Class switch recombination (CSR) in B cells involves deletion‐recombination at switch (S) region DNA and is important for the diversification of antibody isotypes during an immune response. Here, we identify two NME [NM23/NDPK (nucleoside diphosphate kinase)] isoforms, NME1 and NME2, as novel players in this process. Knockdown of NME2 leads to decreased CSR, while knockdown of the highly homologous NME1 results in increased CSR. Interestingly, these NME proteins also display differential occupancy at S regions during CSR despite their homology; NME1 binds to S regions prior to stimulation, while NME2 binds to S regions only after stimulation. To the best of our knowledge, this represents the first report of a role for these proteins in the regulation of CSR.

Adaptive immune responses require the generation of a diverse repertoire of immunoglobulins (Igs) that can recognize and neutralize a seemingly infinite number of antigens. V(D)J recombination creates the primary Ig repertoire, which subsequently is modified by somatic hypermutation (SHM) and class switch recombination (CSR). SHM promotes Ig affinity maturation whereas CSR alters the effector function of the Ig. Both SHM and CSR require activation-induced cytidine deaminase (AID) to produce dU:dG mismatches in the Ig locus that are transformed into untemplated mutations in variable coding segments during SHM or DNA double-strand breaks (DSBs) in switch regions during CSR. Within the Ig locus, DNA repair pathways are diverted from their canonical role in maintaining genomic integrity to permit AID-directed mutation and deletion of gene coding segments. Recently identified proteins, genes, and regulatory networks have provided new insights into the temporally and spatially coordinated molecular interactions that control the formation and repair of DSBs within the Ig locus. Unravelling the genetic program that allows B cells to selectively alter the Ig coding regions while protecting non-Ig genes from DNA damage advances our understanding of the molecular processes that maintain genomic integrity as well as humoral immunity.

BackgroundAcute kidney injury (AKI) is a risk factor for progression to chronic kidney disease, with even subclinical AKI episodes progressing to chronic kidney disease. Several risk factors such as preexisting kidney disease, hyperglycemia, and hypertension may aggravate renal disease after AKI. However, mechanisms underlying the progression of AKI are still unclear. This study identified the effect of human cluster of differentiation 36 (CD36) overexpression on the progression of folic acid-induced AKI.MethodsPax8–rtTA/tetracycline response element–human CD36 transgenic mice were used to elucidate the effect of human CD36 overexpression in the proximal tubules on folic acid-induced AKI.ResultsResults of histological analysis showed severely dilated tubules with casts and albuminuria in folic acid-treated transgenic mice overexpressing human CD36 compared with folic acid-treated wild-type mice. In addition, analysis of mRNA expression showed a significant increase in the collagen 3a1 gene in folic acid-treated transgenic mice overexpressing human CD 36 compared with folic acid-treated wild type mice.ConclusionHuman CD36-overexpressing transgenic mice showed severe pathological changes and albuminuria compared with wild-type mice. Moreover, mRNA expression of the collagen 3a1 gene increased in folic acid-treated transgenic mice. These results suggest that human CD36 overexpression is a risk factor of AKI and its progression to chronic kidney disease.

Activation-induced cytidine deaminase (AID) generates U:G mismatches in immunoglobulin genes that can be converted into untemplated mutations during somatic hypermutation (SHM) or DNA double-strand breaks (DSB) during class switch recombination (CSR). Null mutations in UNG and MSH2 demonstrate the complementary roles of the base excision repair (BER) and mismatch repair (MMR) pathways, respectively, in CSR. Phosphorylation of AID at serine-38 (pS38-AID) was previously hypothesized to regulate BER during CSR, as the AID phosphorylation mutant, AID(S38A), cannot interact with APE1, a BER protein. Consistent with these findings, we observe a complete block in CSR in AID S38A/S38A MSH2 −/− mouse B cells that correlates with an impaired mutation frequency at 5'Sμ. Similarly, SHM is almost negligible at the JH4 intron in AID S38A/S38A MSH2 −/− mouse B cells and, consistent with this, NP-specific affinity maturation in AID S38A/S38A MSH2 −/− mice is not significantly elevated in response to NP-CGG immunization. Surprisingly, AID S38A/S38A UNG −/− mouse B cells also cannot complete CSR or affinity maturation despite accumulating significant mutations in 5'Sμ as well as the JH4 intron. These data identify a novel role for pS38-AID in MMR-dependent CSR and affinity maturation.