Neutrophils are the
most prevalent immune cells in synovial fluid in inflamed joints of children
with oligoarticular juvenile idiopathic arthritis (JIA).
Despite this, little
is known about neutrophil function at the site of inflammation in JIA and how
local neutrophils contribute to disease pathogenesis. This study aimed to
characterize phenotype and function of synovial fluid neutrophils in
from paired blood and synovial fluid from patients with active oligoarticular
JIA were investigated phenotypically (n=17) and functionally (phagocytosis and
oxidative burst, n=13) by flow cytometry. In a subset of patients (n=6), blood
samples were also obtained during inactive disease at a follow-up visit. Presence of CD206-expressing neutrophils was
investigated in synovial biopsies from four patients by Immune florescence.
synovial fluid had an activated phenotype, characterized by increased CD66b and
CD11b levels, and most neutrophils had a CD16hi CD62Llow
aged phenotype. A large proportion of the
synovial fluid neutrophils expressed CD206, a mannose receptor not
commonly expressed by neutrophils but by monocytes, macrophages and dendritic
cells. CD206 expressing neutrophils were also
found in synovial tissue biopsies. The synovial fluid neutrophil
phenotype was not dependent on transmigration alone. Functionally, synovial fluid
neutrophils had reduced phagocytic capacity and a trend towards impaired
oxidative burst compared to blood neutrophils. In
addition, the effector functions of the synovial fluid neutrophils correlated
negatively with the proportion of CD206+ neutrophils.
Neutrophils in the inflamed
joint in oligoarticular JIA were altered, both regarding phenotype and
function. Neutrophils in the synovial fluid were activated, had an aged
phenotype, had gained monocyte-like features and had impaired phagocytic
capacity. The impairment in
phagocytosis and oxidative burst was associated to the phenotype shift. We
speculate that these neutrophil alterations might play a role in the sustained
joint inflammation seen in JIA.