title: Bystander activation of cytotoxic T cells: studies on the mechanism and evaluation of in vivo significance in a transgenic mouse model.
abstract: Bystander activation, i.e., activation of T cells specific for an antigen X during an immune response against antigen Y may occur during viral infections. However, the low frequency of bystander-activated T cells has rendered it difficult to define the mechanisms and possible in vivo relevance of this nonspecific activation. This study uses transgenic mice expressing a major histocompatibility complex class I-restricted TCR specific for glycoprotein peptide 33-41 of lymphocytic choriomeningitis virus (LCMV) to overcome this limitation. CD8+ T cells from specific pathogen-free maintained, unimmunized "naive" TCR transgenic mice can differentiate into LCMV-specific cytolytic effector CTL during infections with vaccinia virus or Listeria monocytogenes in vivo or mixed lymphocyte culture in vitro. We show that in these model situations (a) nonspecifically activated CTL are able to confer antiviral protection in vivo, (b) bystander activation is largely independent of the expression of a second T cell receptor of different specificity, (c) bystander activation is not mediated by a broadly cross-reactive TCR, but rather by cytokines, (d) bystander activation can be mediated by cytokines such as IL-2, but not alpha/beta-IFN in vitro; (e) bystander activation is, overall, a rare event, occuring in vivo in roughly 1 in 200 of the LCMV-specific CTL during infection of TCR transgenic mice with vaccinia virus; (f) bystander activation does not have a significant functional impact on nontransgenic CTL memory under the conditions tested; and (g) even in the TCR transgenic situation, where unphysiologically high numbers of T cells of a single specificity are present, bystander activation is not sufficient to cause clinically manifest autoimmune disease in a transgenic mouse model of diabetes. We conclude that although bystander activation via cytokines may generate cytolytically active CTL from naive precursors, quantitative considerations suggest that this is usually not of major biological consequence.
authors.affiliation: Institute of Experimental Immunology, Department of Pathology, University of Zurich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Ehl
authors.forename: J
authors.initials: J
authors.lastname: Hombach
authors.forename: P
authors.initials: P
authors.lastname: Aichele
authors.forename: H
authors.initials: H
authors.lastname: Hengartner
authors.forename: R M
authors.initials: RM
authors.lastname: Zinkernagel
mesh_terms: D000818:Animals; D000956:Antigens, Viral; D001324:Autoantigens / Q000276:immunology; D002454:Cell Differentiation; D018920:Coculture Techniques; D003429:Cross Reactions; D016207:Cytokines / Q000276:immunology; D003602:Cytotoxicity, Immunologic; D003921:Diabetes Mellitus, Experimental / Q000209:etiology / Q000276:immunology; D006023:Glycoproteins / Q000276:immunology*; D007156:Immunologic Memory; D008088:Listeriosis / Q000276:immunology; D008213:Lymphocyte Activation; D008217:Lymphocytic choriomeningitis virus / Q000276:immunology*; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D010446:Peptide Fragments / Q000276:immunology*; D011948:Receptors, Antigen, T-Cell / Q000235:genetics / Q000276:immunology; D013602:T-Lymphocytes, Cytotoxic / Q000276:immunology*; D014615:Vaccinia / Q000276:immunology; D014764:Viral Proteins
chemical_list: D000956:Antigens, Viral; D001324:Autoantigens; D016207:Cytokines; D006023:Glycoproteins; D010446:Peptide Fragments; D011948:Receptors, Antigen, T-Cell; D014764:Viral Proteins; C094244:glycoprotein peptide 33-41, Lymphocytic choriomeningitis virus