CD4+ and CD8+ mediated cellular immune response to recombinant influenza nucleoprotein

Impact of patient characteristics on innate immune responses and inflammasome activation in ex vivo human lung tissues infected with influenza A virus

Background

Influenza A virus (IAV) infection poses a persistent global health challenge, necessitating a nuanced grasp of host immune responses for optimal interventions. While the interplay between aging, immunosenescence, and IAV is recognized as key in severe lower respiratory tract infections, the role of specific patient attributes in shaping innate immune reactions and inflammasome activity during IAV infection remains under-investigated. In this study, we utilized an ex vivo infection model of human lung tissues with H3N2 IAV to discern relationships among patient demographics, IAV nucleoprotein (NP) expression, toll-like receptor (TLR) profiles, PD-1/PD-L1 markers, and cytokine production.

Methods

Our cohort consisted of thirty adult patients who underwent video-assisted thoracoscopic surgery during 2018-2019. Post-surgical lung tissues were exposed to H3N2 IAV for ex vivo infections, and the ensuing immune responses were profiled using flow cytometry.

Results

We observed pronounced IAV activity within lung cells, as indicated by marked NP upregulation in both epithelial cells (P = 0.022) and macrophages (P = 0.003) in the IAV-exposed group relative to controls. Notably, interleukin-2 levels correlated with variations in TLR1 expression on epithelial cells and PD-L1 markers on macrophages. Age emerged as a modulating factor, dampening innate immune reactions, as evidenced by reduced interleukin-2 and interferon-γ concentrations (both adjusted P < 0.05). Intriguingly, a subset of participants with pronounced tumor necrosis factor-alpha post-mock infection (Cluster 1) showed attenuated cytokine responses in contrast to their counterparts in Cluster 2 and Cluster 3 (all adjusted P < 0.05). Individuals in Cluster 2, characterized by a low post-mock infection NP expression in macrophages, exhibited reduced variations in both NP and TLR1-3 expressions on these cells and a decreased variation in interleukin-2 secretion in comparison to their Cluster 3 counterparts, who were identified by their elevated NP macrophage expression (all adjusted P < 0.05).

Conclusion

Our work elucidates the multifaceted interplay of patient factors, innate immunity, and inflammasome responses in lung tissues subjected to ex vivo H3N2 IAV exposure, reflecting real-world lower respiratory tract infections. While these findings provide a foundation for tailored therapeutic strategies, supplementary studies are requisite for thorough validation and refinement.

Self-assembled multiepitope nanovaccine based on NoV P particles induces effective and lasting protection against H3N2 influenza virus

Current seasonal influenza vaccines confer only limited coverage of virus strains due to the frequent genetic and antigenic variability of influenza virus (IV). Epitope vaccines that accurately target conserved domains provide a promising approach to increase the breadth of protection; however, poor immunogenicity greatly hinders their application. The protruding (P) domain of the norovirus (NoV), which can self-assemble into a 24-mer particle called the NoV P particle, offers an ideal antigen presentation platform. In this study, a multiepitope nanovaccine displaying influenza epitopes (HMN-PP) was constructed based on the NoV P particle nanoplatform. Large amounts of HMN-PP were easily expressed in Escherichia coli in soluble form. Animal experiments showed that the adjuvanted HMN-PP nanovaccine induced epitope-specific antibodies and haemagglutinin (HA)-specific neutralizing antibodies, and the antibodies could persist for at least three months after the last immunization. Furthermore, HMN-PP induced matrix protein 2 extracellular domain (M2e)-specific antibody-dependent cell-mediated cytotoxicity, CD4+ and CD8+ T-cell responses, and a nucleoprotein (NP)-specific cytotoxic T lymphocyte (CTL) response. These results indicated that the combination of a multiepitope vaccine and self-assembled NoV P particles may be an ideal and effective vaccine strategy for highly variable viruses such as IV and SARS-CoV-2.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at 10.1007/s12274-023-5395-6.

The Role of Nucleoprotein in Immunity to Human Negative-Stranded RNA Viruses—Not Just Another Brick in the Viral Nucleocapsid

Negative-stranded RNA viruses (NSVs) are important human pathogens, including emerging and reemerging viruses that cause respiratory, hemorrhagic and other severe illnesses. Vaccine design traditionally relies on the viral surface glycoproteins. However, surface glycoproteins rarely elicit effective long-term immunity due to high variability. Therefore, an alternative approach is to include conserved structural proteins such as nucleoprotein (NP). NP is engaged in myriad processes in the viral life cycle: coating and protection of viral RNA, regulation of transcription/replication processes and induction of immunosuppression of the host. A broad heterosubtypic T-cellular protection was ascribed very early to this protein. In contrast, the understanding of the humoral immunity to NP is very limited in spite of the high titer of non-neutralizing NP-specific antibodies raised upon natural infection or immunization. In this review, the data with important implications for the understanding of the role of NP in the immune response to human NSVs are revisited. Major implications of the elicited T-cell immune responses to NP are evaluated, and the possible multiple mechanisms of the neglected humoral response to NP are discussed. The intention of this review is to remind that NP is a very promising target for the development of future vaccines.

Universal influenza vaccines: from viruses to nanoparticles

INTRODUCTION

The current seasonal influenza vaccine confers only limited protection due to waning antibodies or the antigenic shift and drift of major influenza surface antigens. A universal influenza vaccine which induces broad cross-protection against divergent influenza viruses with a comparable or better efficacy to seasonal influenza vaccines against matched strains will negate the need for an annual update of vaccine strains and protect against possible influenza pandemics.

AREAS COVERED

In this review, we summarize the recent progress in nanoparticle-based universal influenza vaccine development. We compared the most potent nanoparticle categories, focusing on how they encapsulate conserved influenza epitopes, stimulate the innate and adaptive immune systems, exhibit antigen depot effect, extend the period for antigen-processing and presentation, and exert an intrinsic adjuvant effect on inducing robust immune responses.

EXPERT COMMENTARY

The development of an effective universal influenza vaccine is an urgent task. Traditional influenza vaccine approaches are not sufficient for preventing recurrent epidemics or occasional pandemics. Nanoparticles are compatible with different immunogens and immune stimulators and can overcome the intrinsically low immunogenicity of conserved influenza virus antigens. We foresee that an affordable universal influenza vaccine will be available within ten years by integrating nanoparticles with other targeted delivery and controlled release technology.

Immunosenescence-Related Transcriptomic and Immunologic Changes in Older Individuals Following Influenza Vaccination

The goal of annual influenza vaccination is to reduce mortality and morbidity associated with this disease through the generation of protective immune responses. The objective of the current study was to examine markers of immunosenescence and identify immunosenescence-related differences in gene expression, gene regulation, cytokine secretion, and immunologic changes in an older study population receiving seasonal influenza A/H1N1 vaccination. Surprisingly, prior studies in this cohort revealed weak correlations between immunosenescence markers and humoral immune response to vaccination. In this report, we further examined the relationship of each immunosenescence marker (age, T cell receptor excision circle frequency, telomerase expression, percentage of CD28⁻ CD4⁺ T cells, percentage of CD28⁻ CD8⁺ T cells, and the CD4/CD8 T cell ratio) with additional markers of immune response (serum cytokine and chemokine expression) and measures of gene expression and/or regulation. Many of the immunosenescence markers indeed correlated with distinct sets of individual DNA methylation sites, miRNA expression levels, mRNA expression levels, serum cytokines, and leukocyte subsets. However, when the individual immunosenescence markers were grouped by pathways or functional terms, several shared biological functions were identified: antigen processing and presentation pathways, MAPK, mTOR, TCR, BCR, and calcium signaling pathways, as well as key cellular metabolic, proliferation and survival activities. Furthermore, the percent of CD4⁺ and/or CD8⁺ T cells lacking CD28 expression also correlated with miRNAs regulating clusters of genes known to be involved in viral infection. Integrated (DNA methylation, mRNA, miRNA, and protein levels) network biology analysis of immunosenescence-related pathways and genesets identified both known pathways (e.g., chemokine signaling, CTL, and NK cell activity), as well as a gene expression module not previously annotated with a known function. These results may improve our ability to predict immune responses to influenza and aid in new vaccine development, and highlight the need for additional studies to better define and characterize immunosenescence.

Antibodies to influenza nucleoprotein cross-react with human hypocretin receptor 2

The sleep disorder narcolepsy is linked to the HLA-DQB1*0602 haplotype and dysregulation of the hypocretin ligand-hypocretin receptor pathway. Narcolepsy was associated with Pandemrix vaccination (an adjuvanted, influenza pandemic vaccine) and also with infection by influenza virus during the 2009 A(H1N1) influenza pandemic. In contrast, very few cases were reported after Focetria vaccination (a differently manufactured adjuvanted influenza pandemic vaccine). We hypothesized that differences between these vaccines (which are derived from inactivated influenza viral proteins) explain the association of narcolepsy with Pandemrix-vaccinated subjects. A mimic peptide was identified from a surface-exposed region of influenza nucleoprotein A that shared protein residues in common with a fragment of the first extracellular domain of hypocretin receptor 2. A significant proportion of sera from HLA-DQB1*0602 haplotype-positive narcoleptic Finnish patients with a history of Pandemrix vaccination (vaccine-associated narcolepsy) contained antibodies to hypocretin receptor 2 compared to sera from nonnarcoleptic individuals with either 2009 A(H1N1) pandemic influenza infection or history of Focetria vaccination. Antibodies from vaccine-associated narcolepsy sera cross-reacted with both influenza nucleoprotein and hypocretin receptor 2, which was demonstrated by competitive binding using 21-mer peptide (containing the identified nucleoprotein mimic) and 55-mer recombinant peptide (first extracellular domain of hypocretin receptor 2) on cell lines expressing human hypocretin receptor 2. Mass spectrometry indicated that relative to Pandemrix, Focetria contained 72.7% less influenza nucleoprotein. In accord, no durable antibody responses to nucleoprotein were detected in sera from Focetria-vaccinated nonnarcoleptic subjects. Thus, differences in vaccine nucleoprotein content and respective immune response may explain the narcolepsy association with Pandemrix.

Incorporation of conserved nucleoprotein into influenza virus-like particles could provoke a broad protective immune response in BALB/c mice and chickens

We engineered influenza A/goose/GD/1996 (H5N1) (clade 0) virus-like particles (VLPs) by coinfecting Sf9 cells with triple/quadruple recombinant baculovirus that expressed hemagglutinin (HA), neuraminidase (NA), and matrix 1 (M1) with or without nucleoprotein (NP). VLP3 (HA, NA, and M1) and VLP4 (HA, NA, M1, and NP) vaccines (containing 1 μg HA) with oil emulsion were administered to mice and chickens by intramuscular injection, and the immune responses were analyzed. The VLP-vaccinated mice demonstrated high antigen specific antibody titers and effective cellular immune responses. The mice and chickens vaccinated with VLP4 demonstrated more robust humoral and cellular immune responses than those vaccinated with VLP3. The VLP4 vaccine afforded 100% protection against a heterologous lethal influenza virus challenge (clade 2.3.4) whereas the VLP3 vaccine conferred 50% protection in chickens. These results implied that the incorporation of conserved NP protein into the VLPs could elicit a broad protective immune response in BALB/c mice and chickens. To the best of our knowledge, this study is the first report describing the immunological profile of the NP-containing VLPs vaccines in mice and chicken models, and the results demonstrate that the non-infectious, genome less VLPs, particularly those containing NP, represent a promising strategy for the development of a safe and effective vaccine to control pandemic influenza.

Nucleoprotein of influenza A virus is a major target of immunodominant CD8+ T-cell responses

Influenza A virus causes annual epidemics and sporadic pandemics, resulting in significant morbidity and mortality worldwide. Vaccines are currently available; however, they induce a non-strain-cross protective humoral immune response directed against the rapidly mutating surface glycoproteins, and thus need to be updated annually. As T cells are directed against more conserved internal influenza proteins, a T-cell-based vaccine has the potential to induce long-lasting and cross-strain protective CD8(+) T-cell immunity, and in that way minimize the severity of influenza infection. However, to rationally design such vaccines, we need to identify immunogenic T-cell regions within the most antigenic viral proteins. In this study, we have used a systematic approach to identify immunodominant peptides in HLA-A2-negative donors. A broad range of CD8(+) T-cell responses were observed and 6/7 donors had an immunodominant response against the relatively conserved internal nucleoprotein (NP). Dissecting the minimal epitope regions within the immunogenic NP led to the identification of six novel immunodominant epitopes, which include a 12-mer and an 8-mer peptides. The majority of immunodominant epitopes was clustered within the carboxyl terminal 2/3 of the NP protein and were highly conserved. We also subjected NP to three common computer algorithms for epitope prediction and found that most of the novel epitopes would not have been predicted. Our study emphasizes the importance of using a systematic approach to identify immunodominant CD8(+) T-cell responses and suggests that the epitope-rich regions within NP present a promising target for the T-cell-mediated multi-strain influenza vaccine.

Development of universal influenza vaccines based on influenza virus M and NP genes

PURPOSE

Vaccination is the safest and most effective measure against influenza virus infections. However, traditional influenza vaccines cannot respond effectively to an unforeseen epidemic or pandemic caused by a virus with antigenic drifts or antigenic shifts. Therefore, developing a universal influenza vaccine (UIV) that induces broad-spectrum and long-term immunity has become a major trend in influenza vaccine research and development.

METHODS

This article reviews the development of UIVs based on these conserved influenza virus proteins.

RESULTS AND CONCLUSION

The matrix protein (M1, M2) and nucleoprotein (NP) of influenza viruses have highly conserved sequences, and they become the major target antigens of current UIV studies.

Hexon-modified recombinant E1-deleted adenovirus vectors as dual specificity vaccine carriers for influenza virus

To determine if an ordered and repetitive display of an epitope promoted induction of superior antibody responses, we compared B-cell responses to an influenza A virus epitope that was either encoded as a transgene by an adenovirus (Ad) vector or expressed on the vector's surface. To this end, we constructed a panel of influenza A virus vaccines based on chimpanzee-derived replication-defective adenovirus (AdC) vectors of serotype SAd-V25 also called AdC68. AdC68 vectors were modified to express a linear B-cell epitope of the ectodomain of matrix 2 (M2e) within variable regions 1 (VR1) or 4 (VR4) of the adenovirus hexon. Additional vectors with wild-type or M2e-modified hexon encoded M2e fused to the influenza A virus nucleoprotein (NP) as a transgene product. Hexon-modified vectors were tested for immunogenicity and efficacy in mice in comparison to vectors with native hexon expressing the M2e-NP fusion protein. Upon priming, vectors expressing M2e within VR1 of hexon induced M2e-specific antibody responses of higher magnitude and avidity than those carrying M2e within VR4 or vectors expressing the M2e as part of a transgene product. CD8(+) T-cell responses to the transgenic NP were comparable between vectors. M2e-specific antibody responses could be boosted by a second dose of the VR1 hexon-modified vector but not by repeated immunization with the VR4 hexon-modified vector.

Endogenously expressed matrix protein M1 and nucleoprotein of influenza A are efficiently presented by class I and class II major histocompatibility complexes

Current influenza vaccines containing primarily hypervariable haemagglutinin and neuraminidase proteins must be prepared against frequent new antigenic variants. Therefore, there is an ongoing effort to develop influenza vaccines that also elicit strong and sustained cytotoxic responses against highly conserved determinants such as the matrix (M1) protein and nucleoprotein (NP). However, their antigenic presentation properties in humans are less defined. Accordingly, we analysed MHC class I and class II presentation of endogenously processed M1 and NP in human antigen presenting cells and observed expansion of both CD8+- and CD4+-specific effector T lymphocytes secreting gamma interferon and tumour necrosis factor. Further enhancement of basal MHC-II antigenic presentation did not improve CD4+ or CD8+ T-cell quality based on cytokine production upon challenge, suggesting that endogenous M1 and NP MHC-II presentation is sufficient. These new insights about T-lymphocyte expansion following endogenous M1 and NP MHC-I and -II presentation will be important to design complementary heterosubtypic vaccination strategies.

Human influenza vaccines and assessment of immunogenicity

Influenza is responsible for the infection of approximately 20% of the population every season and for an annual death toll of approximately half a million people. The most effective means for controlling infection and thereby reducing morbidity and mortality is vaccination by injection with an inactivated vaccine, or by intranasal administration of a live-attenuated vaccine. Protection is not always optimal and there is a need for the development of new vaccines with improved efficacy and for the expansion of enrollment into vaccination programs. An overview of old and new vaccines is presented. Methods of monitoring immune responses such as hemagglutination-inhibition, ELISA and neutralization tests are evaluated for their accuracy in the assessment of current and new-generation vaccines.

Protection against mouse and avian influenza A strains via vaccination with a combination of conserved proteins NP, M1 and NS1

BACKGROUND

Experimental data accumulated over more than a decade indicate that cross-strain protection against influenza may be achieved by immunization with conserved influenza proteins. At the same time, the efficacy of immunization schemes designed along these lines and involving internal influenza proteins, mostly NP and M1, has not been sufficient.

OBJECTIVE

To test the immunogenicity and protective efficacy of DNA vaccination with a combination of NP, M1 and NS1 genes of influenza virus.

METHODS

The immunogenicity and protective efficacy of DNA vaccination with NP, M1 and NS1 was tested in mice and chickens. Mice were challenged with mouse-adapted viral strains H3N2 and H5N2 and chicken challenged with avian H5N3 virus.

RESULTS

In these settings, wild-type NS1 did not impede the cellular and humoral response to NP/M1 immunization in vivo. Moreover, addition of NS1-encoding plasmid to the NP/M1 immunization protocol resulted in a significantly increased protective efficacy in vivo.

CONCLUSIONS

The addition of NS1 to an influenza immunization regimen based on conserved proteins bears promise. It is feasible that upon further genetic modification of these and additional conserved influenza proteins, providing for their higher safety, expression and immunogenicity, a recombinant vaccine based on several structural and non-structural proteins or their epitopes will offer broad anti-influenza protection in a wide range of species.

Immunization with plasmid DNA encoding influenza A virus nucleoprotein fused to a tissue plasminogen activator signal sequence elicits strong immune responses and protection against H5N1 challenge in mice

DNA vaccination is an effective means of eliciting both humoral and cellular immunity. Most of influenza vaccines targeted at hemagglutinin (HA) show efficient immunogenicity for protecting subjects against influenza virus infection. However, major antigenic variations of HA may facilitate the virus in developing resistance against such vaccines. DNA vaccines encoding conserved antigens protect animals against diverse viral subtypes, but their potency requires further improvement. In the present study, a DNA vaccine encoding the conserved nucleoprotein (NP) with a tissue plasminogen activator (tPA) signal sequence (ptPAs/NP) was generated, and immune responses were examined in vaccinated mice. A higher level of NP expression and secretion was observed in lysates and supernatants of the cells transfected with ptPAs/NP when compared to a plasmid encoding the wild-type full-length NP (pflNP). Immunofluorescence studies showed the cytoplasmic localization of the NP protein expressed from ptPAs/NP, but not from pflNP. In mice, the ptPAs/NP vaccine elicited higher levels of the NP-specific IgG and CD8(+) T cell-stimulating responses than that of pflNP. Vaccination with ptPAs/NP efficiently cleared the homologous H5N1 influenza virus in the infected lungs and induced partial cross-protection against heterologous, highly pathogenic H5N1 strains in mice. Our results may contribute to the development of protective immunity against diverse, highly pathogenic H5N1 virus subtypes.

Structure of the influenza virus A H5N1 nucleoprotein: implications for RNA binding, oligomerization, and vaccine design

The threat of a pandemic outbreak of influenza virus A H5N1 has become a major concern worldwide. The nucleoprotein (NP) of the virus binds the RNA genome and acts as a key adaptor between the virus and the host cell. It, therefore, plays an important structural and functional role and represents an attractive drug target. Here, we report the 3.3-A crystal structure of H5N1 NP, which is composed of a head domain, a body domain, and a tail loop. Our structure resolves the important linker segments (residues 397-401, 429-437) that connect the tail loop with the remainder of the molecule and a flexible, basic loop (residues 73-91) located in an arginine-rich groove surrounding Arg150. Using surface plasmon resonance, we found the basic loop and arginine-rich groove, but mostly a protruding element containing Arg174 and Arg175, to be important in RNA binding by NP. We also used our crystal structure to build a ring-shaped assembly of nine NP subunits to model the miniribonucleoprotein particle previously visualized by electron microscopy. Our study of H5N1 NP provides insight into the oligomerization interface and the RNA-binding groove, which are attractive drug targets, and it identifies the epitopes that might be used for universal vaccine development.

Immunodominant peptides from conserved influenza proteins – A tool for more efficient vaccination in the elderly?

Influenza-specific CD8+ T cells are important for the clearance of infection especially in high risk groups such as elderly persons. Activation of these cells by immunization might therefore be a useful tool for a better protection of this specific age group. We therefore analyzed the frequency, phenotype and function of CD8+ T cells with specificity to the influenza M1(58-66) peptide in young, middle-aged and elderly persons ex vivo and after in vitro stimulation. Significantly lower numbers of M1(58-66)-specific CD8+ T cells were detected in the middle-aged and elderly compared to young donors. M1(58-66)-specific CD8+ T cells were either CD45RA(low)CD45RO(low) or CD45RA-CD45RO+, expressed CD28 and CD62L and did not produce perforin. There was no difference in the phenotype of influenza-specific CD8+ T cells between the three age groups. Despite the initially low numbers of M1(58-66)-specific CD8+ T cells in the older age groups, these cells could be expanded in vitro following peptide stimulation. They also acquired a CD45RO+CD28+ CD62L(+/-) phenotype and produced perforin. Our results demonstrate that although initially low in number, M1(58-66)-specific CD8+ T cells from elderly persons can be propagated and differentiated into perforin producing effector cells upon appropriate stimulation. M1(58-66) peptide or other immunodominant peptides derived from conserved influenza proteins could therefore be useful in future influenza vaccines in order to render elderly persons better protected against disease, in particular in the case of an influenza pandemic.

Immunization with influenza A NP-expressing vaccinia virus recombinant protects mice against experimental infection with human and avian influenza viruses

Two-fold immunization of Balb/c mice with a vaccinia virus recombinant expressing the NP protein of influenza A/PR8/34 (H1N1) virus under the control of a strong synthetic promoter induced specific antibodies and protected animals against low-dose challenge by mouse-adapted heterosubtypic variants of human A/Aichi2/68 (H3N2) and avian A/Mallard/Pennsylvania/10218/84 (H5N2) influenza virus strains. The surviving immunized animals had lower anti-hemagglutinin antibody titers compared to non-immunized mice. There was no difference in viral titers in lungs of immunized and non-immunized animals that succumbed to the infection. In order to try to increase immune system presentation of NP-protein-derived peptides, and thereby increase their immunogenicity, we constructed another vaccinia-based NP-expressing recombinant containing a rapid proteolysis signal covalently bound to the NP protein. This sequence, derived from the mouse ornithine decarboxylase gene has been shown to increase degradation of various proteins. However, we found that when used as part of a recombinant NP, this signal neither increased its proteolytic degradation, nor was it more efficient in the induction of a protective response against influenza infection.

T cells from elderly persons respond to neoantigenic stimulation with an unimpaired IL-2 production and an enhanced differentiation into effector cells

We analysed the capacity of T cells from young and elderly persons to produce IL-2 and IFN-gamma after in vitro stimulation with two neoantigens, namely inactivated rabies virus and recombinant Etr protein of tick-borne encephalitis virus (TBEV). Soluble antigens should per definition primarly stimulate CD4(+) naïve T cells. Cytokine production was analysed by ELISPOT technology. T cells from elderly and young donors produced similar amounts of IL-2 after priming with both neoantigens. In contrast, IFN-gamma production was induced earlier and at lower antigenic concentrations in T cells from elderly persons than from young controls indicating an enhanced capacity of primed T cells to differentiate into effector cells. In both age groups the response pattern to neoantigenic stimulation was the same whether unfractionated blood mononuclear cells or purified CD4(+)CD45RA(+) T cells with autologous DC as APC were used. The magnitude of the response was, however slightly lower in isolated cells. Autologous DC still induced an MLR in purified CD4(+)CD45RA(+) cells, which was more pronounced in the young than in the elderly group. Our results demonstrate that the ability of CD4(+) T cells from elderly persons to respond to neoantigenic stimulation is intact and that their capacity to differentiate into effector cells is even enhanced. This is in good agreement with earlier reports on alterations in the homing receptor pattern of naïve T cells in old age. Rapid generation of effector cells from naïve cells may at least partly counterbalance the decreasing size of the naïve T cell pool in elderly persons.

Conjugated Linoleic Acid Ameliorates Viral Infectivity in a Pig Model of Virally Induced Immunosuppression

We investigated the cellular and molecular immunoregulatory actions of conjugated linoleic acid (CLA) of relevance to viral disease pathogenesis and antiviral responses. To test the hypothesis that CLA ameliorates viral disease, we developed a viral challenge model by infecting pigs with type-2 porcine circovirus (PCV2). After 42 d of dietary supplementation with either soybean oil (n = 16) or CLA (n = 16), half of the pigs in each group were challenged with PCV2. We examined the effect of CLA on the development of lesions (i.e., lymphoid depletion and pneumonia) and observed the kinetics of the immune responses against PCV2. The viral infection depleted immature B cells (IgM+SWC3+) and favored proapoptotic mRNA expression profiles [i.e., suppressed B-cell leukemia/lymphoma-xl (Bcl-xl) and stimulated Bcl-2 homologous antagonist/killer (Bak)] in the external inguinal lymph nodes. B-cell depletion was more accentuated in pigs fed the control diet, whereas interleukin (IL)-2 mRNA expression was downregulated. Histopathological examination of the lungs revealed that the interstitial pneumonia tended to be more severe in infected pigs fed the control diet, which were also affected by growth retardation. CD8+ T cells were the primary cellular targets of CLA action in peripheral blood (CD8+CD29low and CD8+CD45RC+) and thymus (CD8+ and CD4+CD8+). CLA interacted with PCV2 to increase the proliferation of CD8+ T cells and to suppress PCV2-specific interferon (IFN)-gamma production in CD4+ T cells. At the molecular level, these cellular immunoregulatory properties were associated with differential patterns of peroxisome proliferator-activated receptor (alpha and gamma) mRNA expression between diets in virally infected pigs.