statistical analyses were conducted using a two - tailed unpaired student ’ s t - test, with p values indicated ( ns, p > 0. 05 ). g immunoblot of cox - 2 and ythdf2 in hacat cells with or without ythdf2 knockdown. h – k qpcr analysis of tnf - α, il - 6, cox - 2 and il - 1β mrnas in hacat cells transduced with shnc ( short hairpin rna negative control ), shdf2 - 1 or shdf2 - 2 ( short hairpin rnas targeting ythdf2 ), with or without uvb. l. immunoblot analysis of ythdf2 and cox - 2 in hacat cells with or without ythdf2 knockdown, with or without uvb. m – q qpcr analysis of ythdf2, tnf - α, il - 6, cox - 2 and il - 1β in nhek cells transfected with sinc ( small interfering rna negative control ) or sidf2 ( sirna targeting ythdf2 ), with or without uvb. r immunoblot of ythdf2 in hacat cells expressing ev ( empty vector ) or oe df2 ( ythdf2 overexpression ), with or without uvb. s – u. qpcr analysis of il - 1β, il 6 and tnf - α in cells as in r. v immunoblot of cox - 2 in cells as in r. β - actin served as internal control for qpcr ( h – k, m – q, s – u ). statistical analyses were conducted using a two - tailed unpaired student ’ s t - test, with p values indicated ( ns, p > 0. 05 ). data are shown as mean ± se ( n = 5 for e; n = 4 for h – k, m – q, s – u ) or mean ± sd ( n = 3 for d ). all experiments were conducted using biologically independent samples. a pathway enrichment of genes upregulated upon ythdf2 knockdown in hacat cells ( rna - seq, q < 0. 05, deseq2 ). b overlap between the top 20 uvb - induced pathways ( rna - seq ) and the top 10 pathways of ythdf2 - interacting proteins ( mass spectrometry, 1 h post - sham or - uvb ). c representative h & e staining of skin sections 24 h after sham or uvb exposure in wt ( ythdf2 ) and df2 cko ( k14cre; ythdf2 ) mice. n = 3 mice per group. d quantification of epidermal thickness ( n = 3 ). e cd45 cell counts in skin of sham - and uvb - irradiated wt and df2 cko mice ( n = 5 ). f box plots of ythdf2 expression across systemic lupus erythematosus ( sle ) datasets. centre line, median; box, interquartile range; whiskers, minimum to maximum. statistical analyses were conducted using a two - tailed unpaired student ’ s t - test, with p values indicated ( ns, p > 0. 05 ). g immunoblot of cox - 2 and ythdf2 in hacat cells with or without ythdf2 knockdown. h – k qpcr analysis of tnf - α, il - 6, cox - 2 and il - 1β mrnas in hacat cells transduced with shnc ( short hairpin rna negative control ), shdf2 - 1 or shdf2 - 2 ( short hairpin rnas targeting ythdf2 ), with or without uvb. l. immunoblot analysis of ythdf2 and cox - 2 in hacat cells with or without ythdf2 knockdown, with or without uvb. m – q qpcr analysis of ythdf2, tnf - α, il - 6, cox - 2 and il - 1β in nhek cells transfected with sinc ( small interfering rna negative control ) or sidf2 ( sirna targeting ythdf2 ), with or without uvb. r immunoblot of ythdf2 in hacat cells expressing ev ( empty vector ) or oe df2 ( ythdf2 overexpression ), with or without uvb. s – u. qpcr analysis of il - 1β, il 6 and tnf - α in cells as in r. v immunoblot of cox - 2 in cells as in r. β - actin served as internal control for qpcr ( h – k, m – q, s – u ). statistical analyses were conducted using a two - tailed unpaired student ’ s t - test, with p values indicated ( ns, p > 0. 05 ). data are shown as mean ± se ( n = 5 for e; n = 4 for h – k, m – q, s – u ) or mean ± sd ( n = 3 for d ). all experiments were conducted using biologically independent samples. uvb damage induces inflammation, which clinically presents as sunburn. to determine whether ythdf2 regulates uvb - induced inflammation in vivo, we generated a mouse model with skin - specific conditional knockout of ythdf2 ( df2 cko ) and assessed differences in uvb - induced histological alteration between wild - type ( wt ) and df2 cko mice. we noted that skin - specific ythdf2 deletion increased epidermal thickness upon sham or uvb irradiation ( fig. 1c, d ), suggesting that ythdf2 loss enhances uvb - induced inflammation. furthermore, flow cytometric analysis showed that skin - specific ythdf2 deletion specifically increased the number of cd45 cells and tcrγδ t cells following uvb irradiation in the skin ( fig. 1e, supplementary fig. s2a, b ); however, it did not significantly alter the number of other immune cells analyzed ( supplementary fig. s2b – d ). next, we analyzed the effect of skin - specific ythdf2 deletion on the systemic immune system in the spleen, blood, and lymph node ( ln ). skin - specific ythdf2 deletion did not significantly alter spleen weight or immune cell counts in the spleen or ln ( supplementary fig. s3a – c ). it is worthwhile to note that skin - specific ythdf2 deletion increased the number of cd11b myeloid cells in both the blood and spleen, pmn - mdscs and ly6c high cells in the blood, and m - mdsc cells in the spleen, while it decreased ly6c low cells in the blood ( supplementary fig. s4a – c ). these findings indicate that skin ythdf2 deletion augments skin inflammation and likely alters the systemic inflammatory response following uvb irradiation. given the role of ythdf2 in modulating skin and systemic inflammation, we hypothesized that its expression may be altered in autoimmune disor ders, where inflammation plays a central role in disease progression. indeed, ythdf2 expression was decreased in systemic lupus erythematosus ( sle ) and type i diabetes as compared with healthy controls ( fig. 1f, supplementary fig. s5a ). in contrast, we did not observe consistent changes in other autoimmune diseases ( supplementary fig. s5b – d ). these findings implicate that ythdf2 may play an important, yet selective, role in autoimmune diseases, likely associated with its role in inflammation. further investigations are warranted to define the role of ythdf2 in the pathogenesis of sle and type i diabetes. prompted by the observed role of ythdf2 in skin inflammation in vivo, we hypothesized that keratinocyte - intrinsic ythdf2 controls inflammatory gene expression and suppresses uvb - induced inflammation. indeed, we found that knockdown of ythdf2 increased the expression of inflammatory mediators such as tnf - α, il - 6, cox - 2 / ptgs2, and il - 1β in both non-tumorigenic human hacat keratinocytes and normal human epidermal keratinocytes ( nhek ) following uvb irradiation; it also increased the baseline expression of multiple inflammatory genes in a431 skin cancer cells ( fig. 1g – q and supplementary fig. s6a – c ). furthermore, we found that skin - specific ythdf2 deletion increased cox - 2 expression upon sham or uvb irradiation ( supplementary fig. s6d ), supporting our findings in human cells that ythdf2 loss enhances uvb - induced inflammatory gene expression. conversely, forced overexpression of ythdf2 inhibited uvb - induced expression of these inflammatory mediators ( fig. 1r – v ). as an mrna ma reader, ythdf2 destabilizes ma - containing mrnas. intriguingly, although ythdf2 knockdown increased mrna levels of pro - inflammatory genes, it had no effect on the mrna stability of il - 8, tgfβ, vegf, cox - 2, gm - csf, il - 6, il - 1α, or il - 1β, while it decreased the mrna stability of tnf - α and il - 16 in hacat cells ( supplementary fig. s6e ). these findings suggest that ythdf2 loss may not directly inhibit mrna decay of pro - inflammatory genes, but rather may indirectly enhance pro - inflammatory gene expression by activating upstream pathways that promote the expression of these genes. to determine the molecular mechanism by which ythdf2 regulates pro - inflammatory gene expression, we analyzed mrna ma modification levels in polyadenylated rnas using ultra - high - performance liquid chromatography - tandem mass spectrometry ( uhplc - ms / ms ). indeed, loss of ythdf2 increased mrna ma enrichment in hacat cells ( fig. 2a ). next, we performed ma - seq to identify potential transcriptome - wide ma - modified gene targets for ythdf2.