Influenza viruses are T cell-independent B cell mitogens

Single-cell genome-wide association reveals that a nonsynonymous variant in ERAP1 confers increased susceptibility to influenza virus

During pandemics, individuals exhibit differences in risk and clinical outcomes. Here, we developed single-cell high-throughput human in vitro susceptibility testing (scHi-HOST), a method for rapidly identifying genetic variants that confer resistance and susceptibility. We applied this method to influenza A virus (IAV), the cause of four pandemics since the start of the 20th century. scHi-HOST leverages single-cell RNA sequencing (scRNA-seq) to simultaneously assign genetic identity to cells in mixed infections of cell lines of European, African, and Asian origin, reveal associated genetic variants for viral burden, and identify expression quantitative trait loci. Integration of scHi-HOST with human challenge and experimental validation demonstrated that a missense variant in endoplasmic reticulum aminopeptidase 1 (ERAP1; rs27895) increased IAV burden in cells and human volunteers. rs27895 exhibits population differentiation, likely contributing to greater permissivity of cells from African populations to IAV. scHi-HOST is a broadly applicable method and resource for decoding infectious-disease genetics.

Cytokine induction in human cord blood lymphocytes after pulsing with UV-inactivated influenza viruses

Mitogenic activity of UV-inactivated influenza viruses in cord blood lymphocytes (CBL), as measured by cytokine release, was investigated. Using prototype viruses of subtype H3N2 (A/Aichi/68), H2N2 (A/Japan/57), and H1N1 (A/Puerto Rico/34) for influenza A virus, and B/Lee/40 for influenza B virus, the results indicated that both Th1 and Th2 cytokines were induced. Stimulation indices were significantly higher for IFNgamma, IL-4 and IL-10 by influenza A viruses than by influenza B virus. Stimulation indices for IL-2 and IL-6 were lower, as these two cytokines were spontaneously released by cord blood lymphocytes in culture. Alignment of the amino acid sequences of the HA for the viruses used in this study indicated that influenza B virus lacked sequence homology to the antigenic sites identified for influenza A virus. Therefore, the antigenic sites may play a role in the mitogenic property, and cord blood lymphocytes could provide a system to compare this property for clinical isolates of influenza virus.

Distinct requirements for IL-6 in polyclonal and specific Ig production induced by microorganisms

The role of IL-6 in Ig production induced in the mouse by lactate dehydrogenase-elevating virus (LDV), Toxoplasma gondii or lipopolysaccharide (LPS) was assessed. Following infection with LDV, a strong activator of B cells, an early and transient IL-6 production was observed, that originated predominantly from macrophages. Whereas LDV-induced B lymphocyte proliferation appeared independent of IL-6, mice deficient for this cytokine showed a marked reduction in their total T-dependent IgG2a production when compared to their normal counterparts. By contrast, specific responses directed against either LDV or non-viral antigens administered at the time of infection were not decreased in the absence of IL-6. Similarly, polyclonal, but not anti-parasite IgG2a production triggered by T. gondii infection was strongly dependent on the presence of IL-6. Finally, T-independent total IgG3 secretion triggered by LPS was also markedly reduced in IL-6-deficient mice. These results suggest that IL-6 plays a major role in T-dependent and T-independent polyclonal Ig production following B lymphocyte activation by viruses, and parasites, but not in specific antibody responses induced by the same microorganisms.

Superstimulatory influenza virus and highly organized BCR-ligands act synergistically on B cell activation

The influenza virus glycoprotein hemagglutinin (HA) behaves as a superstimulatory protein for B lymphocytes from various species. Polyclonal B cell stimulation mediated by HA can be blocked by soluble anti-Ig antibodies. We here report that, if presented in a highly organized form, i.e., as anti-Ig mAb coupled to dextran (anti-Ig-Dex), conventional BCR-ligands and influenza viruses act synergistically on murine B cell activation. Proliferative responses of both spellen-derived and peritoneal B cells mediated by suboptimal amounts of HA were significantly augmented by costimulation with anti-Ig-Dex, and vice versa. Similarly, anti-Ig-Dex, which on its own cannot induce Ig production in the absence of added cytokines, significantly enhanced Ig synthesis in response to superstimulatory HA. By contrast, poorly organized BCR-ligands (i.e. the same anti-Ig mAb in a soluble form) had either no, or a strong inhibitory effect on virus-triggered lymphocyte activation. Assays with various second messenger-antagonists, however, revealed clear differences in the signaling pathway employed by anti-Ig-Dex and HA, suggesting that the functional synergy between the two multimeric agents is mediated by engagement of distinct transducing elements. Taken together, these results indicate that the superstimulatory function of influenza virus HA represents a molecular strategy to mimick B cell activation by conventional, highly organized particulate-antigens.

Isolation and characterization of IgG2a-reactive autoantibodies from influenza virus-infected BALB/c mice

Repeated influenza virus infection induces the production of dominantly IgG2a-type virus-specific antibodies as well as the appearance of IgG2a-reactive autoantibodies in BALB/c mice characterized by low spontaneous rheumatoid factor-type autoantibody production. IgG2a-reactive autoantibody-producing hybridomas could be isolated from the spleen of influenza virus-infected BALB/c mice. The mAb produced by these clones represent not only IgM but also IgG and IgA isotypes and show strong isotype or isoallotype specificity. The common functional property of these autoantibodies is their preferential- and high-affinity binding to complexed, solid-phase-bound or heat-aggregated IgG2a when compared to native soluble or cell-bound IgG2a. The mechanism of induction and the possible biological function of these autoantibodies are discussed in the light of their fine specificity and functional properties.

In vivo manipulation of IgG2a production by isotype-specific autoantibodies

Repeated intranasal infection of Balb/c mice with A/PR/8 influenza virus induced an intense antiviral IgG response dominated by the IgG2a subclass, and accompanied by the appearance of IgG2a reactive autoantibodies. Cells producing IgG2a reactive autoantibodies could then be cloned as hybridomas from the virus infected animals. Monoclonal antibodies produced by selected hybridomas U28, Z26 and Z41 produced IgM-type antibodies with strong specificity for the IgG2a isotype bearing "a" allotypic determinants on the Fc region. These IgG2a specific autoantibodies showed highly preferred binding to solid phase bound or aggregated IgG2a, compared to soluble native IgG2a. Based on these characteristics they were classified as mono-reactive rheumatoid factor (RF)-like autoantibodies. Passive administration of IgM type IgG2a-specific autoantibodies to influenza virus infected animals resulted in a long-term reduction in the secondary antiviral response. This could be demonstrated by decreased virus neutralizing activity of the serum and diminished level of IgG2a-type anti-viral antibodies. A similar effect was observed in Balb/c mice contact sensitized with oxazolone: passive administration of RF-like antibodies resulted in reduced IgG2a response to oxazolone while the level of antibodies belonging to other isotypes was not influenced. These results suggest an isotype-specific regulatory function of these RF-like autoantibodies presumably acting via antigen-antibody complexes.

In vivo polyclonal B-lymphocyte activation elicited by murine viruses

Viruses such as lactate dehydrogenase-elevating virus and adenovirus induce in vivo a polyclonal activation of murine B lymphocytes, followed by a marked increase in the production of immunoglobulin G2a (IgG2a). The role of T lymphocytes in this phenomenon was studied by injection of an anti-CD4 monoclonal antibody able to inhibit the T-helper function. This treatment profoundly depressed the production of IgG2a, whereas it had no effect on the proliferation of B cells. Activated B cells obtained from such infected and treated mice remained able to produce various immunoglobulin isotypes after exposure to an appropriate stimulus. In particular, gamma interferon, which is known to be secreted after viral infection, induced the production of IgG2a. These observations support the hypothesis that the influence of viruses on the switch of immunoglobulins is mediated by T-helper lymphocytes.

Human anti-p68 autoantibodies recognize a common epitope of U1 RNA containing small nuclear ribonucleoprotein and influenza B virus

Autoantibodies from patients with systemic rheumatic diseases were used to map antigenic sites on the 68-kD autoantigen (p68) associated with (U1)RNA-containing small nuclear ribonucleoprotein (snRNP) particles. With truncated recombinant fusion proteins and synthetic peptides, a subset of anti-p68 autoantibodies was found to recognize the amino acid sequence motif Glu-Arg-Lys-Arg-Arg (ERKRR). To investigate the possible involvement of epitopes shared by microbial antigens and host self-components in initiation of autoimmunity (molecular mimicry), a sequence data bank was screened for proteins containing an amino acid motif identical or related to ERKRR. The identical motif was found on the M1 matrix protein of influenza B viruses, and affinity-purified human anti-ERKRR autoantibodies recognized this epitope also in the viral amino acid sequence context. The common epitope recognized by human autoantibodies suggests that influenza B virus infection may play a role in initiation of the anti-p68 and anti-(U1)RNP autoimmune response.

Production of an equine monoclonal antibody specific for the H7 hemagglutinin of equine influenza virus

Peripheral blood leucocytes from a pony previously exposed to equine influenza virus (H3, N8) and vaccinated with killed virus (H3, N8 and H7, N7 subtypes) were cultured in vitro with live A/equine/Prague/56 (H7, N7). On the sixth day of culture, cells were harvested and fused with mouse myeloma cells (X63-Ag8.653). From this fusion, one hemagglutinin specific, equine IgG monoclonal antibody secreting hybridoma was identified and cloned twice by limiting dilution. The antibody inhibited hemagglutination by nine H7 equine influenza virus isolates obtained over a 21-year period, but did not inhibit A/equine/Miami/63 (H3, N8), or A/PR/8/34 (H1, N1). The neutralizing titer of hybridoma induced, nude mouse ascitic fluid was 10(-4.5) when tested in eggs against 100 egg infective doses (EID50) A/equine/Prague/1/56. The hybridoma continued to synthesize antibody during more than 4 months in continuous culture.

Activated B cells can deliver help for the in vitro generation of antiviral cytotoxic T cells

The experiments described in this paper show that activated B cells can deliver help for antiviral cytotoxic T (Tc) cell responses in vitro. This conclusion is based on four observations. (i) Influenza viruses induced secondary Tc cell responses in vitro in the absence of CD4+ T cells. This capacity correlated with the B-cell mitogenicity of these viruses. (ii) Depletion of both CD4+ T cells and B cells prevented the generation of anti-influenza Tc cell responses, whereas depletion of either CD4+ T cells or B cells alone failed to do so. In addition, supplementation of unprimed B cells restored the Tc cell responsiveness of primed splenocytes that had been depleted of both CD4+ T cells and B cells. (iii) Contact between T and B cells was not obligatory for the delivery of B-cell helper signal, and hence help was mediated by a soluble factor(s). (iv) Lipopolysaccharide-activated B cells could replace the CD4+ T-cell requirement in the induction of Tc cell responses to nonmitogenic influenza virus in vitro.

The non-specific stimulation of avian peripheral blood lymphocytes from uninfected chickens by paramyxoviruses and influenza viruses

Avian paramyxovirus-3 was mitogenic to peripheral blood lymphocytes (PBL) from about half the normal birds sampled from 3 inbred flocks. Eight other myxoviruses including Newcastle disease virus, Sendai virus and influenza virus were also irregularly mitogenic. This could complicate in vitro assays for specific immunity.

Characterization of subtype-specific and cross-reactive helper-T-cell clones recognizing influenza virus hemagglutinin

The specificity and function of two T-cell clones derived from A/Memphis/1/71 (H3) influenza virus (Mem 71)-immune BALB/c spleen cells have been compared. One clone, X-31 clone 1, was subtype specific, proliferating in response to influenza strains of the H3 subtype only. The other, Jap clone 3, cross-reacted in proliferation assays with heterologous subtypes of influenza A, but not type B. Both clones recognized the HA1 chain of the hemagglutinin (HA) molecule and their proliferation in response to detergent-disrupted virus could be specifically inhibited by monoclonal antibodies to the HA. The T-cell clones were of the L3T4+ phenotype. Both recognized antigen in association with I-Ed, as indicated by studies with H-2 recombinant strains of mice and by blocking with monoclonal anti-I-E antibody. In vivo, both clones elicited a delayed-type hypersensitivity (DTH) reaction when inoculated into mouse footpads together with virus, X-31 clone 1 again displaying subtype specificity and Jap clone 3 being cross-reactive. The clones were also able to provide factor-mediated help in vitro to virus-primed B cells in an anti-HA antibody response. The cross-reactive T-cell clone provided help not only for B cells primed with influenza A subtype H3 and responding to H3 virus in culture, but also for H2 virus-primed B cells making anti-H2 antibody.

In vitro and in vivo stimulation of murine lymphocytes by human respiratory syncytial virus strains

Respiratory syncytial virus (RSV) strains of subtype A (A2, WV9894, and WV12138) and of subtype B (WV1293, WV4843, and WV6873) are mitogenic in vitro for unprimed BALB/c spleen cells. The virus also triggered splenocytes in vitro to secrete immunoglobulins. Plaque-purified and UV-irradiated materials of both RSV subtypes produced comparable levels of DNA synthesis. Infectious materials of both subtypes also induced pronounced responses. Lymphocyte activation with UV-inactivated RSV strain A2 was dose-dependent and maximal responses occurred after 4-5 days of incubation. The virus preparations were mitogenic for spleen cells depleted of T lymphocytes by treatment with anti-Thy 1.2 and complement and for lymphocytes of congenitally athymic mice (nu-nu). They were also mitogenic for highly purified T lymphocytes separated by panning of spleen cells on anti-mouse Ig-coated Petri dishes, suggesting that both B and T lymphocytes respond to the mitogenic activity of RSV. Moreover, mice infected intranasally with RSV strain A2 generated local as well as peripheral cellular and humoral responses.

Direct role of viral hemagglutinin in B-cell mitogenesis by influenza viruses

The mitogenic activity of influenza virus is a function of the hemagglutinin (HA) molecule. Purified HA is mitogenic for murine B lymphocytes but not T lymphocytes. Furthermore, like the intact virus, HA of the H2 (but not H3) subtype is mitogenic only for B cells expressing the class II major histocompatibility complex glycoprotein I-E. Since virus bearing uncleaved HA is as mitogenic as virus bearing cleaved HA, the membrane fusion activity of the HA molecule is not involved.

Relationship between mitogenic activity of influenza viruses and the receptor-binding specificity of their hemagglutinin molecules

The relationship between the mitogenic activity of influenza type A viruses for murine B lymphocytes and the receptor-binding specificity of their hemagglutinin was examined. Receptor-binding specificity was determined by the ability of the virus to agglutinate erythrocytes that had been sialidase treated and then enzymatically resialylated to contain sialyloligosaccharides with defined sequences. Distinct differences in receptor-binding specificity were observed between strongly and weakly mitogenic viruses of the H3 subtype, with strong mitogenic activity correlating with the ability of the virus to recognize the sequence N-glycolylneuraminic acid alpha 2,6 galactose (NeuGc alpha 2,6Gal). Viruses isolated early in the evolution of the H3 subtype (from 1968 to 1971) are relatively weak mitogens and recognize the sequence N-acetylneuraminic acid alpha 2,6 galactose (NeuAc alpha 2,6Gal) but not NeuGc alpha 2,6Gal. H3 viruses isolated since 1972 are strongly mitogenic, and these viruses recognize both NeuGc alpha 2,6Gal and NeuAc alpha 2,6Gal. The amino acid substitution of Tyr for Thr at residue 155 of HA1 may be critical to this change in receptor-binding specificity and mitogenic activity of the later H3 viruses. Horse serum-resistant variants of H3 viruses, which bind preferentially to the sequence NeuAc alpha 2,3Gal, are poorly mitogenic. Differences were also observed between the receptor-binding specificity of the strongly mitogenic H3 viruses and viruses of the H2 and H6 subtypes, the mitogenic activity of which is limited to strains of mice that express the class II major histocompatibility complex glycoprotein I-E. The results indicate that the receptor-binding specificity of the hemagglutinin plays a critical role in determining the mitogenic activity of influenza viruses.

Functional analysis of influenza-specific helper T cell clones in vivo. T cells specific for internal viral proteins provide cognate help for B cell responses to hemagglutinin

We compared the effects of adoptively transferred Th cell clones specific for the influenza hemagglutinin (HA), matrix (M), or nucleoprotein (NP) on the antibody response of nude mice infected with A/PR/8/34 influenza virus. We show that the production of antibodies to the HA absolutely requires the presence of virus-specific Th cells. Further, transfer of a Th clone specific for the internal proteins, M or NP, was as effective as was transfer of an HA-specific clone in supporting an antibody response to the HA. With each of the clones, the kinetics of the response were accelerated by approximately 3 d compared with the antibody response of normal BALB/c mice. The HA- and M-specific clones supported an isotype switch from IgM to IgG and IgA similar to that which occurs during a normal antibody response. Finally, as shown by coinfection experiments, the response required a cognate T-B interaction whether the determinants recognized by the Th and B cell are located on the same viral protein or on different viral proteins within the same virus particle. The implications of these findings for understanding the T-B interactions that occur during an effective antiviral antibody response are discussed.

Murine helper T lymphocyte response to influenza virus: recognition of haemagglutinin by subtype-specific and cross-reactive T cell clones

Helper T cel lines specific for influenza virus were established by continuous culture of Mem 71-Bel (H3) virus-immune spleen cells in the presence of virus and antigen-presenting cells and their specificity assessed in proliferation experiments. A line stimulated in vitro with Mem 71-Bel virus was able to proliferate in response to viruses of the same, and also of different, type A haemagglutinin (HA) subtypes as the immunizing virus but not to a type B influenza virus. A component of this cross-reactivity was due to recognition of the HA molecule. Lines stimulated in vitro with purified disrupted H3 or H2 viruses showed a higher degree of cross-reactivity. Of nine clones isolated from these lines, seven were directed against the HA molecule and recognized the HA1 chain. The HA-specific T cell clones were either subtype-specific T cell clones were either subtype-specific, recognizing only viruses of the H3 subtype, or cross-reactive, also recognizing viruses of the H2 subtype of type A (but not type B). Subtype-specific and cross-reactive T cell clones were shown to function as helper T cells in vitro. In addition to collaborating with H3 virus-primed B cells responding to H3 virus in culture, the cross-reactive T cell clone could also provide help for H2 virus-primed B cells making anti-HA antibody in response to H2 virus.

Mitogenic activity of influenza virus and haemagglutinin

Influenza A viruses behave as T cell-independent B cell mitogens in vitro. The magnitude of the proliferation induced varies with the haemagglutinin subtype of the virus, the order being H2 greater than H6 greater than H3 greater than H1 for Balb/c mice. H3 viruses are equally mitogenic for all strains of mice tested. In contrast, the mitogenic response to H2 and H6 viruses is controlled by the I-E subregion of the major histocompatibility complex. These viruses are mitogenic only for strains of mice that express surface I-E antigen (haplotypes a, d, k, p, r), and not for haplotypes b, f, q, s, which fail to synthesize a normal E alpha chain and do not express surface I-E antigen. Mitogenesis by H2 and H6 viruses may involve direct interaction of virus with I-E molecules on the B lymphocyte or an accessory cell, whereas mitogenesis by H3 viruses does not appear to involve I-E.