Tatyana V. Golovkina

Rapid fucosylation of intestinal epithelium sustains host-commensal symbiosis in sickness.

Systemic infection induces conserved physiological responses that include both resistance and ‘tolerance of infection’ mechanisms. Temporary anorexia associated with an infection is often beneficial, reallocating energy from food foraging towards resistance to infection or depriving pathogens of nutrients. However, it imposes a stress on intestinal commensals, as they also experience reduced substrate availability; this affects host fitness owing to the loss of caloric intake and colonization resistance (protection from additional infections). We hypothesized that the host might utilize internal resources to support the gut microbiota during the acute phase of the disease. Here we show that systemic exposure to Toll-like receptor (TLR) ligands causes rapid α(1,2)-fucosylation of small intestine epithelial cells (IECs) in mice, which requires the sensing of TLR agonists, as well as the production of interleukin (IL)-23 by dendritic cells, activation of innate lymphoid cells and expression of fucosyltransferase 2 (Fut2) by IL-22-stimulated IECs. Fucosylated proteins are shed into the lumen and fucose is liberated and metabolized by the gut microbiota, as shown by reporter bacteria and community-wide analysis of microbial gene expression. Fucose affects the expression of microbial metabolic pathways and reduces the expression of bacterial virulence genes. It also improves host tolerance of the mild pathogen Citrobacter rodentium. Thus, rapid IEC fucosylation appears to be a protective mechanism that utilizes the host’s resources to maintain host–microbial interactions during pathogen-induced stress.

Successful Transmission of a Retrovirus Depends on the Commensal Microbiota

Commensal microflora promote the pathogenesis of mucosally acquired viruses. To establish chronic infections, viruses must develop strategies to evade the host’s immune responses. Many retroviruses, including mouse mammary tumor virus (MMTV), are transmitted most efficiently through mucosal surfaces rich in microbiota. We found that MMTV, when ingested by newborn mice, stimulates a state of unresponsiveness toward viral antigens. This process required the intestinal microbiota, as antibiotic-treated mice or germ-free mice did not transmit infectious virus to their offspring. MMTV-bound bacterial lipopolysaccharide triggered Toll-like receptor 4 and subsequent interleukin-6 (IL-6)–dependent induction of the inhibitory cytokine IL-10. Thus, MMTV has evolved to rely on the interaction with the microbiota to induce an immune evasion pathway. Together, these findings reveal the fundamental importance of commensal microbiota in viral infections.

Transgenic Mouse Mammary Tumor Virus Superantigen Expression Prevents Viral Infection

Endogenous mouse mammary tumor virus (MMTV) proviruses have recently been shown to cosegregate genetically with the minor lymphocyte-stimulating loci, also termed self-superantigens. The antigenic activity has been localized to the open reading frame (ORF) protein encoded in the long terminal repeat of MMTV. We show here that unlike their nontransgenic littermates, transgenic mice expressing high levels of an ORF protein derived from the C3H exogenous MMTV specifically delete their V beta 14+ T cells and do not become infected with this virus when it is present in their mothers milk. Thus, it appears that MMTV utilizes cells of the immune system in its infection pathway, and mice that retain endogenous MMTVs should be immune to infection by exogenous virus. These results offer possible new approaches to anti-viral therapy or immunization.

Subversion of the innate immune system by a retrovirus.

Retroviruses evolve rapidly to avoid the immune response of the infected host. We show here that the wild-type mouse mammary tumor virus MMTV(C3H) persisted indefinitely in C3H/HeN mice. However, it was rapidly lost in mice of the closely related C3H/HeJ strain and was replaced by a virus recombinant with an endogenous Mtv provirus. Maintenance of the wild-type virus was dependent on Toll-like receptor-4 (TLR4) signaling, which triggered production of the immunosuppressive cytokine interleukin-10. In the presence of mutant TLR4 in C3H/HeJ mice, wild-type virus was eliminated by the cytotoxic immune response, promoting selection of the immune escape recombinant MMTV variants. Thus, subversion of the innate immune system is yet another survival strategy used by retroviruses.

An Essential Role of Th1 Responses and Interferon-gamma in Infection-Mediated Suppression of Neoplastic Growth

We had previously demonstrated that in mice acute toxoplasmosis leads to systemic inhibition of angiogenesis and, consequently, strong suppression of neoplastic growth. Here we investigated the role of Th1 cytokines, in particular interferon gamma (IFNg), in this phenomenon. Besides toxoplasma, neoplastic growth was readily blocked during acute infection with other Th1 response-inducing pathogens such as Listeria monocytogenes and lymphocytic choriomeningitis virus (LCMV). In contrast, chronic infection with LCMV (when Th1 responses were strongly suppressed) and acute infection with Schistosoma mansoni (when Th2 responses predominated) afforded no anti-tumor protection. To corroborate the involvement of Th1 cytokines in infection-mediated suppression of neoplastic growth, we utilized mice deficient in interleukin-10 (IL10), a suppressor of Th1 responses. When challenged with B16 cells concomitantly with toxoplasma infection, both IL10-null and wild type mice exhibited resistance to neoplastic growth. However, tumors borne by IL10-null animals were even smaller than those borne by their wild type counterparts. This enhanced resistance correlated with dramatically elevated levels of circulating IFNg, a principal Th1 cytokine. Furthermore, while interleukin-12 and tumor necrosis factor g were dispensable for tumor suppression, in animals deficient in IFNg production or signaling, tumor growth and neovascularization were markedly enhanced. Interestingly, the enhancement was also apparent in uninfected animals suggesting that IFNg and its anti-angiogenic effects underlie both infection-dependent and -independent tumor surveillance.

Unique Resistance of I/LnJ Mice to a Retrovirus Is Due to Sustained Interferon γ–dependent Production of Virus-neutralizing Antibodies

Selection of immune escape variants impairs the ability of the immune system to sustain an efficient antiviral response and to control retroviral infections. Like other retroviruses, mouse mammary tumor virus (MMTV) is not efficiently eliminated by the immune system of susceptible mice. In contrast, MMTV-infected I/LnJ mice are capable of producing IgG2a virus-neutralizing antibodies, sustain this response throughout their life, and secrete antibody-coated virions into the milk, thereby preventing infection of their progeny. Antibodies were produced in response to several MMTV variants and were cross-reactive to them. Resistance to MMTV infection was recessive and was dependent on interferon (IFN)-γ production, because I/LnJ mice with targeted deletion of the INF-γ gene failed to produce any virus-neutralizing antibodies. These findings reveal a novel mechanism of resistance to retroviral infection that is based on a robust and sustained IFN-γ–dependent humoral immune response.

Cutting Edge: Systemic Inhibition of Angiogenesis Underlies Resistance to Tumors During Acute Toxoplasmosis

The ability of various infections to suppress neoplastic growth has been well documented. This phenomenon has been traditionally attributed to infection-induced concomitant, cell-mediated antitumor immunity. We found that infection with Toxoplasma gondii effectively blocked neoplastic growth of a nonimmunogenic B16.F10 melanoma. Moreover, this effect was independent of cytotoxic T or NK cells, production of NO by macrophages, or the function of the cytokines IL-12 and TNF-α. These findings suggested that antitumor cytotoxicity was not the primary mechanism of resistance. However, infection was accompanied by strong, systemic suppression of angiogenesis, both in a model system and inside the nascent tumor. This suppression resulted in severe hypoxia and avascular necrosis that are incompatible with progressive neoplastic growth. Our results identify the suppression of tumor neovascularization as a novel mechanism critical for infection-induced resistance to tumors.

Genetics of Mouse Mammary Tumor Virus-Induced Mammary Tumors: Linkage of Tumor Induction to the gag Gene

ABSTRACT Retroviruses are believed to induce tumors by acting as insertional mutagens that activate expression of cellular protooncogenes. Indeed, almost 90% of mouse mammary tumor virus (MMTV)-induced mammary tumors in C3H/He mice show upregulation of Int protooncogenes. We have analyzed three different MMTV variants [MMTV(C3H), MMTV(HeJ), and a genetically engineered MMTV hybrid provirus (HP)] for tumorigenicity in mice from two distinct genetic backgrounds. All three viruses were tumor causing in BALB/cJ mice. However, only MMTV(C3H), but not MMTV(HeJ) or HP, induced mammary tumors in C3H/He mice. All of the viruses were infectious on either background and up-regulated expression of Int genes in tumors they induced. Like HP, MMTV(HeJ) was found to be a genetic recombinant between endogenousMtv1 provirus and exogenous MMTV(C3H). Sequence comparison of MMTV variants linked the tumorigenicity of MMTV(C3H) to thegag region of the retrovirus.

Influence of Microbiota on Viral Infections

Most pathogens gain access to the host through surfaces of the body that are exposed to the surrounding environment and rife with resident microorganisms, termed microbiota. Microbiota play an integral role in modulating host health. One significant benefit of the microbiota is that they provide protection against incoming bacterial pathogens [1]. Commensals make their immediate environment inhospitable to many pathogens by producing biosurfactants, by competing for sites of attachment and nutrients, and by excreting metabolites with antimicrobial effects [1]. Furthermore, the presence of commensals promotes maturation of secondary lymphoid organs in the intestine, which are the first line of defense in the intestinal mucosa [2]. Therefore, when a pathogen infiltrates the host, it is not entering a sterile environment, but one that has been shaped by a dynamic commensal community. Although many interactions between bacterial pathogens and the microbiota have been characterized [1], little is known about the interplay between viral pathogens and the natural flora of the host. Are viral pathogens blind to the commensal microbes surrounding them? Judging from recent publications, this appears not to be the case. There is strong evidence that the microbiota can either protect the host from virally induced disease or promote viral propagation/transmission, through direct or indirect mechanisms.

Common Threads in Persistent Viral Infections

ABSTRACT Most viral infections are self-limiting, resulting in either clearance of the pathogen or death of the host. However, a subset of viruses can establish permanent infection and persist indefinitely within the host. Even though persisting viruses are derived from various viral families with distinct replication strategies, they all utilize common mechanisms for establishment of long-lasting infections. Here, we discuss the commonalities between persistent infections with herpes-, retro-, flavi-, arena-, and polyomaviruses that distinguish them from acutely infecting viral pathogens. These shared strategies include selection of cell subsets ideal for long-term maintenance of the viral genome, modulation of viral gene expression, viral subversion of apoptotic pathways, and avoidance of clearance by the immune system.