HLA and anti-citrullinated protein antibodies: Building blocks in RA

Antibodies against citrullinated proteins (ACPAs) are specific for rheumatoid arthritis (RA). ACPA-positive RA is a chronic inflammatory disease resulting from the complex interaction between genetic (mainly HLA class II genes) and environmental factors (mainly smoking). Recent findings have offered new insights into where, when and how anti-citrulline immunity develops. Some studies have found that a mucosal site, such as the lungs, may function as the initiating site for the immune response against citrullinated proteins, in line with the known association between smoking and ACPA. Other studies, focusing rather on the HLA associations, have suggested that cross-reactivity between microbial sequences and citrullinated self-proteins may lead to ACPA formation. Once ACPAs have developed, they can circulate throughout the body and upon reaching the joints exert direct pathogenic effects themselves. ACPAs can target first the bone compartment of the joints to activate osteoclasts and release interleukin (IL)-8 that in turn will promote bone loss and pain-like behaviour. In the current review, we will present the current understanding of the genetic associations in RA contributing to ACPA occurrence and offer insight in the latest findings explaining how and why autoimmunity generated in the lungs of genetically susceptible hosts might lead to chronic inflammation in the joints.