The fusion protein, but not rS450–650 or rCRT/39–27, successfully induced S450–650-specific IgG production in nude mice (Fig. 4). The potent adjuvanticity of rCRT/39–272 can be partially explained by its direct activating effect on B lymphocytes (12). However, there are other possible ways for it to enhance target Ag-specific humoral lresponses in vivo. After all, adjuvants are typically characterized by their ability to activate professional APCs, Erlotinib such as DCs and macrophages, rather than B or T cells. Bone marrow-derived mouse DCs were stimulated
with rCRT/39–272, or rS450–650-CRT, or LPS, or rEGFP for 24 hrs and then analyzed by flow cytometry for CD40 expression, which is regarded as a marker for DC maturation (17). As illustrated in Figure 5, the percentage of CD40+ cells of the groups treated with rCRT/39–272 (24.5%) or rS450–650-CRT (18.6%) was considerably higher than that of the rEGFP control group (6.8%), thus confirming rCRT/39–272 and rS450–650-CRT as potent activators of murine DCs. This is further supported by the fact that rS450–650-CRT as well as rCRT/39–272, but not rEGFP, were able to induce production of IL-12 and IL-1β by DCs in vitro (data not shown). The ability of rCRT/39–272 and rS450–650-CRT
to activate DC is not due to endotoxin contamination because the recombinant proteins used in this study PS-341 in vitro were passed through polymyxin B agarose to remove endogenous LPS that could have come from the E. coli system. Newly emerging pathogens such as SRAS-CoV and avian influenza viruses are of major concern for public health today. The development of more effective vaccines (and adjuvants) against such infectious agents is urgently needed. Our results reported herein show that fusion protein rS450–650-CRT exhibits much more potent immunogenicity than rS450–650 alone in terms of
eliciting rS450–650-specific IgG responses in vivo. It should be noted, however, that whether such Abs exhibit any neutralizing effect against SARS-CoV infection remains to be tested by using either live virus or pseudo-virus systems. Physical linkage between rS450–650 and CRT/39–272 is necessary for the improved immunogenicity, because a mixture of rS450–650 and of rCRT/39–272 was no more immunogenic than rS450–650 alone (Fig. 2). Another advantage of rS450–650-CRT over rS450–650 as an immunogen is its better hydrophilicity. When preparing rS450–650, renaturation steps were necessary after Ni-column purification and the resultant product had to be maintained at a relatively low concentration in order to avoid protein aggregation and precipitation. By contrast, no renaturation steps are necessary for preparation of rS450–650-CRT and the final product is less likely to form aggregates in PBS.