Mucosal immunity to reovirus infection

Leukocyte-derived IFN-α/β and epithelial IFN-λ constitute a compartmentalized mucosal defense system that restricts enteric virus infections

Epithelial cells are a major port of entry for many viruses, but the molecular networks which protect barrier surfaces against viral infections are incompletely understood. Viral infections induce simultaneous production of type I (IFN-α/β) and type III (IFN-λ) interferons. All nucleated cells are believed to respond to IFN-α/β, whereas IFN-λ responses are largely confined to epithelial cells. We observed that intestinal epithelial cells, unlike hematopoietic cells of this organ, express only very low levels of functional IFN-α/β receptors. Accordingly, after oral infection of IFN-α/β receptor-deficient mice, human reovirus type 3 specifically infected cells in the lamina propria but, strikingly, did not productively replicate in gut epithelial cells. By contrast, reovirus replicated almost exclusively in gut epithelial cells of IFN-λ receptor-deficient mice, suggesting that the gut mucosa is equipped with a compartmentalized IFN system in which epithelial cells mainly respond to IFN-λ that they produce after viral infection, whereas other cells of the gut mostly rely on IFN-α/β for antiviral defense. In suckling mice with IFN-λ receptor deficiency, reovirus replicated in the gut epithelium and additionally infected epithelial cells lining the bile ducts, indicating that infants may use IFN-λ for the control of virus infections in various epithelia-rich tissues. Thus, IFN-λ should be regarded as an autonomous virus defense system of the gut mucosa and other epithelial barriers that may have evolved to avoid unnecessarily frequent triggering of the IFN-α/β system which would induce exacerbated inflammation.

Virus-induced interferon consists of two distinct families of molecules, IFN-α/β and IFN-λ. IFN-α/β family members are key antiviral molecules that confer protection against a large number of viruses infecting a wide variety of cell types. By contrast, IFN-λ responses are largely confined to epithelial cells due to highly restricted expression of the cognate receptor. Interestingly, virus resistance of the gut epithelium is not dependent on IFN-α/β but rather relies on IFN-λ, questioning the prevailing view that receptors for IFN-α/β are expressed ubiquitously. Here we demonstrate that the IFN-α/β system is unable to compensate for IFN-λ deficiency during infections with epitheliotropic viruses because intestinal epithelial cells do not express functional receptors for IFN-α/β. We further demonstrate that virus-infected intestinal epithelial cells are potent producers of IFN-λ, indicating that the gut mucosa possesses a compartmentalized IFN system in which epithelial cells predominantly respond to IFN-λ, whereas other cells of the gut mainly rely on IFN-α/β for antiviral defense. We suggest that IFN-λ may have evolved as an autonomous virus defense system of the gut mucosa to avoid unnecessarily frequent triggering of the IFN-α/β system which, due to its potent activity on immune cells, would induce exacerbated inflammation.

Reovirus serotypes elicit distinctive patterns of recall immunity in humans

Mammalian orthoreoviruses (reoviruses) are ubiquitous viral agents that infect cells in respiratory and enteric tracts. The frequency and nature of human cellular immunoregulatory responses against reovirus are unknown. Here we establish systems to detect and quantify reovirus-induced cytokine and chemokine recall responses using primary cultures of virus-infected peripheral blood mononuclear cells (PBMC) and two widely used reovirus serotypes, type 1 Lang (T1L) and type 3 Dearing (T3D) reexposure in vitro. In cultures from 44 healthy adults, reovirus induced exceptionally strong CD4 and CD8 T-cell-dependent gamma interferon (IFN-gamma) recall responses concomitant with intense interleukin 10 (IL-10) production. These responses were elicited independently of viral replication. Surprisingly, paired analyses of subject responses to these two common serotypes revealed that while both elicit intense Th1-dominated immunity, median T3D-driven responses were 2.2-fold weaker (P = 0.0004) than those elicited by T1L. Recall responses evoked by these viral serotypes differed markedly in their mechanism of regulation. T3D IL-10 and IFN-gamma responses were CD4 and CD8 dependent and blocked by interfering with CD86 costimulation but were CD80 independent. T1L responses were consistently CD28 and CD80/86 independent. Thus, despite extensive genetic and morphological similarities between reovirus serotypes, the nature and intensity of the human recall responses as well as the control mechanisms regulating them are clearly distinct.

Viremia and nasal and rectal shedding of rotavirus in gnotobiotic pigs inoculated with Wa human rotavirus

Respiratory symptoms with rotavirus shedding in nasopharyngeal secretions have been reported in children with and without gastrointestinal symptoms (Zheng et al., 1991, J. Med. Virol. 34:29-37). To investigate if attenuated and virulent human rotavirus (HRV) strains cause upper respiratory tract infections or viremia in gnotobiotic pigs, we inoculated them with attenuated or virulent HRV intranasally, intravenously, or orally or via feeding tube (gavage) and assayed virus shedding. After oral or intranasal inoculation with attenuated HRV, the pigs remained asymptomatic, but 79 to 95% shed virus nasally and 5 to 17% shed virus rectally. After inoculation by gavage, no pigs shed virus nasally or rectally, but all pigs seroconverted with antibodies to HRV. No viremia was detected through postinoculation day 10. Controls inoculated intranasally with nonreplicating rotavirus-like particles or mock inoculated did not shed virus. In contrast, 100% of pigs inoculated with virulent HRV (oral, intranasal, or gavage) developed diarrhea, shed virus nasally and rectally, and had viremia. The infectivity of sera from the viremic virulent HRV-inoculated pigs was confirmed by inoculating gnotobiotic pigs orally with pooled HRV-positive serum. Serum-inoculated pigs developed diarrhea and fecal and nasal virus shedding and seroconverted with serum and intestinal HRV antibodies. Pigs inoculated intravenously with serum or intestinal contents from the viremic virulent HRV-inoculated pigs developed diarrhea, virus shedding, and viremia, similar to the orally inoculated pigs. This study provides new evidence that virulent HRV causes transient viremia and upper respiratory tract infection in addition to gastrointestinal infection in gnotobiotic pigs, confirming previous reports of rotavirus antigenemia (Blutt et al., Lancet 362:1445-1449, 2003). Our data also suggest that intestinal infection might be initiated from the basolateral side of the epithelial cells via viremia. Additionally, virus shedding patterns indicate a different pathogenesis for attenuated versus virulent HRV.

Role of interleukin-4 (IL-4) and IL-10 in serum immunoglobulin G antibody responses following mucosal or systemic reovirus infection

Mucosal and parenteral immunizations elicit qualitatively distinct immune responses, and there is evidence that mucosal immunization can skew the balance of T helper 1 and T helper 2 responses. However, a clear picture of the effect of the route of infection on the balance of the T helper responses has not yet emerged. Our laboratory previously demonstrated that oral reovirus infection elicits specific serum immunoglobulin G2a (IgG2a), while parenteral reovirus infection elicits the mixed production of specific serum IgG2a and IgG1 in mice of the H-2(d) haplotype. Knowing that IgG2a production is indicative of a T helper 1 response and IgG1 production is indicative of a T helper 2 response, we hypothesized that the route of infection influences the development of T helper 1 and T helper 2 responses. Using quantitative reverse transcription-PCR, we found that mRNA for the T helper 1 cytokines gamma interferon and interleukin-12 (IL-12) were expressed in draining lymphoid tissues following both oral and parenteral infections. However, we observed that mRNA for the T helper 2 cytokine IL-10 was suppressed in the Peyer's patches and mesenteric lymph nodes and IL-4 mRNA was suppressed in the mesenteric lymph nodes compared to noninfected controls, following oral infection. Using recombinant cytokines and cytokine knockout mice, we confirmed that IL-4 plays a major role in mediating the route-of-infection-dependent differences in serum IgG subclass responses. Therefore, the route of infection needs to be taken into consideration when developing vaccines and adjuvant therapies.

Mucosal T cell response to reovirus

During the last three decades, immunologists and gastroenterologists have witnessed the formation of mucosal immunology as a discipline in biomedical science, and studies of reovirus infection have substantially contributed to this evolution. We have focused on mucosal T cell responses induced by reovirus in conventional, germfree, nude, and NF-kappaB deficient mice. Several major facets of T cell function in the immune responses to this mucosal pathogen have been examined, including viral selection of oligoclonal-T cells, extrathymic T cell development, and distinct signaling pathways used by CD8 sublineages. In addition, our findings with virus-specific T cells selected in the mucosa have suggested novel mechanisms for the rearrangement, selection, and expansion of TCR genes. With the increasing application of molecular tools, reovirus will continue to be a useful model pathogen to study mucosal immunology and will further our understanding of mucosal immunity in health and disease.

Lineage-specific differences among CD8+ T cells in their dependence of NF-kappa B/Rel signaling

Whereas most CD8+ T cells in lymph nodes and spleen express the CD8alpha beta heterodimer and depend absolutely on thymic competence for their development, a substantial population of T cells expressing CD8alpha alpha matures extrathymically. Although the existence of these CD8 sublineages is well established, relatively little is known about differences that might exist among CD8 cells in their requirement for particular transcriptional pathways during the development and maintenance of normal populations. Transgenic mice whose T lineage expresses an IkappaBalpha mutant exhibited decreased NF-kappaB signaling and a diminution in mature CD8 T cells. We now have determined that although TCR-dependent CD69 induction by CD8alpha alpha and CD8alpha beta T cells was unaffected by inhibition of NF-kappaB, TCRalpha beta CD8alpha beta T cells were preferentially reduced compared to their TCRalpha beta CD8alpha alpha or TCRgamma delta counterparts. This finding was most prominent in spleen, but was also apparent in Peyer's patches of transgenic mice. In addition, diminished antiviral cytotoxic responses of CD8alpha beta intraepithelial lymphocytes were observed after enteric reovirus infection. Taken together, these results indicate that NF-kappaB signaling is more important for the thymus-dependent TCRalpha beta CD8alpha beta population than for other CD8 lineages, and thus regulates the number, function, and normal balance of CD8 subsets in the periphery.