Klhl6 Deficiency Impairs Transitional B Cell Survival and Differentiation.

Klhl6 belongs to the KLHL gene family, which is composed of an N-terminal BTB-POZ domain and four to six Kelch motifs in tandem. Several of these proteins function as adaptors of the Cullin3 E3 ubiquitin ligase complex. In this article, we report that Klhl6 deficiency induces, as previously described, a 2-fold reduction in mature B cells. However, we find that this deficit is centered on the inability of transitional type 1 B cells to survive and to progress toward the transitional type 2 B cell stage, whereas cells that have passed this step generate normal germinal centers (GCs) upon a T-dependent immune challenge. Klhl6-deficient type 1 B cells showed a 2-fold overexpression of genes linked with cell proliferation, including most targets of the anaphase-promoting complex/cyclosome complex, a set of genes whose expression is precisely downmodulated upon culture of splenic transitional B cells in the presence of BAFF. These results thus suggest a delay in the differentiation process of Klhl6-deficient B cells between the immature and transitional stage. We further show, in the BL2 Burkitt's lymphoma cell line, that KLHL6 interacts with Cullin3, but also that it binds to HBXIP/Lamtor5, a protein involved in cell-cycle regulation and cytokinesis. Finally, we report that KLHL6, which is recurrently mutated in B cell lymphomas, is an off-target of the normal somatic hypermutation process taking place in GC B cells in both mice and humans, thus leaving open whether, despite the lack of impact of Klhl6 deficiency on GC B cell expansion, mutants could contribute to the oncogenic process.

Specific over-expression of deltex and a new Kelch-like protein in human germinal center B cells.

Ig gene hypermutation was originally described as the molecular process underlying B cell affinity maturation following a T-dependent immune response. Somatic hypermutation is also used in some species such as sheep, to generate diversity during formation of the primary antibody repertoire. In sheep, B cells mutate their Ig receptor during antigen-independent development in the lymphoid follicles of ileal Peyer's patches, but this process is arrested when these same B cells are cultured in vitro. We have used these differences between in vivo and in vitro B cell development to perform a cDNA subtraction between these two cell populations, in order to search for genes that might be involved in the hypermutation process. We describe in this paper the characterization of two genes, highly expressed in sheep ileal Peyer's patch B cells and also in centroblasts of human tonsils: deltex (Drosophila) homolog 1 (DTX1), which is related to the Notch pathway and a new Kelch-like protein, KLHL6. The putative role of these proteins, which are more likely involved in the germinal center B cell differentiation pathway than in the hypermutation mechanism per se, is discussed.

Cutting edge: DNA polymerases mu and lambda are dispensable for Ig gene hypermutation.

Mutations arising in Ig V genes during an immune response are most likely introduced by one or several error-prone DNA polymerases. Many of the recently described nonreplicative DNA polymerases have an intrinsic fidelity compatible with such an activity, the strongest candidates being polymerase (pol) eta, pol iota, pol zeta, and pol mu. We report in this work that mice inactivated for either of the two polymerases related to pol beta (i.e., pol mu and pol lambda) are viable and fertile and display a normal hypermutation pattern.