Abstract
We propose a model by which an increase in the genomic modification, 5-hydroxymethylcytosine (5hmC), contributes to B cell death within the chicken bursa of Fabricus (BF) infected with infectious bursal disease virus (IBDV). Our findings indicate that, following an IBDV infection, Rhode Island Red (RIR) chickens have fewer surviving B cells and higher levels of 5hmC in the BF than the more resistant 15l line of birds. Elevated genomic 5hmC levels within the RIR BF are associated with markers of immune responses: infiltrating T cells and increased expression of CD40L, FasL and iNOS. Such changes correlate with genomic fragmentation and the presence of IBDV capsid protein, VP2. To explore the effects of CD40L, the immature B cell line, DT40, was exposed to recombinant chicken CD40L that resulted in changes in nuclear 5hmC distribution. Collectively, our observations suggest that T cell infiltration exacerbates early immunopathology within the BF during an IBDV infection contributing to B cell genomic instability and death to facilitate viral egress and immunosuppression.
Highlights
The gut associated lymphoid tissue (GALT) is a critical site for B cell development for several species, including the illeal Peyer's patches (IPP) of cattle and sheep, the appendix of the rabbit, and most famously the bursa of Fabricius (BF) in birds (Parra et al, 2013; Ratcliffe, 2006)
Activated T cells may interact with other immune cells including B cells through a range of secreted and cell surface signaling factors, and while cytokine responses have been analysed in Infectious bursal disease virus (IBDV) infections (Liu et al, 2010; Wang et al, 2014), the levels of cell surface molecules have been given less attention. We found that both FasL and CD40 ligand (CD40L) mRNA levels were significantly increased within 48 hpi in BF samples from Rhode Island Red (RIR) birds compared to mock infected RIR birds, with no significant increase in either mRNA between mock and infected BF in samples taken prior to 48 hpi (Fig. 1D)
After 48 hpi, Bu-1þ cells decrease dramatically but remaining cells that visibly have some Bu-1 cell surface expression have 5hmC immunofluorescence that is much brighter with a more expansive nuclear distribution (Fig. 4D), than Bu-1þ cells from a mock infected BF (Fig. 4C). In this current study we propose a model to explain the events in the BF during early pathogenesis of a very virulent IBDV, UK661 infection
Summary
The gut associated lymphoid tissue (GALT) is a critical site for B cell development for several species, including the illeal Peyer's patches (IPP) of cattle and sheep, the appendix of the rabbit, and most famously the bursa of Fabricius (BF) in birds (Parra et al, 2013; Ratcliffe, 2006). Methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) are key to protecting the genome in these cells (Ciccone et al, 2014). The modification 5mC is reported to maintain genomic stability and integrity (Hackett and Surani, 2013), and we find that B cells in the BF have high levels of genomic 5mC. A modification involved in complete genomic demethylation, 5-hydroxymethylcytosine (5hmC), along with mRNAs encoding the enzymes that catalyse the formation of 5hmC, Tet (Ten-11 translocation 1) and Tet (Ciccone et al, 2014) are progressively reduced during bursal B cell development
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