Activation of measles virus from silently infected human lymphocytes

Inflammatory responses to infection: the Dutch contribution

At any given moment, our body is under attack by a large variety of pathogens, which aim to enter and use our body to propagate and disseminate. The extensive cellular and molecular complexity of our immune system enables us to efficiently eliminate invading pathogens or at least develop a condition in which propagation of the microorganism is reduced to a minimum. Yet, the evolutionary pressure on pathogens to circumvent our immune defense mechanisms is immense, which continuously leads to the development of novel pathogenic strains that challenge the health of mankind. Understanding this battle between pathogen and the immune system has been a fruitful area of immunological research over the last century and will continue to do so for many years. In this review, which has been written on the occasion of the 50th anniversary of the Dutch Society for Immunology, we provide an overview of the major contributions that Dutch immunologists and infection biologists have made in the last decades on the inflammatory response to viral, bacterial, fungal or parasitic infections. We focus on those studies that have addressed both the host and the pathogen, as these are most interesting from an immunological point of view. Although it is not possible to completely cover this comprehensive research field, this review does provide an interesting overview of Dutch research on inflammatory responses to infection.

Mitogen-induced upregulation of hepatitis C virus expression in human lymphoid cells

Considering growing evidence indicating that hepatitis C virus (HCV) replicates in lymphoid cells, establishment of a reliable and sensitive method for detection of HCV in these cells may provide means for monitoring the infection and the efficacy of sterilizing antiviral therapy. In this study, conditions for ex vivo augmentation and detection of the HCV genome in peripheral blood mononuclear cells (PBMCs) from patients with chronic hepatitis C (CHC) or after a sustained virological response (SVR) to antiviral treatment were assessed. Following stimulation with combinations of mitogens and/or cytokines, PBMCs and, in certain cases, affinity-purified T and B cells were examined for HCV positive- and negative-strand RNA by using RT-PCR followed by nucleic acid hybridization, while the presence of viral NS3 protein was determined by flow cytometry. HCV RNA augmentation was assessed by quantification of Southern and dot-blot hybridization signals. The results showed that treatment of peripheral lymphoid cells with mitogens stimulating T- and B-cell proliferation and with cytokines supporting their growth significantly increased HCV RNA detection in patients with both CHC and SVR. This enhancement was up to 100-fold for the HCV genome and fivefold for the NS3 protein compared with untreated cells. In conclusion, HCV RNA can be readily detected in circulating lymphoid cells in progressing hepatitis C and following SVR after ex vivo cell stimulation. As such, this method offers a new investigative tool to study HCV lymphotropism and to monitor virus presence during the course of HCV infection.

Measles virus infection causes transient depletion of activated T cells from peripheral circulation

BACKGROUND

Natural measles virus infection as well as vaccination with attenuated measles virus induce temporary immunosuppression, which is responsible for part of the morbidity and mortality associated with measles. The underlying molecular mechanisms are not known. Recently, in vitro studies have revealed a marked increase of LFA-1 expression of lymphocytes in the presence of infectious measles virus.

OBJECTIVES

In order to further investigate immune dysfunction in measles we analyzed the expression of leukocyte function-associated antigen 1 (LFA-1) on ex vivo derived circulating human T cells.

STUDY DESIGN

Expression of LFA-1 was measured by flow cytometry using monoclonal antibodies directed against CD11a and CD18. LFA-1 expression was followed in the course of infection in four adult seronegative vaccinees and in four patients with natural measles.

RESULTS

There was a remarkable loss of LFA-1-bright cells during natural measles and after measles vaccination. The number of LFA-1-bright cells reached a minimum on day 7-14 after vaccination, and at the onset of rash during natural measles infection, and approached normal levels within 3-5 weeks.

CONCLUSION

It is suggested that measles virus infection interferes with lymphocyte trafficking and reallocation. Disruption of recirculation and random homing of lymphocytes might contribute to the immunosuppression, which is characteristic for measles virus infection.

Rabies virus infects mouse and human lymphocytes and induces apoptosis

Attenuated and highly neurovirulent rabies virus strains have distinct cellular tropisms. Highly neurovirulent strains such as the challenge virus standard (CVS) are highly neurotropic, whereas the attenuated strain ERA also infects nonneuronal cells. We report that both rabies virus strains infect activated murine lymphocytes and the human lymphoblastoid Jurkat T-cell line in vitro. The lymphocytes are more permissive to the attenuated ERA rabies virus strain than to the CVS strain in both cases. We also report that in contrast to that of the CVS strain, ERA viral replication induces apoptosis of infected Jurkat T cells, and cell death is concomitant with viral glycoprotein expression, suggesting that this protein has a role in the induction of apoptosis. Our data indicate that (i) rabies virus infects lymphocytes, (ii) lymphocyte infection with the attenuated rabies virus strain causes apoptosis, and (iii) apoptosis does not hinder rabies virus production. In contrast to CVS, ERA rabies virus and other attenuated rabies virus vaccines stimulate a strong immune response and are efficient live vaccines. The paradoxical finding that a rabies virus triggers a strong immune response despite the fact that it infects lymphocytes and induces apoptosis is discussed in terms of the function of apoptosis in the immune response.

Measles virus infects human dendritic cells and blocks their allostimulatory properties for CD4+ T cells

Measles causes a profound immune suppression which is responsible for the high morbidity and mortality induced by secondary infections. Dendritic cells (DC) are professional antigen-presenting cells required for initiation of primary immune responses. To determine whether infection of DC by measles virus (MV) may play a role in virus-induced suppression of cell-mediated immunity, we examined the ability of CD1a+ DC derived from cord blood CD34+ progenitors and Langerhans cells isolated from human epidermis to support MV replication. Here we show that both cultured CD1a+ DC and epidermal Langerhans cells can be infected in vitro by both vaccine and wild type strains of MV. DC infection with MV resulted within 24-48 h in cell-cell fusion, cell surface expression of hemagglutinin, and virus budding associated with production of infectious virus. MV infection of DC completely abrogated the ability of the cells to stimulate the proliferation of naive allogeneic CD4+ T cell as early as day 2 of mixed leukocyte reaction (MLR) (i.e., on day 4 of DC infection). Mannose receptor-mediated endocytosis and viability studies indicated that the loss of DC stimulatory function could not be attributed to the death or apoptosis of DC. This total loss of DC stimulatory function required viral replication in the DC since ultraviolet (UV)-inactivated MV or UV-treated supernatant from MV-infected DC did not alter the allostimulatory capacity of DC. As few as 10 MV- infected DC could block the stimulatory function of 10(4) uninfected DC. More importantly, MV-infected DC, in which production of infectious virus was blocked by UV treatment or paraformaldehyde fixation, actively suppressed allogeneic MLR upon transfer to uninfected DC-T-cultures. Thus, the mechanisms which contribute to the loss of the allostimulatory function of DC include both virus release and active suppression mediated by MV-infected DC, independent of virus production. These data suggest that carriage of MV by DC may facilitate virus spreading to secondary lymphoid organs and that MV replication in DC may play a central role in the general immune suppression observed during measles.

Replication of measles virus in human monocytes and T cells

Replication of measles virus (MV) in populations of peripheral blood mononuclear cells enriched for T cells and monocytes was studied using a temperature-sensitive mutant, MV ts38, and the parent counterpart, MV Lec. Stimulation of the cells was required for a full cycle of virus replication in both cell types. More infectious virus was released after stimulation from MV-infected populations enriched for T cells, T cell-enriched than from monocyte-enriched populations. However, similar amounts of viral mRNA, genomic RNA, and viral proteins of the expected size were found in both cell populations. The results indicate that MV-specific macromolecular synthesis is similar in both T cells and monocytes, but the assembly and (or) release of infectious virus is greatly reduced in monocytes as compared with T cells.

Measles virus-induced down-regulation of CD46 is associated with enhanced sensitivity to complement-mediated lysis of infected cells

CD46, the major component of the measles virus (MV) receptor complex and a member of the regulators of complement activity (RCA) gene cluster, is down-regulated in MV-infected cells. We investigated whether the reduction of surface CD46 correlates with enhanced sensitivity of lymphoid and monocytic cells to lysis by activated complement. On human U937 cells, acutely or persistently infected with MV-Edmonston (ED) vaccine strain, infection-dependent down-regulation of CD46 confers sensitivity to activated complement, regardless of the pathway of activation and the specificity of the activating antibodies. Interestingly, down-regulation of CD46 alone is sufficient to confer susceptibility of cells to complement lysis despite the continued surface expression of other RCA proteins such as CD35 and CD55. In primary cultures, both peripheral blood lymphocytes and macrophages are efficiently lysed in the presence of complement activated via the alternative pathway after MV infection. In contrast to the MV-ED infection, infection of cells with the lymphotropic MV wild-type strain WTF does not down-regulate CD46. Cells infected with MV-WTF do not exhibit enhanced susceptibility to complement lysis. These data suggest that MV strains similar to WTF that do not down-regulate CD46 may have an enhanced potential for replication and dissemination within the human host, whereas complement-mediated elimination of cells infected with CD46-down-regulating strains of MV, such as ED, may limit the spread of MV infection, and could thus represent an attenuating factor for MV.

Cell proteins bind to sites within the 3' noncoding region and the positive-strand leader sequence of measles virus RNA

The genomic 3' noncoding region (NCR) of nonsegmented negative-strand RNA viruses contains recognition site(s) for the polymerase complex, while the RNA plus-strand leader sequence (LS) is probably involved in RNA encapsidation. It is known that host-encoded factors play a role in transcription and replication of some of this group of viruses. Here we report that cellular proteins interact with the genomic 3' NCR and with the plus-strand LS RNA of an important human pathogen, measles virus (MV), a member of the family Paramyxoviridae. Using gel retardation assay and RNA footprinting analysis, we demonstrated that in Vero cells, host-encoded proteins bind specifically to domains within these two sequences. A polypeptide of about 20 kDa binding to the 3' NCR and two polypeptides of about 22 and 30 kDa interacting with plus-strand LS were detected by RNA-protein UV cross-linking. Different RNA-binding activities were found in cells differing in permissiveness to MV replication. The results suggest a role for host-encoded proteins in MV replication.

Measles virus inhibits mitogen-induced T cell proliferation but does not directly perturb the T cell activation process inside the cell

Measles virus (MV) inhibits lymphocyte function in patients, as well as in cells infected in vitro. The proliferation of phytohemagglutinin-stimulated T lymphocytes is suppressed by in vitro MV infection, as shown by the diminished incorporation of [3H]thymidine into DNA and the reduced frequency of cells in the S phase of the cell cycle, as compared with mock-infected cells. MV infection itself, however, does not completely block DNA synthesis in infected cells, because infected T cells expressing MV antigens on the cell surface, isolated by fluorescence-activated cell sorter, could still proliferate. Northern blot analysis indicated that the expression of genes induced during T cell activation, such as those encoding interleukin 2 (IL-2), c-myc, IL-2 receptor, IL-6, c-myb, and cdc-2, was not significantly suppressed in MV-infected cells, suggesting that MV does not interfere with the T cell activation process. When anti-MV serum or carbobenzoxy-D-Phe-L-Phe-Gly, a synthetic oligopeptide known to inhibit MV-induced fusion, was added 24 hr after infection, the inhibition of T cell proliferation was reversed in a dose-dependent manner. From these results we propose a model for the inhibition of T cell proliferation by MV; MV glycoproteins expressed on the cell surface of infected cells interact with the MV receptor or other molecules on the cell membrane of adjacent T cells, which in turn affects the proliferation of those T cells.

Effect of interferon-alpha on measles virus replication in human peripheral blood mononuclear cells

We analyzed the effect of exogenous human leukocyte interferon (IFN)-alpha on measles virus (MV) replication in human peripheral blood mononuclear cells (PBMC). The release of infectious virus was progressively reduced by increasing concentrations of IFN-alpha, and blocked with an IFN-alpha concentration of 1000 U/ml. In order to detect a possible target of this inhibitory effect, viral transcription and translation events were analyzed. The synthesis of MV mRNAs was reduced, but not blocked, in the presence of IFN-alpha. However, this effect was not specific on the viral RNAs, but due to a general inhibition of RNA synthesis in IFN-treated PBMC. The expression of viral polypeptides was also inhibited in a dose-dependent manner by exogenous IFN-alpha, but a low level of protein synthesis was detected by both Western blotting and immunofluorescence techniques, even with the maximum amount of IFN-alpha used (1000 U/ml). These findings account for a partial maintenance of the viral replicative cycle, even when the production of infectious virus is blocked. Moreover, the effect of IFN-alpha is not specifically targeted on the virus macromolecular synthesis.

Responses of mice to murine coronavirus immunization

Oral and/or intranasal inoculation of susceptible mouse genotypes with the JHM strain of mouse hepatitis virus (MHV-JHM) consistently results in T cell dysfunction as reflected by in vitro proliferative responses to mitogens or allogeneic cells. One approach to examining the mechanism responsible for the observed functional T cell suppression is to determine whether virus replication is required for its induction. To this end, mice were inoculated oronasally with MHV-JHM that was inactivated with short-wave ultraviolet light, beta-propiolactone or psoralen. Mice were also inoculated with live MHV-JHM after recovery from homotypic or heterotypic MHV infection. Spleen cells from BALB mice inoculated oronasally with inactivated MHV-JHM yielded extremely variable in vitro proliferative responses after concanavalin A stimulation. MHV-susceptible mice exposed oronasally or intraperitoneally to virus inactivated by any of the minimum effective treatments failed to seroconvert. Immunization with psoralen-treated virus intraperitoneally in Freund's complete adjuvant or oronasally failed to protect from live virus challenge, but survivors had elevated virus-specific serum IgG antibody titers compared to mock-immunized controls at two weeks post-challenge. Spleen cells from mice that were challenged after recovery from homotypic live virus infection did not exhibit the profound in vitro T cell suppression normally observed during the acute stage of primary infection. In contrast, MHV-JHM challenge of mice vaccinated with heterotypic live MHV-S resulted in significantly depressed in vitro T cell function. The combined data suggest that either virus replication or exposure to more concentrated antigen may be required for induction of the dramatic T cell dysfunction that occurs as a consequence of MHV-JHM infection as well as for a detectable MHV-specific humoral response.

Expression of measles virus RNA in peripheral blood mononuclear cells of patients with measles, SSPE, and autoimmune diseases

In order to characterize measles virus (MV) infection in peripheral blood mononuclear cells (PBMCs), RNA was isolated from PBMCs after PHA-stimulation for 72 hr of 9 patients with acute measles, 16 patients with subacute sclerosing panencephalitis (SSPE), 13 patients with various autoimmune diseases, and 16 healthy control donors. The RNA obtained was screened for the presence of MV N (nucleocapsid) gene specific transcripts of either positive or negative orientation in a S1 nuclease protection assay. The sensitivity of this assay allowed us to detect one infected cell in 20,000 PBMCs or 0.1 to 0.05 copies of MV-specific RNA per cell. Using single-stranded DNA or RNA probes expression of MV genomic RNA of negative polarity could be detected in only one case of acute measles and one healthy control donor. Conversely, N-specific transcripts of positive polarity, indicating active transcription, could only be detected in patients with acute measles. In addition, in infected PBMCs and in a persistently MV-infected B cell line positive stranded N-specific transcripts containing leader usually present at very low frequency have been found in relatively increased amounts in comparison with transcripts lacking leader. Whereas the ratio of these RNA species during lytic infection with MV in Vero cells is about 1:50, the ratio found here ranges from 1:3 to 1:10. This altered ratio indicates a specific regulation of MV specific transcription in cells of lymphoid origin that has not been found in any other cell system analyzed.

Restriction of measles virus gene expression in acute and subacute encephalitis of Lewis rats

Measles virus (MV) replication in brain tissue of Lewis rats with acute (AE) and subacute (SAME) encephalitis was characterized by biochemical techniques. Messenger RNAs specific for measles virus nucleocapsid (N), phospho (P)-, matrix (M), fusion (F), and haemagglutinin (H) protein were detected in all brain extracts examined. The quantity of the individual MV mRNA species was quite different in comparison to lytically infected Vero cells. A steep gradient of MV transcripts was found in brain tissue which is most likely due to strongly attenuated transcription of mRNAs along the viral genome, representing particularly low transcription of the glycoprotein genes. In addition, in vitro translation assays only revealed synthesis of N and P protein in consistent fashion. The mRNAs for the glycoproteins did not direct the synthesis of detectable viral proteins whereas the M mRNA revealed some activity in animals with AE. The data indicate a strong restriction of the MV envelope gene expression in infected brain tissue, which is independent of the incubation time and type of the central nervous system (CNS) disease. This phenomenon which is similar to the findings observed in measles inclusion body encephalitis and subacute sclerosing panencephalitis suggest that host factors may initially be responsible for the initiation of transcriptional and translational alterations.

Infection of human peripheral blood mononuclear cells with a temperature-sensitive mutant of measles virus

A stable temperature-sensitive mutant of measles virus (MV ts38) was used to study the mechanism of virus-mediated immune suppression of peripheral blood mononuclear cells in vitro. Both unstimulated and phytohemagglutinin-stimulated cultures released infectious virus at 32 degrees C, whereas no virus was released at 37 degrees C, although both viral RNA and viral proteins were synthesized. However, the response of the lymphoid cells to phytohemagglutinin, concanavalin A, and herpes simplex virus antigen was decreased in the presence of MV ts38 at 37 degrees C. The viability of infected cells was not diminished, therefore excluding cell death as a reason for immunosuppression. Interleukin 2 did not play a role in the inhibitory effect of MV ts38. Antibodies to alpha interferon partially reversed the inhibitory effect of the virus infection on lymphocyte mitogenesis, thus implying that alpha interferon plays a role in the immunosuppression. Depletion experiments indicated that adherent cells play a greater role in the measles virus-induced immunosuppression than nonadherent cells. However, monocyte maturation to macrophages had no effect on the degree of immunosuppression.

Measles virus infection of unstimulated blood mononuclear cells in vitro: antigen expression and virus production preferentially in monocytes

Synthesis of measles virus antigens occurred only in a small percentage of peripheral blood mononuclear cells infected in vitro with measles virus without mitogenic stimulation. The infection cycle was restricted as only low amounts of infectious virus were released but all the major structural viral proteins were present as shown by immunofluorescence with monoclonal antibodies. Cells with viral antigen synthesis were characterized by double labelling and by infecting selectively depleted subpopulations. In 3-day cultures, up to 80% of the cells with measles virus antigen were shown to be monocytes by specific staining with anti-MMA and anti-Leu M3 monoclonal antibodies and up to 40% of the monocytes were infected. Less than 10% of the cells expressing virus antigens carried the lymphocyte marker OKT3, the majority of these cells belonging to the Leu3a (helper) population. Anti-alpha-interferon treatment increased the number of measles-positive cells and the release of infectious virus in preparations enriched for monocytes, but had no significant effect on infection of lymphocytes.

Growth of malignant rabbit fibroma virus in lymphoid cells

To understand better the immunosuppressive capacity of malignant rabbit fibroma virus (MV), we characterized MV growth in lymphoid cells. Replication of MV occurs in unstimulated normal spleen cells in vitro and is enhanced by adding T- or B-lymphocyte mitogens. In splenic T-lymphocyte preparations, comparable results are found: virus growth in the absence of mitogen, augmented by adding Con A. Unlike mature T cells, thymic lymphocytes support MV replication only when mitogen is added. When spleen cells from rabbits infected with MV in vivo are removed and cultured without mitogen, MV growth is again observed, with virus titer increasing about 10-fold per day of culture. In spleen cell populations from MV tumor-bearing rabbits, MV grows best in T lymphocytes, moderately in B lymphocytes, and least efficiently in adherent cells. When spleen cells are examined immediately following sacrifice, MV antigens are expressed solely on T lymphocytes from rabbits infected in vivo with MV 7 days previously. However, following overnight incubation in vitro a population of non-T lymphocytes displays cell membrane virus antigens. MV adapts itself somewhat to growth in lymphocytes, showing significantly greater growth in lymphocytes following passage in lymphocytes than is observed for non-lymphocyte-propagated virus. MV-infected lymphocytes also elaborate a factor that enhances MV growth in lymphocytes. Thus, MV replicates preferentially in mature T lymphocytes but will grow well in B cells as well. In vivo infection produces relatively small amounts of recoverable virus. However, when these lymphocytes are cultured in vitro virus replicates very well without added mitogens. These growth patterns may help to understand MV-induced immunologic dysfunction.

The persistence of infectious pancreatic necrosis virus in Atlantic salmon

Atlantic salmon leucocytes from Infectious Pancreatic Necrosis Virus (IPNV) carriers showed a suppressed response to phytohemagglutinin stimulation compared with uninfected controls. A significant degree of inhibition of DNA synthesis was observed using 3H-thymidine incorporation. IPNV was isolated from 41% of the stimulated leucocyte cultures supernatants, while only 6% of the unstimulated cultures were found to be positive.

Detection of measles virus RNA in lymphocytes from peripheral-blood and brain perivascular infiltrates of patients with subacute sclerosing panencephalitis

To clarify the relation between lymphocytes and measles virus in subacute sclerosing panencephalitis, we used in situ hybridization and a cloned measles virus DNA probe, specific for nucleocapsid protein, to detect measles virus RNA sequences in circulating lymphocytes and brain perivascular cuffs of patients with subacute sclerosing panencephalitis. Seventy to 90 per cent of peripheral mononuclear cells from three such patients were found to contain measles virus RNA sequences. In contrast, only a few infected cells were observed in four seropositive adults (0.1 to 5 per cent) and three age-matched children (10 to 15 per cent) used as controls. In one sample of brain tissue from a patient with subacute sclerosing panencephalitis, viral RNA sequences were also detected in nerve cells and in numerous cells from the perivascular infiltrates. In contrast, no hybridization was observed in brain tissue from a patient with herpetic encephalitis and from a patient with postlymphoma encephalitis. We conclude that measles virus has a strong tropism for lymphocytes and nerve cells in subacute sclerosing panencephalitis and that lymphocytes may be involved in the pathogenesis of the disease.

Replication of measles virus in human lymphocytes

Replication of measles virus was restricted in human peripheral blood mononuclear cells (PBMC). However, in in vitro-infected, unstimulated cells, active synthesis of viral RNA and proteins occurred, while the release of infectious virus could not be detected. Stimulation with PHA caused a productive infection cycle comparable to the lytic infection. Replication of viral RNA was demonstrated in both T and B cells, and in both OKT4+- and OKT8+-depleted T cell subsets. The presence of measles virus RNA was detected in PBMC isolated from measles patients, and the production of the immunoreactive hemagglutinin protein was defective in these cells.

Effect of papaverine treatment on replication of measles virus in human neural and nonneural cells

The replication of measles virus in human neural and nonneural cell lines in terms of growth and cytopathic effect was affected by treatment of the cells with papaverine, which increases endogenous cyclic AMP. Suppression of virus growth was most prominent in neuroblastoma cells, followed by that in epidermoid carcinoma and glioblastoma cells, whereas the suppressive effect was relatively weak in oligodendroglioma cells. The papaverine-induced suppression of virus growth in neuroblastoma cells was studied in detail. The suppression that occurred was dependent on the dose of papaverine and was reversible. By treatment with 10 microM papaverine, virus-cell interactions were modified as follows: (i) early replication steps such as adsorption, penetration, and uncoating of the virus were not affected; (ii) synthesis of viral RNAs, including genomic RNA and mRNA, was inhibited; (iii) translation of viral proteins from mRNA was not blocked; and (iv) glycosylation and transport of viral glycoproteins to the cell membrane were not inhibited, but phosphorylation was blocked. The significance of suppressed virus replication in neural cells is discussed in relation to the persistence mechanisms of measles virus in the central nervous system.

The lymphoproliferative response to measles virus in twins with multiple sclerosis

The cellular immune response to measles virus, as measured by lymphocyte proliferation in normal individuals, is considerably lower than that to mumps or vaccinia viruses, and stable multiple sclerosis patients do not differ significantly from the norm. The response to these viruses was studied in 28 twin sets both concordant and discordant for multiple sclerosis. Normal responses to mumps and vaccinia viruses occurred throughout. Seven affected twins manifested a persistently elevated response to measles virus, whereas the unaffected twins had a (normal) low response. The differences were unrelated to differences in T cell subsets, unusual kinetics of the response, or differential susceptibility of lymphocytes to the effects of measles virus infection in vitro. The specificity of the response resides in an E+ subpopulation, and the addition of low-responder E+ cells to high-responder E+ cells failed to identify an active low-responder suppressor population. These findings suggest the presence of clonally expanded measles-specific T cell populations in the high responders with multiple sclerosis.

Response of human lymphocytes to measles virus after natural infection

The lymphoproliferative response to measles, mumps, and vaccinia virus-infected monolayers measured in seropositive adults by thymidine incorporation demonstrated that only 5% of individuals responded well to measles virus (stimulation index, greater than 5). Possible explanations for this occurrence include a lack of sensitization, active suppression, or failure in long-term stimulation. To distinguish among these possibilities, we studied the responses to measles virus in 22 immunocompetent individuals during early convalescence from natural measles infection. Substantial responses occurred (stimulation index, 7.03), particularly in a smaller group which included those individuals with milder cases of the disease. The level of responsiveness declined over a period of weeks. Responder and nonresponder cell mixing showed no active cellular suppression. These studies indicate that the low responses to measles virus found in late convalescence represent a lack of prolonged stimulation of the cell population measured in this assay.

Enhanced yields of measles virus from cultured cells

Conditions for increasing the yield of measles virus from cultured cells have been investigated. Important factors for high virus yields were found to be cell type, incubation at 33 degrees C, concentration of serum in the medium, use of virus as inoculum within five passages of plaque purification, and repeated harvesting of the virus-containing medium at 12 h intervals after extensive virus-induced cell fusion had occurred. Under these conditions, Vero cells infected with the Edmonston strain of measles virus yielded a total of 2.98 X 10(9) plaque-forming units (p.f.u.) of released virus, which is equivalent to 270 p.f.u./cell. A similar amount of cell-associated virus was produced.

Characterization of measles virus-specific antibodies in sera from patients with chronic active hepatitis

Measles virus-specific antibodies in sera from patients with HBsAg-negative chronic active hepatitis and raised antibody titres against measles virus, have been examined by crossed immunoelectrophoresis. The immunoprecipitates were further analysed by SDS-polyacrylamide gel electrophoresis. Five measles virus-specific precipitation lines were demonstrated using measles virus-infected cells solubilized with Triton X-100. The three major precipitation lines were analysed by SDS-PAGE and contained the virus polypeptides: nucleoprotein, NP (MW approximately 60 000); haemoagglutinin, H (MW approximately 80 000) and fusion protein F1 (MW approximately 40 000). Considerably higher amounts of antibodies against these three virus polypeptides were demonstrated in the patient sera than in sera from healthy controls. By SDS-PAGE analysis of radiolabelled immune complexes adsorbed to Sepharose-protein A, antibodies against five measles virus polypeptides: NP, H, F1, P protein (MW approximately 70 000) and matrix protein, M (MW approximately 37 000) were demonstrated in the patient sera.

Reversible repression and activation of measles virus infection in neural cells

Conversion of acute measles virus infection to an indolent state has been achieved by treatment of infected cells of neural origin with agents that affect cyclic nucleotide metabolism. Striking results were obtained with papaverine, an inhibitor of cAMP phosphodiesterase that is capable of enhancing neural differentiation. In papaverine-treated cultures, decreased production of infectious virus was accompanied by selective disappearance of intracellular matrix proton, as detected by immunofluorescence. Viral nucleocapsid protein was enhanced in the cytoplasm while three other structural proteins--polymerase, hemagglutinin, and fusion protein--showed little change in distribution or intensity of staining. These results were specific for cells of neural origin and not observed in CV-1 or Vero cultures. cAMP, dibutyryl cAMP, 8-bromo-cAMP, and isobutylmethylxanthine all inhibited replication of virus but less so than did papaverine. Inhibition of virus replication by any of these agents was rapidly reversible, either by removal of the agent or by addition of cGMP to the culture medium and was accompanied by reappearance of the matrix protein. These results suggest that measles virus replication in neural cells depends on host factors, particularly those affecting endogenous cAMP and cGMP. Viral persistence may thus be related to the state of neural differentiation. This model system may yield information on mechanisms of recrudescence observed in some chronic diseases of the nervous system.

Mumps virus replication in human lymphoid cell lines and in peripheral blood lymphocytes: preference for T cells

The replication of mumps virus was studied in human continuous lymphoblastoid cell lines (LCLs) with T or B characteristics and in lymphocyte subpopulations derived from peripheral blood. T-LCLs supported effective virus replication as shown by high titers of free and cell-associated virus over 1 to 4 days after infection. By immunofluorescence analysis, the majority of cells were positive for mumps virus antigens. In contrast, the B-cell lines produced low titers of infectious virus, and only a small percentage expressed viral antigens. This resistance of the B-LCLs was found with several mumps virus strains. Cultures of peripheral blood mononuclear cells also supported mumps virus replication. Very high titers of infectious virus (10(8) PFU/ml) were observed in cultures prestimulated with phytohaemagglutinin. Studies with enriched T and B cells point to the activated T lymphocyte as the major virus-producing cell.

Selective immunosuppressive effects of measles virus infection

The antibody response to diphtheria toxoid by cultured tonsil cells was suppressed by measles virus, which productively infected a small percentage of both T and B lymphocytes. However, infection of cultures spontaneously secreting antibody and immunoglobulin (with Herpes simplex virus) or measles virus did not result in immunosuppression. Immunosuppression by measles virus may thus be attributed to a selective effect on the inductive phase of the response and suggest that infection of immunoglobulin-producing B lymphocytes does not alter the ability of these cells to secrete antibody.

Failure to cleave measles virus fusion protein in lymphoid cells

The host-directed cleavage of measles virus fusion protein on infected lymphoid cells was studied to understand the mechanism of viral persistence in lymphoid cells in vivo. Several lymphoblastoid cell lines were infected with measles virus, and the viral glycoproteins expressed on the cell's surface were radiolabeled and analyzed for cleavage of fusion (F(0)) to F(1) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Daudi and Ramos lymphoblastoid cells were deficient in their ability to cleave measles virus fusion protein and correspondingly produced low titers of infectious measles virus, Daudi cells being more defective than Ramos cells. In contrast, other lymphoblastoid cells studied, Victor, Raji, Wi-L2, RPMI 8866, and Seraphine, cleaved the fusion polypeptide and made significantly more infectious virus. Despite their defect in cleaving F protein, Daudi cells were able to assemble and release (noninfectious) measles virus particles into the fluid phase. The deficit in Daudi cells was corrected by fusing infected Daudi cells with cleavage-competent cells such as Victor or Raji. Furthermore, the cleavage event performed by competent cells could be mimicked at the plasma membrane by treating infected Daudi cells with trypsin, implicating the role of a plasma membrane enzyme in cleaving F(0) to F(1) during measles virus infection. Hence, lymphoid cells deficient in the plasma membrane enzyme required to cleave F protein are permissive for measles virus, maintain viral gene products, produce mostly noninfectious virus, and fail to place the biologic activity F(1) protein on their surfaces.

Lymphoid cell infection by measles virus in newborn hamsters. Role for monocytes in virus spread to distant sites

Following intraperitoneal inoculation of measles (HBS) virus into newborn hamsters widespread but variable productive lymphoid tissue infection was detected by a sensitive viral isolation technique. Peritoneal wash cells and spleen were the most common sites of infection. Virus was frequently isolated from the bone marrow and was commonly found in multiple lymph nodes. Thymic infection was only rarely demonstrated. A mononuclear cell associated viremia was demonstrated by Ficollhypaque fractionation of peripheral blood from animals receiving both high- and low-dose virus. Infection was present in both plastic adherent and non-adherent fractions of the blood mononuclear cells. The infected cell population in the spleen had both nylon wool and plastic adherent characteristics and infection was therefore thought to be macrophage-associated. The possible relevance of such macrophage associated infection is discussed with regard to the pathogenesis of natural measles infection and the immunosuppression observed in measles-infected hosts.