Human cytotoxic T-lymphocyte antigen 4 (hCTLA4), an immunosuppressive agent, was expressed as a fusion protein to produce a multimeric form. The fusion partner was Hepatitis B surface antigen (HBsAg), which was fused to the N-terminal extracellular domain of hCTLA4. The expression vector, pPIC3.5hc, constructed for the insertion of the fusion gene into the pPIC3.5 vector with the AOX promoter, was introduced into Pichia pastoris GS115. The expressed fusion protein, HBsAg-hCTLA4, had a molecular size of similar to 39 kDa, which was detected using anti-hCTLA4 and anti-HBsAg antibodies by Western blot analysis. Transmission electron microscopic analysis of the expressed fusion protein revealed two kinds of particles including 40 nm fused particles and 22 nm HBsAg particles. The 40 nm particles were assumed to be particles fused with hCTLA4 displayed as a multimeric form on the outer surface of the HBsAg particles. The HBsAg in the HBsAg-hCTLA4 particles were shown to have a low antigenicity in reversed passive hemagglutination assay (RPHA). Accordingly, the HBsAg was proven to be an effective fusion partner. In addition, hCTLA4 fused with HBsAg could possibly be displayed as a multimeric form. (c) 2006 Elsevier Inc. All rights reserved.