Study On The Mechanism Of Endocytic Precursor B Cell Receptor Controlling ? Light Chain To ? Light Chain Ratio

B cells are a population of lymphocytes that can produce antibodies and thus play a critical role in the humoral immune response.The development of B cells can be divided into different stages based on the B cell receptor(BCR)rearrangement.At the pro-B stage,the immunoglobulin(Ig)heavy(H)chain locus opens up and starts to rearrange.A successfully rearranged H chain combines with the surrogate light chain and forms the pre-BCR on the large pre-B cell surface.Signals emanating from the pre-BCR promote the recombination of the light(L)chains and the transition of pre-B cells into immature B cells.At this stage,immature B cells express recombined H and L chains and then migrate from the bone marrow into the peripheral lymphoid organs for further maturation.In the periphery,transitional B cells develop into mature B cells.In mice,L chains have two isotypes,the kappa(?)chain and the lambda(?)chain.B cells can express either ? or? L chains and the ratio of Ig L ?/? is constant.Alteration of the ratio is an indication of B-cell malignancies.Two proposed explanations for the constant ratio formation are the ordered rearrangement theory and the stochastic theory.However,the exact molecular mechanism that determines the ratio of two isotypes L chains remains unknown.Dynamin2 is a GTPase that controls the endocytosis/internalization of cell surface receptors.In this study,we generated Mb1Cre-mediated B cell-specific Dynamin 2conditional knockout mice and found that the B cell development was impaired in the mutant mice.Dynamin2-deficient mice lacked mature B cell and exhibited a marked increase in the ?/? ratio of immature B cells.Upon engagement,the endocytosis of the pre-BCR or BCR was reduced and signaling of the pre BCR and BCR was altered in the mutant B cells.The germline expression of the entire ? L chain locus was markedly decreased whereas the germline transcription of the ? L chain locus was selectively reduced in the Dynamin-2-deficient pre-B cells relative to the wild-type counterpart cells.In addition,the diversity of both ? and ? L chains was reduced in the mutant immature B cells.Furthermore,high-throughput RNA sequencing analyses revealed that the expression of the target genes of the methyltransferase EZH2 that methylates histone H3 on dimethyl lysine-27 to generate H3K27-trimethyl(H3K27me3),an epigenetic mark associated with gene repression,was decreased in the mutant relative to control wild-type B cells.Taken together,these data demonstrate that Dynamin-2-mediated internalization of the pre-BCR alters signals to regulate the germline expression of the ? and ? loci through the epigenetic modifier and thus control the ratio of ?/? light chains and the diversity of ? and ? L chains.