Tolerogenic activity of polyethylene glycol-conjugated lysozyme distinct from that of the native counterpart

Conjugation of proteins with polyethylene glycol (PEG) has been reported to make the proteins tolerogenic. Native proteins are also potentially tolerogenic when given without adjuvants. We compared immunotolerogenic activities between PEG-conjugated and native hen egg-white lysozyme (HEL). BALB/c mice were given consecutive weekly intraperitoneal administrations of PEG-conjugated HEL, unmodified HEL or phosphate-buffered saline (PBS), for 3 weeks, then challenged with HEL in Freund's complete adjuvant. The pretreatment with PEG-HEL tolerized both T-cell and humoral responses, while native HEL tolerized only the T-cell response. Immunoglobulin G1 (IgG1) antibody was already elevated in HEL-pretreated mice prior to challenge immunization, followed by suppressed IgG2a and IgG2b, but spared IgG1 production after the antigen challenge, whereas PEG-HEL-pretreated mice produced no antibody in all IgG subclasses prior and subsequent to the antigen-challenge. Production of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) by lymphoid cells in response to HEL in vitro was markedly suppressed in both the antigen-pretreated groups, while suppression of IL-4 production was evident only in PEG-HEL-, not in HEL-pretreated animals. Involvement of suppressor cells in these tolerance states was found to be unlikely, and the immunological property of PEG-HEL differed from deaggregated HEL that was similar to the original HEL. These results suggest a unique immunotolerogenic activity of PEG-conjugated proteins to suppress both T-helper type-1 (Th1)- and Th2-type responses, the result being extensive inhibition of all IgG subclass responses, while tolerance induction by unconjugated soluble proteins tends to be targeted on Th1-, but spares Th2-type responses.