MMTV superantigens coerce an unconventional topology between the TCR and MHC class II

Intact, recombinant, and spliced forms of endogenous mouse mammary tumor viruses in inbred and wild mice

Endogenous retroviruses (ERVs) are chromosomally integrated viral copies that represent relics of past infections. Analysis of the sequenced genomes of 17 mouse strains, Mus musculus subspecies, and Mus spretus identified 29 ERVs of mouse mammary tumor viruses (MMTVs), termed Mtvs. The 15 laboratory mouse Mtvs are each present in multiple strains reflecting their common breeding history; most predate the development of inbred strains and were likely acquired by Mus musculus domesticus progenitors but have no orthologs in wild mice, whereas four, including the intact Mtv1, were likely endogenized more recently. One of the 14 Mtvs found in wild mice was distributed over a broad geographic range in southeast Asia. Most Mtvs are full-length, with multiple open reading frames, but Mtvs from many wild mice have an unusual envelope deletion corresponding to an intron of the viral rem accessory gene, suggesting its derivation from spliced MMTV cDNAs. These deleted envs have open reading frames, are found in globally distributed mice, and show subspecies-specific sequence variation consistent with their recurrent generation. The highly variable MMTV sag gene, responsible for resistance to exogenous infection, exhibits evidence of recombination as well as positive selection, consistent with its role in antiviral defense. In contrast, the spread of Mtvs in Mus musculus populations is not marked by an active arms race pitting the MMTV envelope against its cellular receptor. Thus, the acquisition of potentially disease-inducing Mtvs is a recent and ongoing process in Mus accompanied by recombination, positive selection, and a recurrent envelope deletion.

IMPORTANCE

Endogenous retroviruses (ERVs) are copies of viral genomes inserted into host chromosomes, producing a fossil record of past infections and virus-host co-adaptations. ERVs of mouse mammary tumor viruses (Mtvs) were found in all common laboratory strains, all Mus musculus subspecies, and a sister species, Mus spretus. Most laboratory mouse Mtvs predate inbred strain origins and were acquired by M. musculus domesticus, but although widely shared among strains, none of these were found in wild mice. Among wild mouse Mtvs, only one showed a broad geographic distribution. All M. musculus subspecies carry Mtvs with a large envelope deletion corresponding to the processed mRNA for the viral rem gene; such Mtvs likely derive from spliced viral mRNA. The Mtv sag gene responsible for resistance to exogenous infection is under purifying selection and has been subject to recombination, whereas the Mtv envelope and its cellular receptor show no evidence of genetic conflicts.

Intestinal Immune System and Amplification of Mouse Mammary Tumor Virus

Mouse mammary tumor virus (MMTV) is a virus that induces breast cancer in mice. During lactation, MMTV can transmit from mother to offspring through milk, and Peyer's patches (PPs) in mouse intestine are the first and specific target organ. MMTV can be transported into PPs by microfold cells and then activate antigen-presenting cells (APCs) by directly binding with Toll-like receptors (TLRs) whereas infect them through mouse transferrin receptor 1 (mTfR1). After being endocytosed, MMTV is reversely transcribed and the cDNA inserts into the host genome. Superantigen (SAg) expressed by provirus is presented by APCs to cognate CD4+ T cells via MHCII molecules to induce SAg response, which leads to substantial proliferation and recruitment of related immune cells. Both APCs and T cells can be infected by MMTV and these extensively proliferated lymphocytes and recruited dendritic cells act as hotbeds for viral replication and amplification. In this case, intestinal lymphatic tissues can actually become the source of infection for the transmission of MMTV in vivo, which results in mammary gland infection by MMTV and eventually lead to the occurrence of breast cancer.

Extrinsic allospecific signals of hematopoietic origin dictate iNKT cell lineage-fate decisions during development

Invariant NKT (iNKT) cells are critical to the maintenance of tolerance toward alloantigens encountered during postnatal life pointing to the existence of a process for self-education. However, the impact of developmentally encountered alloantigens in shaping the phenotype and function of iNKT cells has not been described. To better understand this process, the current report examined naïve iNKT cells as they matured in an allogeneic environment. Following the prenatal transfer of fetal hematopoietic cells between age-matched allogeneic murine fetuses, cell-extrinsic signals appeared to dictate allospecific patterns of Ly49 receptor expression and lineage diversity in developing iNKT cells. Regulation for this process arose from cells of hematopoietic origin requiring only rare exposure to facilitate broad changes in developing iNKT cells. These findings highlight surprisingly asymmetric allospecific alterations in iNKT cells as they develop and mature in an allogeneic environment and establish a new paradigm for study of the self-education of iNKT cells.

Convergent capture of retroviral superantigens by mammalian herpesviruses

Horizontal gene transfer from retroviruses to mammals is well documented and extensive, but is rare between unrelated viruses with distinct genome types. Three herpesviruses encode a gene with similarity to a retroviral superantigen gene (sag) of the unrelated mouse mammary tumour virus (MMTV). We uncover ancient retroviral sags in over 20 mammals to reconstruct their shared history with herpesviral sags, revealing that the acquisition is a convergent evolutionary event. A retrovirus circulating in South American primates over 10 million years ago was the source of sag in two monkey herpesviruses, and a different retrovirus was the source of sag in a Peruvian rodent herpesvirus. We further show through a timescaled phylogenetic analysis that a cross-species transmission of monkey herpesviruses occurred after the acquisition of sag. These results reveal that a diverse range of ancient sag-containing retroviruses independently donated sag twice from two separate lineages that are distinct from MMTV.

Horizontal gene transfer from retroviruses to mammals is rare between unrelated viruses. Here the authors show the convergent acquisition by herpesviruses of a virulence gene of ancient retroviruses, which occurred at least twice from different donor lineages, to distinct herpesviruses that infect mammals.

Prenatal Allogeneic Tolerance in Mice Remains Stable Despite Potent Viral Immune Activation

Transplanting stem cells before birth offers an unparalleled opportunity to initiate corrective treatment for numerous childhood diseases with minimal or no host conditioning. Although long-term engraftment has been demonstrated following in utero hematopoietic cellular transplantation during immune quiescence, it is unclear if prenatal tolerance becomes unstable with immune activation such as during a viral syndrome. Using a murine model of in utero hematopoietic cellular transplantation, the impact of an infection with lymphocytic choriomeningitis virus on prenatal allospecific tolerance was examined. The findings in this report illustrate that established mechanisms of donor-specific tolerance are strained during potent immune activation. Specifically, a transient reversal in the anergy of alloreactive lymphocytes is seen in parallel with the global immune response toward the virus. However, these changes return to baseline following resolution of the infection. Importantly, prenatal engraftment remains stable during and after immune activation. Collectively, these findings illustrate the robust nature of allospecific tolerance in prenatal mixed chimerism compared with models of postnatal chimerism and provides additional support for the prenatal approach to the treatment of congenital benign cellular disease.