Costimulation by B7 modulates specificity of cytotoxic T lymphocytes: a missing link that explains some bystander T cell activation

Non-cognate bystander cytolysis by clonal epitope-specific CTL lines through CD28-CD80 interaction inhibits antibody production: A potential caveat to CD8 T-cell immunotherapy

Adoptive transfer of virus epitope-specific CD8 T cells is an immunotherapy option to control cytomegalovirus (CMV) infection and prevent CMV organ disease in immunocompromised solid organ transplantation (SOT) and hematopoietic cell transplantation (HCT) recipients. The therapy aims at an early, selective recognition and cytolysis of infected cells for preventing viral spread in tissues with no adverse immunopathogenic side-effects by attack of uninfected bystander cells. Here we describe that virus epitope-specific, cloned T-cell lines lyse target cells that present the cognate antigenic peptide to the TCR, but simultaneously have the potential to lyse uninfected cells expressing the CD28 ligand CD80 (B7-1). While TCR-mediated cytolysis requires co-receptor CD8 and depends on perforin, the TCR-independent and viral epitope-independent cytolysis through CD28-CD80 signaling does not require CD8 on the effector cells and is perforin-independent. Importantly, this non-cognate cytolysis pathway leads to bystander cytolysis of CD80-expressing B-cell blasts and thereby inhibits pan-specific antibody production.

Aryl hydrocarbon receptor activation reduces dendritic cell function during influenza virus infection

It has long been known that activation of the aryl hydrocarbon receptor (AhR) by ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suppresses T cell-dependent immune responses; however, the underlying cellular targets and mechanism remain unclear. We have previously shown that AhR activation by TCDD reduces the proliferation and differentiation of influenza virus-specific CD8(+) T cells through an indirect mechanism; suggesting that accessory cells are critical AhR targets during infection. Respiratory dendritic cells (DCs) capture antigen, migrate to lymph nodes, and play a key role in activating naive CD8(+) T cells during respiratory virus infection. Herein, we report an examination of how AhR activation alters DCs in the lung and affects their trafficking to and function in the mediastinal lymph nodes (MLN) during infection with influenza virus. We show that AhR activation impairs lung DC migration and reduces the ability of DCs isolated from the MLN to activate naive CD8(+) T cells. Using novel AhR mutant mice, in which the AhR protein lacks its DNA-binding domain, we show that the suppressive effects of TCDD require that the activated AhR complex binds to DNA. These new findings suggest that AhR activation by chemicals from our environment impacts DC function to stimulate naive CD8(+) T cells and that immunoregulatory genes within DCs are critical targets of AhR. Moreover, our results reinforce the idea that environmental signals and AhR ligands may contribute to differential susceptibilities and responses to respiratory infection.

Intranasal immunization with inactivated influenza virus enhances immune responses to coadministered simian-human immunodeficiency virus-like particle antigens

Intranasal immunization with inactivated influenza virus vaccine can provide protective immunity, whereas many other antigens are less effective when used for mucosal immunization. To determine whether influenza virus could enhance immune responses to an antigen coadministered to a mucosal surface, we studied the intranasal immunization of mice with a mixture of simian-human immunodeficiency virus (SHIV) virus-like particles (VLPs) and inactivated influenza virus. Compared to mice immunized with SHIV VLPs alone, mice coimmunized with SHIV VLPs and inactivated influenza virus showed significant increases in serum immunoglobulin G (IgG) and mucosal IgA antibodies specific to the human immunodeficiency virus envelope protein, neutralizing activities, numbers of gamma interferon- and interleukin 4-secreting lymphocytes, and cytotoxic-T-lymphocyte activities. The levels of enhancement of immune response by coimmunization with inactivated influenza virus were equivalent to those induced by inclusion of immunostimulatory CpG oligodeoxynucleotides (CpG DNA). We also observed that SHIV VLPs bind to influenza virus virions, forming mixed aggregates. These results indicate that inactivated influenza virus can play a role as a mucosal adjuvant to coadministered antigens.

Immune modulation by flaviviruses

Flaviviruses cause pleomorphic disease with significant morbidity and mortality worldwide. Interestingly, in contrast to most viruses, which subvert or avoid host immune systems, members of the neurotropic Japanese encephalitis serocomplex cause functional changes associated with increased efficacy of the immune response. These viruses induce increased cell surface expression of immune recognition molecules, including class I and II major histocompatibility complex (MHC) and various adhesion molecules. Increases are functional: infected cells are significantly more susceptible to both virus- and MHC-specific cytotoxic T cell lysis. Induced changes are modulated positively or negatively by Th1 and Th2 cytokines, as well as by cell cycle position and adherence status at infection. Infection also increases costimulatory molecule expression on Langerhans cells in the skin. Local interleukin-1 beta production causes accelerated migration of phenotypically altered Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of upregulation is unclear, but changes are associated with NF-kappa B activation and increased MHC and ICAM-1 gene transcription, independently of interferon (IFN) or other proinflammatory cytokines. Increased MHC and adhesion molecule expression may contribute to the pathogenesis of flavivirus encephalitis. Results from a murine model of flavivirus encephalitis developed in this laboratory suggest that fatal disease is immunopathological in nature, with IFN-gamma playing a crucial role. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity T cells, which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly upregulate cell surface molecules.

Modulation of CD8+ T cell response to antigen by the levels of self MHC class I

The response of H-Y-specific TCR-transgenic CD8(+) T cells to Ag is characterized by poor proliferation, cytolytic activity, and IFN-gamma secretion. IFN-gamma secretion, but not cytotoxic function, can be rescued by the B7.1 molecule, suggesting that costimulation can selectively enhance some, but not all, effector CD8(+) T cell responses. Although the H-Y epitope binds H-2D(b) relatively less well than some other epitopes, it can induce potent CTL responses in nontransgenic mice, suggesting that the observed poor responsiveness of transgenic CD8(+) T cells cannot be ascribed to the epitope itself. Previously reported reactivity of this TCR to H-2A(b) is also not the cause of the poor responsiveness of the H-Y-specific CD8(+) T cells, as H-Y-specific CD8(+) T cells obtained from genetic backgrounds lacking H-2A(b) also responded poorly. Rather, reducing the levels of H-2(b) class I molecules by breeding the mice to (C57BL/6 x B10.D2)F(1) or TAP1(+/-) backgrounds partially restored cytotoxic activity and enhanced proliferative responses. These findings demonstrate that the self MHC class I gene dosage may regulate the extent of CD8(+) T cell responsiveness to Ag.

Dimensions of antigen recognition and levels of immunological specificity

Although recognition and specificity are among the most fundamental concepts in immunology, there is a common tendency to equate these notions with the fit, especially in terms of molecular shape, between interacting molecules. Even in the case of monovalent recognition, there are factors that contribute to the energetics of the interaction that are not readily accounted for by detailed structural analysis of the interacting (epitopic and paratopic) molecular surfaces. Consequently, recognition involves more than just the three spatial dimensions and time. Factors such as solute-solvent interactions, molecular crowding, and confinement, not directly related to the details of the intermolecular interface, can play crucial roles in determining both intrinsic affinity and differential intrinsic affinity. Furthermore, stating that a given structural subunit (e.g., amino acid) is recognized in a given noncovalent interaction does not clarify whether the structural subunit in question participates in the interaction through van der Waals contact, contribution to intrinsic affinity, or differential contribution to relative intrinsic affinities for two or more different ligands. Additional factors become relevant in considering the specificity exhibited in multivalent interactions, cell activation, and activation of the whole immune system. Therefore, specificity as defined for a monovalent binding event can diverge from specificity as it is defined for higher-order interactions. A corollary of this conclusion is that the composition of epitopes and paratopes, defined in terms of the structural elements for which substitutions have an effect on the specificity-defining measurement, can differ in different contexts despite complete conservation of the structures that physically make direct contact. An analysis of specificity at the organismal level suggests that the immune system does not recognize or respond to substances that correspond precisely to either nonself substances or to dangerous substances. An alternative notion for the molecular origins of immunological discrimination does not require that there be any single reason for immune responsiveness. This concept of what the immune system recognizes and responds to derives from the recognition that the ultimate function of the immune system is to contribute to survival and reproductive success through any available means.

Tumor cells expressing membrane-bound form of IL-4 induce antitumor immunity

Local cytokine concentrations are required for inhibition of tumor growth with less toxic side-effects. However, genetically engineered tumor cells secreting cytokines still induce toxicity and activate bystander cells. To circumvent such problems, membrane-bound forms of IL-4 (IL-4m) were expressed on MethA fibrosarcoma tumor cells. Chimeric forms of IL-4 with the type I transmembrane protein CD4 or type II transmembrane protein TNF were designed to express IL-4 in opposite orientations on the tumor cell surface. The IL-4m on tumor clones was able to support cell growth of the IL-4 dependent cytotoxic cell line (CT.4S) and the Th2 cell clone (D10). Furthermore, the IL-4m tumor clones stimulated proliferation of 2C TCR transgenic spleen cells which are responsive to Ld MHC class I molecules. Expression of the IL-4/TNF chimeric protein on MethA cells elicited antitumor immunity and protected from MethA tumor challenge. The proposed tumor vaccine may serve as an effective gene therapy method to avoid the toxicity of recombinant cytokines and bulk bystander leukocyte stimulation encountered in conventional cytokine gene therapy.

TCR Signaling Thresholds Regulating T Cell Development and Activation Are Dependent upon SHP-1

An examination of thymocytes and peripheral T cells from SHP-1-deficient motheaten mice possessing a transgenic MHC class I-restricted TCR has implicated SHP-1 in regulating TCR signaling thresholds at three checkpoints in T cell development and activation. First, in the population of CD4−CD8− double negative thymocytes, SHP-1 appears capable of regulating signals from TCR complexes that control the maturation and proliferation of double negative thymocytes. Second, the loss of SHP-1 increased the number of CD4+CD8+ double positive thymocytes capable of maturing as TCRhigh single positive thymocytes. Third, the loss of SHP-1 altered the basal level of activation of naive lymph node T cells. Accordingly, SHP-1-deficient lymph node T cells bearing the transgenic TCR demonstrated a hyperresponsiveness to stimulation with cognate peptide. However, the loss of SHP-1 did not alter the cytolytic ability of mature effector cytotoxic T lymphocytes. Together these results suggest that SHP-1 contributes to establishing thresholds for TCR signaling in thymocytes and naive peripheral T cells.

Control of murine cytomegalovirus in the lungs: relative but not absolute immunodominance of the immediate-early 1 nonapeptide during the antiviral cytolytic T-lymphocyte response in pulmonary infiltrates

The lungs are a major organ site of cytomegalovirus (CMV) infection, pathogenesis, and latency. Interstitial CMV pneumonia represents a critical manifestation of CMV disease, in particular in recipients of bone marrow transplantation (BMT). We have employed a murine model for studying the immune response to CMV in the lungs in the specific scenario of immune reconstitution after syngeneic BMT. Control of pulmonary infection was associated with a vigorous infiltration of the lungs, which was characterized by a preferential recruitment and massive expansion of the CD8 subset of alpha/beta T cells. The infiltrate provided a microenvironment in which the CD8 T cells differentiated into mature effector cells, that is, into functionally active cytolytic T lymphocytes (CTL). This gave us the opportunity for an ex vivo testing of the antigen specificities of CTL present at a relevant organ site of viral pathogenesis. The contribution of the previously identified immediate-early 1 (IE1) nonapeptide of murine CMV was evaluated by comparison with the CD3epsilon-redirected cytolytic activity used as a measure of the overall CTL response in the lungs. The IE1 peptide was detected by pulmonary CTL, but it accounted for a minor part of the response. Interestingly, no additional viral or virus-induced antigenic peptides were detectable among naturally processed peptides derived from infected lungs, even though infected fibroblasts were recognized in a major histocompatibility complex-restricted manner. We conclude that the antiviral pulmonary immune response is a collaborative function that involves many antigenic peptides, among which the IE1 peptide is immunodominant in a relative sense.

Persisting viruses and chronic inflammation: understanding their relation to autoimmunity

Viral infections may induce and sustain autoimmune processes via several and overlapping mechanisms. We outline how chronic inflammation, sustained by persisting viruses, may be "the prerequisite" for initiation and maintenance of the multistep process leading to autoimmunity. Chronic inflammation may favour priming of autoreactive T cells which have escaped thymic tolerance and are able to mount a cross-reactive response to self-mimicking antigens carried by viruses in the periphery. Moreover, chronic inflammation and persisting viruses can synergistically support autoimmunity through other relevant mechanisms: unveiling of cryptic self-epitopes, determinant spreading, activation of dendritic cells, constant priming of new autoreactive T cells, and efficient generation and restimulation of memory cells. Therefore, viruses seem to play a key role among the many environmental factors which, together with the genetic background, have been implicated in the pathogenesis of autoimmune diseases. We will also discuss some hypotheses explaining why autoimmunity is a rare event.