Many citrullinated proteins are known autoantigens in rheumatoid arthritis, a disease mediated by inflammatory cytokines, such as tumor necrosis factor-alpha (TNFalpha). Citrullinated proteins are generated by converting peptidylarginine to peptidylcitrulline, a process catalyzed by the peptidylarginine deiminases (PADs), including PAD1 to PAD4 and PAD6. Several major risk factors for rheumatoid arthritis are associated with heightened citrullination. However, the physiological role of citrullination in immune cells is poorly understood. We report that suppression of PAD activity attenuates Toll-like receptor-induced expression of interleukin-1beta (IL-1beta) and TNFalpha by neutrophils in vivo and in vitro but not their global transcription activity. Mechanistically, PAD4 directly citrullinates nuclear factor kappaB (NF-kappaB) p65 and enhances the interaction of p65 with importin alpha3, which brings p65 into the nucleus. The citrullination-enhanced interaction of p65 with importin alpha3 and its nuclear translocation and transcriptional activity can be attributed to citrullination of four arginine residues located in the Rel homology domain of p65. Furthermore, a rheumatoid arthritis-prone variant of PAD4, carrying three missense mutations, is more efficient in interacting with p65 and enhancing NF-kappaB activity. Together, these data not only demonstrate a critical role of citrullination in an NF-kappaB-dependent expression of IL-1beta and TNFalpha but also provide a molecular mechanism by which heightened citrullination propagates inflammation in rheumatoid arthritis. Accordingly, attenuating p65-mediated production of IL-1beta and TNFalpha by blocking the citrullination of p65 has great therapeutic potential in rheumatoid arthritis.