Quantitation of influenza virus antigens on infected target cells and their recognition by cross-reactive cytotoxic T cells

Monoclonal antibodies detect M-protein epitopes on the surface of influenza virions

Various data obtained with activable hydrophobic probes, proteolytic treatments and anti M-protein polyclonal antibodies strongly suggest that M-protein of influenza A is an integral part of the lipid bilayer of native virions and somehow spans at the surface of the virions. Therefore we have looked for the presence of M-protein epitopes on the surface of influenza A virion by using four type A M-protein monoclonal antibodies. We developed a specific and sensitive competition ELISA where intact virions, dodecyl-sulfate disrupted virions and spikeless particles obtained after proteolytic treatment with caseinase C were used to test their ability to inhibit the reaction between these monoclonal antibodies and pure M-protein. Intact virions or SDS disrupted virions prevented three monoclonal antibodies from reacting with the M-protein. Spikeless particles also inhibited the specific binding of two of these antibodies, whereas the other fourth antibody was inhibited by contact with SDS disrupted particles only. Data presented show that at least three distinct M-protein epitopes were detected, of which at least two are exposed on the surface of intact virions. Of these two epitopes, one is inactivated by the proteolytic treatment. The third epitope could only react with its monoclonal antibody when the virus particles were solubilized with SDS. This work provides a clear demonstration that a substantial part of the M-protein spans the lipid bilayer and that the rest, protected by lipids, resists proteolytic enzymes and is prevented from binding with anti M-protein monoclonal antibodies.

Searching for MHC-restricted anti-viral antibodies: antibodies recognizing the nucleoprotein of influenza virus dominate the serological response of C57BL/6 mice to syngeneic influenza-infected cells

An attempt has been made to generate monoclonal antibodies which recognize the same target structures on influenza-infected cells as those seen by cytotoxic T lymphocyte (CTL) receptors. Such antibodies, if they mimicked the T cell receptor specificity, would be expected to be both virus specific and restricted in their binding by the major histocompatibility complex (MHC) antigens. Approximately 200 hybridomas from C57BL/6 (H-2b) mice primed and boosted with influenza virus (X-31)-infected EL4 (a C57BL/6 T cell lymphoma) were screened for reactivity on infected and uninfected cells of different MHC haplotypes. Of the 10 hybridoma antibodies which were identified as being reactive with X-31-infected EL4, but not uninfected EL4, all reacted equally well with X-31-infected cells of H-2b, H-2d and H-2k haplotypes, indicating a lack of MHC restriction in their recognition of the infected cells. Unexpectedly, 7 of the 10 monoclonal antibodies were found to react specifically with the purified influenza virus nucleoprotein (NP), a predominant viral antigen in CTL recognition of infected cells. Fluorescence-activated flow cytometry confirmed that these antibodies were able to recognize NP serological determinants on the surface of viable, infected cells, but the anti-NP antibodies were unable to block the lytic activity of an NP-specific CTL clone.

Fine specificity analysis of human influenza-specific cloned cell lines

Influenza-specific human-T-cell clones, proliferating in the presence of virus-infected cells with restriction by class II molecules and displaying class II-restricted CTL activity or specific helper activity in antibody synthesis, have been analyzed for antigenic specificities. All of them were obtained by in vitro stimulation against influenza A/Texas virus. In all cases the virus specificity appeared identical in cytolytic and proliferative responses. Three of the clones were broadly cross-reactive, recognizing all or almost all type A influenza strains. The three remaining clones were subtype specific when tested with human strains and recognized the surface glycoproteins of influenza virus. One of these lines reacted with an epitope of the neuraminidase N2 while the other two recognized the hemagglutinin H3. By using a large panel of mammalian and avian influenza strains, it can be demonstrated that hemagglutinin-specific human T cells can recognize a cross-reacting determinant shared by H3 and H4 subtypes of hemagglutinin which has never been detected with antibodies.

Influenza A virus nucleoprotein is a major target antigen for cross-reactive anti-influenza A virus cytotoxic T lymphocytes

Influenza A virus-specific cytotoxic T lymphocytes (CTL) capable of lysing cells infected with any influenza A virus ("cross-reactive CTL") constitute a major portion of the host CTL response to influenza. The viral nucleoprotein (NP), a major internal virion structural protein, has been implicated as a possible target antigen for cross-reactive CTL. To directly examine CTL recognition of NP, a vaccinia virus recombinant containing a DNA copy of an influenza A virus NP gene was constructed. We found that murine cells infected with this virus were efficiently lysed in a major histocompatibility complex-restricted manner by cross-reactive CTL populations obtained by immunization with a variety of influenza A virus subtypes. In addition, the recombinant vaccinia virus containing the PR8 NP gene was able to both stimulate and prime for a vigorous secondary cross-reactive CTL response. Significantly, splenocytes from mice primed by inoculation with the recombinant vaccinia virus containing the PR8 NP gene could be stimulated by influenza A viruses of all three major human subtypes. Finally, unlabeled target competition experiments suggest that NP is a major, but not the sole, viral target antigen recognized by cross-reactive CTL.

Influenza virus M2 protein is an integral membrane protein expressed on the infected-cell surface

The influenza A virus M2 protein is expressed abundantly at the cell surface, and in addition to the hemagglutinin (HA) and neuraminidase (NA), is a third virus-specific membrane protein. M2 has an internal hydrophobic membrane anchorage domain and associates with the same cellular membrane fractions as HA and NA. Trypsin treatment of infected cells and immunoprecipitation with site-specific antisera indicate that a minimum of 18 NH2-terminal amino acids of M2 are exposed at the cell surface. Ten NH2-terminal residues are conserved in all strains of influenza A virus for which sequences are available. Antibodies can recognize M2 on the cell surface and therefore it may be an infected-cell surface antigen. We discuss properties of M2 that match it to the elusive major target molecule on influenza A virus-infected cells for cross-reactive cytotoxic T cells.

Monoclonal antibodies against microorganisms

The recent spread of hybridoma technology among laboratories has promoted the development of monoclonal antibodies against a wide variety of infectious disease agents. While monoclonal antibodies theoretically represent an excellent (perhaps superior) alternative to conventional antisera as diagnostic, therapeutic or laboratory reagents, traditional antisera may be preferable to monoclonal antibody in some circumstances because of the fixed affinity and specificity as well as the limited functional capacities of some antibodies. The acceptance of monoclonal antibodies by the clinical microbiologist and physician must await proof of their reliability, safety and efficacy.

The influenza A virus nucleoprotein gene controls the induction of both subtype specific and cross-reactive cytotoxic T cells

Using genetically typed recombinant influenza A viruses that differ only in their genes for nucleoprotein, we have demonstrated that repeated stimulation in vitro of C57BL/6 spleen cells primed in vivo with E61-13-H17 (H3N2) virus results in the selection of a population of cytotoxic T lymphocytes (CTL) whose recognition of infected target cells maps to the gene for nucleoprotein of the 1968 virus. Influenza A viruses isolated between 1934 and 1979 fall into two groups defined by their ability to sensitize target cells for lysis by these CTL: 1934-1943 form one group (A/PR/8/34 related) and 1946-1979 form the second group (A/HK/8/68 related). These findings complement and extend our previous results with an isolated CTL clone with specificity for the 1934 nucleoprotein (27, 28). It is also shown that the same spleen cells derived from mice primed with E61-13-H17 virus in vivo, but maintained in identical conditions by stimulation with X31 virus (which differs from the former only in the origin of its gene for NP) in vitro, results in the selection of CTL that cross-react on target cells infected with A/PR/8/1934 (H1N1) or A/Aichi/1968 (H3N2). These results show that the influenza A virus gene for NP can play a role in selecting CTL with different specificities and implicate the NP molecule as a candidate for a target structure recognized by both subtype-directed and cross-reactive influenza A-specific cytotoxic T cells.

Cytotoxic T lymphocyte recognition of the influenza hemagglutinin gene product expressed by DNA-mediated gene transfer

We have used the technique of DNA-mediated gene transfer to examine cytotoxic T lymphocyte (CTL) recognition of the product of the cloned A/JAPAN/305/57 hemagglutinin (HA) gene in murine (L929) cells. Using both heterogeneous and homogeneous (clonal) populations of type A influenza-specific CTL, we have demonstrated that the HA molecule can serve as a target antigen for both the subtype-specific and the cross-reactive subpopulations of influenza-specific CTL. Our results also raise the possibility that other virus-specified polypeptides may serve as target molecules for cross-reactive CTL.

Most influenza A virus-specific memory cytotoxic T lymphocytes react with antigenic epitopes associated with internal virus determinants

This paper shows that most murine (C57BL/6) influenza A virus-specific memory cytotoxic T lymphocyte (CTL) clones tested in limiting dilution did not react with the influenza A virus surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). This lysis of syngeneic target cells infected with the influenza A virus strains, Aichi (H3N2), PR8 (H1N1), or recombinant strain X31 (H3N2) indicates that most antigenic epitopes recognized are associated with internal virus determinants. X31 and PR8 share the internal, and X31 and Aichi the external, viral determinants. Extensive CTL cross-reactivity was observed in experiments with target cells infected with virus carrying internal determinants homologous with the priming virus. In contrast, when the internal viral determinants differed between the priming virus and the virus used to infect the target cells, and although HA and NA were homologous, we found almost complete CTL-specificity for the priming virus. Thus, the predominant reactivity of influenza A virus-specific CTL differs from that of anti-influenza A antibodies, which are primarily directed towards epitopes on the virus surface glycoproteins. This finding may be relevant for the role of influenza A virus-specific CTL in recurrent infections with different influenza A viruses.

Immunological surveillance of tumors in the context of major histocompatibility complex restriction of T cell function

The immunological surveillance hypothesis was formulated prior to the realization of the fact that an individual's effector T cells generally only see neoantigen if it is appropriately presented in the context of self MHC glycoproteins. The biological consequence of this mechanism is that T lymphocytes are focused onto modified cell-surface rather than onto free antigen. The discovery of MHC-restricted T cell recognition, and the realization that T cell-mediated immunity is of prime importance in promoting recovery from infectious processes, has thus changed the whole emphasis of the surveillance argument. Though the immunological surveillance hypothesis generated considerable discussion and many good experiments, there is no point in continuing the debate in the intellectual context that seemed reasonable in 1970. It is now much more sensible to think of "natural surveillance" and "T cell surveillance," without excluding the probability that these two systems have elements in common. We can now see that T cell surveillance probably operates well in some situations, but is quite ineffective in many others. Part of the reason for this may be that the host response selects tumor clones that are modified so as to be no longer recognized by cytotoxic T cells. The possibility that this reflects changes in MHC phenotype has been investigated, and found to be the case, for some experimental tumors. In this regard, it is worth remembering that many "mutations" in MHC genes that completely change the spectrum of T cell recognition are serologically silent. The availability of molecular probes for investigating the status of MHC genes in tumor cells, together with the capacity to develop cloned T cell lines, monoclonal antibodies to putative tumor antigens, and cell lines transfected with genes coding for these molecules, indicates how T cell surveillance may profitably be explored further in both experimental and human situations.

Stimulation of cytotoxic T cells by liposomes containing influenza virus or its components

Since inactivated virus preparations are poor inducers of influenza-specific cytotoxic T cells (Tc), studies were undertaken utilizing artificial vesicles (liposomes) as a means of delivering viral and H-2 antigens in a multivalent form and oriented with respect to a lipid bilayer. Liposomes prepared from extracted mouse cell lipids efficiently incorporated influenza-viral proteins and were not toxic in culture. Using polybrene to promote greater contact of liposomes with cells, liposomes prepared from whole virus could effectively stimulate memory Tc from spleens of intranasally infected mice in vitro. H-2 was not required in the liposomes to obtain stimulation, and its presence did not improve responses, which were always lower than in parallel stimulations using virally infected syngeneic cells. Liposomes prepared from purified influenza virion subunits (haemagglutinin, neuraminidase, matrix protein) were only slightly stimulatory in vitro, and were unable to prime mice for significant Tc memory.

The segregation of specific clones of cytotoxic lymphocytes in an in vitro primary response against influenza virus

CBA spleen cells have been stimulated in vitro with A/Jap influenza virus-infected CBA spleen cells to generate a 'primary' cytotoxic lymphocyte (CL) response. The culture conditions were devised to allow the segregation of individual clones of CL and cytotoxicity measured by the lysis of infected or non-infected L-929 cells. The specificity was assessed by splitting clones and measuring the ability of the clones to discriminate between pairs of targets. Influenza A/FMI and A/Jap strains were used. Subsets of clones were detected which could lyze either A/Jap-infected or A/FMI-infected target cells. In addition CL clones were found which lyzed uninfected L-929 cells and a fourth category were clones which could not discriminate between A/FMI-and A/Jap-infected targets.

Evidence for two T-helper populations with distinct specificity in the humoral response to influenza A viruses

Virus specificity of T-helper cells for the humoral antibody response to influenza A viruses was studied with a hapten-carrier secondary adoptive transfer system, using whole virus, or viral components inserted into liposomes as carrier with B cells primed to DNP human gamma globulin. Evidence was obtained for two distinct T-helper cell populations from mice primed by influenza infection: a T-helper cell cross-reactive for all type A influenza viruses and a second T-helper population specific for the variant haemagglutinin. In vivo the virus cross-reactive T helpers recognized whole virus, but did not recognize isolated surface glycoproteins or internal virus proteins.

Immunologic studies on the influenza A virus nonstructural protein NS1

We purified the major influenza virus nonstructural protein, designated NS1, from cytoplasmic inclusions that were solubilized and used to raise antisera in rabbits. One of the antisera was found to be specific for NS1 by complement fixation tests and analyses of immune precipitates. Antiserum to NS1 isolated from cells infected with A/WSN/33 virus specifically precipitated NS1 from extracts of cells infected with seven distinct isolates of influenza A virus representing five different antigenic subtypes. These included A/WSN/33, A/PR/8/34, A/FW/5/50, A/USSR/90/77, A/RI/5+/57, A/Victoria/3/75, and A/Swine /1977/31; however, NS1 from cells infected with B/Lee/40 virus was not precipitated. Radioimmunoassays using radioiodinated NS1 protein from A/WSN virus-infected cells and unlabeled cytoplasmic extracts of cells infected with various strains of influenza virus as competitors indicated significant antigenic cross-reactivities for the NS1 proteins of all influenza A viruses tested. The results suggest a gradual antigenic drift over the 45 yr separating the earliest and most recent virus isolates examined. Thus, compared with the virion neuraminidase and hemagglutinin antigens, NS1 appears to be highly conserved in different influenza A virus isolates.

H-2 and viral haemagglutinin expression by influenza-infected cells; the proteins are close but do not cocap

The proximity of H-2K and D antigens and influenza virus haemagglutinin (HA) molecules on the surface of infected target cells was assessed by a topographical study using monoclonal antibodies to H-2 and to HA. The effect of pretreatment of fixed, infected cells with excess of one monoclonal antibody on the subsequent binding of a second radiolabelled antibody was measured. Using CBA mouse B lymphoblasts which were paraformaldehyde fixed 5 hr postinfection with influenza virus (A/USSR/90/77), pretreatment with monoclonal antibody 30/3 to H-2Kk and Dk partially blocked (Approximately equal to 37%) the binding of one radiolabelled monoclonal anti-HA antibody (264/2). A different monoclonal IgG (W18/1) directed to the same HA molecule was not blocked by similar pretreatment of cells with the anti-H-2 antibody. Interaction of monoclonal antibodies with their sites is highly specific, and mutual blocking of two antibodies requires very closely located sites even if the antibodies are directed to the same molecule. We therefore have evidence for proximity of H-2 and HA molecules; however, we were unable to demonstrate cocapping of H-2K and D antigens with influenza HA.

Regulation of delayed-type hypersensitivity to pathogens and alloantigens

Recent studies reinforce the notion that delayed-type hypersensitivity plays a key role in the host defence against microbial and intracellular parasitic infection, and in the rejection of skin allografts. F. Y. Liew reviews these studies and discusses the observation that this T-cell mediated immunity is profoundly regulated by antigen-specific suppressor T cells, some of which are restricted by products of the I-J subregion of the MHC.

Surface expression of a nonstructural antigen on influenza A virus-infected cells

Rabbit antiserum to the influenza A virus nonstructural protein, NS1, was used for indirect immunofluorescence studies of infected cells. Nonstructural antigens were detected on surfaces of P815 cells as early as 4 h after infection with A/WSN/40 virus. Adsorption of the serum with virion structural proteins did not affect the observed fluorescence, and a progressive increase in surface fluorescence at later times postinfection indicated that the surface antigen was newly synthesized during the replication cycle.

Hybridomas: a new dimension in biological analyses

Since the first report of hybridomas producing monoclonal antibodies by Kohler and Milstein in 1975, this technique has spread to nearly all areas of biological, biochemical, and biomedical research. Watching the use of these methods spread from immunologists to cell biologists, developmental biologists, biochemists and to other biological disciplines and observing the nearly logarithmic increase in publications using these reagents has been in itself fascinating and informative. An overview of the development of this technology and its applications is presented including the use of monoclonal antibodies to study cell surface molecules, differentiation antigens, receptors, and histocompatibility antigens. The use of these antibodies to analyze microorganisms and parasitic antigens as well as their use in the genetic analysis of human cell surface antigens and the detection of polymorphic variation in enzymes and other proteins is discussed. Examples of the application of monoclonal reagents to the study of tumor cell biology including the labeling of metastatic tumor cells and the detection of cell surface molecules implicated in the regulation of growth control and cell division are provided.

A clearance test in mice using non-adapted viruses to determine the immunogenicity of influenza strains

A test for the immunogenicity of influenza viruses is described, which is based upon the intranasal vaccinating dose required to induce inhibition of multiplication of unadapted influenza viruses in the lungs of mice. This test is more sensitive than an antigen extinction procedure, in which immunogenicity is measured according to the dose required to induce the formation of hemagglutination-inhibition antibody. The clearance test has been used to demonstrate that a) influenza A/Northern Territory/60/68 virus is a better immunogen than A/Victoria/3/75 and both are probably superior to A/U.S.S.R./92/77; b) for A/Northern Territory/60/68, vaccination by the intranasal route in 25 g mice is at least 43,600 times more efficient than by the intraperitoneal route and c) common immunogenic relationships exist between various H3N2 viruses and an H1N1 strain.

Effect of helper T cells on the primary in vitro production of delayed-type hypersensitivity to influenza virus

Injection of mice with infectious or noninfectious preparations of influenza virus induces the formation of T cells which, when added to primary tissue cultures of normal spleen cells exposed to influenza virus, enhance the generation of effector T cells which mediate delayed-type hypersensitivity (DTH) reaction. The enhancing cells possess Thy-1 and Ly-1 surface antigens are radioresistant and antigen-specific. If infectious virus was used to stimulate the DTH response in vitro, help was delivered whether homologous or heterologous A strain influenza virus was used to generate the helper T cells (Th) in vivo. In contrast, only Th cells generated using homologous virus were effective if noninfectious virus was used to stimulate the DTH response in vitro. Peak helper activity occurred 2 d after virus injection and the Th cells were only effective if added to the primary cultures within 24 h after addition of the stimulating antigen. The Th cells enhanced the generation of both classes of DTH effector cells, i.e., those that are Ly-1 positive and IA-subregion restricted and those that are Ly-2,3 positive and K,D-region restricted. The activity of the Th cells was found to be IA-subregion restricted and this was shown to operate at the level of the stimulator cells so that the delivery of help to the responder cells was not H-2 restricted. The possibility that the Th cells might be a precursor to the Ly-1 positive IA subregion-restricted DTH effector cells is discussed.

The detection of influenza A virus antigens in cultured cells by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) was employed to investigate the expression of influenza A/Hong Kong/68 (H3N3) virus structural proteins on the surface of infected MDCK cells, and to detect viral antigens in culture media and cell extracts. Infected cells were fixed with 0.1 per cent glutaraldehyde before being examined for the presence of cell-surface antigens. Viral antigens were first observed on the surface of cells 4 hours after infection and reached a maximum 10-12 hours after infection, when measured by haemadsorption with chicken erythrocytes and by ELISA and immunofluorescence with hyperimmune antiserum to Hong Kong virus. A good correlation was found between the three assay systems. The presence of individual virion structural proteins on the cell surface was determined by ELISA using specific antibodies purified by differential affinity chromatography. Either or both or the internal matrix and nucleoprotein antigens were expressed from 2 to 6 hours after infection, with maximum expression after 2 hours, and the strain-specific and common antigenic determinants of haemagglutinin were observed on the cell surface from 4 hours after infection, and reached a maximum 8 to 10 hours after infection. Low levels of neuraminidase were detected between 4 and 8 hours after infection. Culture media and cell extracts were titrated by infectivity and haemagglutination assays, and by ELISA. Titres obtained from the culture media showed a close correlation between the three assay methods, with peak titres being attained 24 hours after infection. Viral antigens were first observed in cell extracts by ELISA 4 hours after infection, and infectious virions and haemagglutinin 2 hours later, but whereas maximum titres of infectious virus and haemagglutinin were found 10 hours after infection, the ELISA titre continued to rise until 24 hours after infection, which suggested that virus structural proteins were being accumulated in the cells after most of the progeny virions had been released. The results are discussed in terms of the potential use of ELISA in rapid virus diagnosis.

Roles of influenza virus infectivity and glycosylation of viral antigen for recognition of target cells by cytolytic T lymphocytes

The influenza virus strains A/JAP (H2N2) and the recombinant strain A/JAP/BEL (H2N1) were tested before and after UV-light inactivation for their ability to sensitize target cells for cytotoxic T-cell lysis (CTL). Infectious preparations were efficient sensitizers for both specific and cross-reactive CTL, exposure of the cells to even low doses of virus resulting in almost maximum susceptibility. When inactivated, however, A/JAP/BEL was about 10 times more efficient than A/JAP at sensitizing the cells for specific CTL; neither sensitized the cells for cross-reactive CTL. Thus factors other than or in addition to a cleaved haemagglutinin (HA) molecule are important in the fusion of the virus with the cell membrane. Target cells which were infected with virus and exposed to different concentrations of tunicamycin, which inhibits glycosylation, became susceptible to CTL by both specific and cross-reactive effector cells through to a lesser extent than controls. Infected cells showed both strong haemadsorption and cocapping of the HA with K, D gene products. Both of these properties were greatly diminished in the presence of even low concentrations of tunicamycin. Analysis of binding studies using labelled monoclonal anti-HA IgG showed that, in the presence of tunicamycin, the total amount of HA expressed at the cell surface was not reduced, but there was an increase in the dissociation constant of the reaction between expressed HA and antibody. This latter finding was thought to reflect a conformational change in the HA antigen, which might be the reason for the reduced susceptibility to CTL.

Cross-reactivity for different type A influenza viruses of a cloned T-killer cell line

Spleen cytotoxic T cells killing influenza virus-infected target cells are cross-reactive for the different type A influenza viruses, in contrast to the circulating antibodies, which show fine specificity for each A virus subtype variant. This finding has raised the question of whether a single T cell can recognize cells infected with all type A viruses. T-killer cell lines with specificity for alloantigens and the male Y antigen can be selected by means of growth factors present in the supernatant of T cells stimulated with concanavalin A (refs 3-7). We report here that we have been able to establish clones of mouse T cells killing target cells infected with influenza virus. Our cell line maintains the same specificity as the heterogeneous spleen cell population from infected mice, in as far as the T-killer cells are specific for A influenza virus, but do not discriminate between the different type A viruses. The cell line maintains H-2 restriction and does not kill cells infected with B influenza virus. The cells grow in the presence of T-cell growth factor and do not require antigen for growth although they maintain their receptors for type A virus. They can also be stimulated by irradiated T-helper cells from mice primed by type A influenza infection in the presence of type A virus-infected cells.