Cytokines, IgG Subclasses and Costimulation in a Mouse Model of Thyroid Autoimmunity Induced by Injection of Fibroblasts Co-expressing MHC Class II and Thyroid Autoantigens

AKR/N mice injected with fibroblasts expressing MHC class II (RT4.15HP cells) and the TSH receptor (TSHR) develop antibodies similar to those in Graves' disease. We were unable to analyse the subclass of these antibodies because of unexpectedly high non-specific binding by ELISA or flow cytometry. The non-specific binding reflected generalized immune activation which occurred even when the fibroblasts did not express the TSHR. However, the IgG subclasses were determined for thyroid peroxidase (TPO) antibodies induced using TPO-expressing RT4.14HP cells and found to be IgG2a > IgG1. This Thl pattern is consistent with spontaneous secretion of interferon-gamma (but not IL-4 or IL-10) by splenocytes from injected mice. The Th1 bias was related to fibroblast injection because conventional immunization of the same mouse strain with purified TPO and adjuvant induced a Th2 response (IgG1 >> IgG2a). Further, untransfected fibroblasts themselves induced powerful, non-specific proliferative responses when used as antigen-presenting cells (APC) in vitro. Flow cytometry revealed that the RT4.15HP fibroblasts (and TSHR- and TPO-transfected derivatives) expressed B7-1. Unexpected constitutive expression of this key molecule may bypass the requirement for up-regulation of other costimulatory molecules involved in T cell stimulation. Our data support the concept that RT4.15HP fibroblasts present the TSHR (or TPO), at least for initiating the immune response. However, the accompanying generalized immune stimulation creates difficulties for analysis of TSHR-specific T and B lymphocytes. On the other hand, extension of the model to TPO, an easier antigen to study, will facilitate analysis of murine T cell responses likely to resemble those in human thyroid autoimmunity.