Resurrection of endogenous retroviruses in antibody-deficient mice

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The mammalian host has developed a long-standing symbiotic relationship with a considerable number of microbial species. These include the microbiota on environmental surfaces, such as the respiratory and gastrointestinal tracts1, and also endogenous retroviruses (ERVs), comprising a substantial fraction of the mammalian genome2, 3. The long-term consequences for the host of interactions with these microbial species can range from mutualism to parasitism and are not always completely understood. The potential effect of one microbial symbiont on another is even less clear. Here we study the control of ERVs in the commonly used C57BL/6 (B6) mouse strain, which lacks endogenous murine leukaemia viruses (MLVs) able to replicate in murine cells. We demonstrate the spontaneous emergence of fully infectious ecotropic4 MLV in B6 mice with a range of distinct immune deficiencies affecting antibody production. These recombinant retroviruses establish infection of immunodeficient mouse colonies, and ultimately result in retrovirus-induced lymphomas. Notably, ERV activation in immunodeficient mice is prevented in husbandry conditions associated with reduced or absent intestinal microbiota. Our results shed light onto a previously unappreciated role for immunity in the control of ERVs and provide a potential mechanistic link between immune activation by microbial triggers and a range of pathologies associated with ERVs, including cancer.

At a glance


  1. eMLV activation in antibody-deficient mice.
    Figure 1: eMLV activation in antibody-deficient mice.

    a, Significantly upregulated (>4-fold) genes in CD11b+ MHC-IIhiB220Gr1 macrophages from Rag1−/− mice compared with macrophages from wild-type (WT) mice. Triplicate microarrays from cells isolated from 40 mice are shown. b, eMLV spliced env mRNA expression in the same cells as in a. Each symbol represents macrophages from 20 mice (P = 0.024; paired Student’s t-test). c, eMLV spliced env mRNA expression in indicated organs from wild-type or Rag1−/− mice (spleen: P = 0.020; ileum: P = 0.032; colon: P = 0.004; lung: P = 0.001; muscle: P = 0.016; and kidney: P = 0.009; unpaired Student’s t-test). d, e, MLV SU expression (detected using the 83A25 monoclonal antibody; see Methods) in splenocytes (d) or indicated cell types (e) from wild-type or Rag1−/− mice. f, eMLV spliced env mRNA expression in the spleens of the indicated strains (P<0.001 between wild-type and either Ighm−/− or Ighm−/− MD4 mice; one-way analysis of variance (ANOVA)). g, MLV SU expression in splenic lymphocytes from wild-type or Ighm−/− MD4 mice. h, eMLV spliced env mRNA expression in the spleens of the indicated strains (P<0.001 between wild-type and either Myd88−/− or Tlr7−/− mice; one-way ANOVA). In c, f and h, each symbol is an individual mouse. In d, e and g, plots are representative of four mice per group. In f and h, values above 103 were considered high and are indicated by red-filled symbols.

  2. Retroviraemia and leukaemias/lymphomas in antibody-deficient mice.
    Figure 2: Retroviraemia and leukaemias/lymphomas in antibody-deficient mice.

    a, Detection of infectious MLV (RARV-5/XG7) from the plasma of a representative Rag1−/− mouse by restoring infectivity of the green fluorescent protein (GFP)-expressing XG7 retroviral vector in the indicated cell type. Numbers within the plots denote the percentage of retrovirally transduced (GFP+) cells. b, Fv1 tropism of RARVs isolated from 6 (R2–R4)- or 25 (R5–R8)-week-old healthy Rag1−/− mice, shown as the ratio of infectivity in B-3T3 to N-3T3 cells (B:N ratio). B- and N-tropic strains of Friend MLV (F-MLV) are shown for comparison. c, Amino acid residues of capsid positions 105–113 (CA105–113) deduced from the nucleotide sequence of Emv2 and the same RARVs as in b. Dots indicate identities. d, Phylogenetic tree of the same RARVs as in b. The scale indicates the probability of base substitution per site. e, eMLV spliced env mRNA expression in the spleens of Rag1−/− mice or vertically infected Rag1−/− Emv2−/− mice. Each dot is an individual mouse (P<0.001; unpaired Student’s t-test). Values above 103 were considered high and are indicated by red-filled symbols. f, MLV SU expression in splenocytes from Rag1−/− or vertically infected Rag1−/− Emv2−/− mice (representative of nine mice per group). g, eMLV DNA copy numbers per haploid genome, determined by qPCR for the pol or ecotropic env gene, in DNA from the spleens of healthy Rag1−/− mice (right) or vertically infected Rag1−/− Emv2−/− mice (left). Symbols represent individual mice, grouped according to their age. The sensitivity limit of this PCR method was determined as a median of 0.0003 copies per haploid genome, using Emv2−/− mice. eMLV DNA copy numbers for Rag1−/− mice include Emv2 (1/N). h, Tumour (leukaemias/lymphomas) incidence in cohorts of wild-type (n = 37), Rag1−/− (n = 38) or vertically infected Rag1−/− Emv2−/− mice (n = 23) at the NIMR SPF facility (P<0.000001 between wild-type and Rag1−/− mice; P = 0.00025 between wild-type and Rag1−/− Emv2−/− mice; log-rank survival analysis).

  3. Mouse ERV activation by microbial products.
    Figure 3: Mouse ERV activation by microbial products.

    a, ERV/retroelement-reporting probe set (Supplementary Table 3) signals in a publicly available Affymetrix HT mouse genome 430A microarray data set20 (ArrayExpress accession E-GEOD-17721) of wild-type B6 bone marrow-derived dendritic cells after stimulation with microbial products. Black arrows indicate the probe sets that are significantly regulated (P<0.05) more than twofold by at least one stimulus. Emv2-specific probe sets (annotated as Mela) are also indicated by grey arrows for comparison. ETn, early transposon. b, Mean log2 fold change in the MLV-reporting probe set in the same data set. c, MLV SU expression in wild-type or Emv2−/− splenocytes before (open histograms) or after (filled histograms) stimulation with 10μgml−1 LPS for 48h, according to forward scatter (Fsc) and CD19 expression. Numbers in the plots denote the percentage of cells within each gate and represent two donors each analysed in duplicate.

  4. eMLV activation in antibody-deficiency depends on husbandry conditions.
    Figure 4: eMLV activation in antibody-deficiency depends on husbandry conditions.

    a, eMLV spliced env mRNA expression in the spleens of Rag1−/− mice on neutral pH (SPF) or acidified water (pH2.5) at the NIMR, on acidified water (pH2.8) at JAX, on neutral pH at RCHCI, or in germ-free (GF) facilities at UMICH (P<0.016 between Rag1−/− mice at SPF NIMR and all other groups; one-way ANOVA). b, eMLV spliced env mRNA expression in the spleens of Ighm−/−, Myd88−/− or Tlr7−/− mice on neutral pH water (SPF) at the NIMR or on acidified water (pH 2.8) at JAX (P = 0.005 and P = 0.029 for Ighm−/− and Tlr7−/− mice, respectively; unpaired Student’s t-test). Each dot is an individual mouse and values above 103 were considered high and are indicated by red-filled symbols.

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Change history

Corrected online 28 November 2012
Affiliation 2 was corrected; formatting of the H2dlAb1-Ea allele in the main text was also corrected.


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Author information


  1. Division of Immunoregulation, MRC National Institute for Medical Research, The Ridgeway, London NW7 1AA, UK

    • George R. Young,
    • Urszula Eksmond &
    • George Kassiotis
  2. Basic Science Program, SAIC-Frederick, Inc. and Cancer and Inflammation Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Frederick, Maryland 21701, USA

    • Rosalba Salcedo
  3. Centre d’Immunologie de Marseille-Luminy (CIML), Aix-Marseille University-UM2, INSERM-U1104, CNRS-UMR7280, Marseille, France

    • Lena Alexopoulou
  4. Division of Virology, MRC National Institute for Medical Research, The Ridgeway, London NW7 1AA, UK

    • Jonathan P. Stoye


G.R.Y., J.P.S. and G.K. designed the study. G.R.Y. and U.E. carried out experiments and analysed data. R.S. and L.A. provided data or study samples. G.R.Y., J.P.S. and G.K. prepared the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Primary microarray data from triplicate arrays were deposited at ArrayExpress under accession E-MEXP-3623.

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