Variable Region Identical Immunoglobulins Differing in Isotype Express Different Paratopes

The finding that the antibody (Ab) constant (C) region can influence fine specificity suggests that isotype switching contributes to the generation of Ab diversity and idiotype restriction. Despite the centrality of this observation for diverse immunological effects such as vaccine responses, isotype-restricted antibody responses, and the origin of primary and secondary responses, the molecular mechanism(s) responsible for this phenomenon are not understood. In this study, we have taken a novel approach to the problem by probing the paratope with 15N label peptide mimetics followed by NMR spectroscopy and fluorescence emission spectroscopy. Specifically, we have explored the hypothesis that the C region imposes conformational constraints on the variable (V) region to affect paratope structure in a V region identical IgG1, IgG2a, IgG2b, and IgG3 mAbs. The results reveal isotype-related differences in fluorescence emission spectroscopy and temperature-related differences in binding and cleavage of a peptide mimetic. We conclude that the C region can modify the V region structure to alter the Ab paratope, thus providing an explanation for how isotype can affect Ab specificity.

The Influence of Cross-Linking and Plasminogen on Kinetics of Fibrinolysis

The influence of cross-linking upon the kinetics of fibrinolysis has been studied in an in vitro system by monitoring the release of radioactive label, peptide materials, fragment X and D-dimer. Cross-linked (XL) and non-cross-linked (NXL) fibrin clots were prepared by clotting citrated plasma containing 125-I labelled fibrinogen with human thrombin, in the presence respectively of 40 mM Ca C12 or 2 mM EDTA, and incubating for 2 hours at 37°C. After washing, clots were placed in buffer containing human plasminogen (glu- or lys-) in concentrations ranging from 0 to 10 caseinolytic units per ml. Clots were transferred after 30 minutes to a solution of streptokinase (1000 units per ml) and the supernatants subsampled serially for 3 hours. NXL fibrin lysed progressively and sometimes completely. Maximal lysis rates were achieved with intermediate concentrations of plasminogen. XL fibrin lysed slowly and significant lysis was obtained only after exposure to the higher concentrations of lys-plasminogen. XL-fibrin lysed more readily after exposure to glu-plasminogen than to lys-plasminogen. Analysis of the chains in XL fibrin clots by SDS-polyacrylamide electrophoresis revealed the presence of a small residuum of NXL fibrin and incomplete cross-linkage of α chains. The NXL component lysed preferentially in streptokinase and the terminal clots contained only XL fibrin. Parallel effects may operate in vivo. The data support the contention that fibrinolytic mechanisms readily deal with NXL fibrin, and that only fibrin which has been XL occurs in thrombi and is of pathologic significance.