The role of CD40-CD40 ligand (CD154) interactions in immunoglobulin light chain repertoire generation and somatic mutation

To determine whether CD40 ligation influences the molecular and selective mechanisms that govern the development of the human Ig light chain repertoire, analysis of the Vκ and Vλ repertoires of CD19⁺ B cells obtained from a patient with X-linked hyper IgM syndrome (XHIM) and a nonfunctional CD154 was carried out. The nonproductive Vκ and Vλ repertoires were largely comparable to that of the normals with respect to V gene and J segment distribution as well as CDR3 length and V_LJ_L joint complexity. Comparison of the nonproductive and productive repertoires indicated that a limited number of V_L genes were positively and negatively selected in the XHIM patient. Although mutations were observed in the XHIM V_L repertoires, the frequency of mutations was significantly lower than in normals. Typical targeting of these mutations into RGYW/WRCY motifs was significantly reduced and subsequent selection of RGYW/WRCY mutations, which is normally observed, was not found. These results indicate that CD40 ligation is not required for generation of the light chain repertoire, positive selection of some Vk rearrangements, negative selection of specific V_L genes, and some degree of somatic mutation. Importantly, however, targeting of mutations to RGYW/WRCY motifs and subsequent selection of these mutated motifs does not occur in the absence of CD40 ligation.