Mechanisms of recovery from a generalized viral infection: mousepox. I. The effects of anti-thymocyte serum

Resistance To Poxvirus Lethality Does Not Require the Necroptosis Proteins RIPK3 or MLKL

Receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like pseudokinase (MLKL) are proteins that are critical for necroptosis, a mechanism of programmed cell death that is both activated when apoptosis is inhibited and thought to be antiviral. Here, we investigated the role of RIPK3 and MLKL in controlling the Orthopoxvirus ectromelia virus (ECTV), a natural pathogen of the mouse. We found that C57BL/6 (B6) mice deficient in RIPK3 (Ripk3^-/-) or MLKL (Mlkl^-/-) were as susceptible as wild-type (WT) B6 mice to ECTV lethality after low-dose intraperitoneal infection and were as resistant as WT B6 mice after ECTV infection through the natural footpad route. Additionally, after footpad infection, Mlkl^-/- mice, but not Ripk3^-/- mice, endured lower viral titers than WT mice in the draining lymph node (dLN) at three days postinfection and in the spleen or in the liver at seven days postinfection. Despite the improved viral control, Mlkl^-/- mice did not differ from WT mice in the expression of interferons or interferon-stimulated genes or in the recruitment of natural killer (NK) cells and inflammatory monocytes (iMOs) to the dLN. Additionally, the CD8 T-cell responses in Mlkl^-/- and WT mice were similar, even though in the dLNs of Mlkl^-/- mice, professional antigen-presenting cells were more heavily infected. Finally, the histopathology in the livers of Mlkl^-/- and WT mice at 7 dpi did not differ. Thus, the mechanism of the increased virus control by Mlkl^-/- mice remains to be defined. IMPORTANCE The molecules RIPK3 and MLKL are required for necroptotic cell death, which is widely thought of as an antiviral mechanism. Here we show that C57BL/6 (B6) mice deficient in RIPK3 or MLKL are as susceptible as WT B6 mice to ECTV lethality after a low-dose intraperitoneal infection and are as resistant as WT B6 mice after ECTV infection through the natural footpad route. Mice deficient in MLKL are more efficient than WT mice at controlling virus loads in various organs. This improved viral control is not due to enhanced interferon, natural killer cell, or CD8 T-cell responses. Overall, the data indicate that deficiencies in the molecules that are critical to necroptosis do not necessarily result in worse outcomes following viral infection and may improve virus control.

The Pathogenesis and Immunobiology of Mousepox

Ectromelia virus is a mouse-specific orthopoxvirus that, following footpad infection or natural transmission, causes mousepox in most strains of mice, while a few strains, such as C57BL/6, are resistant to the disease but not to the infection. Mousepox is an acute, systemic, highly lethal disease of remarkable semblance to smallpox, caused by the human-specific variola virus. Starting in 1929 with its discovery by Marchal, work with ECTV has provided essential information for our current understanding on how viruses spread lympho-hematogenously, the genetic control of antiviral resistance, the role of different components of the innate and adaptive immune system in the control of primary and secondary infections with acute viruses, and how the mechanisms of immune evasion deployed by the virus affect virulence in vivo. Here, I review the literature on the pathogenesis and immunobiology of ECTV infection in vivo.

Vaccine-induced protection against orthopoxvirus infection is mediated through the combined functions of CD4 T cell-dependent antibody and CD8 T cell responses

Antibody production by B cells in the absence of CD4 T cell help has been shown to be necessary and sufficient for protection against secondary orthopoxvirus (OPV) infections. This conclusion is based on short-term depletion of leukocyte subsets in vaccinated animals, in addition to passive transfer of immune serum to naive hosts that are subsequently protected from lethal orthopoxvirus infection. Here, we show that CD4 T cell help is necessary for neutralizing antibody production and virus control during a secondary ectromelia virus (ECTV) infection. A crucial role for CD4 T cells was revealed when depletion of this subset was extended beyond the acute phase of infection. Sustained depletion of CD4 T cells over several weeks in vaccinated animals during a secondary infection resulted in gradual diminution of B cell responses, including neutralizing antibody, contemporaneous with a corresponding increase in the viral load. Long-term elimination of CD8 T cells alone delayed virus clearance, but prolonged depletion of both CD4 and CD8 T cells resulted in death associated with uncontrolled virus replication. In the absence of CD4 T cells, perforin- and granzyme A- and B-dependent effector functions of CD8 T cells became critical. Our data therefore show that both CD4 T cell help for antibody production and CD8 T cell effector function are critical for protection against secondary OPV infection. These results are consistent with the notion that the effectiveness of the smallpox vaccine is related to its capacity to induce both B and T cell memory.

IMPORTANCE

Smallpox eradication through vaccination is one of the most successful public health endeavors of modern medicine. The use of various orthopoxvirus (OPV) models to elucidate correlates of vaccine-induced protective immunity showed that antibody is critical for protection against secondary infection, whereas the role of T cells is unclear. Short-term leukocyte subset depletion in vaccinated animals or transfer of immune serum to naive, immunocompetent hosts indicates that antibody alone is necessary and sufficient for protection. We show here that long-term depletion of CD4 T cells over several weeks in vaccinated animals during secondary OPV challenge reveals an important role for CD4 T cell-dependent antibody responses in effective virus control. Prolonged elimination of CD8 T cells alone delayed virus clearance, but depletion of both T cell subsets resulted in death associated with uncontrolled virus replication. Thus, vaccinated individuals who subsequently acquire T cell deficiencies may not be protected against secondary OPV infection.

Susceptibility of BALB/c-nu/nu mice and BALB/c mice to equine herpesvirus 9 infection

This study aimed to clarify the timing and infectivity of equine herpesvirus 9 (EHV-9) infection in BALB/c-nu/nu mice and their immunocompetent counterpart (BALB/c). Following intranasal inoculation with 10(5) PFU of EHV-9, specimens from 8 mice per group were collected at different times postinoculation (PI) and assessed using histopathology, immunohistochemistry for viral antigen, and quantitative real-time polymerase chain reaction for ORF30 gene expression. In BALB/c-nu/nu mice, EHV-9 antigen was abundant in olfactory epithelia of all inoculated animals, and in the olfactory bulb of 1 animal. In contrast, only 1 BALB/c mouse per time point had rhinitis, with mild to moderate immunopositivity starting from 12 to 48 h PI, followed by a gradual virus clearance at 72 h PI. Statistically, significant differences were noted in the immunohistochemistry reactions between the 2 mouse strains, indicating that BALB/c-nu/nu is more susceptible to infection. Relative expression levels of ORF30 gene in olfactory epithelia were significantly different between the 2 groups, with the exception of 12 h PI, when BALB/c-nu/nu animals showed dramatic increases in ORF30 gene expression level until 48 h PI, followed by a decline in expression level until the end of experiment. In contrast, the expression level in brains showed no differences between mouse strain except at 96 h PI. In both strains, the highest messenger RNA expression was detected at 48 h PI, followed by a decline in BALB/c mice, proving a rapid clearance of virus in BALB/c and a gradual slowing down of the increased expression levels in BALB/c-nu/nu.

Comparable polyfunctionality of ectromelia virus- and vaccinia virus-specific murine T cells despite markedly different in vivo replication and pathogenicity

Vaccinia virus (VACV) stimulates long-term immunity against highly pathogenic orthopoxvirus infection of humans (smallpox) and mice (mousepox [ectromelia virus {ECTV}]) despite the lack of a natural host-pathogen relationship with either of these species. Previous research revealed that VACV is able to induce polyfunctional CD8(+) T-cell responses after immunization of humans. However, the degree to which the functional profile of T cells induced by VACV is similar to that generated during natural poxvirus infection remains unknown. In this study, we monitored virus-specific T-cell responses following the dermal infection of C57BL/6 mice with ECTV or VACV. Using polychromatic flow cytometry, we measured levels of degranulation, cytokine expression (gamma interferon [IFN-γ], tumor necrosis factor alpha [TNF-α], and interleukin-2 [IL-2]), and the cytolytic mediator granzyme B. We observed that the functional capacities of T cells induced by VACV and ECTV were of a similar quality in spite of the markedly different replication abilities and pathogenic outcomes of these viruses. In general, a significant fraction (≥50%) of all T-cell responses were positive for at least three functions both during acute infection and into the memory phase. In vivo killing assays revealed that CD8(+) T cells specific for both viruses were equally cytolytic (∼80% target cell lysis after 4 h), consistent with the similar levels of granzyme B and degranulation detected among these cells. Collectively, these data provide a mechanism to explain the ability of VACV to induce protective T-cell responses against pathogenic poxviruses in their natural hosts and provide further support for the use of VACV as a vaccine platform able to induce polyfunctional T cells.

Direct CD28 costimulation is required for CD8+ T cell-mediated resistance to an acute viral disease in a natural host

Previous studies have suggested that, differing from model Ags, viruses that replicate extensively in the host still induce normal CD8+ T cell responses in the absence of CD28 costimulation. Because these studies were performed with viruses that do not normally cause acute disease, an important remaining question is whether CD28 costimulation is required for CD8+ T cell-mediated resistance to widely replicating but pathogenic viruses. To address this question, we studied the role of CD28 costimulation in CD8+ T cell-mediated resistance to mousepox, a disease of the mouse caused by the natural mouse pathogen, the ectromelia virus (ECTV). C57BL/6 (B6) mice are naturally resistant to mousepox, partly due to a fast and strong CD8+ T cell response. We found that B6 mice deficient in CD28 (CD28 knockout (KO)) are highly susceptible to lethal mousepox during the early stages of ECTV infection but can be protected by immunization with the antigenically related vaccinia virus (VACV) or by adoptive transfer of CD28 KO anti-VACV memory CD8+ cells. Of interest, a thorough comparison of the CD8+ T cell responses to ECTV and VACV suggests that the main reason for the susceptibility of CD28 KO mice to mousepox is a reduced response at the early stages of infection. Thus, while in the absence of CD28 costimulation the end point strength of the T cell responses to nonpathogenic viruses may appear normal, CD28 costimulation increases the speed of the T cell response and is essential for resistance to a life-threatening acute viral disease.

CD8(+) T cell-mediated immune responses in West Nile virus (Sarafend strain) encephalitis are independent of gamma interferon

The flavivirus West Nile virus (WNV) can cause fatal encephalitis in humans and mice. It has recently been demonstrated, in an experimental model using WNV strain Sarafend and C57BL/6 mice, that both virus- and immune-mediated pathology is involved in WNV encephalitis, with CD8(+) T cells being the dominant subpopulation of lymphocyte infiltrates in the brain. Here, the role of activated WNV-immune CD8(+) T cells in mouse WNV encephalitis was investigated further. Passive transfer of WNV-immune CD8(+) T cells reduced mortality significantly and prolonged survival times of mice infected with WNV. Early infiltration of WNV-immune CD8(+) T cells into infected brains is shown, suggesting a beneficial contribution of these lymphocytes to recovery from encephalitis. This antiviral function was not markedly mediated by gamma interferon (IFN-gamma), as a deficiency in IFN-gamma did not affect mortality to two strains of WNV (Sarafend and Kunjin) or brain virus titres significantly. The cytolytic potential, as well as precursor frequency, of WNV-immune CD8(+) T cells were not altered by the absence of IFN-gamma. This was reflected in transfer experiments of WNV-immune CD8(+) T cells from IFN-gamma(-/-) mice into WNV-infected wild-type mice, which showed that IFN-gamma-deficient T cells were as effective as those from WNV-immune wild-type mice in ameliorating disease outcome. It is speculated here that one of the pleiotropic functions of IFN-gamma is mimicked by WNV-Sarafend-mediated upregulation of cell-surface expression of major histocompatibility complex antigens, which may explain the lack of phenotype of IFN-gamma(-/-) mice in response to WNV.

Protection against vaccinia virus challenge by CD8 memory T cells resolved by molecular mimicry

Live vaccinia virus (VV) vaccination has been highly successful in eradicating smallpox. However, the mechanisms of immunity involved in mediating this protective effect are still poorly understood, and the roles of CD8 T-cell responses in primary and secondary VV infections are not clearly identified. By applying the concept of molecular mimicry to identify potential CD8 T-cell epitopes that stimulate cross-reactive T cells specific to lymphocytic choriomeningitis virus (LCMV) and VV, we identified after screening only 115 peptides two VV-specific immunogenic epitopes that mediated protective immunity against VV. An immunodominant epitope, VV-e7r130, did not generate cross-reactive T-cell responses to LCMV, and a subdominant epitope, VV-a11r198, did generate cross-reactive responses to LCMV. Infection with VV induced strong epitope-specific responses which were stable into long-term memory and peaked at the time virus was cleared, consistent with CD8 T cells assisting in the control of VV. Two different approaches, direct adoptive transfer of VV-e7r-specific CD8 T cells and prior immunization with a VV-e7r-expressing ubiquitinated minigene, demonstrated that memory CD8 T cells alone could play a significant role in protective immunity against VV. These studies suggest that exploiting cross-reactive responses between viruses may be a useful tool to complement existing technology in predicting immunogenic epitopes to large viruses, such as VV, leading to a better understanding of the role CD8 T cells play during these viral infections.

Antibodies and CD8+T Cells Are Complementary and Essential for Natural Resistance to a Highly Lethal Cytopathic Virus

It is believed that CD8+ T lymphocytes or Abs can independently clear many primary viral infections, including those caused by Orthopoxviruses (OPV), a genus that includes the human pathogens variola and monkeypox and the vaccine species vaccinia virus. However, most experiments addressing the role of Abs and CD8+ T cells in protection have used viruses that are not specific for the host. In the present study, we used the mouse-specific OPV ectromelia virus and mice deficient in CD40, B cells, or CD8+ T cells and adoptive transfers of CD8+ T or B lymphocytes to show that the protection afforded by CD8+ T cells is incomplete. Despite sustained CD8+ T cell responses, in the absence of Ab responses ectromelia virus persists. This results in delayed disease and inexorably leads to death. Therefore, CD8+ T lymphocytes and Abs are not redundant but complementary and essential to survive infections with a highly pathogenic viruses in the natural host.

Immunization with a single extracellular enveloped virus protein produced in bacteria provides partial protection from a lethal orthopoxvirus infection in a natural host

Subunit vaccines that use the vaccinia virus extracellular envelope protein A33R alone or combined with other structural proteins are excellent candidates for a new smallpox vaccine. Since a new smallpox vaccine would be used in humans, who are the natural hosts for the Orthopoxvirus variola, the agent of smallpox, it would be important to determine whether a prospective smallpox vaccine can protect from a lethal Orthopoxvirus infection in a natural host. We addressed this question using the mouse-specific Orthopoxvirus ectromelia virus. We demonstrate that immunization with recombinant ectromelia virus envelope protein EVM135 or its ortholog vaccinia virus A33R produced in E. coli protects susceptible mice from a lethal ectromelia virus infection. This is the first report that a subunit vaccine can provide protection to a lethal Orthopoxvirus infection in its natural host.

Vaccinia virus H3L envelope protein is a major target of neutralizing antibodies in humans and elicits protection against lethal challenge in mice

The smallpox vaccine is the prototypic vaccine, yet the viral targets critical for vaccine-mediated protection remain unclear in humans. We have produced protein microarrays of a near-complete vaccinia proteome and used them to determine the major antigen specificities of the human humoral immune response to the smallpox vaccine (Dryvax). H3L, an intracellular mature virion envelope protein, was consistently recognized by high-titer antibodies in the majority of human donors, particularly after secondary immunization. We then focused on examining H3L as a valuable human antibody target. Purified human anti-H3L antibodies exhibited substantial vaccinia virus-neutralizing activity in vitro (50% plaque reduction neutralization test [PRNT50] = 44 microg/ml). Mice also make an immunodominant antibody response to H3L after vaccination with vaccinia virus, as determined by vaccinia virus protein microarray. Mice were immunized with recombinant H3L protein to examine H3L-specific antibody responses in greater detail. H3L-immunized mice developed high-titer vaccinia virus-neutralizing antibodies (mean PRNT50 = 1:3,760). Importantly, H3L-immunized mice were subsequently protected against lethal intranasal challenges with 1 or 5 50% lethal doses (LD50) of pathogenic vaccinia virus strain WR, demonstrating the in vivo value of an anti-H3L response. To formally demonstrate that neutralizing anti-H3L antibodies are protective in vivo, we performed anti-H3L serum passive-transfer experiments. Mice receiving H3L-neutralizing antiserum were protected from a lethal challenge with 3 LD50 of vaccinia virus strain WR (5/10 versus 0/10; P < 0.02). Together, these data show that H3L is a major target of the human anti-poxvirus antibody response and is likely to be a key contributor to protection against poxvirus infection and disease.

Identification of poxvirus CD8+ T cell determinants to enable rational design and characterization of smallpox vaccines

The large size of poxvirus genomes has stymied attempts to identify determinants recognized by CD8⁺ T cells and greatly impeded development of mouse smallpox vaccination models. Here, we use a vaccinia virus (VACV) expression library containing each of the predicted 258 open reading frames to identify five peptide determinants that account for approximately half of the VACV-specific CD8⁺ T cell response in C57BL/6 mice. We show that the primary immunodominance hierarchy is greatly affected by the route of VACV infection and the poxvirus strain used. Modified vaccinia virus ankara (MVA), a candidate replacement smallpox vaccine, failed to induce responses to two of the defined determinants. This could not be predicted by genomic comparison of viruses and is not due strictly to limited MVA replication in mice. Several determinants are immunogenic in cowpox and ectromelia (mousepox) virus infections, and immunization with the immunodominant determinant provided significant protection against lethal mousepox. These findings have important implications for understanding poxvirus immunity in animal models and bench-marking immune responses to poxvirus vaccines in humans.

Can we really learn from model pathogens?

Cytolytic T (Tc) lymphocytes are the first order response of the adaptive immune system in the recovery from primary viral infections. These effector cells execute their function either by direct cytotoxicity through the Fas or perforin pathway and/or by the release of cytokines that either directly or indirectly exert antiviral activity. Mice respond to infection by closely related viruses with a vigorous Tc response, which is characterized by extensive crossreactivity on target cells infected with these viruses. However, the action of these cells can be beneficial, detrimental or neutral. From our current state of knowledge, no generalizations as to protective or detrimental effects of cytolytic effector functions in recovery from virus infections can be made. Thus, virus-host immune interactions have to be assessed individually and cannot be generalized.

Granzymes in cytolytic lymphocytes--to kill a killer?

Granzymes (gzm) are major components of the granules of cytolytic lymphocytes, natural killer and cytotoxic T cells. Their generally accepted mode of action consists of their directed secretion towards a virus-infected or neoplastic target cell and perforin-dependent delivery to the target cell cytosol, where they engage in various actions resulting in target cell apoptosis. Here, based on observations of infection of gzmAxB(-/-) mice with ectromelia virus, mousepox, we propose an additional--and distinct--function for gzmA and B. In this model, gzm constitute one of the first lines of defence of immune cells against virus infection of immune cells themselves. Accordingly, endogenous gzm interfere with viral replication in cytolytic lymphocytes either directly, as a result of their proteolytic activity, leading to destruction of viral proteins, or indirectly, via: (i) processes akin to the caspase cascade when acting as effector molecules in the induction of target cell apoptosis; or (ii) their capacity to induce early inflammatory mediators. We discuss the predictions of the model in the light of available data.

Memories of virus-specific CD8+ T cells

This brief review focuses on the way that our understanding of virus-specific CD8(+) T-cell-mediated immunity evolved, giving particular attention to the early impact of the program at the Australian National University. The story developed through a sequence of distinct eras, each of which can be defined in the context of the technologies available at that time. The progress has been enormous, but there is a great deal still to be learned. A particular challenge is to use what we know for human benefit.

Vaccinia virus motility

Vaccinia virus (VV), the virus smallpox vaccine, replicates in the cytoplasm of infected cells. The intracellular movement of this large virus would be inefficient without specific transport mechanisms; therefore, VV uses microtubules for movement during both entry and egress. In addition, the dissemination of virus from infected cells to adjacent cells is promoted by the polymerization of actin beneath cell surface virions to drive virus particles away from the cell. Last, the roles of different VV particles in virus movement within and between hosts are discussed.

Cell-mediated cytotoxicity in recovery from poxvirus infections

The availability of mutant and gene targeted knockout mice with defects in components of cellular cytotoxicity mediated by either the Fas or the exocytosis pathway permitted an analysis of their role in recovery from poxvirus infections. Ectromelia (EV), a natural mouse pathogen causing mousepox, the closely related orthopoxviruses cow pox (CPV) and vaccinia virus (VV), each encode serpins that inhibit Fas mediated apoptosis and lysis of target cells. Nevertheless, distinct differences were seen when the three viruses were inoculated into perforin-deficient mice: highly resistant C57Bl/6 mice became susceptible to low doses of EV; resistance to CPV increased whereas there was no effect on VV infections. Absence of the cytolytic granule associated granzymes (gzm) A and B rendered C57Bl/6 mice increasingly more susceptible to EV infections. Lack of both gzms rendered them as susceptible as perforin deficient mice, despite the presence of functionally active perforin. Elevated EV titres in liver and spleen of gzmA x B deficient mice, early after infection and before cytotoxic T cells were detectable, strongly suggests that these two gzms exert an antiviral effect by a mechanism distinct from effector molecules of NK and cytotoxic T cells.

Viral interference with antigen presentation

CD8+ T cells play an important role in immunity to viruses. Just how important these cells are is demonstrated by the evolution of viral strategies for blocking the generation or display of peptide-major histocompatibility complex class I complexes on the surfaces of virus-infected cells. Here, we focus on viral interference with antigen presentation; in particular we consider the importance (and difficulty) of establishing the evolutionary significance (that is, the ability to enhance viral transmission) of viral gene products that interfere with antigen presentation in vitro.

The pas de deux of viruses and CD8 T cells

The vertebrate immune system has evolved to deal with parasitic life forms, so it is hardly surprising that experiments with pathogens have proved illuminating for immunology. Those of us who have worked for years with infectious processes are acutely conscious that we do little more than probe the vast 'experiment of nature.' There is no place for doctrinaire rigidity in this extraordinarily complex area of biology. Though we have obviously tried to do rational experiments, much of the novelty that has been brought in to immunology from the analysis of the virus-specific host response has, in a very real sense, been the product of what we now call 'discovery science'. The following relates some of the research that I was involved in personally and attempts to put it in the context of both the history of the field and the events of the time. Virus infections, particularly HIV, pose enormous challenges for the future. It is generally helpful to know a little of what went before.

Cell death mediated by alloreactive cytotoxic T cells via the granule exocytosis or the Fas pathway is independent of p34cdc2 kinase: Fas dependent killing of cells arrested in the cell cycle

Inappropriate activation of p34cdc2 kinase has been shown to occur during apoptosis induced by cytotoxic T-cell derived perforin and fragmentin. We analysed the effect of two inhibitors of p34cdc2 kinase on alloreactive Tc-cell-mediated lysis and DNA fragmentation of P815 and L1210 target cells. Olomoucine, a specific inhibitor of cyclin dependent kinases, did not affect DNA fragmentation in the target cells. Lysis of olomoucine-treated target cells as assessed by 51Cr release over a typical 8-h period was also unaffected. We also examined the effects of thapsigargin on target cell death. This toxin causes increased intracellular calcium rises that then result in irreversible inhibition of cyclin dependent kinases, including p34cdc2 kinase. The same extent of specific cell lysis was induced by cytotoxic T cells from perforin(-/-), granzyme B(-/-), granzyme A(-/-), perforin(-/-) X granzymeB(-/-) X granzymeA(-/-) KO mice or normal mice in untreated target cells or target cells treated with either olomoucine or thapsigargin. Similarly DNA fragmentation measured by release of tritiated DNA was also unaffected. Thus inhibition of p34cdc2 kinase affects neither the Fas nor the perforin/granzyme pathways of alloreactive cytotoxic T-cell killing as measured by DNA fragmentation or chromium release. P815 cells treated with olomoucine were arrested in the cell cycle after 12-16 h exposure to the toxin. After cell cycle arrest, target cells now showed enhanced 51Cr release induced by effector cytotoxic T cells (CTL) derived from perforin(-/-) mice compared to untreated cells. This lysis was accompanied by an increase in cell surface Fas expression. Olomoucine induced cell cycle arrest and expression of Fas was reversible and when cells re-entered the cell cycle, surface expression of Fas was lost.

Enhanced resistance in STAT6-deficient mice to infection with ectromelia virus

We inoculated BALB/c mice deficient in STAT6 (STAT6(-/-)) and their wild-type (wt) littermates (STAT6(+/+)) with the natural mouse pathogen, ectromelia virus (EV). STAT6(-/-) mice exhibited increased resistance to generalized infection with EV when compared with STAT6(+/+) mice. In the spleens and lymph nodes of STAT6(-/-) mice, T helper 1 (Th1) cytokines were induced at earlier time points and at higher levels postinfection when compared with those in STAT6(+/+) mice. Elevated levels of NO were evident in plasma and splenocyte cultures of EV-infected STAT6(-/-) mice in comparison with STAT6(+/+) mice. The induction of high levels of Th1 cytokines in the mutant mice correlated with a strong natural killer cell response. We demonstrate in genetically susceptible BALB/c mice that the STAT6 locus is critical for progression of EV infection. Furthermore, in the absence of this transcription factor, the immune system defaults toward a protective Th1-like response, conferring pronounced resistance to EV infection and disease progression.

Characterization of the ectromelia virus serpin, SPI-2

Poxviruses encode multiple proteins that enable them to evade host responses. Among these are serine protease inhibitors (serpins). One of the earliest serpins described, cowpox virus crmA, acts to inhibit inflammation and apoptosis. crmA homologous serpins, known as SPI-2, are conserved in rabbitpox, vaccinia and variola viruses. Here, we describe the characterization of ectromelia virus (EV) SPI-2. EV SPI-2 encodes a protein of approximately 38 kDa showing >94% identity with other poxviral homologues. Conservative changes in amino acid sequence were found within the reactive site loop and the serpin backbone. Like crmA, transient expression of SPI-2 protected cells from tumour necrosis factor-mediated apoptosis and inhibited the activity of caspases-1 and -8 but not caspases-3, -6 or granzyme B. Overall, this study demonstrates that EV SPI-2 is functionally similar to crmA, based on in vitro assays.

Differential pathogenesis of lethal mousepox in congenic DBA/2 mice implicates natural killer cell receptor NKR-P1 in necrotizing hepatitis and the fifth component of complement in recruitment of circulating leukocytes to spleen

Innate resistance of C57BL/6 (B6) mice to lethal mousepox is controlled by multiple genes. Previously, four resistance genes were localized to specific subchromosomal regions and transferred onto a susceptible DBA/2 (D2) background by serial backcrossing and intercrossing to produce congenic strains. Intraperitoneally inoculated ectromelia virus was uniformly lethal and achieved similar titers in B6 and D2 mice but elicited differential responses in liver, spleen, and circulating blood leukocytes. The distribution of these response phenotypes in congenic strains linked control of phenotypes with specific subchromosomal regions. D2.R1 mice, which carried a differential segment of chromosome 6, exhibited a B6 liver response and intermediate spleen and circulating leukocyte responses. D2.R2 and D2.R4 mice, which carried differential segments of chromosomes 2 and 1, respectively, exhibited a D2 liver response, a B6 spleen response, and an intermediate circulating leukocyte response. The localization of control of liver response phenotypes to chromosome 6 implicates cells that express natural killer (NK) cell receptor NKR-P1 alloantigens. The localization of control of spleen and circulating leukocyte responses to chromosomes 1, 2, and 6 implicates NK cells, the fifth component of complement, and a gene near the selectin gene complex in recruitment of circulating leukocytes to spleen.

Myxoma virus induces extensive CD4 downregulation and dissociation of p56lck in infected rabbit CD4+ T lymphocytes

Myxoma virus is a pathogenic poxvirus that induces extensive dysregulation of cellular immunity in infected European rabbits. Infection of a rabbit CD4+ T-cell line (RL-5) with myxoma virus results in dramatic reductions of cell surface levels of CD4 as monitored by flow cytometry. The virus-induced downregulation of CD4 requires early but not late viral gene expression and could not be inhibited by staurosporine, an inhibitor of protein kinase C, which effectively blocks phorbol 12-myristate-13-acetate-induced downregulation of CD4. The decrease in total cellular levels of CD4 during myxoma virus infection could be inhibited by the lysosomotrophic agent NH4Cl, suggesting a lysosomal fate for CD4 during myxoma virus infection. Steady-state levels of the CD4-associated protein tyrosine kinase p56lck remained unchanged during myxoma virus infection, suggesting that p56lck dissociates from CD4 prior to CD4 degradation in virus infected cells. Total p56lck kinase activity was unaffected during myxoma virus infection, although the amount of p56lck physically associated with CD4 declined in parallel with the loss of CD4. Thus, myxoma virus infection of CD4+ T lymphocytes triggers CD4 downregulation via a protein kinase C-independent pathway, causing the dissociation of p56lck and the degradation of CD4 in lysosomal vesicles.

Antigen specificity of the ovine cytotoxic T lymphocyte response to bluetongue virus

Bluetongue virus (BTV), an arbovirus transmitted by midges, can cause serious disease in sheep. Both virus neutralizing antibody and cytotoxic T lymphocytes (CTL) have been shown to have a role in protective immunity. In this study, the antigen specificity of CTL from BTV-immune sheep has been determined using recombinant vaccinia viruses expressing individual BTV antigens. The results show that, in the sheep studied thus far, the serotype-specific outer coat protein, VP2, and the non-structural protein, NS1 are major immunogens for CTL, with VP5 (an outer coat protein) and NS3 being minor immunogens. No VP7 (a major group-reactive inner coat protein) specific CTL were detected. The CTL from sheep immunized with serotype 1 were cross-reactive and able to recognize target cells infected with other BTV serotypes. Further work demonstrated that the cross-reactive CTL recognized NS1, but not VP2.

Vaccinia virus expression of the VP7 protein of South African bluetongue virus serotype 4 and its use as an antigen in a capture ELISA

Recombinant vaccinia viruses expressing the VP7 core protein of South African bluetongue virus serotype 4 (SA-BTV4) were identified by polymerase chain reaction amplification. Expression of VP7 was verified by radio-immunoprecipitation and a F(ab')2-based ELISA. Antibodies to VP7 were detected in sera from sheep that had been infected with 20 different virulent BTV serotypes by using the vaccinia virus (VV) expressed VP7 as antigen in a capture ELISA. F(ab')2-immobilised VV-expressed SA-BTV4 VP7 cross-reacted with sera directed against all 9 African horsesickness virus serotypes and epizootic haemorrhagic disease virus serotype 2.

Effects of virally expressed interleukin-10 on vaccinia virus infection in mice

To investigate the in vivo role of interleukin-10 (IL-10) in viral infection, we compared infections with a recombinant vaccinia virus (VV) expressing IL-10 (VV-IL10) under control of the VV P7.5 promoter and a control virus (VV-beta gal) in normal and severe combined immunodeficient mice. In normal mice, VV-IL10 infection resulted in less natural killer cell activity at 3 days postinfection and less VV-specific cytotoxic T-cell activity at 6 or 7 days postinfection than VV-beta gal infection. However, the use of dermal scarification or intraperitoneal, intranasal, or intracerebral inoculation into immunocompetent mice resulted in no difference between VV-IL10 and VV-beta gal in visible lesions, mortality, protective immunity to a 100-fold lethal VV challenge, or VV-specific antibody response. In the immunodeficient mice, VV-IL10 infection resulted in greater natural killer cell activity and lower virus replication than VV-beta gal infection. These in vivo effects were subtler and more complex than had been anticipated. From the VV-IL10 murine model, the Epstein-Barr virus-encoded homolog of human IL-10, BCRF1, may provide a selective advantage by blunting the early human natural killer cell and cytotoxic T-cell responses so that Epstein-Barr virus can establish a well-contained latent infection in B lymphocytes.

Ectromelia virus replication in major target organs of innately resistant and susceptible mice after intravenous infection

The kinetics of ectromelia virus replication in the spleen and liver and of alpha/beta interferon production in the spleen were determined during the first 3 days after intravenous infection with the virulent Moscow strain in resistant C57 BL/6 and susceptible DBA/2 mice. Virus replication in the spleen as measured by assays for virus DNA and infectious centers was suppressed in C57BL/6 mice relative to DBA/2 mice within the first 1 or 2 days of infection. Infectious centers increased in DBA/2 mice but not in C57 BL/6 mice. Differences in virus replication between strains were less discrete when spleens were assayed for infectious virus than when they were assayed for infectious centers because infectious centers of most C57 BL/6 mice had more infectious virus than infectious centers of DBA/2 mice. Virus replication in the liver, the major target organ, as measured by virus DNA and infectious virus assays, was suppressed in C57 BL/6 mice relative to DBA/2 mice 3 days after infection but not before that interval. The results indicate that genetic control of ectromelia virus replication begins within the first 1 or 2 days of infection in the spleen but is delayed in the liver and that genetic control is directed at the prevention of virus spread more than at virus replication.

Cytokine production in virus-infected mice: a single cell analysis

Antiviral activity of IFN-gamma and TNF has been demonstrated in vitro and in vivo. Recombinant vaccinia viruses that encode either of these cytokines are highly attenuated in vivo and, indeed, are no longer lethal when they are used to infect athymic nude mice. Thus, these cytokines are able to replace the requirement for CD8 T cells in immunodeficient mice. We believe that the antiviral properties of some cytokines are a central component in the physiological control of virus infection. In this report, we used flow cytofluorimetric analysis to simultaneously detect cytokine production and T cell phenotype. Importantly, this method allowed the analysis of cytokine production in cells freshly isolated from tissues. In contrast to other procedures, restimulation of the cells in vitro was not necessary. We demonstrate that CD8 IFN-gamma+ cells were predominantly induced early in the response to vaccinia virus infection and, at the cellular peak of the response, CD8 IFN-gamma+ and CD4 TNF+ T cells were present at equal frequencies. Hence, these findings support the concept that effector T cells produce cytokines that are known to be antiviral, and that these cytokines are an important component of effector T cell activity.

Induction of primary anti-viral cytotoxic T cells by in vitro stimulation with short synthetic peptide and interleukin-7

The present study investigated whether a short synthetic peptide NPP, with a modified sequence (147-158 R156-) derived from influenza A virus nucleoprotein with high affinity for Kd major histocompatibility complex class I molecules, could induce primary influenza virus-specific cytotoxic T (Tc) cells in vitro. Naive BALB/c (H-2d) splenocytes did not respond to the stimulation with only NPP with the generation of effector Tc cells specific for influenza A virus-infected target cells in vitro. However, they were able to do so if cultured with NPP in the presence of IL-7. IL-7 activity in this system differed significantly from IL-2 activity in the specificity of the effect. The use of exogenous IL-2, instead of IL-7, with NPP resulted in the induction of lytic cells that lysed both influenza virus-infected and uninfected syngeneic target cells. These results suggest that IL-7 is a potent regulatory cytokine in the antigen-specific activation of resting naive Tc cell precursors and may provide the necessary conditions for the induction of human primary Tc cells in vitro.

Beta 2-microglobulin-, CD8+ T-cell-deficient mice survive inoculation with high doses of vaccinia virus and exhibit altered IgG responses

Transgenic mice lacking an intact beta 2-microglobulin (beta 2m) gene fail to express major histocompatibility complex (MHC) class I proteins on the cell surface and, as a result, are virtually devoid of CD4- CD8+ lymphocytes. These animals provide a unique model system for directly assessing the role of CD8+ lymphocytes in the modulation of viral infection in vivo. beta 2m- CD8- mice and their normal littermates were inoculated at the base of the tail with the WR strain of vaccinia virus and monitored for serum antibody and lesion formation. Both groups developed similar lesions in response to a broad virus dose range, and all animals had completely recovered by day 28 after inoculation. Isotype-specific immunoglobulin levels were determined for each animal on day 7 and day 14 after primary inoculation, and again 7 days after a virus challenge. The virus-specific IgG1, IgG2a, and IgG2b levels were significantly different in the beta 2m-/- group (20-, 9-, and 30-fold lower, respectively, on day 7 after challenge) compared with the beta 2m+/- group. Virus-specific serum IgM levels for both groups remained similar throughout the experiment. In a separate experiment, beta 2m-/- mice were immunized with a nonviral antigen, 2,4,6-trinitrophenyl-conjugated keyhole limpet hemocyanin, and both total and antigen-specific isotype-specific immunoglobulin titers were determined. Total IgG1, IgG2a, IgG2b, and IgG3 tended to be lower overall in the beta 2m-/- mice compared with beta 2m+/- littermates. In contrast, total and antigen-specific IgE titers were similar in the two groups. These data indicate that CD8+ lymphocytes are not required to clear high doses of vaccinia virus, and they suggest that beta 2m-/- mice are less efficient at antigen-specific IgG production than their beta 2m+/- littermates.

Acute inflammatory response to cowpox virus infection of the chorioallantoic membrane of the chick embryo

The chick embryo chorioallantoic membrane was used to study the acute inflammatory response in the absence of contributions from the immune system. In preliminary experiments, lesions of wild-type cowpox virus strain Brighton (CPV-BR) and a 38K gene deletion mutant of CPV-BR (CPV-BR.D1) were compared with vaccinia virus (strains WR and Copenhagen), fowlpox virus, laryngotracheitis virus, and infectious tenosynovitis virus, and were ranked for degree of induced inflammation. The maximal and minimal inflammatory responses were observed with CPV-BR.D1 and CPV-BR viruses, respectively. CPV-BR.D1 lacks a 38K gene which encodes an anti-inflammatory 38-kDa protein that has homology to SERPINs. The kinetics and character of the inflammatory response were examined further in the wild-type CPV-BR and mutant CPV-BR.D1 infections using cell counts, electron microscopy, and assays for inflammatory cell activation. CPV-BR virus infection rapidly spread through the ectoderm, uniformly infecting all cells with the production of large amounts of virions and viral-induced cytopathic effect, but evoking little or no inflammatory response until 144 hr p.i. The CPV-BR.D1 infection, on the other hand, was rapidly contained by a dexamethasone-sensitive inflammatory response mainly of activated heterophils which was advanced by 36 hr p.i. Both infections resulted in disseminated disease with similar numbers of liver lesions and only a slight difference in the LD50, with the CPV-BR.D1 values being higher than that for CPV-BR virus. In this model, the acute inflammatory response alone is unable to prevent disseminated disease and associated mortality.

Vaccinia virus-specific human CD4+ cytotoxic T-lymphocyte clones

Vaccinia virus-specific cytotoxic T-lymphocyte (CTL) clones were established from a healthy donor, who had been immunized with vaccinia virus vaccine, by stimulation of peripheral blood lymphocytes with UV-inactivated vaccinia virus antigen. The phenotype of all of the clones established was CD3+ CD4+ CD8- Leu11-. We used a panel of allogenic vaccinia virus-infected B-lymphoblastoid cell lines and demonstrated that some of the clones recognized vaccinia virus epitopes presented by human leukocyte antigen (HLA) class II molecules. Monoclonal antibodies specific for either HLA-DP or HLA-DR determinant reduced the cytotoxicity of specific clones. The HLA-restricted cytotoxicity of the clones is vaccinia virus specific, because vaccinia virus-infected but not influenza virus-infected autologous target cells were lysed. Using vaccinia virus deletion mutants, we found that some of the CTL clones recognize an epitope(s) that lies within the HindIII KF regions of the vaccinia virus genome. These results indicate that heterogeneous CD4+ CTL clones specific for vaccinia virus are induced in response to infection and may be important in recovery from and protection against poxvirus infections.

Resistance to ectromelia virus infection in mice. Analysis of H-2-linked gene effects

The mechanism for resistance to ectromelia infection has been investigated in B6 and B10 congenic strains of mice which carry different alleles at the H-2 major histocompatibility locus. Greater susceptibility in some B10 congenic strains of mice has been associated with higher viral titres in the draining popliteal and inguinal lymph nodes as well as spleen at 3 days post infection. T cells which develop cytotoxic function following in vitro culture in the presence of T cell growth factors have also been detected in the popliteal lymph nodes of B6/B10 congenic strains of mice as early as 3 days post infection. Greater cytotoxicity has been detected in cultures of cells from resistant B6/B10 mice than from the susceptible B10 congenic strain B10.G, or other semi-resistant B10 congenic strains which differ at the H-2 locus. The early activation of T cells appears to be under H-2 gene control and activated T cells may play an "early" role in controlling viral replication within the lymphoid system.

Local production of tumor necrosis factor encoded by recombinant vaccinia virus is effective in controlling viral replication in vivo

Tumor necrosis factor (TNF) has pleiotropic effects on a wide variety of cell types. In vitro studies have demonstrated that TNF has antiviral properties and is induced in response to viral infections. However, a role for TNF in the antiviral immune response of the host has yet to be demonstrated. Here we describe the construction of and studies using a recombinant vaccinia virus that encodes the gene for murine TNF-alpha. By comparing the replication of and immune responses elicited by the TNF-encoding virus to a similarly constructed control virus, we hoped to observe immunobiological effects of TNF in the host. The in vivo experiments with this recombinant virus demonstrate that the localized production of TNF-alpha during a viral infection leads to the rapid and efficient clearance of the virus in normal mice and attenuates the otherwise lethal pathogenicity of the virus in immunodeficient animals. This attenuation occurs early in the infection (by postinfection hour 24) and is not due to the enhancement of cellular or antibody responses by the vaccinia virus-encoded TNF. This evidence suggests that attenuation of the recombinant virus is due to a direct antiviral effect of TNF on cells at the site of infection. Therefore, these results support the suggestion that TNF produced by immune cells may be an important effector mechanism of viral clearance in vivo.

Poxvirus pathogenesis

Poxviruses are a highly successful family of pathogens, with variola virus, the causative agent of smallpox, being the most notable member. Poxviruses are unique among animal viruses in several respects. First, owing to the cytoplasmic site of virus replication, the virus encodes many enzymes required either for macromolecular precursor pool regulation or for biosynthetic processes. Second, these viruses have a very complex morphogenesis, which involves the de novo synthesis of virus-specific membranes and inclusion bodies. Third, and perhaps most surprising of all, the genomes of these viruses encode many proteins which interact with host processes at both the cellular and systemic levels. For example, a viral homolog of epidermal growth factor is active in vaccinia virus infections of cultured cells, rabbits, and mice. At least five virus proteins with homology to the serine protease inhibitor family have been identified and one, a 38-kDa protein encoded by cowpox virus, is thought to block a host pathway for generating a chemotactic substance. Finally, a protein which has homology with complement components interferes with the activation of the classical complement pathway. Poxviruses infect their hosts by all possible routes: through the skin by mechanical means (e.g., molluscum contagiosum infections of humans), via the respiratory tract (e.g., variola virus infections of humans), or by the oral route (e.g., ectromelia virus infection of the mouse). Poxvirus infections, in general, are acute, with no strong evidence for latent, persistent, or chronic infections. They can be localized or systemic. Ectromelia virus infection of the laboratory mouse can be systemic but inapparent with no mortality and little morbidity, or highly lethal with death in 10 days. On the other hand, molluscum contagiosum virus replicates only in the stratum spinosum of the human epidermis, with little or no involvement of the dermis, and does not spread systemically from the site of infection. The host response to infection is progressive and multifactorial. Early in the infection process, interferons, the alternative pathway of complement activation, inflammatory cells, and natural killer cells may contribute to slowing the spread of the infection. The cell-mediated response involving learned cytotoxic T lymphocytes and delayed-type hypersensitivity components appears to be the most important in recovery from infection. A significant role for specific antiviral antibody and antibody-dependent cell-mediated cytotoxicity has yet to be demonstrated in recovery from a primary infection, but these responses are thought to be important in preventing reinfection.

Poxvirus pathogenesis

Poxviruses are a highly successful family of pathogens, with variola virus, the causative agent of smallpox, being the most notable member. Poxviruses are unique among animal viruses in several respects. First, owing to the cytoplasmic site of virus replication, the virus encodes many enzymes required either for macromolecular precursor pool regulation or for biosynthetic processes. Second, these viruses have a very complex morphogenesis, which involves the de novo synthesis of virus-specific membranes and inclusion bodies. Third, and perhaps most surprising of all, the genomes of these viruses encode many proteins which interact with host processes at both the cellular and systemic levels. For example, a viral homolog of epidermal growth factor is active in vaccinia virus infections of cultured cells, rabbits, and mice. At least five virus proteins with homology to the serine protease inhibitor family have been identified and one, a 38-kDa protein encoded by cowpox virus, is thought to block a host pathway for generating a chemotactic substance. Finally, a protein which has homology with complement components interferes with the activation of the classical complement pathway. Poxviruses infect their hosts by all possible routes: through the skin by mechanical means (e.g., molluscum contagiosum infections of humans), via the respiratory tract (e.g., variola virus infections of humans), or by the oral route (e.g., ectromelia virus infection of the mouse). Poxvirus infections, in general, are acute, with no strong evidence for latent, persistent, or chronic infections. They can be localized or systemic. Ectromelia virus infection of the laboratory mouse can be systemic but inapparent with no mortality and little morbidity, or highly lethal with death in 10 days. On the other hand, molluscum contagiosum virus replicates only in the stratum spinosum of the human epidermis, with little or no involvement of the dermis, and does not spread systemically from the site of infection. The host response to infection is progressive and multifactorial. Early in the infection process, interferons, the alternative pathway of complement activation, inflammatory cells, and natural killer cells may contribute to slowing the spread of the infection. The cell-mediated response involving learned cytotoxic T lymphocytes and delayed-type hypersensitivity components appears to be the most important in recovery from infection. A significant role for specific antiviral antibody and antibody-dependent cell-mediated cytotoxicity has yet to be demonstrated in recovery from a primary infection, but these responses are thought to be important in preventing reinfection.

Immunodeficient mice recover from infection with vaccinia virus expressing interferon-gamma

Recombinant vaccinia viruses (VV)-encoding murine interferon-gamma (IFN-gamma) were constructed and the effect of virus-encoded IFN-gamma on the immune response towards VV in vivo investigated. In athymic nude mice and sublethally irradiated euthymic mice, IFN-gamma expression by VV enabled the mice to recover from the infection, whereas mice infected with the control virus died. In normal CBA/H mice also, the growth of VV was greatly reduced and it was cleared faster from mouse organs than the control virus. Natural killer (NK) cell responses in these mice were not enhanced suggesting that this recovery is not NK cell mediated. Other possible mechanisms and implications of this observation are discussed.

Immune response of lambs to experimental infection with Orf virus

A group of six specific pathogen free (SPF) lambs were infected epidermally with Orf virus. Seven weeks later they were reinfected. For a period of 4 weeks after each inoculation they were observed clinically and blood was collected for analysis of virus specific antibody measured by ELISA and peripheral blood lymphocyte (PBL) proliferative response to various viral antigens. After the primary infection all animals showed clinical signs of Orf, namely vesicle formation which became pustular followed by scabbing; this steadily became heavier prior to shedding and the resolution of the infection by about 4 weeks. The severity of infection varied within the group. Little lymphoproliferative activity was recorded during the primary infection, although five/six test animals had positive lymphoproliferative responses to an sodium dodecyl sulphate (SDS) solubilised scab purified Orf virus preparation at some point between days 7 and 14 after inoculation. All animals seroconverted to Orf virus, lymphoproliferative activity always preceding specific antibody detection. Resolution of the secondary infection was very rapid. Vesicles were visible by day 2 after inoculation which became pustular followed by scab formation and resolution in the majority of animals by day 8. All animals showed a significant (greater than four-fold) rise in specific antibody titre following secondary inoculation. The proliferative activity of PBL's was much greater than that recorded for the primary infection although the magnitude of this response varied greatly between individuals.

Evidence that NK cells and interferon are required for genetic resistance to lethal infection with ectromelia virus

C 57 BL/6 mice developed resistance to lethal intravenous challenge with virulent (Moscow strain) ectromelia virus between 2 and 3 weeks of age. The fraction of C57 BL/6 mice in which virus was detected in spleen was significantly lower than for DBA/2 mice by day 3. Thereafter, C 57 BL/6 mice had significantly reduced virus titers in spleen compared with those of DBA/2 mice. Resistance was abrogated by treatment with anti-asialo GM1 gammaglobulin, which blocks NK cell activity, or with anti-interferon (IFN) alpha, beta. C 57 BL/6 mice carrying the bg/bg mutation, associated with a deficiency of NK cells, were highly susceptible to lethal infection as were athymic mice derived from a resistant genetic background. Virus titers in spleens of C 57 BL/6 mice treated with anti-asialo GM1 or anti-IFN alpha, beta were significantly higher 4 days after virus challenge than were titers in C 57 BL/6 mice treated with normal rabbit serum. The results strongly suggest that genetic resistance to lethal ectromelia virus infection requires non-specific host defenses such as NK cells and IFN alpha, beta that are activated during the first 3 to 4 days of infection.

Efficacy of influenza haemagglutinin and nucleoprotein as protective antigens against influenza virus infection in mice

Influenza nucleoprotein (NP)-specific cytotoxic T lymphocytes (CTL) stimulated by immunization of mice with VV-PR8-NP6, a recombinant vaccinia virus expressing A/PR/8/34 NP, did not protect mice against challenge with A/PR/8/34 4 days later. Neither were secondary NP-specific CTL stimulated by reimmunization able to protect mice. These results contrast with the ability of transferred, in vitro-cultured and stimulated, NP-specific CTL to protect recipient mice from challenge with A/PR/8/34. Immunization of mice with a recombinant vaccinia virus expressing A/PR/8/34 HA protected mice challenged 4 days later, either via the small amount of antibody already present, or via HA-specific CTL that would have to be more efficient than NP-specific CTL in either trafficking to the infected lung or in effector function.