ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, can infect both CD4+ and CD8+ T cells but requires CD4+ T cells in order to cause paralysis and immunodeficiency.

When neonatal FVB/N mice were inoculated with ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, they developed a progressive bilateral hindlimb paralysis and immunodeficiency leading to death 4 to 6 weeks after inoculation. T lymphocytes have been shown to be primarily responsible for this ts1-induced syndrome. Here we compare the role played by each subset of T lymphocytes, i.e., CD4+ and CD8+ T cells, in disease development. Mice were depleted of a specific subset for the first 10 days of their lives by using either anti-CD4 or anti-CD8 monoclonal antibodies in vivo. Disease development in these mice was then monitored. Depletion of CD4+ T cells significantly attenuated the ts1-induced syndrome: virus replication was decreased, disease latency was extended, and death was prevented in 60% of the mice. Similar treatment with anti-CD8 antibody had almost no effect on disease progression. However, when depletion was begun 2 weeks after neonatal ts1 inoculation, CD4+ T cell depletion did not affect disease development. ts1 infected CD4+ and CD8+ T lymphocytes equally well in vivo, as shown by flow cytometric analysis, but virus replication was restricted primarily to the CD4+ subset of T cells, as found by in vitro assay. Hence, CD4+ T lymphocytes play an important role in the development of ts1-induced paralysis and immunodeficiency. The mechanism of this CD4+ T-cell-mediated disease production by ts1 is not clear; however, increased replication of ts1 in the CD4+ T cells, especially in the early stages of the disease, seems to play a crucial role.     

Role of CD4+ and CD8+ T cells in murine resistance to street rabies virus.

Mice of the SJL/J and BALB/cByJ inbred strains are naturally resistant to street rabies virus (SRV) injected via the intraperitoneal route. To determine the cellular mechanism of resistance, monoclonal antibodies specific for CD4+ or CD8+ subsets of T cells were used to deplete the respective cell population in SRV-infected animals. Elimination of CD4+ T-helper cells abrogated the production of immunoglobulin G (IgG) neutralizing antibodies in response to rabies virus infection and reversed the resistant status of SJL/J and BALB/cByJ mice. In contrast, in vivo depletion of CD8+ cytotoxic T cells had no measurable effect on host resistance to SRV. These results indicate that serum neutralizing antibodies of the IgG class are a primary immunological mechanism of defense against rabies virus infection in this murine model of disease. CD8+ cytotoxic T lymphocytes, which have been shown to transfer protection in other rabies virus systems, appear to have no role in protecting mice against intraperitoneally injected SRV.     

Paralysis of street rabies virus-infected mice is dependent on T lymphocytes.

Street rabies virus (SRV)-infected T-lymphocyte-deficient (nude) mice, in contrast to euthymic mice, did not develop hindlimb paralysis prior to death. To document the role of T lymphocytes in rabies virus-associated paralysis, 10(8) spleen cells from normal immunocompetent euthymic mice were transferred to nude mice and the recipient mice were challenged with SRV. One hundred percent of the reconstituted mice developed paralysis and died. Depletion of T cells from the donor spleen suspension prior to transfer abrogated the development of paralysis but did not prevent the deaths of the recipient animals. Mice receiving 10(8) rabies virus-immune spleen cells did not become paralyzed and did not die. Nude mice inoculated with either rabies virus-immune or normal mouse serum prior to and following SRV inoculation did not develop paralysis. Immune serum protected the mice, whereas animals inoculated with normal serum died. Central nervous system inflammatory responses in nude mice immunologically reconstituted with normal spleen cells were characterized by diffuse cellular infiltrates in the parenchyma and extensive perivascular cuffing. Perivascular infiltrates included CD8+ and CD4+ T lymphocytes and Mac-1+ macrophage-microglial cells. Inflammatory cells in the parenchyma were limited to CD8+ lymphocytes and Mac-1+ cells. These observations indicate that paralysis of SRV-infected mice is dependent on T lymphocytes. Whether injury leading to paralysis is mediated by T lymphocytes or by an influence of T lymphocytes on macrophage-microglial cells or other cells remains to be determined.     

Adoptive immunotherapy of murine cytomegalovirus adrenalitis in the immunocompromised host: CD4-helper-independent antiviral function of CD8-positive memory T lymphocytes derived from latently infected donors.

The ability of memory T lymphocytes derived from latently infected mice to control murine cytomegalovirus disease in the immunocompromised host was studied by adoptive transfer experiments. At a stage of pathogenesis when virus had already colonized target tissues, a therapeutic antiviral function could be ascribed to the CD8+ subset. This in vivo function was not restricted to sites in which intravenously infused lymphocytes usually are trapped or home in, such as the lungs or the spleen, respectively, but was also evident in the adrenal glands, a site to which antiviral effector cells have to specifically migrate. Specific infiltration of adrenal gland cortical tissue by donor-derived CD8+ memory T lymphocytes was demonstrated. CD4+ memory T lymphocytes had no antiviral effect by themselves and also were not required for the function of the CD8+ effector cells in this short-term immunotherapy model. These findings should help settle the debate about which subset of T lymphocytes comprises the effector cells that can directly control cytomegalovirus infection in the murine model system.     

Virus-specific CD8+ T-cell responses in mice transgenic for a T-cell receptor beta chain selected at random.

The consequences of severely limiting the T-cell receptor (TCR) repertoire available for the response to intranasal infection with an influenza A virus or with Sendai virus have been analyzed by using H-2k mice (TG8.1) transgenic for a TCR beta-chain gene (V beta 8.1D beta 2J beta 2.3C beta 2). Analyzing the prevalence of V beta 8.1+ CD8+ T cells in lymph node cultures from nontransgenic (non-TG) H-2k controls primed with either virus and then stimulated in vitro with the homologous virus or with anti-CD3 epsilon showed that this TCR is not normally selected from the CD8+ T-cell repertoire during these infections. However, the TG8.1 mice cleared both viruses and generated virus-specific effector cytotoxic T lymphocytes (CTL) and memory CTL precursors, though the responses were delayed compared with the non-TG controls. Depletion of the CD4+ T-cell subset had little effect on the course of influenza virus infection but substantially slowed the development of the Sendai virus-specific CTL response and virus elimination in both the TG8.1 and non-TG mice, indicating that CD4+ helpers are promoting the CD8+ T-cell response in the Sendai virus model. Even so, restricting the available T-cell repertoire to lymphocytes expressing a single TCR beta chain still allows sufficient TCR diversity for CD8+ T cells (acting in the presence or absence of the CD4+ subset) to limit infection with an influenza A virus and a parainfluenza type 1 virus.     

Efficacious control of cytomegalovirus infection after long-term depletion of CD8+ T lymphocytes.

Although the relative contribution of different immune effector functions to clearing tissues of cytomegalovirus is controversial, the contribution of CD8+ T lymphocytes has generally been accepted as essential. In this report, we show that under certain conditions the CD8+ T-lymphocyte subset can be dispensable for clearance of cytomegalovirus. Mice depleted of the CD8+ T-lymphocyte subset eliminated infectious virus with a clearance kinetics similar to that of normal mice. Adoptive transfer studies revealed that the limitation of virus spread required the cooperation between the CD4+ subset and other cells. Comparison between protective functions generated in fully immunocompetent and in CD8- mice demonstrated that elimination of the CD8+ subset before infection altered the quality of the antiviral immune response. The compensatory protective activity gained by CD4+ cells in CD8- mice was absent in normal mice recovering from virus infection.     

CD8-positive T lymphocytes specific for murine cytomegalovirus immediate-early antigens mediate protective immunity.

We have shown in a murine model system for acute, lethal cytomegalovirus (CMV) disease in the immunocompromised natural host that control of virus multiplication in tissues, protection from virus-caused tissue destruction, and survival are mediated by virus-specific CD8+ CD4-T lymphocytes. Protection from a lethal course of disease did not result in a rapid establishment of virus latency, but led to a long-lasting, persistent state of infection. The CD8- CD4+ subset of T lymphocytes was not effective by itself in controlling murine CMV (MCMV) multiplication in tissue or essential for the protective function of the CD8+ CD4- effector cells. The antiviral efficacy of the purified CD8+ CD4- subset was not impaired by preincubation with fibroblasts that presented viral structural antigens, but was significantly reduced after depletion of effector cells specific for the nonstructural immediate-early antigens of MCMV, which are specified by the first among a multitude of viral genes expressed during MCMV replication in permissive cells. Thus, MCMV disease provides the first example of a role for nonstructural herpesvirus immediate-early antigens in protective immunity.     

Nipah virus infects specific subsets of porcine peripheral blood mononuclear cells

Nipah virus (NiV), a zoonotic paramyxovirus, is highly contagious in swine, and can cause fatal infections in humans following transmission from the swine host. The main viral targets in both species are the respiratory and central nervous systems, with viremia implicated as a mode of dissemination of NiV throughout the host. The presented work focused on the role of peripheral blood mononuclear cells (PBMC) in the viremic spread of the virus in the swine host. B lymphocytes, CD4-CD8-, as well as CD4+CD8- T lymphocytes were not permissive to NiV, and expansion of the CD4+CD8- cells early post infection was consistent with functional humoral response to NiV infection observed in swine. In contrast, significant drop in the CD4+CD8- T cell frequency was observed in piglets which succumbed to the experimental infection, supporting the hypothesis that antibody development is the critical component of the protective immune response. Productive viral replication was detected in monocytes, CD6+CD8+ T lymphocytes and NK cells by recovery of infectious virus in the cell supernatants. Virus replication was supported by detection of the structural N and the non-structural C proteins or by detection of genomic RNA increase in the infected cells. Infection of T cells carrying CD6 marker, a strong ligand for the activated leukocyte cell adhesion molecule ALCAM (CD166) highly expressed on the microvascular endothelial cell of the blood-air and the blood-brain barrier may explain NiV preferential tropism for small blood vessels of the lung and brain.     

CD4 T cell-dependent CD8 T cell maturation

We have investigated the contribution of CD4 T cells to the optimal priming of functionally robust memory CD8 T cell subsets. Intranasal infection of CD4 T cell-deficient (CD4(-/-)) mice with lymphocytic choriomeningitis virus resulted in the elaboration of virus-specific CD8 T cell responses that cleared the infection. However, by comparison with normal mice, the virus-specific CD8 T cells in CD4(-/-) mice were quantitatively and qualitatively different. In normal mice, lymphocytic choriomeningitis virus-specific memory CD8 T cells are CD44(high), many are CD122(high), and a majority of these cells regain expression of CD62L overtime. These cells produce IFN-gamma and TNF-alpha, and a subset also produces IL-2. In the absence of CD4 T cell help, a distinct subset of memory CD8 T cells develops that remains CD62L(low) up to 1 year after infection and exhibits a CD44(int)CD122(low) phenotype. These cells are qualitatively different from their counterparts in normal hosts, as their capacity to produce TNF-alpha and IL-2 is diminished. In addition, although CD4-independent CD8 T cells can contain the infection following secondary viral challenge, their ability to expand is impaired. These findings suggest that CD4 T cell responses not only contribute to the optimal priming of CD8 T cells in chronically infected hosts, but are also critical for the phenotypic and functional maturation of CD8 T cell responses to Ags that are more rapidly cleared. Moreover, these data imply that the development of CD62L(high) central memory CD8 T cells is arrested in the absence of CD4 T cell help.     

Pivotal role of antibody and subsidiary contribution of CD8+ T cells to recovery from infection in a murine model of Japanese encephalitis

The immunological correlates for recovery from primary Japanese encephalitis virus (JEV) infection in humans and experimental animals remain poorly defined. To investigate the relative importance of the adaptive immune responses, we have established a mouse model for Japanese encephalitis in which a low-dose virus inoculum was administered into the footpads of adult C57BL/6 mice. In this model, ~60% of the mice developed a fatal encephalitis and a virus burden in the central nervous system (CNS). Using mice lacking B cells (μMT(-/-) mice) and immune B cell transfer to wild-type mice, we show a critically important role for humoral immunity in preventing virus spread to the CNS. T cell help played an essential part in the maintenance of an effective antibody response necessary to combat the infection, since mice lacking major histocompatibility complex class II showed truncated IgM and blunted IgG responses and uniformly high lethality. JEV infection resulted in extensive CD8(+) T cell activation, judged by upregulation of surface markers CD69 and CD25 and cytokine production after stimulation with a JEV NS4B protein-derived H-2D(b)-binding peptide and trafficking of virus-immune CD8(+) T cells into the CNS. However, no significant effect of CD8(+) T cells on the survival phenotype was found, which was corroborated in knockout mice lacking key effector molecules (Fas receptor, perforin, or granzymes) of cytolytic pathways triggered by T lymphocytes. Accordingly, CD8(+) T cells are mostly dispensable for recovery from infection with JEV. This finding highlights the conflicting role that CD8(+) T cells play in the pathogenesis of JEV and closely related encephalitic flaviviruses such as West Nile virus.     

Yellow fever virus encephalitis: properties of the brain-associated T-cell response during virus clearance in normal and gamma interferon-deficient mice and requirement for CD4+ lymphocytes

Viral encephalitis caused by neuroadapted yellow fever 17D virus (PYF) was studied in parental and gamma interferon (IFN-gamma)-deficient (IFN-gamma knockout [GKO]) C57BL/6 mice. The T-cell responses which enter the brain during acute fatal encephalitis of nonimmunized mice, as well as nonfatal encephalitis of immunized mice, were characterized for relative proportions of CD4+ and CD8+ cells, their proliferative responses, and antigen-specific expression of cytokines during stimulation in vitro. Unimmunized mice accumulated only low levels of T cells within the brain during fatal disease, whereas the brains of immunized mice contained higher levels of both T-cell subsets in response to challenge, with CD8+ cells increased relative to the CD4+ subset. The presence of T cells correlated with the time at which virus was cleared from the central nervous system in both parental and GKO mice. Lymphocytes isolated from the brains of challenged immunized mice failed to proliferate in vitro in response to T-cell mitogens or viral antigens; however, IFN-gamma, interleukin 4 (IL-4), and, to a lesser extent, IL-2 were detectable after stimulation. The levels of IFN-gamma, but not IL-2 or IL-4, were augmented in response to viral antigen, and this specificity was detectable in the CD4+ compartment. When tested for the ability to survive both immunization and challenge with PYF virus, GKO and CD8 knockout mice did not differ from parental mice (80 to 85% survival), although GKO mice exhibited a defect in virus clearance. In contrast, CD4 knockout and Igh-6 mice were unable to resist challenge. The data implicate antibody in conjunction with CD4+ lymphocytes bearing a Th1 phenotype as the critical factors involved in virus clearance in this model.     

Suppression of simian immunodeficiency virus replication in vitro by CD8+ lymphocytes

The AIDS-like disease in rhesus monkeys induced by the simian immunodeficiency virus (SIV) has been used as a model to explore the nature of the T lymphocyte response after infection with viruses of the human immunodeficiency virus family. Activated CD8+ lymphocytes are present in increased numbers in the paracortex of lymph nodes of SIV-infected rhesus monkeys with a lymphadenopathy syndrome. We demonstrate that SIV is more readily isolated from CD8+ lymphocyte-depleted PBL of SIV-infected animals than from their unfractionated PBL. Rather than reflecting the fact that the CD8+ lymphocyte-depleted cell populations are simply enriched for CD4+ lymphocytes, this indicates that CD8+ cells themselves are critical in this regulatory interaction. In fact, CD8+ lymphocytes from SIV-infected but not uninfected rhesus monkeys can block SIV replication in vitro in PBL populations. A T lymphocyte population that blocks replication of viruses of the HIV family may contribute to containing the progression of AIDS.     

The role of CD4+ cells in sustaining lymphocyte proliferation during lymphocytic choriomeningitis virus infection.

The murine immune response to lymphocytic choriomeningitis virus (LCMV) infection involves the activation of CD8+, class I MHC-restricted and virus-specific CTL. At times coinciding with CTL activation, high levels of IL-2 gene expression and production occur, the IL-2R is expressed, and T cell blastogenesis and proliferation are induced. We have previously found that, although both CD4+ and CD8+ T cell subsets transcribe IL-2, the CD4+ subset appears to be the major producer of IL-2 whereas the CD8+ subset appears to be the major proliferating population when the subsets are separated after activation in vivo. The studies presented here were undertaken to examine the contribution made by the CD4+ subset to lymphocyte proliferation in vivo. Responses to LCMV infection were examined in intact mice and in mice depleted of CD4+ or CD8+ subsets by antibody treatments in vivo. Protocols were such that in vivo treatments with anti-CD4 or anti-CD8 depleted the respective subset by greater than 90%. In situ hybridizations demonstrated that the IL-2 gene was expressed in non-B lymphocytes isolated from either CD4+ cell-depleted or CD8+ cell-depleted mice on day 7 post-infection with LCMV. When placed in culture, however, cells from CD8+ cell-depleted mice produced significantly higher levels of detectable IL-2 than did cells isolated from CD4+ cell-depleted mice on day 7 post-infection. IL-2 was apparently produced in vivo in mice depleted of either CD4+ or CD8+ cells, as expression of the gene for the p55 chain of the IL-2R, IL-2 responsiveness, and lymphocyte proliferation were observed with cells isolated from both sets of mice. Lymphocyte proliferation was shown to be sustained in mice depleted of CD4+ cells in vivo by three criteria: 1) non-B lymphocytes isolated from infected mice depleted of CD4+ cells underwent more DNA synthesis than did those isolated from uninfected mice or from infected mice depleted of CD8+ cells; 2) leukocyte yields were expanded during infection of CD4+ cell-depleted mice; and 3) CD8+ cell numbers were increased during infection of CD4+ cell-depleted mice. The majority of non-B lymphocytes having the characteristics of blast lymphocytes was recovered in the CD8+ populations isolated from infected CD4+ cell-depleted mice. These findings suggest that the requirement for the CD4+ subset to sustain CD8+ lymphocyte proliferation in vivo is limited, and that CD4+ and CD8+ cell types can function independently in many aspects of their responses to viral infections.     

Vaccination against persistent viral infection exacerbates CD4+ T-cell-mediated immunopathological disease.

Lymphocytic choriomeningitis virus (LCMV) infection of normal mice results in a fatal immunopathologic meningitis mediated by CD8+ cytotoxic T lymphocytes (CTL). We have previously shown that female beta2-microglobulin-deficient (beta2m-/-) mice, which are also deficient in CD8+ T cells, are susceptible to LCMV-induced immune-mediated meningitis, characterized by significant weight loss and mortality. This LCMV disease in beta2m-/- mice is mediated by CD4+ T lymphocytes. Our previous studies have also demonstrated that male beta2m-/- mice are less susceptible than female beta2m-/- mice to LCMV-induced, immune-mediated mortality and weight loss. In this report, we show that vaccination of male beta2m-/- mice enhances immunopathology following intracranial infection with LCMV. We observed increased production of gamma interferon (IFN-gamma), an increase in CD4+ CTL precursor frequency, and an increased frequency of IFN-gamma-producing cells from spleen cells of vaccinated male beta2m-/- mice. Vaccinated male beta2m-/- mice also had significantly increased inflammation in the cerebrospinal fluid (CSF), characterized by a large CD4+ T-cell infiltrate. CSF cells from vaccinated mice showed increased production of IFN-gamma on day 7 postchallenge. Neither vaccinated nor control beta2m-/- mice were able to clear virus, and the two groups had similarly high levels of virus early after infection. These results suggest that the magnitude of the early immune response is more important than the level of virus in the brain in determining the outcome of immunopathology in beta2m-/- mice. We show here that vaccination can increase CD4+ T-cell-dependent immunopathology to a persistent viral infection.     

Frequencies of circulating cytolytic,CD45RA+CD27-, CD8+ T lymphocytes depend on infection with CMV

Viral infections may cause serious disease unless the adaptive immune system is able to clear the viral agents through its effector arms. Recent identification and functional characterization of subpopulations of human CD8(+) T cells has set the stage to study the correlation between the appearance of particular subsets and common viral infections during childhood, i.e., EBV, CMV, varicella-zoster virus (VZV), and the attenuated measles-mumps-rubella (MMR) vaccine strains. In a cohort of 220 healthy children we analyzed lymphocytes and subpopulations of CD4(+) and CD8(+) T cells. The presence of the cytolytic CD45RA(+)CD27(-) subset of CD8(+) T cells correlated with prior CMV infection as defined by seroconversion (p < 0.0001). The number of this CD8(+) T cell subset remained stable during follow-up over 3 years in 40 children. The CD45RA(+)CD27(-) subset of CD8(+) T cells first appeared during acute CMV infection and subsequently stabilized at an individual set-point defined by age and immunocompetence. The functional importance of these cells in CMV surveillance was reflected by their increased numbers in immunosuppressed pediatric kidney transplant patients. Preferential expansion of CD8(+)CD45RA(+)CD27(-) cytolytic T cells seems unique for CMV.     

Differential dynamics of CD4(+) and CD8(+) T-lymphocyte proliferation and activation in acute simian immunodeficiency virus infection

Although lymphocyte turnover in chronic human immunodeficiency virus and simian immunodeficiency virus (SIV) infection has been extensively studied, there is little information on turnover in acute infection. We carried out a prospective kinetic analysis of lymphocyte proliferation in 13 rhesus macaques inoculated with pathogenic SIV. A short-lived dramatic increase in circulating Ki-67(+) lymphocytes observed at 1 to 4 weeks was temporally related to the onset of SIV replication. A 5- to 10-fold increase in Ki-67(+) CD8(+) T lymphocytes and a 2- to 3-fold increase in Ki-67(+) CD3(-) CD8(+) natural killer cells accounted for >85% of proliferating lymphocytes at peak proliferation. In contrast, there was little change in the percentage of Ki-67(+) CD4(+) T lymphocytes during acute infection, although transient increases in Ki-67(-) and Ki-67(+) CD4(+) T lymphocytes expressing CD69, Fas, and HLA-DR were observed. A two- to fourfold decline in CD4(+) T lymphocytes expressing CD25 and CD69 was seen later in SIV infection. The majority of Ki-67(+) CD8(+) T lymphocytes were phenotypically CD45RA(-) CD49d(hi) Fas(hi) CD25(-) CD69(-) CD28(-) HLA-DR(-) and persisted at levels twofold above baseline 6 months after SIV infection. Increased CD8(+) T-lymphocyte proliferation was associated with cell expansion, paralleled the onset of SIV-specific cytotoxic T-lymphocyte activity, and had an oligoclonal component. Thus, divergent patterns of proliferation and activation are exhibited by CD4(+) and CD8(+) T lymphocytes in early SIV infection and may determine how these cells are differentially affected in AIDS.     

Rabies virus ocular disease: T-cell-dependent protection is under the control of signaling by the p55 tumor necrosis factor alpha receptor, p55TNFR

Following brain infection, the Challenge Virus Standard strain of rabies virus infects the retina. Rabies virus ocular infection induces the infiltration of neutrophils and predominantly T cells into the eye. The role of tumor necrosis factor alpha (TNF-alpha)-lymphotoxin signaling in the control of rabies virus ocular infection and inflammatory cell infiltration was assessed using mice lacking the p55 TNF-alpha receptor (p55TNFR(-/-) mice). The incidence of ocular disease and the intensity of retinal infection were greater in p55TNFR(-/-) mice than in C57BL/6 mice: the aggravation correlated with less neutrophil and T-cell infiltration. This indicates that cellular infiltration is under the control of the p55 TNF-alpha receptor and suggests that inflammatory cells may protect the eye against rabies virus ocular infection. The role of T cells following rabies virus ocular disease was assessed by comparison of rabies virus infection in nude mice with their normal counterparts. Indeed, the incidence and severity of the rabies virus ocular disease were higher in athymic nude mice than in BALB/c mice, indicating that T lymphocytes are protective during rabies virus ocular infection. Moreover, few T cells and neutrophils underwent apoptosis in rabies virus-infected retina. Altogether, these data suggest that T lymphocytes and neutrophils are able to enter the eye, escape the immune privilege status, and limit rabies virus ocular disease. In conclusion, rabies virus-mediated eye disease provides a new model for studying mechanisms regulating immune privilege during viral infection.     

异型流感病毒感染相关T淋巴细胞增殖活性及IFN-γ阳性Ｔ淋巴细胞水平研究

为探讨机体异型流感病毒间交叉保护作用机制,将实验动物随机分成实验组和对照组,测定异型流感病毒感染后病毒载量,T淋巴细胞增殖活性和IFN-γ阳性CD3＋CD8＋及CD3＋CD4＋淋巴细胞水平的变化。结果显示,异型流感病毒感染后产生的交叉免疫应答反应可能与T淋巴细胞增殖有关;与CTL及Th1类淋巴细胞水平相关,并有时间限制性;IL-2可以加强异型流感病毒感染后IFN-γ阳性CD3＋CD8＋淋巴细胞水平。本研究为制备能够抵御变异流感病毒感染的疫苗提供了理论基础。