Reilly B D, Levine R P, Skanes V M
Faculty of Medicine, Memorial University of Newfoundland, St. John's, Canada.
J Immunol. 1991 Nov 1;147(9):3018-23.
The C4A and C4B isotypes of human C4 show certain functional differences that stem from their relative preference for transacylation to amino (-NH2) vs hydroxyl (-OH) nucleophiles, respectively, on complement-activating surfaces. Comparison of amino acid sequences of the alpha-chain fragment of C4, C4d, has shown C4A- and C4B-specific sequences at residues 1101-1106 are the only consistent structural difference between isotype, i.e., Pro, Cys, Pro, Val, Leu, Asp in C4A and Leu, Ser, Pro, Val Ile, His in C4B. These residues may be responsible either in part or entirely for properties associated with isotype. To examine the functional role of residues 1101-1106 in C4B-mediated hemolysis, whole serum or immunopurified human C4 with allotypes, A3B1, A3, B2B1, or B1 were preincubated in the presence or absence of an antipeptide mAb (BII-1) specific for amino acid residues 1101-1105 of C4B. Sensitized sheep E and C4-deficient guinea pig serum was then added and lysis measured by absorbance at 415 nm. Our results show lysis of antibody-sensitized sheep E is inhibited by antibody and C4B2B1, C4B1, or C4A3B1 but not antibody and C4A3. The interference of hemolysis by BII-1 could not be explained by inhibition of activation of C4B or inhibition of C3 or C5 convertase activity. Furthermore, results from uptake experiments show that BII-1 interferes with the covalent binding activity of C4B, indicating residues 1101-1105 play a role in the covalent binding reaction of C4B to the target E-antibody complex.
人类C4的C4A和C4B同种型表现出某些功能差异，这些差异分别源于它们在补体激活表面上对氨基（-NH2）与羟基（-OH）亲核试剂进行转酰化反应的相对偏好。对C4的α链片段C4d的氨基酸序列进行比较后发现，C4A和C4B在1101 - 1106位残基处的特异性序列是同种型之间唯一一致的结构差异，即C4A中的脯氨酸、半胱氨酸、脯氨酸、缬氨酸、亮氨酸、天冬氨酸，以及C4B中的亮氨酸、丝氨酸、脯氨酸、缬氨酸、异亮氨酸、组氨酸。这些残基可能部分或完全决定了与同种型相关的特性。为了研究C4B介导溶血过程中1101 - 1106位残基的功能作用，将含有A3B1、A3、B2B1或B1等异型的全血清或免疫纯化的人C4在存在或不存在针对C4B 1101 - 1105位氨基酸残基的抗肽单克隆抗体（BII - 1）的情况下进行预孵育。然后加入致敏绵羊红细胞和C4缺陷型豚鼠血清，并通过在415nm处的吸光度测量溶血情况。我们的结果表明，抗体和C4B2B1、C4B1或C4A3B1可抑制抗体致敏绵羊红细胞的溶血，而抗体和C4A3则不能。BII - 1对溶血的干扰无法通过抑制C4B的激活或抑制C3或C5转化酶活性来解释。此外，摄取实验结果表明，BII - 1干扰了C4B的共价结合活性，表明110l - 1105位残基在C4B与靶标E - 抗体复合物的共价结合反应中起作用。
