the accompanying paper II, glycyrrhizin (GL) is shown to be inhibitory on the CD34+CD31+ IMC-mediated suppression of HBD-1 production by normal human epidermal keratinocytes (NHEK). In this study, the inhibitory effect of GL on CD34+CD31+ IMC-mediated suppression in HBD-1 production by NHEK was tested in human immune system mice (HIS mice) (X-irradiated NOD-SCID mice inoculated i.d. with NHEK and CD34+CD31+ IMC). Two days after the cell inoculation, inoculation site skin tissues were biopsied (8-mm diameter) from HIS mice treated with or without GL (Minophagen Pharmaceutical Co., Ltd., Tokyo, Japan) 6 and 24 hrs after the cell inoculation. Five biopsied tissue samples were homogenized together on ice in PBS. The concentrations of HBD-2 in the homogenates were measured by ELISA. In the results, 4.9 μg/ml of HBD-2 was detected in skin homogenates derived from NOD-SCID mice inoculated with NHEK, while HBD-2 was not detected in skin homogenates from HIS mice. When HIS mice were treated with 10 mg/kg of GL, HBD-2 was detected in IMC-inoculation site tissues of these mice. The amount of HBD-2 detected in this homogenate was corresponded to that detected in tissues of NOD-SCID mice inoculated with NHEK alone. These results indicate that GL improves the suppressed HBD-2 production in HIS mice. This study was supported by SHC #8610.

Regulated and Multiple miRNA and siRNA Delivery Into Primary Cells by a Lentiviral Platform

RNA interference (RNAi) has tremendous potential for investigating gene function and developing new therapies. However, the design and validation of proficient vehicles for stable and safe microRNA (miR) and small interfering RNA (siRNA) delivery into relevant target cells remains an active area of investigation. Here, we developed a lentiviral platform to efficiently coexpress one or more natural/artificial miR together with a gene of interest from constitutive or regulated polymerase-II (Pol-II) promoters. By swapping the stem-loop (sl) sequence of a selected primary transcript (pri-miR) with that of other miR or replacing the stem with an siRNA of choice, we consistently obtained robust expression of the chimeric/artificial miR in several cell types. We validated our platform transducing a panel of engineered cells stably expressing sensitive reporters for miR activity and on a natural target. This approach allowed us to quantitatively assess at steady state the target suppression activity and expression level of each delivered miR and to compare it to those of endogenous miR. Exogenous/artificial miR reached the concentration and activity typical of highly expressed natural miR without perturbing endogenous miR maturation or regulation. Finally, we demonstrate the robust performance of the platform reversing the anergic/suppressive phenotype of human primary regulatory T cells (Treg) by knocking-down their master gene Forkhead Transcription Factor P3 (FOXP3).

Evaluation of safety and efficacy of RNAi against HIV-1 in the human immune system (Rag-2-/-γc-/-) mouse model

RNA interference (RNAi) gene therapy against HIV-1 by stable expression of antiviral short hairpin RNAs (shRNAs) can potently inhibit viral replication in T cells. Recently, a mouse model with a human immune system (HIS) was developed that can be productively infected with HIV-1. In this in vivo model, in which Rag-2(-/-)gamma(c)(-/-) mice are engrafted with human CD34(+)CD38(-) hematopoietic precursor cells, we evaluated an anti-HIV RNAi gene therapy. Human hematopoietic stem cells were transduced with a lentiviral vector expressing an shRNA against the HIV-1 nef gene (shNef) or the control vector. We observed normal development of the different cell subsets of the immune system. However, although initial transduction efficiencies were similar for both vectors, a reduced percentage of transduced human immune cells was observed for the shNef vector after establishment of the HIS in vivo. Further studies are required to fully evaluate the safety implications. When we infected the mature human CD4(+) T cells from the HIS mouse ex vivo with HIV-1, potent inhibition of viral replication was scored in shNef-expressing cells, confirming efficacy. When challenged with an shNef-resistant HIV-1 variant, equal replication was scored in control and shNef-expressing cells, confirming sequence-specificity of the RNAi therapy. We thus demonstrated that an antiviral RNAi-based gene therapy on blood stem cells leads to HIV-1-resistant T cells in vivo, an important proof of concept in the clinical development of RNAi against HIV-1.

Human CD4-major histocompatibility complex class II (DQw6) transgenic mice in an endogenous CD4/CD8-deficient background: reconstitution of phenotype and human-restricted function.

To reconstitute the human immune system in mice, transgenic mice expressing human CD4 and human major histocompatibility complex (MHC) class II (DQw6) molecules in an endogenous CD4- and CD8-deficient background (mCD4/8-/-), after homologous recombination, have been generated. We report that expression of human CD4 molecule in mCD4/8-/- mice rescues thymocyte development and completely restores the T cell compartment in peripheral lymphoid organs. Upon vesicular stomatitis virus (VSV) challenge, the reconstituted mature T cell population effectively provide T help to B cells in immunoglobulin class switching from IgM to specific IgG-neutralizing antibodies. Human CD4+DQw6+ double transgenic mice are tolerant to DQw6 and the DQw6 molecule functions in antigen presentation, effectively generating a human MHC class II-restricted T cell response to streptococcal M6C2 peptide. These data show that both the hCD4 and DQw6 molecules are functional in mCD4/8-/- mice, fully and stably reconstituting this limb of the human immune system in mice. This animal model provides a powerful in vivo tool to dissect the human CD4-human class II MHC interaction, especially its role in human autoimmune diseases, superantigen-mediated diseases, and acquired immunodeficiency syndrome (AIDS).

Transfer of a functional human immune system to mice with severe combined immunodeficiency.

The pressing need for a better experimental system for AIDS research has brought into sharp focus the shortcomings of available animal models and the practical and ethical limitations of studies of immune responses and viral pathogenesis in humans. Current studies of the human immune responses are limited to relatively restrictive in vivo experiments and several in vitro systems that, although useful, allow only short-term studies and support responses to a few antigens. Neither model is particularly amenable to studies of the pathogenesis of diseases of the immune system. We report here that injection of human peripheral blood leukocytes (PBL) can result in the stable long-term reconstitution of a functional human immune system in mice with severe combined immunodeficiency (SCID). Human PBL transplanted to SCID mice increase in number and survive for at least six months; reconstituted mice show spontaneous secretion of human immunoglobulin and a specific human antibody response is induced following immunization with tetanus toxoid. All of the major cell populations present in PBL are found in the lymphoid tissue and blood of SCID recipients, although the relative proportions of B cells, T-cell subsets and monocytes/macrophages in long-term recipients differ from those found in normal PBL and, in mice transplanted with 50 x 10(6) or more PBL from Epstein-Barr virus (EBV)-seropositive donors, EBV-positive B-cell lymphomas often develop. Our results suggest that xenogeneic transplantation of human lymphoid cells into SCID mice may provide a useful model for the study of normal human immune function, the response of the immune system to pathogenic agents and early events in lymphomagensis.