Ig knock-in mice producing anti-carbohydrate antibodies: breakthrough of B cells producing low affinity anti-self antibodies.

Natural Abs specific for the carbohydrate Ag Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) play an important role in providing protective host immunity to various pathogens; yet little is known about how production of these or other anti-carbohydrate natural Abs is regulated. In this study, we describe the generation of Ig knock-in mice carrying functionally rearranged H chain and L chain variable region genes isolated from a B cell hybridoma producing alphaGal-specific IgM Ab that make it possible to examine the development of B cells producing anti-carbohydrate natural Abs in the presence or absence of alphaGal as a self-Ag. Knock-in mice on a alphaGal-deficient background spontaneously developed alphaGal-specific IgM Abs of a sufficiently high titer to mediate rejection of alphaGal expressing cardiac transplants. In the spleen of these mice, B cells expressing alphaGal-specific IgM are located in the marginal zone. In knock-in mice that express alphaGal, B cells expressing the knocked in BCR undergo negative selection via receptor editing. Interestingly, production of low affinity alphaGal-specific Ab was observed in mice that express alphaGal that carry two copies of the knocked in H chain. We suggest that in these mice, receptor editing functioned to lower the affinity for self-Ag below a threshold that would result in overt pathology, while allowing development of low affinity anti-self Abs.