The circular dichroism of phosphocholine-specific mouse hybridoma and myeloma proteins: unusual properties of the hybridoma protein 101.6G6

Non-random conformation of a mouse IgG2a monoclonal antibody at low pH

The pH dependence of the conformation of a mouse IgG2a, kappa monoclonal antibody (MN12) was investigated by several physical techniques, including fluorescence spectroscopy, near-ultraviolet and far-ultraviolet CD, and electric-field-induced transient birefringence measurements. The intensity of the intrinsic tryptophan fluorescence remained constant in the pH range from 3.5 to 10.0. A conformational alteration in the MN12 molecule was observed in the pH region between pH 3.5 and 2.5, as reflected by a substantial enhancement of the fluorescence quantum yield. This effect was more pronounced at high ionic strengths. The fluorescence emission was unaltered, indicating that the acid-induced conformational state is different from a completely unfolded state. This was confirmed by CD and fluorescence polarisation measurements. Iodide and acrylamide fluorescence quenching studies indicated a gradually increasing accessibility of MN12 tryptophan residues with decreasing pH. At low pH precipitation was observed in the presence of iodide. One rotational relaxation time (0.16-0.18 microseconds) was observed for MN12 by electric-field-induced transient birefringence measurements at low ionic strength. After exposure of MN12 to low pH for 1 h, the relaxation time was increased to 0.23 microseconds; a further increase to 0.30 microseconds was observed after 24 h. The combined results suggest an acid-induced expansion and enhanced flexibility of MN12, which eventually leads to irreversible aggregation.

The role of the VL- and VH-segments in the preferential reassociation of immunoglobulin subunits

Competitive reassociation experiments, in which equimolar amounts of two different L-chains were allowed to compete for a limiting amount of H-chain, were performed to assess the role of the V kappa- and J kappa-segments on the ability of an L-chain to compete. Using H- and L-chains from the murine anti-phosphorylcholine (PC) myelomas, TEPC15, MOPC167 and MCPC603, and a series of V kappa 21 L-chains, it was found that the V kappa 21 L-chains competed uniformly better than the anti-PC L-chains, when the anti-PC H-chains were used, despite any differences in the J-segments of the competing L-chains. In addition, when the anti-PC L-chains, which all employ identical J kappa-segments but very diverse V kappa-segments, were in competition against each other, a hierarchy of competitive ability existed which was independent of whether the chains were autologous or heterologous and independent of antigen binding activity. Competitive reassociation experiments between the V kappa 21 and anti-PC L-chains were also performed using the heterologous anti-lysozyme monoclonal HyHEL-10 H-chain or the anti-galactan J539 H-chain, and it was found that the relative competitive ability of the V kappa 21 L-chains with respect to the anti-PC L-chains was dependent on which H-chain was employed. The results suggested that the main factor favouring preferential reassociation by any particular L-chain was the V kappa-segment and that the effects of the J kappa-segment could not be observed where a high degree of diversity in the V-segments existed. Furthermore, while the results implied that specific pairs of VH- and VL-domains had a higher affinity for each other, this was not a necessary criterion in the formation of autologous pairs of H- and L-chains as demonstrated by the preferential heterologous reassociation of the V kappa 21 L-chains over the autologous anti-PC L-chains. These results were consistent with the independent, random rearrangement of immunoglobulin H- and L-chain V-domain gene segments and predict that the hypothetical repertoire of antibodies is not limited by the selection of specific pairs of high-affinity VH-VL domains.