We recently reported that human VDJ genes associated with different IgG subclasses have significant differences in their levels of somatic point mutations, and the ranking of mean mutation numbers corresponds to the position of the C gamma constant region genes within the IGH gene locus. These data are not easily explained if class switching is the outcome of summed T cell commands. Rather, the data suggest a process of programmed, sequential switches between each of the IgG subclass-encoding IGHG genes. This would result in predictable differences in the relative antigen-binding affinities of the different IgG subclasses, and we recently described a Temporal Model of human antibody function [1] in which these relative affinities are critical to the proposed roles of the IgG subclasses.
We have now investigated the possibility that isotype switch pathways could also be intrinsic to murine B cell biology. We applied mutation analysis to explore the relationships between murine IgG subclasses in C57BL/6 and BALB/c mice, using public sequence databases and sequences generated using next generation 454 pyrosequencing. Highly significant differences were seen between isotypes. VDJ genes associated with the most 3’ BALB/c IGHG gene (encoding IgG2a) had the highest mutation level, but interestingly, VDJ genes associated with the most 3’ C57BL/6 IGHG gene (encoding IgG2c) had the lowest mutation level. IgG2a antibodies are therefore likely to have the highest relative affinity of the IgG subclasses in BALB/c mice, while IgG2c antibodies are likely to have the lowest affinity in C57BL/6 mice. These two isotypes are generally assumed to be functionally equivalent, but clearly this is not so. Overall, the data support the existence of intrinsic class switch pathways, but they are incompatible with a simple model of downstream sequential switching. Instead, we propose that two parallel switch pathways have evolved in the mouse through gene duplication.
1. Collins, A.M. and K.J. Jackson. Front Immunol, 2013. 4: p. 235.