The majority of cells infected with the defective murine AIDS virus belong to the B-cell lineage

Murine AIDS is initiated in the lymph nodes draining the site of inoculation, and the infected B cells influence T cells located at distance, in noninfected organs

The infection of cells which belong to the B-cell lineage is thought to be the primary event leading to the phenotypic and functional alterations seen in the murine AIDS (M. Huang, C. Simard, D. Kay, and P. Jolicoeur, J. Virol. 65:6562-6571, 1991). Using in situ hybridization, we studied the time course of the anatomic distribution of the murine AIDS-infected B cells in C57BL/6 mice inoculated intraperitoneally or in the foot pad with helper-free stocks of the defective murine AIDS virus. The local lymph nodes draining the injection site (the mediastinal or popliteal lymph nodes) were the primary organs in which infected B cells could be detected. From this initial site, the proliferating infected B cells were found to migrate progressively to most of the other lymph nodes and to the spleen. The bone marrow cells (containing the precursor B cells) were not found to be infected by the virus. These results suggest that the defective murine AIDS virus infects mature Ly-1- B cells present in lymph nodes. We compared the concanavalin A response of the T cells at an early time postinoculation, before all lymphoid organs are infiltrated with infected B cells. In lymphoid organs free of infected B cells, T cells were found to be hyperresponsive. In lymphoid organs in which infected B cells were present, T cells were hyporesponsive. These data suggest that infected B cells influence distant T cells, maybe by the release of a circulating factor or through another uninfected cell population activated by the infected B cells.

Population dynamics of CD4+ T cells lacking Thy-1 in murine retrovirus-induced immunodeficiency syndrome (MAIDS)

Increased numbers of CD4+ Thy-1- cells have been described in the spleen (SP) of mice with retrovirus-induced immunodeficiency (MAIDS). Since this phenotypic abnormality might have considerable functional importance, the expansion of the CD4+ Thy-1- subset in MAIDS was characterized further. CD4+ Thy-1- and Thy-1+ T-cells from infected mice expressed similar densities of CD3 and TCR alpha/beta. In contrast, the Thy-1- subset was uniformly CD44hi, even early in the disease when part of Thy-1+ cells were still CD44lo. The emergence of CD4+ Thy-1- cells occurred first in SP and lymph nodes and was observed later in thymus. The important fraction of CD4+ cells lacking Thy-1 normally present in Peyer's patches was only weakly modified. Despite the major expansion of the CD4+ Thy-1- phenotype, the proliferating fraction was not higher in this subset than in CD4+ Thy-1+ cells from infected mice. Persistence after hydroxyurea administration was identical in both subsets, indicating similar mean cell lifespans. Taken together, these results show that the major expansion of CD4+ Thy-1- T-cells in MAIDS is not ascribable solely to increased proliferation within this subset. Phenotypic analysis suggests that CD4+ Thy-1- cells result from the differentiation of Thy-1+ cells induced by activation signals related to retroviral infection.

Identification of the infected target cell type in spongiform myeloencephalopathy induced by the neurotropic Cas-Br-E murine leukemia virus

The Cas-Br-E murine leukemia virus (MuLV) induces a progressive hindlimb paralysis accompanied by a spongiform myeloencephalopathy in susceptible mice. In order to better understand the pathological process leading to these neurodegenerative lesions, we have investigated the nature of the cell type(s) infected by the virus during the course of the disease in CFW/D and SWR/J mice. For this purpose, we used in situ hybridization with virus-specific probes in combination with cell-type-specific histochemical (lectin) and immunological markers as well as morphological assessment. In the early stage of infection, endothelial cells represented the main cell type expressing viral RNA in the central nervous system (CNS). With disease progression and the appearance of lesions, microglial cells became the major cell type infected, accounting for up to 65% of the total infected cell population in diseased areas. Morphologically, these cells appeared activated and were frequently found in clusters. Infection and activation of microglial cells were almost exclusively restricted to diseased regions of the CNS. Neurons in diseased regions were not discernibly infected with virus at either early or late times of disease progression. Similarly, the proportion of infected astrocytes was typically < 1%. Although some endothelial cells and oligodendrocytes were infected by the virus, their infection was not limited to diseased CNS regions. These results are consistent with a model of indirect motor neuron degeneration, subsequent to the infection of nonneuronal CNS cells and especially of microglial cells. Infected microglial cells may play a role in the disease process by releasing not only virions or viral env-gene-encoded gp70 proteins but also other factors which may be directly or indirectly toxic to neurons. Parallels between microglial cell infection by MuLV and by lentiviruses, and specifically by human immunodeficiency virus, are discussed.

The MA (p15) and p12 regions of the gag gene are sufficient for the pathogenicity of the murine AIDS virus

Inoculation of the replication-defective retrovirus DEF27 (BM5d), packaged as an amphotropic virus pseudotype, into C57BL/6J mice leads to development of murine AIDS. Disease development showed a long incubation period (20 to 24 weeks), was associated with amplification of the BM5d provirus in splenocytes and lymph nodes, and was independent of the presence of exogenous or endogenous replication-competent helper viruses. However, both the onset of disease and amplification of the defective provirus were significantly enhanced by coinfection with the replication-competent B-cell-tropic ecotropic helper virus BM5e. The part of the BM5d viral genome that was essential for the pathogenicity was determined by making precisely engineered alterations in the reading frame of the gag and pol genes of BM5d proviral DNA and examining the ability of the altered amphotropic BM5d pseudotypes to induce the disease in C57BL/6J mice. The results show that expression of the MA (p15) and p12 regions of the gag gene is sufficient for pathogenicity of the BM5d retrovirus.

High expression of NK-1.1 antigen is induced by infection with murine AIDS virus

Spleen cells from mice infected with LP-BM5 MuLV, a causative agent of murine acquired immunodeficiency syndrome (MAIDS), were tested for frequency of NK-1.1+ cells and natural killer (NK) activity. During the first 3 weeks following infection, NK activity was well conserved, but by 9-12 weeks post-infection (p.i.), killer activity was depressed; however, the frequency of NK-1.1+ cells increased within 4 weeks of infection and remained elevated thereafter, even following the decline in functional killing activity. Since the absolute number of NK-1.1+ cells increased after infection, the ability of each NK-1.1+ cell to kill the targets seems drastically impaired. Extraordinary expansion of NK-1.1-positive cells was induced by infection with LP-BM5-defective virus (BM5def), a crucial element for MAIDS induction, but not with a helper non-pathogenic virus. With advance of MAIDS the NK-1.1 antigen (Ag) was preferentially expressed on B220+ and Thy-1+ cells, in contrast to CD4+ and CD8+ cells, and among activated large cells a higher proportion was NK-1.1+ than NK-1.1-. Mice with graft-versus-host disease (GVHD) due to class II major histocompatibility complex (MHC) Ag disparity showed a high frequency of NK-1.1 expression in association with other phenotypic alterations, very similar to those seen in mice with MAIDS. In contrast, B6-lpr/lpr mice developed similar activation of B cells but did not exhibit enhanced expression of the NK-1.1 marker. Thus, enhanced expression of the NK-1.1 Ag might be associated with chronic activation of lymphocytes through a common but not universal pathway.

Delayed progression of a murine retrovirus-induced acquired immunodeficiency syndrome in X-linked immunodeficient mice

The murine acquired immunodeficiency syndrome (MAIDS) caused by defective LP-BM5 murine leukemia virus (MuLV) is a disease that shows severe immunodeficiency with abnormal lymphoproliferation, and hypergammaglobulinemia in susceptible C57BL/6 (B6) mice. To examine the cellular mechanisms of development of MAIDS, we injected LP-BM5 MuLV intraperitoneally into B6 mice bearing the X chromosome-linked immunodeficiency (xid). xid mice lack functionally mature B cells including Ly-1 B cells (also known as B-1 cells). All B6 mice died by 20 wk after LP-BM5 MuLV inoculation. In marked contrast, xid mice have continued to survive without any sign of MAIDS-related symptoms till at least 20 wk after the inoculation. The delayed progression of MAIDS in xid mice appears to depend on xid mutation, according to our experiments using both sexes of (B6.xid x B6)F1 and (B6 x B6.xid)F1 mice. Furthermore, Ly-1 B cells, enriched by a FACS, were shown to integrate the defective genome and appeared to be a major virus-infected B cell population. Our data corroborate that Ly-1 B cells play an important role in the induction and progression of MAIDS.

Dissociation between lymphoproliferative responses and virus replication in mice with different sensitivities to retrovirus-induced immunodeficiency

Murine AIDS (MAIDS) is induced by a replication-defective virus (BM5d). In susceptible mice (C57BL/6J), inoculation with LP-BM5 murine leukemia virus, which consists of the BM5d virus and replication-competent B-tropic ecotropic (BM5e) and milk cell focus-inducing (BM5-MCF) helper viruses results in the polyclonal proliferation of T and B cells, immunodeficiency, and the expansion of B cells containing the BM5d provirus followed by the development of B-cell lymphomas. Several strains of mice that are resistant to LP-BM5-induced murine AIDS have been identified, and major histocompatibility complex genes as well as non-major histocompatibility complex genes were shown to play a role in this resistance. In the present study, we have examined and compared the replication of the BM5d and BM5e viruses after inoculation of LP-BM5 into sensitive (C57BL/6J) and resistant (C57BL/KSJ) mice. Using a specific polymerase chain reaction, we could detect the BM5d and BM5e proviruses as early as 1 week postinfection in the sensitive mice, and the levels of both viruses increased significantly with the progression of the disease. In contrast, in the resistant C57BL/KSJ mice, replication of BM5d and BM5e was restricted and no BM5d and only very low levels of the BM5e provirus could be detected either at early or late times postinoculation with the LP-BM5 virus mixture. Inoculation with LP-BM5 did not lead to the production of antibodies that could recognize the BM5d-encoded Pr60gag in either the sensitive or resistant mice; however, production of antibodies recognizing the env-related proteins of the helper virus was detected in the resistant but not in the sensitive mice at late times postinfection. Interestingly, inoculation with LP-BM5 increased polyclonal stimulation of spleen cells and decreased mitogen stimulation in both strains of mice. This stimulation of splenocytes persisted in the sensitive mice but decreased after a few weeks in the resistant mice. These results show an early block in BM5d and BM5e replication in the resistant C57BL/KSJ mice and indicate that resistance is a consequence of the inhibition of an onset of the BM5d virus infection and its expansion. However, initial responses to virus infection such as proliferation of spleen cells and response to mitogen are similar in both strains of mice and are therefore not necessarily related to the development of the disease.

Presence of transplantable T-lymphoid cells in C57BL/6 mice infected with murine AIDS virus

The Duplan strain of murine leukemia virus induces murine AIDS in C57BL/6 mice. When spleen cells from C57BL/6 mice infected with the virus were transplanted into nude mice, subcutaneous solid tumors at the transplanted sites were formed and splenomegaly and lymphadenopathy were induced. These transplantable cells were Thy-1- CD4+ alpha-beta T-cell receptor-positive T cells and integrated with the pathogenic defective viral genome. These results indicate that neoplastic cells of T-cell lineage were induced by infecting C57BL/6 mice with murine AIDS virus.

Susceptibility of inbred strains of mice to murine AIDS (MAIDS) correlates with target cell expansion and high expression of defective MAIDS virus

Murine AIDS (MAIDS) is readily induced by the Duplan strain of defective murine leukemia virus in susceptible C57BL/6 mice. To identify mouse strains resistant to MAIDS, and to understand the genetic factors controlling susceptibility to the disease, we screened more than 20 inbred strains of mice for their susceptibility to MAIDS. For this study, mice of the Fv-1n/n, Fv-1b/b, or Fv-1n/b genotype were inoculated with stocks of defective MAIDS virus pseudotyped with N-tropic, B-tropic, or NB-tropic helper murine leukemia virus, respectively. Strains could be classified as susceptible, resistant, or moderately resistant. None of the individual H-2 haplotypes examined appears to explain resistance to MAIDS by itself. However, a very good correlation between the susceptibility or resistance phenotype and the presence or absence of defective proviral DNA and RNA in the spleen of these animals was found. Since the presence of defective proviral DNA and RNA reflects the oligoclonal proliferation of the cells infected by the defective MAIDS virus, our results strongly suggest that this target cell expansion is genetically controlled and is necessary and perhaps even sufficient for the development of the disease.