Horizontal transmission of murine retroviruses

Milk Transmission of Mammalian Retroviruses

The transmission of viruses from one host to another typically occurs through horizontal or vertical pathways. The horizontal pathways include transmission amongst individuals, usually through bodily fluids or excretions, while vertical transmission transpires from mother to their offspring, either during pregnancy, childbirth, or breastfeeding. While there are more than 200 human pathogenic viruses to date, only a small number of them are known to be transmitted via breast milk, including cytomegalovirus (CMV), human immunodeficiency virus type 1 (HIV-1), and human T cell lymphotropic virus type 1 (HTLV-1), the latter two belonging to the family Retroviridae. Breast milk transmission is a common characteristic among mammalian retroviruses, but there is a lack of reports summarizing our knowledge regarding this route of transmission of mammalian retroviruses. Here, we provide an overview of the transmission of mammalian exogenous retroviruses with a focus on Orthoretrovirinae, and we highlight whether they have been described or suspected to be transmitted through breast milk, covering various species. We also elaborate on the production and composition of breast milk and discuss potential entry sites of exogenous mammalian retroviruses during oral transmission.

The Viral Origin of Human Breast Cancer: From the Mouse Mammary Tumor Virus (MMTV) to the Human Betaretrovirus (HBRV)

A Human Betaretrovirus (HBRV) has been identified in humans, dating as far back as about 4500 years ago, with a high probability of it being acquired by our species around 10,000 years ago, following a species jump from mice to humans. HBRV is the human homolog of the MMTV (mouse mammary tumor virus), which is the etiological agent of murine mammary tumors. The hypothesis of a HMTV (human mammary tumor virus) was proposed about 50 years ago, and has acquired a solid scientific basis during the last 30 years, with the demonstration of a robust link with breast cancer and with PBC, primary biliary cholangitis. This article summarizes most of what is known about MMTV/HMTV/HBRV since the discovery of MMTV at the beginning of last century, to make evident both the quantity and the quality of the research supporting the existence of HBRV and its pathogenic role. Here, it is sufficient to mention that scientific evidence includes that viral sequences have been identified in breast-cancer samples in a worldwide distribution, that the complete proviral genome has been cloned from breast cancer and patients with PBC, and that saliva contains HBRV, as a possible route of inter-human infection. Controversies that have arisen concerning results obtained from human tissues, many of them outdated by new scientific evidence, are critically discussed and confuted.

In vivo imaging of retrovirus infection reveals a role for Siglec-1/CD169 in multiple routes of transmission

Early events in retrovirus transmission are determined by interactions between incoming viruses and frontline cells near entry sites. Despite their importance for retroviral pathogenesis, very little is known about these events. We developed a bioluminescence imaging (BLI)-guided multiscale imaging approach to study these events in vivo. Engineered murine leukemia reporter viruses allowed us to monitor individual stages of retrovirus life cycle including virus particle flow, virus entry into cells, infection and spread for retroorbital, subcutaneous, and oral routes. BLI permitted temporal tracking of orally administered retroviruses along the gastrointestinal tract as they traversed the lumen through Peyer’s patches to reach the draining mesenteric sac. Importantly, capture and acquisition of lymph-, blood-, and milk-borne retroviruses spanning three routes was promoted by a common host factor, the I-type lectin CD169, expressed on sentinel macrophages. These results highlight how retroviruses co-opt the immune surveillance function of tissue-resident sentinel macrophages for establishing infection.

Murine Leukemia Virus Exploits Innate Sensing by Toll-Like Receptor 7 in B-1 Cells To Establish Infection and Locally Spread in Mice

Lymph-borne Friend murine leukemia virus (FrMLV) exploits the sentinel macrophages in the draining popliteal lymph node (pLN) to infect highly permissive innate-like B-1 cells and establish infection in mice. The reason for FrMLV sensitivity of B-1 cells and their impact on viral spread is unknown. Here we demonstrate that Toll-like receptor 7 (TLR7) sensing and type I interferon (IFN-I) signaling in B-1 cells contribute to FrMLV susceptibility. FrMLV infection in B-1 cell-deficient mice (bumble; IκBNS dysfunctional) was significantly lower than that in the wild-type mice and was rescued by adoptive transfer of wild-type B-1 cells. This rescue of FrMLV infection in bumble mice was dependent on intact TLR7 sensing and IFN-I signaling within B-1 cells. Analyses of infected cell types revealed that the reduced infection in bumble mice was due predominantly to compromised virus spread to the B-2 cell population. Our data reveal how FrMLV exploits innate immune sensing and activation in the B-1 cell population for infection and subsequent spread to other lymphocytes.IMPORTANCE Viruses establish infection in hosts by targeting highly permissive cell types. The retrovirus Friend murine leukemia virus (FrMLV) infects a subtype of B cells called B-1 cells that permit robust virus replication. The reason for their susceptibility had remained unknown. We found that innate sensing of incoming virus and the ensuing type I interferon response within B-1 cells are responsible for their observed susceptibility. Our data provide insights into how retroviruses coevolved with the host to co-opt innate immune sensing pathways designed to fight virus infections for establishing infection. Understanding early events in viral spread can inform antiviral intervention strategies that prevent the colonization of a host.

Origins of the endogenous and infectious laboratory mouse gammaretroviruses

The mouse gammaretroviruses associated with leukemogenesis are found in the classical inbred mouse strains and in house mouse subspecies as infectious exogenous viruses (XRVs) and as endogenous retroviruses (ERVs) inserted into their host genomes. There are three major mouse leukemia virus (MuLV) subgroups in laboratory mice: ecotropic, xenotropic, and polytropic. These MuLV subgroups differ in host range, pathogenicity, receptor usage and subspecies of origin. The MuLV ERVs are recent acquisitions in the mouse genome as demonstrated by the presence of many full-length nondefective MuLV ERVs that produce XRVs, the segregation of these MuLV subgroups into different house mouse subspecies, and by the positional polymorphism of these loci among inbred strains and individual wild mice. While some ecotropic and xenotropic ERVs can produce XRVs directly, others, especially the pathogenic polytropic ERVs, do so only after recombinations that can involve all three ERV subgroups. Here, I describe individual MuLV ERVs found in the laboratory mice, their origins and geographic distribution in wild mouse subspecies, their varying ability to produce infectious virus and the biological consequences of this expression.

Transmission of chimeric HIV by mating in conventional mice: prevention by pre-exposure antiretroviral therapy and reduced susceptibility during estrus

Heterosexual transmission accounts for the majority of new human immunodeficiency virus (HIV) cases worldwide. The current approach to investigate HIV heterosexual transmission in animals involves application of virus stock to the vaginal surface, a method that does not reproduce the physiological conditions of vaginal intercourse that influence the rate of transmission. We have previously described efficient infection of conventional mice using EcoHIV/NL4-3 and EcoHIV/NDK, chimeric HIV molecular clones constructed to express all HIV structural and regulatory genes except envelope, which is replaced by a rodent-tropic envelope gene. Here we investigated whether EcoHIV/NDK-infected male mice transmit virus to females during coitus, and the sensitivity of this transmission to HIV pre-exposure prophylaxis and the estrus state. Our general approach was to allow mating between EcoHIV/NDK-infected male mice and uninfected females for 1–7 nights. At 1–6 weeks after mating, mice were euthanized and virus burdens were measured by quantitative PCR (qPCR) amplification of HIV RNA or DNA in peritoneal macrophages, inguinal lymph node cells, spleen cells or vas deferens, or by ELISA for antibodies to HIV Gag. We found that 70–100% of female mice mated to EcoHIV/NDK-infected males acquired infection. Pericoital treatment of females with either 2′,3′-dideoxcytidine (ddC) or tenofovir largely prevented their EcoHIV/NDK infection by mating (P<0.05 and P<0.003, respectively). In males, T cells were dispensable for virus transmission. The rate of EcoHIV/NDK sexual transmission to females in estrus declined sharply (P=0.003) but their infection by injection was unaffected, indicating that the local environment in the female reproductive tract influences susceptibility to HIV. We conclude that this system of EcoHIV/NDK transmission during mouse mating reproduces key features of heterosexual transmission of HIV in humans and can be used to investigate its biology and control.

The avian XPR1 gammaretrovirus receptor is under positive selection and is disabled in bird species in contact with virus-infected wild mice

Xenotropic mouse leukemia viruses (X-MLVs) are broadly infectious for mammals except most of the classical strains of laboratory mice. These gammaretroviruses rely on the XPR1 receptor for entry, and the unique resistance of laboratory mice is due to two mutations in different putative XPR1 extracellular loops. Cells from avian species differ in susceptibility to X-MLVs, and 2 replacement mutations in the virus-resistant chicken XPR1 (K496Q and Q579E) distinguish it from the more permissive duck and quail receptors. These substitutions align with the two mutations that disable the laboratory mouse XPR1. Mutagenesis of the chicken and duck genes confirms that residues at both sites are critical for virus entry. Among 32 avian species, the 2 disabling XPR1 mutations are found together only in the chicken, an omnivorous, ground-dwelling fowl that was domesticated in India and/or Southeast Asia, which is also where X-MLV-infected house mice evolved. The receptor-disabling mutations are also present separately in 5 additional fowl and raptor species, all of which are native to areas of Asia populated by the virus-infected subspecies Mus musculus castaneus. Phylogenetic analysis showed that the avian XPR1 gene is under positive selection at sites implicated in receptor function, suggesting a defensive role for XPR1 in the avian lineage. Contact between bird species and virus-infected mice may thus have favored selection of mouse virus-resistant receptor orthologs in the birds, and our data suggest that similar receptor-disabling mutations were fixed in mammalian and avian species exposed to similar virus challenges.

Endogenous gammaretrovirus acquisition in Mus musculus subspecies carrying functional variants of the XPR1 virus receptor

The xenotropic and polytropic mouse leukemia viruses (X-MLVs and P-MLVs, respectively) have different host ranges but use the same functionally polymorphic receptor, XPR1, for entry. Endogenous retroviruses (ERVs) of these 2 gammaretrovirus subtypes are largely segregated in different house mouse subspecies, but both MLV types are found in the classical strains of laboratory mice, which are genetic mosaics of 3 wild mouse subspecies. To describe the subspecies origins of laboratory mouse XP-MLV ERVs and their coevolutionary trajectory with their XPR1 receptor, we screened the house mouse subspecies for known and novel Xpr1 variants and for the individual full-length XP-MLV ERVs found in the sequenced C57BL mouse genome. The 12 X-MLV ERVs predate the origins of laboratory mice; they were all traced to Japanese wild mice and are embedded in the 5% of the laboratory mouse genome derived from the Asian Mus musculus musculus and, in one case, in the <1% derived from M. m. castaneus. While all 31 P-MLV ERVs map to the 95% of the laboratory mouse genome derived from P-MLV-infected M. m. domesticus, no C57BL P-MLV ERVs were found in wild M. m. domesticus. All M. m. domesticus mice carry the fully permissive XPR1 receptor allele, but all of the various restrictive XPR1 receptors, including the X-MLV-restricting laboratory mouse Xpr1(n) and a novel M. m. castaneus allele, originated in X-MLV-infected Asian mice. Thus, P-MLV ERVs show more insertional polymorphism than X-MLVs, and these differences in ERV acquisition and fixation are linked to subspecies-specific and functionally distinct XPR1 receptor variants.

In vivo imaging of virological synapses

Retroviruses such as the human immunodeficiency virus, human T-cell lymphotropic virus and murine leukaemia virus are believed to spread via sites of cell-cell contact designated virological synapses. Support for this model is based on in vitro evidence in which infected cells are observed to specifically establish long-lived cell-cell contact with uninfected cells. Whether virological synapses exist in vivo is unknown. Here we apply intravital microscopy to identify a subpopulation of B cells infected with the Friend murine leukaemia virus that form virological synapses with uninfected leucocytes in the lymph node of living mice. In vivo virological synapses are, like their in vitro counterpart, dependent on the expression of the viral envelope glycoprotein and are characterized by a prolonged polarization of viral capsid to the cell-cell interface. Our results validate the concept of virological synapses and introduce intravital imaging as a tool to visualize retroviral spreading directly in living mice.

Complete genome sequences of new xenotropic murine leukemia viruses from the senescence-accelerated mouse (SAM): molecular and phylogenetic analyses

Approximately 10% of the mouse genome is constituted by endogenous retroviruses (ERVs), and a number of mouse ERVs remain active. Many copies of endogenous murine leukemia viruses (MuLVs) are detected in the genomes of inbred mouse strains. Some of these MuLVs are transcriptionally active or produce infectious virus particles. Previously, we identified partial env sequences of new xenotropic MuLVs (X-MuLVs) from a senescence-accelerated mouse (SAM) strain. In the present study, we investigated and characterized the complete sequences of the X-MuLVs. The complete genomes and open reading frames (ORFs) of two X-MuLVs, designated xmlv15 and xmlv18 (accession nos. HQ154630 and HQ154631, respectively), were molecularly cloned from the genome of the SAM mice. We confirmed that the xmlv15 and xmlv18 sequences are distinct from all known MuLV genomes and are most similar to DG-75 MuLV. Moreover, we found that common strains of laboratory mice carry our newly identified xmlvs. Additionally, the expression levels of xmlv15-related sequences were much higher in C57BL and ICR mice than in the SAM strains without any stimulators. Our findings suggest that a specific group of endogenous MuLVs is constitutively expressed in the brain and that they may participate in normal functions and/or pathogenic conditions.

No biological evidence of XMRV infection in cervical smears from HIV/ HPV positive and negative Kenyan women

Background

XMRV (xenotropic murine leukaemia virus-related virus) is a gammaretrovirus first discovered in human prostate carcinomas and later linked to chronic fatigue syndrome (CFS). Emerging conflicting data and lack of reproducibility of results within the scientific community has now led to the association of XMRV with CFS being discounted. Indeed the case for an involvement with any human disease has been questioned with the suggestion that XMRV is a laboratory generated recombinant virus. The fact that not all published positive findings can be easily explained as contamination artefacts coupled with the observation that XMRV may have a sexually transmitted mode of infectivity and can be infectious for primates, where it preferential resides in cells of the reproductive tract, prompted us to look for evidence of XMRV in the cervical cells of a cohort of Kenyan women both with and without pre-existing HIV/HPV infections.

Results

Using a highly sensitive and selective triplex PCR approach we analysed DNA from the liquid based cytology (LBC) cervical smears of 224 Kenyan women. There was no evidence of XMRV expression in any of the sample population irrespective of HPV and/or HIV status.

Conclusions

The data presented show no indication of XMRV infection in any of the cervical samples screened in this study. Approximately 50% of the women were HIV positive but this did not influence the findings signifying that XMRV does not act as an opportunistic infection in this cohort nor is it related to HPV status. Our results therefore support the findings that XMRV is confined to the laboratory and does not currently represent an infectious agent for humans, with a cautionary adage that such potential zoonotic viruses should be carefully monitored in the future.

Long-term infection and vertical transmission of a gammaretrovirus in a foreign host species

Increasing evidence has indicated natural transspecies transmission of gammaretroviruses; however, viral-host interactions after initial xeno-exposure remain poorly understood. Potential association of xenotropic murine leukemia virus-related virus (XMRV) in patients with prostate cancer and chronic fatigue syndrome has attracted broad interests in this topic. Although recent studies have indicated that XMRV is unlikely a human pathogen, further understanding of XMRV xenoinfection would allow in vivo modeling of the initial steps of gammaretroviral interspecies transmission, evolution and dissemination in a new host population. In this study, we monitored the long-term consequences of XMRV infection and its possible vertical transmission in a permissive foreign host, wild-derived Mus pahari mice. One year post-infection, XMRV-infected mice showed no notable pathological changes, while proviral DNA was detected in three out of eight mice. XMRV-infected mice remained seropositive throughout the study although the levels of gp70 Env- and p30 capsid-specific antibodies gradually decreased. When vertical XMRV transmission was assessed, no viremia, humoral immune responses nor endogenization were observed in nine offspring from infected mothers, yet one offspring was found PCR-positive for XMRV-specific sequences. Amplified viral sequences from the offspring showed several mutations, including one amino acid deletion in the receptor binding domain of Env SU. Our results therefore demonstrate long-term asymptomatic infection, low incidence of vertical transmission and limited evolution of XMRV upon transspecies infection of a permissive new host, Mus pahari.

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

Defective eosinophil hematopoiesis ex vivo in inbred Rocky Mountain White (IRW) mice

We examine the proliferation and differentiation of bone marrow (BM) progenitors from inbred Rocky Mountain White (IRW) mice, a strain used primarily for retrovirus infection studies. In contrast to findings with BALB/c and C57BL/6 strains, IRW BM cells cannot proliferate or generate pure eosinophil cultures ex vivo in response to a defined cytokine regimen. Analysis of IRW BM at baseline was unremarkable, including 0.08 ± 0.03% Lin(-)Sca-1(+)c-kit(+) (LSK) hematopoietic stem cells and 5.2 ± 0.3% eosinophils; the percentage of eosinophil progenitors (EoPs; Lin(-)Sca-1(-)c-kit(+)CD34(+)IL-5Rα(+)) was similar in all three mouse strains. Transcripts encoding GM-CSFRα and the IL-3/IL-5/GM-CSF common β chain were detected at equivalent levels in IRW and BALB/c BM, whereas expression of transcripts encoding IL-5Rα, IL-3Rα, and GATA-2 was diminished in IRW BM compared with BALB/c. Expression of membrane-bound IL-5Rα and intracellular STAT5 proteins was also diminished in IRW BM cells. Diminished expression of transcripts encoding IL-5Rα and GATA-2 and immunoreactive STAT5 in IRW BM persisted after 4 days in culture, along with diminished expression of GATA-1. Western blot revealed that cells from IRW BM overexpress nonsignaling soluble IL-5Rα protein. Interestingly, OVA sensitization and challenge resulted in BM and airway eosinophilia in IRW mice; however, the responses were significantly blunted. These results suggest that IRW mice have diminished capacity to generate eosinophils in culture and in vivo, likely as a result of diminished signaling via IL-5Rα.

Of Mice and Men: On the Origin of XMRV

The novel human retrovirus xenotropic murine leukemia virus-related virus (XMRV) is arguably the most controversial virus of this moment. After its original discovery in prostate cancer tissue from North American patients, it was subsequently detected in individuals with chronic fatigue syndrome from the same continent. However, most other research groups, mainly from Europe, reported negative results. The positive results could possibly be attributed to contamination with mouse products in a number of cases, as XMRV is nearly identical in nucleotide sequence to endogenous retroviruses in the mouse genome. But the detection of integrated XMRV proviruses in prostate cancer tissue proves it to be a genuine virus that replicates in human cells, leaving the question: how did XMRV enter the human population? We will discuss two possible routes: either via direct virus transmission from mouse to human, as repeatedly seen for, e.g., Hantaviruses, or via the use of mouse-related products by humans, including vaccines. We hypothesize that mouse cells or human cell lines used for vaccine production could have been contaminated with a replicating variant of the XMRV precursors encoded by the mouse genome.

The mouse "xenotropic" gammaretroviruses and their XPR1 receptor

The xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs) all rely on the XPR1 receptor for entry, but these viruses vary in tropism, distribution among wild and laboratory mice, pathogenicity, strategies used for transmission, and sensitivity to host restriction factors. Most, but not all, isolates have typical xenotropic or polytropic host range, and these two MLV tropism types have now been detected in humans as viral sequences or as infectious virus, termed XMRV, or xenotropic murine leukemia virus-related virus. The mouse xenotropic MLVs (X-MLVs) were originally defined by their inability to infect cells of their natural mouse hosts. It is now clear, however, that X-MLVs actually have the broadest host range of the MLVs. Nearly all nonrodent mammals are susceptible to X-MLVs, and all species of wild mice and several common strains of laboratory mice are X-MLV susceptible. The polytropic MLVs, named for their apparent broad host range, show a more limited host range than the X-MLVs in that they fail to infect cells of many mouse species as well as many nonrodent mammals. The co-evolution of these viruses with their receptor and other host factors that affect their replication has produced a heterogeneous group of viruses capable of inducing various diseases, as well as endogenized viral genomes, some of which have been domesticated by their hosts to serve in antiviral defense.

The tyrosine-based YXXØ targeting motif of murine leukemia virus envelope glycoprotein affects pathogenesis

Retroviruses, such as human and simian immunodeficiency viruses (HIV and SIV), and murine leukemia viruses (MuLV), harbor a tyrosine-based motif in the intracytoplasmic domain of their envelope glycoprotein. This motif can act as an endocytosis signal or as a targeting signal, restricting viral budding at specific cell surface membrane domains. In the present study, proviral DNA of the ecotropic Cas-Br-E strain of MuLV was modified by substitution or deletion of the critical tyrosine residue. Mutant viruses lost basolateral targeting in polarized MDCK epithelial cells while expression level of the glycoprotein at the cell surface was not affected. This suggests that the tyrosine-based motif in MuLV does not act as an endocytosis signal. Only a small delay in the appearance of disease was observed in inoculated mice. In contrast, a striking change in the pathology was observed with enlarged thymus and lymph nodes in animals inoculated with mutant viruses.

A small animal model for mother-to-fetus transmission of ts1, a murine retrovirus

Infection with a murine retrovirus, MoMuLV-TB, ts1 in BALB/c mice has been established as a small animal model for retroviral neurodegenerative disease as shown with infections such as HIV. However, mother-to-pup transmission has never been demonstrated in this model. The current investigation examines vertical transmission of ts1 in this mouse model. A total of 15 females were used to produce 59 pups (16 were used for control, and 43 were used as experimental animals). For experiment 1, 24 5-day-old mice were injected with [0.2 mL of 2.0 x 10(6) ffu/mL ts1] virus. For experiment 2, 19 48-h-old mice were injected with [0.1 mL of 4 x 10(6) ffu/mL ts1] virus. Control groups were injected with DMEM only. PCR and electron microscopy were performed to determine the presence of virus. All mice from experiment 1 injected with ts1 showed viral infection, and retained 100% reproductive capacity. Three out of 102 pups produced by these infected females were infected with ts1. Nine percent of the pups from experiment 2 injected with ts1 retained normal reproductive capacity, and two out of eight (25%) pups had viral infection. Vertical transmission of this unique retrovirus occurs and is dependent, in part, on the timing of maternal infection.

Viruses in the mammalian male genital tract and their effects on the reproductive system

This review describes the various viruses identified in the semen and reproductive tracts of mammals (including humans), their distribution in tissues and fluids, their possible cell targets, and the functional consequences of their infectivity on the reproductive and endocrine systems. The consequences of these viral infections on the reproductive tract and semen can be extremely serious in terms of organ integrity, development of pathological and cancerous processes, and transmission of diseases. Furthermore, of essential importance is the fact that viral infection of the testicular cells may result not only in changes in testicular function, a serious risk for the fertility and general health of the individual (such as a fall in testosteronemia leading to cachexia), but also in the possible transmission of virus-induced mutations to subsequent generations. In addition to providing an exhaustive account of the data available in these domains, this review focuses attention on the fact that the interface between endocrinology and virology has so far been poorly explored, particularly when major health, social and economical problems are posed. Our conclusions highlight the research strategies that need to be developed. Progress in all these domains is essential for the development of new treatment strategies to eradicate viruses and to correct the virus-induced dysfunction of the endocrine system.

Interaction of gametes with exogenous genes: possible opportunities for incorporation into embryonic genome

The mechanism of incorporation of foreign DNA into newly fertilized eggs is poorly understood. It is not known with certainty if S phase DNA replication is required or if integration could occur at other times of the cell cycle that involve DNA strand breaks, such as chromatin rearrangements. We have investigated DNA strand breaks in mouse eggs and zygotes with a sensitive terminal uridine nucleotide end labeling (TUNEL) assay. Greater than 90% of all polar bodies and metaphase II chromosomes in freshly ovulated mouse eggs are TUNEL-assay positive. Approximately one-third of zygotes assayed 6 hr after fertilization contain at least one TUNEL assay positive pro-nucleus and/or decondensing sperm head. These results indicate that early embryonic DNA contains multiple transient DNA breaks that could play a role in the incorporation of foreign DNA.

Testicular germ cells of HIV-seropositive asymptomatic men are infected by the virus

In situ PCR hybridization studies in the testis of infected asymptomatic subjects detected the presence of HIV-1 proviral DNA in the nuclei of germ cells at all stages of differentiation suggesting that HIV-seropositive men produce infected spermatozoa that are released in the genital tract. In all subjects studied spermatogenesis was normal, the presence of provirus was not associated with germ cell damage and a very mild local immune response was observed. The HIV hybridization pattern observed in germ cells supports the hypothesis of a clonal infection. It is suggested the possibility of a direct infection of the germ cells by cell-free virus and that the testis might represent a site of early viral localization, well tolerated because of the immune privilege of this organ.

Sperm-mediated gene transfer in mice

Sperm-mediated DNA transfer to offspring has the potential to markedly simplify the generation of transgenic animals, but the efficiency in mice has been controversial. To determine the basis of the variability of the procedure in mice, we undertook a large, collaborative study of sperm-mediated DNA transfer to mouse eggs in well-established laboratory conditions for in vitro fertilization and offspring development following embryo transfer. Sperm were incubated with plasmid DNA during the capacitation period and then added to freshly ovulated mouse oocytes for fertilization; cleaved embryos were then transferred to the oviducts of pseudopregnant recipients for gestation. From a total of 75 experiments, 13 produced 130 transgenic offspring, amounting to 7.4% of total fetuses. In five experiments, more than 85% of offspring were transgenic, but the factors leading to this high success rate were not discovered. Clustering of such a low frequency event could account for the disparate reports of transgenic success with sperm-mediated DNA transfer to mouse offspring. Discovering the factors important to success would not only allow this simplified approach to become an important tool in the generation of transgenic mice, but could also lead to important insights into natural protective mechanisms against sperm-mediated transfer of foreign DNA.

Presence and cellular distribution of HIV in the testes of seropositive subjects: an evaluation by in situ PCR hybridization

Cellular distribution of HIV-1 proviral DNA has been studied, by in situ PCR hybridization, in the testes of infected men who died at various stages of the disease. In seropositive asymptomatic subjects, HIV-1 proviral DNA was present in the nuclei of germ cells at all stages of their differentiation. The presence of provirus did not induce germ cell damage, was associated with normal spermatogenesis, and was not accompanied by morphologic signs of immune response. The observed HIV hybridization pattern of germ cells suggests clonal infection. Mechanisms responsible for HIV penetration in testicular germ cells remain to be clarified; however, the possibility of a direct infection of the germ cells by cell-free virus is suggested. In the testes of AIDS-deceased men, histologic features of hypoplasia with arrested spermatogenesis were evident, and few infected spermatogonia and spermatocytes were observed. The whole of these data demonstrates that the testis is a site of early viral localization that fails to elicit an immunological response, and that HIV-seropositive men produce infected spermatozoa that are released in the genital tract.

Detection of feline immunodeficiency virus in semen from seropositive domestic cats (Felis catus)

Electroejaculates from experimentally infected domestic cats were evaluated for the presence of feline immunodeficiency virus (FIV). Virus was isolated from cell-free seminal plasma and seminal cells by cocultivation with a feline interleukin-2-dependent CD4+ T-cell line, in which productive infection was demonstrated by syncytium formation and FIV gag p26 antigen secretion. In addition, an 868-bp segment of the FIV gag provirus gene was identified in cocultured cells by PCR and Southern analysis. A 582-bp fragment of the FIV gag provirus genome was detected by nested PCR and Southern analysis in nonfractionated seminal cells and in sperm purified by a swim-up procedure. This is the first report describing the detection of replication-competent FIV in cell-free and cell-associated forms in domestic cat semen.

Hypothesis: a memory lymphocyte-specific soma-to-germline genetic feedback loop

Analysis of recently published DNA sequence data obtained for related germline Ig variable (IgV) genetic elements of several vertebrate species revealed the presence of a number of extremely non-random patterns of sequence variability among these genes. Strikingly, the patterns were also observed in two sets of chicken IgV pseudogenes. Since the observed patterns are clearly incompatible with existing theories of multigene family evolution, a new model that can account for all of the data is presented in this paper. The model is a modification and extension of an earlier proposed mechanism whereby somatically expressed genes can be returned to the germline by endogenous retroviruses that may act as soma-to-germline genetic vectors. The mechanism described proposes that the interactions that may result in the soma-to-germline transfer of somatically selected IgV genes occur in the epididymis of the male reproductive tract and are restricted to memory lymphocytes. This mechanism makes a number of predictions that are amenable to experimental testing. From the data presently available in the literature it is not possible to extend the mechanism to the female reproductive tract.

Demented and nondemented patients with AIDS differ in brain-derived human immunodeficiency virus type 1 envelope sequences

Human immunodeficiency virus (HIV) dementia is a common clinical syndrome of uncertain pathogenesis in patients with AIDS. In several animal models of retrovirus-induced brain disease, specific viral envelope sequences have been found to influence the occurrence of central nervous system disease. Therefore, to search for unique envelope sequences correlated with HIV dementia, we studied 22 HIV-infected patients who were neurologically assessed premortem and classified into demented (HIVD) (n = 14) and nondemented (ND) (n = 8) groups. Using DNA from autopsied brain and spleen, we amplified, cloned, and sequenced a 430-nucleotide region including the V3 loop and flanking regions. All brain-derived clones in both clinical groups showed marked homology to the macrophage-tropic consensus sequence within the V3 loop. Two amino acid positions within (position 305) and outside (position 329) the V3 region showed significant divergence between the two clinical groups. At position 305, a histidine was predominant in the HIVD group and was not observed in the ND group, but a proline was predominant in the ND group and was not observed in the HIVD group. Similarly, at position 329, a leucine was predominant in the HIVD group but rarely observed in the ND group, whereas an isoleucine was predominant in the ND group at this position. In addition, the HIVD group had 21 amino acid residues at specific positions that were unique relative to the ND group, whereas only 2 residues at specific positions were unique to the ND group. These data suggest that distinct HIV envelope sequences are associated with the clinical expression of HIV dementia.

Identification of a sequence in the unique 5' open reading frame of the gene encoding glycosylated Gag which influences the incubation period of neurodegenerative disease induced by a murine retrovirus

Neonatal inoculation of the wild-mouse ecotropic retrovirus CasBrE (clone 15-1) causes a noninflammatory spongiform neurodegenerative disease with an incubation period of > or = 6 months. Introduction of sequences from Friend murine leukemia virus (clone FB29) into the genome of CasBrE results in a marked shortening of the incubation period. The FB29 sequences which influence the incubation period were previously localized to the 5' leader sequence of the viral genome (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). In the current study, we constructed a series of chimeric viruses consisting of the genome of CasBrE containing various segments of the leader sequence from FB29. A 41-nucleotide element (positions 481 through 521) near the 3' end of the leader was found to have a strong influence on the incubation period. This element influenced the kinetics of virus replication and/or spread in nonneuronal tissues, a property which was shown previously to determine the extent of central nervous system infection (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). Curiously, this sequence had no demonstrable effect on virus replication in vitro in a fibroblastic cell line from Mus dunni. This segment encodes 14 of the unique 88-amino-acid N terminus of pr75gag, the precursor of a glycosylated form of the gag polyprotein which is expressed at the cell surface. Previous in vitro studies of mutants of Moloney murine leukemia virus lacking expression of glycosylated Gag failed to reveal a function for this protein in virus replication. We mutated the Kozak consensus sequence around the initiation codon for this protein in the chimeric virus CasFrKP, a virus which induces neurologic disease with a short (18- to 23-day) incubation period. M. dunni cells infected with the mutants lacked detectable cell surface Gag, but, compared with CasFrKP, no effect on replication kinetics in vitro was observed. In contrast, there was a marked slowing of the replication kinetics in vivo and a dramatic attenuation of neurovirulence. These studies indicate that glycosylated Gag has an important function in virus replication and/or spread in the mouse and further suggest that the sequence of its N terminus is a critical, though likely indirect, determinant of neurovirulence.

Immunology of the testicular excurrent ducts

The sperm autoantigen concentration in the epididymis equals or exceeds that in the testis. This makes the epididymis a probable site of initiation of an antisperm autoimmune response. The mechanisms regulating antisperm antibody formation in the testicular excurrent ducts and some related aspects with clinical interest are reviewed.

Transplacental transmission of a leukemogenic murine leukemia virus

Recombinant inbred BXH-2 mice spontaneously produce a B-tropic murine leukemia virus (MuLV) beginning early in life and have a high incidence of spontaneous myeloid leukemia. These traits are not characteristic of the progenitor strains (C57BL/6J and C3H/HeJ) or of 11 other recombinant inbred BXH strains. Genetic analysis has shown that the virus is not transmitted through the germ line, suggesting that the virus is passed from one generation to the next by horizontal transmission. An additional ecotropic proviral locus was detected in some mice of this strain after several generations of inbreeding. We show that BXH ecotropic virus was transmitted to other strains when fostered on viremic BXH-2 mice and that these mice go on to develop tumors of hematopoietic origin. Our earlier finding that virus is expressed early in gestation suggested that the ecotropic MuLV is also transmitted in utero. In order to determine the stage at which the ecotropic MuLV is transmitted in utero, preimplantation stage embryos were transferred to the uteri of recipient ecotropic virus-negative mice. These mice were found to be negative for the presence of the ecotropic MuLV, suggesting that transplacental transmission of the ecotropic virus readily occurs in BXH-2 mice. Although other viruses, including human lentiviruses, are transmitted across the placental barrier, transplacental transmission of MuLV is a rare event. Thus, the BXH-2 mouse strain may contribute to our understanding of the mechanism of transplacental transmission and pathogenesis and offers a potential new model for use in drug therapy of exogenously transmitted viruses related to lentiviruses.

Murine retrovirus-induced spongiform encephalopathy: productive infection of microglia and cerebellar neurons in accelerated CNS disease

We have examined the pathological lesions and sites of infection in mice inoculated with a highly neurovirulent recombinant wild mouse ecotropic retrovirus (FrCasE). The spongiform lesions appeared initially as swollen postsynaptic neuronal processes, progressing to swelling in neuronal cell bodies, all in the absence of detectable gliosis. Infection of neurons in regions of vacuolation was not detected. However, high level infection of cerebellar granule neurons was observed in the absence of cytopathology, wherein viral protein was found associated with both axons and dendrites. Infection of ramified and amoeboid microglial cells was associated with cytopathology in the brain stem, and endothelial cell-pericyte infection was found throughout the CNS. No evidence of defective retroviral expression was observed. These results are consistent with an indirect mechanism of retrovirus-induced neuropathology.

Age-dependent resistance to murine retrovirus-induced spongiform neurodegeneration results from central nervous system-specific restriction of virus replication

The murine retrovirus CasBrE causes a noninflammatory spongiform degeneration of the central nervous system (CNS). Mice inoculated as neonates develop viremia and are susceptible to disease. However, mice inoculated at 10 days of age do not develop viremia and are totally resistant to the neurologic disease. We recently described a highly neurovirulent chimeric virus, FrCasE (J. L. Portis, S. Czub, C. F. Garon, and F. J. McAtee, J. Virol. 64:1648-1656, 1990), which contains the env gene of CasBrE. Mice inoculated at 10 days of age with this virus developed a viremia comparable to that in neonatally inoculated mice but, surprisingly, were still completely resistant to the neurodegenerative disease. A comparison of the tissue distribution of virus replication for mice inoculated at 1 or 10 days of age was determined by Southern blot analysis for the quantification of viral DNA and by infectious-center assay for the quantification of virus-producing cells. The levels of virus replication in the spleens were comparable in the two groups. In contrast, virus replication in the CNS of the resistant 10-day-old mice was markedly restricted (100- to 1,000-fold). Intracerebral inoculation did not overcome this restriction. A similar pattern of CNS-specific restriction of virus replication and resistance to disease was observed in athymic NIH Swiss nude mice inoculated at 10 days of age, suggesting that T-cell immunity was not involved. From our results, we conclude that the age-dependent resistance to disease is a consequence of the restriction of virus replication within the CNS due to the developmental state of the organ.

The R-U5-5' leader sequence of neurovirulent wild mouse retrovirus contains an element controlling the incubation period of neurodegenerative disease

The wild mouse ecotropic retrovirus CasBrE causes a spongiform neurodegenerative disease after neonatal inoculation, with an incubation period ranging from 2 to 12 months. We previously showed that introduction of long terminal repeat (LTR) and gag-pol sequences from a strain of Friend murine leukemia virus (FB29) resulted in a dramatic acceleration of the onset of the disease. The chimeric virus FrCasE, which consisted of the FB29 genome containing 3' pol and env sequences from the wild mouse virus, induced a highly predictable, lethal neurodegenerative disease with an incubation period of only 16 days. Here we report that the sequences which are primary determinants of the length of the incubation period are located in the 5' end of the viral genome between a KpnI site in the R region of the LTR and a PstI site immediately 5' of the start codon for pr65gag (R-U5-5' leader). This region contains the tRNA primer binding site, splice donor site for the subgenomic env mRNA, and the packaging sequence. Computer-assisted sequence analysis failed to find evidence of a consensus sequence for a DNA enhancer in this region. In addition, sequences within a region of the genome between a ClaI site at the 3' end of env to the KpnI site in the R region of the LTR (inclusive of U3) also influenced the incubation period of the disease, but the effect was distinctly weaker than that of the R-U5-5' leader sequence. This U3 effect, however, appeared to be independent of the number of direct repeats, since deletion of one of two duplicated 42-base repeats containing consensus sequences of nuclear-factor binding domains had no effect on the incubation period of the disease. On the basis of Southern blot analysis of total viral DNA in the tissues, the effect of these sequences on the incubation period appeared to be related to the level of virus replication in the central nervous system. All of the chimeric viruses analyzed, irrespective of neurovirulence, replicated to comparable levels in the spleen and induced comparable levels of viremia.

Genital mucosal transmission of simian immunodeficiency virus: animal model for heterosexual transmission of human immunodeficiency virus

An animal model for the heterosexual transmission of human immunodeficiency virus (HIV) was developed by the application of simian immunodeficiency virus (SIV) onto the genital mucosas of both mature and immature, male and female rhesus macaques. Virus preparations were infused into the vaginal vaults or the urethras (males) of the animals through a soft plastic pediatric nasogastric feeding tube. The macaques that were infected by this route (six males and nine females) developed SIV-specific antibodies, and SIV was isolated from peripheral mononuclear cells of all seropositive animals. One male and one female infected by this route developed severe acquired immunodeficiency syndrome-like disease with retroviral giant-cell pneumonia. As few as two inoculations of cell-free SIV containing 50 50% tissue culture infective doses induced persistent viremia. Cell-free virus preparations were capable of producing infection by the genital route. Much higher doses of virus were required to transmit SIV by this route than are required for transmission by intravenous inoculation. Thus, it appears that the mucous membranes of the genital tract act as a barrier to SIV infection. Spermatozoa and seminal plasma were not required for the genital transmission of SIV. Rarely, SIV was recovered from mononuclear cells in semen and vaginal secretions. The SIV-rhesus macaque model is suitable for assessing the role of cofactors in heterosexual transmission of HIV and will be useful for testing the effectiveness of spermicides, pharmacologic agents, and vaccines in preventing the heterosexual transmission of HIV.

Epididymis is a principal site of retrovirus expression in the mouse

High levels of retrovirus particles are present in the reproductive tract of male mice. In this report epithelial cells that line the lumen of the epididymis are shown to be a principal site of virus synthesis. Aggregates of free virus were evident in the epididymal lumen in addition to the sperm-associated virus previously reported. Large intraluminal cells with characteristics of macrophages and engorged with virus particles were also seen. Virus particles were not detected in testis, liver, brain, or spleen. Thus, the epididymal epithelium is a principal reservoir for retrovirus expression. The virus would be ejaculated as free, cell-associated, and sperm-bound particles. The high level of expression and the relative isolation of epididymal virus from the immune system may relate to venereal transmission of retrovirus infections in mice and humans.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

An enhancer sequence instability that diversifies the cell repertoire for expression of a murine leukemia virus

Studies of recombinants between murine leukemia viruses (MuLVs) that cause thymic or erythroid leukemias have shown that enhancer sequences in the long-terminal repeats (LTRs) can determine the target tissues for pathogenesis. It has been inferred that the enhancers may specifically target viral expression into the cells that then become neoplastic. However, the neoplasms in those studies formed after latencies and contained ultimate viruses (called MCFs) that differed from the injected viruses in their enhancer sequences and envelope (env) genes. Transcriptional activities of LTRs from these proximal and ultimate viruses have not been thoroughly analyzed in different hematopoietic lineages. We present evidence that the enhancer of Friend spleen focus-forming virus (SFFV), an ultimate erythroleukemogenic retrovirus, contains an unstable 42-nucleotide direct repeat. Other ultimate erythroleukemogenic MuLVs (Friend MCFs) contain an enhancer nearly identical to that of SFFV both in its sequence and in its specific instability. The instability occurs in sequences that contain inverted repeats and we propose that it occurs by a simple reverse transcriptase hop mechanism. We constructed plasmids that contain the two forms of the SFFV LTR linked to the bacterial chloramphenicol acetyltransferase (CAT) gene, and we compared these in transient transfection assays with LTR-CAT plasmids constructed from Friend and Moloney MuLVs. The assays employed erythroleukemia cells, thymic lymphoma cells, and fibroblasts. The tropisms of expression correlated only weakly with tissue specificities of pathogenesis and each LTR was active in all cells. The SFFV 42-nucleotide duplication reduced expression in erythroid cells and increased expression in fibroblasts. We conclude that retroviral enhancers do not stringently direct gene expression into specific cell lineages, but on the contrary they are leaky and contain replicative instabilities that also may facilitate viral entrenchment throughout the host. These results have important implications for understanding murine retroviral evolution and the multi-step process of leukemogenesis.

Neurotropic retroviruses of wild mice and macaques

A neurotropic retrovirus causes a naturally occurring lower-limb paralysis in wild mice, characterized by a noninflammatory spongiform change located primarily in the lower spinal cord. The causative agent is an ecotropic murine leukemia virus, unique to certain wild mice in southern California. The disease is readily transmitted to newborn susceptible laboratory mice. The paralytogenic property is attributed to direct viral injury to motor neurons and glial cells and is associated with unique amino acids in the murine leukemia virus envelope gp70. This murine model may have relevance to both human T-lymphotropic virus type I, and human immunodeficiency virus infection of human brain. It presents a practical model for testing antiviral agents aimed at retrovirus infection of the mammalian central nervous system. Simian acquired immunodeficiency syndrome type D retrovirus causes a silent infection of the brain in infected macaques. Viral nucleic acids are detected in the brain parenchyma in the absence of viral antigen, neurological symptoms, and neuropathology. Infected choroid plexus epithelial cells are the source of cell-free virus in the cerebrospinal fluid of viremic monkeys. This model adds yet another example of retroviral infection of the central nervous system and points to the choroid plexus as a potential source of infectious virus.

Lymphocyte major histocompatibility complex-encoded class II structures may act as sperm receptors

Human sperm and blood cells were cocultured in vitro to determine whether specific interactions occur between gametes and blood cells. Evidence for cell type-specific sperm binding and penetration of lymphocytes is presented together with findings that suggest that either or both events involve major histocompatibility complex-encoded class II molecules on lymphocytes and a sperm ligand that is immunoreactive with antibodies to T-cell surface antigen T4. Involvement of HLA-DR is suggested by the pattern of sperm interactions with HLA-DR-positive and -negative cells and by inhibition of sperm binding to HLA-DR-positive cells by a monoclonal antibody that identifies a nonpolymorphic determinant on the HLA-DR molecule. That the complementary sperm ligand may be a T4-like structure is suggested by specific inhibition of sperm-lymphocyte binding with monoclonal antibodies OKT4 and OKT4A. The results are discussed in terms of possible roles for immunoglobulin-related structures in human fertilization and in the sexual transmission of the acquired immunodeficiency syndrome.