Effective immunochemotherapy of CALLA+C mu+ human pre-B acute lymphoblastic leukemia in mice with severe combined immunodeficiency using B43 (anti-CD19) pokeweed antiviral protein immunotoxin plus cyclophosphamide.

A highly aggressive human CALLA+C mu+ pre-B acute lymphoblastic leukemia (ALL) cell line (NALM-6-UM1) causes disseminated and invariably fatal leukemia in CB.17 mice with severe combined immunodeficiency (SCID). We used this SCID mouse model of human pre-B ALL to evaluate and compare, in a total of 434 SCID mice, the antileukemic efficacy of B43 (anti-CD19)-pokeweed antiviral protein (PAP) immunotoxin and cyclophosphamide (CPA) as individual reagents and as combined immunochemotherapeutic regimens. B43-PAP plus CPA was superior to either the immunotoxin or drug alone, and combined immunochemotherapy markedly improved the event-free survival (EFS) of SCID mice challenged with NALM-6-UM1 pre-B ALL cells. Notably, 90% to 100% of SCID mice challenged with 1 x 10(6) leukemia cells and then treated with B43-PAP plus CPA combined immunochemotherapy regimens became long-term survivors, a result not achieved with B43-PAP alone or CPA alone. The advantage was particularly evident in mice inoculated with 5 x 10(6) leukemia cells. While neither 15 micrograms B43-PAP (median survival, 58 days) nor 1 mg CPA (median survival, 49 days) resulted in long-term EFS of SCID mice challenged with 5 x 10(6) NALM-6-UM1 pre-B ALL cells, the probability of EFS at 6 months was 50% +/- 16% for SCID mice treated with 15 micrograms B43-PAP plus 1 mg CPA (median survival, greater than 180 days) (P less than .0001). The probability of long-term EFS was only 14% +/- 7% for mice treated with 30 micrograms B43-PAP and 0% +/- 0% for mice treated with 1 mg CPA, but 40% +/- 16% for mice treated with 30 micrograms B43-PAP plus 1 mg CPA (P less than .0001). Similarly, the probability of EFS at 6 months was 40% +/- 16% for mice treated with 2 mg CPA alone, 70% +/- 15% for mice treated with 2 mg CPA plus 15 micrograms B43-PAP, and 70% +/- 15% for mice treated with 2 mg CPA plus 30 micrograms B43-PAP. Ten SCID mice in the B43-PAP plus CPA combined immunochemotherapy arms surviving long term after the inoculation of 5 x 10(6) NALM-6-UM1 pre-B ALL cells were electively killed at 174 to 181 days to assess their leukemia burden. We found no evidence of leukemia in any of the bone marrow specimens by two-color immunofluorescence and multiparameter flow cytometry.(ABSTRACT TRUNCATED AT 400 WORDS)     

In vivo efficacy of B43 (anti-CD19)-pokeweed antiviral protein immunotoxin against human pre-B cell acute lymphoblastic leukemia in mice with severe combined immunodeficiency.

A highly aggressive subclone of the human CALLA+C mu+ pre-B acute lymphoblastic leukemia (ALL) cell line NALM-6 (designated NALM-6-UM1) caused disseminated and fatal leukemia in CB.17 mice with severe combined immunodeficiency (SCID). An intravenous challenge with 1 x 10(6) (NALM-6-UM1 cells caused 15 of 27 (56%) SCID mice to become paraplegic at 31 +/- 2 days (median = 33 days) and 27 of 27 (100%) mice to die of disseminated leukemia at 38 +/- 1 days (median = 39 days). We used this SCID mouse model of aggressive human pre-B ALL to evaluate the in vivo antileukemic efficacy of B43 (anti-CD19)-pokeweed antiviral protein (PAP) immunotoxin. A 3-day treatment with nontoxic doses of B43-PAP markedly reduced the incidence of paraplegia and improved event-free survival (EFS) in SCID mice challenged with 1 x 10(6) NALM-6-UM1 pre-B ALL cells, as reflected by significantly higher cumulative proportions of mice free of paraplegia or alive at 1 to 7 months, as compared with phosphate-buffered saline (PBS) treated control mice. The Kaplan-Meier estimates and standard errors of the probability of developing paraplegia after inoculation of 1 x 10(6) NALM-6-UM1 cells was 64% +/- 10% for PBS-treated mice (median time to paraplegia = 37 days) (N = 27), 18% +/- 8% for mice treated with 15 micrograms B43-PAP (5 micrograms/mouse/d x 3 days) (N = 23) and 5% +/- 5% for mice treated with 30 micrograms B43-PAP (10 micrograms/mouse/d x 3 days) (N = 21). While 27 of 27 PBS-treated control SCID mice died of leukemia at 38 +/- 1 days (range = 24 to 54 days), only 16 of 44 B43-PAP-treated mice developed leukemia at 74 +/- 12 days (range = 30 to 182 days), consistent with greater than or equal to 6 logs kill of clonogenic NALM-6-UM1 cells in 64% of SCID mice. The Kaplan-Meier estimates and standard errors of the probability of long-term EFS after inoculation of 1 x 10(6) NALM-6-UM1 cells were 65% +/- 10% for mice treated with 15 micrograms B43-PAP and 60% +/- 11% for mice treated with 30 micrograms B43-PAP with a median survival time of greater than 7 months for both groups. In contrast, neither unconjugated B43 monoclonal antibody nor the anti-T-cell immunotoxin G17.2 (anti-CD4)-PAP decreased the incidence of paraplegia or improved EFS.(ABSTRACT TRUNCATED AT 400 WORDS)     

Antitumor activity of anti-CD30 immunotoxin (Ber-H2/saporin) in vitro and in severe combined immunodeficiency disease mice xenografted with human CD30+ anaplastic large-cell lymphoma

To develop a novel adjunctive therapy for CD30 (Ki-1)+ anaplastic large- cell lymphoma (ALCL), we investigated in preclinical studies the antitumor activity of an immunotoxin (IT) constructed by coupling the plant ribosome-inactivating protein saporin (SO6) to the monoclonal antibody (MoAb) Ber-H2 that is directed against the CD30 molecule, a new member of the tumor necrosis factor receptor (TNFR) super-family. The activity of Ber-H2/SO6 IT was tested both in vitro against the CD30+ ALCL-derived cell line JB6 and in vivo using our severe combined immunodeficiency disease (SCID) mouse model of human xenografted CD30+ ALCL. In vitro, the Ber-H2/SO6 IT was selectively and highly toxic to the JB6 cell line [50% inhibiting concentration (IC50), 3.23 x 10(-12) mol/L as SO6]. In vivo, a 3-day treatment with nontoxic doses of Ber- H2/SO6 (50% of LD50) induced lasting complete remissions (CR) in 80% of mice when started 24 hours after tumor transplantation. In contrast, injection of the IT at later stages of tumor growth (mice bearing subcutaneous tumors of 40- to 60-mm3 volume), induced CR in only 6 of 21 (approximately 30%) mice and significantly delayed tumor growth rate (P < .01). This finding suggests that maximum effect of the anti-CD30 IT is observed when tumor cell burden is small. Persistent tumors from IT-treated mice consisted of CD30+ cells, thus excluding the possibility that selection of CD30-negative mutant clones during IT therapy was responsible for resistance to treatment. We conclude that Ber-H2/SO6 IT is an effective agent against CD30+ ALCL growing in SCID mice, suggesting its possible role as adjuvant therapy in patients with CD30+ ALCL refractory to standard treatments.     

Developmental hierarchy during early human B-cell ontogeny after autologous bone marrow transplantation using autografts depleted of CD19+ B-cell precursors by an anti-CD19 pan-B-cell immunotoxin containing pokeweed antiviral protein.

Sequential immunophenotypes of bone marrow (BM) and peripheral blood (PBL) lymphoid cells from 15 B-lineage acute lymphoblastic leukemia (ALL) patients who underwent autologous bone marrow transplantation (BMT) during complete remission were determined by dual-color immunofluorescence and multiparameter flow cytometry. Autografts were depleted of CD19+ B-cell precursors by an immunochemopurging protocol that combines B43-PAP, a potent anti-CD19 immunotoxin, and the cyclophosphamide congener 4-hydroperoxycyclophosphamide (4-HC). A marked interpatient variation was observed in the appearance and expansion of B-cell precursors repopulating the posttransplant marrow. The expression of CD10 and CD19 antigens during early B-cell ontogeny post-BMT preceded the expression of CD20, CD21, CD22, CD40, and sIgM. The surface antigen profiles of the emerging B-cell precursors were similar to those of fetal liver or fetal bone marrow B-cell precursors. Our comparisons of BM and PBL samples from patients in the early post-BMT period demonstrated that (1) PBL initially contains fewer B-lineage cells than does BM, and (2) circulating B-lineage lymphoid cells have a more mature immunophenotype than do BM B-lineage lymphoid cells. Comparison of the surface antigen profiles of day 30 versus day 100 or year 1 BM or PBL lymphoid cells showed an increase in the percentages of CD10+CD22- undifferentiated lymphocyte precursors, as well as CD19+sIgM- B-cell precursors (pre-pre-B), consistent with a time-dependent expansion of these B-cell precursor populations post-BMT. Importantly, the percentages of CD10+CD22+ and CD19+sIgM+ B-cell precursor (pre-B) populations also increased between 30 days and 1 year post-BMT, confirming the ability of emerging immature B-cell precursors to differentiate along the B-precursor pathway. The acquisition and expression of B-lineage differentiation antigens at different stages of the post-BMT B-cell ontogeny support the notion that the expression of these antigens is developmentally programmed. Similar to patients in previous autologous BMT studies, recipients of B-cell precursor-depleted autografts had normal or nearly normal serum immunoglobulin levels, suggesting that the maturing B-cell/plasma cell populations can produce and secrete immunoglobulins. The development of a functional CD19+ B-lineage lymphoid compartment in recipients of autografts which were depleted of CD19+ B-cell precursors corroborates the previously postulated existence of CD19- B-lineage lymphoid progenitor cells.     

Efficacy of rituximab (anti-CD20) for refractory systemic lupus erythematosus involving the central nervous system

AIM: Neuropsychiatric systemic lupus erythematosus (NPSLE) is a serious treatment-resistant phenotype of systemic lupus erythematosus. A standard treatment for NPSLE is not available. This report describes the clinical and laboratory tests of 10 patients with NPSLE before and after rituximab treatment, including changes in lymphocyte phenotypes. METHODS: Rituximab was administered at different doses in 10 patients with refractory NPSLE, despite intensive treatment. RESULTS: Treatment with rituximab resulted in rapid improvement of central nervous system-related manifestations, particularly acute confusional state. Rituximab also improved cognitive dysfunction, psychosis and seizure, and reduced the SLE Disease Activity Index Score at day 28 in all 10 patients. These effects lasted for >1 year in five patients. Flow cytometric analysis showed that rituximab down regulated CD40 and CD80 on B cells and CD40L, CD69 and inducible costimulator on CD4+ T cells. CONCLUSIONS: Rituximab rapidly improved refractory NPSLE, as evident by resolution of various clinical signs and symptoms and improvement of radiographic findings. The down regulation of functional molecules on B and T cells suggests that rituximab modulates the interaction of activated B and T cells through costimulatory molecules. These results warrant further analysis of rituximab as treatment for NPSLE.     

Human t(4;11)(q21;q23) acute lymphoblastic leukemia in mice with severe combined immunodeficiency

Mice with severe combined immunodeficiency (SCID) were injected intravenously with primary bone marrow blasts from 12 children with newly diagnosed t(4;11)(q21;q23) acute lymphoblastic leukemia (ALL). Blasts from eight patients caused overt disseminated leukemia, whereas blasts from the other four patients produced occult leukemia that was detectable only by the polymerase chain reaction (PCR) technique. Only one patient among eight whose blasts caused disseminated leukemia in SCID mice remains alive and disease-free at 48.4 months postdiagnosis. In contrast, three of the other four patients whose blasts did not cause overt leukemia in SCID mice remain alive and disease-free at 6.1, 23.6, and 35.9 months, respectively. Thus, the occurrence of overt leukemia in SCID mice may be a predictor of patients' disease-free survival. The described SCID mouse model system may prove useful for designing more effective treatment strategies against therapy- refractory t(4;11) ALL.     

Autologous transplantation of bone marrow purged in vitro with anti-CD7- (WT1-) ricin A immunotoxin in T-cell lymphoblastic leukemia and lymphoma

Seven patients with high-risk acute T-cell lymphoblastic leukemia (T- ALL) and six with T cell lymphoma (T-LL) were treated with autologous bone marrow transplantation (ABMT) after in vitro purging of their bone marrow with WT1 (CD7)-ricin A-chain immunotoxin. CD7 expression on the tumor cells showed large variations between the individual patients and was highly related to the specific cytotoxicity of WT1-ricin A. Incubation of bone marrow with up to 10(-8)mol/L WT1-ricin A in the presence of 6 mmol/L NH4Cl did not compromise the growth potential of the hematopoietic progenitors CFU-GM, CFU-GEMM, and BFU-E. Hematologic engraftment (greater than 10(9) leukocytes/L) occurred within a normal time period (median, 17 days). Seven patients are alive and in complete remission (CR) at 48+, 44+, 40+, 26+, 11+, 7+, and 6+ months after ABMT. Four patients relapsed within 6 months after ABMT. Two of them had the lowest CD7 expression on their tumor cells, the other two were transplanted in CR2 and CR3. Two patients died from transplantation related infections. The immunologic reconstitution was delayed, although the numbers of T cells reached normal levels within 1 month. The number of CD7+ cells remained low up to 1 year after transplantation. The T4/T8-ratio was decreased for at least 6 months. The T-cell response to mitogens recovered to normal levels after 1 year. This study shows that ABMT with WT1-ricin A purged bone marrow in high-risk T-cell malignancies results in a complete hematopoietic and a delayed immunologic reconstitution. The actuarial relapse free survival is 61% at 3 years.     

An in vivo assay for retrovirally transduced human peripheral T lymphocytes using nonobese diabetic/severe combined immunodeficiency mice

OBJECTIVE: Availability of a mouse model to analyze human peripheral lymphocytes genetically modified with retroviral vectors would be useful in T-cell-directed gene transfer studies. To address this issue, we assessed the ability of nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice to maintain such cells in their peripheral blood. MATERIALS AND METHODS: Human peripheral lymphocytes stimulated with recombinant human interleukin-2 (rhIL-2) and anti-CD3 and CD28 antibodies were transduced with the enhanced green fluorescent protein (EGFP) gene using the retroviral vector GCsap(MSCV) and then transplanted into NOD/SCID mice at 1 x 10(8) cells per mouse. RESULTS: Transplanted human peripheral lymphocytes survived and expressed EGFP in the mice over the 6- to 8-week posttransplant period without any signs of graft-vs-host disease. Of importance was that these cells remained at the G(0)/G(1) stage and again proliferated in response to cytokines when cultured in vitro. Interestingly, the mice in which the transduced T lymphocytes remained at the resting stage clearly elucidated the superiority of the murine stem cell virus (MSCV) LTR to maintain the transgene expression by nonproliferating T lymphocytes over the Moloney murine leukemia virus (MoMLV)- and myeloproliferative sarcoma virus (MPSV)-derived LTRs, which was obscure in in vitro culture where the transduced lymphocytes was being stimulated with rhIL-2. CONCLUSIONS: The mouse model and GCsap(MSCV) vector described herein comprise a simple and reliable in vivo assay system for studies of gene and cell therapies employing human peripheral lymphocytes.     

Transfer of Type 1 (insulin-dependent) diabetes mellitus associated autoimmunity to mice with severe combined immunodeficiency (SCID)

Summary Pancreatic beta-cell destruction and development of Type 1 (insulin-dependent) diabetes mellitus are associated with circulating islet cell antibodies. Mice with severe combined immunodeficiency (SCID mice) were reconstituted with peripheral blood mononuclear cells from Type 1 diabetic patients, one who was antibody positive and one antibody negative, and from healthy individuals. Reconstituted mice were subsequently immunized with rat islets in incomplete Freunds adjuvant or adjuvant alone. Seventeen mice received peripheral blood mononuclear cells obtained at three different time points from the islet cell antibody positive patient. Before immunization with rat islets two mice developed antibodies to glutamic acid decarboxylase, a major target for antibodies in Type 1 diabetes, whereas none were positive for cytoplasmic islet cell antibodies. Following immunization with rat islets, glutamic acid decarboxylase antibodies were detected by immunoprecipitation in three additional mice, two of which also became positive for cytoplasmic islet cell antibodies. Of 22 mice which received peripheral blood mononuclear cells from either the islet cell antibody negative patient (n = 5) or from two healthy individuals (n = 17), none were positive for islet cell autoantibodies before or after immunization. None of the islet cell antibody positive mice became hyperglycaemic, showed impaired glucose tolerance or islet cell damage when studied 40 days after immunization (i.e. 100 days after reconstitution). In conclusion these results show that human B lymphocytes producing diabetes-associated autoantibodies can be transferred to SCID mice and remain antigen sensitive, but also that autoantibodies alone are not sufficient to induce beta-cell destruction.     

Evidence for early central nervous system involvement in the acquired immunodeficiency syndrome (AIDS) and other human immunodeficiency virus (HIV) infections. Studies with neuropsychologic testing and magnetic resonance imaging

Although a high prevalence of central nervous system disease is seen in persons with the acquired immunodeficiency syndrome (AIDS), the natural history of brain involvement with human immunodeficiency virus (HIV) remains poorly understood. Neuropsychologic evaluations of 55 ambulatory homosexual men revealed abnormalities in 13 of 15 with AIDS, 7 of 13 [corrected] with AIDS-related complex, 7 of 16 [corrected] with HIV-seropositivity only, and 1 of 11 with HIV-seronegativity. Common neuropsychologic problems included impaired abstracting ability, learning difficulties, and slowed speed of information processing. Magnetic resonance imaging had abnormal findings in 9 of 13 patients with AIDS and 5 of 10 patients with AIDS-related complex who were available for scans. The commonest abnormalities were sulcal and ventricular enlargement and bilateral patchy areas of high signal intensity in the white matter. We postulate that central nervous system involvement by HIV may begin early in the course of AIDS and cause mild cognitive deficits in otherwise asymptomatic persons.     

Monoclonal antibodies specific for the outer surface protein A (OspA) of Borrelia burgdorferi prevent Lyme borreliosis in severe combined immunodeficiency (scid) mice.

We have recently shown that viable Borrelia burgdorferi organisms induce a chronic infection associated with arthritis and carditis in severe combined immunodeficiency (scid) mice but not in immunocompetent mice. The disease is similar to that found in patients suffering from Lyme disease. We now show that B. burgdorferi-specific immune mouse sera as well as a monoclonal antibody to the spirochetal outer surface antigen A (31 kDa) but not monoclonal antibodies specific for the 41-kDa antigenic component of the periplasmic flagella are able to prevent (or mitigate) the development of the disease in scid mice when passively transferred at the time of the bacterial inoculation. The identification of a B. burgdorferi-associated protective antigen suggests that the corresponding spirochetal protein should be tested as a vaccine against Lyme disease.     

A unique pattern of central nervous system leukemia in acute myelomonocytic leukemia associated with inv(16)(p13q22)

Twenty-six patients with inv(16)(p13q22) or del(16)(q22) in association with acute myelomonocytic leukemia (AMML-M4, FAB classification), and abnormal marrow eosinophils have been treated at this institute. Initial bone marrow eosinophilia (greater than or equal to 4%) was observed in 22 of 26 patients (85%), and abnormal eosinophil morphology, characterized by immature cells with some interspersed basophilic granules, was evident in 26 of 26 (100%). Giemsa-banded chromosome analysis performed in all patients revealed 16 cases with inv(16)(p13q22) alone, and ten cases with additional chromosome changes. Twenty-five patients received combination induction chemotherapy, and 23 (92%) achieved complete remission (CR). The median duration of remission was 18 months (range, six to 72 + months), and the median duration of survival was 34 months (range, 0.5 to 133 months). Nine patients (35%) relapsed in the CNS at a median time of 19 months (range, six to 133 months) from first marrow CR. All patients had leptomeningeal disease, and in addition, six of nine (66%) demonstrated two or more enhancing lesions on computed tomography brain scan, consistent with intracerebral myeloblastomas. Review of 384 Giemsa-banded patients with acute myeloid leukemia revealed no other morphologic or cytogenetic subgroup with either an equivalent incidence of CNS leukemia or documented intracerebral myeloblastomas. This series of inv(16)(p13q22)/del(16)(q22) AMML reports a favorable prognosis for such patients and associates a specific clonal cytogenetic subgroup of acute leukemia with a distinct propensity for CNS relapse, manifesting as leptomeningeal disease and intracerebral myeloblastomas.     

Efficacy of an Fc-modified anti-CD123 antibody (CSL362) combined with chemotherapy in xenograft models of acute myelogenous leukemia in immunodeficient mice

The prognosis of older patients with acute myelogenous leukemia is generally poor. The interleukin-3 receptor α-chain (CD123) is highly expressed on the surface of acute leukemia cells compared with normal hematopoietic stem cells. CSL362 is a fully humanized, CD123-neutralizing monoclonal antibody containing a modified Fc structure, which enhances human natural killer cell antibody-dependent cell-mediated cytotoxicity. Six continuous acute myelogenous leukemia xenografts established from patient explants and characterized by cell and molecular criteria, produced progressively lethal disease 42-202 days after transplantation. CSL362 alone reduced engraftment of one of four and three of four acute myelogenous leukemia xenografts in the bone marrow and peripheral organs, respectively. A cytarabine and daunorubicin regimen was optimized using this model to identify potentially synergistic interactions with CSL362. Cytarabine/daunorubicin improved the survival of mice engrafted with four of four acute myelogenous leukemia xenografts by 31–41 days. Moreover, CSL362 extended the survival of cytarabine/daunorubicin-treated mice for two of two acute myelogenous leukemia xenografts, while augmentation of natural killer cell-deficient NSG mice with adoptively transferred human natural killer cells improved survival against a single xenograft. Interestingly, this enhanced CSL362 efficacy was lost in the absence of chemotherapy. This study shows that acute myelogenous leukemia xenografts provide a platform for the evaluation of new therapeutics, simulating complex in vivo interactions, and that the in vivo efficacy of CSL362 supports continued clinical development of this drug.     

The soluble interleukin-6 (IL-6) receptor/IL-6 fusion protein enhances in vitro maintenance and proliferation of human CD34(+)CD38(-/low) cells capable of repopulating severe combined immunodeficiency mice.

In vitro maintenance and proliferation of human hematopoietic stem cells is crucial for many clinical applications. Early hematopoietic cells express low levels of FLT-3 and c-kit receptors, as well as the interleukin-6 (IL-6) receptor signal transducing element, gp130, but do not express IL-6 receptor itself. Therefore, we have attempted to maintain human cord blood or bone marrow CD34(+) cells ex vivo in serum-free cultures containing stem cell factor (SCF) and FLT-3 ligand (FL) alone or together with a new recombinant molecule of soluble IL-6 receptor fused to IL-6 (IL6RIL6 chimera). The effect of IL6RIL6 chimera on the proliferation and differentiation of CD34(+) cells was compared with that of each chimera component added separately. The engraftment potential of in vitro-cultured cells was determined using our recently established functional in vivo assay for primitive human severe combined immunodeficiency (SCID)-repopulating cells (SRC). We report here that IL6RIL6 chimera induced significantly higher levels of progenitors and SRC compared with SCF + FL alone or together with IL-6 and soluble IL-6 receptor. IL6RIL6 chimera prolonged in vitro maintenance of SRC for up to 14 days. Stimulation of CD34(+)CD38(-/low) enriched cells with IL6RIL6 chimera maintained the early CD34(+)CD38(-/low) cell subpopulation, which could be detected in vitro for up to 14 days. Moreover, IL6RIL6 chimera preferentially stimulated the growth of early CD34(+)38(-/low) cells, resulting in significantly higher levels of progenitors compared with more mature CD34(+)38(+) cells. Taken together, these findings demonstrate the importance of IL6RIL6 chimera in stimulating the proliferation of early CD34(+). CD38(-)gp130(+)IL-6R(-) cells in vitro and extended maintenance of progenitors and SRC.     

Gene therapy using a simian virus 40-derived vector inhibits the development of in vivo human immunodeficiency virus type 1 infection of severe combined immunodeficiency mice implanted with human fetal thymic and liver tissue

To evaluate the in vivo efficacy of gene therapy for treating human immunodeficiency virus type 1 (HIV-1) infection, a novel simian virus (SV) 40-derived vector gene delivery system that efficiently transduces human leukocytes was combined with a model using severe combined immunodeficiency mice infected with HIV-1 and implanted with human fetal thymic and liver tissue (thy/liv-SCID-hu mice). The SV40-derived vector, SV(Aw), which encodes a variable fragment antibody recognizing HIV-1 integrase (IN#33),was injected into the human thymic grafts of thy/liv-SCID-hu mice and induced IN#33 expression in most of the thymocytes in the graft. After in vivo challenge with HIV-1, IN#33 expression inhibited in vivo HIV-1 infection, as evidenced by the markedly lower number of HIV-1-infected thymocytes detected in human thymic grafts injected with the SV(Aw) vector, compared with those injected with a control SV40-derived vector. Thus, these findings demonstrate the utility of this new mouse model system for assessing the in vivo efficacy of HIV-1-specific gene therapy. In addition, these data indicate that SV40-derived vectors may provide a system capable of efficient in vivo gene delivery.