Developmentally programmed expression of AID in chicken B cells

A B-cell targeting virus disrupts potentially protective genomic methylation patterns in lymphoid tissue by increasing global 5-hydroxymethylcytosine levels

The mechanisms by which viruses modulate the immune system include changes in host genomic methylation. 5-hydroxymethylcytosine (5hmC) is the catalytic product of the Tet (Ten-11 translocation) family of enzymes and may serve as an intermediate of DNA demethylation. Recent reports suggest that 5hmC may confer consequences on cellular events including the pathogenesis of disease; in order to explore this possibility further we investigated both 5-methylcytosine (5mC) and 5hmC levels in healthy and diseased chicken bursas of Fabricius. We discovered that embryonic B-cells have high 5mC content while 5hmC decreases during bursa development. We propose that a high 5mC level protects from the mutagenic activity of the B-cell antibody diversifying enzyme activation induced deaminase (AID). In support of this view, AID mRNA increases significantly within the developing bursa from embryonic to post hatch stages while mRNAs that encode Tet family members 1 and 2 reduce over the same period. Moreover, our data revealed that infectious bursal disease virus (IBDV) disrupts this genomic methylation pattern causing a global increase in 5hmC levels in a mechanism that may involve increased Tet 1 and 2 mRNAs. To our knowledge this is the first time that a viral infection has been observed to cause global increases in genomic 5hmC within infected host tissues, underlining a mechanism that may involve the induction of B-cell genomic instability and cell death to facilitate viral egress.

Ligation of surface Ig by gut-derived antigen positively selects chicken bursal and peripheral B cells

In many mammals and birds, B cell lymphopoiesis takes place in GALT, such as the avian bursa of Fabricius. Although BCR expression is sufficient for bursal colonization, the role of BCR ligation in the later stages of bursal B cell lymphopoiesis remains elusive. To address this directly, we introduced a surface Ig-related construct with defined Ag specificity containing the Ag-binding portion of a lamprey variable lymphocyte receptor specific for PE fused to a truncated chicken μ-chain (VLR(PE)Tμ) into developing chick embryos. VLR(PE)Tμ expression supports bursal follicle colonization, clonal expansion, and Ig V gene diversification. VLR(PE)Tμ-expressing B cells migrate to the periphery in the absence of the Ag starting from day 18 of embryogenesis. VLR(PE)Tμ-expressing B cells declined rapidly in the bursa and periphery in the absence of Ag after hatch; however, intrabursal injection of PE prolonged survival of VLR(PE)Tμ(+) bursal and peripheral B cells. Intrabursal introduction of Ag increased emigration of short-lived LT2(+) B cells. Peripheral VLR(PE)Tμ(+) B cells were maintained following intrabursal PE application and contained both short-lived LT2(+) and long-lived LT2(-) B cells. In the chicken bursa, the later stages of B cell development occur in the presence of gut-derived Ag; therefore, we conclude that Ag-mediated ligation of BCR in bursal B cells acts to positively select bursal B cells into both short-lived and long-lived peripheral B cell populations.

Establishment of an in vitro system representing the chicken gut-associated lymphoid tissue

The bursa of Fabricius is critical for B cell development and differentiation in chick embryos. This study describes the production in vitro, from dissociated cell suspensions, of cellular agglomerates with functional similarities to the chicken bursa. Co-cultivation of epithelial and lymphoid cells obtained from embryos at the appropriate developmental stage regularly led to agglomerate formation within 48 hours. These agglomerates resembled bursal tissue in having lymphoid clusters overlaid by well organized epithelium. Whereas lymphocytes within agglomerates were predominantly Bu-1a⁺, a majority of those emigrating onto the supporting membrane were Bu-1a⁻ and IgM⁺. Both agglomerates and emigrant cells expressed activation-induced deaminase with levels increasing after 24 hours. Emigrating cells were actively proliferating at a rate in excess of both the starting cell population and the population of cells remaining in agglomerates. The potential usefulness of this system for investigating the response of bursal tissue to avian Newcastle disease virus (strain AF2240) was examined.

Antibody repertoire development in fetal and neonatal piglets. XXIII: fetal piglets infected with a vaccine strain of PRRS Virus display the same immune dysregulation seen in isolator piglets

The Ig levels and antibody repertoire diversification in fetal piglets infected with an attenuated Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) were measured. Serum Ig levels were greatly elevated in PRRSV-infected fetuses; IgG was elevated >50-fold, IgM>5-15-fold and IgA>2-fold compared to control fetuses. Their IgM to IgG to IgA profile was the same as that in isolator piglets infected for the same period with wild-type PRRSV. Fetal animals showed less repertoire diversification than even isolator piglets that were maintained germfree (GF) while the repertoire diversification index (RDI) for PRRSV-infected isolator piglets was 10-fold higher and comparable to littermates infected with swine influenza (S-FLU). However, when expressed as the RDI:Ig ratio, infected fetuses appeared 10-fold less capable of repertoire diversification than uninfected littermates and GF isolator piglets. Compared to S-FLU isolator piglets that resolve the infection, the RDI:Ig of PRRSV-infected isolator piglets was 100-fold lower. Overall, infection of fetuses with an attenuated virus shows the same immune dysregulation seen postnatally in wild type infected isolator piglets, indicating that: (a) attenuation did not alter the ability of the virus to cause dysregulation and (b) the isolator infectious model reflects the fetal disease.

Brca1 in immunoglobulin gene conversion and somatic hypermutation

Defects in Brca1 confer susceptibility to breast cancer and genomic instability indicative of aberrant repair of DNA breaks. Brca1 was previously implicated in the homologous recombination pathway via effects on the assembly of recombinase Rad51. Activation-induced cytidine deaminase (AID) deaminates C to U in B lymphocyte immunoglobulin (Ig) DNA to initiate programmed DNA breaks. Subsequent uracil-glycosylase mediated U removal, and perhaps further processing, leads to four known classes of mutation: Ig class switch recombination that results in a region-specific genomic deletion, Ig somatic hypermutation that introduces point mutations in Ig V-regions, Ig gene conversion in vertebrates that possess Ig pseudo-V genes, and translocations common to B cell lymphomas. We tested the involvement of Brca1 in AID-dependent Ig diversification in chicken DT40 cells. The DT40 cell line diversifies IgVlambda mainly by gene conversion, and less so by point mutation. Brca1-deficiency caused a shift in Vlambda diversification, significantly reducing the proportion of gene conversions relative to point mutations. Thus, Brca1 regulates AID-dependent DNA lesion repair. Interestingly, while Brca1 is required to recruit ubiquitinated FancD2 to DNA damage, the phenotype of Brca1-deficient DT40 differs from the one of FancD2-deficient DT40, in which both gene conversion and non-templated mutations are impaired.

Diversified bursal medullary B cells survive and expand independently after depletion following neonatal infectious bursal disease virus infection

The primary immunoglobulin repertoire of chickens is generated not by gene rearrangement but by a subsequent process of gene conversion in proliferating immature B cells within the follicles of a specialized gut-associated lymphoid organ, the bursa of Fabricius. Neonatal infection with infectious bursal disease virus can eliminate almost the entire bursal B-cell compartment. Thereafter, two types of follicle reappear. Larger follicles, with rapidly proliferating B cells and normal structure, are correlated with partial recovery of antibody response. Smaller follicles, lacking distinct cortex and medulla, appear unable to produce antigen-responsive B cells. To understand the genesis of the two types of follicle, we analysed their VL sequences and activation-induced deaminase mRNA levels. The results provide a model of bursal repopulation in which surviving bursal stem cells generate new follicles with normal morphology and function, while surviving medullary B cells continue to proliferate slowly, under the influence of stromal cells, giving rise to the smaller follicles. The latter remain fixed in a stage of development incapable of further gene diversification.

AID in somatic hypermutation and class switch recombination

Somatic hypermutation and class-switch-recombination are initiated by the deamination of deoxycytosine in DNA by activation-induced-deaminase, AID. Recently, there has been much research into how AID targets double-stranded DNA in sub-regions of Ig genes, the involvement of co-factors and posttranslational modifications in this process, the co-option of DNA 'repair' mechanisms and AID evolution.

Immunoglobulin Gene Diversification

Three processes alter genomic sequence and structure at the immunoglobulin genes of B lymphocytes: gene conversion, somatic hypermutation, and class switch recombination. Though the molecular signatures of these processes differ, they occur by a shared pathway which is induced by targeted DNA deamination by a B cell-specific factor, activation induced cytidine deaminase (AID). Ubiquitous factors critical for DNA repair carry out all downstream steps, creating mutations and deletions in genomic DNA. This review focuses on the genetic and biochemical mechanisms of diversification of immunoglobulin genes.