MHC class II biosynthesis by activated rat CD4+ T cells: development of repression in vitro and modulation by APC-derived signals

Immunisation with an allogeneic peptide promotes the induction of antigen-specific MHC II(pos) CD4+ rat T cells demonstrating immunostimulatory properties

BACKGROUND

The phenomenon of T cell stimulation by MHC class II expressing (MHC II(pos)) CD4+ T cells has been intensively investigated for T cell clones but, so far, not for native T cells. The extensive use of T cell clones may explain the inconsistent outcomes of T cell-mediated antigen-presentation. Therefore, we used freshly isolated primed rat CD4+ T cells induced by immunisation with an allogeneic peptide P1, which is involved in allograft rejection.

METHODS

MHC II(pos) and MHC II(neg) CD4+ T cells were isolated from popliteal lymph nodes of P1-immunised Lewis rats and were purified by combining depletion and positive selection steps. Purified MHC II(pos) CD4+ T cells and MHC II(neg) CD4+ T cells (10⁵ cells per well each) were autostimulated or restimulated with P1-loaded (33 μg/ml peptide P1) and subsequently irradiated (with 20 Gy) autologous DC.

RESULTS

Seven days after immunisation, a small population of MHC II(pos) CD4+ T cells was detectable (approximately 8.0% of total lymph node cells), as well as a large population of MHC II(neg) CD4+ T cells (up to 45%). Antigen-specific proliferation was observed for both T cell populations but only P1-loaded MHC II(pos) CD4+ T cells presented antigen presenting cell (APC) function for P1-primed T cells. Their inability to activate unprimed T cells may be due to impaired surface expression of costimulatory molecules (CD80 and CD86).

CONCLUSION

Immunisation with the allogeneic peptide antigen P1 induced antigen-specific MHC II(pos) CD4+ rat T cells demonstrating perfect APC function for primed T cells in vitro.

Antigen presentation assays to investigate uncharacterized immunoregulatory genes

Antigen presentation to T lymphocytes is the seminal triggering event of the specific immune response, and poxviruses encode immunomodulatory genes that disrupt this process. Discovery of viral proteins that interfere with steps in the antigen presentation process requires a robust, easily manipulated antigen-presenting and T lymphocyte response system. Use of fresh primary antigen-presenting cells (APC) is preferable because cell lines that can present antigen in vitro are often not representative of APC in vivo and are typically weak stimulators. To study immunomodulatory poxvirus genes, we have used infected primary rat macrophages to present a model antigen, the myelin basic protein peptide, to a cognate CD4+ RsL11 T cell clone. Using this system, viruses can be assessed for difference in immunomodulation, and viral gene functions may also be assayed by comparing effects of wild type virus and mutant viruses (e.g., a deletion in the putative immunomodulatory gene). While antigen presentation can be thought of as a single event, it can also be considered as a larger process comprising multiple steps including: antigen acquisition, antigen processing, peptide loading onto MHC molecules, transport to the surface, MHC binding to T cell receptor, interaction of costimulatory molecules, cell signaling, cytokine synthesis by both cells, and proliferation of antigen specific T lymphocytes. This system allows for the initial determination of whether there is a phenotype and then also allows the stepwise deconstruction of the system to analyze this process at several points to focus in on the mechanism of immunomodulation. We have used this model system to elucidate the function of a highly conserved but previously uncharacterized poxvirus gene that we showed was important for virulence in rodents. The experimental system developed should be broadly applicable to analyzing viral effects on immunity.

Experimental autoimmune encephalomyelitis in Lewis rats: IFN-beta acts as a tolerogenic adjuvant for induction of neuroantigen-dependent tolerance

Cytokine-Ag fusion proteins represent a novel approach for induction of Ag-specific tolerance and may constitute an efficient therapy for autoimmune disease. This study addressed whether a fusion protein containing rat IFN-beta and the encephalitogenic 73-87 determinant of myelin basic protein (i.e., the neuroantigen, or NAg) could prevent or treat experimental autoimmune encephalomyelitis (EAE) in Lewis rats. The optimal structure of the fusion protein was comprised of the rat IFN-beta cytokine as the N-terminal domain with an enterokinase (EK) linker to the NAg domain. Both cytokine and NAg domains had full biological activity. Subcutaneous administration of 1 nmol of IFNbeta-NAg fusion protein in saline on days -21, -14, and -7 before encephalitogenic challenge on day 0 resulted in a substantial attenuation of EAE. In contrast, administration of IFN-beta or NAg alone did not affect susceptibility to EAE. The covalent attachment of IFN-beta and NAg was not necessary, because separate injections of IFN-beta and NAg at adjacent sites were as effective as injection of IFNbeta-NAg for prevention of disease. When treatment was initiated after disease onset, the rank order of inhibitory activity was as follows: the IFNbeta-NAg fusion protein > or = a mixture of IFN-beta plus NAg > IFN-beta > NAg. The novel finding that IFN-beta acts as a tolerogenic adjuvant as well as a tolerogenic fusion partner may have significance for development of tolerogenic vaccines.

Different signaling pathways expressed by chicken naive CD4(+) T cells, CD4(+) lymphocytes activated with staphylococcal enterotoxin B, and those malignantly transformed by Marek's disease virus

Proteomics methods, based on liquid chromatography and tandem mass spectrometry, produce large "shotgun" proteomes that are most appropriately compared not at the level of differentially expressed proteins only but at the more comprehensive level of biological networks and pathways. This is now possible with the emergence of functional annotation databases and tools, databases of canonical pathways and molecular interactions and computational text mining tools. Here, we used shotgun proteomics, and the differential proteomics modeling functionalities available in the Pathwaystudio network modeling program to define the cell physiology of Hodgkin's disease antigen-overexpressing (CD30 (hi)) CD4 (+) T cell lymphomas using the unique Marek's disease (MD) natural animal model. CD30 (hi) lymphoma cells have characteristics of activated T cells but are also fundamentally different from their nontransformed healthy counterparts. We compared the cell physiology of naïve, superantigen-activated and MD-transformed CD4 (+) T cell proteomes. While the superantigen-activated cells had signaling pathways associated with cell activation, inflammation, proliferation and cell death, the MD-transformed cells had growth factor, cytokine, adhesion, and transcription factor signaling responses associated with oncogenicity, cell proliferation, angiogenesis, motility, and metastasis.

IL-2/neuroantigen fusion proteins as antigen-specific tolerogens in experimental autoimmune encephalomyelitis (EAE): correlation of T cell-mediated antigen presentation and tolerance induction

The purpose of this study was to assess whether the Ag-targeting activity of cytokine/neuroantigen (NAg) fusion proteins may be associated with mechanisms of tolerance induction. To assess this question, we expressed fusion proteins comprised of a N-terminal cytokine domain and a C-terminal NAg domain. The cytokine domain comprised either rat IL-2 or IL-4, and the NAg domain comprised the dominant encephalitogenic determinant of the guinea pig myelin basic protein. Subcutaneous administration of IL2NAg (IL-2/NAg fusion protein) into Lewis rats either before or after an encephalitogenic challenge resulted in an attenuated course of experimental autoimmune encephalomyelitis. In contrast, parallel treatment of rats with IL4NAg (IL-4/NAg fusion protein) or NAg lacked tolerogenic activity. In the presence of IL-2R(+) MHC class II(+) T cells, IL2NAg fusion proteins were at least 1,000 times more potent as an Ag than NAg alone. The tolerogenic activity of IL2NAg in vivo and the enhanced potency in vitro were both dependent upon covalent linkage of IL-2 and NAg. IL4NAg also exhibited enhanced antigenic potency. IL4NAg was approximately 100-fold more active than NAg alone in the presence of splenic APC. The enhanced potency of IL4NAg also required covalent linkage of cytokine and NAg and was blocked by soluble IL-4 or by a mAb specific for IL-4. Other control cytokine/NAg fusion proteins did not exhibit a similar enhancement of Ag potency compared with NAg alone. Thus, the IL2NAg and IL4NAg fusion proteins targeted NAg for enhanced presentation by particular subsets of APC. The activities of IL2NAg revealed a potential relationship between NAg targeting to activated T cells, T cell-mediated Ag presentation, and tolerance induction.

Reversal of airway hyperresponsiveness by induction of airway mucosal CD4+CD25+ regulatory T cells

An important feature of atopic asthma is the T cell–driven late phase reaction involving transient bronchoconstriction followed by development of airways hyperresponsiveness (AHR). Using a unique rat asthma model we recently showed that the onset and duration of the aeroallergen-induced airway mucosal T cell activation response in sensitized rats is determined by the kinetics of functional maturation of resident airway mucosal dendritic cells (AMDCs) mediated by cognate interactions with CD4⁺ T helper memory cells. The study below extends these investigations to chronic aeroallergen exposure. We demonstrate that prevention of ensuing cycles of T cell activation and resultant AHR during chronic exposure of sensitized rats to allergen aerosols is mediated by CD4⁺CD25⁺Foxp3⁺LAG3⁺ CTLA⁺CD45RC⁺ T cells which appear in the airway mucosa and regional lymph nodes within 24 h of initiation of exposure, and inhibit subsequent Th-mediated upregulation of AMDC functions. These cells exhibit potent regulatory T (T reg) cell activity in both in vivo and ex vivo assay systems. The maintenance of protective T reg activity is absolutely dependent on continuing allergen stimulation, as interruption of exposure leads to waning of T reg activity and reemergence of sensitivity to aeroallergen exposure manifesting as AMDC/T cell upregulation and resurgence of T helper 2 cytokine expression, airways eosinophilia, and AHR.

Activated mouse T-cells synthesize MHC class II, process, and present morbillivirus nucleocapsid protein to primed T-cells

A pivotal step in the initiation of T-cell immunity is the presentation of antigenic peptides by major histocompatibility complex (MHC) expressed on antigen presenting cells. The expression of MHC class II molecules by mouse T-cells has not been shown unequivocally. In the present work, we demonstrate that activated mouse T-cells synthesize MHC class II molecules de novo and express them on their surface. Further, we have demonstrated that in vitro activated T-cells take up extra-cellular soluble nucleocapsid protein of a morbillivirus. The internalized antigen goes to antigen processing compartment as shown by co-localization of antigen and LAMP-1 using confocal microscopy. We show that activated T-cells express H2M, a chaperone molecule known to have a role in antigen presentation. Further, we demonstrate that activated T-cells process and present internalized extra-cellular antigen to primed T-cells as shown by IL-2 secretion and in vitro proliferation. The presentation of antigen by T-cells may have implications in immuno-regulation, control of infection by lymphotropic viruses and maintenance of immunological memory.