Development of the passive immunization for prevention of dental caries using B cell epitopes of PAc.

变形链球菌（Streptococcus mutans）的菌毛样表面蛋白（fimbria like surface protein，PAc）是链球菌的强表面抗原之一，其分子量为190 kDa，并且参与该微生物与牙齿表面的初始附着。因此，该蛋白抗原被认为是抗龋齿疫苗的良好候选物。实际上，它已成功地用作动物实验的疫苗。也有报道用抗PAc的单克隆抗体进行被动免疫可以预防猴子的龋齿。我们利用DNA工程技术构建了7个截短的PAc片段，并提示富含丙氨酸的重复区（A区）从PAc分子的氨基酸残基219延伸至464的氨基酸残基在其表面蛋白与人唾液组分的结合中是重要的，因此，我们合成了24个连续重叠的19个残基的肽段，覆盖了整个A区，并利用一系列B10同源小鼠确定了肽段上的T和B细胞表位。迄今为止，已鉴定出至少3种T细胞表位和3种B细胞表位。最后，为了获得被动免疫用的抗原肽，对16种含有T和/或B细胞表位的人工合成肽进行了免疫试验。其中5种蛋白，特别是PAc（361-379）和PAc（391-409），不仅能诱导产生抗自身肽的抗体，而且能诱导产生抗天然PAc分子的抗体。这两种抗原肽可能用于被动免疫预防龋齿。

A fimbria like surface protein (PAc) of Streptococcus mutans with a molecular mass of 190 kDa is one of the strong surface antigens of the Streptococcus and is involved in the initial attachment of this microorganism to the tooth surface. Thus, the protein antigen has been considered to be a good candidate for an anti-caries vaccine. Actually, it was successfully used as vaccine in the animal experiments. Passive immunization with monoclonal antibodies against the PAc was also reported to prevent dental caries in monkeys.In the present project, we constructed 7 truncated PAc fragments by using a DNA engineering and suggested that the alanine-rich repeating region (A-region) extended from the amino acid residues 219 to 464 of the PAc molecule is important in the binding of its surface protein to human salivary components and is possessed of the dominant antigenicity.We, therefore, synthesized 24 sequential overlapping 19 residues-peptides covering the entirety of A-region and determined T and B cell epitopes on the peptides by using a series of B10 congenic mice. Until now, at least 3 kinds of T cell epitopes and 3 kinds of B cell epitopes were identified. The results clearly indicate that murien immune responses to the PAc are restricted by the MHC class II gene haplotypes.Finally, to obtain the antigenic peptides for the passive immunization, the immunities of 16 synthetic peptides which contain the T and/or B cell epitopes were examined by immunization to the mice. The 5 of them, especially PAc(361-379) and PAc(391-409), can indue the production of antibodies against not only own peptide but also the native PAc molecule. These two antigenic peptides might be useful in the passive immunization for prevention of dental caries.