Regulation of cytokine production by virus-specific CD8 T cells in the lungs

Genetic absence of PD-L1 does not restore CD8+ T cell function during respiratory virus infection and delays virus clearance

A key mediator of T cell impairment during respiratory virus infection is the inhibitory receptor PD-1. PD-1 is induced on T cells following antigen exposure, whereas proinflammatory cytokines upregulate the ligands PD-L1 and PD-L2. Respiratory virus infection leads to upregulation of PD-L1 on airway epithelial cells, dendritic cells, and alveolar macrophages. However, the role of PD-L1 on different cell types in acute respiratory virus infections is not known. We sought to determine the role of PD-L1 on different cell types in CD8+ T cell impairment. We found that PD-L1-/- mice challenged with human metapneumovirus or influenza showed a similar level of CD8+ T cell impairment compared to wild-type (WT) mice. Moreover, virus clearance was delayed in PD-L1-/- mice compared to WT. CD8+ T cells from PD-L1-deficient mice expressed higher levels of inhibitory receptors both at baseline and after respiratory virus infection. The antibody blockade of PD-L2 failed to restore function to the impaired cells. While reciprocal bone marrow chimeras between WT and PD-L1-/- mice did not restore CD8+ T cell function after the respiratory virus challenge, mice that received the PD-L1-/- bone marrow had higher inhibitory receptor expression on CD8+ cells. This discrepancy in the inhibitory receptor expression suggests that cells of the hematopoietic compartment contribute to T cell impairment on CD8+ T cells.IMPORTANCEThe phenomenon of pulmonary CD8+ T cell impairment with diminished antiviral function occurs during acute respiratory virus infection mediated by Programmed Cell Death-1 (PD-1) signaling. Moreover, PD-1 blockade enhances T cell function to hasten viral clearance. The ligand PD-L1 is expressed in many cell types, but which cells drive lung T cell impairment is not known. We used genetic approaches to determine the contribution of PD-L1 on lung T cell impairment. We found that PD-L2 cannot compensate for the loss of PD-L1, and PD-L1-deficient mice exhibit increased expression of other inhibitory receptors. Bone marrow chimeras between PD-L1-deficient and wild-type mice indicated that hematopoietic PD-L1 expression is associated with inhibitory receptor upregulation and impairment.

Construction and immunological evaluation of hepatitis B virus core virus-like particles containing multiple antigenic peptides of respiratory syncytial virus

Respiratory syncytial virus (RSV) infection causes severe disease in the lower respiratory tract of infants and young children. Currently, no licensed vaccine is available. In this study, we generated the chimeric virus-like particles (tHBc/F_E1E2, tHBc/F_E1E2/M2_82-90 and tHBc/F_E1E2/M2_82-90/tG VLPs) containing multiple antigenic peptides of RSV proteins based on a truncated hepatitis B virus core carrier (tHBc). We investigated the immune protection against RSV infection induced by these VLPs in a mouse model. Immunization with the VLPs elicited RSV-specific IgG and neutralizing antibody production and conferred protection against RSV infection in vivo. Compared with UV-RSV or tHBc/F_E1E2/M2_82-90/tG VLPs, the tHBc/F_E1E2 and tHBc/F_E1E2/M2_82-90 VLPs induced significantly decreased Th2 cytokines (IL-4, IL-5) and increased Th1 cytokines (IFN-γ, TNF-α, IL-2) as well as increased IgG2a/IgG1 ratios. tHBc/F_E1E2 and tHBc/F_E1E2/M2_82-90 VLPs also elicited an increased regulatory T (Treg) cell frequency and IL-10 secretion in the lungs of vaccinated mice, thereby relieving pulmonary pathology upon subsequent RSV infection. Our results demonstrate that the VLPs containing antigenic peptides of F protein combined with a CTL epitope of M2 may represent a promising RSV subunit vaccine candidate.

Peculiar Phenotypic and Cytotoxic Features of Pulmonary Mucosal CD8 T Cells in People Living with HIV Receiving Long-Term Antiretroviral Therapy

People living with HIV have high burdens of chronic lung disease, lung cancers, and pulmonary infections despite antiretroviral therapy (ART). The rates of tobacco smoking by people living with HIV vastly exceed that of the general population. Furthermore, we showed that HIV can persist within the lung mucosa despite long-term ART. As CD8 T cell cytotoxicity is pivotal for controlling viral infections and eliminating defective cells, we explored the phenotypic and functional features of pulmonary versus peripheral blood CD8 T cells in ART-treated HIV⁺ and uninfected controls. Bronchoalveolar lavage fluid and matched blood were obtained from asymptomatic ART-treated HIV⁺ smokers (n = 11) and nonsmokers (n = 15) and uninfected smokers (n = 7) and nonsmokers (n = 10). CD8 T cell subsets and phenotypes were assessed by flow cytometry. Perforin/granzyme B content, degranulation (CD107a expression), and cytotoxicity against autologous Gag peptide-pulsed CD4 T cells (Annexin V⁺) following in vitro stimulation were assessed. In all groups, pulmonary CD8 T cells were enriched in effector memory subsets compared with blood and displayed higher levels of activation (HLA-DR⁺) and exhaustion (PD1⁺) markers. Significant reductions in proportions of senescent pulmonary CD28^-CD57⁺ CD8 T cells were observed only in HIV⁺ smokers. Pulmonary CD8 T cells showed lower perforin expression ex vivo compared with blood CD8 T cells, with reduced granzyme B expression only in HIV⁺ nonsmokers. Bronchoalveolar lavage CD8 T cells showed significantly less in vitro degranulation and CD4 killing capacity than blood CD8 T cells. Therefore, pulmonary mucosal CD8 T cells are more differentiated, activated, and exhausted, with reduced killing capacity in vitro than blood CD8 T cells, potentially contributing to a suboptimal anti-HIV immune response within the lungs.

Pre-existing neutralizing antibodies prevent CD8 T cell-mediated immunopathology following respiratory syncytial virus infection

Despite being a leading cause of severe respiratory disease, there remains no licensed respiratory syncytial virus (RSV) vaccine. Neutralizing antibodies reduce the severity of RSV-associated disease, but are not sufficient for preventing reinfection. In contrast, the role of memory CD8 T cells in protecting against a secondary RSV infection is less established. We recently demonstrated that high-magnitude memory CD8 T cells efficiently reduced lung viral titers following RSV infection, but induced fatal immunopathology that was mediated by IFN-γ. To evaluate the ability of RSV-specific neutralizing antibodies to prevent memory CD8 T cell-mediated immunopathology, mice with high-magnitude memory CD8 T cell responses were treated with neutralizing antibodies prior to RSV challenge. Neutralizing antibody treatment significantly reduced morbidity and prevented mortality following RSV challenge compared with IgG-treated controls. Neutralizing antibody treatment restricted early virus replication, which caused a substantial reduction in memory CD8 T cell activation and IFN-γ production, directly resulting in survival. In contrast, therapeutic neutralizing antibody administration did not impact morbidity, mortality, or IFN-γ levels, despite significantly reducing lung viral titers. Therefore, only pre-existing neutralizing antibodies prevent memory CD8 T cell-mediated immunopathology following RSV infection. Overall, our results have important implications for the development of future RSV vaccines.

Vaccination with a Single-Cycle Respiratory Syncytial Virus Is Immunogenic and Protective in Mice

Respiratory syncytial virus (RSV) is the leading cause of severe respiratory tract infection in infants and young children, but no vaccine is currently available. Live-attenuated vaccines represent an attractive immunization approach; however, balancing attenuation while retaining sufficient immunogenicity and efficacy has prevented the successful development of such a vaccine. Recently, a recombinant RSV strain lacking the gene that encodes the matrix (M) protein (RSV M-null) was developed. The M protein is required for virion assembly following infection of a host cell but is not necessary for either genome replication or gene expression. Therefore, infection with RSV M-null produces all viral proteins except M but does not generate infectious virus progeny, resulting in a single-cycle infection. We evaluated RSV M-null as a potential vaccine candidate by determining its pathogenicity, immunogenicity, and protective capacity in BALB/c mice compared with its recombinant wild-type control virus (RSV recWT). RSV M-null-infected mice exhibited significantly reduced lung viral titers, weight loss, and pulmonary dysfunction compared with mice infected with RSV recWT. Despite its attenuation, RSV M-null infection induced robust immune responses of similar magnitude to that elicited by RSV recWT. Additionally, RSV M-null infection generated serum Ab and memory T cell responses that were similar to those induced by RSV recWT. Importantly, RSV M-null immunization provided protection against secondary viral challenge by reducing lung viral titers as efficiently as immunization with RSV recWT. Overall, our results indicate that RSV M-null combines attenuation with high immunogenicity and efficacy and represents a promising novel live-attenuated RSV vaccine candidate.

Identification of Novel Respiratory Syncytial Virus CD4+ and CD8+ T Cell Epitopes in C57BL/6 Mice

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infection and hospitalization in infants. It is well established that both CD4⁺ and CD8⁺ T cells are critical for mediating viral clearance but also contribute to the induction of immunopathology following RSV infection. C57BL/6 mice are often used to study T cell responses following RSV infection given the wide variety of genetically modified animals available. To date, few RSV-derived CD4⁺ and CD8⁺ T cell epitopes have been identified in C57BL/6 mice. Using an overlapping peptide library spanning the entire RSV proteome, intracellular cytokine staining for IFN-γ was performed to identify novel CD4⁺ and CD8⁺ T cell epitopes in C57BL/6 mice. We identified two novel CD4⁺ T cell epitopes and three novel CD8⁺ T cell epitopes located within multiple RSV proteins. Additionally, we characterized the newly described T cell epitopes by determining their TCR Vβ expression profiles and MHC restriction. Overall, the novel RSV-derived CD4⁺ and CD8⁺ T cell epitopes identified in C57BL/6 mice will aid in future studies of RSV-specific T cell responses.

Cytokines and CD8 T cell immunity during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection and hospitalization in infants. In spite of the enormous clinical burden caused by RSV infections, there remains no efficacious RSV vaccine. CD8 T cells mediate viral clearance as well as provide protection against a secondary RSV infection. However, RSV-specific CD8 T cells may also induce immunopathology leading to exacerbated morbidity and mortality. Many of the crucial functions performed by CD8 T cells are mediated by the cytokines they produce. IFN-γ and TNF are produced by CD8 T cells following RSV infection and contribute to both the acceleration of viral clearance and the induction of immunopathology. To prevent immunopathology, regulatory mechanisms are in place within the immune system to inhibit CD8 T cell effector functions after the infection has been cleared. The actions of a variety of cytokines, including IL-10 and IL-4, play a critical role in the regulation of CD8 T cell effector activity. Herein, we review the current literature on CD8 T cell responses and the functions of the cytokines they produce following RSV infection. Additionally, we discuss the regulation of CD8 T cell activation and effector functions through the actions of various cytokines.

The CD8 T Cell Response to Respiratory Virus Infections

Humans are highly susceptible to infection with respiratory viruses including respiratory syncytial virus (RSV), influenza virus, human metapneumovirus, rhinovirus, coronavirus, and parainfluenza virus. While some viruses simply cause symptoms of the common cold, many respiratory viruses induce severe bronchiolitis, pneumonia, and even death following infection. Despite the immense clinical burden, the majority of the most common pulmonary viruses lack long-lasting efficacious vaccines. Nearly all current vaccination strategies are designed to elicit broadly neutralizing antibodies, which prevent severe disease following a subsequent infection. However, the mucosal antibody response to many respiratory viruses is not long-lasting and declines with age. CD8 T cells are critical for mediating clearance following many acute viral infections in the lung. In addition, memory CD8 T cells are capable of providing protection against secondary infections. Therefore, the combined induction of virus-specific CD8 T cells and antibodies may provide optimal protective immunity. Herein, we review the current literature on CD8 T cell responses induced by respiratory virus infections. Additionally, we explore how this knowledge could be utilized in the development of future vaccines against respiratory viruses, with a special emphasis on RSV vaccination.

T Cell Responses during Acute Respiratory Virus Infection

The T cell response is an integral and essential part of the host immune response to acute virus infection. Each viral pathogen has unique, frequently nuanced, aspects to its replication, which affects the host response and as a consequence the capacity of the virus to produce disease. There are, however, common features to the T cell response to viruses, which produce acute limited infection. This is true whether virus replication is restricted to a single site, for example, the respiratory tract (RT), CNS etc., or replication is in multiple sites throughout the body. In describing below the acute T cell response to virus infection, we employ acute virus infection of the RT as a convenient model to explore this process of virus infection and the host response. We divide the process into three phases: the induction (initiation) of the response, the expression of antiviral effector activity resulting in virus elimination, and the resolution of inflammation with restoration of tissue homeostasis.

Lung CD8+ T Cell Impairment Occurs during Human Metapneumovirus Infection despite Virus-Like Particle Induction of Functional CD8+ T Cells

Human metapneumovirus (HMPV) is a major cause of respiratory disease in infants, the elderly, and immunocompromised individuals worldwide. There is currently no licensed HMPV vaccine. Virus-like particles (VLPs) are an attractive vaccine candidate because they are noninfectious and elicit a neutralizing antibody response. However, studies show that serum neutralizing antibodies are insufficient for complete protection against reinfection and that adaptive T cell immunity is important for viral clearance. HMPV and other respiratory viruses induce lung CD8(+) T cell (TCD8) impairment, mediated by programmed death 1 (PD-1). In this study, we generated HMPV VLPs by expressing the fusion and matrix proteins in mammalian cells and tested whether VLP immunization induces functional HMPV-specific TCD8 responses in mice. C57BL/6 mice vaccinated twice with VLPs and subsequently challenged with HMPV were protected from lung viral replication for at least 20 weeks postimmunization. A single VLP dose elicited F- and M-specific lung TCD8s with higher function and lower expression of PD-1 and other inhibitory receptors than TCD8s from HMPV-infected mice. However, after HMPV challenge, lung TCD8s from VLP-vaccinated mice exhibited inhibitory receptor expression and functional impairment similar to those of mice experiencing secondary infection. HMPV challenge of VLP-immunized μMT mice also elicited a large percentage of impaired lung TCD8s, similar to mice experiencing secondary infection. Together, these results indicate that VLPs are a promising vaccine candidate but do not prevent lung TCD8 impairment upon HMPV challenge.

IMPORTANCE

Human metapneumovirus (HMPV) is a leading cause of acute respiratory disease for which there is no licensed vaccine. Virus-like particles (VLPs) are an attractive vaccine candidate and induce antibodies, but T cell responses are less defined. Moreover, HMPV and other respiratory viruses induce lung CD8(+) T cell (TCD8) impairment mediated by programmed death 1 (PD-1). In this study, HMPV VLPs containing viral fusion and matrix proteins elicited epitope-specific TCD8s that were functional with low PD-1 expression. Two VLP doses conferred sterilizing immunity in C57BL/6 mice and facilitated HMPV clearance in antibody-deficient μMT mice without enhancing lung pathology. However, regardless of whether responding lung TCD8s had previously encountered HMPV antigens in the context of VLPs or virus, similar proportions were impaired and expressed comparable levels of PD-1 upon viral challenge. These results suggest that VLPs are a promising vaccine candidate but do not prevent lung TCD8 impairment upon HMPV challenge.

The increased type-1 and type-2 chemokine levels in children with acute RSV infection alter the development of adaptive immune responses

Severe RSV infections and frequent recurrence could be related to the altered polarization of type-2/type-1 T cells. This increases the importance of determining distinctive chemokines and chemokine receptor profiles on memory T cells. We analyzed systemic adaptive T cell response in the acute (n = 17) and convalescent phase (n = 7) of RSV-infected children, in the acute (n = 11) and convalescent phase (n = 6) of children with other viral respiratory infections (adenovirus and influenza virus), and in healthy children (n = 18). Expression of CCR4 and CXCR3 on effector-memory (T_EM) and central-memory (T_CM) T cells was compared between tested groups. Serum concentrations of specific chemokines were determined. High CXCL10 levels were detected in acutely infected children regardless of virus pathogen, whereas increased CCL17 production was RSV-specific. Higher percentages of CCR4⁺ CD4 T_EM cells in acute RSV infection were accompanied with higher percentages of CXCR3⁺ CD8 T_EM cells, whereas the development of long-lived memory CXCR3⁺ CD4 and CD8 T_CM cells seems to be compromised, as only children with other viral infections had higher percentages in the convalescent phase. Presence of type-2 and type-1 adaptive antiviral immune response, together with insufficient development of long-lived type-1 T cell memory, could play an important role in RSV pathogenesis and reinfection.

The effector T cell response to influenza infection

Influenza virus infection induces a potent initial innate immune response, which serves to limit the extent of viral replication and virus spread. However, efficient (and eventual) viral clearance within the respiratory tract requires the subsequent activation, rapid proliferation, recruitment, and expression of effector activities by the adaptive immune system, consisting of antibody producing B cells and influenza-specific T lymphocytes with diverse functions. The ensuing effector activities of these T lymphocytes ultimately determine (along with antibodies) the capacity of the host to eliminate the viruses and the extent of tissue damage. In this review, we describe this effector T cell response to influenza virus infection. Based on information largely obtained in experimental settings (i.e., murine models), we will illustrate the factors regulating the induction of adaptive immune T cell responses to influenza, the effector activities displayed by these activated T cells, the mechanisms underlying the expression of these effector mechanisms, and the control of the activation/differentiation of these T cells, in situ, in the infected lungs.

Determining the breadth of the respiratory syncytial virus-specific T cell response

Respiratory syncytial virus (RSV) is the most common cause of viral lower respiratory tract infections in infants and children under the age of 5. Studies examining RSV infection in susceptible BALB/c mice indicate that both CD4 and CD8 T cells not only contribute to viral clearance but also facilitate RSV-induced disease. However, efforts to understand the mechanisms by which RSV-specific T cells mediate disease following acute RSV infection have been hampered by the lack of defined RSV-specific T cell epitopes. Using an overlapping peptide library spanning each of the RSV-derived proteins, intracellular cytokine staining for gamma interferon was utilized to identify novel RSV-specific CD4 and CD8 T cell epitopes. Five novel CD8 T cell epitopes were revealed within the RSV fusion (F) protein and glycoprotein (G). In addition, five previously unidentified CD4 T cell epitopes were discovered, including epitopes in the phosphoprotein (P), polymerase protein (L), M2-1 protein, and nucleoprotein (N). Though the initial CD4 T cell epitopes were 15 amino acids in length, synthesis of longer peptides increased the frequency of responding CD4 T cells. Our results indicate that CD4 T cell epitopes that are 17 amino acids in length result in more optimal CD4 T cell stimulation than the commonly used 15-mer peptides.

IMPORTANCE

Respiratory syncytial virus (RSV) is the leading cause of hospitalization for lower respiratory tract infection in children. T cells play a critical role in clearing an acute RSV infection, as well as contributing to RSV-induced disease. Here we examined the breadth of the RSV-specific T cell response, using for the first time an overlapping peptide library spanning the entire viral genome. We identified 5 new CD4 and 5 new CD8 T cell epitopes, including a CD8 T cell epitope within the G protein that was previously believed not to elicit a CD8 T cell response. Importantly, we also demonstrated that the use of longer, 17-mer peptides elicits a higher frequency of responding CD4 T cells than the more commonly used 15-mer peptides. Our results demonstrate the breadth of the CD4 and CD8 T cell response to RSV and demonstrate the importance of using longer peptides when stimulating CD4 T cell responses.

The initial draining lymph node primes the bulk of the CD8 T cell response and influences memory T cell trafficking after a systemic viral infection

Lymphocytic choriomeningitis virus (LCMV) causes a systemic infection in mice with virus replication occurring in both peripheral tissues and secondary lymphoid organs. Because of the rapid systemic dissemination of the virus, the secondary lymphoid organs responsible for the induction of the LCMV-specific CD8 T cell response are poorly defined. We show that the mediastinal lymph node (MedLN) serves as the primary draining lymph node following LCMV infection. In addition, we demonstrate that the MedLN is responsible for priming the majority of the virus-specific CD8 T cell response. Following resolution of the acute infection, the draining MedLN exhibits characteristics of a reactive lymph node including an increased presence of germinal center B cells and increased cellularity for up to 60 days post-infection. Furthermore, the reactive MedLN harbors an increased frequency of CD62L(-) effector memory CD8 T cells as compared to the non-draining lymph nodes. The accumulation of LCMV-specific CD62L(-) memory CD8 T cells in the MedLN is independent of residual antigen and is not a unique feature of the MedLN as footpad infection with LCMV leads to a similar increase of virus-specific CD62L(-) effector memory CD8 T cells in the draining popliteal lymph node. Our results indicate that CD62L(-) effector memory CD8 T cells are granted preferential access into the draining lymph nodes for an extended time following resolution of an infection.

Vaccine-elicited CD8+ T cells protect against respiratory syncytial virus strain A2-line19F-induced pathogenesis in BALB/c mice

CD8(+) T cells may contribute to vaccines for respiratory syncytial virus (RSV). Compared to CD8(+) T cells responding to RSV infection, vaccine-elicited anti-RSV CD8(+) T cells are less well defined. We used a peptide vaccine to test the hypothesis that vaccine-elicited RSV-specific CD8(+) T cells are protective against RSV pathogenesis. BALB/c mice were treated with a mixture (previously termed TriVax) of an M2(82-90) peptide representing an immunodominant CD8 epitope, the Toll-like receptor (TLR) agonist poly(I·C), and a costimulatory anti-CD40 antibody. TriVax vaccination induced potent effector anti-RSV CD8(+) cytotoxic T lymphocytes (CTL). Mice were challenged with RSV strain A2-line19F, a model of RSV pathogenesis leading to airway mucin expression. Mice were protected against RSV infection and against RSV-induced airway mucin expression and cellular lung inflammation when challenged 6 days after vaccination. Compared to A2-line19F infection alone, TriVax vaccination followed by challenge resulted in effector CD8(+) T cells with greater cytokine expression and the more rapid appearance of RSV-specific CD8(+) T cells in the lung. When challenged 42 days after TriVax vaccination, memory CD8(+) T cells were elicited with RSV-specific tetramer responses equivalent to TriVax-induced effector CD8(+) T cells. These memory CD8(+) T cells had lower cytokine expression than effector CD8(+) T cells, and protection against A2-line19F was partial during the memory phase. We found that vaccine-elicited effector anti-RSV CD8(+) T cells protected mice against RSV infection and pathogenesis, and waning protection correlated with reduced CD8(+) T cell cytokine expression.

Changes in functional but not structural avidity during differentiation of CD8+ effector cells in vivo after virus infection

By the peak of the CD8(+) T cell response, the effector cell pool consists of a heterogeneous population of cells that includes both those with an increased propensity to become long-lived memory cells (memory precursor effector cells; MPEC) and those that are terminally differentiated cells (short-lived effector cells; SLEC). Numerous studies have established the critical role that functional avidity plays in determining the in vivo efficacy of CD8(+) effector cells. Currently, how functional avidity differs in MPEC versus SLEC and the evolution of this property within these two populations during the expansion and contraction of the response are unknown. The data presented in this study show that at the peak of the effector response generated after poxvirus infection, SLEC were of higher functional avidity than their MPEC counterpart. Over time, however, SLEC exhibited a decrease in peptide sensitivity. This is in contrast to MPEC, which showed a modest increase in peptide sensitivity as the response reached equilibrium. The decrease in functional avidity in SLEC was independent of CD8 modulation or the amount of Ag receptor expressed by the T cell. Instead, the loss in sensitivity was correlated with decreased expression and activation of ZAP70 and Lck, critical components of TCR membrane proximal signaling. These results highlight the potential contribution of avidity in the differentiation and evolution of the T cell effector response after viral infection.

Quantifying antigen-specific CD4 T cells during a viral infection: CD4 T cell responses are larger than we think

The number of virus-specific CD8 T cells increases substantially during an acute infection. Up to 90% of CD8 T cells are virus specific following lymphocytic choriomeningitis virus (LCMV) infection. In contrast, studies identifying virus-specific CD4 T cell epitopes have indicated that CD4 T cells often recognize a broader array of Ags than CD8 T cells, consequently making it difficult to accurately quantify the total magnitude of pathogen-specific CD4 T cell responses. In this study, we show that CD4 T cells become CD11a(hi)CD49d(+) after LCMV infection and retain this expression pattern into memory. During the effector phase, all the LCMV-specific IFN-γ(+) CD4 T cells display a CD11a(hi)CD49d(+) cell surface expression phenotype. In addition, only memory CD11a(hi)CD49d(+) CD4 T cells make IFN-γ after stimulation. Furthermore, upon secondary LCMV challenge, only CD11a(hi)CD49d(+) memory CD4 T cells from LCMV-immune mice undergo proliferative expansion, demonstrating that CD11a(hi)CD49d(+) CD4 T cells are truly Ag specific. Using the combination of CD11a and CD49d, we demonstrate that up to 50% of the CD4 T cells are virus specific during the peak of the LCMV response. Our results indicate that the magnitude of the virus-specific CD4 T cell response is much greater than previously recognized.

Multiple CD4+ T cell subsets produce immunomodulatory IL-10 during respiratory syncytial virus infection

The host immune response is believed to contribute to the severity of pulmonary disease induced by acute respiratory syncytial virus (RSV) infection. Because RSV-induced pulmonary disease is associated with immunopathology, we evaluated the role of IL-10 in modulating the RSV-specific immune response. We found that IL-10 protein levels in the lung were increased following acute RSV infection, with maximum production corresponding to the peak of the virus-specific T cell response. The majority of IL-10-producing cells in the lung during acute RSV infection were CD4(+) T cells. The IL-10-producing CD4(+) T cells included Foxp3(+) regulatory T cells, Foxp3(-) CD4(+) T cells that coproduce IFN-γ, and Foxp3(-) CD4(+) T cells that do not coproduce IFN-γ. RSV infection of IL-10-deficient mice resulted in more severe disease, as measured by increased weight loss and airway resistance, as compared with control mice. We also observed an increase in the magnitude of the RSV-induced CD8(+) and CD4(+) T cell response that correlated with increased disease severity in the absence of IL-10 or following IL-10R blockade. Interestingly, IL-10R blockade during acute RSV infection altered CD4(+) T cell subset distribution, resulting in a significant increase in IL-17A-producing CD4(+) T cells and a concomitant decrease in Foxp3(+) regulatory T cells. These results demonstrate that IL-10 plays a critical role in modulating the adaptive immune response to RSV by limiting T-cell-mediated pulmonary inflammation and injury.

Differentiation-dependent differences in murine T cell susceptibility to negative regulation by the lung

A large number of viral infections are contracted via the respiratory route. Thus, an effective immune response at this site is of vital importance. Past studies in murine models analyzing a number of viruses have reported that CD8(+) effector T cells entering the lung after respiratory infection exhibit significant functional inactivation. The impaired function in these cells has been proposed to be the result of infection-induced changes in the lung; however, we have found that loss of function can occur in effector CD8(+) T cells present in the lung, even in the absence of infection. This functional inactivation takes place within 48 hours of entry into the lung, and is seen only in effector cells residing in the lung parenchyma, and not the airway. In this study, we have extended our findings to show that functional impairment of these effector cells is not initiated by bone marrow-derived cells, and is independent of proliferation in the lung tissue. Of critical importance, we have also determined that the susceptibility to functional inactivation is a common property shared by most effector cells. Finally, we show that the susceptibility to loss of function is actively regulated throughout differentiation. Although naive CD8(+) T cells, like effector cells, are negatively regulated as a result of residence in the lung, memory cells exhibit profound resistance to functional inactivation. The selective resistance of CD8(+) memory cells may allow the host to limit damage during the effector phase while retaining a protective response that can effectively limit subsequent infection.

Effects of human respiratory syncytial virus, metapneumovirus, parainfluenza virus 3 and influenza virus on CD4+ T cell activation by dendritic cells

Background

Human respiratory syncytial virus (HRSV), and to a lesser extent human metapneumovirus (HMPV) and human parainfluenza virus type 3 (HPIV3), re-infect symptomatically throughout life without antigenic change, suggestive of incomplete immunity. One causative factor is thought to be viral interference with dendritic cell (DC)-mediated stimulation of CD4+ T cells.

Methodology, Principal Findings

We infected human monocyte-derived DC with purified HRSV, HMPV, HPIV3, or influenza A virus (IAV) and compared their ability to induce activation and proliferation of autologous CD4+ T cells in vitro. IAV was included because symptomatic re-infection without antigenic change is less frequent, suggesting that immune protection is more complete and durable. We examined virus-specific memory responses and superantigen-induced responses by multiparameter flow cytometry. Live virus was more stimulatory than inactivated virus in inducing DC-mediated proliferation of virus-specific memory CD4+ T cells, suggesting a lack of strong suppression by live virus. There were trends of increasing proliferation in the order: HMPV<HRSV<HPIV3<IAV, and greater production of interferon-γ and tumor necrosis factor-α by proliferating cells in response to IAV, but differences were not significant. Exposure of DC to HRSV, HPIV3, or IAV reduced CD4+ T cell proliferation in response to secondary stimulus with superantigen, but the effect was transitory and greatest for IAV. T cell cytokine production was similar, with no evidence of Th2 or Th17 skewing.

Conclusions, Significance

Understanding the basis for the ability of HRSV in particular to symptomatically re-infect without significant antigenic change is of considerable interest. The present results show that these common respiratory viruses are similar in their ability to induce DC to activate CD4+ T cells. Thus, the results do not support the common model in which viral suppression of CD4+ T cell activation and proliferation by HRSV, HMPV, and HPIV3 is a major factor in the difference in re-infectability compared to IAV.

Foxp3+ CD4 regulatory T cells limit pulmonary immunopathology by modulating the CD8 T cell response during respiratory syncytial virus infection

Regulatory Foxp3(+) CD4 T cells (Tregs) prevent spontaneous inflammation in the lungs, inhibit allergic and asthmatic responses, and contribute to tolerance to inhaled allergens. Additionally, Tregs have previously been shown to suppress the CD8 T cell response during persistent virus infections. However, little is known concerning the role that Tregs play in modulating the adaptive immune response during acute respiratory virus infections. We show following acute respiratory syncytial virus (RSV) infection that Foxp3(+) CD4 Tregs rapidly accumulate in the lung-draining mediastinal lymph nodes and lungs. BrdU incorporation studies indicate that Tregs undergo proliferation that contributes to their accumulation in the lymph nodes and lungs. Following an acute RSV infection, pulmonary Tregs modulate CD25 expression and acquire an activated phenotype characterized as CD11a(high), CD44(high), CD43(glyco+), ICOS(+), and CTLA-4(+). Surprisingly, in vivo depletion of Tregs prior to RSV infection results in delayed virus clearance concomitant with an early lag in the recruitment of RSV-specific CD8 T cells into the lungs. Additionally, Treg depletion results in exacerbated disease severity, including increased weight loss, morbidity, and enhanced airway restriction. In Treg-depleted mice there is an increase in the frequency of RSV-specific CD8 T cells that coproduce IFN-gamma and TNF-alpha, which may contribute to enhanced disease severity. These results indicate that pulmonary Tregs play a critical role in limiting immunopathology during an acute pulmonary virus infection by influencing the trafficking and effector function of virus-specific CD8 T cells in the lungs and draining lymph nodes.

T cell epitope specificity and pathogenesis of mouse hepatitis virus-1-induced disease in susceptible and resistant hosts

Intranasal mouse hepatitis virus-1 (MHV-1) infection of susceptible mouse strains mimics some important pathologic features observed in the lungs of severe acute respiratory syndrome (SARS)-coronavirus-infected humans. The pathogenesis of SARS remains poorly understood, although increasing evidence suggests that immunopathology could play an important role. We previously reported that the adaptive immune response plays an important protective role in MHV-1-infected resistant B6 mice and that both CD4 and CD8 T cells play a significant role in the development of morbidity and lung pathology following intranasal MHV-1 infection of susceptible C3H/HeJ and A/J mice. In this study, we have identified novel CD4 and CD8 epitopes in MHV-1-infected susceptible and resistant strains of mice. Susceptible C3H/HeJ mice mount robust and broad MHV-1-specific CD4 T cell responses, whereas in resistant B6 mice, Ag-specific CD8 T cell responses dominate. We also show that previously immunized susceptible C3H/HeJ mice do not develop any morbidity and are completely protected following a lethal-dose MHV-1 challenge despite mounting only a modest secondary T cell response. Finally, we demonstrate that the resistance displayed by B6 mice is not solely accounted for by the elaboration of a broad and vigorous MHV-1-specific CD8 T cell response, as MHV-1 infection of C3.SW-H2(b)/SnJ mice, which mount an equally robust CD8 T cell response of the same specificity, is still associated with significant morbidity. Thus, identification of novel CD4 and CD8 T cell epitopes for MHV-1 permitted high-resolution analyses of pulmonary T cell responses in a mouse model of SARS.

Mucosal parainfluenza virus-vectored vaccine against Ebola virus replicates in the respiratory tract of vector-immune monkeys and is immunogenic

We previously used human parainfluenza virus type 3 (HPIV3) as a vector to express the Ebola virus (EBOV) GP glycoprotein. The resulting HPIV3/EboGP vaccine was immunogenic and protective against EBOV challenge in a non-human primate model. However, it remained unclear whether the vaccine would be effective in adults due to preexisting immunity to HPIV3. Here, the immunogenicity of HPIV3/EboGP was compared in HPIV3-naive and HPIV3-immune Rhesus monkeys. After a single dose of HPIV3/EboGP, the titers of EBOV-specific serum ELISA or neutralization antibodies were substantially less in HPIV3-immune animals compared to HPIV3-naive animals. However, after two doses, which were previously determined to be required for complete protection against EBOV challenge, the antibody titers were indistinguishable between the two groups. The vaccine virus appeared to replicate, at a reduced level, in the respiratory tract despite the preexisting immunity. This may reflect the known ability of HPIV3 to re-infect and may also reflect the presence of EBOV GP in the vector virion, which confers resistance to neutralization in vitro by HPIV3-specific antibodies. These data suggest that HPIV3/EboGP will be immunogenic in adults as well as children.

Pivotal Advance: Nonfunctional lung effectors exhibit decreased calcium mobilization associated with reduced expression of ORAI1

CD8(+) T cells play a critical role in the clearance of respiratory pathogens. Thus, it is surprising that functional inactivation of lung effectors has been observed in many models of viral infection. Currently, the molecular defect responsible for the shut-off of function in these cells is unknown. In the present study, we addressed this question using a model of respiratory infection with the paramyxovirus SV5. Nonfunctional cells were found to exhibit decreases in SOCE, resulting in reduced NFAT1 activation. Notably, function could be restored by the provision of increased levels of extracellular calcium. The reduced ability to mobilize calcium was associated with reduced expression of ORAI1, the CRAC channel subunit. These findings reveal a previously unknown mechanism for the negative regulation of function in effector T cells.

Systems biology coupled with label-free high-throughput detection as a novel approach for diagnosis of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a treatable and preventable disease state, characterised by progressive airflow limitation that is not fully reversible. Although COPD is primarily a disease of the lungs there is now an appreciation that many of the manifestations of disease are outside the lung, leading to the notion that COPD is a systemic disease. Currently, diagnosis of COPD relies on largely descriptive measures to enable classification, such as symptoms and lung function. Here the limitations of existing diagnostic strategies of COPD are discussed and systems biology approaches to diagnosis that build upon current molecular knowledge of the disease are described. These approaches rely on new 'label-free' sensing technologies, such as high-throughput surface plasmon resonance (SPR), that we also describe.

Immunity to polyomavirus infection: the polyomavirus-mouse model

A ubiquitous clinically silent murine pathogen, polyomavirus has enjoyed long-term co-evolution with the mouse, a highly tractable and genetically and immunologically informative small animal model. Thus, polyomavirus has provided a valuable experimental construct to decipher the host immune mechanisms that come into play to control systemic low-level persistent viral infections. Impaired immunosurveillance for infected cells puts the murine host at risk both to injury resulting from excessive direct virus cytolysis and development of virus-induced tumors. In this review, we present our current understanding of the multifaceted immune response invoked by the mouse to maintain détente with this potentially deleterious persistent natural pathogen, and discuss implications of these studies for therapeutic interventions for human polyomavirus infection.

Impairment of the CD8+ T cell response in lungs following infection with human respiratory syncytial virus is specific to the anatomical site rather than the virus, antigen, or route of infection

Background

A subset of the virus-specific CD8+ cytotoxic T lymphocytes (CTL) isolated from the lungs of mice infected with human respiratory syncytial virus (RSV) is impaired in the ability to secrete interferon γ (IFNγ), a measure of functionality. It was suggested that the impairment specifically suppressed the host cellular immune response, a finding that could help explain the ability of RSV to re-infect throughout life.

Results

To determine whether this effect is dependent on the virus, the route of infection, or the type of infection (respiratory, disseminated, or localized dermal), we compared the CTL responses in mice following intranasal (IN) infection with RSV or influenza virus or IN or intradermal (ID) infection with vaccinia virus expressing an RSV CTL antigen. The impairment was observed in the lungs after IN infection with RSV, influenza or vaccinia virus, and after a localized ID infection with vaccinia virus. In contrast, we observed a much higher percentage of IFNγ secreting CD8+ lymphocytes in the spleens of infected mice in every case.

Conclusion

The decreased functionality of CD8+ CTL is specific to the lungs and is not dependent on the specific virus, viral antigen, or route of infection.