Functionally active NKG2A-expressing natural killer cells are elevated in rheumatoid arthritis patients compared to psoriatic arthritis patients and healthy donors

OBJECTIVES:
Natural killer cell receptors (NKR) have been implicated in rheumatoid (RA) and psoriatic arthritis (PsA) pathogenesis. To gain more insight into their role, we characterised NKR (co-)expression patterns on NK and T cells and NK cell function in RA and PsA. 
METHODS:
The frequency of NK and T cells expressing killer like immunoglobulin (KIR) and NKG2 receptors and natural cytotoxicity receptors was assessed by 10-colour flow cytometry in peripheral blood of 23 RA, 12 PsA patients and 18 healthy donors (HD). NK cell cytotoxicity and IFN-gamma production was assessed in 8 RA patients and 8 HD. 
RESULTS:
In RA but not PsA, the frequency of NK cells (median; range) expressing NKG2A (42%; 14-81%) was elevated compared to HD (23%; 9-58%). NKG2A+ NK cells predominantly lack KIR, but display normal cytotoxicity and IFN-γ production. In contrast, RA patients with normal NKG2A+ NK cell frequency have less functional NK cells compared to HD. T cells expressing Fc-gamma receptor CD16 were elevated in RA (median 0.75%) versus HD (0.3%). Furthermore, T cells expressing the KIRs CD158ah in both RA (0.7%) and PsA (0.3%), and CD158e1e2 in RA (1.5%) were elevated compared to HD (0.2% and 0.4%, respectively). In RA, CD4+ T cells expressing the KIRs CD158ah, CD158b1b2j and CD158e1e2 were low (<2%) but significantly elevated compared to HD. 
CONCLUSIONS:
This study demonstrates the presence of an elevated, functionally active NKG2A+ KIR- NK cell population in RA. Together with an elevated frequency of NKR-expressing T cells, these changes may reflect differential pathogenetic involvement.