Protective and Pathologic Roles of the Immune Response to Mouse Hepatitis Virus Type 1: Implications for Severe Acute Respiratory Syndrome

Intranasal mouse hepatitis virus type 1 (MHV-1) infection of mice induces lung pathology similar to that observed in severe acute respiratory syndrome (SARS) patients. However, the severity of MHV-1-induced pulmonary disease varies among mouse strains, and it has been suggested that differences in the host immune response might account for this variation. It has also been suggested that immunopathology may represent an important clinical feature of SARS. Little is known about the host immune response to MHV-1 and how it might contribute to some of the pathological changes detected in infected mice. In this study we show that an intact type I interferon system and the adaptive immune responses are required for controlling MHV-1 replication and preventing morbidity and mortality in resistant C57BL/6J mice after infection. The NK cell response also helps minimize the severity of illness following MHV-1 infection of C57BL/6J mice. In A/J and C3H/HeJ mice, which are highly susceptible to MHV-1-induced disease, we demonstrate that both CD4 and CD8 T cells contribute to morbidity during primary infection, and memory responses can enhance morbidity and mortality during subsequent reexposure to MHV-1. However, morbidity in A/J and C3H/HeJ mice can be minimized by treating them with immune serum prior to MHV-1 infection. Overall, our findings highlight the role of the host immune response in contributing to the pathogenesis of coronavirus-induced respiratory disease.Severe acute respiratory syndrome (SARS) is caused by a zoonotic coronaviral infection that reached epidemic proportions beginning in late 2002 (37, 52, 55, 76, 84, 86). The etiologic agent, SARS-coronavirus (CoV), is a novel group 2 CoV that emerged in the human population exposed to infected animals that were present in wet markets in various provinces of southern China (16, 22, 35, 45, 57, 61). Although the outbreak was quickly contained by the application of aggressive public health measures, it highlighted the deadly potential of this novel pathogen as more than 8,000 people in more than 25 countries were affected, and almost 800 infected individuals died (37, 76, 84, 86). Although there have not been additional outbreaks of this disease in the general population since 2003, due to the continued presence of related viruses in bats and other animals and to cultural practices prevalent in the local population in southern China, the reemergence of this pathogen in the human population may occur in the future (40).Currently, there are no rigorously tested efficacious prophylactic or therapeutic agents targeting this pathogen. Given the lethal potential of this virus, it is imperative to develop specific antiviral therapies that can be rapidly and universally applied. One of the serious drawbacks in the field is the paucity of appropriate animal models that faithfully reproduce the clinical features of SARS (52, 60). Although a mouse-adapted strain of this virus is available, studies with this strain need to be performed in biosafety level 3 facilities (48, 59). Logistical issues associated with such requirements hamper the rapidity and ease with which one can perform a comprehensive and detailed systemic examination of the dynamics of host-pathogen interactions. Recently, it was reported that intranasal infection of certain strains of mice with a related group 2 respiratory CoV, mouse hepatitis virus type 1 (MHV-1), induced pulmonary disease that was very similar to that observed in human subjects infected with SARS-CoV (11). In addition to the phylogenetic proximity of MHV-1 and SARS-CoV, they also share similarities in genome organization and in mechanisms of replication (63, 68). Hence, it is likely that the pathophysiology observed in MHV-1-infected mice mimics important pathological features associated with SARS-CoV infection in humans. A dysregulated immune response characterized by aberrant cytokine production is postulated to contribute to clinical disease in patients with SARS (8, 26, 55, 58, 72, 75, 82, 83). MHV-1 infection of susceptible strains of mice is also associated with an altered cytokine profile, and published reports suggest that the host immune response to the virus is an important contributor to the pathology observed in susceptible strains of mice (11). Examination of the immune response to a pathogen is critical for the purpose of designing rational and effective vaccination approaches. In addition, it also helps identify potentially deleterious effects of the immune response that can subsequently be manipulated to the advantage of the host, thereby maximizing recovery and minimizing morbidity.In the present study we have carried out a comprehensive analysis of the immune response to MHV-1 following intranasal infection of both resistant and susceptible strains of inbred mice. Our observations in alpha/beta interferon (type I IFN) receptor-knockout (IFN-αβR-KO) mice and NK cell-depleted mice shed light on the protective role of these components of the innate immune response in resistant C57BL/6J (B6) mice. And our examination of the adaptive immune responses to MHV-1 shows that they function as a double-edged sword, mediating protection in resistant strains and contributing to pathology in susceptible strains of mice.     

Toll-Like Receptor 4 Deficiency Increases Disease and Mortality after Mouse Hepatitis Virus Type 1 Infection of Susceptible C3H Mice

Severe acute respiratory syndrome (SARS) is characterized by substantial acute pulmonary inflammation with a high mortality rate. Despite the identification of SARS coronavirus (SARS-CoV) as the etiologic agent of SARS, a thorough understanding of the underlying disease pathogenesis has been hampered by the lack of a suitable animal model that recapitulates the human disease. Intranasal (i.n.) infection of A/J mice with the CoV mouse hepatitis virus strain 1 (MHV-1) induces an acute respiratory disease with a high lethality rate that shares several pathological similarities with SARS-CoV infection in humans. In this study, we examined virus replication and the character of pulmonary inflammation induced by MHV-1 infection in susceptible (A/J, C3H/HeJ, and BALB/c) and resistant (C57BL/6) strains of mice. Virus replication and distribution did not correlate with the relative susceptibilities of A/J, BALB/c, C3H/HeJ, and C57BL/6 mice. In order to further define the role of the host genetic background in influencing susceptibility to MHV-1-induced disease, we examined 14 different inbred mouse strains. BALB.B and BALB/c mice exhibited MHV-1-induced weight loss, whereas all other strains of H-2^b and H-2^d mice did not show any signs of disease following MHV-1 infection. H-2^k mice demonstrated moderate susceptibility, with C3H/HeJ mice exhibiting the most severe disease. C3H/HeJ mice harbor a natural mutation in the gene that encodes Toll-like receptor 4 (TLR4) that disrupts TLR4 signaling. C3H/HeJ mice exhibit enhanced morbidity and mortality following i.n. MHV-1 infection compared to wild-type C3H/HeN mice. Our results indicate that TLR4 plays an important role in respiratory CoV pathogenesis.Severe acute respiratory syndrome (SARS) is a disease that was initially observed in 2002 and led to approximately 8,000 affected individuals in multiple countries with over 700 deaths (1, 24, 47, 48). The causative agent of SARS was subsequently identified as a novel coronavirus (CoV) termed SARS-CoV (8, 17, 22, 27, 32, 37). Although SARS-CoV infections following the initial outbreak in 2002 and 2003 have been limited primarily to laboratory personnel, the identification of an animal reservoir for the virus raises concern about the potential for future outbreaks (25).The pathogenesis of SARS has been difficult to study, in part because no animal model is able to fully recapitulate the morbidity and mortality observed in infected humans (35). Infection of a number of inbred mouse strains, including BALB/c, C57BL/6, and 129S, with primary human isolates of SARS-CoV results in the replication of the virus within the lung tissue without the subsequent development of readily apparent clinical disease (11, 16, 41). Infection of aged BALB/c mice results in clinically apparent disease that more closely mimics some aspects of SARS in humans (36). However, immune responses in aged mice are known to be altered (5, 15), and thus, the mechanisms that control the induction of disease may differ between adult and aged mice. Recent work has demonstrated that serial passage of SARS-CoV in mice results in a mouse adaptation that leads to more profound replication of the virus in the lung (28, 34). However, the time to death from this mouse-adapted SARS-CoV is 3 to 5 days, which is much more rapid than the time to mortality observed in fatal cases of SARS in humans.Phylogenetic analysis has revealed that SARS-CoV is most closely related to group 2 CoVs, which include the mouse hepatitis virus (MHV) family (39). Thus, information gathered by infection of mice with closely related members of the group 2 CoVs may further contribute to our understanding of SARS-CoV pathogenesis in humans. While many strains of MHV induce primarily hepatic and central nervous system diseases (6, 7, 12, 18, 21, 23, 40), a recent study demonstrated that intranasal (i.n.) infection of A/J mice with MHV type 1 (MHV-1) induces pulmonary injury that shares several pathological characteristics with SARS-CoV infection of humans (2, 3, 9, 29, 43).In the current study, we examined the relationship between MHV-1 replication in the lungs and the severity of disease in four inbred strains of mice: A/J, BALB/c, C57BL/6, and C3H/HeJ. Our results demonstrate that MHV-1 replicates to similar levels in the lung in each of these inbred strains of mice regardless of their relative levels of susceptibility, as measured by weight loss and clinical illness. Both A/J and C3H/HeJ mice exhibited enhanced weight loss and clinical illness following i.n. MHV-1 infection compared to BALB/c and C57BL/6 mice. Analysis of many different inbred mouse strains confirmed A/J and C3H/HeJ mice as the most susceptible to i.n. MHV-1 infection. Interestingly, C3H/HeJ mice harboring a natural mutation in the gene that encodes Toll-like receptor 4 (TLR4) that disrupts its normal function exhibited greatly increased morbidity and mortality after i.n. MHV-1 infection compared to wild-type C3H/HeN mice. Our results indicate that TLR4 plays an important role in respiratory CoV pathogenesis.     

Open Reading Frame 33 of a Gammaherpesvirus Encodes a Tegument Protein Essential for Virion Morphogenesis and Egress

Tegument is a unique structure of herpesvirus, which surrounds the capsid and interacts with the envelope. Morphogenesis of gammaherpesvirus is poorly understood due to lack of efficient lytic replication for Epstein-Barr virus and Kaposi''s sarcoma-associated herpesvirus/human herpesvirus 8, which are etiologically associated with several types of human malignancies. Murine gammaherpesvirus 68 (MHV-68) is genetically related to the human gammaherpesviruses and presents an excellent model for studying de novo lytic replication of gammaherpesviruses. MHV-68 open reading frame 33 (ORF33) is conserved among Alpha-, Beta-, and Gammaherpesvirinae subfamilies. However, the specific role of ORF33 in gammaherpesvirus replication has not yet been characterized. We describe here that ORF33 is a true late gene and encodes a tegument protein. By constructing an ORF33-null MHV-68 mutant, we demonstrated that ORF33 is not required for viral DNA replication, early and late gene expression, viral DNA packaging or capsid assembly but is required for virion morphogenesis and egress. Although the ORF33-null virus was deficient in release of infectious virions, partially tegumented capsids produced by the ORF33-null mutant accumulated in the cytoplasm, containing conserved capsid proteins, ORF52 tegument protein, but virtually no ORF45 tegument protein and the 65-kDa glycoprotein B. Finally, we found that the defect of ORF33-null MHV-68 could be rescued by providing ORF33 in trans or in an ORF33-null revertant virus. Taken together, our results indicate that ORF33 is a tegument protein required for viral lytic replication and functions in virion morphogenesis and egress.Gammaherpesviruses are associated with tumorigenesis. Like other herpesviruses, they are characterized as having two distinct stages in their life cycle: lytic replication and latency (15, 16, 18, 21, 54). Latency provides the viruses with advantages to escape host immune surveillance and to establish lifelong persistent infection and contributes to transformation and development of malignancies. However, it is through lytic replication that viruses propagate and transmit among hosts to maintain viral reservoirs. Both viral latency and lytic replication play important roles in tumorigenesis. The gammaherpesvirus subfamily includes Epstein-Barr virus (EBV), Kaposi''s sarcoma-associated herpesvirus (KSHV)/human herpesvirus 8 and murine gammaherpesvirus 68 (MHV-68), among others. EBV is associated with Burkitt''s lymphoma, nasopharyngeal carcinoma, Hodgkin''s disease, and lymphoproliferative diseases in immunodeficient patients (28). KSHV is etiologically linked with Kaposi''s sarcoma, primary effusion lymphoma, and multicentric Castleman''s disease (11-13, 22, 52). Neither in vivo nor in vitro studies of EBV and KSHV are convenient due to their propensity to establish latency in cell culture and their limited host ranges.MHV-68 is genetically related to these two human gammaherpesviruses, especially to KSHV, based on the alignment of their genomic sequences and other biological properties (55). As a natural pathogen of wild rodents, MHV-68 also infects laboratory mice (6, 40, 46) and replicates to a high titer in a variety of fibroblast and epithelial cell lines. These advantages make MHV-68 an excellent model for studying the lytic replication of gammaherpesviruses in vitro and certain aspects of virus-host interactions in vivo. In addition, the MHV-68 genome has been cloned as a bacterial artificial chromosome (BAC) that can propagate in Escherichia coli (1, 2, 36, 51), making it convenient to study the function of each open reading frame (ORF) by genetic methods. Exploring the functions of MHV-68 ORFs will likely shed light on the functions of their homologues in human gammaherpesviruses.Gammaherpesviral particles have a characteristic multilayered architecture. An infectious virion contains a double-stranded DNA genome, an icosahedral capsid shell, a thick, proteinaceous tegument compartment, and a lipid bilayer envelope spiked with glycoproteins (14, 30, 47, 49). As a unique structure of herpesviruses, the tegument plays important roles in multiple aspects of the viral life cycle, including virion assembly and egress (38, 48, 53), translocation of nucleocapsids into the nucleus, transactivation of viral immediate-early genes, and modulation of host cell gene expression, innate immunity, and signal transduction (9, 10, 23, 60). Some components of MHV-68 tegument have been identified by a mass spectrometric study (8), and the functions of some tegument proteins have been revealed, such as ORF45, ORF52, and ORF75c (7, 24, 29).MHV-68 ORF33 is conserved among Alpha-, Beta-, and Gammaherpesvirinae subfamilies. Its homologues include human herpes simplex virus type 1 (HSV-1) UL16, human herpes simplex virus type 2 (HSV-2) UL16, human cytomegalovirus (HCMV) UL94, EBV BGLF2, KSHV ORF33, and rhesus monkey rhadinovirus (RRV) ORF33. HSV-1 UL16 has been identified as a tegument protein and may function in viral DNA packaging, virion assembly, budding, and egress (5, 32, 35, 41, 44). HCMV UL94 is a virion associated protein and might function in virion assembly and budding (31, 57). EBV BGLF2, KSHV ORF33, and RRV ORF33 are also virion-associated proteins, but their functions are not clear (26, 43, 59). The mass spectrometric study of MHV-68 did not identify ORF33 as a virion component (8), although ORF33 is found to be essential for viral lytic replication by transposon mutagenesis of the MHV-68 genome cloned as a BAC (51). However, insertion of the 1.2-kbp Mu transposon in that study may influence the expression of ORFs approximate to ORF33. Consequently, the role ORF33 plays in viral replication needs to be confirmed, preferably through site-directed mutagenesis. Whether ORF33 is a tegument protein and the exact viral replication stage in which it functions also need to be investigated.We determined that MHV-68 ORF33 encodes a tegument protein and is expressed with true late kinetics. To explore the function of ORF33 in viral lytic phase, we used site-directed mutagenesis and generated an ORF33-null mutant, taking advantage of the MHV-68 BAC system. We showed that the ORF33-null mutant is capable of viral DNA replication, early and late gene expression, capsid assembly, and DNA packaging, but incapable of virion release. The defect of ORF33-null mutant can be rescued in trans by an ORF33 expression plasmid.     

CD4 T-Cell Help Programs a Change in CD8 T-Cell Function Enabling Effective Long-Term Control of Murine Gammaherpesvirus 68: Role of PD-1-PD-L1 Interactions

We previously showed that agonistic antibodies to CD40 could substitute for CD4 T-cell help and prevent reactivation of murine gammaherpesvirus 68 (MHV-68) in the lungs of major histocompatibility complex (MHC) class II^−/− (CII^−/−) mice, which are CD4 T cell deficient. Although CD8 T cells were required for this effect, no change in their activity was detected in vitro. A key question was whether anti-CD40 treatment (or CD4 T-cell help) changed the function of CD8 T cells or another cell type in vivo. To address this question, in the present study, we showed that adoptive transfer of CD8 T cells from virus-infected wild-type mice or anti-CD40-treated CII^−/− mice caused a significant reduction in lung viral titers, in contrast to those from control CII^−/− mice. Anti-CD40 treatment also greatly prolonged survival of infected CII^−/− mice. This confirms that costimulatory signals cause a change in CD8 T cells enabling them to maintain effective long-term control of MHV-68. We investigated the nature of this change and found that expression of the inhibitory receptor PD-1 was significantly increased on CD8 T cells in the lungs of MHV-68-infected CII^−/−, CD40^−/−, or CD80/86^−/− mice, compared with that in wild-type or CD28/CTLA4^−/− mice, correlating with the level of viral reactivation. Furthermore, blocking PD-1-PD-L1 interactions significantly reduced viral reactivation in CD4 T-cell-deficient mice. In contrast, the absence of another inhibitory receptor, NKG2A, had no effect. These data suggest that CD4 T-cell help programs a change in CD8 T-cell function mediated by altered PD-1 expression, which enables effective long-term control of MHV-68.Murine gammaherpesvirus 68 (MHV-68) is a naturally occurring rodent pathogen which is closely related to Epstein-Barr virus (EBV) and Kaposi''s sarcoma-associated herpesvirus (KSHV) (17, 64). Intranasal administration of MHV-68 to mice results in acute productive infection of lung epithelial cells and a latent infection in various cell types, including B lymphocytes, dendritic cells, epithelial cells, and macrophages (18, 19, 52, 53, 61, 65). The virus induces an inflammatory infiltrate in the lungs, lymph node enlargement, splenomegaly, and mononucleosis comprising increased numbers of activated CD8 T cells in the blood (53, 58). It has also been reported to induce lymphoproliferative disease/lymphoma in immunocompromised mice (30, 55, 60). Thus, the pathogenesis resembles that of EBV in humans, although structurally, the virus is more closely related to KSHV.Infectious MHV-68 is cleared from the lungs by a T-cell-dependent mechanism 10 to 15 days after infection (18, 53, 56). In wild-type mice, the lungs remain clear of replicating virus thereafter. Although CD4 T cells are not essential for primary clearance of replicating virus, they are required for effective long-term control (11). Thus, major histocompatibility complex (MHC) class II^−/− mice that lack CD4 T cells or mice rendered CD4 deficient by antibody treatment initially clear infectious virus from the lungs. However, infectious virus reactivates in the lungs 10 to 15 days later and gradually increases in titer (11, 43). The infected CD4-deficient mice eventually die, apparently from long-term lung damage due to continuing lytic viral replication (11). MHC class II^−/− mice do not produce antibody to T-dependent antigens (10). Cytotoxic T-lymphocyte (CTL) epitopes have been identified in open reading frame (ORF) 6 (p56, H-2D^b-restricted), and ORF 61 (p79, H-2K^b-restricted) gene products, which appear to encode early lytic-phase proteins (32, 49). The epitopes are presented during two distinct phases during MHV-68 infection, which changes the pattern of CTL dominance (32, 51). However, there is no significant difference in the numbers of CD8 T cells specific for each epitope in wild-type mice and CD4 T-cell-deficient mice (4, 50). In addition, CTL activity measured in vitro does not differ substantially in the lungs of wild-type mice or CD4 T-cell-deficient mice (4, 11, 50). Furthermore, postexposure vaccination with the p56 epitope failed to prevent viral reactivation in class II^−/− mice, despite dramatically expanding the number of CD8 T cells specific for the peptide (5). In contrast, vaccination of wild-type mice against these epitopes reduced lytic viral titers in the lung dramatically on subsequent challenge with MHV-68. B-cell-deficient mice clear MHV-68 with the kinetics of wild-type mice and do not show viral reactivation in the lungs (13, 61), suggesting that antibody is not essential for control of the virus. Depletion of CD4 T cells during the latent phase of infection in B-cell-deficient mice does not induce viral reactivation, whereas depletion of both CD4 and CD8 T-cell subsets provokes viral reactivation in the lungs (52). Short-term depletion of both CD4 and CD8 T-cell subsets during the latent phase of infection in wild-type mice does not lead to viral reactivation probably due to the presence of neutralizing antibody (11). Taken together, these results suggest that CD4 and CD8 T cells and B cells play overlapping roles in preventing or controlling reactivation of MHV-68 during the latent phase of infection. However, the B-cell- and CD8 T-cell-mediated control mechanisms do not develop in the absence of CD4 T cells.We, and others, have previously shown that the costimulatory molecule CD28 is not required for long-term control of MHV-68 (28, 29). However, interestingly, mice lacking both of the ligands for CD28, CD80 and CD86, show viral reactivation in the lung (21, 35). Our previously published data showed that agonistic antibodies to CD40 could substitute for CD4 T-cell function in the long-term control of MHV-68 (46). CD8 T-cell receptor-positive (TCR+) cells were required for this effect, while antibody production was not restored (45, 46). MHV-68-infected CD40L^−/− mice (7) and CD40^−/− mice (29) also showed viral reactivation in the lungs. However, no change in CD8 CTL activity was detected in in vitro assays following anti-CD40 treatment (46). A key question was whether anti-CD40 treatment (or CD4 T-cell help) caused a direct change in CD8 T-cell function or whether both CD8 T cells and an independent anti-CD40-sensitive step were required for viral control. To address this question, we used adoptive transfer of CD8 T cells from MHV-68-infected wild-type mice, anti-CD40-treated mice, or control MHC class II^−/− mice to MHV-68-infected class II^−/− recipients. We also investigated whether anti-CD40 treatment prolonged survival in addition to reducing lung viral titers. The heterodimeric molecule CD94/NKG2A has been implicated in negatively regulating the CD8 T-cell response to polyomavirus (38) and herpes simplex virus (HSV) (54), while the inhibitory receptor PD-1 (programmed death 1) has been implicated in T-cell exhaustion following infection with several other persistent viruses (2, 15, 20, 22, 26, 36, 39-41, 57, 67). In the present study, we investigated the effect of signaling via various costimulatory molecules on the expression of NKG2A and PD-1 and how these molecules influenced viral control.     

The Spike Protein of Murine Coronavirus Regulates Viral Genome Transport from the Cell Surface to the Endoplasmic Reticulum during Infection

We observed that the nonfusogenic mouse hepatitis virus (MHV) strain MHV-2 reached a titer of ∼2 log₁₀ higher than that of the fusogenic strain A59 in astrocytoma DBT cells. To determine whether the spike protein is responsible for the difference, a recombinant virus, Penn-98-1, that contains the A59 genome with a spike from MHV-2 was used to infect DBT cells. Results showed that Penn-98-1 behaved like MHV-2, thus establishing a role for the spike protein in viral growth. The inverse correlation between viral fusogenicity and growth was further established in four different cell types and with a fusogenic mutant, the S757R mutant, derived from isogenic Penn-98-1. While both A59 and Penn-98-1 entered cells at similar levels, viral RNA and protein syntheses were significantly delayed for A59. Interestingly, when the genomic RNAs were delivered directly into the cells via transfection, the levels of gene expression for these viruses were similar. Furthermore, cell fractionation experiments revealed that significantly more genomic RNAs for the nonfusogenic MHVs were detected in the endoplasmic reticulum (ER) within the first 2 h after infection than for the fusogenic MHVs. Pretreatment of Penn-98-1 with trypsin reversed its properties in syncytium formation, virus production, and genome transport to the ER. These findings identified a novel role for the spike protein in regulating the uncoating and delivery of the viral genome to the ER after internalization.Murine coronavirus mouse hepatitis virus (MHV) is a member of the family Coronaviridae. It is an enveloped, positive-strand-RNA virus. The viral envelope contains three or four structural proteins, depending on the virus strain (21). The spike (S) protein is a glycoprotein with a molecular mass of approximately 180 kDa. For some MHV strains, such as JHM and A59, the S protein is cleaved by a furin-like proteinase into two subunits, the amino-terminal S1 and the carboxyl-terminal S2. The S1 subunit is thought to form the globular head of the spike and is responsible for the initial attachment of the virus to the receptor on the cell surface. The S2 subunit, which forms the stalk portion of the spike and which anchors the S protein to the viral envelope, facilitates the fusion between the viral envelope and the cell membrane and cell-cell fusion (4, 7, 20, 25, 39). In contrast, the S protein of some other MHV strains, such as MHV-2, does not undergo cleavage and usually does not cause cell-cell fusion (15, 34). It appears that the cleavability of the MHV S protein is associated usually, though not always, with its fusogenicity (10, 36). It has been suggested that the fusogenicity of the S protein may determine the route of virus entry, i.e., via direct fusion with plasma membranes or following endocytosis (11, 34), although the mechanism for virus-induced cell-cell fusion may differ from that for virus-cell fusion during entry (8). The S protein also elicits the induction of neutralizing antibodies and cell-mediated immunity in infected hosts (3). It is therefore an important determinant for viral infectivity, pathogenicity, and virulence (2, 5, 31, 38). The hemagglutinin-esterase (HE) protein is present only in certain MHV strains (22, 42) and may play a role in viral pathogenesis (44, 45). The small envelope (E) protein and the membrane (M) protein play a key role in virus assembly (40). The nucleocapsid (N) protein is a phosphoprotein of approximately 50 kDa and is associated with the RNA genome to form the nucleocapsid inside the envelope (21, 37).Infection of host cells by MHV is mediated through the interaction between the S protein and the cellular receptors that are members of the carcinoembryonic antigen (CEA) family of the immunoglobulin superfamily (9). This interaction then triggers fusion between the viral envelope and the plasma membrane or the endosomal membrane, the latter of which follows receptor-mediated endocytosis, thus allowing the nucleocapsid to deliver into the cytoplasm. Direct entry from the plasma membrane appears to be the predominant route for most MHV strains (19, 28), although entry by some mutant MHVs, such as OBLV60 and MHV-2, is low pH dependent, i.e., via endocytosis (11, 34). However, nothing is known about how the genomic RNA is transported to the rough endoplasmic reticulum (ER) for translation. Once on the ER, the viral genomic RNA is translated into a polymerase polyprotein from the 5′-end two open reading frames (two-thirds of the genome) via ribosomal frameshifting. The polymerase polyproteins in turn synthesize genomic and multiple species of subgenomic mRNAs. These mRNAs are then translated into nonstructural and structural proteins, the latter of which are essential for generation of progeny viruses.MHV can infect rodents, causing hepatitis, enteritis, nephritis, and central nervous system diseases. In the mouse central nervous system, some MHV strains, such as JHM and A59, are neurovirulent, causing acute encephalitis and chronic demyelination (1, 13), while others, such as MHV-2, exhibit extremely low neurovirulence, causing only meningitis without apparent encephalitis and demyelination (6, 16, 41). Extensive mutagenesis studies in combination with targeted RNA recombination have identified that the S protein is the major determinant of MHV pathogenicity in animals, although other viral genes also appear to modulate viral pathogenicity (17, 32). For example, the recombinant MHV Penn-98-1, which contains the S protein of MHV-2 in an A59 genome background, causes acute meningoencephalitis similar to that caused by A59 but does not cause demyelination similar to that observed for MHV-2 (6). It has also been shown that the amounts of antigen staining and necrosis in the liver correlate with the viral titer, which is determined largely by the S protein (29). However, how the S protein affects viral titer in cell culture and in animals is not known.In the present study, we initially observed that the levels of production of infectious viruses in an astrocytoma DBT cell line were markedly different among three MHV strains. Using the recombinant MHV Penn-98-1 and its isogenic S757R mutant, we further established that the S protein is responsible for the observed difference. The difference in virus production between A59 and Penn-98-1 was detected as early as 4 to 6 h postinfection (p.i.) and likely occurred during the early stages of the virus life cycle but after virus internalization. Interestingly, when the genomic RNAs were delivered directly into the cells via transfection, the levels of gene expression for these viruses were similar. Furthermore, cell fractionation experiments revealed that significantly more genomic RNAs for nonfusogenic MHVs were delivered to the ER within the first 2 h after infection than for fusogenic MHVs. These results demonstrate that the spike protein of MHV can regulate the intracellular transport of the viral genome to the ER following internalization. To our knowledge, this is the first study identifying a role for a coronavirus S protein in genome delivery in addition to its well-established role in receptor binding and virus-cell and cell-cell fusions during infection.     

Cultivation of Fastidious Bacteria by Viability Staining and Micromanipulation in a Soil Substrate Membrane System

Soil substrate membrane systems allow for microcultivation of fastidious soil bacteria as mixed microbial communities. We isolated established microcolonies from these membranes by using fluorescence viability staining and micromanipulation. This approach facilitated the recovery of diverse, novel isolates, including the recalcitrant bacterium Leifsonia xyli, a plant pathogen that has never been isolated outside the host.The majority of bacterial species have never been recovered in the laboratory (1, 14, 19, 24). In the last decade, novel cultivation approaches have successfully been used to recover “unculturables” from a diverse range of divisions (23, 25, 29). Most strategies have targeted marine environments (4, 23, 25, 32), but soil offers the potential for the investigation of vast numbers of undescribed species (20, 29). Rapid advances have been made toward culturing soil bacteria by reformulating and diluting traditional media, extending incubation times, and using alternative gelling agents (8, 21, 29).The soil substrate membrane system (SSMS) is a diffusion chamber approach that uses extracts from the soil of interest as the growth substrate, thereby mimicking the environment under investigation (12). The SSMS enriches for slow-growing oligophiles, a proportion of which are subsequently capable of growing on complex media (23, 25, 27, 30, 32). However, the SSMS results in mixed microbial communities, with the consequent difficulty in isolation of individual microcolonies for further characterization (10).Micromanipulation has been widely used for the isolation of specific cell morphotypes for downstream applications in molecular diagnostics or proteomics (5, 15). This simple technology offers the opportunity to select established microcolonies of a specific morphotype from the SSMS when combined with fluorescence visualization (3, 11). Here, we have combined the SSMS, fluorescence viability staining, and advanced micromanipulation for targeted isolation of viable, microcolony-forming soil bacteria.     

Proteasome Activator REGγ Enhances Coxsackieviral Infection by Facilitating p53 Degradation

Coxsackievirus B3 (CVB3) is a small RNA virus associated with diseases such as myocarditis, meningitis, and pancreatitis. We have previously demonstrated that proteasome inhibition reduces CVB3 replication and attenuates virus-induced myocarditis. However, the underlying mechanisms by which the ubiquitin/proteasome system regulates CVB replication remain unclear. In this study, we investigated the role of REGγ, a member of the 11S proteasome activator, in CVB3 replication. We showed that overexpression of REGγ promoted CVB3 replication but that knockdown of REGγ led to reduced CVB3 replication. We further demonstrated that REGγ-mediated p53 proteolysis contributes, as least in part, to the proviral function of REGγ. Although total protein levels of REGγ remained unaltered after CVB3 infection, virus infection induced a redistribution of REGγ from the nucleus to the cytoplasm, rendering an opportunity for a direct interaction of REGγ with viral proteins and/or host proteins (e.g., p53), which controls viral growth and thereby enhances viral infectivity. Further analyses suggested a potential modification of REGγ by SUMO following CVB3 infection, which was verified by both in vitro and in vivo sumoylation assays. Sumoylation of REGγ may play a role in its nuclear export during CVB3 infection. Taken together, our results present the first evidence that the host REGγ pathway is utilized and modified during CVB3 infection to promote efficient viral replication.Viruses often adapt to the existing host cellular machinery to complete their own life cycle. The ubiquitin/proteasome system (UPS), a primary intracellular protein degradation system in eukaryotic cells, has emerged as a key modulator in viral infectivity and virus-mediated pathogenesis (6).Coxsackievirus B3 (CVB3) is a small RNA virus associated with diseases such as myocarditis, meningitis, and pancreatitis (36). We have previously studied the function and regulation of the UPS in CVB3 infection and CVB3-induced myocarditis (7, 16, 17, 33). We demonstrated that CVB3 utilizes and manipulates the host UPS to achieve successful replication (17, 33). We provided evidence that proteasome inhibition reduces CVB3 replication and attenuates virus-induced myocarditis (7). However, we recognize the potential toxicity of general inhibition of proteasome function as a therapeutic means. Further investigation to identify specific targets within the UPS utilized during CVB3 infection is urgently needed and will allow for more-precise targeting in drug therapy.The 20S proteasome is a multisubunit protease complex responsible for the degradation of misfolded proteins or short-lived regulatory proteins (16, 18). In the absence of proteasome activators, the 20S proteasome is latent and the protein substrates are barred from entering the 20S proteasome (16, 18). There are at least two families of proteasome activators, the 19S proteasome (also known as PA700) and the 11S proteasome (also known as REG or PA28) (16, 18). The 19S activator binds to proteasome to form the 26S proteasome, which primarily performs degradation of proteins in a ubiquitin-dependent manner.The REG activator binds to and activates the proteasome in an ATP-independent manner to promote mainly ubiquitin-independent protein degradation. Three classes of REG have been identified, REGα, REGβ, and REGγ. REGα/β forms a heteroheptamer which is mainly localized to the cytosol (16, 18). The level of REGα/β is inducible by gamma interferon, and the main function of REGα/β has been implicated in major histocompatibility complex (MHC) class I antigen presentation (16, 18). REGγ exists in a homoheptamer and is primarily found in the nucleus (16, 18). Although the functional significance of REGγ has not been fully defined, studies of REGγ-deficient mice reveal a role for REGγ in the regulation of cell cycle progression and cell survival/apoptosis (1, 27). These effects appear to be related to REGγ-mediated degradation of several important intracellular proteins, such as cyclin-dependent kinase inhibitors p21, p16, and p19 (2, 14) and tumor suppressor p53 (43). Moreover, an interaction between the REGγ system and the viral proteins has recently been reported. It was shown that REGγ binds to and regulates the stability and nuclear retention of hepatitis C core protein (26), contributing to hepatitis C core protein-induced insulin resistance and hepatocarcinoma (24, 25).We have previously reported that gene silencing of ubiquitin reduces viral protein synthesis and viral titers (33). However, such inhibitions are not as potent as by proteasome inhibition, suggesting that 11S proteasome-mediated proteasomal degradation may also play a role. In the present study, we seek to further understand the underlying mechanisms by which the UPS regulates CVB3 replication by investigating the interplay between REGγ and CVB3 infection and exploring the potential mechanisms of how REGγ controls CVB3 replication. Here, we provided the first evidence that the host REGγ pathway was utilized and modulated during CVB3 infection to promote efficient viral replication.     

A New Borrelia Species Defined by Multilocus Sequence Analysis of Housekeeping Genes

Analysis of Lyme borreliosis (LB) spirochetes, using a novel multilocus sequence analysis scheme, revealed that OspA serotype 4 strains (a rodent-associated ecotype) of Borrelia garinii were sufficiently genetically distinct from bird-associated B. garinii strains to deserve species status. We suggest that OspA serotype 4 strains be raised to species status and named Borrelia bavariensis sp. nov. The rooted phylogenetic trees provide novel insights into the evolutionary history of LB spirochetes.Multilocus sequence typing (MLST) and multilocus sequence analysis (MLSA) have been shown to be powerful and pragmatic molecular methods for typing large numbers of microbial strains for population genetics studies, delineation of species, and assignment of strains to defined bacterial species (4, 13, 27, 40, 44). To date, MLST/MLSA schemes have been applied only to a few vector-borne microbial populations (1, 6, 30, 37, 40, 41, 47).Lyme borreliosis (LB) spirochetes comprise a diverse group of zoonotic bacteria which are transmitted among vertebrate hosts by ixodid (hard) ticks. The most common agents of human LB are Borrelia burgdorferi (sensu stricto), Borrelia afzelii, Borrelia garinii, Borrelia lusitaniae, and Borrelia spielmanii (7, 8, 12, 35). To date, 15 species have been named within the group of LB spirochetes (6, 31, 32, 37, 38, 41). While several of these LB species have been delineated using whole DNA-DNA hybridization (3, 20, 33), most ecological or epidemiological studies have been using single loci (5, 9-11, 29, 34, 36, 38, 42, 51, 53). Although some of these loci have been convenient for species assignment of strains or to address particular epidemiological questions, they may be unsuitable to resolve evolutionary relationships among LB species, because it is not possible to define any outgroup. For example, both the 5S-23S intergenic spacer (5S-23S IGS) and the gene encoding the outer surface protein A (ospA) are present only in LB spirochete genomes (36, 43). The advantage of using appropriate housekeeping genes of LB group spirochetes is that phylogenetic trees can be rooted with sequences of relapsing fever spirochetes. This renders the data amenable to detailed evolutionary studies of LB spirochetes.LB group spirochetes differ remarkably in their patterns and levels of host association, which are likely to affect their population structures (22, 24, 46, 48). Of the three main Eurasian Borrelia species, B. afzelii is adapted to rodents, whereas B. valaisiana and most strains of B. garinii are maintained by birds (12, 15, 16, 23, 26, 45). However, B. garinii OspA serotype 4 strains in Europe have been shown to be transmitted by rodents (17, 18) and, therefore, constitute a distinct ecotype within B. garinii. These strains have also been associated with high pathogenicity in humans, and their finer-scale geographical distribution seems highly focal (10, 34, 52, 53).In this study, we analyzed the intra- and interspecific phylogenetic relationships of B. burgdorferi, B. afzelii, B. garinii, B. valaisiana, B. lusitaniae, B. bissettii, and B. spielmanii by means of a novel MLSA scheme based on chromosomal housekeeping genes (30, 48).     

Virus Entry via the Alternative Coreceptors CCR3 and FPRL1 Differs by Human Immunodeficiency Virus Type 1 Subtype

Human immunodeficiency virus type 1 (HIV-1) infects target cells by binding to CD4 and a chemokine receptor, most commonly CCR5. CXCR4 is a frequent alternative coreceptor (CoR) in subtype B and D HIV-1 infection, but the importance of many other alternative CoRs remains elusive. We have analyzed HIV-1 envelope (Env) proteins from 66 individuals infected with the major subtypes of HIV-1 to determine if virus entry into highly permissive NP-2 cell lines expressing most known alternative CoRs differed by HIV-1 subtype. We also performed linear regression analysis to determine if virus entry via the major CoR CCR5 correlated with use of any alternative CoR and if this correlation differed by subtype. Virus pseudotyped with subtype B Env showed robust entry via CCR3 that was highly correlated with CCR5 entry efficiency. By contrast, viruses pseudotyped with subtype A and C Env proteins were able to use the recently described alternative CoR FPRL1 more efficiently than CCR3, and use of FPRL1 was correlated with CCR5 entry. Subtype D Env was unable to use either CCR3 or FPRL1 efficiently, a unique pattern of alternative CoR use. These results suggest that each subtype of circulating HIV-1 may be subject to somewhat different selective pressures for Env-mediated entry into target cells and suggest that CCR3 may be used as a surrogate CoR by subtype B while FPRL1 may be used as a surrogate CoR by subtypes A and C. These data may provide insight into development of resistance to CCR5-targeted entry inhibitors and alternative entry pathways for each HIV-1 subtype.Human immunodeficiency virus type 1 (HIV-1) infects target cells by binding first to CD4 and then to a coreceptor (CoR), of which C-C chemokine receptor 5 (CCR5) is the most common (6, 53). CXCR4 is an additional CoR for up to 50% of subtype B and D HIV-1 isolates at very late stages of disease (4, 7, 28, 35). Many other seven-membrane-spanning G-protein-coupled receptors (GPCRs) have been identified as alternative CoRs when expressed on various target cell lines in vitro, including CCR1 (76, 79), CCR2b (24), CCR3 (3, 5, 17, 32, 60), CCR8 (18, 34, 38), GPR1 (27, 65), GPR15/BOB (22), CXCR5 (39), CXCR6/Bonzo/STRL33/TYMSTR (9, 22, 25, 45, 46), APJ (26), CMKLR1/ChemR23 (49, 62), FPLR1 (67, 68), RDC1 (66), and D6 (55). HIV-2 and simian immunodeficiency virus SIVmac isolates more frequently show expanded use of these alternative CoRs than HIV-1 isolates (12, 30, 51, 74), and evidence that alternative CoRs other than CXCR4 mediate infection of primary target cells by HIV-1 isolates is sparse (18, 30, 53, 81). Genetic deficiency in CCR5 expression is highly protective against HIV-1 transmission (21, 36), establishing CCR5 as the primary CoR. The importance of alternative CoRs other than CXCR4 has remained elusive despite many studies (1, 30, 70, 81). Expansion of CoR use from CCR5 to include CXCR4 is frequently associated with the ability to use additional alternative CoRs for viral entry (8, 16, 20, 63, 79) in most but not all studies (29, 33, 40, 77, 78). This finding suggests that the sequence changes in HIV-1 env required for use of CXCR4 as an additional or alternative CoR (14, 15, 31, 37, 41, 57) are likely to increase the potential to use other alternative CoRs.We have used the highly permissive NP-2/CD4 human glioma cell line developed by Soda et al. (69) to classify virus entry via the alternative CoRs CCR1, CCR3, CCR8, GPR1, CXCR6, APJ, CMKLR1/ChemR23, FPRL1, and CXCR4. Full-length molecular clones of 66 env genes from most prevalent HIV-1 subtypes were used to generate infectious virus pseudotypes expressing a luciferase reporter construct (19, 57). Two types of analysis were performed: the level of virus entry mediated by each alternative CoR and linear regression of entry mediated by CCR5 versus all other alternative CoRs. We thus were able to identify patterns of alternative CoR use that were subtype specific and to determine if use of any alternative CoR was correlated or independent of CCR5-mediated entry. The results obtained have implications for the evolution of env function, and the analyses revealed important differences between subtype B Env function and all other HIV-1 subtypes.     

Murine Cytomegalovirus US22 Protein pM140 Protects Its Binding Partner,pM141, from Proteasome-Dependent but Ubiquitin-Independent Degradation

Stable assembly of murine cytomegalovirus (MCMV) virions in differentiated macrophages is dependent upon the expression of US22 family gene M140. The M140 protein (pM140) exists in complex with products of neighboring US22 genes. Here we report that pM140 protects its binding partner, pM141, from ubiquitin-independent proteasomal degradation. Protection is conferred by a stabilization domain mapping to amino acids 306 to 380 within pM140, and this domain is functionally independent from the region that confers binding of pM140 to pM141. The M140 protein thus contains multiple domains that collectively confer a structure necessary to function in virion assembly in macrophages.Murine cytomegalovirus (MCMV) US22 family genes M36, M139, M140, and M141 promote efficient replication of the virus in macrophages (1, 8, 12, 17). The M139, M140, and M141 genes are clustered within the MCMV genome and appear to function cooperatively (10, 12). During infection, the protein M140 (pM140) forms a stable complex with pM141, and one or more larger complexes are formed by the addition of M139 gene products (15). Although these complexes are evident in infected fibroblasts as well as macrophages, they are required for optimal MCMV replication selectively in macrophages (1, 17). In the absence of M140, virion assembly in macrophages is defective, likely due to the reduced levels of the major capsid protein and tegument protein M25 (11). pM140 also confers stability to its binding partner, pM141; in the absence of the M140 gene, the half-life of pM141 is reduced from 2 h to 1 h (12). Deletion of M141 compromises virus replication in macrophages (12), and pM141 directs pM140 to a perinuclear region of infected macrophages adjacent to an enlarged microtubule organizing center with characteristics of an aggresome (11, 15). Aggresomes are sites where proteins are targeted for degradation by either the proteasome or autophagy (3, 6, 19). We therefore hypothesized that complexing of pM141 to pM140 rescues pM141 from degradation by the proteasome and/or autophagy.