Peripheral tolerance of antigen-specific Th cells induced with polyethylene glycol-conjugate of protein antigen

It has long been known that protein antigen conjugated with polyethylene glycol (PEG), a nonimmunogenic artificial polymer, induces immune tolerance of antigen-specific Th cells. However, the mechanism of this tolerance induction remains unknown. In this study, the response and differentiation of ovalbumin (OVA)-specific CD4 Th cells upon exposure to tolerogenic PEG conjugate of OVA (PEG-OVA) were studied. Naïve OVA-specific Th cells from OT-II mice were labeled with carboxyfluorescein succinimidyl ester (CFSE), transferred into histocompatible C57BL/6 mice, and then subsequently stimulated with either tolerogenic PEG-OVA or with OVA. Upon stimulation with tolerogenic PEGOVA in vivo, these cells showed a robust proliferative response comparable to that observed by stimulation with OVA. Nevertheless, upon prolonged exposure to PEG-OVA, OVA-specific Th cells became anergic, showing a markedly reduced capacity to respond, and to produce IL-2 and other cytokines when stimulated with antigenic OVA323-339 peptide in vitro. There was also a significant reduction of the frequency of clonotypic TCR Vα2CD4 T cells in the spleens of OT-II mice treated with PEG-OVA. These features of response of naïve OVA-specific Th cells upon sustained exposure to PEG-OVA were quite analogous to those reported for the same cells transferred into mice with systemic expression of the transgenic OVA gene. The highly enhanced stability in the circulation that was observed for PEG-OVA was likely the basis of its tolerogenic capacity.