FcγRIIIa Signaling Modulates Endosomal TLR Responses in Human CD4+ T Cells

Abstract

Summary Recognition of antibody-opsonized pathogens by immune cells triggers both toll-like receptor (TLR) and Fc-receptor (FcR) signaling. FcRs endocytose modified nucleic acids bound to antibodies and deliver them to endosomes, where they are recognized by nucleic acid-sensing TLRs (NA-TLRs). We show that in CD4+ T-cells NA-TLRs, TLR3, TLR8 and TLR9 are upregulated by FcγRIIIa-pSyk cosignaling and localize with FcγRIIIa on the cell surface. TLR9 accumulates on the cell surface, where it recognizes CpG oligonucleotide (ODN) 2006. Subcellular location of NA-TLRs is a key determinant in discriminating self vs. viral nucleic acid. Hydroxychloroquine used for treating systemic lupus erythematosus and a Syk inhibitor blocked NA-TLR localization with FcγRIIIa. Engaging TLR9 with CpG ODN contributes to the development of IL17A+ and IL-21+ populations. RNA-seq analysis showed up-regulation of proinflammatory cytokines, NF-κB signaling and heat shock protein pathway RNA transcripts. These data suggest a role for FcγRIIIa-pSyk cosignaling in modulating NA-TLR responses in human CD4+ T-cells by affecting the amounts and cellular distribution. These events are important for understanding of autoimmune pathology.

A contribution of FcγRIIIa cosignaling in TFHsubset development in Systemic Lupus Erythematosus

“…In human B cells, cosynergistic signaling from ITAM and MyD88 enahnces B cell receptor activation [29]. We previously reported that the TLR9 and FcgRIIIa joint cosignaling enhances IL-17A and IL-21 production [18]. Thus, we analyzed IL-17A and IL-21 prodution from synergistic cosignaling in Bcl6 + T FH cells.…”

“…The ICs conjugates were in the range of 23 to 30 µM fluorochrome to-protein ratio. Purification and conjugation was performed as described previously [16,18]. All conjugates were titrated and flow compensation was done using isotype controls.…”

“…FcgRIIIa activation represents a state of stress, since a number of genes that were upregulated are stress response genes. A number of heat shock proteins were upregulated upon FcgRIIIa cosignaling [18]. A significant increase in the expression of G-protein coupled receptor FcgRIIIa cosignaling upregulates miRNAs that are associated with T FH development:…”

“…Chromatin-IgG complexes activate B cells by the dual engagement of IgM and TLRs [46]. In CD4 + T cells, DNA/RNA containing ICs by engaging FcgRIIIa and TLR9 could thus enhance T FH development [18]. CpG 2006 bound to TLR9 on T cell surface and MyD88 protein co-immunoprecipitates with FcgRIIIa protein, which suggest a coordinated signaling in CD4 + T cells [18].…”

“…In CD4 + T cells, DNA/RNA containing ICs by engaging FcgRIIIa and TLR9 could thus enhance T FH development [18]. CpG 2006 bound to TLR9 on T cell surface and MyD88 protein co-immunoprecipitates with FcgRIIIa protein, which suggest a coordinated signaling in CD4 + T cells [18]. In B cells, joint signaling from ITAM (in FcRs) and MyD88 ( in NA-TLRs) is distinct, compared to individual signaling from either ITAM or MyD88 [29].…”