We have previously observed that enforced expression, through transgenesis, of another uracil-excision enzyme (single-stranded monofunctional uracil DNA glycosylase [SMUG] 1) only partially rescues switch recombination in B cells ofung/msh2/mice (9). for specific recruitment of the uracil-excision enzyme or for it to be active on single-stranded DNA. Functional immunoglobulin genes are assembled by V-(D)-J joining, a process catalysed by the RAG1/RAG2 recombinase. The functionally rearranged Ig variable (IgV) regions can then be diversified by somatic hypermutation or gene conversion; the immunoglobulin constant (IgC) region can be exchanged by switch recombination (allowing the switch from IgM to IgG, IgA, or IgE). Somatic hypermutation, gene conversion, and switch recombination are all dependent on activation-induced deaminase (AID) (14). A variety of lines of evidence (for review see recommendations [5,6]) reveal that AID Fluticasone propionate acts by deaminating deoxycytidine residues to deoxyuridine (dU) at sites within the immunoglobulin loci, generating dU:deoxyguanosine (dG) lesions. Thus, genetic and biochemical assays show that Fluticasone propionate AID is able to deaminate cytidine in single-stranded DNA with a local sequence preference that accords with the localization of in vivo somatic mutation hotspots; antibody gene diversification pathways are Fluticasone propionate also altered in B cells that carry disruptions in the pathways that process uracil in DNA (723). Uracil excision by the uracil-DNA glycosylase (uracil-N-glycolase [UNG]) is usually envisioned to play a central role in AID-triggered immunoglobulin gene diversification (79,11,12,17,2123). The transversion mutations at C:G pairs observed during IgV somatic hypermutation are proposed to be a consequence of DNA synthesis over the UNG-generated abasic site in the IgV; switch recombination is usually proposed to be, in large part, achieved through UNG-mediated excision of the AID-generated Fluticasone propionate uracils in the vicinity of the immunoglobulin switch region. Although such a DNA deamination scheme for antibody gene diversification enjoys widespread Fluticasone propionate support, the ability of active-site mutants of UNG to restore switch recombination in in vitro assays was a major surprise, and it has brought on controversy about the importance of uracil excision in antibody gene diversification (2428). We revisit the ability of UNG mutants to partake in immunoglobulin gene diversification, and find that those UNG mutants that, on retroviral delivery, are capable of restoring switch recombination to UNG-deficient B cells still retain uracil-excision activity both in vivo and in vitro. The results suggest that the ability of UNG mutants that retain <1% of the specific uracil-excision of the wild-type enzyme to potentiate efficient switch recombination likely reflects, in part, the heavy overexpression associated with retroviral delivery, but is also an indication that uracil excision at only a small proportion of AID-generated uracils may suffice for switch recombination, a conclusion that is consistent with the proposals of other studies concerning switch-associated DNA breaks (2931). == RESULTS == == Restoration of class switching by retroviral delivery of mutant UNGs == We first sought to recapitulate the findings of Begum et al. (25) that switching could be effectively restored toung/B cells by retroviral delivery of UNG mutants carrying amino acid substitutions at their active sites. Constructs were generated encoding mutant versions of the nuclear isoform of mouse UNG with the choice of mutants guided both by the identity of the mutants studied by Begum et al. (25), as well as by various structural and kinetic analyses of human UNG, to which mouse UNG is usually 95% identical. We find that single amino-acid substitution mutants of residues that participate in catalyzing the cleavage of the glycosidic bond (D147N or D147G, H270L, and N206V [3236]) are all able to complement the switching deficiency of B cells fromung/mice in the Mouse monoclonal to CD35.CT11 reacts with CR1, the receptor for the complement component C3b /C4, composed of four different allotypes (160, 190, 220 and 150 kDa). CD35 antigen is expressed on erythrocytes, neutrophils, monocytes, B -lymphocytes and 10-15% of T -lymphocytes. CD35 is caTagorized as a regulator of complement avtivation. It binds complement components C3b and C4b, mediating phagocytosis by granulocytes and monocytes. Application: Removal and reduction of excessive amounts of complement fixing immune complexes in SLE and other auto-immune disorder retroviral reconstitution assay, whereas no evident restoration is usually obtained with the double UNG mutants (D147N,H270L) and (D147N,N206V) (Fig. 1). This is entirely in keeping with (and extends upon) a previous work (25). Interestingly, combining D147N with mutation of residue L274 still allows efficient restoration of switching. Residue L274 does not partake in the actual hydrolytic catalysis, but instead is usually inserted into the DNA helix upon uracil flipping, with mutation of this residue significantly diminishing UNG activity by reducing the stability of the DNAUNG complex (32,36). == Physique 1. == Class switching inung/B cells after retroviral delivery of mutant UNGs.(A) Experimental strategy with schematic representation of the retroviral vector used for UNG delivery. (B) Representative flow cytometric plots of switching to IgG1 byung/B cells after retroviral delivery of mutant mouse.