Retinoic Acid Improves Defective TLR9/RP105-Induced Immune Responses in Common Variable Immunodeficiency–Derived B Cells

Abstract

Common variable immunodeficiency (CVID) is a disease that is characterized primarily by low levels of serum Igs, resulting in a high incidence of infections. It also has been associated with impaired B cell signaling via TLR9 and reduced serum levels of vitamin A. Given the established link between vitamin A deficiency and increased susceptibility to infections, we investigated the ability of the vitamin A metabolite all-trans retinoic acid (RA) to restore the defective immune responses in CVID-derived B cells activated through the TLRs TLR9 and RP105. We demonstrate that RA almost normalizes proliferation and IL-10 secretion in patient-derived B cells. IgG secretion is also partially restored, but to a more moderate extent. This can be explained by impaired RA-mediated isotype switching in TLR9/RP105-stimulated CVID-derived B cells owing to reduced induction of activation-induced deaminase. Accordingly, these B cells secreted higher levels of IgM than did normal B cells, and RA augmented IgM secretion. The ability of RA to improve critical immune parameters in CVID-derived B cells stimulated through TLR9 and RP105 support the possibility of combining RA with TLR stimulation for the treatment of CVID.

Retinoic acid-induced IgG production in TLR-activated human primary B cells involves ULK1-mediated autophagy

“…Likewise, the RAR antagonist prevented the RA-induced ULK1 expression and LC3B-II formation. As we previously also have shown that RA-induced secretion of IgG in TLR9-and CD180-activated B cells is mediated via RARs, 24 it appears that autophagy is a link connecting RAR-mediated gene expression and RA-induced antibody production. This notion was further strengthened by the fact that 1) blocking RAR-mediated gene expression, inhibited the RA-mediated autophagy, and 2) blocking autophagy by ULK1 siRNA nearly reduced the level of RAinduced IgG to the background level observed in CpG and anti-CD180-stimulated cells.…”

“…17,18 We have previously shown that RA enhances proliferation and Ig-production in B cells stimulated via TLRs. 23,24,31 In the present study we provide a novel mechanism whereby vitamin A may stimulate the immune system, by demonstrating that RA-induced secretion of IgG from TLR9-and CD180-stimulated human B cells involves ULK1-mediated autophagy. Hence, RA promoted both the formation of LC3B-II and the expression of ULK1, whereas blocking autophagy by knockdown of ULK1 reduced the RA-mediated production of IgG.…”

“…8,9 Having previously demonstrated the stimulatory effect of RA on IgG-production in normal human B cells activated via TLR9 alone or in combination with CD180, 23,24,31 we here investigated the possible involvement of autophagy in this process. The MAP1LC3/LC3 (microtubuleassociated protein 1 light chain 3) protein is a widely used and accepted marker of phagophores and autophagosomes, 32 and is commonly used to monitor the levels of autophagy.…”

“…Given the recently acknowledged importance of autophagy in plasma cell formation and Ig secretion, 8,9 and having previously shown that RA potentiates IgG secretion in human peripheral B cells stimulated via TLRs, 23,24,31 we next asked whether autophagy was involved in RA-mediated induction of IgG. To this end, B cells were transfected with siRNA against ULK1 or with a control siRNA, and as shown in Figure 6A, knocking down ULK1 reversed the RA-induced formation of LC3B-II.…”

“…21 TLR9, located in the membrane of endosomes, recognizes CpG motifs of bacterial DNA and is activated in vitro by synthetic unmethylated CpG oligomers. 22 We 23,24 and others 25 have revealed that TLR9-mediated activation of B cells can be significantly enhanced by costimulation with antibodies against the TLR-related receptor CD180/ RP105. CD180 was originally identified as a radio-protective protein of 150 kDa 26,27 and the in vivo ligand for CD180 remains unknown.…”