Transcriptional Regulatory Patterns of Activation-Induced Cytidine Deaminase (AID) Expression in the Context of Plasmodium Falciparum Infection
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Date
2019-07
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Publisher
University of Ghana
Abstract
Activation-Induced cytidine Deaminase (AID) initiates two important immunoglobulin gene
modification processes in B lymphocytes; somatic hypermutation (SHM) and Class Switch
Recommendation (CSR). Despite this important function, the aberrant expression of AID is
associated with a number of cancers. Although the regulation of AID is strict and complex, there
is evidence that Plasmodium falciparum infections can lead to a deregulated expression of the
enzyme. It is not known how infection with the malaria parasite mediate this deregulated
expression and activity. The aim of this work was to investigate the transcriptional regulatory
patterns of AID and transcriptional factors (TFs) associated with AID expression. First, the
transcriptional regulatory patterns of AID expression in P. falciparum-exposed geminal center Bcells
was investigated in tonsillar mononuclear cells (MNCs) obtained from six (6) children from
a malaria holoendemic region (Ghana) and one (1) child from a malaria free region (New Mexico,
USA). Total MNCs were sorted into four distinct sub-populations; naive B lymphocytes (NB),
germinal center B lymphocytes (GC), memory B lymphocytes (MB) and plasmablasts (PB). The
expression patterns of AID and its alternatively spliced variants, as well as the AID related TFs,
Pax5, HoxC4, Bcl6, Bach2, Irf8, Prdm-1, XBP-1, miR181b and miR155 were investigated in the
B-cell sub-populations by qRT-PCR. The Ghanaian tonsils had GC B-cell frequencies ranging
from 14% to 51%, and GC B cells accounted for only 7% of the total MNCs in the New Mexican
tonsil. While the expression of positive AID TFs Pax5, HoxC4, Bcl6, Bach2 and Irf8 were
significantly upregulated (p < 0.0001), negative regulatory TFs of AID, Prdm-1 and XBP-1 were
significantly downregulated (p < 0.0001) in GC B cells compared to PBs. Together with the fulllength
AID mRNA transcript (AIDFL), AID-ΔE4, AID-ΔE4, AID-ΔE4a, and AID-ivs3 were
upregulated (p < 0.0001) in GC B cells compared to NB, MB and PBs. The expression of miR181b was down-regulated in all the B-cell subsets, but miR155 was significantly (p < 0.0001) downregulated
in both MB and GC B lymphocytes. The expression patterns of AID and related TFs
were investigated in children with asymptomatic P. falciparum infection. In asymptomatic infected
children, significantly (p = 0.00197) higher levels of AID transcripts were observed compared to
uninfected children, independent of the EBV infection status. The expression of Bcl6, Pax5 Prdm-
1 and Irf4 were also higher in children with asymptomatic infection compared to uninfected
children, but the expression of XBP-1 and Irf8 did not vary significantly (p > 0.05). Lastly, the
expression patterns of AID and related TFs were assayed in total white blood cells (WBCs) isolated
from whole blood of children between ages 0 -3 years with either severe malaria anemia (SMA),
uncomplicated malaria (Non-SMA) or febrile but aparasitemic (AP). Elevated AID transcripts
levels were observed in children with SMA, which was not associated with Hb levels (Pearson’s
R, p value: AP = -0.36, 0.35, non-SMA = 0.26, 0.16 and SMA = 0.09, 0.62), fever, or body
temperature (Pearson’s coefficient, R, p: AP = 0.29, 0.36, Non-SMA = 0.20, 0.24 and SMA = 0.16,
0.38). The levels of AID transcripts correlated with parasite loads in children with SMA (Pearson’s
R = 0.41, p = 0.018), but not with titers of IgG (Pearson’s coefficient, R, p: Non-SMA = 0.1946,
0.6022 and SMA = -0.0414, 0.1903) and IgM (Pearson’s coefficient, R, p: Non-SMA = -0.1241,
0.6022 and SMA = 0.26, 0.1903) to EBV viral capsid antigen (VCA). While Irf8 and XBP-1 levels
were higher in P. falciparum infected children, irrespective of the SMA status (p < 0.01), Pax5
transcript levels were higher (p < 0.01) in SMA children than non-SMA children. The expression
levels of Prdm-1 and Irf4 were not different (p > 0.05) among SMA, non-SMA and Aparasitemic
children. The expression of miR181b was lowest in SMA children, and miR155 levels were not
different in all three groups. All four splice variants of AID were significantly higher in SMA
children than non-SMA and aparasitemic children. In conclusion, enhanced AID transcription in GC B lymphocytes from Ghanaian tonsils was associated with elevated levels of Pax5, HoxC4,
Bcl6, Bach2 and Irf8, as well as the down-regulation of miR155 and the expression of AID
alternative splice variants. P. falciparum infection is associated with increased levels of AID and
Pax5 transcripts in both asymptomatic and SMA children. This study is the first to document the
effect of asymptomatic and acute P. falciparum infection on the transcriptional regulatory patterns
of AID.
Description
PHD. Molecular Cell Biology of Infectious Diseases
Keywords
Activation-Induced cytidine Deaminase (AID), Somatic hypermutation (SHM), Class Switch Recommendation (CSR), AID, Infection, Regulation