Killing of measles virus-infected cells by human cytotoxic T cells

The Role of Nucleoprotein in Immunity to Human Negative-Stranded RNA Viruses—Not Just Another Brick in the Viral Nucleocapsid

Negative-stranded RNA viruses (NSVs) are important human pathogens, including emerging and reemerging viruses that cause respiratory, hemorrhagic and other severe illnesses. Vaccine design traditionally relies on the viral surface glycoproteins. However, surface glycoproteins rarely elicit effective long-term immunity due to high variability. Therefore, an alternative approach is to include conserved structural proteins such as nucleoprotein (NP). NP is engaged in myriad processes in the viral life cycle: coating and protection of viral RNA, regulation of transcription/replication processes and induction of immunosuppression of the host. A broad heterosubtypic T-cellular protection was ascribed very early to this protein. In contrast, the understanding of the humoral immunity to NP is very limited in spite of the high titer of non-neutralizing NP-specific antibodies raised upon natural infection or immunization. In this review, the data with important implications for the understanding of the role of NP in the immune response to human NSVs are revisited. Major implications of the elicited T-cell immune responses to NP are evaluated, and the possible multiple mechanisms of the neglected humoral response to NP are discussed. The intention of this review is to remind that NP is a very promising target for the development of future vaccines.

Induction of CD4 T cell proliferation and in vitro Th1-like cytokine responses to measles virus

Mechanisms that lead to induction of life-long immunity to measles virus (MV) are poorly understood. In the present study, we have assessed the activation, proliferation and cytokine secreting function of peripheral blood T cells from MV immune individuals. Expression of cell blastogenesis markers, such as increased forward light scatter and CD38 expression, peaked 5-7 days after infection of peripheral blood mononuclear cells (PBMC) with the live attenuated Edmonston strain of MV. Subset analysis revealed that both CD3- and CD3+ cells expressed activation markers but that the CD3+ T cells predominated late in the culture period corresponding to maximal proliferation and cell recovery. The majority of CD3+ T cells consisted of CD4+CD8- cells. IFN-gamma and IL-4 production similarly showed optimal production late in culture. Depletion of CD4 cells prior to culture and MV stimulation completely abrogated both IFN-gamma and IL-4 production, whereas depletion of CD8 cells did not diminish production, suggesting that CD4+CD8- T cells were principally involved in production of these cytokines. Finally, optimal IFN-gamma production was elicited at high MV doses and IL-4 at much lower doses. These results suggest that among MV immune individuals, in vitro responses to measles are dominated by CD4+ T cells that, depending on antigen dose, primarily produce a Th1-like and, to a lesser extent, a Th1/Th2-mixed pattern of cytokine release.

Neutralizing B cell response in measles

Co-evolving mechanisms of immune clearance and of immune suppression are among the hallmarks of measles. B cells are major targets cells of measles virus (MV) infection. Virus interactions with B cells result both in immune suppression and a vigorous antibody response. Although antibodies fully protect against (re)infection, their importance during the disease and in the presence of a potent cellular response is less well understood. Specific serum IgM appears with onset of rash and confirms clinical diagnosis. After isotype switching, IgG1 develops and confers life-long protection. The most abundant antibodies are specific for the nucleoprotein, but neutralizing and protective antibodies are solely directed against the two surface glycoproteins, the hemagglutinin and the fusion protein. Major neutralizing epitopes have been mapped mainly on the hemagglutinin protein with monoclonal antibodies, producing an increasingly comprehensive map of functional domains.

Immune and artificial selection in the haemagglutinin (H) glycoprotein of measles virus

We present a maximum likelihood (ML) analysis of the selection pressures that have shaped the evolution of the large (L) protein and the haemagglutinin (H) glycoprotein of measles virus (MV). A number of amino acid sites that have potentially been subject to adaptive evolution were identified in the H protein using sequences from every known genotype of MV. All but one of these putative positively selected sites reside within the ectodomain of the H protein, where they often show an association with positions of potential B-cell epitopes and sites known to interact with the CD46 receptor. This suggests that MV may be under pressure from the immune system, albeit relatively weakly, to alter sites within epitopes and hence evade the humoral immune response. The positive selection identified at amino acid 546 was shown to correlate with the passage history of MV isolates in Vero cells. We reveal that Vero cell passaging has the potential to introduce an artificial signal of adaptive evolution through selection for changes that increase affinity for the CD46 receptor.

Vaccination against canine distemper virus infection in infant ferrets with and without maternal antibody protection, using recombinant attenuated poxvirus vaccines

Canine distemper virus (CDV) infection of ferrets is clinically and immunologically similar to measles, making this a useful model for the human disease. The model was used to determine if parenteral or mucosal immunization of infant ferrets at 3 and 6 weeks of age with attenuated vaccinia virus (NYVAC) or canarypox virus (ALVAC) vaccine strains expressing the CDV hemagglutinin (H) and fusion (F) protein genes (NYVAC-HF and ALVAC-HF) would induce serum neutralizing antibody and protect against challenge infection at 12 weeks of age. Ferrets without maternal antibody that were vaccinated parenterally with NYVAC-HF (n = 5) or ALVAC-HF (n = 4) developed significant neutralizing titers (log(10) inverse mean titer +/- standard deviation of 2.30 +/- 0.12 and 2.20 +/- 0.34, respectively) by the day of challenge, and all survived with no clinical or virologic evidence of infection. Ferrets without maternal antibody that were vaccinated intranasally (i.n.) developed lower neutralizing titers, with NYVAC-HF producing higher titers at challenge (1.11 +/- 0.57 versus 0.40 +/- 0.37, P = 0.02) and a better survival rate (6/7 versus 0/5, P = 0.008) than ALVAC-HF. Ferrets with maternal antibody that were vaccinated parenterally with NYVAC-HF (n = 7) and ALVAC-HF (n = 7) developed significantly higher antibody titers (1.64 +/- 0. 54 and 1.28 +/- 0.40, respectively) than did ferrets immunized with an attenuated CDV vaccine (0.46 +/- 0.59; n = 7) or the recombinant vectors expressing rabies glycoprotein (RG) (0.19 +/- 0.32; n = 8, P = 7 x 10(-6)). The NYVAC vaccine also protected against weight loss, and both the NYVAC and attenuated CDV vaccines protected against the development of some clinical signs of infection, although survival in each of the three vaccine groups was low (one of seven) and not significantly different from the RG controls (none of eight). Combined i.n.-parenteral immunization of ferrets with maternal antibody using NYVAC-HF (n = 9) produced higher titers (1.63 +/- 0. 25) than did i.n. immunization with NYVAC-HF (0.88 +/- 0.36; n = 9) and ALVAC-HF (0.61 +/- 0.43; n = 9, P = 3 x 10(-7)), and survival was also significantly better in the i.n.-parenteral group (3 of 9) than in the other HF-vaccinated animals (none of 18) or in controls immunized with RG (none of 5) (P = 0.0374). Multiple routes were not tested with the ALVAC vaccine. The results suggest that infant ferrets are less responsive to i.n. vaccination than are older ferrets and raises questions about the appropriateness of this route of immunization in infant ferrets or infants of other species.

Ex vivo analysis of cytotoxic T lymphocytes to measles antigens during infection and after vaccination in Gambian children

The study of cytotoxic T cell responses to measles antigens during infection and after vaccination may provide insight into the immunopathology of the infection. It will also provide a knowledge of the immunity conferred by wild or attenuated virus, which will help in the design of new vaccines. Direct cytotoxic T cell responses, which did not require in vitro restimulation, were measured from peripheral blood by a standard 51Cr-release assay in 35 patients with acute measles, using HLA class I matched allogeneic B cells as targets. 77% showed specific responses to measles fusion protein, 69% to the hemagglutinin, and 50% to the nucleoprotein. These responses, which were related to severity of disease and history of previous vaccination, had waned by 14-24 wk after measles when memory responses to the same antigens could be elicited by restimulation in 71% of the 13 patients tested. A similar pattern followed vaccination: direct cytotoxic responses to fusion and hemagglutinin proteins were shown in 70% of the 20 children tested while 50% responded to the nucleoprotein. These responses, which were mediated by both CD8(+) and CD4(+) cells, faded over 6 wk when memory responses could be restimulated. Thus, a vigorous cytotoxic T lymphocyte response to fusion, hemagglutinin, and nucleoproteins is important in both natural and vaccine-induced immunity to measles.

CTL responses induced by a single immunization with peptide encapsulated in biodegradable microparticles

A synthetic peptide representing a measles virus (MV) cytotoxic T cell epitope (CTL) when encapsulated in poly (D,L-lactide co-glycolide) (PLG) 50:50 microparticles induced a strong CTL response after a single intraperitoneal immunization of mice which was greater than that following administration of the peptide in Freund's complete adjuvant. A 100 micrograms dose of encapsulated peptide was shown to be more effective for CTL priming than 50 and 25 micrograms doses. A vaccine formulation prepared by simply mixing empty 50:50 PLG microparticles with the peptide resulted in the induction of CTL responses comparable to those induced by the encapsulated peptide. Moreover, a CTL response against MV-infected target cells was observed. These findings highlight the potential immunostimulatory effect of PLG microparticles for the induction of MV and peptide-specific CTL responses.

Induction of measles virus-specific cytotoxic T-cell responses after intranasal immunization with synthetic peptides

We have investigated the structural requirements for the induction of cytotoxic T-cell responses (CTL) in vivo after intranasal immunization with an immunodominant CTL epitope from the nucleoprotein of measles virus (MV). For the induction of CTL responses, covalent linkage of the CTL epitope to a helper T-cell epitope was required and the orientation of the epitopes influenced the immunogenicity of the CTL epitope. The presence of two copies as compared with one copy of a T-helper epitope, rendered the CTL epitope more immunogenic and resulted in the in vivo induction of MV-specific CTLs without the need for an adjuvant. The role of CTL responses to this epitope in protection after intranasal administration was evaluated in a mouse model against challenge with a neuroadapted strain of MV. Although a decreased mortality in the peptide immunized compared with that in unimmunized mice was observed, the protection achieved was not significant. These findings highlight the importance of the rational design of synthetic immunogens for the induction of CTL responses and the potential of the intranasal route for immunization.

Demonstration of virus-specific CD8+ memory T cells in measles-seropositive individuals by in vitro peptide stimulation

CD8+ cytotoxic T lymphocytes (CTL) in measles virus infection have been difficult to investigate due to the strong immunosuppressive effects exhibited by infectious measles virus in vitro. In order to circumvent immunosuppression we used predicted peptide epitopes to induce measles virus-specific CTL. This was done by screening the structural proteins of measles virus for HLA-A2.1 peptide-binding motifs with valine in position 2 and leucine in position 9. Synthetic peptides np210-218, np226-234, and np340-348 from the nucleoprotein, peptide hp29-37 from the haemagglutinin protein, and peptide pp 519-527 from the polymerase protein were synthesized and used to expand measles virus-specific CD8+ CTL in vitro. Induction of CTL with synthetic peptides was restricted to HLA-A2-positive peripheral blood mononuclear cells (PBMC) from measles-seropositive individuals. We conclude that this method is a useful tool to demonstrate memory CD8+ CTL in measles-seropositive adults and to evaluate the role of structural proteins in CTL responses against measles.

Rapid screening for Mamu-A1-positive rhesus macaques using a SIVmac Gag peptide-specific cytotoxic T-lymphocyte assay

As part of an ongoing vaccine study using peptide immunogens designed to stimulate simian immunodeficiency virus (SIV)mac-specific cytotoxic T lymphocytes (CTL) it was necessary to identify rhesus macaques within our colony bearing the Mamu-A1 major histocompatibility complex (MHC) class I haplotype. Peripheral blood mononuclear cells (PBMC) from individual monkeys were analysed by immunoelectrofocusing for the presence of a band corresponding to the Mamu-A1 molecule. In addition, PBMC were pulsed with the SIVmac Gag peptide 11 (against which CTL are Mamu-A1 restricted) and analysed for susceptibility to lysis by peptide 11-specific CTL. PBMC from all of the rhesus macaques shown to be Mamu-A1 positive by immunoelectrofocusing were highly sensitive to lysis by the peptide 11-specific CTL. A total of 46% (16 from 35) of the rhesus macaques originating from India were found to be Mamu-A1 positive, whereas none of the Chinese rhesus (0 from 37) macaques possessed this haplotype. Once a peptide-specific CTL is established, screening by CTL assay offers a faster, reliable and more relevant alternative to immunoelectrofocusing for selecting monkeys for use in vaccination trials.

Immune responses during measles virus infection

The characteristic disease features of measles--fever and rash--are associated with the immune response to infection and are coincident with virus clearance. MV-specific antibody and CD4 and CD8 T cell responses are generated and contribute to virus clearance and protection from reinfection. During this same phase of immune activation immunologic abnormalities are also apparent. There is a generalized suppression of cellular immune responses that may contribute to increased susceptibility to other infections. Autoimmune disease may appear in the form of acute disseminated encephalomyelitis. If virus-specific immune responses are inadequate infection may progress with pulmonary or CNS manifestations, but without a rash. The pathogenesis of the rare disease SSPE, that occurs many years after primary infection is not clear, but immune responses show increased antibody to measles and cellular immune responses similar to those seen after uncomplicated infection.

CD4+ T cells are essential in overcoming experimental murine measles encephalitis

Clinical observations and experimental animal models have stressed the importance of the cellular immune response in the recovery from measles virus infection. However, the relative contribution of different T-cell subsets to viral elimination is controversial. The aim of the present study was to define the components of the immune system which contribute to the control of measles virus infection. For this purpose the effect of in vivo depletion of CD4+ and/or CD8+ T lymphocytes in the murine model of experimental acute measles encephalitis was monitored with respect to disease manifestation, survival, neuropathological changes, virus elimination from brain, and antiviral antibody titre. In measles virus-resistant BALB/c mice removal of the CD8+ T-cell subset did not interfere with the clearance of virus from the brain. In contrast, depletion of CD4+ T cells rendered BALB/c mice susceptible to infection. Also, in measles virus-susceptible C3H mice CD4+ T cells played a role in recovery from measles infection, but seemed not to be as effective as CD4+ T cells from resistant BALB/c mice. The data indicate that CD4+ T cells are essential for protection against measles virus-infection of the central nervous system.

Human HLA class I- and HLA class II-restricted cloned cytotoxic T lymphocytes identify a cluster of epitopes on the measles virus fusion protein

The transmembrane fusion (F) glycoprotein of measles virus is an important target antigen of human HLA class I- and class II-restricted cytotoxic T lymphocytes (CTL). Genetically engineered F proteins and nested sets of synthetic peptides spanning the F protein were used to determine sequences of F recognized by a number of F-specific CTL clones. Combined N- and C-terminal deletions of the respective peptides revealed that human HLA class I and HLA class II-restricted CTL efficiently recognize nonapeptides or decapeptides representing epitopes of F. Three distinct sequences recognized by three different HLA class II (DQw1, DR2, and DR4/w53)-restricted CTL clones appear to cluster between amino acids 379 and 466 of F, thus defining an important T-cell epitope area of F. Within this same region, a nonamer peptide of F was found to be recognized by an HLA-B27-restricted CTL clone, as expected on the basis of the structural homology between this peptide and other known HLA-B27 binding peptides.

Augmentation of major histocompatibility complex class I and ICAM-1 expression on glial cells following measles virus infection: evidence for the role of type-1 interferon

An intracellular staining procedure for the cytoskeletal marker, glial fibrillary acidic protein of astrocytes, has been developed which allows flow cytometric phenotyping of astrocytes within complex mixtures of glial cells. Employing this technique, we show here that measles virus infection of rat mixed glial cell cultures results in a rapid augmentation of major histocompatibility complex (MHC) class I and ICAM-1 on the majority of astrocytes in culture. MHC class I levels are increased on macrophages/microglia but ICAM-1 expression is not normally affected on this cell type. Some MHC class II induction is also observed after virus infection but only on astrocytes. A type-I interferon (IFN)-inducible protein, Mx, was identified in cultured glial cells after infection. Qualitatively comparable MHC class I and ICAM-1 enhancement after addition of type-I IFN, supports the conclusion that this cytokine(s) released as a result of virus infection, is responsible for alterations in the expression of molecules on glial cells, that are involved in T cell recognition. Astrocytes after viral infection were more susceptible to alloantigen-specific cytotoxic T lymphocytes and cytotoxic T lymphocyte activity was substantially reduced in the presence of mAb specific for MHC class I, ICAM-1 and LFA-1 but not MHC class II. The relevance of these findings to T cell recognition of virus-infected cells in the central nervous system is discussed.

T-cell recognition of measles virus haemagglutinin studied in a mouse model

BALB/c mice were pretreated with cyclophosphamide and immunized 2 days later with inactivated, purified measles virus (MV) mixed with dimethyl dioctadecyl ammonium bromide (DDA). Seven days later, lymph nodes (LN) were removed and lymphocytes cultured in the presence of purified MV antigens. MV haemagglutinin (H) was found to be a major antigen responsible for proliferation of the lymphocytes. Incorporation of purified H into liposomes significantly enhanced the proliferative response compared with purified H alone. Response to MV nucleocapsid protein was only moderate, and insertion of this protein into liposomes did not improve the response. As an attempt to analyse T-cell epitopes of MV H, three synthetic peptides previously found to elicit a strong antibody response were used both as priming and stimulating antigens. None of the peptides was able to elicit a secondary response when MV-primed LN cells were stimulated in vitro. However, each peptide primed T cells for a secondary challenge with purified, inactivated MV, which was demonstrated by proliferation and a delayed-type hypersensitivity assay and also by transfer experiments with peptide-primed cells.

Immune responses to herpes simplex virus in patients with recurrent herpes labialis: I. Development of cell-mediated cytotoxic responses

Groups of subjects during acute (0-3 days) and convalescent (2-3 weeks) phase of recurrent herpes labialis (RHL), and other subjects seropositive or seronegative for herpes simplex virus type 1 (HSV-1) antibody without any history of RHL, were tested for the appearance of cell-mediated cytotoxic responses by stimulating peripheral blood leukocytes (PBL) in vitro with ultraviolet-inactivated HSV-1 antigen, using the release of radiolabelled chromium (51Cr) from HSV-1-infected autologous, or allogeneic lymphocytes and K562 erythroleukemia cell line as nonspecific targets. Development of HSV specific cytotoxic response using autologous targets was essentially limited to subjects with RHL and in HSV antibody seropositive control subjects. Peak activity was observed during the acute phase of the disease, compared to the activity in the convalescent phase in seropositive subjects with RHL, and was preceded by high lymphoproliferative response to HSV. Higher cytotoxic responses against K562 cells were also observed in RHL subjects compared to the controls. Depletion of Leu-2+, Leu-3+ or Leu-11 effector lymphocytes from HSV-1-stimulated PBL cultures by treatment with complement and appropriate monoclonal antibodies resulted in significant reduction of cytotoxicity to HSV-1-infected autologous cells. However, cytotoxicity to K562 cells was reduced only after depletion of Leu-11+ cells. Low levels of allogeneic restriction were observed for cytotoxicity to HSV-1-infected targets. These observations suggest selective activation of virus specific Leu-2+ and Leu-3+ T cell subsets as well as natural killer cell mediated cytotoxic mechanisms during the active phase of recurrences of herpes simplex virus infection.

The role of IL-2 and T4+ cells in the generation of human influenza virus-specific CTL activity

Stimulation of human peripheral blood lymphocytes (PBL) with influenza A virus leads to the generation of virus-specific cytotoxic T lymphocyte (CTL) activity as well as natural killer (NK)-like activity. In this study, we show that exogenous IL-2 augments the in vitro generation of virus-specific CTL activity, only when added some days after the initiation of the culture. Apparently, the endogenously produced IL-2 can be a limiting factor in the in vitro generation of CTL activity. The increase of influenza virus-specific CTL activity after addition of exogenous IL-2 does not affect the restriction pattern of the CTL response. So, the preferential use of certain HLA antigens as restriction elements is not due to a limiting amount of endogenously produced IL-2. Depletion of T4+ cells completely abrogates the generation of virus-specific CTL activity. Addition of exogenous IL-2 to T4+-cell-depleted cultures fully restores the generation of HLA-restricted virus-specific CTL activity. We conclude that in the in vitro generation of virus-specific CTL activity in bulk cultures of human PBL the sole function of T4+ cells in human virus-specific CTL generation is the production of IL-2, no cognitive cell interaction of T8+ CTL precursors with T4+ cells is required, and in bulk cultures T8+ cells themselves are not able to produce sufficient amounts of IL-2 to ascertain the maturation of virus-specific CTL precursors into cytolytic T cells. Finally, we show that exogenous IL-2 also has a stimulatory effect on the NK-like or lymphokine-activated killer activity, which is always concomitantly induced in virus-specific CTL generation cultures, but has no influence on the levels of IFN produced in such cultures.

IL-2- and IFN gamma-enhanced natural cytotoxic activity: analysis of the role of different lymphoid subsets and implications for activation routes

To elucidate the immunoregulatory mechanisms by which human recombinant interleukin 2 (r.IL-2) and human recombinant interferon (IFN)-gamma influence natural cytotoxic activity of human peripheral blood lymphocytes (PBL), experiments were performed in which we studied: the kinetics of augmentation of natural cytotoxic activity by IL-2 and IFN-gamma; the phenotypes of the natural cytotoxic precursor cells acted upon by IL-2 and IFN-gamma; the role of IL-2-induced IFN-gamma in the natural cytotoxic activity enhancing effects of IL-2 and (d) the surface phenotypes of the natural cytotoxic cells activated by IL-2 or IFN-gamma. Three phenotypical distinct precursor cells can be identified. The phenotypes of the precursor cells sensitive to r.IFN-gamma are T3-, Leu 7+or-, FcR-gamma+. The phenotypes of the precursor cells sensitive to r.IL-2 are T3-, Leu 7-, FcR-gamma+or-. It appears that during prolonged culture relatively more of the FcR-gamma- precursor cells acquire natural cytotoxic activity upon stimulation with r.IL-2. Antibodies to IFN-gamma, known to neutralize both recombinant and natural IFN-gamma, completely inhibited the IFN-gamma-induced augmentation of natural cytotoxic activity but only slightly affected the IL-2-mediated augmentation. This indicates that only part of the augmenting effect of r.IL-2 is mediated through IL-2-induced IFN-gamma. This implies that r.IL-2 augments natural cytotoxic activity predominantly through an IFN-gamma-independent pathway. The phenotype of the effector cells expressing natural cytotoxic activity after a 4-day culture period with IL-2 or IFN-gamma was analyzed by depletion experiments. Virtually all the cells that exerted the IFN-gamma-enhanced natural cytotoxic activity are FcR-gamma+, whereas a portion of the cells that exerted the IFN-gamma-enhanced natural cytotoxic activity are FCR-gamma-. Both natural killer cells and lymphokine-activated killer cells contribute to natural cytotoxic activity. The relative contributions of both activities to the natural cytotoxic activity augmented by IL-2, respectively, IFN-gamma, is discussed.

Kinetics of cytotoxic lymphocytes in efferent lymph from single lymph nodes following immunization with vaccinia virus

Efferent lymphocytes collected from a cannulated lymphatic draining single lymph nodes were studied for their cytotoxic activity following the injection of live vaccinia virus subcutaneously into the drainage site of a lymph node. Injection of virus produced a 40-fold increase in the lymphoblast output 7 days following virus injection. Cytotoxic lymphocytes were detectable in lymph shortly after the appearance of lymphoblasts at 80 h and also reached a maximum during the 7th day. This was followed by a rapid decline of the cytotoxic cells although cytotoxic cells were detectable up to 2 weeks. The cytotoxic activity in lymph was found to be antigen specific, dependent on the effector/target cell ratio, and allogeneically restricted, indicating that it was most likely due to cytotoxic T lymphocytes (CTL). CTL precursors were found in large numbers in efferent lymph and appeared at approximately the same time as the mature CTL. Unlike CTL, the precursors became part of the recirculating lymphocyte pool and were detectable in efferent lymph for at least 2 months. Following a secondary challenge with vaccinia, lymphoblasts and CTL appeared at least 36 h earlier in the lymph. In summary, we have demonstrated that virus specific CTL are found in the efferent lymph collected from a single immunized lymph node in sheep. The kinetics of the CTL and CTL precursors indicate that these lymphocytes are one of the earliest antigen specific cells detectable in efferent lymph and suggests that these cells migrate rapidly from the lymph node into efferent lymph for dissemination throughout the host to sites of virus infection.

Mapping of the major histocompatibility complex and viral antigens on the plasma membrane of a measles virus-infected cell

The two measles virus glycoproteins, the hemagglutinin and fusion protein, are expressed on the surfaces of infected cells. Although the two molecules are chemically distinct, they associate on the cell surface, judging from their ability to comigrate (co-cap). However, neither is directly complexed with the major histocompatibility (MHC) gene products, HLA-A, -B, -C or -D, on the plasma membrane, based on results from three distinct assays. First, in tests of capping, these viral glycoproteins failed to comigrate with any HLA determinant. Second, electron microscopy showed that the viral glycoproteins occupied domains on the plasma membrane distinct from MHC gene products; 125I labeling of cell surface determinants and subsequent analysis by immune precipitation and PAGE confirmed this result. Third, incubation of measles virus-infected cells in the presence of monoclonal or polyclonal antibodies to measles virus glycoproteins removed the viral glycoproteins from the cells' surfaces but did not cause a corresponding decrease in amounts of HLA molecules. These results indicate that the hemagglutinin and fusion polypeptides of measles virus lie in close association on the plasma membrane; however, neither is linked with MHC gene products.

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.