Rheumatoid arthritis-associated peptidylarginine deiminase 4 haplotype citrullinates human immunodeficiency virus-pol and confers antiviral activity

Peptidylarginine deiminase 4 (PADI4)-mediated citrullination modulates immune responses and drives autoantibody formation in rheumatoid arthritis (RA). Two common haplotypes, defined by four exonic SNPs ( PADI4_89 , 90 , 92 , and 104 ), have been described, with haplotype 2 (the risk haplotype associated with increased mRNA stability) conferring increased susceptibility to RA. Citrullination is recognized as a key antimicrobial mechanism shaping host immunity, yet the broader impact of PADI4 polymorphisms on host defense remains unknown. Here, using pseudotyped and replication-competent HIV-1 models, we found that haplotype 2 reduces viral infectivity, suppressing luciferase activity by 88% and p24 production by 66%. Among the four Hap2-associated SNPs examined, all substitutions significantly attenuated the antiviral activity of PADI4 Hap2, with G163A and T349C showing the most pronounced effects. PADI4 knockout in THP-1 cells and PBMCs increased viral replication, supporting an intrinsic antiviral role. Biochemical analyses were consistent with citrullination of the HIV-1 Pol subunits p66 and p51 by PADI4 and with impaired reverse transcription, whereas a catalytic mutant (D473A) abolished these effects. Our findings support an association between PADI4 haplotype 2 and reduced HIV-1 infectivity, and suggest that PADI4-mediated citrullination may contribute to antiviral host defense, highlighting a potential connection between autoimmunity-associated alleles and antiviral immunity.