Properties of mouse leukemia viruses. XIX. Effective antibody therapy of AKR leukemia occurs independently of virus neutralization and produces long-term changes in the virus status of the thymus

Porcine Endogenous Retroviruses and Xenotransplantation, 2021

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and some of them are able to infect human cells. Therefore, PERVs pose a risk for xenotransplantation, the transplantation of pig cells, tissues, or organ to humans in order to alleviate the shortage of human donor organs. Up to 2021, a huge body of knowledge about PERVs has been accumulated regarding their biology, including replication, recombination, origin, host range, and immunosuppressive properties. Until now, no PERV transmission has been observed in clinical trials transplanting pig islet cells into diabetic humans, in preclinical trials transplanting pig cells and organs into nonhuman primates with remarkable long survival times of the transplant, and in infection experiments with several animal species. Nevertheless, in order to prevent virus transmission to the recipient, numerous strategies have been developed, including selection of PERV-C-free animals, RNA interference, antiviral drugs, vaccination, and genome editing. Furthermore, at present there are no more experimental approaches to evaluate the full risk until we move to the clinic.

Prevention of spontaneous AKR T cell lymphomagenesis by elimination of potential lymphoma cells with antibody to specific gp 71 determinants

AKR mice, highly susceptible to spontaneous T cell lymphomagenesis, were protected from developing the disease by a course of daily treatment with antibody 18-5 to gp 71 determinants (administered from birth to 10 days). Potential lymphoma cells (PLC) identified among bone marrow cells of untreated AKR mice since birth (using the transplantation bioassay method) were eliminated following treatment with antibody 18-5. Namely, transplantation of bone marrow cells and thymocytes of 250-day-old untreated AKR mice into (AKR/J x DBA/2)F1 and/or AKR recipients yielded 86-93% T cell lymphoma of AKR origin at a short mean latent period of 38-42 days. In contrast, transfer of lymphoid cells from 18-5-treated mice at a matching age caused only 7-13% T cell lymphoma of AKR origin. The accelerating effect of MCF 247 on lymphoma development in untreated AKR mice was not effective in 18-5-treated mice, probably due to lack of PLC that are promoted by MCF 247 to overt T cell lymphoma. The characteristic changes in thymus subpopulations preceding lymphoma development and coinciding with PLC identification in the thymus of untreated mice was prevented by 18-5. It is suggested that prevention of lymphoma development in AKR mice by passive antiviral immunotherapy involves elimination of PLC representing the initial tumorigenic phase in AKR lymphomagenesis.

The role of bone marrow and thymic elements in the initiation and spread of virus production in the AKR thymus

Passive anti-viral immunotherapy greatly suppresses the incidence of spontaneous leukemia in AKR mice, rendering the thymus of successfully treated animals devoid of infectious ecotropic retrovirus. Reconstitution assays have determined that the thymic and splenic homing cells of the AKR bone marrow become ecotropic virus producers subsequent to their seeding of these hematopoietic organs and that in vitro depletion of gp71 expressing bone marrow cells reduces stem cell numbers without affecting prothymocyte content. In the thymus, a population of radioresistant cells, which phenotypically resemble cortical thymocytes, but are unique in their expression of high levels of H-2Kk antigen, have been found to produce high levels of both ecotropic and MCF virus and have been implicated as a putative therapeutic target cell population of anti-viral treatment. In addition, the failure of treated animals to reconstitute following lethal irradiation suggests that an immunotherapy-induced alteration occurs in the bone marrow of AKR mice.

Major histocompatibility complex class II-regulated immunity to murine leukemia virus protects against early T- but not late B-cell lymphomas

We studied the relative importance of class I and class II major histocompatibility complex (MHC) immunoregulation in the control of T- and B-cell lymphomas induced by murine leukemia virus. Previously, we have described a mink cell focus-inducing (MCF) murine leukemia virus, MCF 1233, which induces not only lymphoblastic T-cell lymphomas but also follicle center cell or lymphoblastic B-cell lymphomas. We now report that the outcome of neonatal infection with MCF 1233 in H-2-congenic C57BL/10 and C57BL/6 mice is decisively influenced by the H-2 I-A locus. A total of 64% of H-2 I-Ak, d mice [B10.BR, B10.D2, B10.A(2R), B10.A(4R), and B10.MBR] developed T-cell lymphomas after MCF 1233 infection (mean latency, 37 weeks). In contrast, H-2 I-Ab [B10, B10.A(5R), B6], H-2 I-Ab/k [(B10.A x B10)F1 and (B10 x B10.A)F1], and H-2 I-Abm12 (bm12) mice were resistant against T-cell lymphomagenesis, but 65% of these H-2 I-Ab, b/k, bm12 animals developed B-cell lymphomas (mean latency, 71 weeks). Animals of T-cell lymphoma-susceptible strains that escaped from T-cell lymphomagenesis developed B-cell lymphomas with similar frequency as animals of T-cell lymphoma-resistant strains, but with a shorter latency. H-2 class II-determined regulation of antiviral immunity was reflected in the presence of high titers of antiviral envelope antibodies in T-cell lymphoma-resistant B-cell lymphoma-susceptible H-2 I-Ab, b/k, bm12 mice, whereas in T-cell lymphoma-susceptible H-2 I-Ak,d mice no antiviral antibodies were found. At week 4 after neonatal MCF 1233 infection, a high percentage of thymocytes were virally infected in both T-cell lymphoma-susceptible and -resistant mice. However, T-cell lymphoma-resistant animals cleared the thymic infection between weeks 4 and 10 of age, coinciding with a sharp rise in serum levels of antiviral antibodies. We conclude that the pleiotropic effects of MCF 1233 infection in H-2-congenic mice result from MHC class II I-A-determined T-cell response differences.

The coenrichment of stem cells, prothymocytes, and stromal elements with ecotropic retrovirus-producing cells from the bone marrow of leukemia-prone AKR mice

Ecotropic virus-producing cells in the bone marrow of the leukemia-prone AKR strain of mice were significantly enriched by fractionation on discontinuous density gradients of Percoll and were found in a low-density population of cells comprised predominantly of medium to large blast cells. The high ecotropic virus-producing low-density bone marrow cell population was also found to be significantly enriched in pluripotent stem cells, prothymocytes, and stromal elements. During the period of time defined by a window for successful leukemosuppressive immunotherapy of AKR mice, virus-producing cells were exclusively detected in this fraction of bone marrow cells, implicating the functional classes of cells coenriched in this fraction as both potential targets of anti-viral immunotherapy and responsible for the seeding of the spleen and thymus with infectious ecotropic virus.

Role of antibodies to murine leukemia virus p15E transmembrane protein in immunotherapy against AKR leukemia: a model for studies in human acquired immunodeficiency syndrome

Previous studies have demonstrated that the onset of AKR leukemia could be dramatically delayed and the overall incidence significantly reduced following treatment with high-titered heterologous antibodies directed against the gp71 major glycoprotein of the virus. However, to be maximally successful, the treatment had to be initiated during the postnatal period of the AKR mouse, encompassing a narrow window representing approximately the first 3 days of life. In the present study we sought to extend this barrier by including antibodies directed against a second envelope component of the virion, the transmembrane protein, p15E. We demonstrate that although neither antibodies to gp71 nor antibodies to p15E could influence the course of leukemia development when applied individually later in life, a combination of the two antibodies was effective even if given as late as 5 months after birth. The significance of these studies is discussed in relation to human retrovirus-associated diseases.

The effects of leukemosuppressive immunotherapy on bone marrow infectious cell centers in AKR mice

The bone marrow of AKR mice is the richest source of infectious ecotropic cell centers (ICCs) during the neonatal period. The bone marrow ICCs reside in a low-density population expressing high levels of viral glycoprotein (gp71) and Class I histocompatibility antigens (H-2Kk). In addition, ICCs are enriched in the lymphoid band of Ficoll-Hypaque-fractionated bone marrow, the adherent population of nylon wool separated cells and among the low-density subpopulation of Percoll-fractionated marrow. The observed dichotomy between viral antigen expression and actual virus production suggests that actively cycling cells may be the primary virus producers in the AKR bone marrow. The phenotypic and physical data indicate that bone marrow stem cells and/or prothymocytes may be among the initial virus producing cells in the AKR bone marrow. Leukemosuppressive antiviral immunotherapy delays the appearance of ICCs in the bone marrow but does not exert any major long-term changes on the populations of cells present.

Anti-GP 120 antibodies from HIV seropositive individuals mediate broadly reactive anti-HIV ADCC

Cytophilic antibodies which mediate antibody dependent cellular cytotoxicity (ADCC) against envelope antigens of human immunodeficiency virus (HIV) can be found in seropositive individuals. In these experiments, sera from a wide spectrum of HIV infected patients ranging from asymptomatic to overt acquired immunodeficiency syndrome (AIDS) were shown to contain high titers of antibodies that mediate ADCC. Not only did patient antibodies bind to surface expressed viral antigens and mediate ADCC against cells chronically infected with human T-lymphotropic virus type IIIB (HTLV-IIIB), but also against cells infected with the divergent HTLV-IIIRF2 and HTLV-IIIMN viral isolates. Similar results were obtained with target cells bearing purified GP 120 from HTLV-IIIB and HTLV-IIIRF2, indicating that a major portion of the activity was mediated by anti-GP 120 antibodies. Consistent with this was the ability to absorb most of the group-specific ADCC activity from the serum of an HIV infected individual using affinity columns bearing purified HTLV-IIIB GP 120. The finding that human antibodies reactive against the HIV envelope glycoprotein mediate ADCC against cells chronically infected with divergent strains of HIV will have important implications in designing rational approaches to passive and active immunotherapy.

Properties of mouse leukemia viruses: XX. Variation of AKR Substrains in response to antibody therapy

In previous reports in this series, we have demonstrated that treatment of young AKR mice with IgG prepared against the viral envelope glycoprotein suppresses the development of spontaneous leukemia. Moreover, animals exhibiting high anti-viral antibody titers can transfer protection to their offspring. In this report we have extended the studies to another strain of AKR mice and find that the ability to transfer protection to offspring was not obtained. Foster nursing experiments were therefore conducted and their outcomes are indicative that maternal factors may be responsible for this phenomenon.

Generation of thymotropic envelope gene recombinant virus and induction of lymphoma by ecotropic Moloney murine leukemia virus

Biologically cloned pure ecotropic Moloney MuLV was used to infect Balb/c and AKR mice to determine the replication of ecotropic virus, the possible generation of recombinant viruses, and the induction of disease. Infectious cell center (ICC) experiments carried out with lymphoid cells of individual Balb/c mice showed that e-M-MuLV rapidly infected up to 30% of lymphoid cells in liver, spleen, and especially in the thymus. No recombinant virus was seen until about Day 35 when a burst of RM-MuLV was observed only in the thymus. New RM-MuLV was found in all 32 preleukemic and leukemic mice tested and persisted at low levels until death. The RM-MuLV recovered early in the preleukemic phase had an env-related M-MuLV but grew very poorly. Cells from a late tumor which grew and cloned readily were examined to see whether the new RM-MuLV was present in every clone. Overtly, most tumor cells did not seem to contain RM-MuLV, but when "unmasking" was performed, every tumor cell contained identical RM-MuLV. In AKR mice, both e-M-MuLV and recombinant M-MuLV caused an acceleration of lymphoma. De novo appearance of a thymotropic RM-MuLV, which was of the Moloney RM-MuLV type and the absence of early detectable endogenous AKR-MCF-type recombinants, suggested that the early lymphoma was due to the induction of a new disease. Several theoretical approaches dealing with viral env-gene permutations are discussed.

Pathogenesis and virus content of lymphomas induced by pure ecotropic Graffi murine leukemia virus

Tumors induced by wild type Graffi murine leukemia virus (Gi-MuLV) contained high titers of MuLV consisting of a predominant ecotropic (e)-MuLV and a scarcer titer recombinant (RM) MuLV component. Each of these was purified by biological cloning and examined for its envelope properties and leukemogenicity. Both the e- and the RM-MuLV's were single isolates and unique in terms of their neutralization profiles and peptide maps. The cloned e-Gi-MuLV was highly leukemogenic in C57Bl mice, inducing a very rapid lethal thymic lymphoma but no myeloid leukemia. e-Gi-MuLV also accelerated thymic lymphoma in AKR mice. The purified RM-MuLV did not induce any tumors. Infectious cell center (ICC) experiments of organs of mice inoculated with e-Gi-MuLV showed that virus replicated very rapidly and reached maximal titers in about one week in C57Bl mice. There was a highly preferential replication in the thymus of the animal so that this e-Gi-MuLV can be considered as thymotropic. Within two weeks after infection of mice, infected cells of the thymus also began to release low levels of a non-ecotropic MuLV. The rapid induction of lymphoma is compared to that induced by other e-MuLV's and their RM-MuLV's, and to the natural AKR-MuLV-associated disease. These findings are discussed in the context of prevailing theories on envelope gene rearrangements in the virus and in the proviral sequences in resulting tumors.