Tropism, cytotoxicity, and inflammatory properties of two envelope genes of murine leukemia virus type-endogenous retroviruses of C57BL/6J mice

Divergent and dynamic activity of endogenous retroviruses in burn patients and their inflammatory potential

Genes constitute ~3% of the human genome, whereas human endogenous retroviruses (HERVs) represent ~8%. We examined post-burn HERV expression in patients' blood cells, and the inflammatory potentials of the burn-associated HERVs were evaluated. Buffy coat cells, collected at various time points from 11 patients, were screened for the expression of eight HERV families, and we identified their divergent expression profiles depending on patient, HERV, and time point. The population of expressed HERV sequences was patient-specific, suggesting HERVs' inherent genomic polymorphisms and/or differential expression potentials depending on characteristics of patients and courses of injury response. Some HERVs were shared among the patients, while the others were divergent. Interestingly, one burn-associated HERV gag gene from a patient's genome induced IL-6, IL-1β, Ptgs-2, and iNOS. These findings demonstrate that injury stressors initiate divergent HERV responses depending on patient, HERV, and disease course and implicate HERVs as genetic elements contributing to polymorphic injury pathophysiology.

Injury-elicited stressors alter endogenous retrovirus expression in lymphocytes depending on cell type and source lymphoid organ

Background

Murine leukemia virus-type endogenous retroviruses (MuLV-ERVs) constitute ~10% of the mouse genome and are associated with various pathophysiologic processes. In this study, we examined whether MuLV-ERVs’ response to burn-elicited stressors is specific for certain lymphocyte populations and/or locations of lymphoid organ.

Results

B- and T-cells, which were sorted from nine lymphoid organs of C57BL/6J mice after burn, were subjected to MuLV-ERV expression analyses. Overall, the post-burn MuLV-ERV expression pattern was dependent on lymphocyte type, time after injury, location of lymphoid organ, and MuLV-ERV type. For instance, the MuLV-ERV expression in T-cells from the thymus and three cervical lymph nodes decreased at 3 hours post-burn while the expression of some MuLV-ERVs was augmented in B-cells derived from the mesenteric lymph node. The MuLV-ERV U3 sequences population of the burn-24 hours group was less diverse in comparison to the no burn and burn-3 hours groups. In addition, it was apparent that at the 24 hours time point, the U3 populations of B-cells from both no burn and burn groups were less heterogeneous than the T-cells’ U3 populations. Using the U3 sequences, some of which were isolated only from specific experimental groups (B- vs. T-cells; no burn vs. burn), as probes, 51 putative MuLV-ERVs, including 16 full-length proviruses, were mapped followed by characterization of their biologic properties.

Conclusion

MuLV-ERVs’ response to burn-elicited stressors may be differentially controlled depending on lymphocyte type, location of lymphoid organ, MuLV-ERV type, and stress duration.

Sgp3 and TLR7 stimulation differentially alter the expression profile of modified polytropic retroviruses implicated in murine systemic lupus

The envelope glycoprotein, gp70, of endogenous retroviruses represents one of the major nephritogenic autoantigens implicated in murine systemic lupus erythematosus. Among different endogenous retroviruses (ecotropic, xenotropic and polytropic), lupus-prone mice express remarkably high levels of modified polytropic (mPT) retroviruses, which are controlled by the Sgp3 (serum gp70 production) locus. To define the contribution of the Sgp3 locus derived from lupus-prone mice to the expression of the specific mPT proviruses, the genetic origin of different mPT viruses expressed in livers and thymi of wild-type and Sgp3 congenic C57BL/6 mice was determined through clonal analysis of their transcripts. Among 13 mPT proviruses present in the C57BL/6 genome, only 3 proviruses (Mpmv6, Mpmv10 and Mpmv13) were selectively but differentially expressed in livers and thymi. This was likely a result of co-regulated expression with host genes because of their integration in the same transcriptional direction. In contrast, Sgp3 induced the steady-state expression of an additional select group of mPT proviruses and, after stimulation of TLR7, the highly upregulated expression of a potentially replication-competent mPT virus Mpmv4. These results indicated that the expression of distinct subpopulations of mPT retroviruses was regulated by Sgp3- and TLR7-dependent mechanisms. The induction of potentially replication-competent mPT viruses and the upregulation of one such virus after stimulation with TLR7 in Sgp3 congenic mice further highlight the implication of Sgp3 in autoimmune responses against nephritogenic serum gp70 through the activation of TLR7.

Sgp3 and Sgp4 control expression of distinct and restricted sets of xenotropic retroviruses encoding serum gp70 implicated in murine lupus nephritis

The envelope glycoprotein gp70 of endogenous retroviruses implicated in murine lupus nephritis is secreted by hepatocytes and its expression is controlled by Sgp3 (serum gp70 production 3) and Sgp4 loci derived from lupus-prone mice. Among three different endogenous retroviruses (ecotropic, xenotropic and polytropic), xenotropic viruses are considered to be the major source of serum gp70. Although the abundance of xenotropic viral gp70 RNA in livers was up-regulated by the presence of these two Sgp loci, it has not yet been clear whether Sgp3 and Sgp4 regulate the expression of a fraction or multiple xenotropic viruses present in mouse genome. To address this question, we determined the genetic origin of xenotropic viral sequences expressed in wild-type and two different Sgp congenic C57BL/6 mice. Among 14 xenotropic proviruses present in the C57BL/6 genome, only two proviruses (Xmv10 and Xmv14) were actively transcribed in wild-type C57BL/6 mice. In contrast, Sgp3 enhanced the transcription of Xmv10 and induced the transcription of three additional xenotropic viruses (Xmv15, Xmv17 and Xmv18), while Sgp4 induced the expression of a different xenotropic virus (Xmv13). Notably, stimulation of TLR7 in Sgp3 congenic C57BL/6 mice led to a highly enhanced expression of potentially replication-competent Xmv18. These results indicated that Sgp3 and Sgp4 independently regulated the transcription of distinct and restricted sets of xenotropic viruses in trans, thereby promoting the production of nephritogenic gp70 autoantigens. Furthermore, the induced expression of potentially replication-competent xenotropic viruses by Sgp3 may contribute to the development of autoimmune responses against gp70 through the activation of TLR7.